feat(extensions): implement multi-catalog stack support

- Add CatalogEntry dataclass to represent catalog entries
- Add get_active_catalogs() reading SPECKIT_CATALOG_URL, project config,
  user config, or built-in default stack (org-approved + community)
- Add _load_catalog_config() to parse .specify/extension-catalogs.yml
- Add _validate_catalog_url() HTTPS validation helper
- Add _fetch_single_catalog() with per-URL caching, backward-compat for DEFAULT_CATALOG_URL
- Add _get_merged_extensions() that merges all catalogs (priority wins on conflict)
- Update search() and get_extension_info() to use merged results
  annotated with _catalog_name and _install_allowed
- Update clear_cache() to also remove per-URL hash cache files
- Add extension_catalogs CLI command to list active catalogs
- Add catalog add/remove sub-commands for .specify/extension-catalogs.yml
- Update extension_add to enforce install_allowed=false policy
- Update extension_search to show source catalog per result
- Update extension_info to show source catalog with install_allowed status
- Add 13 new tests covering catalog stack, merge conflict resolution,
  install_allowed enforcement, and catalog metadata

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>

src/specify_cli/__init__.py

@@ -1772,6 +1772,13 @@ extension_app = typer.Typer(
 )
 app.add_typer(extension_app, name="extension")
 
+catalog_app = typer.Typer(
+    name="catalog",
+    help="Manage extension catalogs",
+    add_completion=False,
+)
+extension_app.add_typer(catalog_app, name="catalog")
+
 
 def get_speckit_version() -> str:
     """Get current spec-kit version."""
@@ -1837,6 +1844,155 @@ def extension_list(
         console.print("  [cyan]specify extension add <name>[/cyan]")
 
 
+@extension_app.command("catalogs")
+def extension_catalogs():
+    """List all active extension catalogs."""
+    from .extensions import ExtensionCatalog, ValidationError
+
+    project_root = Path.cwd()
+
+    specify_dir = project_root / ".specify"
+    if not specify_dir.exists():
+        console.print("[red]Error:[/red] Not a spec-kit project (no .specify/ directory)")
+        console.print("Run this command from a spec-kit project root")
+        raise typer.Exit(1)
+
+    catalog = ExtensionCatalog(project_root)
+
+    try:
+        active_catalogs = catalog.get_active_catalogs()
+    except ValidationError as e:
+        console.print(f"[red]Error:[/red] {e}")
+        raise typer.Exit(1)
+
+    console.print("\n[bold cyan]Active Extension Catalogs:[/bold cyan]\n")
+    for entry in active_catalogs:
+        install_str = (
+            "[green]install allowed[/green]"
+            if entry.install_allowed
+            else "[yellow]discovery only[/yellow]"
+        )
+        console.print(f"  [bold]{entry.name}[/bold] (priority {entry.priority})")
+        console.print(f"     URL: {entry.url}")
+        console.print(f"     Install: {install_str}")
+        console.print()
+
+    config_path = project_root / ".specify" / "extension-catalogs.yml"
+    if config_path.exists():
+        console.print(f"[dim]Config: {config_path.relative_to(project_root)}[/dim]")
+    else:
+        console.print("[dim]Using built-in default catalog stack.[/dim]")
+        console.print(
+            "[dim]Add .specify/extension-catalogs.yml to customize.[/dim]"
+        )
+
+
+@catalog_app.command("add")
+def catalog_add(
+    url: str = typer.Argument(help="Catalog URL (must use HTTPS)"),
+    name: str = typer.Option(..., "--name", help="Catalog name"),
+    priority: int = typer.Option(10, "--priority", help="Priority (lower = higher priority)"),
+    install_allowed: bool = typer.Option(
+        False, "--install-allowed/--no-install-allowed",
+        help="Allow extensions from this catalog to be installed",
+    ),
+):
+    """Add a catalog to .specify/extension-catalogs.yml."""
+    from .extensions import ExtensionCatalog, ValidationError
+
+    project_root = Path.cwd()
+
+    specify_dir = project_root / ".specify"
+    if not specify_dir.exists():
+        console.print("[red]Error:[/red] Not a spec-kit project (no .specify/ directory)")
+        console.print("Run this command from a spec-kit project root")
+        raise typer.Exit(1)
+
+    # Validate URL
+    tmp_catalog = ExtensionCatalog(project_root)
+    try:
+        tmp_catalog._validate_catalog_url(url)
+    except ValidationError as e:
+        console.print(f"[red]Error:[/red] {e}")
+        raise typer.Exit(1)
+
+    config_path = specify_dir / "extension-catalogs.yml"
+
+    # Load existing config
+    if config_path.exists():
+        try:
+            config = yaml.safe_load(config_path.read_text()) or {}
+        except Exception:
+            config = {}
+    else:
+        config = {}
+
+    catalogs = config.get("catalogs", [])
+
+    # Check for duplicate name
+    for existing in catalogs:
+        if existing.get("name") == name:
+            console.print(f"[yellow]Warning:[/yellow] A catalog named '{name}' already exists.")
+            console.print("Use 'specify extension catalog remove' first, or choose a different name.")
+            raise typer.Exit(1)
+
+    catalogs.append({
+        "name": name,
+        "url": url,
+        "priority": priority,
+        "install_allowed": install_allowed,
+    })
+
+    config["catalogs"] = catalogs
+    config_path.write_text(yaml.dump(config, default_flow_style=False, sort_keys=False))
+
+    install_label = "install allowed" if install_allowed else "discovery only"
+    console.print(f"\n[green]✓[/green] Added catalog '[bold]{name}[/bold]' ({install_label})")
+    console.print(f"  URL: {url}")
+    console.print(f"  Priority: {priority}")
+    console.print(f"\nConfig saved to {config_path.relative_to(project_root)}")
+
+
+@catalog_app.command("remove")
+def catalog_remove(
+    name: str = typer.Argument(help="Catalog name to remove"),
+):
+    """Remove a catalog from .specify/extension-catalogs.yml."""
+    project_root = Path.cwd()
+
+    specify_dir = project_root / ".specify"
+    if not specify_dir.exists():
+        console.print("[red]Error:[/red] Not a spec-kit project (no .specify/ directory)")
+        console.print("Run this command from a spec-kit project root")
+        raise typer.Exit(1)
+
+    config_path = specify_dir / "extension-catalogs.yml"
+    if not config_path.exists():
+        console.print(f"[red]Error:[/red] No catalog config found. Nothing to remove.")
+        raise typer.Exit(1)
+
+    try:
+        config = yaml.safe_load(config_path.read_text()) or {}
+    except Exception:
+        console.print("[red]Error:[/red] Failed to read catalog config.")
+        raise typer.Exit(1)
+
+    catalogs = config.get("catalogs", [])
+    original_count = len(catalogs)
+    catalogs = [c for c in catalogs if c.get("name") != name]
+
+    if len(catalogs) == original_count:
+        console.print(f"[red]Error:[/red] Catalog '{name}' not found.")
+        raise typer.Exit(1)
+
+    config["catalogs"] = catalogs
+    config_path.write_text(yaml.dump(config, default_flow_style=False, sort_keys=False))
+
+    console.print(f"[green]✓[/green] Removed catalog '{name}'")
+    if not catalogs:
+        console.print("\n[dim]No catalogs remain in config. Built-in defaults will be used.[/dim]")
+
+
 @extension_app.command("add")
 def extension_add(
     extension: str = typer.Argument(help="Extension name or path"),
@@ -1925,6 +2081,19 @@ def extension_add(
                     console.print("  specify extension search")
                     raise typer.Exit(1)
 
