Move scratchpad injection to tail of message list for cache efficiency

[chinmaymk]

Summary

Refactored the scratchpad middleware to inject the scratchpad message just before the final user message (in the tail region) instead of after the first user message (in the pinned prefix zone). This optimization keeps the long, stable prefix (system prompt + context files + conversation history) byte-identical across turns, maximizing provider prompt-cache hit rates.

Key Changes

• Injection position: Changed from inserting after the first user message to inserting before the last user message
• Stripping logic: Updated to strip ALL previously injected scratchpad blocks (iterating backward) instead of stopping after the first removal, preventing stale copies from accumulating even when compaction merges messages
• Loop control: Changed break statements to continue in the stripping loop to ensure all scratchpad blocks are removed before re-injection
• Search direction: Reversed the loop that finds the insertion point to search backward from the end of the message list

Implementation Details

• The scratchpad now lives in the "tail region" of the message list, keeping the prefix stable for caching
• All previously injected scratchpad blocks are stripped first (backward iteration ensures splice indices remain valid)
• Updated test expectations to reflect the new injection position (messages[1] instead of messages[2] in most cases, with adjustments for scenarios where new scratchpad is inserted before stripped messages)
• Comments updated to clarify the caching rationale and the comprehensive stripping behavior

https://claude.ai/code/session_011wxfX5yFjds1KXZSzrRYhC