feat(transport): async prompt + SSE streaming from opencode events

- POST /session/:id/prompt_async fires the prompt (204 immediate)
- GET /event SSE stream picks up message.part.updated with real text deltas
- Filters events by session ID, computes delta from cumulative text
- Breaks on message completed or session idle

app/main.py

@@ -273,23 +273,27 @@ async def _ensure_opencode_session(email: str) -> str:
         return sid
 
 
-async def _send_to_opencode(
+async def _stream_opencode(
     user_text: str,
     system_prompt: str,
     session_id: str,
     ws: WebSocket,
 ) -> str:
-    """Send a message to an opencode session and stream the response to the WebSocket.
+    """Send a message to opencode and stream the response via SSE.
 
-    Uses POST /session/:id/message (synchronous — waits for full LLM response).
+    1. POST /session/:id/prompt_async  — fire the prompt (returns 204 immediately)
-    Then streams the text word-by-word to the WebSocket for the typewriter effect.
+    2. GET /event (SSE)                — stream message.part.updated events with text deltas
-    Returns the full response text.
+
+    The SSE stream emits events for ALL sessions. We filter by our session ID
+    and track the assistant message parts to extract text deltas.
     """
     full_response = ""
+    prev_text = ""  # track cumulative text to compute deltas
+
     try:
-        async with httpx.AsyncClient(timeout=120.0) as client:
+        async with httpx.AsyncClient(timeout=5.0) as fire_client:
-            resp = await client.post(
+            resp = await fire_client.post(
-                f"{OPENCODE_URL}/session/{session_id}/message",
+                f"{OPENCODE_URL}/session/{session_id}/prompt_async",
                 auth=_opencode_auth(),
                 headers={"x-opencode-directory": "/home/madcat"},
                 json={
@@ -301,27 +305,65 @@ async def _send_to_opencode(
                     },
                 },
             )
-            if resp.status_code != 200:
+            if resp.status_code not in (200, 204):
-                raise RuntimeError(
+                raise RuntimeError(f"prompt_async HTTP {resp.status_code}: {resp.text[:200]}")
-                    f"opencode HTTP {resp.status_code}: {resp.text[:200]}"
-                )
-            data = resp.json()
-            # Extract text from response parts
-            parts = data.get("parts", [])
-            for part in parts:
-                if part.get("type") == "text":
-                    full_response += part.get("text", "")
 
-        # Stream text word-by-word to the WebSocket for typewriter effect
+        # Now stream SSE events until the assistant message is done
-        if full_response:
+        async with httpx.AsyncClient(timeout=180.0) as sse_client:
-            words = full_response.split(" ")
+            async with sse_client.stream(
-            for i, word in enumerate(words):
+                "GET",
-                delta = word if i == 0 else " " + word
+                f"{OPENCODE_URL}/event",
-                await ws.send_json({"role": "assistant", "delta": delta, "done": False})
+                auth=_opencode_auth(),
-                await asyncio.sleep(0.03)  # ~30ms per word — fast typewriter
+                headers={"Accept": "text/event-stream", "x-opencode-directory": "/home/madcat"},
+            ) as sse_resp:
+                if sse_resp.status_code != 200:
+                    body = await sse_resp.aread()
+                    raise RuntimeError(f"SSE HTTP {sse_resp.status_code}: {body[:200]}")
+
+                async for line in sse_resp.aiter_lines():
+                    if not line.startswith("data: "):
+                        continue
+                    payload = line[6:].strip()
+                    if not payload:
+                        continue
+                    try:
+                        evt = json.loads(payload)
+                    except json.JSONDecodeError:
+                        continue
+
+                    evt_type = evt.get("type", "")
+                    props = evt.get("properties", {})
+
+                    # Filter to our session
+                    if props.get("sessionID") != session_id:
+                        continue
+
+                    # message.part.updated — contains text deltas
+                    if evt_type == "message.part.updated":
+                        part = props.get("part", {})
+                        if part.get("type") == "text":
+                            new_text = part.get("text", "")
+                            if len(new_text) > len(prev_text):
+                                delta = new_text[len(prev_text):]
+                                prev_text = new_text
+                                full_response = new_text
+                                await ws.send_json({"role": "assistant", "delta": delta, "done": False})
+
+                    # message.updated with role=assistant + completed status = done
+                    if evt_type == "message.updated":
+                        msg = props.get("message", props)
+                        role = msg.get("role", "")
+                        status = msg.get("status", "")
+                        if role == "assistant" and status in ("completed", "done", "error"):
+                            break
+
+                    # session status idle = agent finished
+                    if evt_type == "session.status":
+                        if props.get("status") == "idle":
+                            break
 
     except Exception as e:
-        log.error("opencode error: %s", e)
+        log.error("opencode stream error: %s", e)
         await ws.send_json({
             "role": "system",
             "content": f"upstream error: {e} — try again",
@@ -824,9 +866,9 @@ async def chat_ws(ws: WebSocket) -> None:
 
             system_prompt = _pick_system_prompt(bound_slug, cart) + eems_context
 
-            # Send to opencode and stream response
+            # Send to opencode and stream response via SSE
             oc_session = await _ensure_opencode_session(user["email"])
-            response_text = await _send_to_opencode(user_msg, system_prompt, oc_session, ws)
+            response_text = await _stream_opencode(user_msg, system_prompt, oc_session, ws)
 
             if response_text:
                 history.append({"role": "assistant", "content": response_text})