fix(mistral): reject file purposes Mistral lacks instead of mapping them to batch

The proxy runs batch-file validation and guardrails only for purpose=batch,
so a purpose such as assistants that was silently rewritten to batch on the
way to Mistral let an upload skip both. Only batch, fine-tune and ocr pass
through now; anything else is a 400.

litellm/llms/mistral/files/transformation.py

@@ -89,11 +89,14 @@ def _to_openai_purpose(purpose: MistralFilePurpose) -> OpenAIFilesPurpose:
 
 
 def _to_mistral_purpose(purpose: str) -> MistralFilePurpose:
+    """Only Mistral's own purposes pass through. Silently mapping anything else to ``batch``
+    would let an upload skip the proxy's batch-file validation and guardrails, which only
+    run when the caller says ``purpose=batch``."""
     match purpose:
-        case "fine-tune" | "ocr":
+        case "batch" | "fine-tune" | "ocr":
             return purpose
         case _:
-            return "batch"
+            raise ValueError(f"Mistral does not support purpose={purpose!r}. Use one of: batch, fine-tune, ocr")
 
 
 def _api_base_from(litellm_params: Mapping[str, object]) -> str:
@@ -166,7 +169,7 @@ class MistralFilesConfig(BaseFilesConfig):
         content_type: Final = extracted.get("content_type") or "application/octet-stream"
         upload: Final = MistralMultipartUpload(
             file=(filename, extracted["content"], content_type),
-            purpose=(None, _to_mistral_purpose(create_file_data.get("purpose", "batch"))),
+            purpose=(None, _to_mistral_purpose(create_file_data.get("purpose") or "batch")),
         )
         return dict(upload)  # mutable-ok: BaseFilesConfig signature
 

tests/test_litellm/llms/mistral/files/test_mistral_files_transformation.py

@@ -91,18 +91,28 @@ def test_upload_request_is_multipart_with_batch_purpose(config):
     }
 
 
-@pytest.mark.parametrize(
-    "openai_purpose,mistral_purpose",
-    [("batch", "batch"), ("fine-tune", "fine-tune"), ("ocr", "ocr"), ("assistants", "batch"), ("user_data", "batch")],
-)
-def test_upload_request_maps_purpose_onto_mistral_enum(config, openai_purpose, mistral_purpose):
+@pytest.mark.parametrize("purpose", ["batch", "fine-tune", "ocr"])
+def test_upload_request_passes_mistral_purposes_through(config, purpose):
     body = config.transform_create_file_request(
         model="",
-        create_file_data=CreateFileRequest(file=("f.bin", b"x"), purpose=openai_purpose),
+        create_file_data=CreateFileRequest(file=("f.bin", b"x"), purpose=purpose),
         optional_params={},
         litellm_params={},
     )
-    assert body["purpose"] == (None, mistral_purpose)
+    assert body["purpose"] == (None, purpose)
+
+
+@pytest.mark.parametrize("purpose", ["assistants", "user_data", "vision", "evals"])
+def test_upload_request_rejects_purposes_mistral_lacks(config, purpose):
+    """Regression: these used to be silently rewritten to ``batch``, so an upload that skipped the
+    proxy's batch-only validation and guardrails still landed on Mistral as a batch input file."""
+    with pytest.raises(ValueError, match=f"purpose={purpose!r}"):
+        config.transform_create_file_request(
+            model="",
+            create_file_data=CreateFileRequest(file=("f.bin", b"x"), purpose=purpose),
+            optional_params={},
+            litellm_params={},
+        )
 
 
 def test_upload_request_requires_file(config):
@@ -181,6 +191,11 @@ def test_list_request_filters_by_mapped_purpose(config):
     assert no_params == {}
 
 
+def test_list_request_rejects_purposes_mistral_lacks(config):
+    with pytest.raises(ValueError, match="purpose='assistants'"):
+        config.transform_list_files_request(purpose="assistants", optional_params={}, litellm_params={})
+
+
 def test_list_response(config):
     out = config.transform_list_files_response(
         raw_response=_response(