Read-state is lost across auto-compaction → `File has not been read yet` on files already read

Type: bug

Environment

• Claude Code version: 2.1.220
• OS: macOS 26.5.2 (Darwin 25.5.0)
• Platform: Claude subscription
• Model: claude-opus-5 / claude-opus-4-8

What happened

Edit and Write fail with:

<tool_use_error>File has not been read yet. Read it first before writing to it.</tool_use_error>

on files the session had already read — the read simply happened before an auto-compaction. The guard itself is correct and valuable (it has never once let through a write to an unread file in our data). The problem is that its bookkeeping does not survive compaction, so a long session pays a redundant Read for every file it touches after each compact.

Measured in this transcript store: the error appears in 340 of 5 096 transcript files. Over one 26-day window it occurred 155 times across ~2 006 transcripts (0.077/transcript), down from 231 across ~1 084 transcripts (0.213/transcript) in the previous window — so it is improving, but it is still the most common recoverable tool error we see after the one in draft 1.

Expected

Either of:

• Read-state is carried across compaction for files the session has already read and that have not changed on disk (mtime/size check at write time), or
• The guard offers an automatic fallback for small files: re-read transparently instead of erroring, at least below some size threshold.

Repro

1. Start a session in a large project; read a file.
2. Work until auto-compaction fires (or force it).
3. Edit that same file without re-reading it.
4. Observe File has not been read yet even though the file is unchanged and was read in this same session.

Impact

One wasted round trip per file per compaction. In long-running agentic sessions with several compactions this is a steady, invisible tax; it also causes the model to re-read large files it already summarized, which pushes the context back up and makes the next compaction arrive sooner.