+                # Enforce install_allowed policy
+                if not ext_info.get("_install_allowed", True):
+                    catalog_name = ext_info.get("_catalog_name", "community")
+                    console.print(
+                        f"[red]Error:[/red] '{extension}' is available in the "
+                        f"'{catalog_name}' catalog but installation is not allowed from that catalog."
+                    )
+                    console.print(
+                        f"\nTo enable installation, add '{extension}' to an approved catalog "
+                        f"(install_allowed: true) in .specify/extension-catalogs.yml."
+                    )
+                    raise typer.Exit(1)
+
                 # Download extension ZIP
                 console.print(f"Downloading {ext_info['name']} v{ext_info.get('version', 'unknown')}...")
                 zip_path = catalog.download_extension(extension)
@@ -2069,6 +2238,15 @@ def extension_search(
                 tags_str = ", ".join(ext['tags'])
                 console.print(f"  [dim]Tags:[/dim] {tags_str}")
 
+            # Source catalog
+            catalog_name = ext.get("_catalog_name", "")
+            install_allowed = ext.get("_install_allowed", True)
+            if catalog_name:
+                if install_allowed:
+                    console.print(f"  [dim]Catalog:[/dim] {catalog_name}")
+                else:
+                    console.print(f"  [dim]Catalog:[/dim] {catalog_name} [yellow](discovery only — not installable)[/yellow]")
+
             # Stats
             stats = []
             if ext.get('downloads') is not None:
@@ -2082,8 +2260,15 @@ def extension_search(
             if ext.get('repository'):
                 console.print(f"  [dim]Repository:[/dim] {ext['repository']}")
 
-            # Install command
+            # Install command (show warning if not installable)
+            if install_allowed:
                 console.print(f"\n  [cyan]Install:[/cyan] specify extension add {ext['id']}")
+            else:
+                console.print(f"\n  [yellow]⚠[/yellow]  Not directly installable from '{catalog_name}'.")
+                console.print(
+                    f"  Add to an approved catalog with install_allowed: true, "
+                    f"or use: specify extension add --from <url>"
+                )
             console.print()
 
     except ExtensionError as e:
@@ -2132,6 +2317,12 @@ def extension_info(
         # Author and License
         console.print(f"[dim]Author:[/dim] {ext_info.get('author', 'Unknown')}")
         console.print(f"[dim]License:[/dim] {ext_info.get('license', 'Unknown')}")
+
+        # Source catalog
+        if ext_info.get("_catalog_name"):
+            install_allowed = ext_info.get("_install_allowed", True)
+            install_note = "" if install_allowed else " [yellow](discovery only)[/yellow]"
+            console.print(f"[dim]Source catalog:[/dim] {ext_info['_catalog_name']}{install_note}")
         console.print()
 
