fix: robust UX for remote agent errors

packages/core/src/agents/a2a-client-manager.test.ts

@@ -242,7 +242,7 @@ describe('A2AClientManager', () => {
       expect(call.message.taskId).toBe(expectedTaskId);
     });
 
-    it('should throw prefixed error on failure', async () => {
+    it('should propagate the original error on failure', async () => {
       sendMessageStreamMock.mockImplementationOnce(() => {
         throw new Error('Network error');
       });
@@ -252,9 +252,7 @@ describe('A2AClientManager', () => {
         for await (const _ of stream) {
           // consume
         }
-      }).rejects.toThrow(
-        '[A2AClientManager] sendMessageStream Error [TestAgent]: Network error',
-      );
+      }).rejects.toThrow('Network error');
     });
 
     it('should throw an error if the agent is not found', async () => {

packages/core/src/agents/a2a-client-manager.ts

@@ -25,6 +25,7 @@ import {
 import { v4 as uuidv4 } from 'uuid';
 import { Agent as UndiciAgent } from 'undici';
 import { debugLogger } from '../utils/debugLogger.js';
+import { classifyAgentError } from './a2a-errors.js';
 
 // Remote agents can take 10+ minutes (e.g. Deep Research).
 // Use a dedicated dispatcher so the global 5-min timeout isn't affected.
@@ -109,18 +110,22 @@ export class A2AClientManager {
       },
     );
 
-    const factory = new ClientFactory(options);
-    const client = await factory.createFromUrl(agentCardUrl, '');
-    const agentCard = await client.getAgentCard();
+    try {
+      const factory = new ClientFactory(options);
+      const client = await factory.createFromUrl(agentCardUrl, '');
+      const agentCard = await client.getAgentCard();
 
-    this.clients.set(name, client);
-    this.agentCards.set(name, agentCard);
+      this.clients.set(name, client);
+      this.agentCards.set(name, agentCard);
 
-    debugLogger.debug(
-      `[A2AClientManager] Loaded agent '${name}' from ${agentCardUrl}`,
-    );
+      debugLogger.debug(
+        `[A2AClientManager] Loaded agent '${name}' from ${agentCardUrl}`,
+      );
 
-    return agentCard;
+      return agentCard;
+    } catch (error: unknown) {
+      throw classifyAgentError(name, agentCardUrl, error);
+    }
   }
 
   /**
@@ -161,19 +166,9 @@ export class A2AClientManager {
       },
     };
 
-    try {
-      yield* client.sendMessageStream(messageParams, {
-        signal: options?.signal,
-      });
-    } catch (error: unknown) {
-      const prefix = `[A2AClientManager] sendMessageStream Error [${agentName}]`;
-      if (error instanceof Error) {
-        throw new Error(`${prefix}: ${error.message}`, { cause: error });
-      }
-      throw new Error(
-        `${prefix}: Unexpected error during sendMessageStream: ${String(error)}`,
-      );
-    }
+    yield* client.sendMessageStream(messageParams, {
+      signal: options?.signal,
+    });
   }
 
