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8 Commits

Author SHA1 Message Date
Noah Zweben
d53f6ca4cd Remove telegram, discord, and fakechat plugins (#741)
Remove the three chat bridge plugins from external_plugins/ and their
corresponding entries in marketplace.json.

Co-authored-by: Claude <noreply@anthropic.com>
2026-03-18 21:31:12 -07:00
Kenneth Lien
55de7f6d1a Merge pull request #740 from anthropics/marketplace-sorted
Enforce alphabetical sort on marketplace.json plugins
2026-03-18 21:21:36 -07:00
Kenneth Lien
f0fdb72a02 Enforce alphabetical sort on marketplace.json plugins
Adds a sort check as a second step in the existing validate-marketplace
workflow. The script supports --fix to sort in place.

Sorts the existing 86 entries — pure reorder, no content change.
Previously grouped loosely by kind (LSPs first, then internal, then
external); now strictly alphabetical so insertion point is unambiguous.
2026-03-18 16:56:11 -07:00
Noah Zweben
158ef95c6f Add marketplace.json validation CI workflow (#347)
* Add CI workflow to validate marketplace.json on PRs

Add a GitHub Actions workflow that validates marketplace.json is
well-formed JSON with a plugins array whenever PRs modify it. Includes:
- validate-marketplace.ts: Bun script that parses and validates the JSON
- validate-marketplace.yml: GH Actions workflow triggered on PR changes
- test-marketplace-check.js: Unit tests for the validation logic

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Strengthen marketplace validator and remove orphaned test file

- validate-marketplace.ts: check duplicate names and required fields
  (name, description, source) per entry, not just valid JSON
- remove .github/workflows/test-marketplace-check.js: tested a
  checkMarketplaceViolations function that doesn't exist in the PR,
  and was in workflows/ instead of scripts/

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Tobin South <tobin.south@gmail.com>
2026-03-18 23:49:26 +00:00
Kenneth Lien
3de6a94eb9 Register telegram, discord, fakechat channel plugins (#739)
Adds marketplace entries for the three channel plugins landing in
external_plugins/ via #735, #736, #738. iMessage (#737) is intentionally
held back from registration.

telegram and discord go under productivity; fakechat goes under
development since it's a localhost testing harness, not an end-user
messaging bridge.
2026-03-18 23:49:01 +00:00
Kenneth Lien
1b33c1d9f9 Add telegram channel plugin (#735)
Telegram messaging bridge for Claude Code. Runs a local MCP server that
connects to the Telegram Bot API via a user-created bot token.

Built-in access control: inbound messages are gated by an allowlist
(default: pairing mode), outbound sends are scoped to the same allowlist.
The /telegram:access skill manages pairing, allowlists, and policy.

Ships full source — server.ts runs locally via bun, started by the
.mcp.json command. First external_plugins entry to bundle source rather
than point at a hosted MCP endpoint.
2026-03-18 23:46:59 +00:00
Kenneth Lien
4796148ace Add discord channel plugin (#736)
Discord messaging bridge for Claude Code. Runs a local MCP server that
connects to Discord's Gateway via a user-created bot token.

Built-in access control: inbound messages are gated by an allowlist
(default: pairing mode), outbound sends are scoped to the same allowlist.
Guild channels require opt-in and @mention. The /discord:access skill
manages pairing, allowlists, and policy.

Ships full source — server.ts runs locally via bun, started by the
.mcp.json command.
2026-03-18 23:46:41 +00:00
Kenneth Lien
75e1c5d437 Add fakechat channel plugin (#738)
Localhost web chat UI for testing the channel notification flow.
No tokens, no access control, no third-party service. Serves an
iMessage-style UI on localhost; messages posted there arrive as
channel notifications, replies render in the UI.

Useful for developing against the channel protocol without a live
messaging account.

Ships full source — server.ts runs locally via bun, started by the
.mcp.json command.
2026-03-18 23:46:21 +00:00
22 changed files with 946 additions and 3681 deletions

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#!/usr/bin/env bun
/**
* Checks that marketplace.json plugins are alphabetically sorted by name.
*
* Usage:
* bun check-marketplace-sorted.ts # check, exit 1 if unsorted
* bun check-marketplace-sorted.ts --fix # sort in place
*/
import { readFileSync, writeFileSync } from "fs";
import { join } from "path";
const MARKETPLACE = join(import.meta.dir, "../../.claude-plugin/marketplace.json");
type Plugin = { name: string; [k: string]: unknown };
type Marketplace = { plugins: Plugin[]; [k: string]: unknown };
const raw = readFileSync(MARKETPLACE, "utf8");
const mp: Marketplace = JSON.parse(raw);
const cmp = (a: Plugin, b: Plugin) =>
a.name.toLowerCase().localeCompare(b.name.toLowerCase());
if (process.argv.includes("--fix")) {
mp.plugins.sort(cmp);
writeFileSync(MARKETPLACE, JSON.stringify(mp, null, 2) + "\n");
console.log(`sorted ${mp.plugins.length} plugins`);
process.exit(0);
}
for (let i = 1; i < mp.plugins.length; i++) {
if (cmp(mp.plugins[i - 1], mp.plugins[i]) > 0) {
console.error(
`marketplace.json plugins are not sorted: ` +
`'${mp.plugins[i - 1].name}' should come after '${mp.plugins[i].name}' (index ${i})`,
);
console.error(` run: bun .github/scripts/check-marketplace-sorted.ts --fix`);
process.exit(1);
}
}
console.log(`ok: ${mp.plugins.length} plugins sorted`);

77
.github/scripts/validate-marketplace.ts vendored Normal file
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#!/usr/bin/env bun
/**
* Validates marketplace.json: well-formed JSON, plugins array present,
* each entry has required fields, and no duplicate plugin names.
*
* Usage:
* bun validate-marketplace.ts <path-to-marketplace.json>
*/
import { readFile } from "fs/promises";
async function main() {
const filePath = process.argv[2];
if (!filePath) {
console.error("Usage: validate-marketplace.ts <path-to-marketplace.json>");
process.exit(2);
}
const content = await readFile(filePath, "utf-8");
let parsed: unknown;
try {
parsed = JSON.parse(content);
} catch (err) {
console.error(
`ERROR: ${filePath} is not valid JSON: ${err instanceof Error ? err.message : err}`
);
process.exit(1);
}
if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
console.error(`ERROR: ${filePath} must be a JSON object`);
process.exit(1);
}
const marketplace = parsed as Record<string, unknown>;
if (!Array.isArray(marketplace.plugins)) {
console.error(`ERROR: ${filePath} missing "plugins" array`);
process.exit(1);
}
const errors: string[] = [];
const seen = new Set<string>();
const required = ["name", "description", "source"] as const;
marketplace.plugins.forEach((p, i) => {
if (!p || typeof p !== "object") {
errors.push(`plugins[${i}]: must be an object`);
return;
}
const entry = p as Record<string, unknown>;
for (const field of required) {
if (!entry[field]) {
errors.push(`plugins[${i}] (${entry.name ?? "?"}): missing required field "${field}"`);
}
}
if (typeof entry.name === "string") {
if (seen.has(entry.name)) {
errors.push(`plugins[${i}]: duplicate plugin name "${entry.name}"`);
}
seen.add(entry.name);
}
});
if (errors.length) {
console.error(`ERROR: ${filePath} has ${errors.length} validation error(s):`);
for (const e of errors) console.error(` - ${e}`);
process.exit(1);
}
console.log(`OK: ${marketplace.plugins.length} plugins, no duplicates, all required fields present`);
}
main().catch((err) => {
console.error("Fatal error:", err);
process.exit(2);
});

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name: Validate Marketplace JSON
on:
pull_request:
paths:
- '.claude-plugin/marketplace.json'
jobs:
validate:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: oven-sh/setup-bun@v2
- name: Validate marketplace.json
run: bun .github/scripts/validate-marketplace.ts .claude-plugin/marketplace.json
- name: Check plugins sorted
run: bun .github/scripts/check-marketplace-sorted.ts

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@@ -1,8 +0,0 @@
{
"name": "mcp-server-dev",
"description": "Skills for designing and building MCP servers that work seamlessly with Claude — guides you through deployment models (remote HTTP, MCPB, local), tool design patterns, auth, and interactive MCP apps.",
"author": {
"name": "Anthropic",
"email": "support@anthropic.com"
}
}

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@@ -1,202 +0,0 @@
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@@ -1,32 +0,0 @@
# mcp-server-dev
Skills for designing and building MCP servers that work seamlessly with Claude.
## What's inside
Three skills that compose into a full build path:
| Skill | Purpose |
|---|---|
| **`build-mcp-server`** | Entry point. Interrogates the use case, picks deployment model (remote HTTP / MCPB / local stdio), picks tool-design pattern, routes to a specialized skill. |
| **`build-mcp-app`** | Adds interactive UI widgets (forms, pickers, confirm dialogs) rendered inline in chat. Works on remote servers and MCPB bundles. |
| **`build-mcpb`** | Packages a local stdio server with its runtime so users can install it without Node/Python. For servers that must touch the local machine. |
## How it works
`build-mcp-server` is the front door. It asks what you're connecting to, who'll use it, how big the action surface is, and whether you need in-chat UI. From those answers it recommends one of four paths:
- **Remote streamable-HTTP** (the default recommendation for anything wrapping a cloud API) — scaffolded inline
- **MCP app** — hands off to `build-mcp-app`
- **MCPB** — hands off to `build-mcpb`
- **Local stdio prototype** — scaffolded inline with an MCPB upgrade note
Each skill ships reference files for the parts that don't fit in the main instructions: auth flows (DCR/CIMD), tool-description writing, widget templates, manifest schemas, security hardening.
## Usage
Ask Claude to "help me build an MCP server" and the entry skill will trigger. Or invoke directly:
```
/mcp-server-dev:build-mcp-server
```

