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Story Crater Bot 1991291bc9 feat: add cache-aligned prompt builder for LLM API cost savings
PROBLEM:
- PromptBuilder.build() puts everything in a single user message
- System + query + memory + chunk all change together
- LLM prompt caching gets 0% hits (entire message differs per call)
- For a 50-chunk ingestion run, we pay full input price 50 times

SOLUTION: PromptBuilder.build_cache_aligned()
- Splits prompt into 3 separate messages:
  1. SYSTEM: instructions (stable across ALL calls) → CACHED
  2. USER[0]: query/problem (stable per run) → CACHED
  3. USER[1]: memory + chunk (varies per call) → not cached
- Cache prefix (system + query) reused across all chunks in a run
- Estimated 30-70% cache hit ratio depending on chunk sizes
- ~50% input token cost savings for multi-chunk ingestion

TEMPLATES:
- templates/gru-mem-system.txt (instructions only, 840B)
- templates/gru-mem-query.txt (problem wrapper, 29B)
- templates/gru-mem-turn.txt (memory + section, 57B)
- templates/gru-mem.txt (legacy, unchanged)

API:
- PromptBuilder::build() — legacy, backward compatible
- PromptBuilder::build_cache_aligned() → PromptMessages
- PromptMessages.cache_prefix_tokens() — cacheable token count
- PromptMessages.total_tokens() — total estimated tokens
- PromptMessages.headroom() — tokens available for response

TESTS: 11 unit + 3 integration = 14 new tests
- test_cache_aligned_produces_two_user_messages
- test_cache_prefix_is_stable_across_chunks
- test_cache_prefix_is_stable_across_memory_changes
- test_cache_prefix_tokens_positive
- test_headroom_positive_under_budget
- test_legacy_build_still_works
- test_cache_aligned_contains_query
- test_cache_aligned_memory/chunk_budget_exceeded
- a8_cache_prefix_stable_across_50_chunks
- a9_cache_aligned_headroom
- a10_cache_savings_estimate

TOTAL: 64 mem-core tests passing (52 unit + 12 integration)
2026-08-28 08:24:38 -07:00

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You are a memory gate for a recurrent update system. You evaluate whether new evidence sections contain information relevant to a standing problem, and update the running memory accordingly.
Rules:
1. Retain all relevant details from the previous memory while adding new useful information.
2. Judge whether you have collected enough information to answer the problem.
3. Reason about the new section between <think> and </think>.
4. If the section contains useful information: output <check>yes</check>, then update memory between <update> and </update>.
5. If the section does NOT contain useful information: output <check>no</check>, then keep previous memory unchanged between <update> and </update>.
6. If more information is needed: return <next>continue</next>.
7. ONLY when enough information is collected: return <next>end</next>.