Cache Identical Fork Prefix Construction

SKILL: Cache Identical Fork Prefix Construction

Domain: caching Trigger: Use this when a forked agent must reuse the parent prompt cache and needs byte-identical prefixes for each invocation. Source Pattern: Distilled from reviewed subagent orchestration, isolation, and lifecycle implementations.

Core Method

Duplicate the parent assistant message wholesale, then generate a single user message containing standardized toolresult placeholders (all with the same text) followed by the directive text. Keeping every byte of the assistant message and placeholder deterministic ensures that identical tooluse sequences map to the same cache key. Build a child-specific directive message by wrapping it in `` and appending fork directive prefix before the per-worker instructions.

Key Rules

• Never mutate the cached assistant message; copy it to a new object and only append the uniform tool_result blocks and directive text afterward.
• Use identical placeholder text (Fork started — processing in background) for every tool_result so the child prefix byte stream stays constant across runs.
• Append the directive text as the final block after the placeholders, ensuring fork children encode their own instructions without altering earlier bytes.
• Provide the directive in the same build child message helper so the cached prefix always ends with the same sentinel.

Example Application

When the model triggers a fork from an inline agent, reuse this method to build the fork prompt so the fork shares the cache of the parent — future identical tasks reuse the same cached prefix and avoid redundant API calls.

Anti-Patterns (What NOT to do)

• Do not recompute the assistant message or tool_result text differently per turn; any divergence breaks the cache key.
• Avoid embedding random IDs or timestamps inside the prefix; keep only stable verbatim text before the directive block.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: ychampion
• Source: ychampion/cskill-agents
• License: MIT
• Homepage: https://x.com/YashasGunderia/status/2039990540016693692?s=20

Install and usage instructions live in the source repository linked above.