fix(api): send tool progress as custom SSE event to prevent model corruption (#6972)

Tool progress markers (e.g. `⏰ list`) were injected directly into
SSE delta.content chunks. OpenAI-compatible frontends (Open WebUI,
LobeChat, etc.) store delta.content verbatim as the assistant message
and send it back on subsequent requests. After enough turns, the model
learns to emit these markers as plain text instead of issuing real tool
calls — silently hallucinating tool results without ever running them.

Fix: Send tool progress as a custom `event: hermes.tool.progress` SSE
event instead of mixing it into delta.content. Per the SSE spec, clients
that don't understand a custom event type silently ignore it, so this is
backward-compatible. Frontends that want to render progress indicators
can listen for the custom event without persisting it to conversation
history.

The /v1/runs endpoint already uses structured events — this aligns the
/v1/chat/completions streaming path with the same principle.

Closes #6972

gateway/platforms/api_server.py

@@ -644,15 +644,35 @@ class APIServerAdapter(BasePlatformAdapter):
                     _stream_q.put(delta)
 
             def _on_tool_progress(event_type, name, preview, args, **kwargs):
-                """Inject tool progress into the SSE stream for Open WebUI."""
+                """Send tool progress as a separate SSE event.
+
+                Previously, progress markers like ``⏰ list`` were injected
+                directly into ``delta.content``.  OpenAI-compatible frontends
+                (Open WebUI, LobeChat, …) store ``delta.content`` verbatim as
+                the assistant message and send it back on subsequent requests.
+                After enough turns the model learns to *emit* the markers as
+                plain text instead of issuing real tool calls — silently
+                hallucinating tool results.  See #6972.
+
+                The fix: push a tagged tuple ``("__tool_progress__", payload)``
+                onto the stream queue.  The SSE writer emits it as a custom
+                ``event: hermes.tool.progress`` line that compliant frontends
+                can render for UX but will *not* persist into conversation
+                history.  Clients that don't understand the custom event type
+                silently ignore it per the SSE specification.
+                """
                 if event_type != "tool.started":
-                    return  # Only show tool start events in chat stream
+                    return
                 if name.startswith("_"):
-                    return  # Skip internal events (_thinking)
+                    return
                 from agent.display import get_tool_emoji
                 emoji = get_tool_emoji(name)
                 label = preview or name
-                _stream_q.put(f"\n`{emoji} {label}`\n")
+                _stream_q.put(("__tool_progress__", {
+                    "tool": name,
+                    "emoji": emoji,
+                    "label": label,
+                }))
 
             # Start agent in background.  agent_ref is a mutable container
             # so the SSE writer can interrupt the agent on client disconnect.
@@ -763,6 +783,29 @@ class APIServerAdapter(BasePlatformAdapter):
             }
             await response.write(f"data: {json.dumps(role_chunk)}\n\n".encode())
 
+            # Helper — route a queue item to the correct SSE event.
+            async def _emit(item):
+                """Write a single queue item to the SSE stream.
+
+                Plain strings are sent as normal ``delta.content`` chunks.
+                Tagged tuples ``("__tool_progress__", payload)`` are sent
+                as a custom ``event: hermes.tool.progress`` SSE event so
+                frontends can display them without storing the markers in
+                conversation history.  See #6972.
+                """
+                if isinstance(item, tuple) and len(item) == 2 and item[0] == "__tool_progress__":
+                    event_data = json.dumps(item[1])
+                    await response.write(
+                        f"event: hermes.tool.progress\ndata: {event_data}\n\n".encode()
+                    )
+                else:
+                    content_chunk = {
+                        "id": completion_id, "object": "chat.completion.chunk",
+                        "created": created, "model": model,
+                        "choices": [{"index": 0, "delta": {"content": item}, "finish_reason": None}],
+                    }
+                    await response.write(f"data: {json.dumps(content_chunk)}\n\n".encode())
+
             # Stream content chunks as they arrive from the agent
             loop = asyncio.get_event_loop()
             while True:
@@ -776,12 +819,7 @@ class APIServerAdapter(BasePlatformAdapter):
                                 delta = stream_q.get_nowait()
                                 if delta is None:
                                     break
-                                content_chunk = {
-                                    "id": completion_id, "object": "chat.completion.chunk",
-                                    "created": created, "model": model,
-                                    "choices": [{"index": 0, "delta": {"content": delta}, "finish_reason": None}],
-                                }
-                                await response.write(f"data: {json.dumps(content_chunk)}\n\n".encode())
+                                await _emit(delta)
                             except _q.Empty:
                                 break
                         break
@@ -790,12 +828,7 @@ class APIServerAdapter(BasePlatformAdapter):
                 if delta is None:  # End of stream sentinel
                     break
 
-                content_chunk = {
-                    "id": completion_id, "object": "chat.completion.chunk",
-                    "created": created, "model": model,
-                    "choices": [{"index": 0, "delta": {"content": delta}, "finish_reason": None}],
-                }
-                await response.write(f"data: {json.dumps(content_chunk)}\n\n".encode())
+                await _emit(delta)
 
             # Get usage from completed agent
             usage = {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0}

tests/gateway/test_api_server.py

@@ -464,7 +464,7 @@ class TestChatCompletionsEndpoint:
 
     @pytest.mark.asyncio
     async def test_stream_includes_tool_progress(self, adapter):
-        """tool_progress_callback fires → progress appears in the SSE stream."""
+        """tool_progress_callback fires → progress appears as custom SSE event, not in delta.content."""
         import asyncio
 
         app = _create_app(adapter)
@@ -495,8 +495,26 @@ class TestChatCompletionsEndpoint:
                 assert resp.status == 200
                 body = await resp.text()
                 assert "[DONE]" in body
-                # Tool progress message must appear in the stream
-                assert "ls -la" in body
+                # Tool progress must appear as a custom SSE event, not in
+                # delta.content — prevents model from learning to imitate
+                # markers instead of calling tools (#6972).
+                assert "event: hermes.tool.progress" in body
+                assert '"tool": "terminal"' in body
+                assert '"label": "ls -la"' in body
+                # The progress marker must NOT appear inside any
+                # chat.completion.chunk delta.content field.
+                import json as _json
+                for line in body.splitlines():
+                    if line.startswith("data: ") and line.strip() != "data: [DONE]":
+                        try:
+                            chunk = _json.loads(line[len("data: "):])
+                        except _json.JSONDecodeError:
+                            continue
+                        if chunk.get("object") == "chat.completion.chunk":
+                            for choice in chunk.get("choices", []):
+                                content = choice.get("delta", {}).get("content", "")
+                                # Tool emoji markers must never leak into content
+                                assert "ls -la" not in content or content == "Here are the files."
                 # Final content must also be present
                 assert "Here are the files." in body
 
@@ -532,10 +550,12 @@ class TestChatCompletionsEndpoint:
                 )
                 assert resp.status == 200
                 body = await resp.text()
-                # Internal _thinking event should NOT appear
+                # Internal _thinking event should NOT appear anywhere
                 assert "some internal state" not in body
-                # Real tool progress should appear
-                assert "Python docs" in body
+                # Real tool progress should appear as custom SSE event
+                assert "event: hermes.tool.progress" in body
+                assert '"tool": "web_search"' in body
+                assert '"label": "Python docs"' in body
 
     @pytest.mark.asyncio
     async def test_no_user_message_returns_400(self, adapter):