OpenAI cache writes always reported as 0

[dalemartyn]

Description

Since 5.6 OpenAI is charging for cache writes. Opencode is reporting them as 0.

Cache reads work fine, so it feels like the write count isn't being picked up, which only matters now that they have started charging for them.

Plugins

No response

OpenCode version

1.18.3

Steps to reproduce

opencode stats shows read > 0, write = 0

Screenshot and/or share link

Operating System

No response

Terminal

No response

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• DeepSeek provider: prompt_cache_miss_tokens not mapped to cache.write #35974: DeepSeek provider cache write tokens always 0 (same symptom — cache reads work, writes reported as 0)
• fix: cache write tokens always 0 for Anthropic models behind an OpenAI-compatible provider #36749: Cache write tokens always 0 for OpenAI-compatible providers (same root cause discussion, includes a proposed fix)
• OpenAI-compatible providers report zero tokens_cache_read despite upstream returning cached_tokens #34022: OpenAI-compatible providers report zero cache read tokens (related cache accounting issue on the same code path)

[rekram1-node]

are u using openai provider? What provider are u using? Need more info, are u using plugins? Etc etc

[dalemartyn]

Yeah, I'm using the normal openai provider, signed in with a subscription (chat got plus). No plugins, vanilla install.

[johannes-engler-mw]

Independent reproduction, with a reconciliation against the provider's actual billing and a control group that isolates the cause to the missing cache.write count.

Setup: opencode 1.18.5, macOS, opencode-go provider (@ai-sdk/openai-compatible), headless opencode run --format json, no plugins, isolated XDG_* dirs. 63 automated sessions on 2026-07-31 across three models (21 each).

Result for gpt-5.6-luna: summing opencode's self-reported per-step cost gives $0.1459. The provider's own per-model daily billing view shows $0.41 for the same model on the same day — a 2.8× understatement.

Token totals opencode recorded across those 21 luna sessions:

input 414,350 · output 79,877 · reasoning 30,620 · cache.read 3,813,750 · cache.write 0

The gap is $0.2641. At luna's $0.125/M cache-write rate that implies ~2.11M unreported cache-write tokens, against 3.81M recorded reads — a ~1.8:1 read:write ratio, which is what you'd expect from sessions with growing context. (As #36749 puts it: something had to write the prefix that is being read.)

Control group — same gateway, same day, same harness:

model	cache-write rate	opencode self-reported	provider billing UI
gpt-5.6-luna	$0.125/M	$0.1459	$0.41 ✗
deepseek-v4-flash	none	$0.1138	$0.12 ✓
mimo-v2.5	none	$0.0882	$0.11 ✓

This is the part I think is useful for triage: the two models that don't bill for cache writes reconcile with billing to the cent, and only the model that does is wrong. So this isn't general cost drift or a gateway-wide pricing mismatch — the error tracks exactly with whether cache writes are billable, which points at the missing cache.write count rather than at any rate.

Blast radius on this gateway is wider than luna: qwen3.5-plus, qwen3.6-plus, qwen3.7-plus and qwen3.7-max all carry cache-write rates as well, with qwen3.7-max at $3.125/M — above its own $2.50/M input rate.

Related: #39891 reports the same luna discrepancy via the TUI-vs-web comparison, and #36749 has the root cause on this code path (@ai-sdk/openai-compatible hardcodes cacheWrite: void 0, and usage() only reads inputTokenDetails.cacheWriteTokens) along with an offered fix.