Show upstream ACP JSON-RPC errors instead of [object Object]

* Preserve ACP JSON-RPC error details

* Unslop ACP error summarization

packages/server/src/server/agent/agent-manager.test.ts

@@ -4195,6 +4195,8 @@ test("turn_failed surfaces provider code and diagnostic in system error message"
 
   await expect(manager.runAgent(agent.id, "hello")).rejects.toThrow("Provider execution failed");
 
+  expect(manager.getAgent(agent.id)?.lastError).toBe("Provider execution failed");
+
   const systemError = manager
     .getTimeline(agent.id)
     .find(

packages/server/src/server/agent/providers/acp-agent.test.ts

@@ -1,7 +1,13 @@
 import { type ChildProcess } from "node:child_process";
 import { EventEmitter } from "node:events";
 import { afterEach, describe, expect, test, vi } from "vitest";
-import type {
+import {
+  AgentSideConnection,
+  ClientSideConnection,
+  PROTOCOL_VERSION,
+  RequestError,
+  ndJsonStream,
+  type Agent,
   PermissionOption,
   PromptResponse,
   RequestPermissionRequest,
@@ -1575,4 +1581,79 @@ describe("ACPAgentSession", () => {
     });
     expect(asInternals<ACPSessionInternals>(session).activeForegroundTurnId).toBeNull();
   });
+
+  test("startTurn preserves JSON-RPC error details from a real ACP prompt response", async () => {
+    const session = createSession();
+    const clientToAgent = new TransformStream();
+    const agentToClient = new TransformStream();
+    const upstreamMessage =
+      "Authentication failed: Please authenticate to continue. Run `/login` to log in.";
+    const upstreamData = {
+      cause: "auth_required",
+      errorMessage: "Please authenticate to continue. Run `/login` to log in.",
+    };
+    const agent: Agent = {
+      async initialize() {
+        return {
+          protocolVersion: PROTOCOL_VERSION,
+          agentCapabilities: {},
+          authMethods: [{ id: "windsurf-api-key", name: "API Key" }],
+        };
+      },
+      async newSession() {
+        return { sessionId: "session-1" };
+      },
+      async prompt() {
+        throw new RequestError(-32000, upstreamMessage, upstreamData);
+      },
+      async authenticate() {},
+      async cancel() {},
+    };
+    const agentConnection = new AgentSideConnection(
+      () => agent,
+      ndJsonStream(agentToClient.writable, clientToAgent.readable),
+    );
+    const connection = new ClientSideConnection(
+      () => ({
+        async requestPermission() {
+          return { outcome: { outcome: "cancelled" } };
+        },
+        async sessionUpdate() {},
+      }),
+      ndJsonStream(clientToAgent.writable, agentToClient.readable),
+    );
+    await connection.initialize({
+      protocolVersion: PROTOCOL_VERSION,
+      clientCapabilities: {},
+      clientInfo: { name: "Paseo test", version: "dev" },
+    });
+    expect(agentConnection.signal.aborted).toBe(false);
+    const sessionResponse = await connection.newSession({
+      cwd: "/tmp/paseo-acp-test",
+      mcpServers: [],
+    });
+    const turnFailed = new Promise<Extract<AgentStreamEvent, { type: "turn_failed" }>>(
+      (resolve) => {
+        session.subscribe((event) => {
+          if (event.type === "turn_failed") {
+            resolve(event);
+          }
+        });
+      },
+    );
+
+    asInternals<ACPSessionInternals>(session).sessionId = sessionResponse.sessionId;
+    asInternals<ACPSessionInternals>(session).connection = connection;
+
+    await session.startTurn("hello");
+
+    await expect(turnFailed).resolves.toMatchObject({
+      error: expect.stringContaining(upstreamMessage),
+      code: "-32000",
+      diagnostic: expect.stringContaining("auth_required"),
+    });
+    await expect(turnFailed).resolves.toMatchObject({
+      error: expect.not.stringContaining("[object Object]"),
+    });
+  });
 });

packages/server/src/server/agent/providers/acp-agent.ts

@@ -11,6 +11,7 @@ import {
   ClientSideConnection,
   PROTOCOL_VERSION,
   type AgentCapabilities as ACPAgentCapabilities,
+  type Error as ACPError,
   type AnyMessage,
   type Client as ACPClient,
   type ClientCapabilities as ACPClientCapabilities,
@@ -107,6 +108,33 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return value != null && typeof value === "object" && !Array.isArray(value);
 }
 
+function isACPError(value: unknown): value is ACPError {
+  return isRecord(value) && typeof value.message === "string" && typeof value.code === "number";
+}
+
+function summarizeACPRequestError(error: unknown): {
+  message: string;
+  code?: string;
+  diagnostic?: string;
+} {
+  // Promise rejections are untyped, but the ACP SDK rejects JSON-RPC failures as response.error.
+  if (isACPError(error)) {
+    const code = String(error.code);
+    const data = error.data === undefined ? "" : ` | data=${JSON.stringify(error.data)}`;
+    return {
+      message: error.message,
+      code,
+      diagnostic: `${error.message} | code=${code}${data}`,
+    };
+  }
+
+  if (error instanceof Error) {
+    return { message: error.message };
+  }
+
+  return { message: String(error) };
+}
+
 function resolveTerminalCommand(
   command: string,
   args?: string[],
@@ -1045,12 +1073,13 @@ export class ACPAgentSession implements AgentSession, ACPClient {
         return;
       })
       .catch((error) => {
-        const message = error instanceof Error ? error.message : String(error);
+        const summary = summarizeACPRequestError(error);
         this.finishTurn({
           type: "turn_failed",
           provider: this.provider,
-          error: message,
+          error: summary.message,
-          diagnostic: this.collectDiagnostic(message),
+          code: summary.code,
+          diagnostic: this.collectDiagnostic(summary.diagnostic ?? summary.message),
           turnId,
         });
       });