feat(mcp): external registry + agent ctx + Streamable HTTP server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/mcp/context.js

@@ -16,3 +16,25 @@ export function buildCtxFromEnv(env = process.env) {
     actor: { kind: 'user', id: null }
   };
 }
+
+/**
+ * Builds the tool ctx for an authenticated EXTERNAL agent. The Space is taken
+ * from the agent's own scope (never client-supplied) and `spaceScoped` is set
+ * so read() denies entities it can't prove are in-Space.
+ * @param {{id:string, capabilities?:object, scopes?:object}} agent
+ */
+export function buildCtxFromAgent(agent) {
+  const actor = {
+    kind: 'agent',
+    id: agent.id,
+    capabilities: agent.capabilities || {},
+    scopes: agent.scopes || {}
+  };
+  return {
+    agent: actor,
+    space_id: (agent.scopes && agent.scopes.space_id) || null,
+    view: null,
+    spaceScoped: true,
+    actor
+  };
+}

lib/mcp/external-registry.js

@@ -0,0 +1,15 @@
+// Curated registry exposed to EXTERNAL agents over MCP HTTP. Deliberately
+// separate from companionRegistry (Dross) so new Dross tools never auto-leak
+// to the internet. Read + suggest-only: search/read/context + propose_change
+// (which always routes to the pending_changes inbox).
+import { createRegistry } from '../ai/agent/registry.js';
+import { searchTool } from '../ai/agent/tools/search.js';
+import { readTool } from '../ai/agent/tools/read.js';
+import { contextTool } from '../ai/agent/tools/context.js';
+import { proposeChangeTool } from '../ai/agent/tools/propose_change.js';
+
+export const externalRegistry = createRegistry();
+externalRegistry.registerTool(searchTool);
+externalRegistry.registerTool(readTool);
+externalRegistry.registerTool(contextTool);
+externalRegistry.registerTool(proposeChangeTool);

lib/mcp/http.js

@@ -0,0 +1,54 @@
+import { Server } from '@modelcontextprotocol/sdk/server/index.js';
+import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
+import { ListToolsRequestSchema, CallToolRequestSchema } from '@modelcontextprotocol/sdk/types.js';
+import { externalRegistry } from './external-registry.js';
+import { buildCtxFromAgent } from './context.js';
+import { recordAudit } from '../db/repos/audit_stub.js';
+
+// --- transport-free helpers (exported for tests) ---
+export function listExternalTools() {
+  return externalRegistry.listTools().map(({ name, description, input_schema }) =>
+    ({ name, description, input_schema }));
+}
+export async function callExternalTool(name, args, ctx) {
+  const tool = externalRegistry.getTool(name);
+  if (!tool) throw new Error(`Unknown tool: ${name}`);
+  return tool.handler(args, ctx);
+}
+
+// --- MCP server factory (one per request in stateless mode) ---
+export function createExternalMcpServer(ctx) {
+  const server = new Server(
+    { name: 'void-external', version: '1.0.0' },
+    { capabilities: { tools: {} } }
+  );
+  server.setRequestHandler(ListToolsRequestSchema, () => ({
+    tools: listExternalTools().map(({ name, description, input_schema }) =>
+      ({ name, description, inputSchema: input_schema }))
+  }));
+  server.setRequestHandler(CallToolRequestSchema, async (request) => {
+    const { name, arguments: args = {} } = request.params;
+    try {
+      const result = await callExternalTool(name, args, ctx);
+      recordAudit(ctx.actor, 'mcp_tool_call', 'agent', ctx.agent.id, null,
+        { tool: name, space_id: ctx.space_id }).catch(() => {});
+      return { content: [{ type: 'text', text: JSON.stringify(result) }], structuredContent: result };
+    } catch (err) {
+      return { content: [{ type: 'text', text: err.message ?? String(err) }], isError: true };
+    }
+  });
+  return server;
+}
+
+// --- Express handler: stateless Streamable HTTP. Requires req.mcpAgent (mcpAuth). ---
+export async function handleMcp(req, res) {
+  const ctx = buildCtxFromAgent(req.mcpAgent);
+  const server = createExternalMcpServer(ctx);
+  const transport = new StreamableHTTPServerTransport({
+    sessionIdGenerator: undefined,   // stateless
+    enableJsonResponse: true
+  });
+  res.on('close', () => { try { transport.close(); } catch { /* */ } try { server.close(); } catch { /* */ } });
+  await server.connect(transport);
+  await transport.handleRequest(req, res, req.body);
+}