         # Requirements
@@ -2188,12 +2379,21 @@ def extension_info(
 
         # Installation status and command
         is_installed = manager.registry.is_installed(ext_info['id'])
+        install_allowed = ext_info.get("_install_allowed", True)
         if is_installed:
             console.print("[green]✓ Installed[/green]")
             console.print(f"\nTo remove: specify extension remove {ext_info['id']}")
-        else:
+        elif install_allowed:
             console.print("[yellow]Not installed[/yellow]")
             console.print(f"\n[cyan]Install:[/cyan] specify extension add {ext_info['id']}")
+        else:
+            catalog_name = ext_info.get("_catalog_name", "community")
+            console.print("[yellow]Not installed[/yellow]")
+            console.print(
+                f"\n[yellow]⚠[/yellow]  '{ext_info['id']}' is available in the '{catalog_name}' catalog "
+                f"but not in your approved catalog. Add it to .specify/extension-catalogs.yml "
+                f"with install_allowed: true to enable installation."
+            )
 
     except ExtensionError as e:
         console.print(f"\n[red]Error:[/red] {e}")

src/specify_cli/extensions.py

@@ -8,9 +8,11 @@ without bloating the core framework.
 
 import json
 import hashlib
+import os
 import tempfile
 import zipfile
 import shutil
+from dataclasses import dataclass
 from pathlib import Path
 from typing import Optional, Dict, List, Any
 from datetime import datetime, timezone
@@ -36,6 +38,15 @@ class CompatibilityError(ExtensionError):
     pass
 
 
+@dataclass
+class CatalogEntry:
+    """Represents a single catalog entry in the catalog stack."""
+    url: str
+    name: str
+    priority: int
+    install_allowed: bool
+
+
 class ExtensionManifest:
     """Represents and validates an extension manifest (extension.yml)."""
 
@@ -976,6 +987,7 @@ class ExtensionCatalog:
     """Manages extension catalog fetching, caching, and searching."""
 
     DEFAULT_CATALOG_URL = "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.json"
+    COMMUNITY_CATALOG_URL = "https://raw.githubusercontent.com/github/spec-kit/main/extensions/catalog.community.json"
     CACHE_DURATION = 3600  # 1 hour in seconds
 
     def __init__(self, project_root: Path):
@@ -990,43 +1002,82 @@ class ExtensionCatalog:
         self.cache_file = self.cache_dir / "catalog.json"
         self.cache_metadata_file = self.cache_dir / "catalog-metadata.json"
 
-    def get_catalog_url(self) -> str:
-        """Get catalog URL from config or use default.
+    def _validate_catalog_url(self, url: str) -> None:
+        """Validate that a catalog URL uses HTTPS (localhost HTTP allowed).
 
-        Checks in order:
-        1. SPECKIT_CATALOG_URL environment variable
-        2. Default catalog URL
-
-        Returns:
-            URL to fetch catalog from
+        Args:
+            url: URL to validate
 
         Raises:
-            ValidationError: If custom URL is invalid (non-HTTPS)
+            ValidationError: If URL is invalid or uses non-HTTPS scheme
         """
-        import os
-        import sys
         from urllib.parse import urlparse
 
-        # Environment variable override (useful for testing)
-        if env_value := os.environ.get("SPECKIT_CATALOG_URL"):
-            catalog_url = env_value.strip()
-            parsed = urlparse(catalog_url)
-
-            # Require HTTPS for security (prevent man-in-the-middle attacks)
-            # Allow http://localhost for local development/testing
+        parsed = urlparse(url)
         is_localhost = parsed.hostname in ("localhost", "127.0.0.1", "::1")
         if parsed.scheme != "https" and not (parsed.scheme == "http" and is_localhost):
             raise ValidationError(
-                    f"Invalid SPECKIT_CATALOG_URL: must use HTTPS (got {parsed.scheme}://). "
+                f"Catalog URL must use HTTPS (got {parsed.scheme}://). "
                 "HTTP is only allowed for localhost."
             )
-
         if not parsed.netloc:
-                raise ValidationError(
-                    "Invalid SPECKIT_CATALOG_URL: must be a valid URL with a host."
-                )
+            raise ValidationError("Catalog URL must be a valid URL with a host.")
 
