[sentry]

Summary

Fixes an issue where ChatSession.rebuildVisibleBlocks (called on the main actor via completeRunCleanup when a run finishes) hangs the UI for over 3 seconds parsing multi-megabyte file_read tool payloads via ToolEnvelope.successPayload.

Changes

• Behavior change
• UI change
• Refactor / chore
• Tests
• Docs

1. Cap file_read payload parsing: In ContentBlock.knownFileContents, guard result.utf8.count <= knownFileParseByteLimit (1 MB) before invoking ToolEnvelope.successPayload. The index only retains the first 256 KB of text per file anyway, so parsing multi-megabyte JSON payloads on the main actor was wasteful and caused UI hangs.
2. Memoize known-files index per rebuild: In ContentBlock.generateBlocks, build the known-files index lazily at most once per rebuild instead of re-evaluating it for every failed or streaming file edit.
3. Defer known-files evaluation: In FileDiff.inferredEditPath, make knownFiles an @autoclosure so the index is only evaluated when the streaming old_string excerpt is actually long enough (≥24 chars) to disambiguate a path.
4. Unit tests: Added regression tests covering deferral in FileDiffStreamingPreviewTests and size-cap filtering in FileReadImageAndFormatContractTests.

Test Plan

Regression tests were added in FileDiffStreamingPreviewTests (deferral of known-files evaluation) and FileReadImageAndFormatContractTests (size-cap filtering).

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed
• I verified build on macOS with Xcode 16.4+

Fixes APPLE-MACOS-3VR

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