Add Skills extension

docs/advanced/skills.md

@@ -0,0 +1,109 @@
+# Skills
+
+[SEP-2640](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640) defines a
+convention for serving [Agent Skills](https://agentskills.io/) over MCP: a skill is a directory
+of files — minimally a `SKILL.md` with YAML frontmatter — exposed as ordinary MCP resources,
+conventionally under a `skill://` URI. A server enumerates its skills with `skills/list`,
+answers for any one of them by URI with `skills/get`, and — optionally — lists a directory's
+direct children with `resources/directory/read`.
+
+The SDK ships this as the built-in `Skills` extension (`io.modelcontextprotocol/skills`). If
+[Extensions](extensions.md) are new to you, skim that page first.
+
+`Skills` provides the **protocol** primitives: request/response handling, capability
+advertisement, and SEP-2640 conformance validation. It does not discover, read, or hash skills
+from a filesystem — you supply handlers that answer from wherever your catalog actually lives
+(a database, a generated index, an in-memory list, or a directory you walk yourself), and serve
+each skill's files as ordinary resources through `MCPServer.add_resource` or an
+`@mcp.resource(...)` template handler.
+
+## Serving a skill
+
+```python title="server.py" hl_lines="31-41 44-51 55"
+--8<-- "docs_src/skills/tutorial001.py"
+```
+
+Three moves:
+
+* `Skill(uri=..., frontmatter=..., resources=[...])`: one entry, identical in shape whether it
+  comes back from `skills/list` or `skills/get`. `resources` is the skill's complete file
+  manifest — every file, `SKILL.md` included, each with a `sha256:...` digest and byte size — or
+  the string `"dynamic"` for content generated on demand.
+* `list_skills`/`get_skill`: plain async callables, invoked per request. `get_skill` **must**
+  answer for a skill even if a real `list_skills` implementation omitted it — SEP-2640 requires
+  a server to answer by URI for every skill it serves, listed or not.
+* `mcp.add_resource(TextResource(uri=SKILL_URI, ...))`: the skill's actual file content, served
+  through the SDK's ordinary resource machinery. `Skills` never reads or writes resource content
+  itself.
+
+`Skills(list_skills=..., get_skill=...)` is all a server needs; `resources/directory/read` is
+optional (below).
+
+## Fetching a skill
+
+```python title="client.py" hl_lines="5"
+--8<-- "docs_src/skills/tutorial001_client.py"
+```
+
+`list_skills` and `read_directory` follow `nextCursor` to completion, so you get every page's
+skills or resources in one call; `get_skill` costs exactly one request. These three validate the
+server's response against the SEP-2640 conformance rules before returning it — a name that doesn't
+match its URI, a digest in the wrong shape, or an incomplete manifest raises `ValueError` rather
+than reaching your code. `read_skill_uri` is the exception: a thin, discoverable alias for
+`resources/read` that returns a `ReadResourceResult` (text or blob contents) and validates nothing
+itself (see the next paragraph).
+
+`verify_skill_resource(skill, uri, content)` checks a file's bytes — size, then SHA-256 digest —
+against the entry you hold for it. Call it after `read_skill_uri` and before treating the content
+as trustworthy: `resources/read` returns whatever bytes the server sends *right now*, verification
+is what ties those bytes back to the manifest you already validated. It applies to a static
+manifest only — a `"dynamic"` skill carries no digests, so calling it on one raises `ValueError`.
+
+!!! warning
+    Skill content is untrusted model input, exactly like any other server-provided text. SEP-2640
+    requires a host to tag it with its originating server before it reaches the model, and to
+    never grant the frontmatter's `allowed-tools` field (or any other permission-widening field)
+    without explicit per-skill user approval. Both are host responsibilities the SDK cannot
+    discharge for you — see the SEP's [Security Implications](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640)
+    section before building a host on top of this extension.
+
+## Directory reads
+
+A skill's instructions often point at a directory rather than a file ("pick the matching
+template from `templates/`"). `resources/list` cannot answer that — it enumerates a server's
+entire resource space, not one subtree — so SEP-2640 adds `resources/directory/read`, gated
+behind the `directoryRead` capability setting:
+
+```python
+mcp = MCPServer(
+    "catalog",
+    extensions=[
+        Skills(
+            list_skills=list_skills,
+            get_skill=get_skill,
+            read_directory=read_directory,  # lists uri's direct children
+        )
+    ],
+)
+```
+
+Supplying `read_directory` advertises `{"directoryRead": true}` under the extension's
+capabilities; omitting it advertises neither the setting nor the method — a client calling
+`resources/directory/read` against such a server gets `METHOD_NOT_FOUND`.
+`mcp.client.skills.read_directory` raises before sending if the connected server hasn't
+advertised the setting.
+
+## Protocol version and caching
+
+In protocol version `2026-07-28` and later, `skills/list` and `skills/get` results carry the base
+protocol's caching fields, [`ttlMs` and `cacheScope`](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2549) — the
+same freshness hint `tools/list`, `resources/list`, and `resources/read` carry. `Skills` fills
+`cacheScope` with `"public"` when your handler leaves it unset, and omits both fields entirely on an
+older connection, so you don't have to branch on protocol version yourself.
+
+## What this SDK doesn't do
+
+`Skills` is a protocol adapter, not a skills provider. It has no opinion on where a skill's
+bytes live, how they're indexed, or when a catalog is refreshed — that's for a higher-level
+library, or your own handler, to decide. If you're looking for "scan this directory and serve
+whatever's in it," you're looking for a provider built on top of `Skills`, not `Skills` itself.