-            # Warn users when using a non-default catalog (once per instance)
+    def _load_catalog_config(self, config_path: Path) -> Optional[List[CatalogEntry]]:
+        """Load catalog stack configuration from a YAML file.
+
+        Args:
+            config_path: Path to extension-catalogs.yml
+
+        Returns:
+            Ordered list of CatalogEntry objects, or None if file doesn't exist
+            or contains no valid catalog entries.
+        """
+        if not config_path.exists():
+            return None
+        try:
+            data = yaml.safe_load(config_path.read_text()) or {}
+            catalogs_data = data.get("catalogs", [])
+            if not catalogs_data:
+                return None
+            entries: List[CatalogEntry] = []
+            for idx, item in enumerate(catalogs_data):
+                url = str(item.get("url", "")).strip()
+                if not url:
+                    continue
+                self._validate_catalog_url(url)
+                entries.append(CatalogEntry(
+                    url=url,
+                    name=str(item.get("name", f"catalog-{idx + 1}")),
+                    priority=int(item.get("priority", idx + 1)),
+                    install_allowed=bool(item.get("install_allowed", True)),
+                ))
+            entries.sort(key=lambda e: e.priority)
+            return entries if entries else None
+        except (yaml.YAMLError, OSError):
+            return None
+
+    def get_active_catalogs(self) -> List[CatalogEntry]:
+        """Get the ordered list of active catalogs.
+
+        Resolution order:
+        1. SPECKIT_CATALOG_URL env var — single catalog replacing all defaults
+        2. Project-level .specify/extension-catalogs.yml
+        3. User-level ~/.specify/extension-catalogs.yml
+        4. Built-in default stack (org-approved + community)
+
+        Returns:
+            List of CatalogEntry objects sorted by priority (ascending)
+
+        Raises:
+            ValidationError: If a catalog URL is invalid
+        """
+        import sys
+
+        # 1. SPECKIT_CATALOG_URL env var replaces all defaults for backward compat
+        if env_value := os.environ.get("SPECKIT_CATALOG_URL"):
+            catalog_url = env_value.strip()
+            self._validate_catalog_url(catalog_url)
             if catalog_url != self.DEFAULT_CATALOG_URL:
                 if not getattr(self, "_non_default_catalog_warning_shown", False):
                     print(
@@ -1035,11 +1086,160 @@ class ExtensionCatalog:
                         file=sys.stderr,
                     )
                     self._non_default_catalog_warning_shown = True
+            return [CatalogEntry(url=catalog_url, name="custom", priority=1, install_allowed=True)]
 
-            return catalog_url
+        # 2. Project-level config overrides all defaults
+        project_config_path = self.project_root / ".specify" / "extension-catalogs.yml"
+        catalogs = self._load_catalog_config(project_config_path)
+        if catalogs is not None:
+            return catalogs
 
