Accept MCP elicitation requests so headless MCP tool calls work

plugins/codex/scripts/lib/app-server.mjs

@@ -22,6 +22,64 @@ const PLUGIN_MANIFEST = JSON.parse(fs.readFileSync(PLUGIN_MANIFEST_URL, "utf8"))
 export const BROKER_ENDPOINT_ENV = "CODEX_COMPANION_APP_SERVER_ENDPOINT";
 export const BROKER_BUSY_RPC_CODE = -32001;
 
+/**
+ * Server-initiated request Codex uses to gate MCP tool calls. The same method
+ * also carries ordinary form and URL elicitations from configured MCP servers.
+ * See `ServerRequest` in `codex-rs/app-server-protocol`.
+ */
+export const MCP_ELICITATION_REQUEST_METHOD = "mcpServer/elicitation/request";
+
+/**
+ * Discriminator that marks a privileged Codex approval rather than a plain MCP
+ * elicitation. See `APPROVAL_KIND_KEY` / `APPROVAL_KIND_MCP_TOOL_CALL` in
+ * `codex-rs/protocol/src/mcp_approval_meta.rs`.
+ */
+const APPROVAL_KIND_KEY = "codex_approval_kind";
+const APPROVAL_KIND_MCP_TOOL_CALL = "mcp_tool_call";
+
+/** @param {unknown} meta */
+function isMcpToolCallApproval(meta) {
+  return (
+    typeof meta === "object" &&
+    meta !== null &&
+    meta[APPROVAL_KIND_KEY] === APPROVAL_KIND_MCP_TOOL_CALL
+  );
+}
+
+/**
+ * Build the reply to a server-initiated request.
+ *
+ * Codex gates MCP tool calls behind an MCP elicitation rather than the
+ * exec/patch approval channel, so the `approvalPolicy: "never"` that every
+ * thread started by this client already declares does not cover them.
+ * Replying with an error leaves the elicitation unresolved, and core maps that
+ * to `ReviewDecision::Abort`, surfaced as "user rejected MCP tool call". Runs
+ * driven by this client are headless, so no one can accept interactively and
+ * every MCP tool call would fail. Accept those explicitly.
+ *
+ * The same method also delivers ordinary form and URL elicitations from
+ * configured MCP servers. A headless client cannot render a form or complete a
+ * URL flow, so accepting one with empty content would hand the server bogus
+ * consent or invalid input. Decline those instead, which is a valid protocol
+ * answer and lets the server fail cleanly.
+ *
+ * @param {{ id: unknown, method?: string, params?: { _meta?: unknown } }} message
+ */
+export function buildServerRequestResponse(message) {
+  if (message.method === MCP_ELICITATION_REQUEST_METHOD) {
+    const action = isMcpToolCallApproval(message.params?._meta) ? "accept" : "decline";
+    return {
+      id: message.id,
+      result: { action, content: null, _meta: null }
+    };
+  }
+
+  return {
+    id: message.id,
+    error: buildJsonRpcError(-32601, `Unsupported server request: ${message.method}`)
+  };
+}
+
 /** @type {ClientInfo} */
 const DEFAULT_CLIENT_INFO = {
   title: "Codex Plugin",
@@ -154,10 +212,7 @@ class AppServerClientBase {
   }
 
   handleServerRequest(message) {
-    this.sendMessage({
-      id: message.id,
-      error: buildJsonRpcError(-32601, `Unsupported server request: ${message.method}`)
-    });
+    this.sendMessage(buildServerRequestResponse(message));
   }
 
   handleExit(error) {

tests/app-server.test.mjs

@@ -0,0 +1,106 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  MCP_ELICITATION_REQUEST_METHOD,
+  buildServerRequestResponse
+} from "../plugins/codex/scripts/lib/app-server.mjs";
+
+test("accepts MCP tool-call approvals so headless MCP tool calls are not denied", () => {
+  const response = buildServerRequestResponse({
+    id: 12,
+    method: MCP_ELICITATION_REQUEST_METHOD,
+    params: {
+      threadId: "thread-1",
+      turnId: "turn-1",
+      serverName: "example",
+      mode: "form",
+      message: "Allow this tool call?",
+      requestedSchema: { type: "object", properties: {} },
+      _meta: { codex_approval_kind: "mcp_tool_call" }
+    }
+  });
+
+  assert.deepEqual(response, {
+    id: 12,
+    result: { action: "accept", content: null, _meta: null }
+  });
+});
+
+test("declines plain form elicitations this headless client cannot fill in", () => {
+  const response = buildServerRequestResponse({
+    id: 13,
+    method: MCP_ELICITATION_REQUEST_METHOD,
+    params: {
+      threadId: "thread-1",
+      turnId: "turn-1",
+      serverName: "example",
+      mode: "form",
+      message: "Enter your project name",
+      requestedSchema: {
+        type: "object",
+        properties: { project: { type: "string" } },
+        required: ["project"]
+      },
+      _meta: null
+    }
+  });
+
+  assert.equal(response.result.action, "decline");
+  assert.equal(response.result.content, null);
+});
+
+test("declines URL elicitations, e.g. an auth flow nobody can complete", () => {
+  const response = buildServerRequestResponse({
+    id: 14,
+    method: MCP_ELICITATION_REQUEST_METHOD,
+    params: {
+      threadId: "thread-1",
+      serverName: "example",
+      mode: "url",
+      message: "Authorize this connector",
+      url: "https://example.com/oauth",
+      elicitationId: "elicit-1"
+    }
+  });
+
+  assert.equal(response.result.action, "decline");
+});
+
+test("declines approvals of a kind this client does not implement", () => {
+  const response = buildServerRequestResponse({
+    id: 15,
+    method: MCP_ELICITATION_REQUEST_METHOD,
+    params: {
+      serverName: "example",
+      mode: "form",
+      message: "Add this tool?",
+      _meta: { codex_approval_kind: "tool_suggestion" }
+    }
+  });
+
+  assert.equal(response.result.action, "decline");
+});
+
+test("still rejects server requests the client does not implement", () => {
+  const response = buildServerRequestResponse({
+    id: "req-7",
+    method: "item/tool/requestUserInput"
+  });
+
+  assert.equal(response.id, "req-7");
+  assert.equal(response.result, undefined);
+  assert.equal(response.error.code, -32601);
+  assert.match(response.error.message, /item\/tool\/requestUserInput/);
+});
+
+test("preserves the request id type for string ids", () => {
+  const response = buildServerRequestResponse({
+    id: "elicitation-42",
+    method: MCP_ELICITATION_REQUEST_METHOD,
+    params: { _meta: { codex_approval_kind: "mcp_tool_call" } }
+  });
+
+  assert.equal(response.id, "elicitation-42");
+  assert.equal(response.result.action, "accept");
+});