fix(plugins): discover MCP servers from mcp.json at plugin root

.github/skills/create-canvas-extension/SKILL.md

@@ -72,7 +72,7 @@ Create `plugins/<extension-id>/plugin.json` with this shape:
 }
 ```
 
-Keep Agent Plugins fields at the manifest top level. Repository composition belongs only under `extensions.com.github.awesome-copilot`; do not put `agents`, `commands`, `hooks`, `mcpServers`, or `skills` at the top level or directly under `extensions`. Do not add `x-awesome-copilot`, `standalone`, or other repository-specific top-level fields.
+Keep Agent Plugins fields at the manifest top level. Repository composition belongs only under `extensions.com.github.awesome-copilot`; do not put `agents`, `commands`, `hooks`, or `skills` at the top level or directly under `extensions`. MCP servers are declared in `mcp.json` at the plugin root, never in `plugin.json`. Do not add `x-awesome-copilot`, `standalone`, or other repository-specific top-level fields.
 
 For an existing parent plugin, create or update:
 

.github/workflows/validate-plugins.yml

@@ -52,7 +52,8 @@ jobs:
               'All internal plugins and extensions must include:',
               '- `"$schema": "https://agent-plugins.org/schemas/1.0.0/plugin.schema.json"` in `plugin.json`',
               '- A valid `name`, `description`, and `version`',
-              '- Repository composition (`agents`, `commands`, `hooks`, `mcpServers`, `skills`, and reusable `extensions`) under `extensions.com.github.awesome-copilot`',
+              '- Repository composition (`agents`, `commands`, `hooks`, `skills`, and reusable `extensions`) under `extensions.com.github.awesome-copilot`',
+              '- MCP servers declared in `mcp.json` at the plugin root, not in `plugin.json`',
               '- For **extensions**: `extensions.com.github.copilot.logo` must be set to `"assets/preview.png"`',
               '',
               'Do not put repository composition fields at the manifest top level or directly under `extensions`; they must be nested under `extensions.com.github.awesome-copilot`.',

AGENTS.md

@@ -121,6 +121,7 @@ All agent files (`*.agent.md`) and instruction files (`*.instructions.md`) must
 - plugin.json must have `description` field (describing the plugin's purpose)
 - plugin.json must have `version` field (semantic version, e.g., "1.0.0")
 - Plugin content is defined declaratively in plugin.json under `extensions.com.github.awesome-copilot` using source-only composition fields (`agents`, `hooks`, `skills`, and `extensions`). Source files live in top-level directories and are materialized into plugins by CI. This namespace is stripped from the served manifest — skills use the standard `skills/` directory and Copilot-specific content uses `com.github.copilot/`.
+- MCP servers are **not** a composition field. Per the Agent Plugins spec they are declared in an `mcp.json` file at the plugin root, which is committed alongside `plugin.json` and shipped as-is. Do not add `mcpServers` to `plugin.json`, and do not use the legacy `.mcp.json` filename.
 - The `marketplace.json` file is automatically generated from all plugins during build
 - Plugins are discoverable and installable via GitHub Copilot CLI
 
@@ -331,6 +332,7 @@ For plugins (plugins/\*/):
 - [ ] Directory name is lower case with hyphens
 - [ ] If `keywords` is present, it is an array of lowercase hyphenated strings
 - [ ] If composition arrays are present under `extensions.com.github.awesome-copilot`, each entry is a valid relative path
+- [ ] If the plugin ships MCP servers, they are declared in `mcp.json` at the plugin root (with the `mcp.schema.json` `$schema`), not in `plugin.json` or `.mcp.json`
 - [ ] The plugin does not reference non-existent files
 - [ ] Run `npm run plugin:validate` and `npm run build` to verify the plugin passes all checks
 

eng/agent-plugin-schema.mjs

@@ -23,3 +23,135 @@ export function validateAgentPluginManifest(manifest) {
   return validate(manifest) ? [] : (validate.errors ?? []).map((error) =>
     `${error.instancePath || "manifest"} ${error.message}`);
 }
+
+export const AGENT_PLUGIN_MCP_SCHEMA_URL = "https://agent-plugins.org/schemas/1.0.0/mcp.schema.json";
+export const AGENT_PLUGIN_MCP_SCHEMA = {
+  $schema: "https://json-schema.org/draft/2020-12/schema",
+  $id: AGENT_PLUGIN_MCP_SCHEMA_URL,
+  title: "Agent Plugins MCP Configuration",
+  type: "object",
+  properties: {
+    $schema: { const: AGENT_PLUGIN_MCP_SCHEMA_URL },