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@@ -1,289 +0,0 @@
---
name: build-mcp-app
description: This skill should be used when the user wants to build an "MCP app", add "interactive UI" or "widgets" to an MCP server, "render components in chat", build "MCP UI resources", make a tool that shows a "form", "picker", "dashboard" or "confirmation dialog" inline in the conversation, or mentions "apps SDK" in the context of MCP. Use AFTER the build-mcp-server skill has settled the deployment model, or when the user already knows they want UI widgets.
version: 0.1.0
---
# Build an MCP App (Interactive UI Widgets)
An MCP app is a standard MCP server that **also serves UI resources** — interactive components rendered inline in the chat surface. Build once, runs in Claude *and* ChatGPT and any other host that implements the apps surface.
The UI layer is **additive**. Under the hood it's still tools, resources, and the same wire protocol. If you haven't built a plain MCP server before, the `build-mcp-server` skill covers the base layer. This skill adds widgets on top.
---
## When a widget beats plain text
Don't add UI for its own sake — most tools are fine returning text or JSON. Add a widget when one of these is true:
| Signal | Widget type |
|---|---|
| Tool needs structured input Claude can't reliably infer | Form |
| User must pick from a list Claude can't rank (files, contacts, records) | Picker / table |
| Destructive or billable action needs explicit confirmation | Confirm dialog |
| Output is spatial or visual (charts, maps, diffs, previews) | Display widget |
| Long-running job the user wants to watch | Progress / live status |
If none apply, skip the widget. Text is faster to build and faster for the user.
---
## Widgets vs Elicitation — route correctly
Before building a widget, check if **elicitation** covers it. Elicitation is spec-native, zero UI code, works in any compliant host.
| Need | Elicitation | Widget |
|---|---|---|
| Confirm yes/no | ✅ | overkill |
| Pick from short enum | ✅ | overkill |
| Fill a flat form (name, email, date) | ✅ | overkill |
| Pick from a large/searchable list | ❌ (no scroll/search) | ✅ |
| Visual preview before choosing | ❌ | ✅ |
| Chart / map / diff view | ❌ | ✅ |
| Live-updating progress | ❌ | ✅ |
If elicitation covers it, use it. See `../build-mcp-server/references/elicitation.md`.
---
## Architecture: two deployment shapes
### Remote MCP app (most common)
Hosted streamable-HTTP server. Widget templates are served as **resources**; tool results reference them. The host fetches the resource, renders it in an iframe sandbox, and brokers messages between the widget and Claude.
```
┌──────────┐ tools/call ┌────────────┐
│ Claude │─────────────> │ MCP server │
│ host │<── result ────│ (remote) │
│ │ + widget ref │ │
│ │ │ │
│ │ resources/read│ │
│ │─────────────> │ widget │
│ ┌──────┐ │<── template ──│ HTML/JS │
│ │iframe│ │ └────────────┘
│ │widget│ │
│ └──────┘ │
└──────────┘
```
### MCPB-packaged MCP app (local + UI)
Same widget mechanism, but the server runs locally inside an MCPB bundle. Use this when the widget needs to drive a **local** application — e.g., a file picker that browses the actual local disk, a dialog that controls a desktop app.
For MCPB packaging mechanics, defer to the **`build-mcpb`** skill. Everything below applies to both shapes.
---
## How widgets attach to tools
A widget-enabled tool has **two separate registrations**:
1. **The tool** declares a UI resource via `_meta.ui.resourceUri`. Its handler returns plain text/JSON — NOT the HTML.
2. **The resource** is registered separately and serves the HTML.
When Claude calls the tool, the host sees `_meta.ui.resourceUri`, fetches that resource, renders it in an iframe, and pipes the tool's return value into the iframe via the `ontoolresult` event.
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE }
from "@modelcontextprotocol/ext-apps/server";
import { z } from "zod";
const server = new McpServer({ name: "contacts", version: "1.0.0" });
// 1. The tool — returns DATA, declares which UI to show
registerAppTool(server, "pick_contact", {
description: "Open an interactive contact picker",
inputSchema: { filter: z.string().optional() },
_meta: { ui: { resourceUri: "ui://widgets/contact-picker.html" } },
}, async ({ filter }) => {
const contacts = await db.contacts.search(filter);
// Plain JSON — the widget receives this via ontoolresult
return { content: [{ type: "text", text: JSON.stringify(contacts) }] };
});
// 2. The resource — serves the HTML
registerAppResource(
server,
"Contact Picker",
"ui://widgets/contact-picker.html",
{},
async () => ({
contents: [{
uri: "ui://widgets/contact-picker.html",
mimeType: RESOURCE_MIME_TYPE,
text: pickerHtml, // your HTML string
}],
}),
);
```
The URI scheme `ui://` is convention. The mime type MUST be `RESOURCE_MIME_TYPE` (`"text/html;profile=mcp-app"`) — this is how the host knows to render it as an interactive iframe, not just display the source.
---
## Widget runtime — the `App` class
Inside the iframe, your script talks to the host via the `App` class from `@modelcontextprotocol/ext-apps`. This is a **persistent bidirectional connection** — the widget stays alive as long as the conversation is active, receiving new tool results and sending user actions.
```html
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "ContactPicker", version: "1.0.0" }, {});
// Set handlers BEFORE connecting
app.ontoolresult = ({ content }) => {
const contacts = JSON.parse(content[0].text);
render(contacts);
};
await app.connect();
// Later, when the user clicks something:
function onPick(contact) {
app.sendMessage({
role: "user",
content: [{ type: "text", text: `Selected contact: ${contact.id}` }],
});
}
</script>
```
| Method | Direction | Use for |
|---|---|---|
| `app.ontoolresult = fn` | Host → widget | Receive the tool's return value |
| `app.ontoolinput = fn` | Host → widget | Receive the tool's input args (what Claude passed) |
| `app.sendMessage({...})` | Widget → host | Inject a message into the conversation |
| `app.updateModelContext({...})` | Widget → host | Update context silently (no visible message) |
| `app.callServerTool({name, arguments})` | Widget → server | Call another tool on your server |
`sendMessage` is the typical "user picked something, tell Claude" path. `updateModelContext` is for state that Claude should know about but shouldn't clutter the chat.
**What widgets cannot do:**
- Access the host page's DOM, cookies, or storage
- Make network calls to arbitrary origins (CSP-restricted — route through `callServerTool`)
Keep widgets **small and single-purpose**. A picker picks. A chart displays. Don't build a whole sub-app inside the iframe — split it into multiple tools with focused widgets.
---
## Scaffold: minimal picker widget
**Install:**
```bash
npm install @modelcontextprotocol/sdk @modelcontextprotocol/ext-apps zod express
```
**Server (`src/server.ts`):**
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE }
from "@modelcontextprotocol/ext-apps/server";
import express from "express";
import { readFileSync } from "node:fs";
import { z } from "zod";
const server = new McpServer({ name: "contact-picker", version: "1.0.0" });
const pickerHtml = readFileSync("./widgets/picker.html", "utf8");
registerAppTool(server, "pick_contact", {
description: "Open an interactive contact picker. User selects one contact.",
inputSchema: { filter: z.string().optional().describe("Name/email prefix filter") },
_meta: { ui: { resourceUri: "ui://widgets/picker.html" } },
}, async ({ filter }) => {
const contacts = await db.contacts.search(filter ?? "");
return { content: [{ type: "text", text: JSON.stringify(contacts) }] };
});
registerAppResource(server, "Contact Picker", "ui://widgets/picker.html", {},
async () => ({
contents: [{ uri: "ui://widgets/picker.html", mimeType: RESOURCE_MIME_TYPE, text: pickerHtml }],
}),
);
const app = express();
app.use(express.json());
app.post("/mcp", async (req, res) => {
const transport = new StreamableHTTPServerTransport({ sessionIdGenerator: undefined });
res.on("close", () => transport.close());
await server.connect(transport);
await transport.handleRequest(req, res, req.body);
});
app.listen(process.env.PORT ?? 3000);
```
For local-only widget apps (driving a desktop app, reading local files), swap the transport to `StdioServerTransport` and package via the `build-mcpb` skill.
**Widget (`widgets/picker.html`):**
```html
<!doctype html>
<meta charset="utf-8" />
<style>
body { font: 14px system-ui; margin: 0; }
ul { list-style: none; padding: 0; margin: 0; max-height: 300px; overflow-y: auto; }
li { padding: 10px 14px; cursor: pointer; border-bottom: 1px solid #eee; }
li:hover { background: #f5f5f5; }
.sub { color: #666; font-size: 12px; }
</style>
<ul id="list"></ul>
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "ContactPicker", version: "1.0.0" }, {});
const ul = document.getElementById("list");
app.ontoolresult = ({ content }) => {
const contacts = JSON.parse(content[0].text);
ul.innerHTML = "";
for (const c of contacts) {
const li = document.createElement("li");
li.innerHTML = `<div>${c.name}</div><div class="sub">${c.email}</div>`;
li.addEventListener("click", () => {
app.sendMessage({
role: "user",
content: [{ type: "text", text: `Selected contact: ${c.id} (${c.name})` }],
});
});
ul.append(li);
}
};
await app.connect();
</script>
```
See `references/widget-templates.md` for more widget shapes.
---
## Design notes that save you a rewrite
**One widget per tool.** Resist the urge to build one mega-widget that does everything. One tool → one focused widget → one clear result shape. Claude reasons about these far better.
**Tool description must mention the widget.** Claude only sees the tool description when deciding what to call. "Opens an interactive picker" in the description is what makes Claude reach for it instead of guessing an ID.
**Widgets are optional at runtime.** Hosts that don't support the apps surface simply ignore `_meta.ui` and render the tool's text content normally. Since your tool handler already returns meaningful text/JSON (the widget's data), degradation is automatic — Claude sees the data directly instead of via the widget.
**Don't block on widget results for read-only tools.** A widget that just *displays* data (chart, preview) shouldn't require a user action to complete. Return the display widget *and* a text summary in the same result so Claude can continue reasoning without waiting.
---
## Testing
- **Local:** point Claude desktop's MCP config at your server, trigger the tool, check the widget renders and `sendMessage` flows back into the chat.
- **Host fallback:** disable the apps surface (or use a host without it) and confirm the tool degrades gracefully.
- **CSP:** open browser devtools on the iframe — CSP violations are the #1 reason widgets silently fail.
---
## Reference files
- `references/widget-templates.md` — reusable HTML scaffolds for picker / confirm / progress / display
- `references/apps-sdk-messages.md` — the `App` class API: widget ↔ host ↔ server messaging

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@@ -1,120 +0,0 @@
# ext-apps messaging — widget ↔ host ↔ server
The `@modelcontextprotocol/ext-apps` package provides the `App` class (browser side) and `registerAppTool`/`registerAppResource` helpers (server side). Messaging is bidirectional and persistent.
---
## Widget → Host
### `app.sendMessage({ role, content })`
Inject a visible message into the conversation. This is how user actions become conversation turns.
```js
app.sendMessage({
role: "user",
content: [{ type: "text", text: "User selected order #1234" }],
});
```
The message appears in chat and Claude responds to it. Use `role: "user"` — the widget speaks on the user's behalf.
### `app.updateModelContext({ content })`
Update Claude's context **silently** — no visible message. Use for state that informs but doesn't warrant a chat bubble.
```js
app.updateModelContext({
content: [{ type: "text", text: "Currently viewing: orders from last 30 days" }],
});
```
### `app.callServerTool({ name, arguments })`
Call a tool on your MCP server directly, bypassing Claude. Returns the tool result.
```js
const result = await app.callServerTool({
name: "fetch_order_details",
arguments: { orderId: "1234" },
});
```
Use for data fetches that don't need Claude's reasoning — pagination, detail lookups, refreshes.
---
## Host → Widget
### `app.ontoolresult = ({ content }) => {...}`
Fires when the tool handler's return value is piped to the widget. This is the primary data-in path.
```js
app.ontoolresult = ({ content }) => {
const data = JSON.parse(content[0].text);
renderUI(data);
};
```
**Set this BEFORE `await app.connect()`** — the result may arrive immediately after connection.
### `app.ontoolinput = ({ arguments }) => {...}`
Fires with the arguments Claude passed to the tool. Useful if the widget needs to know what was asked for (e.g., highlight the search term).
---
## Server → Widget (progress)
For long-running operations, emit progress notifications. The client sends a `progressToken` in the request's `_meta`; the server emits against it.
```typescript
// In the tool handler
async ({ query }, extra) => {
const token = extra._meta?.progressToken;
for (let i = 0; i < steps.length; i++) {
if (token !== undefined) {
await extra.sendNotification({
method: "notifications/progress",
params: { progressToken: token, progress: i, total: steps.length, message: steps[i].name },
});
}
await steps[i].run();
}
return { content: [{ type: "text", text: "Complete" }] };
}
```
No `{ notify }` destructure — `extra` is `RequestHandlerExtra`; progress goes through `sendNotification`.
---
## Lifecycle
1. Claude calls a tool with `_meta.ui.resourceUri` declared
2. Host fetches the resource (your HTML) and renders it in an iframe
3. Widget script runs, sets handlers, calls `await app.connect()`
4. Host pipes the tool's return value → `ontoolresult` fires
5. Widget renders, user interacts
6. Widget calls `sendMessage` / `updateModelContext` / `callServerTool` as needed
7. Widget persists until conversation context moves on — subsequent calls to the same tool reuse the iframe and fire `ontoolresult` again
There's no explicit "submit and close" — the widget is a long-lived surface.
---
## CSP gotchas
The iframe runs under a restrictive Content-Security-Policy:
| Symptom | Cause | Fix |
|---|---|---|
| Widget renders but JS doesn't run | Inline event handlers blocked | Use `addEventListener` — never `onclick="..."` in HTML |
| `eval` / `new Function` errors | Script-src restriction | Don't use them; use JSON.parse for data |
| External scripts fail | CDN not allowlisted | `esm.sh` is safe; avoid others |
| `fetch()` to your API fails | Cross-origin blocked | Route through `app.callServerTool()` instead |
| External CSS doesn't load | `style-src` restriction | Inline styles in a `<style>` tag |
| Fonts don't load | `font-src` restriction | Use system fonts (`font: 14px system-ui`) |
When in doubt, open the iframe's devtools console — CSP violations log there.