-        # TODO: Support custom catalogs from .specify/extension-catalogs.yml
-        return self.DEFAULT_CATALOG_URL
+        # 3. User-level config
+        user_config_path = Path.home() / ".specify" / "extension-catalogs.yml"
+        catalogs = self._load_catalog_config(user_config_path)
+        if catalogs is not None:
+            return catalogs
+
+        # 4. Built-in default stack
+        return [
+            CatalogEntry(url=self.DEFAULT_CATALOG_URL, name="org-approved", priority=1, install_allowed=True),
+            CatalogEntry(url=self.COMMUNITY_CATALOG_URL, name="community", priority=2, install_allowed=False),
+        ]
+
+    def get_catalog_url(self) -> str:
+        """Get the primary catalog URL.
+
+        Returns the URL of the highest-priority catalog. Kept for backward
+        compatibility. Use get_active_catalogs() for full multi-catalog support.
+
+        Returns:
+            URL of the primary catalog
+
+        Raises:
+            ValidationError: If a catalog URL is invalid
+        """
+        active = self.get_active_catalogs()
+        return active[0].url if active else self.DEFAULT_CATALOG_URL
+
+    def _fetch_single_catalog(self, entry: CatalogEntry, force_refresh: bool = False) -> Dict[str, Any]:
+        """Fetch a single catalog with per-URL caching.
+
+        For the DEFAULT_CATALOG_URL, uses legacy cache files (self.cache_file /
+        self.cache_metadata_file) for backward compatibility. For all other URLs,
+        uses URL-hash-based cache files in self.cache_dir.
+
+        Args:
+            entry: CatalogEntry describing the catalog to fetch
+            force_refresh: If True, bypass cache
+
+        Returns:
+            Catalog data dictionary
+
+        Raises:
+            ExtensionError: If catalog cannot be fetched or has invalid format
+        """
+        import urllib.request
+        import urllib.error
+
+        # Determine cache file paths (backward compat for default catalog)
+        if entry.url == self.DEFAULT_CATALOG_URL:
+            cache_file = self.cache_file
+            cache_meta_file = self.cache_metadata_file
+            is_valid = not force_refresh and self.is_cache_valid()
+        else:
+            url_hash = hashlib.sha256(entry.url.encode()).hexdigest()[:16]
+            cache_file = self.cache_dir / f"catalog-{url_hash}.json"
+            cache_meta_file = self.cache_dir / f"catalog-{url_hash}-metadata.json"
+            is_valid = False
+            if not force_refresh and cache_file.exists() and cache_meta_file.exists():
+                try:
+                    metadata = json.loads(cache_meta_file.read_text())
+                    cached_at = datetime.fromisoformat(metadata.get("cached_at", ""))
+                    age = (datetime.now(timezone.utc) - cached_at).total_seconds()
+                    is_valid = age < self.CACHE_DURATION
+                except (json.JSONDecodeError, ValueError, KeyError):
+                    pass
+
+        # Use cache if valid
+        if is_valid:
+            try:
+                return json.loads(cache_file.read_text())
+            except json.JSONDecodeError:
+                pass
+
+        # Fetch from network
+        try:
+            with urllib.request.urlopen(entry.url, timeout=10) as response:
+                catalog_data = json.loads(response.read())
+
+            if "schema_version" not in catalog_data or "extensions" not in catalog_data:
+                raise ExtensionError(f"Invalid catalog format from {entry.url}")
+
+            # Save to cache
+            self.cache_dir.mkdir(parents=True, exist_ok=True)
+            cache_file.write_text(json.dumps(catalog_data, indent=2))
+            cache_meta_file.write_text(json.dumps({
+                "cached_at": datetime.now(timezone.utc).isoformat(),
+                "catalog_url": entry.url,
+            }, indent=2))
+
+            return catalog_data
+
+        except urllib.error.URLError as e:
+            raise ExtensionError(f"Failed to fetch catalog from {entry.url}: {e}")
+        except json.JSONDecodeError as e:
+            raise ExtensionError(f"Invalid JSON in catalog from {entry.url}: {e}")
+
+    def _get_merged_extensions(self, force_refresh: bool = False) -> List[Dict[str, Any]]:
+        """Fetch and merge extensions from all active catalogs.
+
+        Higher-priority (lower priority number) catalogs win on conflicts
+        (same extension id in two catalogs). Each extension dict is annotated with:
+          - _catalog_name: name of the source catalog
+          - _install_allowed: whether installation is allowed from this catalog
+
+        Catalogs that fail to fetch are skipped. Raises ExtensionError only if
+        ALL catalogs fail.
+
+        Args:
+            force_refresh: If True, bypass all caches
+
+        Returns:
+            List of merged extension dicts
+
+        Raises:
+            ExtensionError: If all catalogs fail to fetch
+        """
+        import sys
+
+        active_catalogs = self.get_active_catalogs()
+        merged: Dict[str, Dict[str, Any]] = {}
+        any_success = False
+
+        for catalog_entry in active_catalogs:
+            try:
+                catalog_data = self._fetch_single_catalog(catalog_entry, force_refresh)
+                any_success = True
+            except ExtensionError as e:
+                print(
+                    f"Warning: Could not fetch catalog '{catalog_entry.name}': {e}",
+                    file=sys.stderr,
+                )
+                continue
+
+            for ext_id, ext_data in catalog_data.get("extensions", {}).items():
+                if ext_id not in merged:  # Higher-priority catalog wins
+                    merged[ext_id] = {
+                        "id": ext_id,
+                        **ext_data,
+                        "_catalog_name": catalog_entry.name,
+                        "_install_allowed": catalog_entry.install_allowed,
+                    }
+
+        if not any_success and active_catalogs:
+            raise ExtensionError("Failed to fetch any extension catalog")
+
+        return list(merged.values())
 
     def is_cache_valid(self) -> bool:
         """Check if cached catalog is still valid.
@@ -1116,7 +1316,7 @@ class ExtensionCatalog:
         author: Optional[str] = None,
         verified_only: bool = False,
     ) -> List[Dict[str, Any]]:
-        """Search catalog for extensions.
+        """Search catalog for extensions across all active catalogs.
 
         Args:
             query: Search query (searches name, description, tags)
@@ -1125,14 +1325,16 @@ class ExtensionCatalog:
             verified_only: If True, show only verified extensions
 
         Returns:
-            List of matching extension metadata
+            List of matching extension metadata, each annotated with
+            ``_catalog_name`` and ``_install_allowed`` from its source catalog.
         """
-        catalog = self.fetch_catalog()
-        extensions = catalog.get("extensions", {})
+        all_extensions = self._get_merged_extensions()
 
         results = []
 
-        for ext_id, ext_data in extensions.items():
+        for ext_data in all_extensions:
+            ext_id = ext_data["id"]
+
             # Apply filters
             if verified_only and not ext_data.get("verified", False):
                 continue
@@ -1158,25 +1360,26 @@ class ExtensionCatalog:
                 if query_lower not in searchable_text:
                     continue
 
-            results.append({"id": ext_id, **ext_data})
+            results.append(ext_data)
 
         return results
 
     def get_extension_info(self, extension_id: str) -> Optional[Dict[str, Any]]:
         """Get detailed information about a specific extension.
 
