OpenCode Go endpoint: no prompt caching on DeepSeek (cache_hit always 0 even on repeated identical prompts)

Description

The OpenCode Go subscription endpoint (https://opencode.ai/zen/go/v1/chat/completions) does not perform prompt caching on DeepSeek models. Every request returns prompt_cache_hit_tokens: 0, even when the exact same prompt is sent repeatedly, so the full prompt is re-processed from scratch on each call. This adds significant per-turn latency on real coding sessions, where large instruction files (AGENTS.md/CLAUDE.md/CONVENTIONS.md + memory) are re-sent on every tool loop.

The OpenAI-compatible /v1 API exposes prompt_cache_hit_tokens / prompt_cache_miss_tokens and prompt_tokens_details.cached_tokens in the usage object, but cache is never hit.

Plugins

None (default). Custom provider block in opencode.json pointing at the Go subscription:

"opencode-go": {
  "npm": "@ai-sdk/openai-compatible",
  "options": {
    "baseURL": "https://opencode.ai/zen/go/v1",
    "apiKey": "..."
  },
  "models": {
    "deepseek-v4-flash": {},
    "deepseek-v4-pro": {}
  }
}

OpenCode version

1.18.15

Steps to reproduce

1. Send POST https://opencode.ai/zen/go/v1/chat/completions with model deepseek-v4-flash and a fixed prompt (e.g. "Explain in 2 lines what a PHP email-validation function does").
2. Repeat the exact same request 3 times (a few seconds apart).
3. Inspect usage in each response.

Result (all 3 identical calls):

"usage":{"prompt_tokens":101,"completion_tokens":100,"total_tokens":201,"prompt_cache_hit_tokens":0,"prompt_cache_miss_tokens":101,"prompt_tokens_details":{"cached_tokens":0}}

Expected: on call #2 and #3 the prefix should be cache-hit (DeepSeek's API does support automatic prompt caching, with hits reported via prompt_tokens_details.cached_tokens). Instead cache is never engaged.

Impact: full prompt re-processing on every turn → higher latency and token cost on long coding sessions, and no benefit from DeepSeek's automatic prefix caching.

OS / Terminal

Linux (Kubuntu) / bash + curl