[RFC 003] [1/n] Implement basic local MCP server & add MCP client #224

Darktex · 2025-11-25T02:16:56Z

As part of implementing RFC 003, I'm adding support for MCP.

At Meta, we typically work with Stacked PRs as a way to stay sane and allow for easier review.

For this reason, this is just the first PR in the stack, but it is already adding the MCP functionality.

Implementation Phases:

PR 1 (Current): Core MCP infrastructure + echo_env conversion
PR 2 (Future): Migrate remaining environments
PR 3 (Future): CodeAct environment with MCP integration
PR 4 (Future): Fix double marshaling with callable introspection

Darktex · 2025-11-25T02:18:21Z

rfcs/RFC-003-implementation-journal.md

@@ -0,0 +1,329 @@
+# RFC-003 Implementation Journal


This was written by Claude. Keeping this here while I implement, but I will remove it when the stack lands.

Darktex · 2025-11-25T02:19:52Z

src/envs/echo_env/client.py



-class EchoEnv(HTTPEnvClient[EchoAction, EchoObservation]):
+class EchoEnv(HTTPEnvClient[CallToolAction, Observation]):


For now I followed the HTTPEnvClient convention, but to me this is the weakest part of OpenEnv as we require that people write boilerplate that we can generate for them.

I will tackle this when we go into supporting ToolCall and CodeAct at the same time, as that is a natural fit for generating this boilerplate on the fly.

Darktex · 2025-11-25T02:20:45Z

src/core/env_server/mcp_environment.py

+        Raises:
+            ValueError: If MCP client not configured or action type invalid
+        """
+        from .types import CallToolObservation, ListToolsObservation


Claude this, but me no like. Will fix.

Darktex · 2025-11-25T02:21:26Z

src/core/env_server/http_server.py

-            observation = await loop.run_in_executor(
-                self._executor, self.env.step, action
-            )
+            # Handle MCP actions asynchronously (don't use thread pool for async operations)


@pankit-eng this is where I need your eyes the most. Does this make sense?

Copilot

Pull request overview

This PR implements the core MCP (Model Context Protocol) infrastructure for OpenEnv as the first phase of RFC-003. It introduces a new MCPEnvironment base class that allows environments to expose tools via the MCP protocol, and converts the echo_env as a reference implementation. The changes enable both gym-style orchestration through HTTP endpoints and direct MCP JSON-RPC access for production agents.

Key Changes:

Added MCP action types (ListToolsAction, CallToolAction) and corresponding observations to the core type system
Implemented MCPEnvironment base class that handles MCP client/server communication boilerplate
Extended HTTP server with /mcp JSON-RPC endpoint for direct tool access alongside existing /step endpoint
Refactored echo_env to use MCP pattern, replacing custom EchoAction/EchoObservation with MCP tools

Reviewed changes

Copilot reviewed 15 out of 17 changed files in this pull request and generated 16 comments.

Show a summary per file

File	Description
`src/core/env_server/types.py`	Added MCP action and observation types (`ListToolsAction`, `CallToolAction`, `ListToolsObservation`, `CallToolObservation`)
`src/core/env_server/mcp_environment.py`	New base class providing MCP infrastructure for environments
`src/core/env_server/http_server.py`	Added `/mcp` JSON-RPC endpoint and MCP action routing in `/step` handler
`src/core/env_server/__init__.py`	Exported new MCP types and `MCPEnvironment` class
`src/core/pyproject.toml`	Added `mcp` and `fastmcp` dependencies
`src/envs/echo_env/server/mcp_server.py`	New MCP tool definitions for echo environment
`src/envs/echo_env/server/echo_environment.py`	Refactored to extend `MCPEnvironment` instead of custom implementation
`src/envs/echo_env/server/app.py`	Updated to use base `Action`/`Observation` types for MCP support
`src/envs/echo_env/models.py`	Removed custom `EchoAction` and `EchoObservation` (deprecated in favor of MCP)
`src/envs/echo_env/client.py`	Updated to handle MCP actions and parse MCP observations
`src/envs/echo_env/__init__.py`	Removed exports of deprecated custom action/observation types
`tests/core/mcp/test_mcp.py`	New test file for MCP client/server integration
`examples/test_mcp_integration.py`	Integration test script demonstrating MCP usage
`examples/echo_mcp_demo.py`	Demo script showing MCP tool interaction
`rfcs/RFC-003-implementation-journal.md`	Implementation journal documenting decisions and progress

Comments suppressed due to low confidence (1)

src/core/env_server/http_server.py:17

Import of 'json' is not used.

import json

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Copilot · 2025-11-25T02:24:16Z

src/core/env_server/http_server.py

+            Returns:
+                JSON-RPC 2.0 response
+            """
+            if self.env.mcp_client is None:


The /mcp endpoint assumes self.env.mcp_client exists, but not all Environment instances will have this attribute. This will raise an AttributeError for non-MCP environments. Add a hasattr(self.env, 'mcp_client') check or check if the env is an instance of MCPEnvironment first.

Suggested change

if self.env.mcp_client is None:

if not hasattr(self.env, "mcp_client") or self.env.mcp_client is None:

@pankit-eng this is good catch by copilot. Do you think we should just add that line?

Copilot · 2025-11-25T02:24:17Z

src/envs/echo_env/client.py

+        >>> tools = client.list_tools()
+        >>> print(tools)  # [{"name": "echo_message", ...}]
+        >>>
+        >>> # Call echo_message tool
+        >>> result = client.echo_message("Hello!")
+        >>> print(result["echoed_message"])  # "Hello!"


