🧠 pi-readcache

A pi extension that overrides the built-in read tool with hash-based, replay-aware caching.

It reduces token usage and context bloat from repeated file reads while preserving correctness as session state evolves.

Correctness is maintained across:

• range reads (path:START-END)
• tree navigation (/tree)
• compaction boundaries
• restart/resume replay

What you get

pi-readcache runs automatically in the background by overriding read and managing replay trust for you. Refresh/invalidation can also be triggered by the model itself via the readcache_refresh tool when it decides a fresh baseline is needed. You can still run /readcache-refresh manually for explicit control.

When the extension is active, read may return:

• full content (mode: full)
• unchanged marker (mode: unchanged)
• unchanged range marker (mode: unchanged_range)
• unified diff for full-file reads (mode: diff)
• baseline fallback (mode: baseline_fallback)

Plus:

• /readcache-status to inspect replay/coverage/savings
• /readcache-refresh <path> [start-end] to invalidate trust for next read
• readcache_refresh tool (same semantics as command)

Install

Preferred (npm):

pi install npm:pi-readcache

Alternative (git):

pi install git:https://github.com/Gurpartap/pi-readcache

After installation, you can use pi normally. If pi is already running when you install or update, run /reload in that session.

Day-to-day usage

Action	Command	Expected result
Baseline read	read src/foo.ts	mode: full or mode: baseline_fallback
Repeat read (no file change)	read src/foo.ts	[readcache: unchanged, ...]
Range read	read src/foo.ts:1-120	mode: full, baseline_fallback, or unchanged_range
Inspect replay/cache state	/readcache-status	tracked scopes, replay window, mode counts, estimated savings
Invalidate full scope	/readcache-refresh src/foo.ts	next full read re-anchors
Invalidate range scope	/readcache-refresh src/foo.ts 1-120	next range read re-anchors

Important behavior notes

• Sensitive-path bypass: readcache does not cache/diff these patterns and falls back to baseline read output: .env*, *.pem, *.key, *.p12, *.pfx, *.crt, *.cer, *.der, *.pk8, id_rsa, id_ed25519, .npmrc, .netrc.
• Compaction is a strict replay barrier for trust reconstruction:
  • replay starts at the latest active compaction + 1.
  • pre-compaction trust is not used after that barrier.
• First read after that barrier for a path/scope will re-anchor with baseline (full/baseline_fallback).
• For exact current file text, the assistant should still perform an actual read in current context.

Diff usefulness gate (why you sometimes get full content instead of a diff)

For full-file reads, mode: diff is emitted only when the generated patch is clearly more useful than baseline text.

Current defaults:

• MAX_DIFF_TO_BASE_RATIO = 0.9
  • if diffBytes >= selectedBytes * 0.9, diff is considered not useful and falls back to baseline (mode: baseline_fallback)
• MAX_DIFF_TO_BASE_LINE_RATIO = 0.85
  • if patch line count is greater than selectedRequestedLines * 0.85, diff is considered too large/noisy and falls back to baseline

Why these defaults:

• avoids near-full-file patch spam on high-churn edits
• improves readability for the model versus very large hunks
• keeps token savings meaningful when diff mode is used

---

For extension developers (and curious cats)

Design docs

• IMPLEMENTATION_SPEC.md
• IMPLEMENTATION_PLAN.md
• EVOLUTION_PLAN_1.md
• EVOLUTION_PLAN_2.md

High-level architecture

flowchart TD
  A[LLM calls read] --> B[read override tool]
  B --> C[Run baseline built-in read]
  B --> D[Load current file bytes/text + sha256]
  B --> E[Rebuild replay knowledge for active leaf]
  E --> F{base trust exists?}
  F -- no --> G[mode=full, attach metadata]
  F -- yes + same hash --> H[mode=unchanged/unchanged_range]
  F -- yes + full scope + useful diff --> I[mode=diff]
  F -- otherwise --> J[mode=baseline_fallback]
  G --> K[persist object + overlay trust]
  H --> K
  I --> K
  J --> K
  K --> L[tool result + readcache metadata]

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Runtime model

• Trust key: (pathKey, scopeKey) where scope is:
  • full
  • r:<start>:<end>
• Trust value: { hash, seq }
• Replay source:
  • prior read tool result metadata (details.readcache)
  • custom invalidation entries (customType: "pi-readcache")
• Overlay:
  • in-memory, per (sessionId, leafId), high seq namespace for same-turn freshness

Compaction/tree semantics

flowchart LR
  R[root] --> C1[compaction #1]
  C1 --> N1[reads...]
  N1 --> C2[compaction #2]
  C2 --> L[active leaf]

  B[replay start] --> S[latest compaction + 1 on active path]

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Rules:

• replay boundary = latest compaction on active branch path + 1
• if no compaction on path, replay starts at root
• tree/fork/switch/compact/shutdown clear in-memory memo/overlay caches

File map

• index.ts - extension entrypoint + lifecycle reset hooks
• src/tool.ts - read override decision engine
• src/replay.ts - replay reconstruction, trust transitions, overlay
• src/meta.ts - metadata/invalidation validators and extractors
• src/commands.ts - /readcache-status, /readcache-refresh, readcache_refresh
• src/object-store.ts - content-addressed storage (.pi/readcache/objects)
• src/diff.ts - unified diff creation + usefulness gating
• src/path.ts - path/range parsing and normalization
• src/telemetry.ts - replay window/mode/savings reporting

Tool-override compatibility contract

Because this overrides built-in read, it must preserve:

• same tool name + parameters (path, offset?, limit?)
• baseline-compatible content shapes (including image passthrough)
• truncation behavior and details.truncation compatibility

Development

npm install
npm run typecheck
npm test

Targeted suites:

npm test -- test/unit/replay.test.ts
npm test -- test/integration/compaction-boundary.test.ts
npm test -- test/integration/tree-navigation.test.ts
npm test -- test/integration/selective-range.test.ts
npm test -- test/integration/refresh-invalidation.test.ts
npm test -- test/integration/restart-resume.test.ts

License

MIT © 2026 Gurpartap Singh (https://x.com/Gurpartap)