provider: v2 always sends prompt_cache_key to OpenAI-compatible providers; setCacheKey option ignored

Summary

opencode v2 (beta) unconditionally sends prompt_cache_key in Chat Completions requests to all OpenAI-compatible providers. Gateways that strictly validate request fields (e.g. third-party relay/relay-style APIs) reject the request with HTTP 400 (未知请求字段：prompt_cache_key / unknown request field), making every session fail. Additionally, the documented setCacheKey provider option ("Enable promptCacheKey for this provider (default false)") has no runtime effect in v2 — it appears only in the config schema, so there is no way to opt out.

opencode 1.x does not have this problem: it only sets promptCacheKey for specific provider packages (@ai-sdk/deepinfra, @ai-sdk/cerebras, @ai-sdk/openai, @ai-sdk/azure, @ai-sdk/xai, @ai-sdk/mistral, venice-ai-sdk-provider), or when setCacheKey: true is explicitly set. Providers using @ai-sdk/openai-compatible are not affected.

Environment

• opencode version: opencode2 v0.0.0-beta-18230 (broken) vs opencode 1.18.23 (works)
• OS: Linux 6.18.33.2-microsoft-standard-WSL2 (x64)
• Terminal: tmux (TERM=xterm-256color)
• Shell: zsh
• Install/channel: npm @opencode-ai/cli (beta)
• Active plugins: opencode-workspace-scope@0.1.1

Reproduction

1. Configure a custom provider in ~/.config/opencode/opencode.jsonc using "npm": "@ai-sdk/openai-compatible" pointing at an OpenAI-compatible gateway that strictly validates fields (in my case a third-party relay service).
2. Set "setCacheKey": false in the provider's options.
3. Start any session with one of its models.

The gateway receives a request body containing "prompt_cache_key": "<session-id>" and responds:

HTTP 400
{"code":"UNKNOWN_FIELD","message":"未知请求字段：prompt_cache_key","data":{"field":"prompt_cache_key"}}

Minimal curl proof that the field itself triggers the rejection against this gateway:

# rejected
curl https://<gateway>/v1/chat/completions -d '{"model":"...","messages":[{"role":"user","content":"hi"}],"max_tokens":5,"prompt_cache_key":"test"}'
# → {"code":"UNKNOWN_FIELD","message":"未知请求字段：prompt_cache_key"}

# works
curl https://<gateway>/v1/chat/completions -d '{"model":"...","messages":[{"role":"user","content":"hi"}],"max_tokens":5}'
# → normal completion

Source analysis on the v2 branch:

1. The key is attached unconditionally to every session request — packages/core/src/session/model-request.ts:331:

   // TODO: Persist cache lineage so nested forks reuse the root session's cache key.
   promptCacheKey: promptCacheKey(session.fork?.sessionID ?? session.id),
   

2. Every OpenAI-chat-compatible route (including custom openai-compatible providers) lowers it to the wire field — packages/ai/src/protocols/openai-chat.ts:702:

   ...(cacheKey ? { prompt_cache_key: cacheKey } : {}),
   

3. The only opt-out is request.cache === "none" — packages/ai/src/protocols/shared.ts:32. Nothing in core ever sets it; only tests do. Notably openai-chat.ts already special-cases non-standard providers for store ("Non-standard providers omit store entirely", line ~701) but not for prompt_cache_key.
4. setCacheKey has zero consumers on the v2 branch: it appears only in the v1 compat schema (packages/core/src/v1/config/provider.ts:93) and in docs (config.mdx). The per-provider gating logic that v1 had was lost during the request-construction refactors (#42680, #43958).

Expected Behavior

1. setCacheKey: false in provider options should suppress prompt_cache_key (the option is still documented in the schema), or be removed from the schema if intentionally dropped. Minimal fix: honor the provider option at model-request.ts:331 by omitting promptCacheKey (or mapping to cache: "none").
2. Ideally, prompt_cache_key should not be sent to generic openai-compatible providers by default (matching v1 behavior), since it is an OpenAI-specific extension that many compatible gateways reject.

Actual Behavior

Every request to the provider fails with HTTP 400 UNKNOWN_FIELD: prompt_cache_key. No configuration can disable it in v2, so affected providers are completely unusable.

Log excerpt:

level=ERROR message="Failed to drain Session"
cause="AI.Error: RequestExecutor.execute: Provider request failed with HTTP 400: 未知请求字段：prompt_cache_key"

Additional Context

• Frequency: 100% reproducible with the affected provider on every v2 release I tested; 0% on opencode 1.18.23 with identical config.
• Current workaround: none within v2 (config option is inert). Rolling back to opencode 1.x restores access.
• Related issues: #25984 (setCacheKey sends the wrong cache format for openai-compatible Bedrock proxies), #42246 (V2 drops prompt cache affinity for OpenRouter/OpenAI Chat) — together these suggest the v2 request-construction rewrite lost v1's per-provider cache-key gating in both directions. Same failure class has been reported against other gateways, e.g. strict relays rejecting prompt_cache_key with HTTP 400.
• Suggestion for other gateway vendors would be to ignore unknown fields like the official OpenAI API does, but opencode should honor its own documented opt-out regardless.