fix(langchain): require full fastmcp for the mcp extra

A multi-server `MCPConfig` mounts its backends on a router, which lives in
the server half of FastMCP. The extra pinned `fastmcp-slim[client]`, so the
documented multi-server example failed at connect time with "MCP configs with
multiple servers require the full `fastmcp` package".

The existing multi-server test only asserted on the transport it built, never
dialing it, which is why this went unnoticed — so the new test drives two real
stdio servers through one adapter.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

libs/langchain_v1/langchain/mcp/adapter.py

@@ -21,10 +21,7 @@ try:
     # not an explicit re-export.
     from mcp.server.mcpserver import MCPServer
 except ImportError as _import_error:
-    msg = (
-        "Please install the fastmcp client to use `MCPAdapter` — "
-        '`pip install "fastmcp-slim[client]"`.'
-    )
+    msg = "Please install FastMCP to use `MCPAdapter` — `pip install fastmcp`."
     raise ImportError(msg) from _import_error
 
 
@@ -34,10 +31,16 @@ if TYPE_CHECKING:
     from fastmcp import FastMCP
     from langchain_core.tools import BaseTool
 else:
-    # In-process FastMCP servers live in the server half of FastMCP, which the
-    # lightweight `fastmcp-slim[client]` install does not pull in. FastMCP degrades
-    # this annotation the same way in `fastmcp.client.transports.inference`.
-    FastMCP = Any
+    try:
+        from fastmcp import FastMCP
+    except ImportError:
+        # In-process FastMCP servers, and multi-server `MCPConfig` targets, live
+        # in the server half of FastMCP. The `mcp` extra installs it, but a
+        # caller who reached this module through a client-only install
+        # (`fastmcp-slim[client]`) can still use every other target type — so
+        # the annotation degrades rather than the import failing, the same way
+        # FastMCP degrades it in `fastmcp.client.transports.inference`.
+        FastMCP = Any
 
 
 MCPAdapterTarget: TypeAlias = (

libs/langchain_v1/pyproject.toml

@@ -49,7 +49,7 @@ deepseek = ["langchain-deepseek"]
 xai = ["langchain-xai"]
 perplexity = ["langchain-perplexity"]
 meta = ["langchain-meta"]
-mcp = ["fastmcp-slim[client]>=4.0.0b4,<5.0.0"]
+mcp = ["fastmcp>=4.0.0b4,<5.0.0"]
 
 [project.urls]
 Homepage = "https://docs.langchain.com/"

libs/langchain_v1/tests/unit_tests/mcp/test_adapter.py

@@ -3,7 +3,9 @@
 from __future__ import annotations
 
 import asyncio
+import sys
 from typing import TYPE_CHECKING, Any
+from unittest.mock import ANY
 
 import pytest
 from fastmcp import Client, FastMCP
@@ -103,6 +105,58 @@ def test_one_adapter_can_serve_several_servers() -> None:
     assert transport.name_as_prefix is True
 
 
+_STDIO_SERVER = """
+import sys
+from mcp.server.mcpserver import MCPServer
+
+mcp = MCPServer("{name}")
+
+
+@mcp.tool()
+def whoami() -> str:
+    \"\"\"Name the server answering this call.\"\"\"
+    return "{name}"
+
+
+mcp.run()
+"""
+
+
+@pytest.fixture
+def two_stdio_servers(tmp_path: Path) -> dict[str, Any]:
+    """Write two single-tool stdio servers and return a config naming both."""
+    servers = {}
+    for name in ("alpha", "beta"):
+        script = tmp_path / f"{name}.py"
+        script.write_text(_STDIO_SERVER.format(name=name))
+        servers[name] = {"command": sys.executable, "args": [str(script)]}
+    return {"mcpServers": servers}
+
+
+@pytest.mark.asyncio
+async def test_several_servers_connect_and_keep_their_prefixes(
+    two_stdio_servers: dict[str, Any],
+) -> None:
+    """A multi-server config connects, and each backend's tools stay namespaced.
+
+    Connecting is the point. Mounting several backends behind one client needs
+    the server half of FastMCP, which a client-only install does not provide —
+    a config that builds a valid transport can still fail the moment it dials.
+    So this drives real servers rather than asserting on the transport.
+    """
+    config = two_stdio_servers
+
+    async with MCPAdapter(config) as adapter:
+        tools = await adapter.get_tools()
+
+    assert sorted(tool.name for tool in tools) == ["alpha_whoami", "beta_whoami"]
+
+    by_name = {tool.name: tool for tool in tools}
+    assert await by_name["alpha_whoami"].ainvoke({}) == [
+        {"type": "text", "text": "alpha", "id": ANY}
+    ]
+
+
 def test_prebuilt_client_is_used_as_is() -> None:
     client: Client[Any] = Client("https://example.com/mcp")
 

libs/langchain_v1/uv.lock

@@ -2329,7 +2329,7 @@ huggingface = [
     { name = "langchain-huggingface" },
 ]
 mcp = [
-    { name = "fastmcp-slim", extra = ["client"] },
+    { name = "fastmcp" },
 ]
 meta = [
     { name = "langchain-meta" },
@@ -2388,7 +2388,7 @@ typing = [
 
 [package.metadata]
 requires-dist = [
-    { name = "fastmcp-slim", extras = ["client"], marker = "extra == 'mcp'", specifier = ">=4.0.0b4,<5.0.0" },
+    { name = "fastmcp", marker = "extra == 'mcp'", specifier = ">=4.0.0b4,<5.0.0" },
     { name = "langchain-anthropic", marker = "extra == 'anthropic'", editable = "../partners/anthropic" },
     { name = "langchain-aws", marker = "extra == 'aws'" },
     { name = "langchain-azure-ai", marker = "extra == 'azure-ai'" },
@@ -2446,7 +2446,7 @@ typing = [
 
 [[package]]
 name = "langchain-anthropic"
-version = "1.6.1"
+version = "1.7.0"
 source = { editable = "../partners/anthropic" }
 dependencies = [
     { name = "anthropic" },
@@ -2456,7 +2456,7 @@ dependencies = [
 
 [package.metadata]
 requires-dist = [
-    { name = "anthropic", specifier = ">=0.120.0,<1.0.0" },
+    { name = "anthropic", specifier = ">=0.120.0,<2.0.0" },
     { name = "langchain-core", editable = "../core" },
     { name = "pydantic", specifier = ">=2.7.4,<3.0.0" },
 ]
@@ -2479,7 +2479,7 @@ test = [
     { name = "pytest-watcher", specifier = ">=0.3.4,<1.0.0" },
     { name = "pytest-xdist", specifier = ">=3.8.0,<4.0.0" },
     { name = "syrupy", specifier = ">=5.0.0,<6.0.0" },
-    { name = "vcrpy", specifier = ">=8.0.0,<9.0.0" },
+    { name = "vcrpy", specifier = ">=8.2.0,<9.0.0" },
 ]
 test-integration = [{ name = "requests", specifier = ">=2.32.3,<3.0.0" }]
 typing = [