prompt-cache-skills

Most popular OSS agent harnesses (Cline, Roo Code, Continue, OpenCode,
Aider) leave 30-90% off your API bill on the table because their
prompt-caching code is subtly wrong, off-by-default, or just missing
for some providers.

This repo is a set of drop-in skills that any AI coding agent
(Claude Code, Codex, Cline, Cursor, Devin, Gemini CLI, OpenCode…) can
read and apply on its own.

You don't read the diffs. You point your agent at this repo and say:

"Apply every skill in this repo that matches the harnesses I use."

The agent reads each SKILL.md, checks if it applies to your setup,
lands the diff, and verifies the fix on the wire. You go from broken
or partial caching to 80-99% cache hit rates without doing the
research yourself.

What you actually save

One row per completed audit, so the coverage matches the scorecard:

13 skills total cover the verified patchable OSS bugs. See
skills/README.md for the full index.

How to use it

Option A — point any AI coding agent at this repo

In your agent of choice (Claude Code, Codex, Cline, Cursor, Devin, etc.):

Read https://github.com/OnlyTerp/prompt-cache-skills

Apply every skill in skills/ that matches the harnesses I currently
use. For each one:
1. Confirm the target file exists in my project at the cited path.
2. Apply the diff.
3. Run the SKILL's Verify steps and confirm the assertion passes.
4. If verify fails, revert and tell me why.

That's it. The agent picks up the rest from each SKILL.md's
machine-readable frontmatter and instructions.

Option B — install as a skill bundle in Claude Code / Devin / etc.

If you use one of the agents that supports a skills directory:

# Claude Code
git clone https://github.com/OnlyTerp/prompt-cache-skills ~/.claude/skills/prompt-cache-skills

# Devin
git clone https://github.com/OnlyTerp/prompt-cache-skills ~/.config/devin/skills/prompt-cache-skills

# OpenCode
git clone https://github.com/OnlyTerp/prompt-cache-skills ~/.config/opencode/skills/prompt-cache-skills

Then ask your agent:

Run the prompt-cache-skills bundle on this codebase.

Option C — read and apply by hand

Each skills/<name>/SKILL.md is a complete fix: target,
symptom, diff, verification. Apply the relevant ones manually if you
don't trust your agent to do it.

What's in here

prompt-cache-skills/
├── skills/                       ← the fixes (this is what your agent reads)
│   ├── cline-fix-volatile-msg/
│   ├── cline-openai-cache-key/
│   ├── cline-pin-timestamp/
│   ├── roo-fix-volatile-msg/
│   ├── roo-bedrock-custom-arn/
│   ├── continue-fix-volatile-msg/
│   ├── continue-enable-defaults/
│   ├── continue-gemini-explicit/
│   ├── opencode-detect-openai-compat/
│   ├── opencode-bedrock-doc-blocks/
│   ├── opencode-mistral-cache-key/
│   ├── aider-1h-ttl/
│   └── aider-cache-default-on/
├── audits/                       ← evidence: completed audits + queued stubs
│   ├── cline.md
│   ├── roo-code.md
│   ├── aider.md
│   ├── opencode.md
│   ├── continue.md
│   ├── codex-cli.md              ← (reference, already correct)
│   ├── claude-code.md
│   ├── hermes-nous.md
│   ├── codex-desktop.md
│   ├── devin-cli.md
│   ├── windsurf-cascade.md
│   ├── antigravity.md
│   ├── grok-cli.md
│   └── queued stubs: crush, goose, aichat, gptme, avante-nvim, kilo-code
├── docs/                         ← the underlying API mechanics
│   ├── concepts/                 ← per-provider caching reference
│   ├── gotchas.md                ← 16 numbered footguns
│   ├── verification.md           ← how to confirm caching on wire
│   └── scorecard.md              ← completed audits graded at a glance
├── tools/                        ← scripts to verify caching + doc consistency
│   ├── check_cache.py            ← fire request twice, dump cache_* fields
│   ├── check_docs_consistency.py ← assert counts/tables/links don't drift
│   ├── audit_harness.sh
│   └── replay_harness.md
└── AGENTS.md                     ← entry point for AI agents reading this repo

Why this exists

If your agent harness sends 30,000 tokens of system prompt + tools per
turn, on Claude 4.7 Opus that's $0.15 per turn uncached vs $0.015
cached — a 10x difference. A 50-turn coding session costs $7.50 vs
$0.75. You're paying 10x what you should be because the harness
you use either:

• doesn't set cache_control at all,
• sets it on volatile content that thrashes the cache,
• doesn't set prompt_cache_key for OpenAI,
• has caching gated behind a config flag you never set, or
• just doesn't implement it for one of your providers.

None of these are hard to fix. They're all 5-15 line diffs. The
hard part is knowing which one applies to your harness and getting it
right. This repo does that work for you.

The grade card

13 completed harness audits, dated 2026-05-27. The original 7 include the default Claude Desktop Code baseline, source-recon audits for Codex CLI, Aider, OpenCode, Roo Code, Cline, and Continue, plus extended source/wire/local-install audits for Hermes/Nous, Codex Desktop, Devin CLI, Windsurf/Cascade, Antigravity, and Grok CLI. Six more files in audits/ are queued stubs, not completed audits.

* RE-backed or inferred from captured/companion wire shape where public source is unavailable; see the linked audit for caveats.

Full per-provider breakdown with file:line citations in
docs/scorecard.md.

Headline findings

1. The "last 2 user messages" pattern is a copy-paste bug that
   propagated Cline → Roo → Continue. All three burn a breakpoint on
   the volatile current turn. Same one-line fix in each.
2. Cline OpenAI native is silently broken — no prompt_cache_key,
   no prefix-stability work. Users on Cline+OpenAI pay full price.
3. Gemini explicit caching is universally unimplemented. Only
   implicit (best-effort, free) caching engages, even on long
   sessions with massive stable system prompts where explicit gives
   a guaranteed 75% discount.
4. Codex CLI is the reference for OpenAI-side caching — thread_id
   as cache key, preserved across compaction and into sub-agents.
5. OpenCode's system-prompt split is the best Anthropic pattern.
6. Hermes / Nous has real multi-provider cache plumbing — source
   covers Anthropic/OpenRouter/Nous/Qwen and xAI wire capture showed
   prompt_cache_key, x-grok-conv-id, and non-zero cached tokens.
7. Closed managed surfaces need transport-aware capture. Devin,
   Windsurf, Antigravity, and Grok CLI are marked unverified where the
   model channel was protobuf, desktop-only, or proxy-invisible.

Trust but verify

Every skill ships with a Verify section that captures the wire and
confirms the fix landed. Don't take our word for it — the
tools/check_cache.py script fires any
request body twice (cold + warm) and prints the diff of cache_*
token fields.

Run it before and after applying a skill. You should see
cache_read_input_tokens (Anthropic) or cached_tokens (OpenAI) or
cachedContentTokenCount (Gemini) go from 0 to most of your input.

Contributing

We accept new skills, new harness audits, and corrections. See
CONTRIBUTING.md. The bar is: a captured request
body + a verified hit-rate change. We don't take vibe submissions.

By participating you agree to our Code of Conduct.

Security

Found something that looks like a credential leak path, a request
construction bug that leaks user secrets, or any other security
issue? See SECURITY.md for the disclosure process.
Don't open a public issue.

Changelog

Releases tracked in CHANGELOG.md.

License

Skills and audit prose: CC-BY-4.0. Code (tools/): MIT.

If this saved you money, star the repo and share it.
_{The whole point is that everyone gets caching working at once.}