Promote mcp-types to a core fastmcp-slim dependency

Bare `import fastmcp` loads mcp_types via _sdk_patches and _compat, so a
bare slim install (no [mcp] extra) hit ModuleNotFoundError. mcp-types only
pulls pydantic + typing-extensions (already core), so graduating it to core
fixes the import while the full mcp SDK stays in the [mcp] extra.

fastmcp_slim/pyproject.toml

@@ -4,6 +4,7 @@ dynamic = ["version", "optional-dependencies"]
 description = "The dependency-slim FastMCP package."
 authors = [{ name = "Jeremiah Lowin" }]
 dependencies = [
+    "mcp-types==2.0.0b1",
     "platformdirs>=4.0.0",
     "pydantic[email]>=2.12.0",
     "pydantic-settings>=2.0.0",
@@ -79,7 +80,6 @@ mcp = [
     "exceptiongroup>=1.2.2",
     "httpx>=0.28.1,<1.0",
     "mcp==2.0.0b1",
-    "mcp-types==2.0.0b1",
     "opentelemetry-api>=1.28.0",
     # starlette floor: transitive via mcp (which only requires >=0.27).
     # Pin past CVE-2026-48710, which was patched in 1.0.1.

tests/client/test_slim_package_boundaries.py

@@ -2,6 +2,9 @@ from __future__ import annotations
 
 import builtins
 import contextlib
+import subprocess
+import sys
+import textwrap
 import types
 from collections.abc import Mapping, Sequence
 from typing import Any, cast
@@ -59,3 +62,55 @@ async def test_multiserver_config_requires_server_for_now() -> None:
         ):
             async with transport.connect_session():
                 pass
+
+
+def test_bare_slim_import_needs_only_mcp_types() -> None:
+    """A bare `fastmcp-slim` install ships `mcp-types` but not the full `mcp` SDK.
+
+    `mcp-types` is a core dependency (it only pulls pydantic + typing-extensions),
+    while the full `mcp` package lives in the `[mcp]` extra pulled by
+    `[client]`/`[server]`. With `mcp` absent but `mcp-types` present, `import
+    fastmcp`, `import fastmcp.settings`, and `import fastmcp.types` must all
+    succeed, while `fastmcp.FastMCP` and `fastmcp.Client` raise the friendly
+    install-hint ImportError (they need `mcp.*` from the server/client extras).
+    """
+    script = textwrap.dedent(
+        """
+        import sys
+        import importlib.abc
+
+        class BlockFullMcp(importlib.abc.MetaPathFinder):
+            # Block the full `mcp` package but leave `mcp_types` importable,
+            # exactly as a bare `fastmcp-slim` install would present.
+            def find_spec(self, name, path, target=None):
+                if name == "mcp" or name.startswith("mcp."):
+                    raise ImportError(f"blocked: {name}")
+                return None
+
+        sys.meta_path.insert(0, BlockFullMcp())
+
+        import fastmcp
+        import fastmcp.settings
+        import fastmcp.types
+
+        assert "mcp_types" in sys.modules, "mcp_types should load on a bare install"
+        assert "mcp" not in sys.modules, "full mcp must not load on a bare install"
+
+        for attr in ("FastMCP", "Client"):
+            try:
+                getattr(fastmcp, attr)
+            except ImportError:
+                pass
+            else:
+                raise AssertionError(f"fastmcp.{attr} should have raised ImportError")
+
+        print("OK")
+        """
+    )
+    result = subprocess.run(
+        [sys.executable, "-c", script],
+        capture_output=True,
+        text=True,
+    )
+    assert result.returncode == 0, result.stderr
+    assert result.stdout.strip().endswith("OK")

uv.lock

@@ -10,7 +10,7 @@ resolution-markers = [
 ]
 
 [options]
-exclude-newer = "2026-06-28T22:01:54.610739Z"
+exclude-newer = "2026-06-29T14:57:29.757967Z"
 exclude-newer-span = "P1W"
 
 [options.exclude-newer-package]
@@ -955,6 +955,7 @@ requires-dist = [{ name = "fastmcp-slim", extras = ["client", "server"], editabl
 name = "fastmcp-slim"
 source = { editable = "fastmcp_slim" }
 dependencies = [
+    { name = "mcp-types" },
     { name = "platformdirs" },
     { name = "pydantic", extra = ["email"] },
     { name = "pydantic-settings" },
@@ -979,7 +980,6 @@ client = [
     { name = "exceptiongroup" },
     { name = "httpx" },
     { name = "mcp" },
-    { name = "mcp-types" },
     { name = "opentelemetry-api" },
     { name = "py-key-value-aio", extra = ["filetree", "keyring", "memory"] },
     { name = "starlette" },
@@ -995,7 +995,6 @@ mcp = [
     { name = "exceptiongroup" },
     { name = "httpx" },
     { name = "mcp" },
-    { name = "mcp-types" },
     { name = "opentelemetry-api" },
     { name = "starlette" },
 ]
@@ -1012,7 +1011,6 @@ server = [
     { name = "jsonref" },
     { name = "jsonschema-path" },
     { name = "mcp" },
-    { name = "mcp-types" },
     { name = "openapi-pydantic" },
     { name = "opentelemetry-api" },
     { name = "packaging" },
@@ -1052,9 +1050,7 @@ requires-dist = [
     { name = "mcp", marker = "extra == 'client'", specifier = "==2.0.0b1" },
     { name = "mcp", marker = "extra == 'mcp'", specifier = "==2.0.0b1" },
     { name = "mcp", marker = "extra == 'server'", specifier = "==2.0.0b1" },
-    { name = "mcp-types", marker = "extra == 'client'", specifier = "==2.0.0b1" },
-    { name = "mcp-types", marker = "extra == 'mcp'", specifier = "==2.0.0b1" },
-    { name = "mcp-types", marker = "extra == 'server'", specifier = "==2.0.0b1" },
+    { name = "mcp-types", specifier = "==2.0.0b1" },
     { name = "openai", marker = "extra == 'openai'", specifier = ">=1.102.0" },
     { name = "openapi-pydantic", marker = "extra == 'server'", specifier = ">=0.5.1" },
     { name = "opentelemetry-api", marker = "extra == 'client'", specifier = ">=1.28.0" },