Summary

Introduces a config-driven post-retrieval rerank pipeline that adjusts search ranking beyond raw cosine distance. Four stages, independently toggleable via ~/.mempalace/config.json:

• Weibull time-decay — newer memories surface higher; old ones decay toward a configurable floor
• Keyword overlap boost — lexical overlap between query and hit text nudges distance down
• Importance boost — hits with higher emotional_weight metadata get a small boost
• LLM rerank (optional) — final top-K pass via Anthropic API

All stages are off by default. No config = identical behavior to before. When any stage is enabled, search_memories over-fetches 3× candidates, runs the pipeline after the existing closet-boost + BM25 hybrid rank (both preserved), re-applies max_distance against the post-rerank fused_distance, and flags the response with reranked: True for transparency.

Why

Raw cosine distance treats every memory as if it were written yesterday. For a memory system where identity and recency matter, that's a regression trap — old notes crowd out fresh ones. The pipeline is modular so users can tune the trade-off per palace (e.g., λ=90 days for a work palace, λ=365 for a personal history wing).

What changes

Example config

{
  "rerank": {
    "weibull_decay":   {"enabled": true,  "k": 1.5, "lambda": 90,  "floor": 0.3},
    "keyword_boost":   {"enabled": true,  "weight": 0.30},
    "importance_boost":{"enabled": false, "weight": 0.15},
    "llm_rerank":      {"enabled": false, "model": "claude-haiku-4-5-20251001", "top_k": 10}
  }
}

Test plan

• pytest tests/test_reranker.py tests/test_searcher.py tests/test_layers.py tests/test_config.py tests/test_mcp_server.py — 186 passed
• ruff check . — all checks passed
• ruff format --check on modified files — clean
• Backward compatibility verified: with no rerank key in config, search_memories returns identical output (no reranked flag, no new fields in response)
• mempalace_search MCP tool accepts rerank: false to bypass the pipeline when desired

Notes for reviewers

• The pipeline composes on top of the existing closet-boost + BM25 hybrid rank, rather than replacing either — earlier drafts that bypassed them regressed on closet-rich corpora
• When rerank is enabled but no stages are configured, the pipeline is an identity function and the reranked flag stays false
• No new dependencies required; llm_rerank stage is gated behind a local import of anthropic and skipped if unavailable

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