[eric] streaming: a session socket that connects mid-reply is handed the text so far after its resume ack, instead of a static Thinking then the whole reply at once

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C9zwUaHucUgrdxvK8FvjYT

backend/apps/agents/core/stream_snapshot.py

@@ -0,0 +1,27 @@
+"""The text of an assistant reply that is streaming RIGHT NOW, for a session socket that just connected.
+
+Every delta before the per-session socket connects is lost to that client: the ring replays them, but
+the client drops replayed stream frames on purpose (they predate its resume ack). Until now the
+transcript stayed on a static "Thinking..." and then received the whole reply at once, which is the
+"streams halfway, wipes, then retypes everything fast" Eric kept seeing. The manager already keeps the
+accumulated text per session (`live_partial`, for the crash snapshot); this hands it to the socket.
+"""
+
+from typing import Dict, Optional
+
+from typeguard import typechecked
+
+from backend.apps.agents.manager.streaming.PartialReply import PartialReply
+
+
+@typechecked
+def stream_snapshot_payload(session_id: str, live_partial: Dict[str, PartialReply]) -> Optional[dict]:
+    partial = live_partial.get(session_id)
+    if partial is None or not partial.msg_id or not partial.text:
+        return None
+    return {
+        "session_id": session_id,
+        "message_id": partial.msg_id,
+        "role": "assistant",
+        "text": partial.text,
+    }

backend/main.py

@@ -209,6 +209,16 @@ async def websocket_session(websocket: WebSocket, session_id: str):
                         "ack": ack,
                     },
                 }))
+                # AFTER the ack, never before: the client drops stream frames until it has the ack.
+                from backend.apps.agents.agent_manager import agent_manager as p_am
+                from backend.apps.agents.core.stream_snapshot import stream_snapshot_payload
+                snapshot = stream_snapshot_payload(session_id, p_am.live_partial)
+                if snapshot is not None:
+                    await websocket.send_text(json.dumps({
+                        "event": "agent:stream_snapshot",
+                        "session_id": session_id,
+                        "data": snapshot,
+                    }))
             elif event == "client:ping":
                 # Heartbeat. Cheap, keeps NATs/firewalls from silently dropping the connection. Carry the client's nonce back so it can match pong→ping for round-trip latency tracking if it wants.
                 await websocket.send_text(json.dumps({

backend/tests/test_stream_snapshot.py

@@ -0,0 +1,30 @@
+"""A session socket that connects mid-reply gets the text so far, once, and nothing when there is none."""
+
+import pathlib
+
+from backend.apps.agents.core.stream_snapshot import stream_snapshot_payload
+from backend.apps.agents.manager.streaming.PartialReply import PartialReply
+
+
+def test_mid_reply_connect_gets_the_accumulated_text():
+    live = {"s1": PartialReply(msg_id="m1", text="The Eiffel Tower is a wrought-iron", branch_id="main")}
+    assert stream_snapshot_payload("s1", live) == {
+        "session_id": "s1", "message_id": "m1", "role": "assistant", "text": "The Eiffel Tower is a wrought-iron",
+    }
+
+
+def test_no_reply_in_flight_means_no_snapshot():
+    assert stream_snapshot_payload("s1", {}) is None
+    assert stream_snapshot_payload("s1", {"s1": PartialReply(msg_id="m1", text="", branch_id="main")}) is None
+    assert stream_snapshot_payload("s1", {"other": PartialReply(msg_id="m1", text="x", branch_id="main")}) is None
+
+
+def test_the_snapshot_is_sent_after_the_hello_ack_not_before():
+    """The client drops every stream frame that arrives before its resume ack, so a snapshot sent
+    earlier would be dropped exactly like the replayed deltas it exists to replace."""
+    src = pathlib.Path("backend/main.py").read_text()
+    hello = src.index('"event": "server:hello"')
+    snapshot = src.index('"event": "agent:stream_snapshot"')
+    assert hello < snapshot
+    handler = src[src.index('if event == "client:hello":'):src.index('elif event == "client:ping":')]
+    assert "stream_snapshot_payload(session_id, p_am.live_partial)" in handler