fix(fireworks_ai): restore Content-Type application/json header (fixes 415) (#33929)

* fix(fireworks_ai): set Content-Type application/json in validate_environment

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* refactor(fireworks_ai): delegate chat validate_environment to OpenAIGPTConfig

Instead of re-adding the JSON Content-Type default inside FireworksAIMixin,
FireworksAIConfig now delegates header construction to OpenAIGPTConfig and only
layers the Fireworks-specific x-session-affinity header on top, so the
Content-Type default can no longer drift away from the OpenAI base and reintroduce
the 415.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* test(fireworks_ai): cover missing api key error path in chat validate_environment

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Krrish Dholakia <krrishdholakia@berri.ai>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
(cherry picked from commit 3fcd19d7ad)

litellm/llms/fireworks_ai/chat/transformation.py

@@ -133,6 +133,32 @@ class FireworksAIConfig(FireworksAIMixin, OpenAIGPTConfig):
     def get_config(cls):
         return super().get_config()
 
+    def validate_environment(
+        self,
+        headers: dict,
+        model: str,
+        messages: List[AllMessageValues],
+        optional_params: dict,
+        litellm_params: dict,
+        api_key: str | None = None,
+        api_base: str | None = None,
+    ) -> dict:
+        api_key = self._get_api_key(api_key)
+        if api_key is None:
+            raise ValueError("FIREWORKS_API_KEY is not set")
+
+        validated_headers = OpenAIGPTConfig.validate_environment(
+            self,
+            headers=headers,
+            model=model,
+            messages=messages,
+            optional_params=optional_params,
+            litellm_params=litellm_params,
+            api_key=api_key,
+            api_base=api_base,
+        )
+        return self._add_session_affinity_header(validated_headers, litellm_params)
+
     def get_supported_openai_params(self, model: str):
         # Base parameters supported by all models
         supported_params = [

litellm/llms/fireworks_ai/common_utils.py

@@ -64,9 +64,16 @@ class FireworksAIMixin:
         if api_key is None:
             raise ValueError("FIREWORKS_API_KEY is not set")
 
-        validated_headers = {"Authorization": "Bearer {}".format(api_key), **headers}
-        if not any(key.lower() == "x-session-affinity" for key in validated_headers):
-            session_id = get_fireworks_session_id(litellm_params)
-            if session_id:
-                validated_headers["x-session-affinity"] = session_id
-        return validated_headers
+        auth_headers = {"Authorization": "Bearer {}".format(api_key), **headers}
+        content_type_header = (
+            {} if any(key.lower() == "content-type" for key in auth_headers) else {"Content-Type": "application/json"}
+        )
+        return self._add_session_affinity_header({**auth_headers, **content_type_header}, litellm_params)
+
+    def _add_session_affinity_header(self, headers: dict, litellm_params: dict) -> dict:
+        if any(key.lower() == "x-session-affinity" for key in headers):
+            return headers
+        session_id = get_fireworks_session_id(litellm_params)
+        if not session_id:
+            return headers
+        return {**headers, "x-session-affinity": session_id}

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

@@ -123,6 +123,90 @@ def test_validate_environment_preserves_explicit_session_affinity_header():
     assert headers["x-session-affinity"] == "explicit-session"
 
 
+def test_validate_environment_sets_json_content_type():
+    config = FireworksAIConfig()
+
+    headers = config.validate_environment(
+        headers={},
+        model="accounts/fireworks/models/test-model",
+        messages=[],
+        optional_params={},
+        litellm_params={},
+        api_key="test-key",
+    )
+
+    assert headers["Content-Type"] == "application/json"
+
+
+def test_validate_environment_preserves_explicit_content_type():
+    config = FireworksAIConfig()
+
+    headers = config.validate_environment(
+        headers={"content-type": "multipart/form-data"},
+        model="accounts/fireworks/models/test-model",
+        messages=[],
+        optional_params={},
+        litellm_params={},
+        api_key="test-key",
+    )
+
+    assert headers["content-type"] == "multipart/form-data"
+    assert "Content-Type" not in headers
+
+
+def test_validate_environment_sets_json_content_type_with_session_affinity():
+    config = FireworksAIConfig()
+
+    headers = config.validate_environment(
+        headers={},
+        model="accounts/fireworks/models/test-model",
+        messages=[],
+        optional_params={},
+        litellm_params={"litellm_session_id": "session-123"},
+        api_key="test-key",
+    )
+
+    assert headers["Content-Type"] == "application/json"
+    assert headers["Authorization"] == "Bearer test-key"
+    assert headers["x-session-affinity"] == "session-123"
+
+
+def test_validate_environment_resolves_api_key_from_env_and_sets_content_type(monkeypatch):
+    monkeypatch.setenv("FIREWORKS_API_KEY", "fw-env-key")
+    config = FireworksAIConfig()
+
+    headers = config.validate_environment(
+        headers={},
+        model="accounts/fireworks/models/test-model",
+        messages=[],
+        optional_params={},
+        litellm_params={},
+    )
+
+    assert headers["Authorization"] == "Bearer fw-env-key"
+    assert headers["Content-Type"] == "application/json"
+
+
+def test_validate_environment_raises_without_api_key(monkeypatch):
+    for env_var in (
+        "FIREWORKS_API_KEY",
+        "FIREWORKS_AI_API_KEY",
+        "FIREWORKSAI_API_KEY",
+        "FIREWORKS_AI_TOKEN",
+    ):
+        monkeypatch.delenv(env_var, raising=False)
+    config = FireworksAIConfig()
+
+    with pytest.raises(ValueError, match="FIREWORKS_API_KEY is not set"):
+        config.validate_environment(
+            headers={},
+            model="accounts/fireworks/models/test-model",
+            messages=[],
+            optional_params={},
+            litellm_params={},
+        )
+
+
 def test_get_fireworks_session_id_prefers_litellm_session_id_over_trace_id():
     assert (
         get_fireworks_session_id(