fix(plugins): classify stale-call circuit breaker as failover, not retry

The cross-turn stale-call circuit breaker (_check_stale_giveup in
agent/chat_completion_helpers.py) raises a RuntimeError when the
provider has been unresponsive for N consecutive stale attempts.
This error was classified as FailoverReason.unknown (retryable=True,
should_fallback=False), causing the retry loop to burn all max_retries
against the same dead provider — each retry hitting the circuit breaker
instantly with zero network overhead — before fallback was attempted.

Add a classification rule (section 7b in classify_api_error) that
recognizes the stale-breaker RuntimeError by its signature phrases and
classifies it as FailoverReason.timeout with retryable=False,
should_fallback=True. This makes the retry loop skip retries and go
straight to fallback provider activation on the first hit.

Test: test_stale_breaker_runtime_error_triggers_fallback_not_retry

agent/error_classifier.py

@@ -840,12 +840,34 @@ def classify_api_error(
             )
         return _result(FailoverReason.timeout, retryable=True)
 
-    # ── 7. Transport / timeout heuristics ───────────────────────────
+    # ── 7b. Stale-call circuit breaker → failover immediately ──────
+    # _check_stale_giveup() in agent/chat_completion_helpers.py raises a
+    # RuntimeError when the provider has been unresponsive for N
+    # consecutive stale attempts (default 5).  The error is NOT a transport
+    # timeout — the circuit breaker fires *before* any network call to avoid
+    # an indefinite stall.  Without this classification the RuntimeError
+    # falls through to FailoverReason.unknown (retryable=True), which burns
+    # all max_retries against the same dead provider (each retry hitting the
+    # circuit breaker instantly with zero network overhead) before fallback
+    # is attempted.  Classify as non-retryable + should_fallback so the
+    # retry loop activates the next fallback provider on the first hit.
+    if (
+        error_type == "RuntimeError"
+        and "consecutive stale attempts" in error_msg
+        and "aborting this call" in error_msg
+    ):
+        return _result(
+            FailoverReason.timeout,
+            retryable=False,
+            should_fallback=True,
+        )
+
+    # ── 8. Transport / timeout heuristics ───────────────────────────
 
     if error_type in _TRANSPORT_ERROR_TYPES or isinstance(error, (TimeoutError, ConnectionError, OSError)):
         return _result(FailoverReason.timeout, retryable=True)
 
-    # ── 8. Fallback: unknown ────────────────────────────────────────
+    # ── 9. Fallback: unknown ────────────────────────────────────────
 
     return _result(FailoverReason.unknown, retryable=True)
 

tests/agent/test_error_classifier.py

@@ -2067,3 +2067,26 @@ class Test408RequestTimeout:
         assert result.retryable is True
         assert result.should_compress is False
 
+    def test_stale_breaker_runtime_error_triggers_fallback_not_retry(self):
+        # The cross-turn stale-call circuit breaker (_check_stale_giveup in
+        # chat_completion_helpers.py) raises a RuntimeError when the provider
+        # has been unresponsive for N consecutive stale attempts.  This must
+        # be classified as non-retryable + should_fallback so the retry loop
+        # activates the fallback provider immediately instead of burning all
+        # max_retries against the same dead provider (each retry hitting the
+        # circuit breaker instantly with zero network overhead).
+        e = RuntimeError(
+            "Provider has been unresponsive (no response received) for "
+            "6 consecutive stale attempts — aborting this call to "
+            "avoid an indefinite stall. Switch models or start a new "
+            "session, then retry."
+        )
+        result = classify_api_error(
+            e, provider="openrouter", model="anthropic/claude-fable-5",
+            approx_tokens=126327, context_length=200000, num_messages=274,
+        )
+        assert result.reason == FailoverReason.timeout
+        assert result.retryable is False
+        assert result.should_fallback is True
+        assert result.should_compress is False
+