   /**

packages/core/src/agents/a2a-errors.test.ts

@@ -0,0 +1,298 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import {
+  A2AAgentError,
+  AgentCardNotFoundError,
+  AgentCardAuthError,
+  AgentAuthConfigMissingError,
+  AgentConnectionError,
+  classifyAgentError,
+} from './a2a-errors.js';
+
+describe('A2A Error Types', () => {
+  describe('A2AAgentError', () => {
+    it('should set name, agentName, and userMessage', () => {
+      const error = new A2AAgentError('my-agent', 'internal msg', 'user msg');
+      expect(error.name).toBe('A2AAgentError');
+      expect(error.agentName).toBe('my-agent');
+      expect(error.message).toBe('internal msg');
+      expect(error.userMessage).toBe('user msg');
+    });
+  });
+
+  describe('AgentCardNotFoundError', () => {
+    it('should produce a user-friendly 404 message', () => {
+      const error = new AgentCardNotFoundError(
+        'my-agent',
+        'https://example.com/card',
+      );
+      expect(error.name).toBe('AgentCardNotFoundError');
+      expect(error.agentName).toBe('my-agent');
+      expect(error.userMessage).toContain('404');
+      expect(error.userMessage).toContain('https://example.com/card');
+      expect(error.userMessage).toContain('agent_card_url');
+    });
+  });
+
+  describe('AgentCardAuthError', () => {
+    it('should produce a user-friendly 401 message', () => {
+      const error = new AgentCardAuthError(
+        'secure-agent',
+        'https://example.com/card',
+        401,
+      );
+      expect(error.name).toBe('AgentCardAuthError');
+      expect(error.statusCode).toBe(401);
+      expect(error.userMessage).toContain('401');
+      expect(error.userMessage).toContain('Unauthorized');
+      expect(error.userMessage).toContain('"auth" configuration');
+    });
+
+    it('should produce a user-friendly 403 message', () => {
+      const error = new AgentCardAuthError(
+        'secure-agent',
+        'https://example.com/card',
+        403,
+      );
+      expect(error.statusCode).toBe(403);
+      expect(error.userMessage).toContain('403');
+      expect(error.userMessage).toContain('Forbidden');
+    });
+  });
+
+  describe('AgentAuthConfigMissingError', () => {
+    it('should list missing config fields', () => {
+      const error = new AgentAuthConfigMissingError(
+        'api-agent',
+        'API Key (x-api-key): Send x-api-key in header',
+        [
+          'Authentication is required but not configured',
+          "Scheme 'api_key' requires apiKey authentication",
+        ],
+      );
+      expect(error.name).toBe('AgentAuthConfigMissingError');
+      expect(error.requiredAuth).toContain('API Key');
+      expect(error.missingFields).toHaveLength(2);
+      expect(error.userMessage).toContain('API Key');
+      expect(error.userMessage).toContain('no auth is configured');
+      expect(error.userMessage).toContain('Missing:');
+    });
+  });
+
+  describe('AgentConnectionError', () => {
+    it('should wrap the original error cause', () => {
+      const cause = new Error('ECONNREFUSED');
+      const error = new AgentConnectionError(
+        'my-agent',
+        'https://example.com/card',
+        cause,
+      );
+      expect(error.name).toBe('AgentConnectionError');
+      expect(error.userMessage).toContain('ECONNREFUSED');
+      expect(error.userMessage).toContain('https://example.com/card');
+    });
+
+    it('should handle non-Error causes', () => {
+      const error = new AgentConnectionError(
+        'my-agent',
+        'https://example.com/card',
+        'raw string error',
+      );
+      expect(error.userMessage).toContain('raw string error');
+    });
+  });
+
+  describe('classifyAgentError', () => {
+    it('should classify a 404 error message', () => {
+      const raw = new Error('HTTP 404: Not Found');
+      const result = classifyAgentError(
+        'agent-a',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentCardNotFoundError);
+      expect(result.agentName).toBe('agent-a');
+    });
+
+    it('should classify a "not found" error message (case-insensitive)', () => {
+      const raw = new Error('Agent card not found at the given URL');
+      const result = classifyAgentError(
+        'agent-a',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentCardNotFoundError);
+    });
+
+    it('should classify a 401 error message', () => {
+      const raw = new Error('Request failed with status 401');
+      const result = classifyAgentError(
+        'agent-b',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentCardAuthError);
+      expect((result as AgentCardAuthError).statusCode).toBe(401);
+    });
+
+    it('should classify an "unauthorized" error message', () => {
+      const raw = new Error('Unauthorized access to agent card');
+      const result = classifyAgentError(
+        'agent-b',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentCardAuthError);
