Add experimental MCP skills extension support (SEP-2640)

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- New `skills-list` and `skills-get` commands implementing the experimental MCP skills extension (`io.modelcontextprotocol/skills`, [SEP-2640](https://github.com/modelcontextprotocol/experimental-ext-skills)). Skills are discovered via the well-known `skill://index.json` resource, falling back to scanning `skill://*/SKILL.md` URIs. `skills-get <name>` reads a skill's `SKILL.md`; pass `--raw` to print just the markdown for piping to LLMs or files. The session overview now lists `skills` under capabilities when a server advertises the extension under either `capabilities.extensions` (per the spec) or `capabilities.experimental` (the SDK-preserved escape hatch). JSON shape: `[{ name, description, type, url }]` for `skills-list`, full `ReadResourceResult` for `skills-get`.
 - `mcpc connect` (with no arguments) now auto-discovers standard MCP config files (`.mcp.json`, `mcp.json`, `.cursor/mcp.json`, `.vscode/mcp.json`, `~/.claude.json`, Claude Desktop, Windsurf, Kiro, etc.) in the current directory and home directory, and connects every server defined across them. Entries with duplicate session names are deduplicated (project-scoped files win over global ones). VS Code's `"servers"` key is also supported.
 - `mcpc connect` auto-connects to `mcp.apify.com` as `@apify` when the `APIFY_API_TOKEN` environment variable is set, using it as a Bearer token. Existing live sessions are reused without restart.
 

README.md

@@ -166,6 +166,8 @@ MCP session commands (after connecting):
   <@session> resources-subscribe <uri>
   <@session> resources-unsubscribe <uri>
   <@session> resources-templates-list
+  <@session> skills-list
+  <@session> skills-get <name> [--raw]
   <@session> tasks-list
   <@session> tasks-get <taskId>
   <@session> tasks-result <taskId>

