Bug: Copilot CLI 1.0.82 BYOK silently disables prompt caching (~5x cost)

Summary

Copilot CLI 1.0.82 in BYOK mode sends chat requests that carry no prompt-cache declaration, so prompt caching is effectively disabled for the whole session. Every turn re-sends the entire growing context at full price — provider usage shows cached_tokens=0 and cache_write/cache_creation_tokens=0 on every request across the session, even though the prefix is stable.

Net effect: ~5x cost for normal multi-turn BYOK sessions.

Setup

• Copilot CLI 1.0.82, BYOK mode (custom OpenAI-compatible /chat/completions endpoint serving an Anthropic model, claude-sonnet-5)
• Compare baseline: same machine, same endpoint/model, 1.0.80 BYOK → prompt caching works (~97% hit)
• Also tested 1.0.82 in GitHub-subscription mode on the same machine → caching works, so the build is cache-capable

Expected

Once a conversation prefix stabilizes, the provider should report cached_tokens > 0 and small cache_creation_tokens for the new tail (that's what 1.0.80 BYOK and subscription mode report).

Actual

Every request reports zero cache metrics:

"prompt_tokens_details": {
  "text_tokens": 146022,
  "cached_tokens": 0,
  "cache_write_tokens": 0,
  "cache_creation_tokens": 0,
  "cache_creation_token_details": { "ephemeral_5m_input_tokens": 0 }
},
"cache_read_input_tokens": 0

Over a 150-request session, text_tokens climbed monotonically 118,966 → 285,085 while cached_tokens and cache_write/creation_tokens stayed 0 every time. The zero cache-write/creation is the strong signal: even a cold miss should write the prefix so the next turn can hit; a persistent 0 write means the request never asks for a cache at all.

Evidence

With normal caching, 30.1M input ≈ $11.4; at 0% it costs ≈ $60.2 → ~$49 (~5x) wasted in a single session.

Root-cause hypothesis

The 1.0.82 BYOK request builder omits the prompt-cache declaration (cache_control / ephemeral-cache policy) that subscription mode injects. Because BYOK calls the user's endpoint directly, the missing declaration is only visible to BYOK users.

Severity

High — silent runaway cost on BYOK setups; no CLI error, only a much larger bill. Requires monitoring provider cache hit-rate to detect.

Suggested fix

Make the BYOK path send the same prompt-cache declaration used by subscription mode.

Repro note

I cannot reproduce right now (downgraded to 1.0.80 where caching works). Reproducing requires running 1.0.82 in BYOK against an Anthropic-model endpoint and reading provider cache accounting. Happy to help test a newer build backported if there's a fix candidate.

Xác định BYOK request builder và so sánh cách xây dựng request của nó với subscription path và hành vi của 1.0.80 được mô tả trong issue. Kiểm tra xem prompt-cache declaration có tồn tại hay không, sau đó thực hiện một phiên BYOK multi-turn với một Anthropic-model endpoint và xác minh rằng các metric cache-write và cache-read không còn bằng 0.