feat: Emit native contexts in snapshot summary (#2375)

Emits native contexts and their sizes with the get_heapsnapshot_summary
MCP tool. This should help the agent get an overview about which native
contexts consume the most memory.

Co-authored-by: Dominik Inführ <dinfuehr@chromium.org>

docs/tool-reference.md

@@ -568,7 +568,7 @@ in the DevTools Elements panel (if any).
 
 ### `get_heapsnapshot_summary`
 
-**Description:** Loads a memory heapsnapshot and returns snapshot summary stats. (requires flag: --memoryDebugging=true)
+**Description:** Loads a memory heapsnapshot and returns snapshot summary stats, including native contexts and their sizes. (requires flag: --memoryDebugging=true)
 
 **Parameters:**
 

src/HeapSnapshotManager.ts

@@ -120,6 +120,13 @@ export class HeapSnapshotManager {
     return snapshot.staticData;
   }
 
+  async getNativeContextSizes(
+    filePath: string,
+  ): Promise<DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes> {
+    const snapshot = await this.getSnapshot(filePath);
+    return await snapshot.getNativeContextSizes();
+  }
+
   async getOrCreateIdForClassKey(
     filePath: string,
     classKey: string,

src/McpContext.ts

@@ -668,6 +668,12 @@ export class McpContext implements Context {
     return await this.#heapSnapshotManager.getStaticData(filePath);
   }
 
+  async getHeapSnapshotNativeContextSizes(
+    filePath: string,
+  ): Promise<DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes> {
+    return await this.#heapSnapshotManager.getNativeContextSizes(filePath);
+  }
+
   async getHeapSnapshotNodesById(
     filePath: string,
     id: number,

src/McpResponse.ts

@@ -83,6 +83,7 @@ export class McpResponse implements Response {
     pagination?: PaginationOptions;
     stats?: DevTools.HeapSnapshotModel.HeapSnapshotModel.Statistics;
     staticData?: DevTools.HeapSnapshotModel.HeapSnapshotModel.StaticData | null;
+    nativeContextSizes?: DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes;
     nodes?: DevTools.HeapSnapshotModel.HeapSnapshotModel.ItemsRange;
     retainingPaths?: DevTools.HeapSnapshotModel.HeapSnapshotModel.RetainingPaths;
     dominators?: DevTools.HeapSnapshotModel.HeapSnapshotModel.DominatorChain;
@@ -330,12 +331,14 @@ export class McpResponse implements Response {
   setHeapSnapshotStats(
     stats: DevTools.HeapSnapshotModel.HeapSnapshotModel.Statistics,
     staticData: DevTools.HeapSnapshotModel.HeapSnapshotModel.StaticData | null,
+    nativeContextSizes: DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes,
   ) {
     this.#heapSnapshotOptions = {
       ...this.#heapSnapshotOptions,
       include: true,
       stats,
       staticData,
+      nativeContextSizes,
     };
   }
 
@@ -730,6 +733,7 @@ export class McpResponse implements Response {
       heapSnapshot?: {
         stats?: object;
         staticData?: object;
+        nativeContextSizes?: object;
         aggregateStats?: {
           objectCount: number;
           totalSelfSize: number;
@@ -1035,6 +1039,15 @@ Call ${handleDialog.name} to handle it before continuing.`);
         structuredContent.heapSnapshot = structuredContent.heapSnapshot || {};
         structuredContent.heapSnapshot.staticData = staticData;
       }
+      const nativeContextSizes = this.#heapSnapshotOptions.nativeContextSizes;
+      if (nativeContextSizes) {
+        response.push('### Native Contexts');
+        response.push(
+          HeapSnapshotFormatter.formatNativeContextSizes(nativeContextSizes),
+        );
+        structuredContent.heapSnapshot = structuredContent.heapSnapshot || {};
+        structuredContent.heapSnapshot.nativeContextSizes = nativeContextSizes;
+      }
       const aggregateData = this.#heapSnapshotOptions.aggregateData;
       if (aggregateData) {
         const sortedEntries = HeapSnapshotFormatter.sort(

src/bin/chrome-devtools-cli-options.ts

@@ -557,7 +557,7 @@ export const commands: Commands = {
   },
   get_heapsnapshot_summary: {
     description:
-      'Loads a memory heapsnapshot and returns snapshot summary stats. (requires flag: --memoryDebugging=true)',
+      'Loads a memory heapsnapshot and returns snapshot summary stats, including native contexts and their sizes. (requires flag: --memoryDebugging=true)',
     category: 'Memory',
     args: {
       filePath: {

src/formatters/HeapSnapshotFormatter.ts

@@ -137,6 +137,28 @@ export class HeapSnapshotFormatter {
     return lines.join('\n');
   }
 