docs_src/skills/tutorial001.py

@@ -0,0 +1,54 @@
+import hashlib
+from typing import Any
+
+from mcp.server.context import ServerRequestContext
+from mcp.server.mcpserver import MCPServer
+from mcp.server.mcpserver.resources import TextResource
+from mcp.server.skills import Skills
+from mcp.shared.exceptions import MCPError
+from mcp.shared.skills import (
+    GetSkillParams,
+    GetSkillResult,
+    ListSkillsParams,
+    ListSkillsResult,
+    Skill,
+    SkillResource,
+)
+from mcp.types import INVALID_PARAMS
+
+SKILL_URI = "skill://git-workflow/SKILL.md"
+SKILL_MD = """\
+---
+name: git-workflow
+description: Follow this team's Git conventions for branching and commits
+---
+
+Branch from `main` using `type/short-description`. Write commit subjects in the
+imperative mood, under 72 characters.
+"""
+
+GIT_WORKFLOW = Skill(
+    uri=SKILL_URI,
+    frontmatter={"name": "git-workflow", "description": "Follow this team's Git conventions for branching and commits"},
+    resources=[
+        SkillResource(
+            uri=SKILL_URI,
+            digest=f"sha256:{hashlib.sha256(SKILL_MD.encode()).hexdigest()}",
+            size=len(SKILL_MD.encode()),
+        )
+    ],
+)
+
+
+async def list_skills(ctx: ServerRequestContext[Any, Any], params: ListSkillsParams) -> ListSkillsResult:
+    return ListSkillsResult(skills=[GIT_WORKFLOW])
+
+
+async def get_skill(ctx: ServerRequestContext[Any, Any], params: GetSkillParams) -> GetSkillResult:
+    if params.uri != SKILL_URI:
+        raise MCPError(code=INVALID_PARAMS, message=f"unknown skill: {params.uri}")
+    return GetSkillResult(skill=GIT_WORKFLOW)
+
+
+mcp = MCPServer("catalog", extensions=[Skills(list_skills=list_skills, get_skill=get_skill)])
+mcp.add_resource(TextResource(uri=SKILL_URI, name="SKILL.md", mime_type="text/markdown", text=SKILL_MD))

docs_src/skills/tutorial001_client.py

@@ -0,0 +1,22 @@
+import anyio
+
+from mcp import Client
+from mcp.client.skills import get_skill, list_skills, read_skill_uri, verify_skill_resource
+from mcp.types import TextResourceContents
+
+
+async def main() -> None:
+    async with Client("http://localhost:8000/mcp") as client:
+        for skill in await list_skills(client.session):
+            print(skill.uri, skill.frontmatter["description"])
+
+        skill = await get_skill(client.session, "skill://git-workflow/SKILL.md")
+        result = await read_skill_uri(client.session, skill.uri)
+        content = result.contents[0]
+        if isinstance(content, TextResourceContents):
+            verify_skill_resource(skill, skill.uri, content.text.encode())
+            print(content.text)
+
+
+if __name__ == "__main__":
+    anyio.run(main)

mkdocs.yml

@@ -67,6 +67,7 @@ nav:
       - Middleware: advanced/middleware.md
       - Extensions: advanced/extensions.md
       - MCP Apps: advanced/apps.md
+      - Skills: advanced/skills.md
   - Troubleshooting: troubleshooting.md
   - Translations: translations.md
   - Migration Guide: migration.md