src/cli/commands/skills.ts

@@ -0,0 +1,323 @@
+/**
+ * Skills command handlers — implements the experimental MCP skills extension
+ * (SEP-2640: io.modelcontextprotocol/skills).
+ *
+ * Skills are not a new MCP primitive — they are a URI convention layered on
+ * top of the existing Resources primitive:
+ *
+ *   - Each skill lives at `skill://<skill-path>/SKILL.md` (markdown + YAML
+ *     frontmatter), optionally with supporting files under the same prefix.
+ *   - Servers MAY expose a discovery index at `skill://index.json` listing
+ *     `{ name, description, type, url }` entries.
+ *   - Servers MAY advertise the extension via
+ *     `capabilities.extensions["io.modelcontextprotocol/skills"]`.
+ *
+ * These commands are sugar on top of `resources-read`, so they work against
+ * any compliant server without requiring server-side awareness of mcpc.
+ *
+ * Spec: https://github.com/modelcontextprotocol/experimental-ext-skills
+ */
+
+import type { CommandOptions, IMcpClient } from '../../lib/types.js';
+import type { ReadResourceResult, Resource } from '@modelcontextprotocol/sdk/types.js';
+import { ServerError, ClientError } from '../../lib/errors.js';
+import { withMcpClient } from '../helpers.js';
+import { formatOutput, formatSkills, formatSkillDetail } from '../output.js';
+
+/**
+ * URI of the well-known skills discovery index.
+ */
+export const SKILLS_INDEX_URI = 'skill://index.json';
+
+/**
+ * Capability key under `capabilities.extensions` advertising skills support.
+ */
+export const SKILLS_EXTENSION_KEY = 'io.modelcontextprotocol/skills';
+
+/**
+ * Single entry in the skills discovery index. Mirrors the Agent Skills
+ * discovery schema with mcpc-relevant fields kept.
+ *
+ * Per SEP-2640, `name` is required for `type: "skill-md"` entries but
+ * optional for `type: "mcp-resource-template"` (parameterized namespaces
+ * may not have a single concrete name). When `name` is absent, mcpc
+ * derives a display name from the URL.
+ */
+export interface Skill {
+  /** Skill name. Derived from URL for nameless `mcp-resource-template` entries. */
+  name: string;
+  /** Human-readable description. */
+  description: string;
+  /**
+   * Entry type, either `"skill-md"` (concrete skill) or
+   * `"mcp-resource-template"` (parameterized namespace).
+   */
+  type?: string;
+  /** MCP resource URI of the skill's `SKILL.md`, or an RFC 6570 URI template. */
+  url: string;
+}
+
+interface RawIndexEntry {
+  name?: unknown;
+  description?: unknown;
+  type?: unknown;
+  url?: unknown;
+}
+
+interface RawIndex {
+  skills?: unknown;
+}
+
+/**
+ * Extract the readable text from a `ReadResourceResult`. Skills resources are
+ * always text (`text/markdown` or `application/json`), so we ignore blobs.
+ *
+ * @internal exported for tests
+ */
+export function extractTextContent(result: ReadResourceResult): string | undefined {
+  for (const item of result.contents) {
+    if ('text' in item && typeof item.text === 'string') {
+      return item.text;
+    }
+  }
+  return undefined;
+}
+
+/**
+ * Parse and validate a discovery index into a list of `Skill` objects. Drops
+ * malformed entries silently rather than failing — the spec instructs hosts
+ * to be permissive about what they accept.
+ *
+ * @internal exported for tests
+ */
+export function parseIndex(text: string): Skill[] {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(text);
+  } catch (err) {
+    throw new ServerError(
+      `Skills index at ${SKILLS_INDEX_URI} is not valid JSON: ${(err as Error).message}`
+    );
+  }
+
+  if (!parsed || typeof parsed !== 'object') {
+    throw new ServerError(
+      `Skills index at ${SKILLS_INDEX_URI} is not a JSON object (got ${typeof parsed})`
+    );
+  }
+
+  const raw = (parsed as RawIndex).skills;
+  if (!Array.isArray(raw)) {
+    return [];
+  }
+
+  const skills: Skill[] = [];
+  for (const entry of raw) {
+    if (!entry || typeof entry !== 'object') continue;
+    const e = entry as RawIndexEntry;
+    if (typeof e.url !== 'string') continue;
+
+    // SEP-2640: `name` is required for `skill-md` but optional for
+    // `mcp-resource-template` (parameterized namespaces).
+    const type = typeof e.type === 'string' ? e.type : undefined;
+    const isTemplate = type === 'mcp-resource-template';
+    let name: string;
+    if (typeof e.name === 'string' && e.name.length > 0) {
+      name = e.name;
+    } else if (isTemplate) {
+      // Derive a display name from the template URL for nameless templates.
+      name = displayNameFromUrl(e.url);
+    } else {
+      // skill-md without a name violates the spec — drop silently.
+      continue;
+    }
+
+    skills.push({
+      name,
+      description: typeof e.description === 'string' ? e.description : '',
+      ...(type !== undefined && { type }),
+      url: e.url,
+    });
+  }
+
+  return skills;
+}
+
+/**
+ * Derive a display name from an index URL when the entry has no `name` field
+ * (e.g. an `mcp-resource-template` entry). Picks the last meaningful segment
+ * before `SKILL.md` or the last segment of the path.
+ */
+function displayNameFromUrl(url: string): string {
+  // Strip scheme + authority; we only care about the path.
+  const schemeEnd = url.indexOf('://');
+  const path = schemeEnd >= 0 ? url.slice(schemeEnd + 3) : url;
