Merge pull request #2574 from jlowin/task-proxy-mount-tests

Forbid task execution through proxies, add mount/proxy task tests

Author: chrisguidry

pyproject.toml

@@ -10,7 +10,7 @@ dependencies = [
     "mcp>=1.23.1",
     "openapi-pydantic>=0.5.1",
     "platformdirs>=4.0.0",
-    "pydocket>=0.14.0",
+    "pydocket>=0.15.2",
     "rich>=13.9.4",
     "cyclopts>=4.0.0",
     "authlib>=1.6.5",

src/fastmcp/cli/tasks.py

@@ -82,7 +82,7 @@ def worker(
 
     # Load server to get task functions
     try:
-        config, resolved_spec = load_and_merge_config(server_spec)
+        config, _resolved_spec = load_and_merge_config(server_spec)
     except FileNotFoundError:
         sys.exit(1)
 

src/fastmcp/client/client.py

@@ -849,9 +849,12 @@ async def _read_resource_as_task(
         )
 
         # Check if server accepted background execution
-        if result.meta and "modelcontextprotocol.io/task" in result.meta:
+        # If response includes task metadata with taskId, server accepted background mode
+        # If response includes returned_immediately=True, server declined and executed sync
+        task_meta = (result.meta or {}).get("modelcontextprotocol.io/task", {})
+        if task_meta.get("taskId"):
             # Background execution accepted - extract server-generated taskId
-            server_task_id = result.meta["modelcontextprotocol.io/task"]["taskId"]
+            server_task_id = task_meta["taskId"]
             # Track this task ID for list_tasks()
             self._submitted_task_ids.add(server_task_id)
 
@@ -1055,9 +1058,12 @@ async def _get_prompt_as_task(
         )
 
         # Check if server accepted background execution
-        if result.meta and "modelcontextprotocol.io/task" in result.meta:
+        # If response includes task metadata with taskId, server accepted background mode
+        # If response includes returned_immediately=True, server declined and executed sync
+        task_meta = (result.meta or {}).get("modelcontextprotocol.io/task", {})
+        if task_meta.get("taskId"):
             # Background execution accepted - extract server-generated taskId
-            server_task_id = result.meta["modelcontextprotocol.io/task"]["taskId"]
+            server_task_id = task_meta["taskId"]
             # Track this task ID for list_tasks()
             self._submitted_task_ids.add(server_task_id)
 
@@ -1397,10 +1403,12 @@ async def _call_tool_as_task(
         )
 
         # Check if server accepted background execution
-        # If response includes task metadata, server accepted background mode
-        if result.meta and "modelcontextprotocol.io/task" in result.meta:
+        # If response includes task metadata with taskId, server accepted background mode
+        # If response includes returned_immediately=True, server declined and executed sync
+        task_meta = (result.meta or {}).get("modelcontextprotocol.io/task", {})
+        if task_meta.get("taskId"):
             # Background execution accepted - extract server-generated taskId
-            server_task_id = result.meta["modelcontextprotocol.io/task"]["taskId"]
+            server_task_id = task_meta["taskId"]
             # Track this task ID for list_tasks()
             self._submitted_task_ids.add(server_task_id)
 
@@ -1415,8 +1423,8 @@ async def _call_tool_as_task(
             return task_obj
         else:
             # Server declined background execution (graceful degradation)
-            # Executed synchronously - wrap the immediate result
-            # Need to convert mcp.types.CallToolResult to our CallToolResult
+            # or returned_immediately=True - executed synchronously
+            # Wrap the immediate result
             parsed_result = await self._parse_call_tool_result(name, result)
             # Use a synthetic task ID for the immediate result
             synthetic_task_id = task_id or str(uuid.uuid4())

src/fastmcp/server/proxy.py

@@ -35,6 +35,7 @@
 from fastmcp.server.context import Context
 from fastmcp.server.dependencies import get_context
 from fastmcp.server.server import FastMCP
+from fastmcp.server.tasks.config import TaskConfig
 from fastmcp.tools.tool import Tool, ToolResult
 from fastmcp.tools.tool_manager import ToolManager
 from fastmcp.tools.tool_transform import (
@@ -259,6 +260,8 @@ class ProxyTool(Tool, MirroredComponent):
     A Tool that represents and executes a tool on a remote server.
     """
 
