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The ext-skills repository contains the published specification for the official Skills extension (io.modelcontextprotocol/skills). SEP-2640 is Final. The Skills Over MCP Working Group maintains the extension. SDK and host support is still being implemented; see the client support matrix and implementations list for current coverage.

modelcontextprotocol/ext-skills

Specification and documentation for Skills over MCP.

The Skills extension allows MCP servers to expose workflow instructions and supporting files to clients. Clients can discover available skills, retrieve their metadata, and read their contents using the existing Resources primitive. A skill is a directory containing a SKILL.md file and optional supporting files, following the Agent Skills specification. This extension defines discovery and retrieval over MCP. Skills are useful for reusable workflows that combine several tools or require supporting references, such as code review or document processing. Serving them over MCP keeps those instructions with the service they describe. Clients can discover available workflows from metadata and load instructions and supporting files only when needed.

User Interaction Model

Host applications determine how skills are exposed to the model and user. Skills can be selected by the model based on their names and descriptions, or explicitly by the user. The extension does not mandate a specific user interaction model. Reading SKILL.md through resources/read does not itself activate a skill. To load the skill, the host routes the read through its skill-loading path, which verifies the content and applies any required user approval before loading it into model context.

Capabilities

Servers that support Skills MUST declare both the resources capability and the io.modelcontextprotocol/skills extension in server/discover:

{
  "capabilities": {
    "resources": {},
    "extensions": {
      "io.modelcontextprotocol/skills": {
        "directoryRead": true
      }
    }
  }
}

Servers that declare this extension MUST implement skills/list and skills/get. Skill files are served through resources/read. The optional directoryRead setting indicates support for resources/directory/read and defaults to false. An empty extension object indicates support without directory reading. Clients issue skills/list and skills/get only after observing the server’s declaration.

These examples use protocol revision 2026-07-28 or later. For brevity, the request examples omit _meta. Every request MUST include the required request metadata.

Protocol Messages

Listing Skills

To discover available skills, clients send a skills/list request. This operation supports pagination and caching. Request:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "skills/list",
  "params": {}
}

Response:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "resultType": "complete",
    "skills": [
      {
        "uri": "skill://code-review/SKILL.md",
        "frontmatter": {
          "name": "code-review",
          "description": "Review code using the team's checklist."
        },
        "resources": [
          {
            "uri": "skill://code-review/SKILL.md",
            "digest": "sha256:d2489d6c182e8df9c178563ff9f2eac7998a0c1bb1278c52a7fd5306e4259160",
            "size": 149
          },
          {
            "uri": "skill://code-review/references/checklist.md",
            "digest": "sha256:dbab2de7bf7db9cc95cb6ce681c3690b26184db39f75b958a0287ca6d9e24e20",
            "size": 65
          }
        ]
      }
    ],
    "ttlMs": 300000,
    "cacheScope": "public"
  }
}

Each entry contains:

Field	Meaning
`uri`	The resource URI of the skill’s `SKILL.md`.
`frontmatter`	All YAML frontmatter fields, unchanged, including `name` and `description`.
`resources`	The complete file manifest, or `"dynamic"` for generated content.

A manifest MUST include SKILL.md and every supporting file, with each file’s URI, SHA-256 digest, and byte size. Each entry returned by skills/list is complete; clients do not need to call skills/get for additional metadata. When a response includes nextCursor, clients pass it as params.cursor to retrieve the next page. List and get results MUST include resultType: "complete", ttlMs, and cacheScope. The cache fields describe freshness and sharing; they do not provide content integrity. Skill identity consists of the originating server’s identity and the skill URI. Names are labels and are not guaranteed to be unique. Hosts MUST preserve both server identity and URI in registries, approvals, and caches. Servers SHOULD use the skill:// scheme, but MAY use another scheme. Hosts MUST NOT identify a resource as a skill solely by its URI scheme.

Getting a Skill

To retrieve a skill entry by URI, clients send a skills/get request. The URI may be supplied by a user, another skill, or server instructions. Request:

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "skills/get",
  "params": {
    "uri": "skill://code-review/SKILL.md"
  }
}

The response contains a skill entry under result.skill, with the same shape as an entry in skills/list, alongside resultType: "complete", ttlMs, and cacheScope. Clients can also use this method to refresh an existing entry. Servers MAY return empty or partial listings, but MUST respond to skills/get for every skill they serve. Hosts MUST support loading by URI, including skills that do not appear in a listing.

Reading Skill Content

To retrieve skill instructions or supporting files, clients send a resources/read request. This example retrieves the SKILL.md from the listing above. Request:

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "resources/read",
  "params": {
    "uri": "skill://code-review/SKILL.md"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "resultType": "complete",
    "contents": [
      {
        "uri": "skill://code-review/SKILL.md",
        "mimeType": "text/markdown",
        "text": "---\nname: code-review\ndescription: Review code using the team's checklist.\n---\n\n# Code review\n\nRead `references/checklist.md`, then review the diff.\n"
      }
    ],
    "ttlMs": 300000,
    "cacheScope": "public"
  }
}

A SKILL.md file MUST begin with YAML frontmatter containing name and description. The final segment of its parent directory’s path MUST match name. Clients resolve relative references against the skill’s root directory. In this example, references/checklist.md resolves to skill://code-review/references/checklist.md. The supporting file is retrieved from the same server using resources/read and contains:

# Review checklist

Check correctness, tests, and compatibility.

Both example files end with a newline; their digests and sizes match the manifest. Hosts MUST NOT retrieve files ahead of need, including on connection, listing, or approval. Approval binds to the manifest without requiring file retrieval.