The docstring mentions list_tools() and echo_message() methods that are not implemented in the EchoEnv class. Either implement these convenience methods or update the documentation to reflect the actual API (using step() with ListToolsAction and CallToolAction).

Copilot · 2025-11-25T02:24:17Z

examples/echo_mcp_demo.py

+
+    # List available tools
+    print("2. Listing available tools...")
+    tools = client.list_tools()


The list_tools() method is not implemented in the EchoEnv client class (see client.py). This call will fail with an AttributeError. Either implement the method in the client or use the step() method with a ListToolsAction.

Copilot · 2025-11-25T02:24:17Z

examples/echo_mcp_demo.py

+
+    # Call echo_message tool using convenience method
+    print("3. Calling echo_message tool...")
+    result = client.echo_message("Hello from MCP!")


The echo_message() method is not implemented in the EchoEnv client class (see client.py). This call will fail with an AttributeError. Either implement the method in the client or use the step() method with a CallToolAction.

Copilot · 2025-11-25T02:24:17Z

src/core/env_server/http_server.py

+                    "id": None,
+                }
+
+            body = await request.json()


The /mcp endpoint lacks error handling for malformed JSON. If request.json() fails (e.g., invalid JSON in request body), it will raise an exception. Consider wrapping the await request.json() call in a try-except block and returning a JSON-RPC error with code -32700 ("Parse error") for invalid JSON.

Copilot · 2025-11-25T02:24:19Z

src/core/env_server/http_server.py

+            return CallToolAction(
+                tool_name=action_data["tool_name"],


Accessing action_data["tool_name"] will raise a KeyError if the tool_name field is missing from the request. Use action_data.get("tool_name") with an appropriate error message if it's None, or validate that the field exists before accessing it.

Suggested change

return CallToolAction(

tool_name=action_data["tool_name"],

tool_name = action_data.get("tool_name")

if tool_name is None:

raise ValueError("Missing required field 'tool_name' for CallToolAction")

return CallToolAction(

tool_name=tool_name,

Copilot · 2025-11-25T02:24:19Z

src/core/env_server/http_server.py

+
+            try:
+                # Reuse MCP client from environment (avoids creating duplicate client)
+                async with self.env.mcp_client:


The MCP client is being used within an async with block in the /mcp endpoint (line 168) and also in _handle_mcp_action (mcp_environment.py line 133). This could lead to issues if the client doesn't support being entered multiple times concurrently. Consider whether the client should be managed at the environment level or if each endpoint should manage its own context.

Copilot · 2025-11-25T02:24:19Z

src/core/env_server/mcp_environment.py

+
+        self.mcp_server = mcp_server
+        self.mcp_client = Client(mcp_server)
+        super().__init__(mcp_client=self.mcp_client)


The Environment.__init__ base class doesn't accept an mcp_client parameter. This call will fail with a TypeError. The base class __init__ signature is def __init__(self, transform: Transform | None = None) (see interfaces.py line 95).

Either:

Remove the mcp_client=self.mcp_client argument here, or

Update the Environment.__init__ signature to accept mcp_client as an optional parameter

Suggested change

super().__init__(mcp_client=self.mcp_client)

super().__init__()

Copilot · 2025-11-25T02:24:19Z

src/envs/echo_env/client.py

+    from core.env_server.types import (
+        CallToolAction,
+        CallToolObservation,
+        ListToolsAction,
+        ListToolsObservation,
+        Observation,
+        State,
+    )


Import of 'ListToolsAction' is not used.

Copilot · 2025-11-25T02:24:20Z

src/envs/echo_env/client.py

+    from openenv_core.env_server.types import (
+        CallToolAction,
+        CallToolObservation,
+        ListToolsAction,
+        ListToolsObservation,
+        Observation,
+        State,
+    )


Import of 'ListToolsAction' is not used.

Darktex · 2025-11-25T19:05:20Z

@copilot I accept all your suggestions. Can you implement these changes?

Copilot · 2025-11-25T19:05:30Z

@Darktex I've opened a new pull request, #226, to work on those changes. Once the pull request is ready, I'll request review from you.

[1/n] Implement RFC 003

84a2919

Darktex requested review from Copilot, pankit-eng and zkwentz November 25, 2025 02:16

meta-cla bot added the CLA Signed This label is managed by the Meta Open Source bot. label Nov 25, 2025

Copilot started reviewing on behalf of Darktex November 25, 2025 02:17 View session

Darktex commented Nov 25, 2025

View reviewed changes

Copilot finished reviewing on behalf of Darktex November 25, 2025 02:22

Copilot AI reviewed Nov 25, 2025

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Copilot AI mentioned this pull request Nov 25, 2025

Address code review feedback for MCP implementation #226

Draft



		class EchoEnv(HTTPEnvClient[EchoAction, EchoObservation]):
		class EchoEnv(HTTPEnvClient[CallToolAction, Observation]):

	if self.env.mcp_client is None:
	if not hasattr(self.env, "mcp_client") or self.env.mcp_client is None:

-            return CallToolAction(
-                tool_name=action_data["tool_name"],
+            tool_name = action_data.get("tool_name")
+            if tool_name is None:
+                raise ValueError("Missing required field 'tool_name' for CallToolAction")
+            return CallToolAction(
+                tool_name=tool_name,

	super().__init__(mcp_client=self.mcp_client)
	super().__init__()

[RFC 003] [1/n] Implement basic local MCP server & add MCP client #224

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Darktex commented Nov 25, 2025 •

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