feat: Emit message-start and content-start events in OCI streaming

Complete the V2 streaming protocol lifecycle:
message-start → content-start → content-delta* → content-end → message-end

Previously only content-delta, content-end, and message-end were emitted,
causing consumers expecting message-start to fail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: fern-support

src/cohere/oci_client.py

@@ -770,13 +770,20 @@ def transform_oci_stream_wrapper(
     """
     Wrap OCI stream and transform events to Cohere V2 format.
 
+    Emits the full V2 streaming lifecycle:
+    message-start -> content-start -> content-delta* -> content-end -> message-end
+
     Args:
         stream: Original OCI stream iterator
         endpoint: Cohere endpoint name
 
     Yields:
         Bytes of transformed streaming events
     """
+    import logging
+
+    generation_id = str(uuid.uuid4())
+    emitted_start = False
     buffer = b""
     for chunk in stream:
         buffer += chunk
@@ -795,14 +802,39 @@ def transform_oci_stream_wrapper(
                 try:
                     oci_event = json.loads(data_str)
                 except json.JSONDecodeError:
-                    import logging
                     logging.warning(
                         "OCI stream: failed to parse SSE event as JSON (endpoint=%s, data=%r)",
                         endpoint, data_str[:200],
                     )
                     continue
 
                 try:
+                    # Emit message-start and content-start before first content delta
+                    if not emitted_start:
+                        # Detect content type from first event
+                        content_type = "text"
+                        if "message" in oci_event and "content" in oci_event["message"]:
+                            content_list = oci_event["message"]["content"]
+                            if content_list and isinstance(content_list, list) and len(content_list) > 0:
+                                oci_type = content_list[0].get("type", "TEXT").upper()
+                                if oci_type == "THINKING":
+                                    content_type = "thinking"
+
+                        message_start = {
+                            "type": "message-start",
+                            "id": generation_id,
+                            "delta": {"message": {"role": "assistant"}},
+                        }
+                        yield b"data: " + json.dumps(message_start).encode("utf-8") + b"\n\n"
+
+                        content_start = {
+                            "type": "content-start",
+                            "index": 0,
+                            "delta": {"message": {"content": {"type": content_type}}},
+                        }
+                        yield b"data: " + json.dumps(content_start).encode("utf-8") + b"\n\n"
+                        emitted_start = True
+
                     cohere_event = transform_stream_event(endpoint, oci_event)
                     yield b"data: " + json.dumps(cohere_event).encode("utf-8") + b"\n\n"
                 except Exception as e:

tests/test_oci_client.py

@@ -555,6 +555,38 @@ def test_load_oci_config_missing_private_key_raises(self):
                 )
             self.assertIn("oci_private_key_path", str(ctx.exception))
 
+    def test_stream_wrapper_emits_full_event_lifecycle(self):
+        """Test that stream emits message-start, content-start, content-delta, content-end, message-end."""
+        import json
+        from cohere.oci_client import transform_oci_stream_wrapper
+
+        chunks = [
+            b'data: {"message": {"content": [{"type": "TEXT", "text": "Hello"}]}}\n',
+            b'data: {"message": {"content": [{"type": "TEXT", "text": " world"}]}, "finishReason": "COMPLETE"}\n',
+            b'data: [DONE]\n',
+        ]
+
+        events = []
+        for raw in transform_oci_stream_wrapper(iter(chunks), "chat"):
+            line = raw.decode("utf-8").strip()
+            if line.startswith("data: "):
+                events.append(json.loads(line[6:]))
+
+        event_types = [e["type"] for e in events]
+        self.assertEqual(event_types[0], "message-start")
+        self.assertEqual(event_types[1], "content-start")
+        self.assertEqual(event_types[2], "content-delta")
+        self.assertEqual(event_types[3], "content-end")
+        self.assertEqual(event_types[4], "message-end")
+
+        # Verify message-start has id and role
+        self.assertIn("id", events[0])
+        self.assertEqual(events[0]["delta"]["message"]["role"], "assistant")
+
+        # Verify content-start has index and type
+        self.assertEqual(events[1]["index"], 0)
+        self.assertEqual(events[1]["delta"]["message"]["content"]["type"], "text")
+
     def test_stream_wrapper_skips_malformed_json_with_warning(self):
         """Test that malformed JSON in SSE stream is skipped (not silently swallowed)."""
         import json
@@ -566,8 +598,8 @@ def test_stream_wrapper_skips_malformed_json_with_warning(self):
             b'data: [DONE]\n',
         ]
         events = list(transform_oci_stream_wrapper(iter(chunks), "chat"))
-        # Should get content-delta + message-end (malformed line skipped)
-        self.assertEqual(len(events), 2)
+        # Should get message-start + content-start + content-delta + message-end (malformed line skipped)
+        self.assertEqual(len(events), 4)
 
     def test_stream_wrapper_raises_on_transform_error(self):
         """Test that transform errors in stream produce OCI-specific error, not opaque httpx error."""