fix(fireworks_ai): strip thinking_blocks from chat messages before Fireworks API call (#27881)

* fix(fireworks_ai): strip thinking_blocks from chat messages before API call

Fireworks OpenAI-compatible ChatMessage schema uses additionalProperties:false
and rejects Anthropic-style messages[].thinking_blocks (e.g. Claude Code replays),
returning invalid_request_error. Remove the field in _transform_messages_helper
alongside provider_specific_fields.

Adds unit test test_transform_messages_helper_strips_thinking_blocks.

Co-authored-by: Cursor <cursoragent@cursor.com>

* chore(fireworks_ai): drop inline comments from message sanitization

Co-authored-by: Cursor <cursoragent@cursor.com>

* docs(fireworks_ai): explain why provider_specific_fields and thinking_blocks are stripped

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

litellm/llms/fireworks_ai/chat/transformation.py

@@ -241,10 +241,13 @@ class FireworksAIConfig(OpenAIGPTConfig):
                                 disable_add_transform_inline_image_block=disable_add_transform_inline_image_block,
                             )
             filter_value_from_dict(cast(dict, message), "cache_control")
-            # Remove fields not permitted by FireworksAI that may cause:
-            # "Not permitted, field: 'messages[n].provider_specific_fields'"
-            if isinstance(message, dict) and "provider_specific_fields" in message:
-                cast(dict, message).pop("provider_specific_fields", None)
+            # Remove fields not permitted by FireworksAI (additionalProperties: false
+            # on their ChatMessage schema) that may cause:
+            # "Extra inputs are not permitted, field: 'messages[n].<field>'"
+            if isinstance(message, dict):
+                m = cast(dict, message)
+                m.pop("provider_specific_fields", None)
+                m.pop("thinking_blocks", None)
 
         return messages
 

tests/test_litellm/llms/fireworks_ai/chat/test_fireworks_ai_chat_transformation.py

@@ -232,3 +232,23 @@ def test_transform_messages_helper_removes_provider_specific_fields():
     )
     for msg in out:
         assert "provider_specific_fields" not in msg
+
+
+def test_transform_messages_helper_strips_thinking_blocks():
+    """thinking_blocks must not be forwarded to Fireworks chat completions."""
+    config = FireworksAIConfig()
+    messages = [
+        {"role": "user", "content": "Translate a poem."},
+        {
+            "role": "assistant",
+            "content": "I can help.",
+            "thinking_blocks": [
+                {"type": "thinking", "thinking": "internal", "signature": ""}
+            ],
+        },
+    ]
+    out = config._transform_messages_helper(
+        messages, model="accounts/fireworks/models/glm-5p1", litellm_params={}
+    )
+    assert "thinking_blocks" not in out[1]
+    assert out[1]["content"] == "I can help."