Heap Snapshot Analysis

Workflow

If the user needs the agent to launch VS Code, drive a scenario, and capture snapshots first, use the VS Code performance workflow skill before returning here for low-level snapshot analysis.

1. Parse Snapshots

Use the helpers in parseSnapshot.ts to load snapshots. The files are often >500MB and too large for JSON.parse as a string — the helpers use Buffer-based extraction. In scratchpad scripts, import helpers from ../helpers/*.ts.

import { parseSnapshot, buildGraph } from '../helpers/parseSnapshot.ts';

const data = parseSnapshot('/path/to/snapshot.heapsnapshot');
const graph = buildGraph(data);

2. Compare Before/After

Use compareSnapshots.ts to diff two snapshots:

import { compareSnapshots } from '../helpers/compareSnapshots.ts';

const result = compareSnapshots('/path/to/before.heapsnapshot', '/path/to/after.heapsnapshot');
// result.topBySize, result.topByCount, result.newObjectGroups, result.summary

3. Find Retainer Paths

Use findRetainers.ts to trace why an object is alive:

import { findRetainerPaths } from '../helpers/findRetainers.ts';

// Find what keeps ChatModel instances alive (skipping weak edges)
findRetainerPaths(graph, 'ChatModel', { maxPaths: 5, maxDepth: 25, maxAttempts: 200 });

4. Write Investigation Scripts

Write investigation-specific scripts in the scratchpad directory. This folder is gitignored — use it freely for one-off analysis.

Organize scratchpad work into dated subfolders named YYYY-MM-DD-short-description/ (e.g., 2026-04-09-chat-model-retainers/). Each subfolder should contain:

• The analysis scripts (.mjs, .mts, etc.)
• A findings.md file documenting the full investigation: all ideas considered, which ones led to changes and which were rejected (and why), before/after measurements, and a summary of the outcome. This lets the user review the agent's reasoning, decide which changes to keep, and follow up on deferred ideas.

Scripts can import the helpers:

cd .github/skills/heap-snapshot-analysis
node --max-old-space-size=16384 scratchpad/2026-04-09-chat-model-retainers/analyze.mjs

Key Concepts

V8 Heap Snapshot Format

The .heapsnapshot file is JSON with these key sections:

• snapshot.meta: Field definitions for nodes and edges
• nodes: Flat array, every N values = one node (N = meta.node_fields.length, typically 6: type, name, id, self_size, edge_count, detachedness)
• edges: Flat array, every M values = one edge (M = meta.edge_fields.length, typically 3: type, name_or_index, to_node)
• strings: String table indexed by name fields in nodes/edges

Edge Types That Matter

Always skip weak edges when tracing retainer paths. WeakMap entries show up as edges from key → backing array, but they don't prevent collection — they're red herrings.

Common VS Code Retention Patterns

1. RowCache templates: ListView's RowCache stores template rows. Templates have currentElement pointing to old viewmodel items. If not cleared on session switch, retains entire model chains.

2. Resource pools: pool.clear() only disposes idle items. If _onDidUpdateViewModel.fire() runs AFTER pool.clear(), released items re-enter the empty pool and are never disposed. Fire event first, then clear.

3. autorunIterableDelta lastValues: The closure captures a Map of previous iteration values. Values stay until the autorun re-runs. Async disposal delays keep models in observable stores longer than expected.

4. HoverService._delayedHovers: Global singleton Map retaining disposed objects via show closure → resolveHoverOptions closure → this. If hover cleanup disposable doesn't fire, the entire object tree is retained.

5. ObjectMutationLog._previous: The incremental serializer keeps a full snapshot of the last-serialized state. Every loaded ChatModel holds 2x its data: live + _previous.

6. _previousModelRef pattern: MutableDisposable setter disposes the old value. Reading .value and storing it elsewhere, then setting .value = undefined, disposes the stored reference. Use clearAndLeak() to extract without disposing.

Defensive Nulling

Null heavy fields in dispose() to break retention chains even when something retains the disposed object:

override dispose() {
    super.dispose();
    this._requests.length = 0;      // conversation data
    this.dataSerializer = undefined;  // serialization snapshot
    this._editingSession = undefined; // editing session + TextModels
    this._session = undefined!;       // back-reference cycles
}

Caveat: Don't null fields on viewmodel items (ChatResponseViewModel._model). The tree's diffIdentityProvider accesses them after the parent viewmodel is disposed but before setChildren replaces them.

False Retainers to Watch For

• DevToolsLogger._aliveInstances (Map): Enabled by VSCODE_DEV_DEBUG_OBSERVABLES env var. Retains ALL observed observables. Check if this is active before investigating observable-rooted paths.
• GCBasedDisposableTracker (FinalizationRegistry): If register(target, held, target) is used (target === unregister token), creates a strong self-reference preventing GC. Currently commented out in production.
• WeakMap backing arrays: Show up in retainer paths but don't prevent collection.

Running Analysis

All helper scripts use ESM and need Node with extra memory:

node --max-old-space-size=16384 scratchpad/analyze.mjs

Typical analysis takes 30-120 seconds per snapshot depending on size.