+    });
+
+    it('should classify a 403 error message', () => {
+      const raw = new Error('HTTP 403 Forbidden');
+      const result = classifyAgentError(
+        'agent-c',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentCardAuthError);
+      expect((result as AgentCardAuthError).statusCode).toBe(403);
+    });
+
+    it('should fall back to AgentConnectionError for unknown errors', () => {
+      const raw = new Error('Something completely unexpected');
+      const result = classifyAgentError(
+        'agent-d',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentConnectionError);
+    });
+
+    it('should classify ECONNREFUSED as AgentConnectionError', () => {
+      const raw = new Error('ECONNREFUSED 127.0.0.1:8080');
+      const result = classifyAgentError(
+        'agent-d',
+        'https://example.com/card',
+        raw,
+      );
+      expect(result).toBeInstanceOf(AgentConnectionError);
+    });
+
+    it('should handle non-Error values', () => {
+      const result = classifyAgentError(
+        'agent-e',
+        'https://example.com/card',
+        'some string error',
+      );
+      expect(result).toBeInstanceOf(AgentConnectionError);
+    });
+
+    describe('cause chain inspection', () => {
+      it('should detect 404 in a nested cause', () => {
+        const inner = new Error('HTTP 404 Not Found');
+        const outer = new Error('fetch failed', { cause: inner });
+        const result = classifyAgentError(
+          'agent-nested',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentCardNotFoundError);
+      });
+
+      it('should detect 401 in a deeply nested cause', () => {
+        const innermost = new Error('Server returned 401');
+        const middle = new Error('Request error', { cause: innermost });
+        const outer = new Error('fetch failed', { cause: middle });
+        const result = classifyAgentError(
+          'agent-deep',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentCardAuthError);
+        expect((result as AgentCardAuthError).statusCode).toBe(401);
+      });
+
+      it('should detect ECONNREFUSED error code in cause chain', () => {
+        const inner = Object.assign(new Error('connect failed'), {
+          code: 'ECONNREFUSED',
+        });
+        const outer = new Error('fetch failed', { cause: inner });
+        const result = classifyAgentError(
+          'agent-conn',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentConnectionError);
+      });
+
+      it('should detect status property on error objects in cause chain', () => {
+        const inner = Object.assign(new Error('Bad response'), {
+          status: 403,
+        });
+        const outer = new Error('agent card resolution failed', {
+          cause: inner,
+        });
+        const result = classifyAgentError(
+          'agent-status',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentCardAuthError);
+        expect((result as AgentCardAuthError).statusCode).toBe(403);
+      });
+
+      it('should detect status on a plain-object cause (non-Error)', () => {
+        const outer = new Error('fetch failed');
+        // Some HTTP libs set cause to a plain object, not an Error instance
+        (outer as unknown as { cause: unknown }).cause = {
+          message: 'Unauthorized',
+          status: 401,
+        };
+        const result = classifyAgentError(
+          'agent-plain-cause',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentCardAuthError);
+        expect((result as AgentCardAuthError).statusCode).toBe(401);
+      });
+
+      it('should detect statusCode on a plain-object cause (non-Error)', () => {
+        const outer = new Error('fetch failed');
+        (outer as unknown as { cause: unknown }).cause = {
+          message: 'Forbidden',
+          statusCode: 403,
+        };
+        const result = classifyAgentError(
+          'agent-plain-cause-403',
+          'https://example.com/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentCardAuthError);
+        expect((result as AgentCardAuthError).statusCode).toBe(403);
+      });
+
+      it('should classify ENOTFOUND as AgentConnectionError, not 404', () => {
+        // ENOTFOUND (DNS resolution failure) should NOT be misclassified
+        // as a 404 despite containing "NOTFOUND" in the error code.
+        const inner = Object.assign(
+          new Error('getaddrinfo ENOTFOUND example.invalid'),
+          {
+            code: 'ENOTFOUND',
+          },
+        );
+        const outer = new Error('fetch failed', { cause: inner });
+        const result = classifyAgentError(
+          'agent-dns',
+          'https://example.invalid/card',
+          outer,
+        );
+        expect(result).toBeInstanceOf(AgentConnectionError);
+        expect(result).not.toBeInstanceOf(AgentCardNotFoundError);
+      });
+    });
+  });
+});