[Error] unhandlederror-PerfSampleError: by <<renderer>> in io#workbench.desktop.main.js:414:15968

[vs-code-engineering]

Error Message

unhandlederror-PerfSampleError: by <<renderer>> in io#workbench.desktop.main.js:414:15968

Stack Trace

    at equals (./src/vs/base/common/objects.ts:112:16)
    at equals (./src/vs/base/common/objects.ts:112:16)
    at equals (./src/vs/base/common/objects.ts:112:16)
    at equals (./src/vs/base/common/objects.ts:112:16)
    at equals (./src/vs/base/common/objects.ts:112:16)
    at _diff (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:422:9)
    at _diffObject (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:453:9)
    at _diffArray (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:493:9)
    at _diff (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:422:9)
    at _diffObject (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:453:9)
    at _diff (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:422:9)
    at write (./src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts:329:1)
    at writeSession (./src/vs/workbench/contrib/chat/common/model/chatSessionStore.ts:348:0)
    at session (./src/vs/workbench/contrib/chat/common/model/chatSessionStore.ts:193:0)

Raw Stack Trace (minified)

 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at io (out/vs/workbench/workbench.desktop.main.js:414:15968) 	 at _diff (out/vs/workbench/workbench.desktop.main.js:2995:549) 	 at _diffObject (out/vs/workbench/workbench.desktop.main.js:2995:973) 	 at _diffArray (out/vs/workbench/workbench.desktop.main.js:2995:1246) 	 at _diff (out/vs/workbench/workbench.desktop.main.js:2995:549) 	 at _diffObject (out/vs/workbench/workbench.desktop.main.js:2995:973) 	 at _diff (out/vs/workbench/workbench.desktop.main.js:2995:549) 	 at write (out/vs/workbench/workbench.desktop.main.js:2993:675) 	 at writeSession (out/vs/workbench/workbench.desktop.main.js:2995:12047) 	 at (anonymous) (out/vs/workbench/workbench.desktop.main.js:2995:9194) 	 at (anonymous) (out/vs/workbench/workbench.desktop.main.js:2995:9156) 	 at (anonymous) (out/vs/workbench/workbench.desktop.main.js:406:73807)

Details

Property	Value
Bucket ID	ab9758cd-8751-eba8-d4af-e619493d586d
Version	1.118.1
Commit	034f571d
Last Seen	Mon May 04 2026 23:57:32 GMT+0000 (Coordinated Universal Time)
Platforms	Windows, Mac, Linux
Product	VSCode

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This issue was automatically created from the VS Code Errors Dashboard

[vs-code-engineering]

Stable Release Anomaly Details

Property	Value
Anomaly type	spike
Spike factor	33.25x increase
Previous stable	1.117.0
Current stable	1.118.1
Last release without this error	1.118.0-insider (present but at low volume)
Commit range	Error present in insider builds — spike correlates with increased stable user base using chat sessions

Analysis

This is a performance regression in objectMutationLog.ts — recursive equals() calls in _diff/_diffObject/_diffArray during writeSession in chatSessionStore.ts. The deep object equality checks become expensive with large chat session objects.

Key call path: equals (objects.ts) → _diff/_diffObject/_diffArray (objectMutationLog.ts) → write (objectMutationLog.ts) → writeSession (chatSessionStore.ts)

The error was present at very low volume in 1.117.0 (8 users) but spiked to 266 users in 1.118.1 (33x increase), likely due to increased chat session complexity or usage patterns.

Generated by errors-stable-scan · ● 7.4M · ◷

[vs-code-engineering]

🔍 Error Investigation

Summary

The PerfSampleError is a stack overflow occurring during chat session persistence. The ObjectMutationLog._diff() method recursively compares previous and current session state using objects.equals(), which itself recurses without depth limits. When chat sessions contain deeply nested data, the combined recursion exceeds the JavaScript call stack limit.

Culprit Commit

This is a pre-existing issue, not a single-commit regression. The error bucket first appeared in v1.116.0 (April 15, 2026) and has been present across all subsequent versions. The objectMutationLog.ts diffing infrastructure was introduced in commit 5438d07d (Jan 13, 2026) by @connor4312 as part of the append-log session storage system. The recursive equals call in _diff has always lacked depth protection.

Code Flow

sequenceDiagram
    participant Store as "ChatSessionStore"
    participant MutLog as "ObjectMutationLog"
    participant DiffFn as "_diff/_diffObject/_diffArray"
    participant EqFn as "objects.equals"

    Store->>MutLog: write(currentSession)
    MutLog->>DiffFn: _diff(transform, path, prev, curr, entries)
    DiffFn->>DiffFn: _diffObject recurses into nested properties
    DiffFn->>EqFn: equals(prev, curr) for Primitive/Key transforms
    Note over EqFn: Recursively compares deeply<br/>nested objects without depth limit
    Note over EqFn: Stack overflow when nesting<br/>exceeds call stack capacity

Loading

Affected Files

File	Role
src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts	Root cause - recursive _diff without depth limit
src/vs/base/common/objects.ts	Crash site - equals() recurses without depth protection
src/vs/workbench/contrib/chat/common/model/chatSessionStore.ts	Trigger - calls writeSession which invokes the diff

Repro Steps

1. Open VS Code and use Copilot Chat extensively in agent mode
2. Accumulate a session with deeply nested response data (large tool call rounds, nested variable data, embedded markdown with complex structures)
3. The session auto-persists, triggering ObjectMutationLog.write() which diffs current state against previous
4. If session data nesting depth is sufficient (14+ recursive equals frames visible in stack), the recursion overflows

Fix Proposal

Fix location: src/vs/workbench/contrib/chat/common/model/objectMutationLog.ts in _diff()
Approach: Add a depth parameter to _diff, _diffObject, and _diffArray that increments on each recursive call. When depth exceeds a safe threshold (e.g., 50), fall back to a full replacement entry (EntryKind.Set) for that subtree instead of continuing to recurse. This prevents stack overflow while still producing correct diff output. Alternatively, replace the equals call at line 431 with a depth-limited comparison.

Recommended Owner

@connor4312 — original author of objectMutationLog.ts and the append-log session storage system, and active maintainer of chat session serialization code.

Generated by errors-investigate · ● 8M · ◷