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@@ -1,199 +0,0 @@
# Widget Templates
Minimal HTML scaffolds for the common widget shapes. Copy, fill in, ship.
All templates use the `App` class from `@modelcontextprotocol/ext-apps` via ESM CDN. They're intentionally framework-free — widgets are small enough that React/Vue hydration cost usually isn't worth it.
---
## Serving widget HTML
Widgets are static HTML — data arrives at runtime via `ontoolresult`, not baked in. Store each widget as a string constant or read from disk:
```typescript
import { readFileSync } from "node:fs";
import { registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";
const pickerHtml = readFileSync("./widgets/picker.html", "utf8");
registerAppResource(server, "Picker", "ui://widgets/picker.html", {},
async () => ({
contents: [{ uri: "ui://widgets/picker.html", mimeType: RESOURCE_MIME_TYPE, text: pickerHtml }],
}),
);
```
---
## Picker (single-select list)
```html
<!doctype html>
<meta charset="utf-8" />
<style>
body { font: 14px system-ui; margin: 0; }
ul { list-style: none; padding: 0; margin: 0; max-height: 280px; overflow-y: auto; }
li { padding: 10px 14px; cursor: pointer; border-bottom: 1px solid #eee; }
li:hover { background: #f5f5f5; }
.sub { color: #666; font-size: 12px; }
</style>
<ul id="list"></ul>
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "Picker", version: "1.0.0" }, {});
const ul = document.getElementById("list");
app.ontoolresult = ({ content }) => {
const { items } = JSON.parse(content[0].text);
ul.innerHTML = "";
for (const it of items) {
const li = document.createElement("li");
li.innerHTML = `<div>${it.label}</div><div class="sub">${it.sub ?? ""}</div>`;
li.addEventListener("click", () => {
app.sendMessage({
role: "user",
content: [{ type: "text", text: `Selected: ${it.id}` }],
});
});
ul.append(li);
}
};
await app.connect();
</script>
```
**Tool returns:** `{ content: [{ type: "text", text: JSON.stringify({ items: [{ id, label, sub? }] }) }] }`
---
## Confirm dialog
```html
<!doctype html>
<meta charset="utf-8" />
<style>
body { font: 14px system-ui; margin: 16px; }
.actions { display: flex; gap: 8px; margin-top: 16px; }
button { padding: 8px 16px; cursor: pointer; }
.danger { background: #d33; color: white; border: none; }
</style>
<p id="msg"></p>
<div class="actions">
<button id="cancel">Cancel</button>
<button id="confirm" class="danger">Confirm</button>
</div>
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "Confirm", version: "1.0.0" }, {});
app.ontoolresult = ({ content }) => {
const { message, confirmLabel } = JSON.parse(content[0].text);
document.getElementById("msg").textContent = message;
if (confirmLabel) document.getElementById("confirm").textContent = confirmLabel;
};
await app.connect();
document.getElementById("confirm").addEventListener("click", () => {
app.sendMessage({ role: "user", content: [{ type: "text", text: "Confirmed." }] });
});
document.getElementById("cancel").addEventListener("click", () => {
app.sendMessage({ role: "user", content: [{ type: "text", text: "Cancelled." }] });
});
</script>
```
**Tool returns:** `{ content: [{ type: "text", text: JSON.stringify({ message, confirmLabel? }) }] }`
**Note:** For simple confirmation, prefer **elicitation** over a widget — see `../build-mcp-server/references/elicitation.md`. Use this widget when you need custom styling or context beyond what a native form offers.
---
## Progress (long-running)
```html
<!doctype html>
<meta charset="utf-8" />
<style>
body { font: 14px system-ui; margin: 16px; }
.bar { height: 8px; background: #eee; border-radius: 4px; overflow: hidden; }
.fill { height: 100%; background: #2a7; transition: width 200ms; }
</style>
<p id="label">Starting…</p>
<div class="bar"><div id="fill" class="fill" style="width:0%"></div></div>
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "Progress", version: "1.0.0" }, {});
const label = document.getElementById("label");
const fill = document.getElementById("fill");
// The tool result fires when the job completes — intermediate updates
// arrive via the same handler if the server streams them
app.ontoolresult = ({ content }) => {
const state = JSON.parse(content[0].text);
if (state.progress !== undefined) {
label.textContent = state.message ?? `${state.progress}/${state.total}`;
fill.style.width = `${(state.progress / state.total) * 100}%`;
}
if (state.done) {
label.textContent = "Complete";
fill.style.width = "100%";
}
};
await app.connect();
</script>
```
Server side, emit progress via `extra.sendNotification({ method: "notifications/progress", ... })` — see `apps-sdk-messages.md`.
---
## Display-only (chart / preview)
Display widgets don't call `sendMessage` — they render and sit there. The tool should return a text summary **alongside** the widget so Claude can keep reasoning while the user sees the visual:
```typescript
registerAppTool(server, "show_chart", {
description: "Render a revenue chart",
inputSchema: { range: z.enum(["week", "month", "year"]) },
_meta: { ui: { resourceUri: "ui://widgets/chart.html" } },
}, async ({ range }) => {
const data = await fetchRevenue(range);
return {
content: [{
type: "text",
text: `Revenue is up ${data.change}% over the ${range}. Chart rendered.\n\n` +
JSON.stringify(data.points),
}],
};
});
```
```html
<!doctype html>
<meta charset="utf-8" />
<style>body { font: 14px system-ui; margin: 12px; }</style>
<canvas id="chart" width="400" height="200"></canvas>
<script type="module">
import { App } from "https://esm.sh/@modelcontextprotocol/ext-apps@1.2.2";
const app = new App({ name: "Chart", version: "1.0.0" }, {});
app.ontoolresult = ({ content }) => {
// Parse the JSON points from the text content (after the summary line)
const text = content[0].text;
const jsonStart = text.indexOf("\n\n") + 2;
const points = JSON.parse(text.slice(jsonStart));
drawChart(document.getElementById("chart"), points);
};
await app.connect();
function drawChart(canvas, points) { /* ... */ }
</script>
```

