From bbc265d912cf5f6518f9fa4c9d3f08d8d050295d Mon Sep 17 00:00:00 2001
From: JingHao-Leon <102573344+JingHao-Leon@users.noreply.github.com>
Date: Fri, 25 Sep 2026 10:12:04 +0800
Subject: [PATCH] fix(core_helpers): map Anthropic
 model_context_window_exceeded stop reason to length
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Anthropic returns stop_reason="model_context_window_exceeded" when generation
stops because the context window filled up. _FINISH_REASON_MAP had no entry for
it, so map_finish_reason logged a warning and returned "stop" — clients cannot
distinguish a truncated response from a complete one, on both chat completions
and the Responses API.

Map it to "length" (consistent with max_tokens and the existing compaction
entry) and cover it in the Anthropic parametrized finish-reason tests.

Fixes #43012
---
 litellm/litellm_core_utils/core_helpers.py         | 1 +
 tests/unit/litellm_core_utils/test_core_helpers.py | 1 +
 2 files changed, 2 insertions(+)

diff --git a/litellm/litellm_core_utils/core_helpers.py b/litellm/litellm_core_utils/core_helpers.py
index b095b4b12c6..5afaad61ec8 100644
--- a/litellm/litellm_core_utils/core_helpers.py
+++ b/litellm/litellm_core_utils/core_helpers.py
@@ -199,6 +199,7 @@ _FINISH_REASON_MAP: Final[dict[str, OpenAIChatCompletionFinishReason]] = {
     "stop_sequence": "stop",
     "end_turn": "stop",
     "max_tokens": "length",
+    "model_context_window_exceeded": "length",
     "tool_use": "tool_calls",
     "refusal": "content_filter",
     "compaction": "length",
diff --git a/tests/unit/litellm_core_utils/test_core_helpers.py b/tests/unit/litellm_core_utils/test_core_helpers.py
index 2a6dd347d5f..ed8a090f064 100644
--- a/tests/unit/litellm_core_utils/test_core_helpers.py
+++ b/tests/unit/litellm_core_utils/test_core_helpers.py
@@ -178,6 +178,7 @@ class TestMapFinishReasonAnthropic:
             ("stop_sequence", "stop"),
             ("end_turn", "stop"),
             ("max_tokens", "length"),
+            ("model_context_window_exceeded", "length"),
             ("tool_use", "tool_calls"),
             ("compaction", "length"),
             ("content_filtered", "content_filter"),