feat(mcp): support custom tool interceptors via extensions_config.json (#2451)

* feat(mcp): support custom tool interceptors via extensions_config.json

Add a generic extension point for registering custom MCP tool
interceptors through `extensions_config.json`. This allows downstream
projects to inject per-request header manipulation, auth context
propagation, or other cross-cutting concerns without modifying
DeerFlow source code.

Interceptors are declared as Python callable paths in a new
`mcpInterceptors` array field and loaded via the existing
`resolve_variable` reflection mechanism:

```json
{
  "mcpInterceptors": [
    "my_package.mcp.auth:build_auth_interceptor"
  ]
}
```

Each entry must resolve to a no-arg builder function that returns an
async interceptor compatible with `MultiServerMCPClient`'s
`tool_interceptors` interface.

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

* test(mcp): add unit tests for custom tool interceptors

Cover all branches of the mcpInterceptors loading logic:

- valid interceptor loaded and appended to tool_interceptors
- multiple interceptors loaded in declaration order
- builder returning None is skipped
- resolve_variable ImportError logged and skipped
- builder raising exception logged and skipped
- absent mcpInterceptors field is safe (no-op)
- custom interceptors coexist with OAuth interceptor

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

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix(mcp): validate mcpInterceptors type and fix lint warnings

Address review feedback:

1. Validate mcpInterceptors config value before iterating:
   - Accept a single string and normalize to [string]
   - Ignore None silently
   - Log warning and skip for non-list/non-string types

2. Fix ruff F841 lint errors in tests:
   - Rename _make_mock_env to _make_patches, embed mock_client
   - Remove unused `as mock_cls` bindings where not needed
   - Extract _get_interceptors() helper to reduce repetition

3. Add two new test cases for type validation:
   - test_mcp_interceptors_single_string_is_normalized
   - test_mcp_interceptors_invalid_type_logs_warning

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

* fix(mcp): validate interceptor return type and fix import mock path

Address review feedback:

1. Validate builder return type with callable() check:
   - callable interceptor → append to tool_interceptors
   - None → silently skip (builder opted out)
   - non-callable → log warning with type name and skip

2. Fix test mock path: resolve_variable is a top-level import in
   tools.py, so mock deerflow.mcp.tools.resolve_variable instead of
   deerflow.reflection.resolve_variable to correctly intercept calls.

3. Add test_custom_interceptor_non_callable_return_logs_warning to
   cover the new non-callable validation branch.

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

* docs(mcp): add mcpInterceptors example and documentation

- Add mcpInterceptors field to extensions_config.example.json
- Add "Custom Tool Interceptors" section to MCP_SERVER.md with
  configuration format, example interceptor code, and edge case
  behavior notes

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

---------

Co-authored-by: IECspace <IECspace@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

backend/packages/harness/deerflow/mcp/tools.py

@@ -12,6 +12,7 @@ from langchain_core.tools import BaseTool
 from deerflow.config.extensions_config import ExtensionsConfig
 from deerflow.mcp.client import build_servers_config
 from deerflow.mcp.oauth import build_oauth_tool_interceptor, get_initial_oauth_headers
+from deerflow.reflection import resolve_variable
 
 logger = logging.getLogger(__name__)
 
@@ -95,6 +96,27 @@ async def get_mcp_tools() -> list[BaseTool]:
         if oauth_interceptor is not None:
             tool_interceptors.append(oauth_interceptor)
 
+        # Load custom interceptors declared in extensions_config.json
+        # Format: "mcpInterceptors": ["pkg.module:builder_func", ...]
+        raw_interceptor_paths = (extensions_config.model_extra or {}).get("mcpInterceptors")
+        if isinstance(raw_interceptor_paths, str):
+            raw_interceptor_paths = [raw_interceptor_paths]
+        elif not isinstance(raw_interceptor_paths, list):
+            if raw_interceptor_paths is not None:
+                logger.warning(f"mcpInterceptors must be a list of strings, got {type(raw_interceptor_paths).__name__}; skipping")
+            raw_interceptor_paths = []
+        for interceptor_path in raw_interceptor_paths:
+            try:
+                builder = resolve_variable(interceptor_path)
+                interceptor = builder()
+                if callable(interceptor):
+                    tool_interceptors.append(interceptor)
+                    logger.info(f"Loaded MCP interceptor: {interceptor_path}")
+                elif interceptor is not None:
+                    logger.warning(f"Builder {interceptor_path} returned non-callable {type(interceptor).__name__}; skipping")
+            except Exception as e:
+                logger.warning(f"Failed to load MCP interceptor {interceptor_path}: {e}", exc_info=True)
+
         client = MultiServerMCPClient(servers_config, tool_interceptors=tool_interceptors, tool_name_prefix=True)
 
         # Get all tools from all servers