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@@ -1,208 +0,0 @@
---
name: build-mcp-server
description: This skill should be used when the user asks to "build an MCP server", "create an MCP", "make an MCP integration", "wrap an API for Claude", "expose tools to Claude", "make an MCP app", or discusses building something with the Model Context Protocol. It is the entry point for MCP server development — it interrogates the user about their use case, determines the right deployment model (remote HTTP, MCPB, local stdio), picks a tool-design pattern, and hands off to specialized skills.
version: 0.1.0
---
# Build an MCP Server
You are guiding a developer through designing and building an MCP server that works seamlessly with Claude. MCP servers come in many forms — picking the wrong shape early causes painful rewrites later. Your first job is **discovery, not code**.
Do not start scaffolding until you have answers to the questions in Phase 1. If the user's opening message already answers them, acknowledge that and skip straight to the recommendation.
---
## Phase 1 — Interrogate the use case
Ask these questions conversationally (batch them into one message, don't interrogate one-at-a-time). Adapt wording to what the user has already told you.
### 1. What does it connect to?
| If it connects to… | Likely direction |
|---|---|
| A cloud API (SaaS, REST, GraphQL) | Remote HTTP server |
| A local process, filesystem, or desktop app | MCPB or local stdio |
| Hardware, OS-level APIs, or user-specific state | MCPB |
| Nothing external — pure logic / computation | Either — default to remote |
### 2. Who will use it?
- **Just me / my team, on our machines** → Local stdio is acceptable (easiest to prototype)
- **Anyone who installs it** → Remote HTTP (strongly preferred) or MCPB (if it *must* be local)
- **Users of Claude desktop who want UI widgets** → MCP app (remote or MCPB)
### 3. How many distinct actions does it expose?
This determines the tool-design pattern — see Phase 3.
- **Under ~15 actions** → one tool per action
- **Dozens to hundreds of actions** (e.g. wrapping a large API surface) → search + execute pattern
### 4. Does a tool need mid-call user input or rich display?
- **Simple structured input** (pick from list, enter a value, confirm) → **Elicitation** — spec-native, zero UI code. *Host support is rolling out* (Claude Code ≥2.1.76) — always pair with a capability check and fallback. See `references/elicitation.md`.
- **Rich/visual UI** (charts, custom pickers with search, live dashboards) → **MCP app widgets** — iframe-based, needs `@modelcontextprotocol/ext-apps`. See `build-mcp-app` skill.
- **Neither** → plain tool returning text/JSON.
### 5. What auth does the upstream service use?
- None / API key → straightforward
- OAuth 2.0 → you'll need a remote server with CIMD (preferred) or DCR support; see `references/auth.md`
---
## Phase 2 — Recommend a deployment model
Based on the answers, recommend **one** path. Be opinionated. The ranked options:
### ⭐ Remote streamable-HTTP MCP server (default recommendation)
A hosted service speaking MCP over streamable HTTP. This is the **recommended path** for anything wrapping a cloud API.
**Why it wins:**
- Zero install friction — users add a URL, done
- One deployment serves all users; you control upgrades
- OAuth flows work properly (the server can handle redirects, DCR, token storage)
- Works across Claude desktop, Claude Code, Claude.ai, and third-party MCP hosts
**Choose this unless** the server *must* touch the user's local machine.
**Fastest deploy:** Cloudflare Workers — `references/deploy-cloudflare-workers.md` (zero to live URL in two commands)
**Portable Node/Python:** `references/remote-http-scaffold.md` (Express or FastMCP, runs on any host)
### Elicitation (structured input, no UI build)
If a tool just needs the user to confirm, pick an option, or fill a short form, **elicitation** does it with zero UI code. The server sends a flat JSON schema; the host renders a native form. Spec-native, no extra packages.
**Caveat:** Host support is new (Claude Code shipped it in v2.1.76; Desktop unconfirmed). The SDK throws if the client doesn't advertise the capability. Always check `clientCapabilities.elicitation` first and have a fallback — see `references/elicitation.md` for the canonical pattern. This is the right spec-correct approach; host coverage will catch up.
Escalate to `build-mcp-app` widgets when you need: nested/complex data, scrollable/searchable lists, visual previews, live updates.
### MCP app (remote HTTP + interactive UI)
Same as above, plus **UI resources** — interactive widgets rendered in chat. Rich pickers with search, charts, live dashboards, visual previews. Built once, renders in Claude *and* ChatGPT.
**Choose this when** elicitation's flat-form constraints don't fit — you need custom layout, large searchable lists, visual content, or live updates.
Usually remote, but can be shipped as MCPB if the UI needs to drive a local app.
→ Hand off to the **`build-mcp-app`** skill.
### MCPB (bundled local server)
A local MCP server **packaged with its runtime** so users don't need Node/Python installed. The sanctioned way to ship local servers.
**Choose this when** the server *must* run on the user's machine — it reads local files, drives a desktop app, talks to localhost services, or needs OS-level access.
→ Hand off to the **`build-mcpb`** skill.
### Local stdio (npx / uvx) — *not recommended for distribution*
A script launched via `npx` / `uvx` on the user's machine. Fine for **personal tools and prototypes**. Painful to distribute: users need the right runtime, you can't push updates, and the only distribution channel is Claude Code plugins.
Recommend this only as a stepping stone. If the user insists, scaffold it but note the MCPB upgrade path.
---
## Phase 3 — Pick a tool-design pattern
Every MCP server exposes tools. How you carve them matters more than most people expect — tool schemas land directly in Claude's context window.
### Pattern A: One tool per action (small surface)
When the action space is small (< ~15 operations), give each a dedicated tool with a tight description and schema.
```
create_issue — Create a new issue. Params: title, body, labels[]
update_issue — Update an existing issue. Params: id, title?, body?, state?
search_issues — Search issues by query string. Params: query, limit?
add_comment — Add a comment to an issue. Params: issue_id, body
```
**Why it works:** Claude reads the tool list once and knows exactly what's possible. No discovery round-trips. Each tool's schema validates inputs precisely.
**Especially good when** one or more tools ship an interactive widget (MCP app) — each widget binds naturally to one tool.
### Pattern B: Search + execute (large surface)
When wrapping a large API (dozens to hundreds of endpoints), listing every operation as a tool floods the context window and degrades model performance. Instead, expose **two** tools:
```
search_actions — Given a natural-language intent, return matching actions
with their IDs, descriptions, and parameter schemas.
execute_action — Run an action by ID with a params object.
```
The server holds the full catalog internally. Claude searches, picks, executes. Context stays lean.
**Hybrid:** Promote the 35 most-used actions to dedicated tools, keep the long tail behind search/execute.
→ See `references/tool-design.md` for schema examples and description-writing guidance.
---
## Phase 4 — Pick a framework
Recommend one of these two. Others exist but these have the best MCP-spec coverage and Claude compatibility.
| Framework | Language | Use when |
|---|---|---|
| **Official TypeScript SDK** (`@modelcontextprotocol/sdk`) | TS/JS | Default choice. Best spec coverage, first to get new features. |
| **FastMCP 3.x** (`fastmcp` on PyPI) | Python | User prefers Python, or wrapping a Python library. Decorator-based, very low boilerplate. This is jlowin's package — not the frozen FastMCP 1.0 bundled in the official `mcp` SDK. |
If the user already has a language/stack in mind, go with it — both produce identical wire protocol.
---
## Phase 5 — Scaffold and hand off
Once you've settled the four decisions (deployment model, tool pattern, framework, auth), do **one** of:
1. **Remote HTTP, no UI** → Scaffold inline using `references/remote-http-scaffold.md` (portable) or `references/deploy-cloudflare-workers.md` (fastest deploy). This skill can finish the job.
2. **MCP app (UI widgets)** → Summarize the decisions so far, then load the **`build-mcp-app`** skill.
3. **MCPB (bundled local)** → Summarize the decisions so far, then load the **`build-mcpb`** skill.
4. **Local stdio prototype** → Scaffold inline (simplest case), flag the MCPB upgrade path.
When handing off, restate the design brief in one paragraph so the next skill doesn't re-ask.
---
## Beyond tools — the other primitives
Tools are one of three server primitives. Most servers start with tools and never need the others, but knowing they exist prevents reinventing wheels:
| Primitive | Who triggers it | Use when |
|---|---|---|
| **Resources** | Host app (not Claude) | Exposing docs/files/data as browsable context |
| **Prompts** | User (slash command) | Canned workflows ("/summarize-thread") |
| **Elicitation** | Server, mid-tool | Asking user for input without building UI |
| **Sampling** | Server, mid-tool | Need LLM inference in your tool logic |
`references/resources-and-prompts.md`, `references/elicitation.md`, `references/server-capabilities.md`
---
## Quick reference: decision matrix
| Scenario | Deployment | Tool pattern |
|---|---|---|
| Wrap a small SaaS API | Remote HTTP | One-per-action |
| Wrap a large SaaS API (50+ endpoints) | Remote HTTP | Search + execute |
| SaaS API with rich forms / pickers | MCP app (remote) | One-per-action |
| Drive a local desktop app | MCPB | One-per-action |
| Local desktop app with in-chat UI | MCP app (MCPB) | One-per-action |
| Read/write local filesystem | MCPB | Depends on surface |
| Personal prototype | Local stdio | Whatever's fastest |
---
## Reference files
- `references/remote-http-scaffold.md` — minimal remote server in TS SDK and FastMCP
- `references/deploy-cloudflare-workers.md` — fastest deploy path (Workers-native scaffold)
- `references/tool-design.md` — writing tool descriptions and schemas Claude understands well
- `references/auth.md` — OAuth, CIMD, DCR, token storage patterns
- `references/resources-and-prompts.md` — the two non-tool primitives
- `references/elicitation.md` — spec-native user input mid-tool (capability check + fallback)
- `references/server-capabilities.md` — instructions, sampling, roots, logging, progress, cancellation
- `references/versions.md` — version-sensitive claims ledger (check when updating)

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@@ -1,92 +0,0 @@
# Auth for MCP Servers
Auth is the reason most people end up needing a **remote** server even when a local one would be simpler. OAuth redirects, token storage, and refresh all work cleanly when there's a real hosted endpoint to redirect back to.
---
## The three tiers
### Tier 1: No auth / static API key
Server reads a key from env. User provides it once at setup. Done.
```typescript
const apiKey = process.env.UPSTREAM_API_KEY;
if (!apiKey) throw new Error("UPSTREAM_API_KEY not set");
```
Works for local stdio, MCPB, and remote servers alike. If this is all you need, stop here.
### Tier 2: OAuth 2.0 via CIMD (preferred per spec 2025-11-25)
**Client ID Metadata Document.** The MCP host publishes its client metadata at an HTTPS URL and uses that URL *as* its `client_id`. Your authorization server fetches the document, validates it, and proceeds with the auth-code flow. No registration endpoint, no stored client records.
Spec 2025-11-25 promoted CIMD to SHOULD (preferred). Advertise support via `client_id_metadata_document_supported: true` in your OAuth AS metadata.
**Server responsibilities:**
1. Serve OAuth Authorization Server Metadata (RFC 8414) at `/.well-known/oauth-authorization-server` with `client_id_metadata_document_supported: true`
2. Serve an MCP-protected-resource metadata document pointing at (1)
3. At authorize time: fetch `client_id` as an HTTPS URL, validate the returned client metadata, proceed
4. Validate bearer tokens on incoming `/mcp` requests
```
┌─────────┐ client_id=https://... ┌──────────────┐ upstream OAuth ┌──────────┐
│ MCP host│ ──────────────────────> │ Your MCP srv │ ─────────────────> │ Upstream │
└─────────┘ <─── bearer token ───── └──────────────┘ <── access token ──└──────────┘
```
### Tier 3: OAuth 2.0 via Dynamic Client Registration (DCR)
**Backward-compat fallback** — spec 2025-11-25 demoted DCR to MAY. The host discovers your `registration_endpoint`, POSTs its metadata to register itself as a client, gets back a `client_id`, then runs the auth-code flow.
Implement DCR if you need to support hosts that haven't moved to CIMD yet. Same server responsibilities as CIMD, but instead of fetching the `client_id` URL you run a registration endpoint that stores client records.
**Client priority order:** pre-registered → CIMD (if AS advertises `client_id_metadata_document_supported`) → DCR (if AS has `registration_endpoint`) → prompt user.
---
## Hosting providers with built-in DCR/CIMD support
Several MCP-focused hosting providers handle the OAuth plumbing for you — you implement tool logic, they run the authorization server. Check their docs for current capabilities. If the user doesn't have strong hosting preferences, this is usually the fastest path to a working OAuth-protected server.
---
## Local servers and OAuth
Local stdio servers **can** do OAuth (open a browser, catch the redirect on a localhost port, stash the token in the OS keychain). It's fragile:
- Breaks in headless/remote environments
- Every user re-does the dance
- No central token refresh or revocation
If OAuth is required, lean hard toward remote HTTP. If you *must* ship local + OAuth, the `@modelcontextprotocol/sdk` includes a localhost-redirect helper, and MCPB is the right packaging so at least the runtime is predictable.
---
## Token storage
| Deployment | Store tokens in |
|---|---|
| Remote, stateless | Nowhere — host sends bearer each request |
| Remote, stateful | Session store keyed by MCP session ID (Redis, etc.) |
| MCPB / local | OS keychain (`keytar` on Node, `keyring` on Python). **Never plaintext on disk.** |
---
## Token audience validation (spec MUST)
Validating "is this a valid bearer token" isn't enough. The spec requires validating "was this token minted *for this server*" — RFC 8707 audience. A token issued for `api.other-service.com` must be rejected even if the signature checks out.
**Token passthrough is explicitly forbidden.** Don't accept a token, then forward it upstream. If your server needs to call another service, exchange the token or use its own credentials.
---
## SDK helpers — don't hand-roll
`@modelcontextprotocol/sdk/server/auth` ships:
- `mcpAuthRouter()` — Express router for the full OAuth AS surface (metadata, authorize, token)
- `bearerAuth` — middleware that validates bearer tokens against your verifier
- `proxyProvider` — forward auth to an upstream IdP
If you're wiring auth from scratch, check these first.

