upstream port: FileReadCache + short-circuit unchanged reads (#3717)

[mabry1985]

Track per-session file-read history; return a file_unchanged placeholder when a file is read again with identical stat metadata. Cuts duplicate context bloat and acts as a correctness guard when a tool re-reads what the model has already seen.

Upstream PR

• feat(core): add FileReadCache and short-circuit unchanged Reads QwenLM/qwen-code#3717 feat(core): add FileReadCache and short-circuit unchanged Reads

Why

Performance + correctness. Our turns regularly re-read the same file inside one conversation; this skips re-emitting identical content. Also reduces the chance the model hallucinates that a file changed when it didn't.

Scope / effort

Medium. Not free — needs a decision on how it interacts with our context-compaction path.

• New file: fileReadCache.ts (~188 lines)
• Modifies: config.ts (cache getter + fileReadCacheDisabled flag), read-file.ts, edit.ts, write-file.ts. We have local edits in read-file.ts and edit.ts.
• Compaction interaction: cache must be cleared on session start; when compaction summarizes prior content, the cache must invalidate or the model will see "file_unchanged" referring to content no longer in its context window.
• Upstream resets the cache in config.startNewSession() — verify our compaction calls into the same lifecycle.

Prerequisites

Decide: ship with fileReadCacheDisabled: true default (safe; gather data) vs ship enabled (perf win, but verify compaction path).

Links

• feat(core): add FileReadCache and short-circuit unchanged Reads QwenLM/qwen-code#3717

[mabry1985]

Closing as already landed on main (verified during the 2026-05 upstream survey): FileReadCache (QwenLM#3717) — packages/core/src/services/fileReadCache.ts is on main. Reopen if the implementation needs the upstream variant.