+        Searches all active catalogs in priority order.
+
         Args:
             extension_id: ID of the extension
 
         Returns:
-            Extension metadata or None if not found
+            Extension metadata (annotated with ``_catalog_name`` and
+            ``_install_allowed``) or None if not found.
         """
-        catalog = self.fetch_catalog()
-        extensions = catalog.get("extensions", {})
-
-        if extension_id in extensions:
-            return {"id": extension_id, **extensions[extension_id]}
-
+        all_extensions = self._get_merged_extensions()
+        for ext_data in all_extensions:
+            if ext_data["id"] == extension_id:
+                return ext_data
         return None
 
     def download_extension(self, extension_id: str, target_dir: Optional[Path] = None) -> Path:
@@ -1236,11 +1439,18 @@ class ExtensionCatalog:
             raise ExtensionError(f"Failed to save extension ZIP: {e}")
 
     def clear_cache(self):
-        """Clear the catalog cache."""
+        """Clear the catalog cache (both legacy and URL-hash-based files)."""
         if self.cache_file.exists():
             self.cache_file.unlink()
         if self.cache_metadata_file.exists():
             self.cache_metadata_file.unlink()
+        # Also clear any per-URL hash-based cache files
+        if self.cache_dir.exists():
+            for extra_cache in self.cache_dir.glob("catalog-*.json"):
+                if extra_cache != self.cache_file:
+                    extra_cache.unlink(missing_ok=True)
+            for extra_meta in self.cache_dir.glob("catalog-*-metadata.json"):
+                extra_meta.unlink(missing_ok=True)
 
 
 class ConfigManager:

tests/test_extensions.py

@@ -6,6 +6,7 @@ Tests cover:
 - Extension registry operations
 - Extension manager installation/removal
 - Command registration
+- Catalog stack (multi-catalog support)
 """
 
 import pytest
@@ -16,6 +17,7 @@ from pathlib import Path
 from datetime import datetime, timezone
 
 from specify_cli.extensions import (
+    CatalogEntry,
     ExtensionManifest,
     ExtensionRegistry,
     ExtensionManager,
@@ -880,10 +882,29 @@ class TestExtensionCatalog:
 
     def test_search_all_extensions(self, temp_dir):
         """Test searching all extensions without filters."""
+        import yaml as yaml_module
+
         project_dir = temp_dir / "project"
         project_dir.mkdir()
         (project_dir / ".specify").mkdir()
 
+        # Use a single-catalog config so community extensions don't interfere
+        config_path = project_dir / ".specify" / "extension-catalogs.yml"
+        with open(config_path, "w") as f:
+            yaml_module.dump(
+                {
+                    "catalogs": [
+                        {
+                            "name": "test-catalog",
+                            "url": ExtensionCatalog.DEFAULT_CATALOG_URL,
+                            "priority": 1,
+                            "install_allowed": True,
+                        }
+                    ]
+                },
+                f,
+            )
+
         catalog = ExtensionCatalog(project_dir)
 