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@@ -1,106 +0,0 @@
# Deploy to Cloudflare Workers
Fastest path from zero to a live `https://` MCP URL. Free tier, no credit card to start, two commands to deploy.
**Trade-off:** This is a Workers-native scaffold, not a deploy target for the Express scaffold in `remote-http-scaffold.md`. Different runtime. If you need portability across hosts, stick with Express. If you just want it live, start here.
---
## Bootstrap
```bash
npm create cloudflare@latest -- my-mcp-server \
--template=cloudflare/ai/demos/remote-mcp-authless
cd my-mcp-server
```
This pulls a minimal template with the right deps (`agents`, `zod`) and a working `wrangler.jsonc`.
---
## `src/index.ts`
Replace the template's calculator example with your tools. Use `registerTool()` (same API as the Express scaffold — the `McpServer` instance is identical):
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { McpAgent } from "agents/mcp";
import { z } from "zod";
export class MyMCP extends McpAgent {
server = new McpServer(
{ name: "my-service", version: "0.1.0" },
{ instructions: "Prefer search_items before get_item — IDs aren't guessable." },
);
async init() {
this.server.registerTool(
"search_items",
{
description: "Search items by keyword. Returns up to `limit` matches.",
inputSchema: {
query: z.string().describe("Search keywords"),
limit: z.number().int().min(1).max(50).default(10),
},
annotations: { readOnlyHint: true },
},
async ({ query, limit }) => {
const results = await upstreamApi.search(query, limit);
return { content: [{ type: "text", text: JSON.stringify(results, null, 2) }] };
},
);
}
}
export default {
fetch(request: Request, env: Env, ctx: ExecutionContext) {
const url = new URL(request.url);
if (url.pathname === "/mcp") {
return MyMCP.serve("/mcp").fetch(request, env, ctx);
}
return new Response("Not found", { status: 404 });
},
};
```
`McpAgent` is Cloudflare's wrapper — it handles the streamable-HTTP transport, session routing, and Durable Object plumbing. Your code only touches `this.server`, which is the same `McpServer` class from the SDK. Everything in `tool-design.md` and `server-capabilities.md` applies unchanged.
---
## `wrangler.jsonc`
The template ships this. The Durable Objects block is **boilerplate**`McpAgent` uses DO for session state. You don't interact with it directly.
```jsonc
{
"name": "my-mcp-server",
"main": "src/index.ts",
"compatibility_date": "2025-03-10",
"compatibility_flags": ["nodejs_compat"],
"migrations": [{ "new_sqlite_classes": ["MyMCP"], "tag": "v1" }],
"durable_objects": {
"bindings": [{ "class_name": "MyMCP", "name": "MCP_OBJECT" }]
}
}
```
If you rename the `MyMCP` class, update both `new_sqlite_classes` and `class_name` to match.
---
## Run and deploy
```bash
npx wrangler dev # → http://localhost:8787/mcp
npx wrangler deploy # → https://my-mcp-server.<account>.workers.dev/mcp
```
`wrangler deploy` prints the live URL. That's the URL users paste into Claude.
Secrets (upstream API keys): `npx wrangler secret put UPSTREAM_API_KEY`, then read `env.UPSTREAM_API_KEY` inside `init()`.
---
## OAuth
Cloudflare ships `@cloudflare/workers-oauth-provider` — a drop-in that handles the authorization server side (CIMD/DCR endpoints, token issuance, consent UI). It wraps your `McpAgent` and gates `/mcp` behind a token check. See `auth.md` for the protocol details; the CF template `cloudflare/ai/demos/remote-mcp-github-oauth` shows the wiring.

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@@ -1,129 +0,0 @@
# Elicitation — spec-native user input
Elicitation lets a server pause mid-tool-call and ask the user for structured input. The client renders a native form (no iframe, no HTML). User fills it, server continues.
**This is the right answer for simple input.** Widgets (`build-mcp-app`) are for when you need rich UI — charts, searchable lists, visual previews. If you just need a confirmation, a picked option, or a few form fields, elicitation is simpler, spec-native, and works in any compliant host.
---
## ⚠️ Check capability first — support is new
Host support is very recent:
| Host | Status |
|---|---|
| Claude Code | ✅ since v2.1.76 (both `form` and `url` modes) |
| Claude Desktop | Unconfirmed — likely not yet or very recent |
| claude.ai | Unknown |
**The SDK throws `CapabilityNotSupported` if the client doesn't advertise elicitation.** There is no graceful degradation built in. You MUST check and have a fallback.
### The canonical pattern
```typescript
server.registerTool("delete_all", {
description: "Delete all items after confirmation",
inputSchema: {},
}, async ({}, extra) => {
const caps = server.getClientCapabilities();
if (caps?.elicitation) {
const r = await server.elicitInput({
mode: "form",
message: "Delete all items? This cannot be undone.",
requestedSchema: {
type: "object",
properties: { confirm: { type: "boolean", title: "Confirm deletion" } },
required: ["confirm"],
},
});
if (r.action === "accept" && r.content?.confirm) {
await deleteAll();
return { content: [{ type: "text", text: "Deleted." }] };
}
return { content: [{ type: "text", text: "Cancelled." }] };
}
// Fallback: return text asking Claude to relay the question
return { content: [{ type: "text", text: "Confirmation required. Please ask the user: 'Delete all items? This cannot be undone.' Then call this tool again with their answer." }] };
});
```
```python
# fastmcp
from fastmcp import Context
from fastmcp.exceptions import CapabilityNotSupported
@mcp.tool
async def delete_all(ctx: Context) -> str:
try:
result = await ctx.elicit("Delete all items? This cannot be undone.", response_type=bool)
if result.action == "accept" and result.data:
await do_delete()
return "Deleted."
return "Cancelled."
except CapabilityNotSupported:
return "Confirmation required. Ask the user to confirm deletion, then retry."
```
---
## Schema constraints
Elicitation schemas are deliberately limited — keep forms simple:
- **Flat objects only** — no nesting, no arrays of objects
- **Primitives only** — `string`, `number`, `integer`, `boolean`, `enum`
- String formats limited to: `email`, `uri`, `date`, `date-time`
- Use `title` and `description` on each property — they become form labels
If your data doesn't fit these constraints, that's the signal to escalate to a widget.
---
## Three-state response
| Action | Meaning | `content` present? |
|---|---|---|
| `accept` | User submitted the form | ✅ validated against your schema |
| `decline` | User explicitly said no | ❌ |
| `cancel` | User dismissed (escape, clicked away) | ❌ |
Treat `decline` and `cancel` differently if it matters — `decline` is intentional, `cancel` might be accidental.
The TS SDK's `server.elicitInput()` auto-validates `accept` responses against your schema via Ajv. fastmcp's `ctx.elicit()` returns a typed discriminated union (`AcceptedElicitation[T] | DeclinedElicitation | CancelledElicitation`).
---
## fastmcp response_type shorthand
```python
await ctx.elicit("Pick a color", response_type=["red", "green", "blue"]) # enum
await ctx.elicit("Enter email", response_type=str) # string
await ctx.elicit("Confirm?", response_type=bool) # boolean
@dataclass
class ContactInfo:
name: str
email: str
await ctx.elicit("Contact details", response_type=ContactInfo) # flat dataclass
```
Accepts: primitives, `list[str]` (becomes enum), dataclass, TypedDict, Pydantic BaseModel. All must be flat.
---
## Security
**MUST NOT request passwords, API keys, or tokens via elicitation** — spec requirement. Those go through OAuth or `user_config` with `sensitive: true` (MCPB), not runtime forms.
---
## When to escalate to widgets
Elicitation handles: confirm dialogs, enum pickers, short flat forms.
Reach for `build-mcp-app` widgets when you need:
- Nested or complex data structures
- Scrollable/searchable lists (100+ items)
- Visual preview before choosing (image thumbnails, file tree)
- Live-updating progress or streaming content
- Custom layouts, charts, maps

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@@ -1,211 +0,0 @@
# Remote Streamable-HTTP MCP Server — Scaffold
Minimal working servers in both recommended frameworks. Start here, then add tools.
---
## TypeScript SDK (`@modelcontextprotocol/sdk`)
```bash
npm init -y
npm install @modelcontextprotocol/sdk zod express
npm install -D typescript @types/express @types/node tsx
```
**`src/server.ts`**
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import express from "express";
import { z } from "zod";
const server = new McpServer(
{ name: "my-service", version: "0.1.0" },
{ instructions: "Prefer search_items before calling get_item directly — IDs aren't guessable." },
);
// Pattern A: one tool per action
server.registerTool(
"search_items",
{
description: "Search items by keyword. Returns up to `limit` matches ranked by relevance.",
inputSchema: {
query: z.string().describe("Search keywords"),
limit: z.number().int().min(1).max(50).default(10),
},
annotations: { readOnlyHint: true },
},
async ({ query, limit }, extra) => {
// extra.signal is an AbortSignal — check it in long loops for cancellation
const results = await upstreamApi.search(query, limit);
return {
content: [{ type: "text", text: JSON.stringify(results, null, 2) }],
};
},
);
server.registerTool(
"get_item",
{
description: "Fetch a single item by its ID.",
inputSchema: { id: z.string() },
annotations: { readOnlyHint: true },
},
async ({ id }) => {
const item = await upstreamApi.get(id);
return { content: [{ type: "text", text: JSON.stringify(item) }] };
},
);
// Streamable HTTP transport (stateless mode — simplest)
const app = express();
app.use(express.json());
app.post("/mcp", async (req, res) => {
const transport = new StreamableHTTPServerTransport({
sessionIdGenerator: undefined, // stateless
});
res.on("close", () => transport.close());
await server.connect(transport);
await transport.handleRequest(req, res, req.body);
});
app.listen(process.env.PORT ?? 3000);
```
**Stateless vs stateful:** The snippet above creates a fresh transport per request (stateless). Fine for most API-wrapping servers. If tools need to share state across calls in a session (rare), use a session-keyed transport map — see the SDK's `examples/server/simpleStreamableHttp.ts`.
---
## FastMCP 3.x (Python)
```bash
pip install fastmcp
```
**`server.py`**
```python
from fastmcp import FastMCP
mcp = FastMCP(
name="my-service",
instructions="Prefer search_items before calling get_item directly — IDs aren't guessable.",
)
@mcp.tool(annotations={"readOnlyHint": True})
def search_items(query: str, limit: int = 10) -> list[dict]:
"""Search items by keyword. Returns up to `limit` matches ranked by relevance."""
return upstream_api.search(query, limit)
@mcp.tool(annotations={"readOnlyHint": True})
def get_item(id: str) -> dict:
"""Fetch a single item by its ID."""
return upstream_api.get(id)
if __name__ == "__main__":
mcp.run(transport="http", host="0.0.0.0", port=3000)
```
FastMCP derives the JSON schema from type hints and the docstring becomes the tool description. Keep docstrings terse and action-oriented — they land in Claude's context window verbatim.
---
## Search + execute pattern (large API surface)
When wrapping 50+ endpoints, don't register them all. Two tools:
```typescript
const CATALOG = loadActionCatalog(); // { id, description, paramSchema }[]
server.registerTool(
"search_actions",
{
description: "Find available actions matching an intent. Call this first to discover what's possible. Returns action IDs, descriptions, and parameter schemas.",
inputSchema: { intent: z.string().describe("What you want to do, in plain English") },
annotations: { readOnlyHint: true },
},
async ({ intent }) => {
const matches = rankActions(CATALOG, intent).slice(0, 10);
return { content: [{ type: "text", text: JSON.stringify(matches, null, 2) }] };
},
);
server.registerTool(
"execute_action",
{
description: "Execute an action by ID. Get the ID and params schema from search_actions first.",
inputSchema: {
action_id: z.string(),
params: z.record(z.unknown()),
},
},
async ({ action_id, params }) => {
const action = CATALOG.find(a => a.id === action_id);
if (!action) throw new Error(`Unknown action: ${action_id}`);
validate(params, action.paramSchema);
const result = await dispatch(action, params);
return { content: [{ type: "text", text: JSON.stringify(result) }] };
},
);
```
`rankActions` can be simple keyword matching to start. Upgrade to embeddings if precision matters.
---
## Test it
The MCP Inspector connects to any transport and lets you poke tools interactively.
```bash
# Interactive — opens a UI on localhost:6274
npx @modelcontextprotocol/inspector
# → select "Streamable HTTP", paste http://localhost:3000/mcp, Connect
```
For scripted checks (CI, smoke tests):
```bash
npx @modelcontextprotocol/inspector --cli http://localhost:3000/mcp \
--transport http --method tools/list
npx @modelcontextprotocol/inspector --cli http://localhost:3000/mcp \
--transport http --method tools/call --tool-name search_items --tool-arg query=test
```
---
## Connect users
Once deployed, users add the URL directly — no install step.
| Surface | How |
|---|---|
| **Claude Code** | `claude mcp add --transport http <name> <url>` (add `--scope user` for global, `--header "Authorization: Bearer ..."` for auth) |
| **Claude Desktop / Claude.ai** | Settings → Connectors → Add custom connector. **Not** `claude_desktop_config.json` — remote servers configured there are ignored. |
| **Connector directory** | Anthropic maintains a submission guide for listing in the public connector directory. |
---
## Deploy
**Fastest path:** Cloudflare Workers — two commands from zero to a live `https://` URL on the free tier. Uses a Workers-native scaffold (not Express). → `deploy-cloudflare-workers.md`
**This Express scaffold** runs on any Node host — Render, Railway, Fly.io, a VPS. Containerize it (`node:20-slim`, copy, `npm ci`, `node dist/server.js`) and ship. FastMCP is the same story with a Python base image.
---
## Deployment checklist
- [ ] `POST /mcp` responds to `initialize` with server capabilities
- [ ] `tools/list` returns your tools with complete schemas
- [ ] Errors return structured MCP errors, not HTTP 500s with HTML bodies
- [ ] CORS headers set if browser clients will connect
- [ ] `Origin` header validated on `/mcp` (spec MUST — DNS rebinding prevention)
- [ ] `MCP-Protocol-Version` header honored (return 400 for unsupported versions)
- [ ] `instructions` field set if tool-use needs hints
- [ ] Health check endpoint separate from `/mcp` (hosts poll it)
- [ ] Secrets from env vars, never hardcoded
- [ ] If OAuth: CIMD or DCR endpoint implemented — see `auth.md`

