fix: add MCP tools cache reset endpoint (#3602)

* fix: add MCP tools cache reset endpoint

* docs: clarify MCP cache reset scope

backend/docs/API.md

@@ -299,6 +299,26 @@ deployment needs additional trusted launchers.
 }
 ```
 
+#### Reset MCP Tools Cache
+
+Clear cached MCP tools and persistent MCP sessions process-wide. This affects
+all threads and users in the current Gateway process. Tools are loaded again
+from configured MCP servers on the next agent run or tool lookup.
+
+```http
+POST /api/mcp/cache/reset
+```
+
+Requires an authenticated admin session.
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "MCP tools cache reset. Tools will reload on next use."
+}
+```
+
 ### Skills
 
 #### List Skills

backend/docs/ARCHITECTURE.md

@@ -427,17 +427,17 @@ SKILL.md Format:
 ### Configuration Reload
 
 ```
-1. Client updates MCP config
+1. Client updates MCP config or requests a cache reset
    PUT /api/mcp/config
+   POST /api/mcp/cache/reset
 
-2. Gateway writes extensions_config.json
-   - Updates mcpServers section
-   - File mtime changes
+2. Gateway updates runtime state
+   - PUT writes extensions_config.json and reloads configuration
+   - Both endpoints reset the MCP tools cache and persistent sessions
 
-3. MCP Manager detects change
-   - get_cached_mcp_tools() checks mtime
-   - If changed: reinitializes MCP client
-   - Loads updated server configurations
+3. MCP Manager reloads on next use
+   - get_cached_mcp_tools() lazily reinitializes MCP tools
+   - Loads current server configurations and tool lists
 
 4. Next agent run uses new tools
 ```