Reading Directories

To list a directory’s direct children, clients send a resources/directory/read request. This method is optional. Clients MUST NOT call it unless the server declares directoryRead: true. Request:

{
  "jsonrpc": "2.0",
  "id": 4,
  "method": "resources/directory/read",
  "params": {
    "uri": "skill://code-review/references"
  }
}

Response:

{
  "jsonrpc": "2.0",
  "id": 4,
  "result": {
    "resultType": "complete",
    "resources": [
      {
        "uri": "skill://code-review/references/checklist.md",
        "name": "checklist.md",
        "mimeType": "text/markdown"
      }
    ]
  }
}

Directory URIs have no trailing slash. Child directories use mimeType: "inode/directory". Clients descend by issuing another request for a child directory. Results support cursor / nextCursor pagination and contain direct children only. For skills with a manifest, hosts MAY answer directory queries from that manifest. Directory reading also supports dynamic skills and other resource trees. Hosts MUST NOT treat a live directory result as extending the retained manifest or expose newly listed files as part of the skill. Access to those files requires an entry refresh and any required user approval.

Message Flow

This example shows loading a skill with a file manifest, after server capabilities have been discovered. The host’s MCP client sends the protocol requests. Skill selection and user approval are host interactions. A known URI can be loaded without listing. An entry returned by skills/list can be reused; otherwise, the host calls skills/get. An unlisted skill may still exist, but an unknown URI returns -32602 (Invalid params) and stops loading. All skill reads use the originating server. If lookup fails, approval is denied, or verification fails, the host does not load or use the content. Recovering from a changed manifest requires refreshing the entry and obtaining any required approval again, as described below.

Integrity and Verification

While acting on a skill, the host retains the entry used to load it. This period extends at least until the skill’s SKILL.md leaves model context. For skills with a manifest, hosts MUST:

Restrict file reads to URIs in the retained manifest.
Verify each file’s raw byte size and SHA-256 digest before use.
Parse SKILL.md frontmatter and compare it field-by-field with the entry’s frontmatter.

Hosts MUST NOT use content that fails verification. To recover from stale metadata, the host refreshes the entry with skills/get. Persisted approval MUST bind to the complete set of file URIs and digests. A changed, added, or removed file revokes that approval; the host MUST obtain approval again before loading or executing. Hosts SHOULD cache verified content on demand. Disk caches MUST either prevent modification by the model, its tools, or other users and keep files immutable, or verify cached bytes on every access. Hosts MUST exclude cached files from filesystem-skill discovery paths and preserve their MCP origin, including after a restart.

Digests establish consistency with the server’s manifest, not trust in its content. For generated content without stable digests, an entry uses "resources": "dynamic". Hosts MAY decline these skills. If accepted, hosts MUST still verify frontmatter and MUST NOT treat persisted approval as covering arbitrary future content.

Implementation Requirements

Servers

Servers MUST:

Serve valid Agent Skills and implement the declared methods, including base Resources support.
Preserve all frontmatter fields and publish complete manifests computed from the bytes served, unless the skill’s resources are declared "dynamic".
Support direct lookup independently of listing. Each skill entry is atomic; its manifest MUST NOT be split across pages.
Support every directory in the served skill namespaces when declaring directoryRead: true.

Servers SHOULD NOT exceed 512 files or 16 MiB per skill, including SKILL.md. Hosts MUST support skills up to these limits and MAY support larger skills.

Security Considerations

Hosts MUST:

Prevent skills with the same name from silently replacing one another.
Tag loaded content with its originating server and bind resource reads to that server using a host-assigned label. Cross-server reads require explicit per-call approval naming both servers.
Treat skill content as untrusted input. Host-side code execution and permission grants such as allowed-tools require explicit per-skill user approval.
Obtain fresh user consent before activating a nested skill. Reading its SKILL.md as supporting content does not activate it or its frontmatter.

See the security requirements for the full approval, origin, and cache rules.

Error Handling

Condition	Handling
Unknown skill/file, or invalid directory URI	JSON-RPC `-32602` (Invalid params).
Internal server failure	JSON-RPC `-32603` (Internal error).
Digest, size, frontmatter, or manifest mismatch	Host rejects content and refreshes the entry.

Verification failures are host-side conditions rather than protocol errors. They are handled as described in Integrity and Verification.

Client Support

See the client matrix for Skills support and links to implementation documentation.

Specification

The full specification is in the ext-skills repository. Development is coordinated by the Skills Over MCP Working Group.

Sunbelt Computer Software

PL/B Language Development and Support

MCP Apps

Authorization Extensions

Tasks

Skills

Skills

modelcontextprotocol/ext-skills

User Interaction Model

Capabilities

Protocol Messages

Listing Skills

Getting a Skill

Reading Skill Content

Reading Directories

Message Flow

Integrity and Verification

Implementation Requirements

Servers

Security Considerations

Error Handling

Client Support

Specification

Sunbelt Computer Software

PL/B Language Development and Support

modelcontextprotocol/ext-skills

​User Interaction Model

​Capabilities

​Protocol Messages

​Listing Skills

​Getting a Skill

​Reading Skill Content

​Reading Directories

​Message Flow

​Integrity and Verification

​Implementation Requirements

​Servers

​Security Considerations

​Error Handling

​Client Support

​Specification

User Interaction Model

Capabilities

Protocol Messages

Listing Skills

Getting a Skill

Reading Skill Content

Reading Directories

Message Flow

Integrity and Verification

Implementation Requirements

Servers

Security Considerations

Error Handling

Client Support

Specification