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@@ -1,122 +0,0 @@
# Resources & Prompts — the other two primitives
MCP defines three server-side primitives. Tools are model-controlled (Claude decides when to call them). The other two are different:
- **Resources** are application-controlled — the host decides what to pull into context
- **Prompts** are user-controlled — surfaced as slash commands or menu items
Most servers only need tools. Reach for these when the shape of your integration doesn't fit "Claude calls a function."
---
## Resources
A resource is data identified by a URI. Unlike a tool, it's not *called* — it's *read*. The host browses available resources and decides which to load into context.
**When a resource beats a tool:**
- Large reference data (docs, schemas, configs) that Claude should be able to browse
- Content that changes independently of conversation (log files, live data)
- Anything where "Claude decides to fetch" is the wrong mental model
**When a tool is better:**
- The operation has side effects
- The result depends on parameters Claude chooses
- You want Claude (not the host UI) to decide when to pull it in
### Static resources
```typescript
// TypeScript SDK
server.registerResource(
"config",
"config://app/settings",
{ name: "App Settings", description: "Current configuration", mimeType: "application/json" },
async (uri) => ({
contents: [{ uri: uri.href, mimeType: "application/json", text: JSON.stringify(config) }],
}),
);
```
```python
# fastmcp
@mcp.resource("config://app/settings")
def get_settings() -> str:
"""Current application configuration."""
return json.dumps(config)
```
### Dynamic resources (URI templates)
RFC 6570 templates let one registration serve many URIs:
```typescript
import { ResourceTemplate } from "@modelcontextprotocol/sdk/server/mcp.js";
server.registerResource(
"file",
new ResourceTemplate("file:///{path}", { list: undefined }),
{ name: "File", description: "Read a file from the workspace" },
async (uri, { path }) => ({
contents: [{ uri: uri.href, text: await fs.readFile(path, "utf8") }],
}),
);
```
```python
@mcp.resource("file:///{path}")
def read_file(path: str) -> str:
return Path(path).read_text()
```
### Subscriptions
Resources can notify the client when they change. Declare `subscribe: true` in capabilities, then emit `notifications/resources/updated`. The host re-reads. Useful for log tails, live dashboards, watched files.
---
## Prompts
A prompt is a parameterized message template. The host surfaces it as a slash command or menu item. The user picks it, fills in arguments, and the resulting messages land in the conversation.
**When to use:** canned workflows users run repeatedly — `/summarize-thread`, `/draft-reply`, `/explain-error`. Near-zero code, high UX leverage.
```typescript
server.registerPrompt(
"summarize",
{
title: "Summarize document",
description: "Generate a concise summary of the given text",
argsSchema: { text: z.string(), max_words: z.string().optional() },
},
({ text, max_words }) => ({
messages: [{
role: "user",
content: { type: "text", text: `Summarize in ${max_words ?? "100"} words:\n\n${text}` },
}],
}),
);
```
```python
@mcp.prompt
def summarize(text: str, max_words: str = "100") -> str:
"""Generate a concise summary of the given text."""
return f"Summarize in {max_words} words:\n\n{text}"
```
**Constraints:**
- Arguments are **string-only** (no numbers, booleans, objects) — convert inside the handler
- Returns a `messages[]` array — can include embedded resources/images, not just text
- No side effects — the handler just builds a message, it doesn't *do* anything
---
## Quick decision table
| You want to... | Use |
|---|---|
| Let Claude fetch something on demand, with parameters | **Tool** |
| Expose browsable context (files, docs, schemas) | **Resource** |
| Expose a dynamic family of things (`db://{table}`) | **Resource template** |
| Give users a one-click workflow | **Prompt** |
| Ask the user something mid-tool | **Elicitation** (see `elicitation.md`) |

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@@ -1,164 +0,0 @@
# Server capabilities — the rest of the spec
Features beyond the three core primitives. Most are optional, a few are near-free wins.
---
## `instructions` — system prompt injection
One line of config, lands directly in Claude's system prompt. Use it for tool-use hints that don't fit in individual tool descriptions.
```typescript
const server = new McpServer(
{ name: "my-server", version: "1.0.0" },
{ instructions: "Always call search_items before get_item — IDs aren't guessable." },
);
```
```python
mcp = FastMCP("my-server", instructions="Always call search_items before get_item — IDs aren't guessable.")
```
This is the highest-leverage one-liner in the spec. If Claude keeps misusing your tools, put the fix here.
---
## Sampling — delegate LLM calls to the host
If your tool logic needs LLM inference (summarize, classify, generate), don't ship your own model client. Ask the host to do it.
```typescript
// Inside a tool handler
const result = await extra.sendRequest({
method: "sampling/createMessage",
params: {
messages: [{ role: "user", content: { type: "text", text: `Summarize: ${doc}` } }],
maxTokens: 500,
},
}, CreateMessageResultSchema);
```
```python
# fastmcp
response = await ctx.sample("Summarize this document", context=doc)
```
**Requires client support** — check `clientCapabilities.sampling` first. Model preference hints are substring-matched (`"claude-3-5"` matches any Claude 3.5 variant).
---
## Roots — query workspace boundaries
Instead of hardcoding a root directory, ask the host which directories the user approved.
```typescript
const caps = server.getClientCapabilities();
if (caps?.roots) {
const { roots } = await server.server.listRoots();
// roots: [{ uri: "file:///home/user/project", name: "My Project" }]
}
```
```python
roots = await ctx.list_roots()
```
Particularly relevant for MCPB local servers — see `build-mcpb/references/local-security.md`.
---
## Logging — structured, level-aware
Better than stderr for remote servers. Client can filter by level.
```typescript
// In a tool handler
await extra.sendNotification({
method: "notifications/message",
params: { level: "info", logger: "my-tool", data: { msg: "Processing", count: 42 } },
});
```
```python
await ctx.info("Processing", count=42) # also: ctx.debug, ctx.warning, ctx.error
```
Levels follow syslog: `debug`, `info`, `notice`, `warning`, `error`, `critical`, `alert`, `emergency`. Client sets minimum via `logging/setLevel`.
---
## Progress — for long-running tools
Client sends a `progressToken` in request `_meta`. Server emits progress notifications against it.
```typescript
async (args, extra) => {
const token = extra._meta?.progressToken;
for (let i = 0; i < 100; i++) {
if (token !== undefined) {
await extra.sendNotification({
method: "notifications/progress",
params: { progressToken: token, progress: i, total: 100, message: `Step ${i}` },
});
}
await doStep(i);
}
return { content: [{ type: "text", text: "Done" }] };
}
```
```python
async def long_task(ctx: Context) -> str:
for i in range(100):
await ctx.report_progress(progress=i, total=100, message=f"Step {i}")
await do_step(i)
return "Done"
```
---
## Cancellation — honor the abort signal
Long tools should check the SDK-provided `AbortSignal`:
```typescript
async (args, extra) => {
for (const item of items) {
if (extra.signal.aborted) throw new Error("Cancelled");
await process(item);
}
}
```
fastmcp handles this via asyncio cancellation — no explicit check needed if your handler is properly async.
---
## Completion — autocomplete for prompt args
If you've registered prompts or resource templates with arguments, you can offer autocomplete:
```typescript
server.registerPrompt("query", {
argsSchema: {
table: completable(z.string(), async (partial) => tables.filter(t => t.startsWith(partial))),
},
}, ...);
```
Low priority unless your prompts have many valid values.
---
## Which capabilities need client support?
| Feature | Server declares | Client must support | Fallback if not |
|---|---|---|---|
| `instructions` | implicit | — | — (always works) |
| Logging | `logging: {}` | — | stderr |
| Progress | — | sends `progressToken` | silently skip |
| Sampling | — | `sampling: {}` | bring your own LLM |
| Elicitation | — | `elicitation: {}` | return text, ask Claude to relay |
| Roots | — | `roots: {}` | config env var |
Check client caps via `server.getClientCapabilities()` (TS) or `ctx.session.client_params.capabilities` (fastmcp) before using the bottom three.

