fix(ai): stdio MCP servers get PATH, HOME, LANG and their configured env only

STC_SECRET, database URLs and API keys are no longer inherited by MCP child
processes. Test: a fake server reports its environment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/feature-ai/src/mcp/client.ts

@@ -25,13 +25,23 @@ function unwrap(r: RpcReply): any {
   return r.result;
 }
 
+/** Inherited by stdio MCP servers. Everything else (STC_SECRET, database URLs, API keys) stays with the server. */
+const INHERITED_ENV = ["PATH", "HOME", "LANG"] as const;
+
+/** The environment for a stdio MCP server: PATH, HOME and LANG from the server, plus exactly the admin-configured variables. */
+export function mcpChildEnv(cfg: McpServerConfig, parent: Record<string, string | undefined> = process.env): Record<string, string> {
+  const env: Record<string, string> = {};
+  for (const k of INHERITED_ENV) if (parent[k] !== undefined) env[k] = parent[k]!;
+  return { ...env, ...(cfg.env ?? {}) };
+}
+
 function stdioTransport(cfg: McpServerConfig, timeoutMs: number): Transport {
   if (!cfg.command) throw new Error(`MCP server "${cfg.name}" has no command`);
   const proc = Bun.spawn([cfg.command, ...(cfg.args ?? [])], {
     stdin: "pipe",
     stdout: "pipe",
     stderr: "ignore",
-    env: { ...process.env, ...(cfg.env ?? {}) },
+    env: mcpChildEnv(cfg),
   });
   let nextId = 1;
   const pending = new Map<number, { resolve(v: RpcReply): void; reject(e: Error): void }>();

packages/feature-ai/test/fixtures/fake-mcp.ts

@@ -8,6 +8,7 @@ export function handle(msg: any): any | null {
     case "tools/list":
       return { jsonrpc: "2.0", id: msg.id, result: { tools: [{ name: "echo", description: "Echo text back", inputSchema: { type: "object", properties: { text: { type: "string" } }, required: ["text"] } }] } };
     case "tools/call":
+      if (msg.params?.name === "env") return { jsonrpc: "2.0", id: msg.id, result: { content: [{ type: "text", text: JSON.stringify(process.env) }] } };
       if (msg.params?.name !== "echo") return { jsonrpc: "2.0", id: msg.id, error: { code: -32602, message: `Unknown tool ${msg.params?.name}` } };
       return { jsonrpc: "2.0", id: msg.id, result: { content: [{ type: "text", text: `echo: ${msg.params.arguments?.text}` }] } };
     default:

packages/feature-ai/test/mcp.test.ts

@@ -55,3 +55,28 @@ test("server selection and tool names", () => {
   expect(mcpToolName("my server", "read file!")).toBe("mcp_my_server__read_file_");
   expect(mcpToolName("a".repeat(40), "b".repeat(40)).length).toBeLessThanOrEqual(64);
 });
+
+test("stdio: the child gets a minimal environment plus only its configured variables", async () => {
+  const saved = { ...process.env };
+  Object.assign(process.env, {
+    STC_SECRET: "server-secret", STC_DATABASE_URL: "postgres://u:p@db/x", DATABASE_URL: "postgres://u:p@db/y",
+    ANTHROPIC_API_KEY: "sk-ant-x", MY_SECRET: "s", AWS_SECRET_ACCESS_KEY: "aws", LANG: "C.UTF-8",
+  });
+  try {
+    const c = await connectMcp({ id: "fake", name: "Fake", transport: "stdio", command: process.execPath, args: [FAKE], env: { MCP_TOKEN: "t", STC_EXPLICIT: "configured" } });
+    try {
+      const env = JSON.parse(await c.call("env", {})) as Record<string, string>;
+      for (const k of ["STC_SECRET", "STC_DATABASE_URL", "DATABASE_URL", "ANTHROPIC_API_KEY", "MY_SECRET", "AWS_SECRET_ACCESS_KEY"]) expect(env[k]).toBeUndefined();
+      expect(env.MCP_TOKEN).toBe("t");
+      expect(env.STC_EXPLICIT).toBe("configured"); // explicitly configured by the admin
+      expect(env.LANG).toBe("C.UTF-8");
+      if (saved.PATH) expect(env.PATH).toBe(saved.PATH);
+      if (saved.HOME) expect(env.HOME).toBe(saved.HOME);
+      const unexpected = Object.keys(env).filter((k) => !["PATH", "HOME", "LANG", "MCP_TOKEN", "STC_EXPLICIT"].includes(k));
+      expect(unexpected.filter((k) => /STC_|DATABASE|SECRET|_KEY$/i.test(k))).toEqual([]);
+    } finally { await c.close(); }
+  } finally {
+    for (const k of Object.keys(process.env)) if (!(k in saved)) delete process.env[k];
+    Object.assign(process.env, saved);
+  }
+});