diff --git a/src/google/adk/models/anthropic_llm.py b/src/google/adk/models/anthropic_llm.py
index 3edbaeb3be0..54d2066ce05 100644
--- a/src/google/adk/models/anthropic_llm.py
+++ b/src/google/adk/models/anthropic_llm.py
@@ -330,12 +330,17 @@ def to_claude_role(role: Optional[str]) -> Literal["user", "assistant"]:
 
 
 # Mapping of Anthropic stop_reason strings to FinishReason enum values
-_STOP_REASON_MAPPING: dict[anthropic_types.StopReason, types.FinishReason] = {
+# Keyed by str: "model_context_window_exceeded" is newer than the minimum
+# supported anthropic SDK's StopReason literal.
+_STOP_REASON_MAPPING: dict[str, types.FinishReason] = {
     "end_turn": types.FinishReason.STOP,
     "stop_sequence": types.FinishReason.STOP,
     "tool_use": types.FinishReason.STOP,
     "pause_turn": types.FinishReason.STOP,
     "max_tokens": types.FinishReason.MAX_TOKENS,
+    # Generation stopped because the context window filled up, so the output
+    # is truncated just as with max_tokens.
+    "model_context_window_exceeded": types.FinishReason.MAX_TOKENS,
     "refusal": types.FinishReason.SAFETY,
 }
 
diff --git a/tests/unittests/models/test_anthropic_llm.py b/tests/unittests/models/test_anthropic_llm.py
index b7a54248f33..f57e7bcd689 100644
--- a/tests/unittests/models/test_anthropic_llm.py
+++ b/tests/unittests/models/test_anthropic_llm.py
@@ -4243,6 +4243,7 @@ async def mock_coro():
         ("stop_sequence", types.FinishReason.STOP),
         ("tool_use", types.FinishReason.STOP),
         ("max_tokens", types.FinishReason.MAX_TOKENS),
+        ("model_context_window_exceeded", types.FinishReason.MAX_TOKENS),
         ("pause_turn", types.FinishReason.STOP),
         ("refusal", types.FinishReason.SAFETY),
         (None, None),