         # Create mock catalog
@@ -1133,3 +1154,373 @@ class TestExtensionCatalog:
 
         assert not catalog.cache_file.exists()
         assert not catalog.cache_metadata_file.exists()
+
+
+# ===== CatalogEntry Tests =====
+
+class TestCatalogEntry:
+    """Test CatalogEntry dataclass."""
+
+    def test_catalog_entry_creation(self):
+        """Test creating a CatalogEntry."""
+        entry = CatalogEntry(
+            url="https://example.com/catalog.json",
+            name="test",
+            priority=1,
+            install_allowed=True,
+        )
+        assert entry.url == "https://example.com/catalog.json"
+        assert entry.name == "test"
+        assert entry.priority == 1
+        assert entry.install_allowed is True
+
+
+# ===== Catalog Stack Tests =====
+
+class TestCatalogStack:
+    """Test multi-catalog stack support."""
+
+    def _make_project(self, temp_dir: Path) -> Path:
+        """Create a minimal spec-kit project directory."""
+        project_dir = temp_dir / "project"
+        project_dir.mkdir()
+        (project_dir / ".specify").mkdir()
+        return project_dir
+
+    def _write_catalog_config(self, project_dir: Path, catalogs: list) -> None:
+        """Write extension-catalogs.yml to project .specify dir."""
+        import yaml as yaml_module
+
+        config_path = project_dir / ".specify" / "extension-catalogs.yml"
+        with open(config_path, "w") as f:
+            yaml_module.dump({"catalogs": catalogs}, f)
+
+    def _write_valid_cache(
+        self, catalog: ExtensionCatalog, extensions: dict, url: str = "http://test.com"
+    ) -> None:
+        """Populate the primary cache file with mock extension data."""
+        catalog_data = {"schema_version": "1.0", "extensions": extensions}
+        catalog.cache_dir.mkdir(parents=True, exist_ok=True)
+        catalog.cache_file.write_text(json.dumps(catalog_data))
+        catalog.cache_metadata_file.write_text(
+            json.dumps(
+                {
+                    "cached_at": datetime.now(timezone.utc).isoformat(),
+                    "catalog_url": url,
+                }
+            )
+        )
+
+    # --- get_active_catalogs ---
+
+    def test_default_stack(self, temp_dir):
+        """Default stack includes org-approved and community catalogs."""
+        project_dir = self._make_project(temp_dir)
+        catalog = ExtensionCatalog(project_dir)
+
+        entries = catalog.get_active_catalogs()
+
+        assert len(entries) == 2
+        assert entries[0].url == ExtensionCatalog.DEFAULT_CATALOG_URL
+        assert entries[0].name == "org-approved"
+        assert entries[0].priority == 1
+        assert entries[0].install_allowed is True
+        assert entries[1].url == ExtensionCatalog.COMMUNITY_CATALOG_URL
+        assert entries[1].name == "community"
+        assert entries[1].priority == 2
+        assert entries[1].install_allowed is False
+
+    def test_env_var_overrides_default_stack(self, temp_dir, monkeypatch):
+        """SPECKIT_CATALOG_URL replaces the entire default stack."""
+        project_dir = self._make_project(temp_dir)
+        custom_url = "https://example.com/catalog.json"
+        monkeypatch.setenv("SPECKIT_CATALOG_URL", custom_url)
+
+        catalog = ExtensionCatalog(project_dir)
+        entries = catalog.get_active_catalogs()
+
+        assert len(entries) == 1
+        assert entries[0].url == custom_url
+        assert entries[0].install_allowed is True
+
+    def test_env_var_invalid_url_raises(self, temp_dir, monkeypatch):
+        """SPECKIT_CATALOG_URL with http:// (non-localhost) raises ValidationError."""
+        project_dir = self._make_project(temp_dir)
+        monkeypatch.setenv("SPECKIT_CATALOG_URL", "http://example.com/catalog.json")
+
+        catalog = ExtensionCatalog(project_dir)
+        with pytest.raises(ValidationError, match="HTTPS"):
+            catalog.get_active_catalogs()
+
+    def test_project_config_overrides_defaults(self, temp_dir):
+        """Project-level extension-catalogs.yml overrides default stack."""
+        project_dir = self._make_project(temp_dir)
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "custom",
+                    "url": "https://example.com/catalog.json",
+                    "priority": 1,
+                    "install_allowed": True,
+                }
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        entries = catalog.get_active_catalogs()
+
+        assert len(entries) == 1
+        assert entries[0].url == "https://example.com/catalog.json"
+        assert entries[0].name == "custom"
+
+    def test_project_config_sorted_by_priority(self, temp_dir):
+        """Catalog entries are sorted by priority (ascending)."""
+        project_dir = self._make_project(temp_dir)
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "secondary",
+                    "url": "https://example.com/secondary.json",
+                    "priority": 5,
+                    "install_allowed": False,
+                },
+                {
+                    "name": "primary",
+                    "url": "https://example.com/primary.json",
+                    "priority": 1,
+                    "install_allowed": True,
+                },
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        entries = catalog.get_active_catalogs()
+
+        assert len(entries) == 2
+        assert entries[0].name == "primary"
+        assert entries[1].name == "secondary"
+
+    def test_project_config_invalid_url_raises(self, temp_dir):
+        """Project config with HTTP (non-localhost) URL raises ValidationError."""
+        project_dir = self._make_project(temp_dir)
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "bad",
+                    "url": "http://example.com/catalog.json",
+                    "priority": 1,
+                    "install_allowed": True,
+                }
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        with pytest.raises(ValidationError, match="HTTPS"):
+            catalog.get_active_catalogs()
+
+    def test_empty_project_config_falls_back_to_defaults(self, temp_dir):
+        """Empty catalogs list in config falls back to default stack."""
+        import yaml as yaml_module
+
+        project_dir = self._make_project(temp_dir)
+        config_path = project_dir / ".specify" / "extension-catalogs.yml"
+        with open(config_path, "w") as f:
+            yaml_module.dump({"catalogs": []}, f)
+
+        catalog = ExtensionCatalog(project_dir)
+        entries = catalog.get_active_catalogs()
+
+        # Falls back to default stack