src/mcp/client/skills.py

@@ -0,0 +1,130 @@
+"""Client-side convenience wrappers for the Skills extension (SEP-2640).
+
+SEP-2640 needs no client-side method registration: `skills/list`, `skills/get`,
+and `resources/directory/read` are ordinary vendor requests sent through
+`ClientSession.send_request`, exactly like [Extension verbs](../advanced/extensions.md#extension-verbs).
+The functions below are the thin, named wrappers SEP-2640's "SDKs: Convenience
+Wrappers" section recommends — each validates the server's advertised support
+before sending, and `list_skills`/`read_directory` follow `nextCursor` to
+completion so a caller sees one page's worth of ergonomics regardless of how
+many requests it took.
+
+    async with Client("http://localhost:8000/mcp") as client:
+        for skill in await list_skills(client.session):
+            print(skill.uri, skill.frontmatter["description"])
+"""
+
+from __future__ import annotations
+
+from mcp_types import ReadResourceResult, Resource
+
+from mcp.client.session import ClientSession
+from mcp.shared.skills import (
+    EXTENSION_ID,
+    GetSkillParams,
+    GetSkillRequest,
+    GetSkillResult,
+    ListSkillsParams,
+    ListSkillsRequest,
+    ListSkillsResult,
+    ReadDirectoryParams,
+    ReadDirectoryRequest,
+    ReadDirectoryResult,
+    Skill,
+    validate_directory_result,
+    validate_list_result,
+    validate_skill,
+)
+from mcp.shared.skills import verify_skill_resource as verify_skill_resource
+
+__all__ = ["get_skill", "list_skills", "read_directory", "read_skill_uri", "verify_skill_resource"]
+
+
+def _require_extension(session: ClientSession, *, directory_read: bool = False) -> None:
+    capabilities = session.server_capabilities
+    settings = (capabilities.extensions or {}).get(EXTENSION_ID) if capabilities else None
+    if settings is None:
+        raise ValueError(f"server does not advertise the {EXTENSION_ID!r} extension")
+    if directory_read and not settings.get("directoryRead"):
+        raise ValueError(f"server does not advertise {EXTENSION_ID!r}'s directoryRead setting")
+
+
+async def list_skills(session: ClientSession, params: ListSkillsParams | None = None) -> list[Skill]:
+    """Call `skills/list`, following `nextCursor` to completion, and validate the result.
+
+    Raises:
+        ValueError: If the server doesn't advertise the Skills extension, or
+            its response is not SEP-2640 conformant.
+    """
+    _require_extension(session)
+    base = params if params is not None else ListSkillsParams()
+    cursor = base.cursor
+    skills: list[Skill] = []
+    seen_cursors: set[str] = {cursor} if cursor is not None else set()
+    while True:
+        page = await session.send_request(
+            ListSkillsRequest(params=base.model_copy(update={"cursor": cursor})), ListSkillsResult
+        )
+        validate_list_result(page)
+        skills.extend(page.skills)
+        if page.next_cursor is None:
+            return skills
+        if page.next_cursor in seen_cursors:
+            raise ValueError(f"server repeated skills/list pagination cursor {page.next_cursor!r}")
+        seen_cursors.add(page.next_cursor)
+        cursor = page.next_cursor
+
+
+async def get_skill(session: ClientSession, uri: str) -> Skill:
+    """Call `skills/get` for `uri` and validate the result.
+
+    Unlike `list_skills`, this succeeds for a skill absent from any listing —
+    per SEP-2640, a server MUST answer `skills/get` for every skill it serves.
+
+    Raises:
+        ValueError: If the server doesn't advertise the Skills extension, its
+            response names a different skill, or the skill is not conformant.
+    """
+    _require_extension(session)
+    result = await session.send_request(GetSkillRequest(params=GetSkillParams(uri=uri)), GetSkillResult)
+    if result.skill.uri != uri:
+        raise ValueError(f"server returned skill {result.skill.uri!r} for requested {uri!r}")
+    validate_skill(result.skill)
+    return result.skill
+
+
+async def read_skill_uri(session: ClientSession, uri: str) -> ReadResourceResult:
+    """Read a skill file's content via `resources/read`.
+
+    A thin, discoverable alias: works for any `skill://` (or other-scheme)
+    file regardless of whether the skill was ever enumerated. Verify the
+    result against a held `Skill` entry with `verify_skill_resource` before
+    treating it as trusted content — this call does not verify anything itself.
+    """
+    return await session.read_resource(uri)
+
+
+async def read_directory(session: ClientSession, uri: str, params: ReadDirectoryParams | None = None) -> list[Resource]:
+    """Call `resources/directory/read` for `uri`, following `nextCursor` to completion.
+
+    Raises:
+        ValueError: If the server doesn't advertise the `directoryRead`
+            setting, or its response is not a valid child listing of `uri`.
+    """
+    _require_extension(session, directory_read=True)
+    base = params if params is not None else ReadDirectoryParams(uri=uri)
+    cursor = base.cursor
+    resources: list[Resource] = []
+    seen_cursors: set[str] = {cursor} if cursor is not None else set()
+    while True:
+        page = await session.send_request(
+            ReadDirectoryRequest(params=base.model_copy(update={"uri": uri, "cursor": cursor})), ReadDirectoryResult
+        )
+        validate_directory_result(uri, page)
+        resources.extend(page.resources)
+        if page.next_cursor is None:
+            return resources
+        if page.next_cursor in seen_cursors:
+            raise ValueError(f"server repeated resources/directory/read pagination cursor {page.next_cursor!r}")
+        seen_cursors.add(page.next_cursor)
+        cursor = page.next_cursor