@@ -26,6 +26,11 @@ vi.mock('@/lib/mcp/service', () => mcpServiceMock)
 
 import { StreamableHTTPError } from '@modelcontextprotocol/sdk/client/streamableHttp.js'
 import { ErrorCode, McpError } from '@modelcontextprotocol/sdk/types.js'
+import {
+  McpDnsResolutionError,
+  McpDomainNotAllowedError,
+  McpSsrfError,
+} from '@/lib/mcp/domain-check'
 import { McpOauthRedirectRequired } from '@/lib/mcp/oauth'
 import { McpConnectionError, McpServerCooldownError } from '@/lib/mcp/types'
 import { GET } from '@/app/api/mcp/tools/discover/route'
@@ -56,6 +61,7 @@ describe('GET /api/mcp/tools/discover', () => {
     )
 
   it.each([
+    ['hostname no longer resolves', () => new McpDnsResolutionError('expired.example.com')],
     ['cannot be reached', () => new McpConnectionError('connect ECONNREFUSED', 'Docs')],
     ['is cooling down after a recent failure', () => new McpServerCooldownError('mcp-1')],
     ['times out on tools/list', () => new McpError(ErrorCode.RequestTimeout, 'Request timed out')],
@@ -72,6 +78,17 @@ describe('GET /api/mcp/tools/discover', () => {
     expect((await response.json()).error).toBe('Failed to discover MCP tools')
   })
 
+  it.each([
+    ['resolves to a blocked address', () => new McpSsrfError('blocked address')],
+    ['is outside ALLOWED_MCP_DOMAINS', () => new McpDomainNotAllowedError('internal.example.com')],
+  ])('answers 403 when the egress policy refuses a server that %s', async (_name, makeError) => {
+    mcpServiceMockFns.mockDiscoverServerTools.mockRejectedValueOnce(makeError())
+
+    const response = await discover()
+
+    expect(response.status).toBe(403)
+  })
+
   it('keeps a 500 for a failure that is not the registered server', async () => {
     mcpServiceMockFns.mockDiscoverServerTools.mockRejectedValueOnce(new Error('pool exhausted'))
 
 
@@ -19,6 +19,7 @@ vi.mock('@/lib/mcp/application/use-cases', () => mcpUseCasesMock)
 
 import { NoWorkspaceAccessError, WorkspaceApiKeyAuthorizationError } from '@/lib/core/application'
 import { OrchestrationError } from '@/lib/core/orchestration/types'
+import { McpDnsResolutionError } from '@/lib/mcp/domain-check'
 import { McpOauthRedirectRequired } from '@/lib/mcp/oauth'
 import {
   McpConnectionError,
@@ -149,6 +150,19 @@ describe('/api/v2/mcp-servers/[mcpServerId]/tools', () => {
     expect(JSON.stringify(body)).not.toContain('ECONNREFUSED')
   })
 
+  it('reports a server whose hostname no longer resolves as a retryable 503', async () => {
+    mockDiscoverMcpServerToolsUseCase.mockRejectedValueOnce(
+      new McpDnsResolutionError('expired.example.com')
+    )
+
+    const response = await GET(request(`workspaceId=${WORKSPACE_ID}`), { ...context })
+    const body = await response.json()
+
+    expect(response.status).toBe(503)
+    expect(body.error.code).toBe('SERVICE_UNAVAILABLE')
+    expect(JSON.stringify(body)).not.toContain('expired.example.com')
+  })
+
   it('reports a stale OAuth grant as a 409 a client can branch on, never as a Sim credential failure', async () => {
     mockDiscoverMcpServerToolsUseCase.mockRejectedValueOnce(
       new McpOauthAuthorizationRequiredError(SERVER_ID, 'Docs server')
 
@@ -4,6 +4,11 @@ import { McpError as McpSdkError } from '@modelcontextprotocol/sdk/types.js'
 import { generateShortId } from '@sim/utils/id'
 import { NextResponse } from 'next/server'
 import { DEFAULT_EXECUTION_TIMEOUT_MS, isTimeoutError } from '@/lib/core/execution-limits'
+import {
+  McpDnsResolutionError,
+  McpDomainNotAllowedError,
+  McpSsrfError,
+} from '@/lib/mcp/domain-check'
 import {
   type McpApiResponse,
   McpConnectionError,
@@ -139,16 +144,18 @@ export function mcpOrchestrationStatus(errorCode: string | undefined): number {
 }
 
 /**
- * Reports whether a registered MCP server failed to answer usefully: it could not be
- * reached, timed out, is cooling down after a recent failure, or answered `tools/list`
- * with a protocol or HTTP error. Typed only, never matched on message text, because
- * `McpConnectionError` interpolates the server's display name into its message. Check
- * authorization errors first: `UnauthorizedError` is not one of these, but a caller that
- * wants to answer reauthorization separately must do so before asking.
+ * Reports whether a registered MCP server failed to answer usefully: its hostname no
+ * longer resolves, it could not be reached, timed out, is cooling down after a recent
+ * failure, or answered `tools/list` with a protocol or HTTP error. Typed only, never
+ * matched on message text, because `McpConnectionError` interpolates the server's display
+ * name into its message. Check authorization errors first: `UnauthorizedError` is not one
+ * of these, but a caller that wants to answer reauthorization separately must do so
+ * before asking.
  */
 export function isMcpServerUnavailableError(error: unknown): boolean {
   return (
     isTimeoutError(error) ||
+    error instanceof McpDnsResolutionError ||
     error instanceof McpConnectionError ||
     error instanceof McpSdkError ||
     error instanceof StreamableHTTPError
@@ -174,6 +181,12 @@ export function categorizeError(error: unknown): { message: string; status: numb
   if (error instanceof McpConnectionError) {
     return { message: 'Connection failed', status: 502 }
   }
+  if (error instanceof McpDnsResolutionError) {
+    return { message: 'Server hostname could not be resolved', status: 502 }
+  }
+  if (error instanceof McpSsrfError || error instanceof McpDomainNotAllowedError) {
+    return { message: 'Server URL is not allowed by egress policy', status: 403 }
+  }
 
   // Fall back to substring matching for SDK / third-party errors we don't
   // own a typed class for.