+    mcpServers: { type: "object", additionalProperties: { $ref: "#/$defs/server" } },
+  },
+  required: ["$schema", "mcpServers"],
+  additionalProperties: false,
+  $defs: {
+    server: {
+      title: "MCP server",
+      oneOf: [
+        { $ref: "#/$defs/stdioServer" },
+        { $ref: "#/$defs/streamableHttpServer" },
+        { $ref: "#/$defs/sseServer" },
+      ],
+    },
+    stdioServer: {
+      title: "stdio MCP server",
+      type: "object",
+      properties: {
+        type: { const: "stdio" },
+        command: { type: "string", minLength: 1 },
+        args: { type: "array", items: { type: "string" } },
+        env: {
+          type: "object",
+          propertyNames: { not: { enum: ["PLUGIN_ROOT", "PLUGIN_DATA"] } },
+          additionalProperties: { type: "string" },
+        },
+        cwd: {
+          type: "string",
+          pattern: "^(?:\\./|\\$\\{PLUGIN_ROOT\\}(?:/|$)|\\$\\{PLUGIN_DATA\\}(?:/|$))",
+        },
+      },
+      required: ["type", "command"],
+      additionalProperties: false,
+    },
+    streamableHttpServer: {
+      title: "Streamable HTTP MCP server",
+      type: "object",
+      properties: {
+        type: { const: "streamable-http" },
+        url: { type: "string", minLength: 1 },
+        headers: { $ref: "#/$defs/headers" },
+      },
+      required: ["type", "url"],
+      additionalProperties: false,
+    },
+    sseServer: {
+      title: "Legacy HTTP+SSE MCP server",
+      type: "object",
+      properties: {
+        type: { const: "sse" },
+        url: { type: "string", minLength: 1 },
+        headers: { $ref: "#/$defs/headers" },
+      },
+      required: ["type", "url"],
+      additionalProperties: false,
+    },
+    headers: { title: "HTTP headers", type: "object", additionalProperties: { type: "string" } },
+  },
+};
+
+const mcpAjv = new Ajv2020({ allErrors: true });
+const validateMcp = mcpAjv.compile(AGENT_PLUGIN_MCP_SCHEMA);
+
+// A bare oneOf failure reports every branch at once, so errors for a server whose
+// `type` is a known discriminator are re-derived from that branch alone.
+const MCP_SERVER_BRANCHES = {
+  stdio: "stdioServer",
+  "streamable-http": "streamableHttpServer",
+  sse: "sseServer",
+};
+const MCP_SERVER_TYPES = Object.keys(MCP_SERVER_BRANCHES);
+
+function formatMcpError(error) {
+  const extra = error.params?.additionalProperty
+    ? ` (${error.params.additionalProperty})`
+    : "";
+  return `${error.instancePath || "config"} ${error.message}${extra}`;
+}
+
+export function validateAgentPluginMcpConfig(config) {
+  if (validateMcp(config)) {
+    return [];
+  }
+  const rawErrors = validateMcp.errors ?? [];
+  const servers = config?.mcpServers;
+  const hasServerObject = typeof servers === "object" && servers !== null && !Array.isArray(servers);
+
+  const messages = [];
+  for (const error of rawErrors) {
+    if (hasServerObject && error.instancePath.startsWith("/mcpServers/")) {
+      continue;
+    }
+    messages.push(formatMcpError(error));
+  }
+
+  if (hasServerObject) {
+    for (const [name, server] of Object.entries(servers)) {
+      if (typeof server !== "object" || server === null || Array.isArray(server)) {
+        messages.push(`/mcpServers/${name} must be an object`);
+        continue;
+      }
+      const branch = MCP_SERVER_BRANCHES[server.type];
+      if (!branch) {
+        messages.push(`/mcpServers/${name}/type must be one of ${MCP_SERVER_TYPES.join(", ")}`);
+        continue;
+      }
+      const branchValidator = mcpAjv.getSchema(`${AGENT_PLUGIN_MCP_SCHEMA_URL}#/$defs/${branch}`);
+      if (branchValidator(server)) {
+        continue;
+      }
+      for (const error of branchValidator.errors ?? []) {
+        if (error.keyword === "not") {
+          continue;
+        }
+        const suffix = error.keyword === "propertyNames"
+          ? ` "${error.params?.propertyName}" is reserved`
+          : formatMcpError(error).slice(error.instancePath.length || "config".length);
+        messages.push(`/mcpServers/${name}${error.instancePath}${suffix}`);
+      }
+    }
+  }
+
+  return messages;
+}

eng/generate-website-data.mjs

@@ -591,30 +591,20 @@ function generatePluginsData(gitDates, resourceIndex = {}) {
         ];
       });
 
-      // Parse mcpServers: supports a path to a .mcp.json file or an inline object
+      // Discover MCP servers from the spec-mandated mcp.json at the plugin root.
