fix(error_classifier): stop empty-response advisories from triggering compression

Provider empty-reply text mentions "very low max_tokens", which used to match
the bare overflow pattern and thrash compress until "Cannot compress further".

agent/error_classifier.py

@@ -269,6 +269,11 @@ _CONTEXT_OVERFLOW_PATTERNS = [
     "context window",
     "prompt is too long",
     "prompt exceeds max length",
+    # NOTE: bare "max_tokens" is load-bearing — the output-cap-retry path keys
+    # off it (e.g. "max_tokens: 65536 > context_window: 200000 ..."). Do NOT
+    # remove it. Provider empty-response advisories also contain "very low
+    # max_tokens", but those are intercepted by _EMPTY_PROVIDER_RESPONSE_PATTERNS
+    # BEFORE this list is consulted, so they never mis-route into compression.
     "max_tokens",
     "maximum number of tokens",
     # vLLM / local inference server patterns
@@ -426,6 +431,19 @@ _THINKING_SIG_PATTERNS = [
 # the exception type is generic (e.g. RuntimeError from a local shim that
 # wraps a subprocess timeout).  Checked before the type-based transport
 # heuristics so custom-provider "timed out" errors don't fall through to
+# Provider empty-response advisories (OpenRouter / nano-gpt / similar).
+# Checked before context-overflow matching because the advisory text often
+# mentions "max_tokens" as a possible cause, which historically sat in
+# _CONTEXT_OVERFLOW_PATTERNS and sent healthy sessions into a compression
+# death spiral ending in "Cannot compress further".
+_EMPTY_PROVIDER_RESPONSE_PATTERNS = [
+    "returned an empty response",
+    "empty response despite retries",
+    "provider returned an empty response",
+    "model returning empty responses",
+    "empty response stream",
+]
+
 # the unknown bucket and get misreported as empty responses.
 _TIMEOUT_MESSAGE_PATTERNS = [
     "timed out",
@@ -1077,6 +1095,14 @@ def _classify_by_status(
         # remaining explicit context-overflow signal routes into the
         # compression-and-retry path (mirroring _classify_400) instead of
         # blind server_error retries that exhaust and drop the turn.
+        # Empty-response advisories that mention "max_tokens" must not enter
+        # that compression path.
+        if any(p in error_msg for p in _EMPTY_PROVIDER_RESPONSE_PATTERNS):
+            return result_fn(
+                FailoverReason.server_error,
+                retryable=True,
+                should_compress=False,
+            )
         if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
             return result_fn(
                 FailoverReason.context_overflow,
@@ -1090,6 +1116,12 @@ def _classify_by_status(
         # Cloudflare/Tailscale hop relabeling the status). Route explicit
         # overflow bodies into compression; otherwise treat as transient
         # overload and retry.
+        if any(p in error_msg for p in _EMPTY_PROVIDER_RESPONSE_PATTERNS):
+            return result_fn(
+                FailoverReason.server_error,
+                retryable=True,
+                should_compress=False,
+            )
         if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
             return result_fn(
                 FailoverReason.context_overflow,
@@ -1215,8 +1247,8 @@ def _classify_400(
     # returns:
     #   "Unsupported parameter: 'max_tokens' is not supported with this model.
     #    Use 'max_completion_tokens' instead."
-    # That string contains the literal substring "max_tokens", which is one of
-    # the _CONTEXT_OVERFLOW_PATTERNS — so without this guard the 400 is
+    # That string contains the literal substring "max_tokens", which historically
+    # sat in _CONTEXT_OVERFLOW_PATTERNS — so without this guard the 400 is
     # misclassified as context_overflow, routed into the compression loop,
     # re-sent with the same bad parameter, and ends in "Cannot compress
     # further".  These errors are deterministic (every retry gets the identical
@@ -1238,6 +1270,17 @@ def _classify_400(
             should_fallback=True,
         )
 
+    # Empty-provider-response advisories must not enter compression. They
+    # often mention "max_tokens" as a possible cause and used to match the
+    # bare overflow pattern, then thrash compress until "Cannot compress
+    # further" on an otherwise healthy session (custom endpoints / nano-gpt).
+    if any(p in error_msg for p in _EMPTY_PROVIDER_RESPONSE_PATTERNS):
+        return result_fn(
+            FailoverReason.server_error,
+            retryable=True,
+            should_compress=False,
+        )
+
     # Context overflow from 400
     if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
         return result_fn(
@@ -1441,6 +1484,15 @@ def _classify_by_message(
             should_fallback=True,
         )
 
+    # Empty-provider-response advisories (often mention "max_tokens") must
+    # retry without compression — see the matching 400-path guard above.
+    if any(p in error_msg for p in _EMPTY_PROVIDER_RESPONSE_PATTERNS):
+        return result_fn(
+            FailoverReason.server_error,
+            retryable=True,
+            should_compress=False,
+        )
+
     # Context overflow patterns
     if any(p in error_msg for p in _CONTEXT_OVERFLOW_PATTERNS):
         return result_fn(