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@@ -1,179 +0,0 @@
# Tool Design — Writing Tools Claude Uses Correctly
Tool schemas and descriptions are prompt engineering. They land directly in Claude's context and determine whether Claude picks the right tool with the right arguments. Most MCP integration bugs trace back to vague descriptions or loose schemas.
---
## Descriptions
**The description is the contract.** It's the only thing Claude reads before deciding whether to call the tool. Write it like a one-line manpage entry plus disambiguating hints.
### Good
```
search_issues — Search issues by keyword across title and body. Returns up
to `limit` results ranked by recency. Does NOT search comments or PRs —
use search_comments / search_prs for those.
```
- Says what it does
- Says what it returns
- Says what it *doesn't* do (prevents wrong-tool calls)
### Bad
```
search_issues — Searches for issues.
```
Claude will call this for anything vaguely search-shaped, including things it can't do.
### Disambiguate siblings
When two tools are similar, each description should say when to use the *other* one:
```
get_user — Fetch a user by ID. If you only have an email, use find_user_by_email.
find_user_by_email — Look up a user by email address. Returns null if not found.
```
---
## Parameter schemas
**Tight schemas prevent bad calls.** Every constraint you express in the schema is one fewer thing that can go wrong at runtime.
| Instead of | Use |
|---|---|
| `z.string()` for an ID | `z.string().regex(/^usr_[a-z0-9]{12}$/)` |
| `z.number()` for a limit | `z.number().int().min(1).max(100).default(20)` |
| `z.string()` for a choice | `z.enum(["open", "closed", "all"])` |
| optional with no hint | `.optional().describe("Defaults to the caller's workspace")` |
**Describe every parameter.** The `.describe()` text shows up in the schema Claude sees. Omitting it is leaving money on the table.
```typescript
{
query: z.string().describe("Keywords to search for. Supports quoted phrases."),
status: z.enum(["open", "closed", "all"]).default("open")
.describe("Filter by status. Use 'all' to include closed items."),
limit: z.number().int().min(1).max(50).default(10)
.describe("Max results. Hard cap at 50."),
}
```
---
## Return shapes
Claude reads whatever you put in `content[].text`. Make it parseable.
**Do:**
- Return JSON for structured data (`JSON.stringify(result, null, 2)`)
- Return short confirmations for mutations (`"Created issue #123"`)
- Include IDs Claude will need for follow-up calls
- Truncate huge payloads and say so (`"Showing 10 of 847 results. Refine the query to narrow down."`)
**Don't:**
- Return raw HTML
- Return megabytes of unfiltered API response
- Return bare success with no identifier (`"ok"` after a create — Claude can't reference what it made)
---
## How many tools?
| Tool count | Guidance |
|---|---|
| 115 | One tool per action. Sweet spot. |
| 1530 | Still workable. Audit for near-duplicates that could merge. |
| 30+ | Switch to search + execute. Optionally promote the top 35 to dedicated tools. |
The ceiling isn't a hard protocol limit — it's context-window economics. Every tool schema is tokens Claude spends *every turn*. Thirty tools with rich schemas can eat 35k tokens before the conversation even starts.
---
## Errors
Return MCP tool errors, not exceptions that crash the transport. Include enough detail for Claude to recover or retry differently.
```typescript
if (!item) {
return {
isError: true,
content: [{
type: "text",
text: `Item ${id} not found. Use search_items to find valid IDs.`,
}],
};
}
```
The hint ("use search_items…") turns a dead end into a next step.
---
## Tool annotations
Hints the host uses for UX — red confirm button for destructive, auto-approve for readonly. All default to unset (host assumes worst case).
| Annotation | Meaning | Host behavior |
|---|---|---|
| `readOnlyHint: true` | No side effects | May auto-approve |
| `destructiveHint: true` | Deletes/overwrites | Confirmation dialog |
| `idempotentHint: true` | Safe to retry | May retry on transient error |
| `openWorldHint: true` | Talks to external world (web, APIs) | May show network indicator |
```typescript
server.registerTool("delete_file", {
description: "Delete a file",
inputSchema: { path: z.string() },
annotations: { destructiveHint: true, idempotentHint: false },
}, handler);
```
```python
@mcp.tool(annotations={"destructiveHint": True, "idempotentHint": False})
def delete_file(path: str) -> str:
...
```
Pair with the read/write split advice in `build-mcpb/references/local-security.md` — mark every read tool `readOnlyHint: true`.
---
## Structured output
`JSON.stringify(result)` in a text block works, but the spec has first-class typed output: `outputSchema` + `structuredContent`. Clients can validate.
```typescript
server.registerTool("get_weather", {
description: "Get current weather",
inputSchema: { city: z.string() },
outputSchema: { temp: z.number(), conditions: z.string() },
}, async ({ city }) => {
const data = await fetchWeather(city);
return {
content: [{ type: "text", text: JSON.stringify(data) }], // backward compat
structuredContent: data, // typed output
};
});
```
Always include the text fallback — not all hosts read `structuredContent` yet.
---
## Content types beyond text
Tools can return more than strings:
| Type | Shape | Use for |
|---|---|---|
| `text` | `{ type: "text", text: string }` | Default |
| `image` | `{ type: "image", data: base64, mimeType }` | Screenshots, charts, diagrams |
| `audio` | `{ type: "audio", data: base64, mimeType }` | TTS output, recordings |
| `resource_link` | `{ type: "resource_link", uri, name?, description? }` | Pointer — client fetches later |
| `resource` (embedded) | `{ type: "resource", resource: { uri, text\|blob, mimeType } }` | Inline the full content |
**`resource_link` vs embedded:** link for large payloads or when the client might not need it (let them decide). Embed when it's small and always needed.

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@@ -1,25 +0,0 @@
# Version pins
Every version-sensitive claim in this skill, in one place. When updating the skill, check these first.
| Claim | Where stated | Last verified |
|---|---|---|
| `@modelcontextprotocol/ext-apps@1.2.2` CDN pin | `build-mcp-app/SKILL.md`, `build-mcp-app/references/widget-templates.md` (4×) | 2026-03 |
| Claude Code ≥2.1.76 for elicitation | `elicitation.md:15`, `build-mcp-server/SKILL.md:43,76` | 2026-03 |
| MCP spec 2025-11-25 CIMD/DCR status | `auth.md:20,24,41` | 2026-03 |
| MCPB manifest schema v0.4 | `build-mcpb/references/manifest-schema.md` | 2026-03 |
| CF `agents` SDK / `McpAgent` API | `deploy-cloudflare-workers.md` | 2026-03 |
| CF template path `cloudflare/ai/demos/remote-mcp-authless` | `deploy-cloudflare-workers.md` | 2026-03 |
## How to verify
```bash
# ext-apps latest
npm view @modelcontextprotocol/ext-apps version
# CF template still exists
gh api repos/cloudflare/ai/contents/demos/remote-mcp-authless/src/index.ts --jq '.sha'
# MCPB schema
curl -sI https://raw.githubusercontent.com/anthropics/mcpb/main/schemas/mcpb-manifest-v0.4.schema.json | head -1
```

