feat(mcp): stdio MCP server exposing the four companion tools

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

lib/mcp/companion-stdio.js

@@ -0,0 +1,110 @@
+/**
+ * companion-stdio.js
+ *
+ * Exposes the four Void companion tools through the MCP stdio protocol.
+ * The `claude` CLI spawns this file as a subprocess and communicates
+ * over stdin/stdout using JSON-RPC.
+ *
+ * Usage (via claude MCP config):
+ *   VOID_AGENT_JSON='...' VOID_SPACE_ID='...' node lib/mcp/companion-stdio.js
+ *
+ * Exported helpers (transport-free — safe to import in tests):
+ *   listMcpTools()          → array of {name, description, input_schema}
+ *   callMcpTool(name, args, ctx) → raw handler result object
+ */
+
+import { Server } from '@modelcontextprotocol/sdk/server/index.js';
+import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js';
+import {
+  ListToolsRequestSchema,
+  CallToolRequestSchema
+} from '@modelcontextprotocol/sdk/types.js';
+import { companionRegistry } from '../ai/agent/tools/index.js';
+import { buildCtxFromEnv } from './context.js';
+
+// ---------------------------------------------------------------------------
+// Transport-free helpers (exported for testing)
+// ---------------------------------------------------------------------------
+
+/**
+ * Returns the list of MCP tool descriptors (no handler) from the companion registry.
+ * @returns {{ name: string, description: string, input_schema: object }[]}
+ */
+export function listMcpTools() {
+  return companionRegistry.listTools().map(({ name, description, input_schema }) => ({
+    name,
+    description,
+    input_schema
+  }));
+}
+
+/**
+ * Calls a tool handler directly (same code path as the MCP server), bypassing transport.
+ * @param {string} name
+ * @param {object} args
+ * @param {object} ctx  – tool context {agent, space_id, view, actor}
+ * @returns {Promise<object>} raw handler result
+ */
+export async function callMcpTool(name, args, ctx) {
+  const tool = companionRegistry.getTool(name);
+  if (!tool) {
+    throw new Error(`Unknown tool: ${name}`);
+  }
+  return tool.handler(args, ctx);
+}
+
+// ---------------------------------------------------------------------------
+// MCP server factory (used when running as a script)
+// ---------------------------------------------------------------------------
+
+function createServer() {
+  const server = new Server(
+    { name: 'void-companion', version: '1.0.0' },
+    { capabilities: { tools: {} } }
+  );
+
+  // tools/list — return all companion tools with their raw JSON schemas
+  server.setRequestHandler(ListToolsRequestSchema, () => ({
+    tools: listMcpTools().map(({ name, description, input_schema }) => ({
+      name,
+      description,
+      inputSchema: input_schema   // MCP protocol field name is inputSchema
+    }))
+  }));
+
+  // tools/call — delegate to the registry handler, wrap result as text content
+  server.setRequestHandler(CallToolRequestSchema, async (request) => {
+    const { name, arguments: args = {} } = request.params;
+    const ctx = buildCtxFromEnv();
+    try {
+      const result = await callMcpTool(name, args, ctx);
+      const text = JSON.stringify(result);
+      return {
+        content: [{ type: 'text', text }],
+        structuredContent: result
+      };
+    } catch (err) {
+      return {
+        content: [{ type: 'text', text: err.message ?? String(err) }],
+        isError: true
+      };
+    }
+  });
+
+  return server;
+}
+
+// ---------------------------------------------------------------------------
+// Entry point — only connect stdio transport when run as a script
+// ---------------------------------------------------------------------------
+
+const isMain =
+  typeof process !== 'undefined' &&
+  process.argv[1] &&
+  import.meta.url === new URL(process.argv[1], 'file://').href;
+
+if (isMain) {
+  const server = createServer();
+  const transport = new StdioServerTransport();
+  await server.connect(transport);
+}