       const mcpItems = [];
-      if (composition.mcpServers) {
-        let mcpServersObj = null;
-        let mcpConfigPath = relPath;
-        if (typeof composition.mcpServers === "string") {
-          const manifestMcpPath = composition.mcpServers.replace(/^\.\//, "");
-          mcpConfigPath = manifestMcpPath ? `${relPath}/${manifestMcpPath}` : relPath;
-          const mcpJsonPath = path.join(pluginDir, manifestMcpPath);
-          if (fs.existsSync(mcpJsonPath)) {
-            try {
-              const mcpJson = JSON.parse(fs.readFileSync(mcpJsonPath, "utf-8"));
-              mcpServersObj = mcpJson.mcpServers || mcpJson;
-            } catch {
-              // ignore parse errors
+      const mcpJsonPath = path.join(pluginDir, "mcp.json");
+      if (fs.existsSync(mcpJsonPath) && fs.statSync(mcpJsonPath).isFile()) {
+        try {
+          const mcpJson = JSON.parse(fs.readFileSync(mcpJsonPath, "utf-8"));
+          const mcpServers = mcpJson.mcpServers;
+          if (mcpServers && typeof mcpServers === "object") {
+            for (const serverName of Object.keys(mcpServers)) {
+              mcpItems.push({ kind: "mcp", path: `${relPath}/mcp.json`, title: serverName });
             }
           }
-        } else if (typeof composition.mcpServers === "object") {
-          mcpServersObj = composition.mcpServers;
-        }
-        if (mcpServersObj) {
-          for (const serverName of Object.keys(mcpServersObj)) {
-            mcpItems.push({ kind: "mcp", path: mcpConfigPath, title: serverName });
-          }
+        } catch {
+          // ignore parse errors
         }
       }
 

eng/validate-plugins.mjs

@@ -6,7 +6,7 @@ import { fileURLToPath } from "url";
 import { ROOT_FOLDER } from "./constants.mjs";
 import { readExternalPlugins } from "./external-plugin-validation.mjs";
 import { validateLicenseField } from "./lib/license.mjs";
-import { AGENT_PLUGIN_SCHEMA_URL, validateAgentPluginManifest } from "./agent-plugin-schema.mjs";
+import { AGENT_PLUGIN_SCHEMA_URL, validateAgentPluginManifest, validateAgentPluginMcpConfig } from "./agent-plugin-schema.mjs";
 
 const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
 const EXTENSIONS_DIR = path.join(ROOT_FOLDER, "extensions");
@@ -212,9 +212,39 @@ function validateExtensionReferences(plugin, pluginDir) {
   return errors;
 }
 
-function validateCompositionNamespace(plugin) {
+export function validateMcpConfig(pluginDir) {
   const errors = [];
-  const compositionFields = ["agents", "hooks", "mcpServers", "skills"];
+  const legacyPath = path.join(pluginDir, ".mcp.json");
+  if (fs.existsSync(legacyPath)) {
+    errors.push("MCP configuration must live at mcp.json in the plugin root, not .mcp.json");
+  }
+
+  const mcpJsonPath = path.join(pluginDir, "mcp.json");
+  if (!fs.existsSync(mcpJsonPath)) {
+    return errors;
+  }
+  if (!fs.statSync(mcpJsonPath).isFile()) {
+    errors.push("mcp.json must be a regular file");
+    return errors;
+  }
+
+  const parsed = parseJsonFile(mcpJsonPath);
+  if (parsed.parseError) {
+    errors.push(`failed to parse mcp.json: ${parsed.parseError}`);
+    return errors;
+  }
+  if (typeof parsed !== "object" || parsed === null || Array.isArray(parsed)) {
+    errors.push("mcp.json must contain a top-level object");
+    return errors;
+  }
+  errors.push(...validateAgentPluginMcpConfig(parsed).map((message) => `mcp.json ${message}`));
+
+  return errors;
+}
+
+export function validateCompositionNamespace(plugin) {
+  const errors = [];
+  const compositionFields = ["agents", "hooks", "skills"];
   const extensions = plugin.extensions;
   const composition = extensions?.[AWESOME_COPILOT_NAMESPACE];
 
@@ -230,6 +260,14 @@ function validateCompositionNamespace(plugin) {
     return errors;
   }
 
+  if (composition?.mcpServers !== undefined) {
+    errors.push(`extensions["${AWESOME_COPILOT_NAMESPACE}"].mcpServers is not supported; declare MCP servers in mcp.json at the plugin root`);
+  }
+
+  if (extensions?.mcpServers !== undefined) {
+    errors.push("extensions.mcpServers is not supported; declare MCP servers in mcp.json at the plugin root");
+  }
+
   for (const field of compositionFields) {
     if (extensions?.[field] !== undefined) {
       errors.push(`extensions.${field} must be moved to extensions["${AWESOME_COPILOT_NAMESPACE}"].${field}`);
@@ -290,12 +328,16 @@ function validatePlugin(folderName) {
 
   // Rule 5b: license (shared with external plugins). Non-SPDX is a warning, not an error.
   const warnings = [];
-  for (const field of ["agents", "hooks", "mcpServers", "skills"]) {
+  if (plugin.mcpServers !== undefined) {
+    errors.push("mcpServers must be declared in mcp.json at the plugin root, not in plugin.json");
+  }
+  for (const field of ["agents", "hooks", "skills"]) {
     if (plugin[field] !== undefined) {
       errors.push(`${field} must be moved to extensions["${AWESOME_COPILOT_NAMESPACE}"].${field}`);
     }
   }
   errors.push(...validateCompositionNamespace(plugin));
+  errors.push(...validateMcpConfig(pluginDir));
   const licenseResult = validateLicenseField(plugin.license, { required: false });
   errors.push(...licenseResult.errors);
   warnings.push(...licenseResult.warnings);