fix(changes): bound the Changes panel by files, not by mutations

[L4XB]

Fixes #1043

The problem

The Changes panel shows files. The cap counted mutation evidence records:

if (this.seen.size >= MAX_RECORDS || this.bytes + bytes > MAX_SESSION_BYTES) {

seen holds one toolCallId per admitted write/edit, so every repeat edit of a file already on screen spent one of the 256 slots. A long SDD session that touches a handful of files over and over reaches the warning while the panel lists maybe five entries, which is what the report describes.

The second half is what made it look frozen rather than merely capped. Once the bound fired, a later edit to a file the panel was already showing was dropped too, so that entry sat at its last admitted state. Refusing it bounded nothing: the FileState it would have updated is already held in files.

The fix

MAX_RECORDS becomes MAX_FILES, checked against the number of distinct paths held, and only for a path that is not held yet:

let files = this.files.get(evidence.root);
const previous = files?.get(evidence.path);
if (!previous && this.trackedFiles >= MAX_FILES) {
  this.notice = FILE_LIMIT_NOTICE;
  return false;
}

An edit to a tracked file is now always admitted. What a long session can accumulate is still bounded, by the 4 MiB snapshot budget, which is unchanged.

The two bounds also stop sharing one sentence. The report asks for this and it is right: they have different remedies, and "additional changes are not displayed" told an operator neither which one filled up nor that the session itself was fine.

files:  Session change capture limit reached; 256 files are shown and later files
        are not tracked. Earlier files keep updating.
bytes:  Session change capture limit reached; the snapshot budget is full and
        later changes are not captured.

Tests

tests/session-changes.test.ts, five cells added, and the existing 257-unique-path cell is untouched and still passes, since the bound is the same number for that shape.

cell	what it stops
many edits to few files never reach the cap	the reported defect: 600 edits across 3 files, no notice, and a fourth file still admitted afterwards
a tracked file keeps updating after the cap fires	the staleness half, which a cap alone would leave in place
the notice names which bound fired	both bounds collapsing back into one sentence
a no-op edit to an unseen path does not spend a slot	before === after being charged for a file it never tracked

Mutation results, 6 of 6 caught:

mutation	result
the cap counts records again (the reported defect)	caught
a known file is refused once the cap is reached	caught
the file counter never advances	caught
the counter advances on every record, not only a new file	caught
both bounds share one notice again	caught
the byte budget is not enforced	caught

The fourth is worth naming. Advancing the counter per record rather than per new file passes every other assertion in the file, and only surfaces as a new file being refused after 256 repeat edits, which is the original defect moved one line over. The first version of my own test did not catch it; the extra assertion in the many-edits cell is there because the mutation run said so.

pnpm run typecheck: 200 recorded diagnostics, no regressions. tests/session-changes.test.ts and tests/session-changes-shell.test.ts: 17 pass.

What this does not change

The cap is still 256 and still hard. A session that genuinely touches more than 256 distinct files gets the same refusal, now with a notice that says so and does not imply the run failed. Coalescing without any bound was the other option in the report; I did not take it, because the panel's memory would then follow the session's file count with nothing holding it.

Summary by CodeRabbit

• Bug Fixes

  • Session changes are now limited by the number of distinct files rather than the number of individual edits.
  • Repeated edits to already tracked files continue to be captured, even after the file limit is reached.
  • Capacity notices now clearly identify whether the file-count or snapshot-size limit was reached.

• Tests

  • Added coverage for file-based limits, continued updates, available capacity, and no-op edits.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Advanced

Run ID: 54dde1b3-3b31-4cc7-a959-ef63c8af6dc7

📥 Commits

Reviewing files that changed from the base of the PR and between 0da9bcc and 6ddf459.

📒 Files selected for processing (2)

• lib/session-changes.ts
• tests/session-changes.test.ts

Included review availability: Your plan provides up to 4 included reviews per hour; 2 remain after this review.

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📝 Walkthrough

Walkthrough

Session change capture now limits distinct tracked files instead of mutation records. Repeated updates to tracked files remain admissible. File-count and byte-budget limits emit separate notices, with regression tests covering both limits and no-op edits.

Changes

Session capture limits

Layer / File(s)	Summary
Distinct-file admission logic lib/session-changes.ts	The capture limit now counts admitted file paths. Updates to tracked files bypass the file-count limit. File-count and byte-budget notices use separate messages.
Limit behavior regression coverage tests/session-changes.test.ts	Tests cover repeated edits, capacity for new files, updates after the cap, distinct notices, and unseen no-op edits.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: alan-thegentleman

Merge Risk: ⚪ Minimal · up to 6ddf4

The updated file-limit behavior is covered without a remaining actionable defect.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: the Changes panel is limited by distinct files instead of mutation records.
Linked Issues check	✅ Passed	The changes satisfy the coding requirements in issue #1043. SessionChanges.record() tracks capacity with trackedFiles, so repeated edits to an existing root/path do not consume the 256-file limit.…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to lib/session-changes.ts and tests/session-changes.test.ts. The implementation and tests directly support issue #1043 by changing file-capacity behavior and valid…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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