No heartbeat event during LLM-streamed tool-argument construction; watchdogs can't distinguish working from hung

Summary

When the model streams a large tool-call argument value (e.g., a 20KB file_text for create), the event stream is functionally silent for 10–30 minutes between tool.execution_start and tool.execution_complete. A stall watchdog watching the event stream cannot distinguish "model is productively building a large output" from "session has hung."

Repro

const session = await client.createSession({ /* ... */ });
session.on((evt) => console.log(evt.type, evt.data));
await session.sendAndWait({
  prompt: "Create a file at ./report.json containing a JSON document " +
          "with at least 20KB of structured analysis data. Use the create tool.",
});

Observe the event stream between tool.execution_start (with empty arguments) and tool.execution_complete (with the full argument). Empirically, no events fire for the duration of argument construction.

Expected

A heartbeat event during argument streaming — byte count, token count, or even a simple tool.argument_progress ping — so consumers can tell a working session from a hung one.

Actual

tool.execution_start fires once, then silence for the duration of LLM argument streaming, then tool.execution_complete fires once when the full tool invocation finishes (which includes both arg construction AND tool execution).

assistant.streaming_delta exists with cumulative byte counts (totalResponseSizeBytes) and fires during response streaming, but it has not been verified whether it continues to fire during the tool-argument construction phase. This would be useful to clarify: if streaming_delta does cover that phase, the gap is consumer-side documentation; if it doesn't, the gap is upstream.

Evidence (SDK source)

nodejs/src/generated/session-events.ts: the SessionEvent union includes:

• assistant.streaming_delta (AssistantStreamingDeltaData.totalResponseSizeBytes) — described as "Streaming response progress with cumulative byte count"
• assistant.reasoning_delta, assistant.message_delta — response-side
• tool.execution_progress, tool.execution_partial_result — fire AFTER execution_start for tools that emit stdout (bash, powershell)

There is no tool.argument_progress or equivalent event specifically scoped to in-flight tool-argument streaming.

Consumer impact

Consumers with long-running tool calls must either tune stall watchdogs to ceilings that mask real failures (15+ minutes) or route artifact completion through filesystem polling instead of session events.

Suggested fix

Either:

• Confirm + document that assistant.streaming_delta continues firing during in-flight tool-argument construction. If so, this is a doc-only fix.
• Emit a tool.argument_progress event during LLM-streamed argument construction carrying byte/token count or a small delta. Even one event every ~10 seconds would let consumers tell working sessions from hung ones.

Related

• #590 — "Long-running prompt suddenly hangs forever with no error or exception" — describes the user-visible symptom. The missing in-flight heartbeat is one root cause: consumers can't distinguish hang-in-progress from work-in-progress without it.
• #1273 — session.disconnect() cooperative — the recovery path when this stall does happen is also broken; the two issues compound.

Environment

- SDK: @github/copilot-sdk@0.3.0
- CLI: @github/copilot@1.0.45
- Node: 22 LTS
- OS: Windows 11
- Model: claude-sonnet-4-6

Bắt đầu với nodejs/src/generated/session-events.ts và các định nghĩa sự kiện assistant.streaming_delta, tool.execution_start và tool.execution_complete. Xác minh liệu streaming_delta có được phát trong khi đang xây dựng các đối số của công cụ hay không; hoàn tất có nghĩa là либо ghi lại hành vi đó trong tài liệu, либо định nghĩa và phát ra một sự kiện tiến trình có phạm vi rõ ràng để consumer có thể phân biệt công việc đang hoạt động với trạng thái bị treo.