Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add ychampion/cskill-agents --skill cache-prefix-stability-for-subagentsgit clone --depth 1 https://github.com/ychampion/cskill-agentsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/ychampion/cskill-agents/cache-prefix-stability-for-subagents)<a href="https://agentmods.dev/skills/ychampion/cskill-agents/cache-prefix-stability-for-subagents"><img src="https://agentmods.dev/badge/skills/ychampion/cskill-agents/cache-prefix-stability-for-subagents.svg" alt="Measured on agentmods" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00028 | $0.00408 |
| Opus 5 | $0.00014 | $0.00204 |
| Sonnet 5 | $0.00006 | $0.00082 |
| Haiku 4.5 | $0.00003 | $0.00041 |
Grade A, and why
cache-prefix-stability-for-subagents scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
What it actually says
SKILL: Cache Prefix Stability For Subagents
Domain: agentic-loop Trigger: Apply when a subagent should share the parent's cached prompt prefix instead of recomputing a divergent request shape. Source Pattern: Distilled from reviewed subagent orchestration, isolation, and lifecycle implementations.
Core Method
Treat prompt-cache compatibility as an explicit contract when building child agent options. Preserve the cache-critical fields that define the API request prefix, including the merged tool pool, thinking configuration, query source, and content-replacement state, instead of letting child defaults drift away from the parent request. This keeps forked or exact-tool subagents eligible for the same cached prefix while still allowing non-cache-sensitive options to vary.
Key Rules
- Only inherit cache-critical request fields when the child is intentionally sharing the parent's prefix, such as the use exact tools path.
- Keep the resolved tool list, thinking configuration, and content-replacement state aligned with the parent request so cache keys do not diverge accidentally.
- Preserve the recursive-fork guard inputs like query source when they are part of the cached request prefix contract.
- Do not inherit unrelated defaults blindly; share only the fields that stabilize the prefix and let other child-specific behavior vary intentionally.
Example Application
If a forked review agent must reuse the parent’s prompt cache, build its options from the parent’s tool list and thinking config instead of resetting them to child defaults; the child can then reuse cached request prefixes while still emitting its own downstream messages.
Anti-Patterns (What NOT to do)
- Do not disable thinking or rebuild the tool pool for a cache-sharing child unless you also accept losing cache alignment.
- Do not rely on accidental structural similarity between parent and child requests; explicitly carry forward the fields that define the cache prefix.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 8d ago First seen · 28 lines · 28 tokens per session scan A 069733d0ce11
cache-prefix-stability-for-subagents is a skill published in the GitHub repository ychampion/cskill-agents (36 stars, last pushed 5mo ago), licensed MIT. It adds 28 tokens to every session and 408 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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