+  const parts = path.split('/').filter(Boolean);
+  if (parts.length === 0) return url;
+  // If the URL ends with SKILL.md, the segment before it is the skill name.
+  if (parts[parts.length - 1] === 'SKILL.md' && parts.length >= 2) {
+    return parts[parts.length - 2] as string;
+  }
+  return parts[parts.length - 1] as string;
+}
+
+/**
+ * Fallback discovery: scan the server's resource list for SKILL.md files
+ * under any `skill://...` prefix. Used when the well-known index is absent.
+ *
+ * @internal exported for tests
+ */
+export function skillsFromResources(resources: Resource[]): Skill[] {
+  // Match `skill://<one-or-more-segments>/SKILL.md`
+  const pattern = /^skill:\/\/((?:[^/]+\/)*[^/]+)\/SKILL\.md$/;
+
+  const skills: Skill[] = [];
+  for (const resource of resources) {
+    const m = pattern.exec(resource.uri);
+    if (!m || !m[1]) continue;
+    // The skill name is the *final* path segment per SEP-2640.
+    const path = m[1];
+    const lastSlash = path.lastIndexOf('/');
+    const name = lastSlash >= 0 ? path.slice(lastSlash + 1) : path;
+
+    skills.push({
+      name: resource.name || name,
+      description: resource.description || '',
+      type: 'skill-md',
+      url: resource.uri,
+    });
+  }
+  return skills;
+}
+
+/**
+ * Discover skills exposed by the server.
+ *
+ * Strategy:
+ * 1. Try to read `skill://index.json` and parse its `skills` array.
+ * 2. If the index is missing (404-style errors), fall back to listing
+ *    resources and matching `skill://*​/SKILL.md` URIs.
+ *
+ * The spec requires that hosts MUST NOT treat an absent index as proof a
+ * server has no skills, hence the fallback.
+ *
+ * @internal exported for tests
+ */
+export async function discoverSkills(client: IMcpClient): Promise<Skill[]> {
+  try {
+    const indexResult = await client.readResource(SKILLS_INDEX_URI);
+    const text = extractTextContent(indexResult);
+    if (text !== undefined) {
+      return parseIndex(text);
+    }
+  } catch {
+    // Index not present — fall through to resource scan.
+  }
+
+  // Fallback: scan all resources, matching `skill://*​/SKILL.md`.
+  const all: Resource[] = [];
+  let cursor: string | undefined;
+  do {
+    const page = await client.listResources(cursor);
+    all.push(...page.resources);
+    cursor = page.nextCursor;
+  } while (cursor);
+
+  return skillsFromResources(all);
+}
+
+/**
+ * Resolve a user-provided identifier into a `SKILL.md` URI.
+ *
+ * Accepts:
+ *   - A bare name (`git-workflow`) → `skill://git-workflow/SKILL.md`
+ *   - A multi-segment path (`acme/billing/refunds`) → `skill://acme/billing/refunds/SKILL.md`
+ *   - A full `skill://...` URI → returned as-is, with `/SKILL.md` appended
+ *     when the URI does not already point at a file.
+ */
+export function resolveSkillUri(input: string): string {
+  const trimmed = input.trim();
+  if (!trimmed) {
+    throw new ClientError('Skill name is required');
+  }
+
+  if (trimmed.startsWith('skill://')) {
+    // Already a URI. If it points at a directory, append SKILL.md.
+    const rest = trimmed.slice('skill://'.length);
+    const lastSegment = rest.slice(rest.lastIndexOf('/') + 1);
+    if (lastSegment.includes('.')) {
+      return trimmed;
+    }
+    return trimmed.endsWith('/') ? `${trimmed}SKILL.md` : `${trimmed}/SKILL.md`;
+  }
+
+  // Strip surrounding slashes; allow nested paths.
+  const path = trimmed.replace(/^\/+/, '').replace(/\/+$/, '');
+  if (!path) {
+    throw new ClientError(`Invalid skill name: ${input}`);
+  }
+  return `skill://${path}/SKILL.md`;
+}
+
+/**
+ * `skills-list` — discover and list skills exposed by the server.
+ *
+ * Tries the well-known `skill://index.json` index first; falls back to
+ * scanning resources for `skill://*​/SKILL.md` URIs.
+ */
+export async function listSkills(target: string, options: CommandOptions): Promise<void> {
+  await withMcpClient(target, options, async (client) => {
+    const skills = await discoverSkills(client);
+
+    if (options.outputMode === 'json') {
+      console.log(formatOutput(skills, 'json'));
+      return;
+    }
+
+    console.log(
+      formatSkills(skills, target, {
+        ...(options.maxChars && { maxChars: options.maxChars }),
+      })
+    );
+  });
+}
+
+/**
+ * `skills-get <name>` — read a skill's SKILL.md.
+ *
+ * Resolves `<name>` to `skill://<name>/SKILL.md` (or accepts a full URI),
+ * reads it via `resources/read`, and renders the markdown.
+ *
+ * With `--raw`, prints just the SKILL.md text (suitable for piping).
+ */
+export async function getSkill(
+  target: string,
+  name: string,
+  options: CommandOptions & { raw?: boolean }
+): Promise<void> {
+  const uri = resolveSkillUri(name);
+
+  await withMcpClient(target, options, async (client) => {
+    const result = await client.readResource(uri);
+
+    if (options.outputMode === 'json') {
+      console.log(formatOutput(result, 'json'));
+      return;
+    }
+
+    if (options.raw) {
+      const text = extractTextContent(result);
+      if (text !== undefined) {
+        console.log(text);
+        return;
+      }
+      // No text content — fall through to formatted view.
+    }
+
+    console.log(
+      formatSkillDetail(uri, result, {
+        ...(options.maxChars && { maxChars: options.maxChars }),
+      })
+    );
+  });
+}