fix: intercept stdout writes to prevent JSON-RPC corruption in stdio mode (#673)

* fix: intercept process.stdout.write to prevent JSON-RPC corruption in stdio mode (#628, #627, #567)

Console method suppression alone was insufficient — native modules, n8n packages,
and third-party code can call process.stdout.write() directly, leaking debug output
(refCount, dbPath, clientVersion, protocolVersion, etc.) into the MCP JSON-RPC stream.

Added stdout write interceptor that only allows JSON-RPC messages through (objects
containing "jsonrpc" field). All other writes are redirected to stderr. This fixes
the flood of "Unexpected token is not valid JSON" warnings on every new Claude
Desktop chat.

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* ci: add Docker Hub login to fix buildx bootstrap rate limiting

GitHub-hosted runners hit Docker Hub anonymous pull limits when
setup-buildx-action pulls moby/buildkit. Add docker/login-action
for Docker Hub before setup-buildx-action in all 4 workflows:
docker-build.yml, docker-build-fast.yml, docker-build-n8n.yml, release.yml.

Uses DOCKERHUB_USERNAME and DOCKERHUB_TOKEN repository secrets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/docker-build-fast.yml

@@ -29,9 +29,15 @@ jobs:
         with:
           lfs: true
         
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
-        
+
       - name: Log in to GitHub Container Registry
         if: github.event_name != 'pull_request'
         uses: docker/login-action@v3

.github/workflows/docker-build-n8n.yml

@@ -55,6 +55,12 @@ jobs:
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 

.github/workflows/docker-build.yml

@@ -71,13 +71,19 @@ jobs:
           "
           echo "✅ Synced package.runtime.json to version $VERSION"
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
-        
+
       - name: Set up Docker Buildx
         id: buildx
         uses: docker/setup-buildx-action@v3
-        
+
       - name: Log in to GitHub Container Registry
         if: github.event_name != 'pull_request'
         uses: docker/login-action@v3
@@ -85,7 +91,7 @@ jobs:
           registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
-          
+
       - name: Extract metadata
         id: meta
         uses: docker/metadata-action@v5
@@ -173,13 +179,19 @@ jobs:
           "
           echo "✅ Synced package.runtime.json to version $VERSION"
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
-        
+
       - name: Set up Docker Buildx
         id: buildx
         uses: docker/setup-buildx-action@v3
-        
+
       - name: Log in to GitHub Container Registry
         if: github.event_name != 'pull_request'
         uses: docker/login-action@v3

.github/workflows/release.yml

@@ -441,12 +441,18 @@ jobs:
           "
           echo "✅ Synced package.runtime.json to version $VERSION"
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v3
-      
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
-      
+
       - name: Log in to GitHub Container Registry
         uses: docker/login-action@v3
         with:

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [2.41.2] - 2026-03-27
+
+### Fixed
+
+- **MCP initialization floods Claude Desktop with JSON parse errors** (Issues #628, #627, #567): Intercept `process.stdout.write` in stdio mode to redirect non-JSON-RPC output to stderr. Console method suppression alone was insufficient — native modules (better-sqlite3), n8n packages, and third-party code can call `process.stdout.write()` directly, corrupting the JSON-RPC stream. Only writes containing valid JSON-RPC messages (`{"jsonrpc":...}`) are now allowed through stdout; everything else is redirected to stderr. This fixes the flood of "Unexpected token is not valid JSON" warnings on every new chat in Claude Desktop, including leaked `refCount`, `dbPath`, `clientVersion`, `protocolVersion`, and other debug strings.
+
+Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en
+
 ## [2.41.1] - 2026-03-27
 
 ### Fixed

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "n8n-mcp",
-  "version": "2.41.1",
+  "version": "2.41.2",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "n8n-mcp",
-      "version": "2.41.1",
+      "version": "2.41.2",
       "license": "MIT",
       "dependencies": {
         "@modelcontextprotocol/sdk": "1.28.0",

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.41.1",
+  "version": "2.41.2",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",

src/mcp/stdio-wrapper.ts

@@ -39,6 +39,25 @@ console.clear = () => {};
 console.count = () => {};
 console.countReset = () => {};
 
+// CRITICAL: Intercept process.stdout.write to prevent non-JSON-RPC output (#628, #627, #567)
+// Console suppression alone is insufficient — native modules (better-sqlite3), n8n packages,
+// and third-party code can call process.stdout.write() directly, corrupting the JSON-RPC stream.
+// Only allow writes that look like JSON-RPC messages; redirect everything else to stderr.
+const originalStdoutWrite = process.stdout.write.bind(process.stdout);
+const stderrWrite = process.stderr.write.bind(process.stderr);
+
+process.stdout.write = function (chunk: any, encodingOrCallback?: any, callback?: any): boolean {
+  const str = typeof chunk === 'string' ? chunk : chunk.toString();
+  // JSON-RPC messages are JSON objects with "jsonrpc" field — let those through
+  // The MCP SDK sends one JSON object per write call
+  const trimmed = str.trimStart();
+  if (trimmed.startsWith('{') && trimmed.includes('"jsonrpc"')) {
+    return originalStdoutWrite(chunk, encodingOrCallback, callback);
+  }
+  // Redirect everything else to stderr so it doesn't corrupt the protocol
+  return stderrWrite(chunk, encodingOrCallback, callback);
+} as typeof process.stdout.write;
+
 // Import and run the server AFTER suppressing output
 import { N8NDocumentationMCPServer } from './server';