+        assert len(entries) == 2
+        assert entries[0].url == ExtensionCatalog.DEFAULT_CATALOG_URL
+
+    # --- _load_catalog_config ---
+
+    def test_load_catalog_config_missing_file(self, temp_dir):
+        """Returns None when config file doesn't exist."""
+        project_dir = self._make_project(temp_dir)
+        catalog = ExtensionCatalog(project_dir)
+
+        result = catalog._load_catalog_config(project_dir / ".specify" / "nonexistent.yml")
+        assert result is None
+
+    def test_load_catalog_config_localhost_allowed(self, temp_dir):
+        """Localhost HTTP URLs are allowed in config."""
+        project_dir = self._make_project(temp_dir)
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "local",
+                    "url": "http://localhost:8000/catalog.json",
+                    "priority": 1,
+                    "install_allowed": True,
+                }
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        entries = catalog.get_active_catalogs()
+
+        assert len(entries) == 1
+        assert entries[0].url == "http://localhost:8000/catalog.json"
+
+    # --- Merge conflict resolution ---
+
+    def test_merge_conflict_higher_priority_wins(self, temp_dir):
+        """When same extension id is in two catalogs, higher priority wins."""
+        import yaml as yaml_module
+
+        project_dir = self._make_project(temp_dir)
+
+        # Write project config with two catalogs
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "primary",
+                    "url": ExtensionCatalog.DEFAULT_CATALOG_URL,
+                    "priority": 1,
+                    "install_allowed": True,
+                },
+                {
+                    "name": "secondary",
+                    "url": ExtensionCatalog.COMMUNITY_CATALOG_URL,
+                    "priority": 2,
+                    "install_allowed": False,
+                },
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+
+        # Write primary cache with jira v2.0.0
+        primary_data = {
+            "schema_version": "1.0",
+            "extensions": {
+                "jira": {
+                    "name": "Jira Integration",
+                    "id": "jira",
+                    "version": "2.0.0",
+                    "description": "Primary Jira",
+                }
+            },
+        }
+        catalog.cache_dir.mkdir(parents=True, exist_ok=True)
+        catalog.cache_file.write_text(json.dumps(primary_data))
+        catalog.cache_metadata_file.write_text(
+            json.dumps({"cached_at": datetime.now(timezone.utc).isoformat(), "catalog_url": "http://test.com"})
+        )
+
+        # Write secondary cache (URL-hash-based) with jira v1.0.0 (should lose)
+        import hashlib
+
+        url_hash = hashlib.sha256(ExtensionCatalog.COMMUNITY_CATALOG_URL.encode()).hexdigest()[:16]
+        secondary_cache = catalog.cache_dir / f"catalog-{url_hash}.json"
+        secondary_meta = catalog.cache_dir / f"catalog-{url_hash}-metadata.json"
+        secondary_data = {
+            "schema_version": "1.0",
+            "extensions": {
+                "jira": {
+                    "name": "Jira Integration Community",
+                    "id": "jira",
+                    "version": "1.0.0",
+                    "description": "Community Jira",
+                },
+                "linear": {
+                    "name": "Linear",
+                    "id": "linear",
+                    "version": "0.9.0",
+                    "description": "Linear from secondary",
+                },
+            },
+        }
+        secondary_cache.write_text(json.dumps(secondary_data))
+        secondary_meta.write_text(
+            json.dumps({"cached_at": datetime.now(timezone.utc).isoformat(), "catalog_url": ExtensionCatalog.COMMUNITY_CATALOG_URL})
+        )
+
+        results = catalog.search()
+        jira_results = [r for r in results if r["id"] == "jira"]
+        assert len(jira_results) == 1
+        # Primary catalog wins
+        assert jira_results[0]["version"] == "2.0.0"
+        assert jira_results[0]["_catalog_name"] == "primary"
+        assert jira_results[0]["_install_allowed"] is True
+
+        # linear comes from secondary
+        linear_results = [r for r in results if r["id"] == "linear"]
+        assert len(linear_results) == 1
+        assert linear_results[0]["_catalog_name"] == "secondary"
+        assert linear_results[0]["_install_allowed"] is False
+
+    def test_install_allowed_false_from_get_extension_info(self, temp_dir):
+        """get_extension_info includes _install_allowed from source catalog."""
+        project_dir = self._make_project(temp_dir)
+
+        # Single catalog that is install_allowed=False
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "discovery",
+                    "url": ExtensionCatalog.DEFAULT_CATALOG_URL,
+                    "priority": 1,
+                    "install_allowed": False,
+                }
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        self._write_valid_cache(
+            catalog,
+            {
+                "jira": {
+                    "name": "Jira Integration",
+                    "id": "jira",
+                    "version": "1.0.0",
+                    "description": "Jira integration",
+                }
+            },
+        )
+
+        info = catalog.get_extension_info("jira")
+        assert info is not None
+        assert info["_install_allowed"] is False
+        assert info["_catalog_name"] == "discovery"
+
+    def test_search_results_include_catalog_metadata(self, temp_dir):
+        """Search results include _catalog_name and _install_allowed."""
+        project_dir = self._make_project(temp_dir)
+        self._write_catalog_config(
+            project_dir,
+            [
+                {
+                    "name": "org",
+                    "url": ExtensionCatalog.DEFAULT_CATALOG_URL,
+                    "priority": 1,
+                    "install_allowed": True,
+                }
+            ],
+        )
+
+        catalog = ExtensionCatalog(project_dir)
+        self._write_valid_cache(
+            catalog,
+            {
+                "jira": {
+                    "name": "Jira Integration",
+                    "id": "jira",
+                    "version": "1.0.0",
+                    "description": "Jira integration",
+                }
+            },
+        )
+
+        results = catalog.search()
+        assert len(results) == 1
+        assert results[0]["_catalog_name"] == "org"
+        assert results[0]["_install_allowed"] is True