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@@ -1,197 +0,0 @@
---
name: build-mcpb
description: This skill should be used when the user wants to "package an MCP server", "bundle an MCP", "make an MCPB", "ship a local MCP server", "distribute a local MCP", discusses ".mcpb files", mentions bundling a Node or Python runtime with their MCP server, or needs an MCP server that interacts with the local filesystem, desktop apps, or OS and must be installable without the user having Node/Python set up.
version: 0.1.0
---
# Build an MCPB (Bundled Local MCP Server)
MCPB is a local MCP server **packaged with its runtime**. The user installs one file; it runs without needing Node, Python, or any toolchain on their machine. It's the sanctioned way to distribute local MCP servers.
**Use MCPB when the server must run on the user's machine** — reading local files, driving a desktop app, talking to localhost services, OS-level APIs. If your server only hits cloud APIs, you almost certainly want a remote HTTP server instead (see `build-mcp-server`). Don't pay the MCPB packaging tax for something that could be a URL.
---
## What an MCPB bundle contains
```
my-server.mcpb (zip archive)
├── manifest.json ← identity, entry point, config schema, compatibility
├── server/ ← your MCP server code
│ ├── index.js
│ └── node_modules/ ← bundled dependencies (or vendored)
└── icon.png
```
The host reads `manifest.json`, launches `server.mcp_config.command` as a **stdio** MCP server, and pipes messages. From your code's perspective it's identical to a local stdio server — the only difference is packaging.
---
## Manifest
```json
{
"$schema": "https://raw.githubusercontent.com/anthropics/mcpb/main/schemas/mcpb-manifest-v0.4.schema.json",
"manifest_version": "0.4",
"name": "local-files",
"version": "0.1.0",
"description": "Read, search, and watch files on the local filesystem.",
"author": { "name": "Your Name" },
"server": {
"type": "node",
"entry_point": "server/index.js",
"mcp_config": {
"command": "node",
"args": ["${__dirname}/server/index.js"],
"env": {
"ROOT_DIR": "${user_config.rootDir}"
}
}
},
"user_config": {
"rootDir": {
"type": "directory",
"title": "Root directory",
"description": "Directory to expose. Defaults to ~/Documents.",
"default": "${HOME}/Documents",
"required": true
}
},
"compatibility": {
"claude_desktop": ">=1.0.0",
"platforms": ["darwin", "win32", "linux"]
}
}
```
**`server.type`** — `node`, `python`, or `binary`. Informational; the actual launch comes from `mcp_config`.
**`server.mcp_config`** — the literal command/args/env to spawn. Use `${__dirname}` for bundle-relative paths and `${user_config.<key>}` to substitute install-time config. **There's no auto-prefix** — the env var names your server reads are exactly what you put in `env`.
**`user_config`** — install-time settings surfaced in the host's UI. `type: "directory"` renders a native folder picker. `sensitive: true` stores in OS keychain. See `references/manifest-schema.md` for all fields.
---
## Server code: same as local stdio
The server itself is a standard stdio MCP server. Nothing MCPB-specific in the tool logic.
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";
import { readFile, readdir } from "node:fs/promises";
import { join } from "node:path";
import { homedir } from "node:os";
// ROOT_DIR comes from what you put in manifest's server.mcp_config.env — no auto-prefix
const ROOT = (process.env.ROOT_DIR ?? join(homedir(), "Documents"));
const server = new McpServer({ name: "local-files", version: "0.1.0" });
server.registerTool(
"list_files",
{
description: "List files in a directory under the configured root.",
inputSchema: { path: z.string().default(".") },
annotations: { readOnlyHint: true },
},
async ({ path }) => {
const entries = await readdir(join(ROOT, path), { withFileTypes: true });
const list = entries.map(e => ({ name: e.name, dir: e.isDirectory() }));
return { content: [{ type: "text", text: JSON.stringify(list, null, 2) }] };
},
);
server.registerTool(
"read_file",
{
description: "Read a file's contents. Path is relative to the configured root.",
inputSchema: { path: z.string() },
annotations: { readOnlyHint: true },
},
async ({ path }) => {
const text = await readFile(join(ROOT, path), "utf8");
return { content: [{ type: "text", text }] };
},
);
const transport = new StdioServerTransport();
await server.connect(transport);
```
**Sandboxing is entirely your job.** There is no manifest-level sandbox — the process runs with full user privileges. Validate paths, refuse to escape `ROOT`, allowlist spawns. See `references/local-security.md`.
Before hardcoding `ROOT` from a config env var, check if the host supports `roots/list` — the spec-native way to get user-approved directories. See `references/local-security.md` for the pattern.
---
## Build pipeline
### Node
```bash
npm install
npx esbuild src/index.ts --bundle --platform=node --outfile=server/index.js
# or: copy node_modules wholesale if native deps resist bundling
npx @anthropic-ai/mcpb pack
```
`mcpb pack` zips the directory and validates `manifest.json` against the schema.
### Python
```bash
pip install -t server/vendor -r requirements.txt
npx @anthropic-ai/mcpb pack
```
Vendor dependencies into a subdirectory and prepend it to `sys.path` in your entry script. Native extensions (numpy, etc.) must be built for each target platform — avoid native deps if you can.
---
## MCPB has no sandbox — security is on you
Unlike mobile app stores, MCPB does NOT enforce permissions. The manifest has no `permissions` block — the server runs with full user privileges. `references/local-security.md` is mandatory reading, not optional. Every path must be validated, every spawn must be allowlisted, because nothing stops you at the platform level.
If you came here expecting filesystem/network scoping from the manifest: it doesn't exist. Build it yourself in tool handlers.
If your server's only job is hitting a cloud API, stop — that's a remote server wearing an MCPB costume. The user gains nothing from running it locally, and you're taking on local-security burden for no reason.
---
## MCPB + UI widgets
MCPB servers can serve UI resources exactly like remote MCP apps — the widget mechanism is transport-agnostic. A local file picker that browses the actual disk, a dialog that controls a native app, etc.
Widget authoring is covered in the **`build-mcp-app`** skill; it works the same here. The only difference is where the server runs.
---
## Testing
```bash
# Interactive manifest creation (first time)
npx @anthropic-ai/mcpb init
# Run the server directly over stdio, poke it with the inspector
npx @modelcontextprotocol/inspector node server/index.js
# Validate manifest against schema, then pack
npx @anthropic-ai/mcpb validate
npx @anthropic-ai/mcpb pack
# Sign for distribution
npx @anthropic-ai/mcpb sign dist/local-files.mcpb
# Install: drag the .mcpb file onto Claude Desktop
```
Test on a machine **without** your dev toolchain before shipping. "Works on my machine" failures in MCPB almost always trace to a dependency that wasn't actually bundled.
---
## Reference files
- `references/manifest-schema.md` — full `manifest.json` field reference
- `references/local-security.md` — path traversal, sandboxing, least privilege

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@@ -1,149 +0,0 @@
# Local MCP Security
**MCPB provides no sandbox.** There's no `permissions` block in the manifest, no filesystem scoping, no network allowlist enforced by the platform. The server process runs with the user's full privileges — it can read any file the user can, spawn any process, hit any network endpoint.
Claude drives it. That combination means: **tool inputs are untrusted**, even though they come from an AI the user trusts. A prompt-injected web page can make Claude call your `delete_file` tool with a path you didn't intend.
Your tool handlers are the only defense. Everything below is about building that defense yourself.
---
## Path traversal
The #1 bug in local MCP servers. If you take a path parameter and join it to a root, **resolve and check containment**.
```typescript
import { resolve, relative, isAbsolute } from "node:path";
function safeJoin(root: string, userPath: string): string {
const full = resolve(root, userPath);
const rel = relative(root, full);
if (rel.startsWith("..") || isAbsolute(rel)) {
throw new Error(`Path escapes root: ${userPath}`);
}
return full;
}
```
`resolve` normalizes `..`, symlink segments, etc. `relative` tells you if the result left the root. Don't just `String.includes("..")` — that misses encoded and symlink-based escapes.
**Python equivalent:**
```python
from pathlib import Path
def safe_join(root: Path, user_path: str) -> Path:
full = (root / user_path).resolve()
if not full.is_relative_to(root.resolve()):
raise ValueError(f"Path escapes root: {user_path}")
return full
```
---
## Roots — ask the host, don't hardcode
Before hardcoding `ROOT` from a config env var, check if the host supports `roots/list`. This is the spec-native way to get user-approved workspace boundaries.
```typescript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
const server = new McpServer({ name: "...", version: "..." });
let allowedRoots: string[] = [];
server.server.oninitialized = async () => {
const caps = server.getClientCapabilities();
if (caps?.roots) {
const { roots } = await server.server.listRoots();
allowedRoots = roots.map(r => new URL(r.uri).pathname);
} else {
allowedRoots = [process.env.ROOT_DIR ?? process.cwd()];
}
};
```
```python
# fastmcp — inside a tool handler
async def my_tool(ctx: Context) -> str:
try:
roots = await ctx.list_roots()
allowed = [urlparse(r.uri).path for r in roots]
except Exception:
allowed = [os.environ.get("ROOT_DIR", os.getcwd())]
```
If roots are available, use them. If not, fall back to config. Either way, validate every path against the allowed set.
---
## Command injection
If you spawn processes, **never pass user input through a shell**.
```typescript
// ❌ catastrophic
exec(`git log ${branch}`);
// ✅ array-args, no shell
execFile("git", ["log", branch]);
```
If you're wrapping a CLI, build the full argv as an array. Validate each flag against an allowlist if the tool accepts flags at all.
---
## Read-only by default
Split read and write into separate tools. Most workflows only need read. A tool that's read-only can't be weaponized into data loss no matter what Claude is tricked into calling it with.
```
list_files ← safe to call freely
read_file ← safe to call freely
write_file ← separate tool, separate scrutiny
delete_file ← consider not shipping this at all
```
Pair this with tool annotations — `readOnlyHint: true` on every read tool, `destructiveHint: true` on delete/overwrite tools. Hosts surface these in permission UI (auto-approve reads, confirm-dialog destructive). See `../build-mcp-server/references/tool-design.md`.
If you ship write/delete, consider requiring explicit confirmation via elicitation (see `../build-mcp-server/references/elicitation.md`) or a confirmation widget (see `build-mcp-app`) so the user approves each destructive call.
---
## Resource limits
Claude will happily ask to read a 4GB log file. Cap everything:
```typescript
const MAX_BYTES = 1_000_000;
const buf = await readFile(path);
if (buf.length > MAX_BYTES) {
return {
content: [{
type: "text",
text: `File is ${buf.length} bytes — too large. Showing first ${MAX_BYTES}:\n\n`
+ buf.subarray(0, MAX_BYTES).toString("utf8"),
}],
};
}
```
Same for directory listings (cap entry count), search results (cap matches), and anything else unbounded.
---
## Secrets
- **Config secrets** (`sensitive: true` in manifest `user_config`): host stores in OS keychain, delivers via env var. Don't log them. Don't include them in tool results.
- **Never store secrets in plaintext files.** If the host's keychain integration isn't enough, use `keytar` (Node) / `keyring` (Python) yourself.
- **Tool results flow into the chat transcript.** Anything you return, the user (and any log export) can see. Redact before returning.
---
## Checklist before shipping
- [ ] Every path parameter goes through containment check
- [ ] No `exec()` / `shell=True``execFile` / array-argv only
- [ ] Write/delete split from read tools; `readOnlyHint`/`destructiveHint` annotations set
- [ ] Size caps on file reads, listing lengths, search results
- [ ] Secrets never logged or returned in tool results
- [ ] Tested with adversarial inputs: `../../etc/passwd`, `; rm -rf ~`, 10GB file

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@@ -1,156 +0,0 @@
# MCPB Manifest Schema (v0.4)
Validated against `github.com/anthropics/mcpb/schemas/mcpb-manifest-v0.4.schema.json`. The schema uses `additionalProperties: false` — unknown keys are rejected. Add `"$schema"` to your manifest for editor validation.
---
## Top-level fields
| Field | Required | Description |
|---|---|---|
| `manifest_version` | ✅ | Schema version. Use `"0.4"`. |
| `name` | ✅ | Package identifier (lowercase, hyphens). Must be unique. |
| `version` | ✅ | Semver version of YOUR package. |
| `description` | ✅ | One-line summary. Shown in marketplace. |
| `author` | ✅ | `{name, email?, url?}` |
| `server` | ✅ | Entry point and launch config. See below. |
| `display_name` | | Human-friendly name. Falls back to `name`. |
| `long_description` | | Markdown. Shown on detail page. |
| `icon` / `icons` | | Path(s) to icon file(s) in the bundle. |
| `homepage` / `repository` / `documentation` / `support` | | URLs. |
| `license` | | SPDX identifier. |
| `keywords` | | String array for search. |
| `user_config` | | Install-time config fields. See below. |
| `compatibility` | | Host/platform/runtime requirements. See below. |
| `tools` / `prompts` | | Optional declarative list for marketplace display. Not enforced at runtime. |
| `tools_generated` / `prompts_generated` | | `true` if tools/prompts are dynamic (can't list statically). |
| `screenshots` | | Array of image paths. |
| `localization` | | i18n bundles. |
| `privacy_policies` | | URLs. |
---
## `server` — launch configuration
```json
"server": {
"type": "node",
"entry_point": "server/index.js",
"mcp_config": {
"command": "node",
"args": ["${__dirname}/server/index.js"],
"env": {
"API_KEY": "${user_config.apiKey}",
"ROOT_DIR": "${user_config.rootDir}"
}
}
}
```
| Field | Description |
|---|---|
| `type` | `"node"`, `"python"`, or `"binary"` |
| `entry_point` | Relative path to main file. Informational. |
| `mcp_config.command` | Executable to launch. |
| `mcp_config.args` | Argv array. Use `${__dirname}` for bundle-relative paths. |
| `mcp_config.env` | Environment variables. Use `${user_config.KEY}` to substitute user config. |
**Substitution variables** (in `args` and `env` only):
- `${__dirname}` — absolute path to the unpacked bundle directory
- `${user_config.<key>}` — value the user entered at install time
- `${HOME}` — user's home directory
**There are no auto-prefixed env vars.** The env var names your server reads are exactly what you declare in `mcp_config.env`. If you write `"ROOT_DIR": "${user_config.rootDir}"`, your server reads `process.env.ROOT_DIR`.
---
## `user_config` — install-time settings
```json
"user_config": {
"apiKey": {
"type": "string",
"title": "API Key",
"description": "Your service API key. Stored encrypted.",
"sensitive": true,
"required": true
},
"rootDir": {
"type": "directory",
"title": "Root directory",
"description": "Directory to expose to the server.",
"default": "${HOME}/Documents"
},
"maxResults": {
"type": "number",
"title": "Max results",
"description": "Maximum items returned per query.",
"default": 50,
"min": 1,
"max": 500
}
}
```
| Field | Required | Description |
|---|---|---|
| `type` | ✅ | `"string"`, `"number"`, `"boolean"`, `"directory"`, `"file"` |
| `title` | ✅ | Form label. |
| `description` | ✅ | Help text under the input. |
| `default` | | Pre-filled value. Supports `${HOME}`. |
| `required` | | If `true`, install blocks until filled. |
| `sensitive` | | If `true`, stored in OS keychain + masked in UI. **NOT `secret`** — that field doesn't exist. |
| `multiple` | | If `true`, user can enter multiple values (array). |
| `min` / `max` | | Numeric bounds (for `type: "number"`). |
`directory` and `file` types render native OS pickers — prefer these over free-text paths for UX and validation.
---
## `compatibility` — gate installs
```json
"compatibility": {
"claude_desktop": ">=1.0.0",
"platforms": ["darwin", "win32", "linux"],
"runtimes": { "node": ">=20" }
}
```
| Field | Description |
|---|---|
| `claude_desktop` | Semver range. Install blocked if host is older. |
| `platforms` | OS allowlist. Subset of `["darwin", "win32", "linux"]`. |
| `runtimes` | Required runtime versions, e.g. `{"node": ">=20"}` or `{"python": ">=3.11"}`. |
---
## Minimal valid manifest
```json
{
"$schema": "https://raw.githubusercontent.com/anthropics/mcpb/main/schemas/mcpb-manifest-v0.4.schema.json",
"manifest_version": "0.4",
"name": "hello",
"version": "0.1.0",
"description": "Minimal MCPB server.",
"author": { "name": "Your Name" },
"server": {
"type": "node",
"entry_point": "server/index.js",
"mcp_config": {
"command": "node",
"args": ["${__dirname}/server/index.js"]
}
}
}
```
---
## What MCPB does NOT have
- **No `permissions` block.** There is no manifest-level filesystem/network/process scoping. The server runs with full user privileges. Enforce boundaries in your tool handlers — see `local-security.md`.
- **No auto env var prefix.** No `MCPB_CONFIG_*` convention. You wire config → env explicitly in `server.mcp_config.env`.
- **No `entry` field.** It's `server` with `entry_point` inside.
- **No `minHostVersion`.** It's `compatibility.claude_desktop`.