+    task_config: TaskConfig = TaskConfig(mode="forbidden")
+
     def __init__(self, client: Client, **kwargs: Any):
         super().__init__(**kwargs)
         self._client = client
@@ -288,21 +291,36 @@ async def run(
         """Executes the tool by making a call through the client."""
         async with self._client:
             context = get_context()
-            # try to get request context meta
-            meta = (
-                dict(context.request_context.meta)
-                if hasattr(context, "request_context")
-                and hasattr(context.request_context, "meta")
-                else None
-            )
+            # Build meta dict from request context
+            meta: dict[str, Any] | None = None
+            if hasattr(context, "request_context"):
+                req_ctx = context.request_context
+                # Start with existing meta if present
+                if hasattr(req_ctx, "meta") and req_ctx.meta:
+                    meta = dict(req_ctx.meta)
+                # Add task metadata if this is a task request
+                if (
+                    hasattr(req_ctx, "experimental")
+                    and hasattr(req_ctx.experimental, "is_task")
+                    and req_ctx.experimental.is_task
+                ):
+                    task_metadata = req_ctx.experimental.task_metadata
+                    if task_metadata:
+                        meta = meta or {}
+                        meta["modelcontextprotocol.io/task"] = task_metadata.model_dump(
+                            exclude_none=True
+                        )
+
             result = await self._client.call_tool_mcp(
                 name=self.name, arguments=arguments, meta=meta
             )
         if result.isError:
             raise ToolError(cast(mcp.types.TextContent, result.content[0]).text)
+        # Preserve backend's meta (includes task metadata for background tasks)
         return ToolResult(
             content=result.content,
             structured_content=result.structuredContent,
+            meta=result.meta,
         )
 
 
@@ -311,6 +329,7 @@ class ProxyResource(Resource, MirroredComponent):
     A Resource that represents and reads a resource from a remote server.
     """
 
+    task_config: TaskConfig = TaskConfig(mode="forbidden")
     _client: Client
     _value: str | bytes | None = None
 
@@ -343,6 +362,7 @@ def from_mcp_resource(
             icons=mcp_resource.icons,
             meta=mcp_resource.meta,
             tags=(mcp_resource.meta or {}).get("_fastmcp", {}).get("tags", []),
+            task_config=TaskConfig(mode="forbidden"),
             _mirrored=True,
         )
 
@@ -366,6 +386,8 @@ class ProxyTemplate(ResourceTemplate, MirroredComponent):
     A ResourceTemplate that represents and creates resources from a remote server template.
     """
 
+    task_config: TaskConfig = TaskConfig(mode="forbidden")
+
     def __init__(self, client: Client, **kwargs: Any):
         super().__init__(**kwargs)
         self._client = client
@@ -386,6 +408,7 @@ def from_mcp_template(  # type: ignore[override]
             parameters={},  # Remote templates don't have local parameters
             meta=mcp_template.meta,
             tags=(mcp_template.meta or {}).get("_fastmcp", {}).get("tags", []),
+            task_config=TaskConfig(mode="forbidden"),
             _mirrored=True,
         )
 
@@ -431,6 +454,7 @@ class ProxyPrompt(Prompt, MirroredComponent):
     A Prompt that represents and renders a prompt from a remote server.
     """
 
+    task_config: TaskConfig = TaskConfig(mode="forbidden")
     _client: Client
 
     def __init__(self, client: Client, **kwargs):
@@ -459,6 +483,7 @@ def from_mcp_prompt(
             icons=mcp_prompt.icons,
             meta=mcp_prompt.meta,
             tags=(mcp_prompt.meta or {}).get("_fastmcp", {}).get("tags", []),
+            task_config=TaskConfig(mode="forbidden"),
             _mirrored=True,
         )