BYOK: autopilot nudge turn re-serializes previously-sent transcript items, breaking prompt caching

[dzamoshchin]

Describe the bug

On the --autopilot completion-nudge turn, the CLI rebuilds the whole input array from parsed internal state instead of resending prior items byte-for-byte (BYOK, responses wire API). Item ids are preserved, bytes are not. The same tool call across requests:

request 1: '{"command": "echo \u00e9", "description": "unicode \u00e9 kept?"}'
request 5: '{"command":"echo é","description":"unicode é kept?"}'

The collapsed escape shows a JSON.parse / JSON.stringify round trip. Tool outputs are also truncated on the same turn.

Prompt caching is exact-prefix, so it breaks at the first rewritten item. Measured against the Responses API (gpt-5-mini), changing only the whitespace in the first tool call's arguments, same prompt_cache_key:

conversation	prompt tokens	cached (identical resend)	cached (after rewrite)	tokens lost
10k	18,060	17,920	5,632	12,288 (68%)
100k	114,820	114,688	5,632	109,056 (95%)

The post-rewrite value is constant (the static prefix ahead of the rewritten item), so the loss grows with session length. This recurs on every nudge turn. Non-nudge turns and sessions without --autopilot are byte-stable.

Separately, on ordinary turns, multi-part reasoning summary arrays are echoed back space-joined into one part.

Affected version

1.0.79

Steps to reproduce the behavior

1. Point the CLI at an OpenAI-compatible endpoint behind a request-logging proxy (COPILOT_PROVIDER_BASE_URL, COPILOT_PROVIDER_TYPE=openai, COPILOT_PROVIDER_WIRE_API=responses)
2. Run copilot -p "..." --allow-all-tools --no-ask-user -s --stream off --autopilot with a prompt that produces a few tool calls, and have the model answer once without calling task_complete so the nudge fires
3. Diff the input arrays across logged requests: on the nudge turn, previously sent function_call.arguments are re-serialized and a rejected call's output is truncated

Expected behavior

Previously sent items are resent byte-for-byte, with only new items appended.