diff --git a/packages/llm/llm-pi-ai/src/stream.ts b/packages/llm/llm-pi-ai/src/stream.ts
index 58bd2e8e3e..59bfdce550 100644
--- a/packages/llm/llm-pi-ai/src/stream.ts
+++ b/packages/llm/llm-pi-ai/src/stream.ts
@@ -39,15 +39,24 @@ export function mapUsage(usage: PiUsage): TokenUsage {
 // wrapper a bare `terminated`, so we are left pattern-matching terse words here.
 // If pi-ai ever forwards the original Error (or a fetch/dispatcher hook that lets
 // us capture the cause ourselves), classify on `code`/`cause` instead of text.
+//
+// pi-ai's SDK-backed providers render an HTTP failure as `<status> <body>`
+// (Anthropic and OpenAI SDK `APIError`, pi-messages `<status> <statusText>: …`).
+// The body can quote another hop's status, such as a gateway 500 that embeds its
+// upstream's 401, so when a message leads with a status the status rules read
+// only that status; other wordings scan the whole message.
+const LEADING_HTTP_STATUS = /^[45]\d\d\b/
+
 function classifyPiAiError(message: string): BuiltinLlmFailureCode {
-  if (/\b(?:401|403)\b/.test(message)) return 'AUTH'
+  const statusSource = LEADING_HTTP_STATUS.exec(message)?.[0] ?? message
+  if (/\b(?:401|403)\b/.test(statusSource)) return 'AUTH'
   if (isQuotaExceededError(message)) return QUOTA_EXCEEDED_CODE
-  if (/\b429\b|rate.?limit/i.test(message)) return 'RATE_LIMIT'
+  if (/\b429\b/.test(statusSource) || /rate.?limit/i.test(message)) return 'RATE_LIMIT'
   // A rejected request body (gateway or provider size cap): resending the
   // same request cannot succeed, so it is invalid, not transient.
-  if (/\b413\b|failed to buffer the request body:\s*length limit exceeded|payload too large|request body too large/i.test(message)) return 'INVALID_REQUEST'
-  if (/\b400\b|invalid.?request/i.test(message)) return 'INVALID_REQUEST'
-  if (/\b5\d\d\b/.test(message)) return 'SERVER'
+  if (/\b413\b/.test(statusSource) || /failed to buffer the request body:\s*length limit exceeded|payload too large|request body too large/i.test(message)) return 'INVALID_REQUEST'
+  if (/\b400\b/.test(statusSource) || /invalid.?request/i.test(message)) return 'INVALID_REQUEST'
+  if (/\b5\d\d\b/.test(statusSource)) return 'SERVER'
   if (/\btime(?:d)?\s*out\b|timeout/i.test(message)) return 'TIMEOUT'
   // A stream truncated before the provider's terminal event: each pi-ai provider
   // throws its own wording when the wire closes mid-response without a terminal
diff --git a/packages/llm/llm-pi-ai/tests/convert.spec.ts b/packages/llm/llm-pi-ai/tests/convert.spec.ts
index fe6d72a2b4..ebdf64fdb0 100644
--- a/packages/llm/llm-pi-ai/tests/convert.spec.ts
+++ b/packages/llm/llm-pi-ai/tests/convert.spec.ts
@@ -939,6 +939,18 @@ describe('mapStopReason / mapUsage', () => {
     }))).toMatchObject({ kind: 'error', failure: { code: 'PI_AI_ERROR' } })
   })
 
+  it.each([
+    // A gateway 500 that relays its upstream's 401 in the body, as the Anthropic SDK renders it.
+    ['500 {"error":{"origin":"upstream","message":"upstream returned an error response: User not found.","type":"upstream_error","code":"upstream_response_error","upstream":{"provider":"custom","status":401,"body":{"error":{"type":"<nil>","message":"User not found."},"type":"error"}}}}', 'SERVER'],
+    ['502 {"error":{"message":"upstream status 400"}}', 'SERVER'],
+    ['404 {"error":{"message":"upstream status 403"}}', 'PI_AI_ERROR'],
+    ['401 {"type":"error","error":{"type":"authentication_error","message":"invalid x-api-key"}}', 'AUTH'],
+    ['429 Too Many Requests: slow down', 'RATE_LIMIT'],
+  ] as const)('classifies %s by its leading response status', (errorMessage, code) => {
+    expect(mapStopReason(assistant({ stopReason: 'error', errorMessage })))
+      .toMatchObject({ kind: 'error', failure: { code } })
+  })
+
   it.each([
     'other side closed',
     'HTTP2 request did not get a response',