diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index e04fee7608c9..7e616ee5a073 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -3959,6 +3959,14 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
             # and the continuation prompt still reads as an interrupted turn.
             if not _partial_text:
                 _partial_text = "[response interrupted]"
+                logger.warning(
+                    "Empty partial-stream stub (0 chars recovered, no tool "
+                    "call) — substituting placeholder content so the assistant "
+                    "message is not persisted empty (would otherwise 400 every "
+                    "later request with 'messages must have non-empty content' "
+                    "/ INVALID_REQUEST_BODY). error=%s",
+                    result["error"],
+                )
             _stub_msg = SimpleNamespace(
                 role="assistant", content=_partial_text, tool_calls=None,
                 reasoning_content=None,
diff --git a/agent/error_classifier.py b/agent/error_classifier.py
index cfe25b18db04..27df08f58ea9 100644
--- a/agent/error_classifier.py
+++ b/agent/error_classifier.py
@@ -1326,6 +1326,14 @@ def _classify_400(
         any(p in error_msg for p in _INVALID_MESSAGE_BODY_PATTERNS)
         or error_code_lower == "invalid_request_body"
     ):
+        logger.warning(
+            "Malformed message array 400 (invalid request body) classified as "
+            "format_error, NOT context overflow — failing fast + falling back "
+            "instead of entering the compression loop. This usually means an "
+            "empty-content assistant stub is in the transcript; num_messages=%s "
+            "approx_tokens=%s. error=%.200s",
+            num_messages, approx_tokens, error_msg,
+        )
         return result_fn(
             FailoverReason.format_error,
             retryable=False,
diff --git a/tests/agent/test_error_classifier.py b/tests/agent/test_error_classifier.py
index 8de35dd9836b..ade8ddf4fc4d 100644
--- a/tests/agent/test_error_classifier.py
+++ b/tests/agent/test_error_classifier.py
@@ -1326,7 +1326,7 @@ class TestClassifyApiError:
         assert result.retryable is False
         assert result.should_compress is not True
 
-    def test_400_litellm_invalid_request_body_shape(self):
+    def test_400_litellm_invalid_request_body_shape(self, caplog):
         """litellm/Bedrock proxy shape (errorMessage/errorCode) → format_error.
 
         The proxy in front of Anthropic surfaces the empty-content rejection
@@ -1335,8 +1335,10 @@ class TestClassifyApiError:
         are not the standard error.message / message, so err_body_msg used to
         come back empty → is_generic=True → mis-routed into compression on a
         large session.  Both the message pattern and the errorCode must be
-        recognized.
+        recognized, and a distinct warning must be logged so the condition is
+        observable in the field.
         """
+        import logging
         proxy_msg = ("The provided request body is invalid: claude "
                      "messages.208: all messages must have non-empty content "
                      "except for the optional final assistant message")
@@ -1350,12 +1352,17 @@ class TestClassifyApiError:
                 "errorArgs": {"reason": "claude messages.208: ..."},
             },
         )
-        result = classify_api_error(
-            e, approx_tokens=66000, context_length=200000, num_messages=219,
-        )
+        with caplog.at_level(logging.WARNING, logger="agent.error_classifier"):
+            result = classify_api_error(
+                e, approx_tokens=66000, context_length=200000, num_messages=219,
+            )
         assert result.reason == FailoverReason.format_error
         assert result.retryable is False
         assert result.should_compress is not True
+        assert any(
+            "Malformed message array 400" in r.getMessage()
+            for r in caplog.records
+        ), "Expected a distinct warning identifying the malformed-body 400"
 
     def test_400_real_context_overflow_still_compresses(self):
         """Guard: the new empty-content guard must NOT swallow real overflows.