Background sub-agent stream failures end the turn without a failure status

[kondv]

Type: Bug

Copilot CLI Version: 1.0.84-5 (also observed on 1.0.81-0 and 1.0.79-9)
OS Version: Windows 11 x64
Model: gpt-6-astra (also observed on an Anthropic model)

Summary

When a model stream ends early, the CLI reports five retries with 5-6 seconds of total retry wait, then stops the background sub-agent's turn. The parent receives a normal completion/idle notification without a failure status.

Found five occurrences in local session history, all on background sub-agent turns.

Steps to Reproduce

No deterministic standalone reproducer yet. The observed workflow was:

1. Start a background sub-agent on a long, tool-heavy task.
2. A model response stream terminates early.
3. Compare the session.error with the following completion/idle notification.

Actual Behaviour

The failing turn reports:

Execution failed: Failed to get response from the AI model; retried 5 times (total retry wait time: 5.00 seconds) Last error: Responses stream ended without a completed response (request ...)

Path	Last error	Reported total retry wait
Responses	Responses stream ended without a completed response	5.00 s
Anthropic messages	stream ended without producing a Message with role=assistant	6.00 s
Native HTTP	unexpected EOF during chunk size line	5.00 s

The wait figure is not the total request duration.

The inspected subagent.completed event contains usage and timing fields, but no success/error field. In one case it arrived 14 seconds after the error, following 57 minutes and 231 tool calls. In another, the agent went idle less than a second after the error.

Expected Behaviour

1. Propagate the failure and its error through the sub-agent completion path.
2. Use configurable backoff for retryable stream failures, without replaying already completed tool actions.

Notes

• Related: when running Codex 5.2, sometimes it errors out with: Error: Stream completed without a response.completed event #1013 and Execution failed: Error: Stream completed without a response.completed event #1372 reported an older stream-completion error. This report adds the retry-wait values and the background sub-agent completion behaviour.
• I have not reproduced this on 1.0.86-2. Finding the same error strings in that binary does not establish that the behaviour is unchanged.
• The failed turn did not produce a usable final response. This does not mean earlier tool changes were rolled back or lost.

Impact

Long background work can stop without a usable final response while the parent receives an ordinary completion/idle signal.