Hashed Edit

Hashed Edit

Purpose

Catch stale-line edit failures BEFORE they corrupt a file. A stale-line edit happens when an agent reads a file, plans an edit, then submits the edit after another tool call has rewritten the line — the edit applies to the wrong content.

Adapted from oh-my-openagent's hashline_edit pattern (which reports reducing stale-line edit failures from ~93% to 32% on Grok Code).

The mechanism: every line read through read-with-hashes.sh is tagged with a content-hash anchor (<lineno>#<hash>|<content>). The edit companion edit-by-hash.sh applies the edit IFF the current line's hash matches the anchor. Mismatch → refuse + diff.

Scope

Two operations, no third:

• Read with hashes — produce tagged output an agent can refer back to
• Edit by hash — apply a single-line replacement (or deletion) gated by hash match

Multi-line edits and structural transforms (move blocks, refactor across files) are out of scope. Use the standard Edit tool for those, or break them into per-line hashed edits where the staleness risk is real.

When to use

• Long sessions on large files. A 2000-line file read at minute 5 may have changed by minute 35. Hash-anchored edits catch the staleness.
• Multi-agent dispatch on shared files. When multiple specialists may touch the same file, hash anchors make race-condition edits fail loud instead of silently corrupting.
• Models that struggle with line numbers. Some models drift between the line they think they're editing and the line they actually reference. Hash anchors catch the drift.
• Risk-asymmetric edits. Editing migrations, secrets-adjacent config, or anything where a wrong-line edit has compounding cost.

When NOT to use

• Single-line edits on a small file in a single agent's session — the standard Edit tool's exact-string-match is enough.
• Files the agent owns end-to-end with no concurrent writers — no staleness risk.
• Operations that don't fit the line-replace shape (full rewrites, block moves, format conversions).

Automated pass

Two scripts under scripts/:

• read-with-hashes.sh <file> [--start N] [--end N] — reads the file (or a line range) and outputs each line in <lineno>#<hash>|<content> form. The hash is a 4-char hex digest (cksum % 65536) over the line content; ~65k buckets is more than enough collision space for a single file.
• edit-by-hash.sh <file> <lineno>#<hash> <new-content> [--delete] — applies the edit IFF the current line's hash matches the anchor. Mismatch produces a stderr diff and exit code 5.

The agent's contract:

1. Read the target region with read-with-hashes.sh.
2. Decide the edit. Reference the line by its anchor (42#a3f1), not its line number alone.
3. Apply via edit-by-hash.sh.
4. On exit code 5 (hash mismatch), re-read the region and retry. Don't paper over the mismatch — the staleness is real.

Manual pass

The lead does the verification the runtime would otherwise do:

1. After a successful edit, re-read the surrounding region. The anchor matched, but read-with-hashes of the post-edit region is the audit trail proving the change is what was intended.
2. On exit code 5, treat the mismatch as signal, not noise. If the file changed since the agent's read, the agent's reasoning was based on stale content. Re-read, re-reason, retry — don't blindly force-apply the edit by re-computing the hash on stale assumptions.
3. Log mismatches. A single mismatch is a race-condition warning; a pattern of mismatches against the same file means concurrent writers and the workflow needs sequencing, not retries.

Known gotchas

• Line endings matter. cksum hashes byte-for-byte. A line that's foo\r\n and one that's foo\n produce different hashes. If your files mix line endings, normalize before hashing or expect the hash check to flag the difference.
• Trailing whitespace matters. cksum includes trailing spaces. A line with a trailing space hashes differently from one without. The hash mismatch you see may be a whitespace edit, not a content edit.
• Empty lines hash deterministically. All empty lines produce the same hash (0000 from cksum of empty input is actually ffff in cksum's representation, but consistent — empty hashes match each other). For edits to empty lines, line number alone disambiguates.
• No multi-line replacement. edit-by-hash.sh replaces ONE line. For multi-line replacement, delete N lines and re-insert via N separate edit calls, or use the standard Edit tool with full context.
• Files without trailing newline. Edge case handled (the script falls back to a tail-based read for the last line) but verify on files where the final-line-no-newline matters semantically.

Future enforcement (v0.3+)

This release ships the helper scripts as opt-in tools. The next step is a PreToolUse hook that intercepts every Edit / MultiEdit call, detects when the agent's old_string doesn't match the file's current content (i.e., the file changed since the agent's last Read), and refuses the edit with a diff. That makes the staleness check the default, not the opt-in.

The hook is deferred to v0.3 for design reasons, not Phase 0.5 reasons:

• The Edit tool's stdin payload shape is already known (hi_old_string, hi_new_string, hi_file_path in lib/hooks/lib/hook-input.sh).
• The interesting design question is what counts as stale — Edit's exact-string-match semantics already catch some cases; the hook should only fire when the agent's old_string matches but represents content that was true at a previous Read.
• Auto-enforcement changes the agent contract; the helper-script shape lets us iterate on the format before locking it.