+  static formatNativeContextSizes(
+    sizes: DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes,
+  ): string {
+    const lines: string[] = [];
+    lines.push('nodeId,nodeName,selfSize,retainedSize,attributedSize');
+    const sortedContexts = [...sizes.nativeContexts].sort(
+      (a, b) => b.attributedSize - a.attributedSize,
+    );
+    for (const nc of sortedContexts) {
+      lines.push(
+        `${nc.nodeId},${nc.nodeName},${DevTools.I18n.ByteUtilities.formatBytesToKb(nc.selfSize)},${DevTools.I18n.ByteUtilities.formatBytesToKb(nc.retainedSize)},${DevTools.I18n.ByteUtilities.formatBytesToKb(nc.attributedSize)}`,
+      );
+    }
+    lines.push(
+      `Shared Size: ${DevTools.I18n.ByteUtilities.formatBytesToKb(sizes.sharedSize)}`,
+    );
+    lines.push(
+      `Unattributed Size: ${DevTools.I18n.ByteUtilities.formatBytesToKb(sizes.noAttributionSize)}`,
+    );
+    return lines.join('\n');
+  }
+
   #getSortedAggregates(): AggregatedInfoWithId[] {
     return Object.values(this.#aggregates).sort((a, b) => b.maxRet - a.maxRet);
   }

src/tools/ToolDefinition.ts

@@ -114,6 +114,7 @@ export interface Response {
   setHeapSnapshotStats(
     stats: DevTools.HeapSnapshotModel.HeapSnapshotModel.Statistics,
     staticData: DevTools.HeapSnapshotModel.HeapSnapshotModel.StaticData | null,
+    nativeContextSizes: DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes,
   ): void;
   setHeapSnapshotNodes(
     nodes: DevTools.HeapSnapshotModel.HeapSnapshotModel.ItemsRange,
@@ -243,6 +244,9 @@ export type Context = Readonly<{
   getHeapSnapshotStaticData(
     filePath: string,
   ): Promise<DevTools.HeapSnapshotModel.HeapSnapshotModel.StaticData | null>;
+  getHeapSnapshotNativeContextSizes(
+    filePath: string,
+  ): Promise<DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes>;
   getHeapSnapshotNodesById(
     filePath: string,
     id: number,

src/tools/memory.ts

@@ -48,7 +48,7 @@ export const takeHeapSnapshot = definePageTool({
 export const getHeapSnapshotSummary = defineTool({
   name: 'get_heapsnapshot_summary',
   description:
-    'Loads a memory heapsnapshot and returns snapshot summary stats.',
+    'Loads a memory heapsnapshot and returns snapshot summary stats, including native contexts and their sizes.',
   annotations: {
     category: ToolCategory.MEMORY,
     readOnlyHint: true,
@@ -64,8 +64,11 @@ export const getHeapSnapshotSummary = defineTool({
     const staticData = await context.getHeapSnapshotStaticData(
       request.params.filePath,
     );
+    const nativeContextSizes = await context.getHeapSnapshotNativeContextSizes(
+      request.params.filePath,
+    );
 
-    response.setHeapSnapshotStats(stats, staticData);
+    response.setHeapSnapshotStats(stats, staticData, nativeContextSizes);
   },
 });
 

tests/formatters/HeapSnapshotFormatter.test.ts

@@ -293,4 +293,43 @@ describe('HeapSnapshotFormatter', () => {
       assert.strictEqual(result, expected);
     });
   });
+
+  describe('formatNativeContextSizes', () => {
+    it('formats native context sizes as CSV with summary lines', () => {
+      const mockSizes: DevTools.HeapSnapshotModel.HeapSnapshotModel.NativeContextSizes =
+        {
+          nativeContexts: [
+            {
+              nodeId: 10,
+              nodeIndex: 1,
+              nodeName: 'system / NativeContext',
+              attributedSize: 500,
+              retainedSize: 1000,
+              selfSize: 100,
+            },
+            {
+              nodeId: 20,
+              nodeIndex: 2,
+              nodeName: 'system / NativeContext / https://example.com',
+              attributedSize: 2000,
+              retainedSize: 5000,
+              selfSize: 200,
+            },
+          ],
+          sharedSize: 300,
+          noAttributionSize: 400,
+        };
+
+      const result = HeapSnapshotFormatter.formatNativeContextSizes(mockSizes);
+      const expected = [
+        'nodeId,nodeName,selfSize,retainedSize,attributedSize',
+        `20,system / NativeContext / https://example.com,${DevTools.I18n.ByteUtilities.formatBytesToKb(200)},${DevTools.I18n.ByteUtilities.formatBytesToKb(5000)},${DevTools.I18n.ByteUtilities.formatBytesToKb(2000)}`,
+        `10,system / NativeContext,${DevTools.I18n.ByteUtilities.formatBytesToKb(100)},${DevTools.I18n.ByteUtilities.formatBytesToKb(1000)},${DevTools.I18n.ByteUtilities.formatBytesToKb(500)}`,
+        `Shared Size: ${DevTools.I18n.ByteUtilities.formatBytesToKb(300)}`,
+        `Unattributed Size: ${DevTools.I18n.ByteUtilities.formatBytesToKb(400)}`,
+      ].join('\n');
+
+      assert.strictEqual(result, expected);
+    });
+  });
 });

tests/tools/memory.test.js.snapshot

@@ -389,4 +389,12 @@ Static Data: {
   "totalSize": 1121496,
   "maxJSObjectId": 54005
 }
+### Native Contexts
+nodeId,nodeName,selfSize,retainedSize,attributedSize
+7249,system / NativeContext / https://example.com,1.2 kB,350 kB,350 kB
+7195,system / NativeContext / https://example.com,1.2 kB,60.1 kB,215 kB
+7307,system / NativeContext,1.2 kB,84.4 kB,84.6 kB
+7199,system / NativeContext,1.2 kB,84.4 kB,84.4 kB
+Shared Size: 22.9 kB
+Unattributed Size: 44.8 kB
 `;