chore(changelog): add anthropic failover entry

Anthropic sometimes returns api_error payloads with message "An unexpected
error occurred while processing the response" during mid-stream failures.
This was not matched by API_ERROR_TRANSIENT_SIGNALS_RE, causing the error
to surface as terminal instead of triggering retry/fallback.

Add "unexpected error" to the transient signal regex. This follows the
same pattern as prior fixes for "Internal server error" (#23193) and
overloaded_error 529 (#34535).

Closes #57010

CHANGELOG.md

@@ -86,6 +86,7 @@ Docs: https://docs.openclaw.ai
 - Cron/announce: preserve all deliverable text payloads for announce mode instead of collapsing to the last chunk, so multi-line cron reports deliver in full to Telegram forum topics.
 - Harden async approval followup delivery in webchat-only sessions (#57359) Thanks @joshavant.
 - Status: fix cache hit rate exceeding 100% by deriving denominator from prompt-side token fields instead of potentially undersized totalTokens. Fixes #26643.
+- Agents/Anthropic failover: treat Anthropic `api_error` payloads with `An unexpected error occurred while processing the response` as transient so retry/fallback can engage instead of surfacing a terminal failure. (#57441) Thanks @zijiess and @vincentkoc.
 
 ## 2026.3.28
 

src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts

@@ -916,6 +916,12 @@ describe("classifyFailoverReason", () => {
         '{"type":"error","error":{"type":"api_error","message":"Service temporarily unavailable"}}',
       ),
     ).toBe("timeout");
+    // Anthropic "unexpected error" variant (#57010)
+    expect(
+      classifyFailoverReason(
+        '{"type":"error","error":{"type":"api_error","message":"An unexpected error occurred while processing the response"}}',
+      ),
+    ).toBe("timeout");
   });
   it("does not classify non-transient api_error payloads as timeout", () => {
     // Context overflow - not transient

src/agents/pi-embedded-helpers/errors.ts

@@ -859,7 +859,7 @@ export function isBillingAssistantError(msg: AssistantMessage | undefined): bool
 // Non-transient api_error payloads (context overflow, validation/schema errors)
 // must NOT be classified as timeout.
 const API_ERROR_TRANSIENT_SIGNALS_RE =
-  /internal server error|overload|temporarily unavailable|service unavailable|unknown error|server error|bad gateway|gateway timeout|upstream error|backend error|try again later|temporarily.+unable/i;
+  /internal server error|overload|temporarily unavailable|service unavailable|unknown error|server error|bad gateway|gateway timeout|upstream error|backend error|try again later|temporarily.+unable|unexpected error/i;
 
 function isJsonApiInternalServerError(raw: string): boolean {
   if (!raw) {