diff --git a/src/vs/platform/agentHost/node/agentHostTelemetryReporter.ts b/src/vs/platform/agentHost/node/agentHostTelemetryReporter.ts
index 5c8417aec68fb5..249f327da8f02c 100644
--- a/src/vs/platform/agentHost/node/agentHostTelemetryReporter.ts
+++ b/src/vs/platform/agentHost/node/agentHostTelemetryReporter.ts
@@ -155,6 +155,7 @@ export interface IAgentHostToolInvokedReport {
 	toolSourceKind: string;
 	result: ToolInvokedResult;
 	invocationTimeMs?: number;
+	resultSizeInCharacters: number;
 }
 
 export interface IAgentHostAskQuestionsToolInvokedEvent {
@@ -811,6 +812,7 @@ export class AgentHostTelemetryReporter {
 			toolSourceKind: report.toolSourceKind,
 			invocationTimeMs: report.invocationTimeMs,
 			provider: report.provider,
+			resultSizeInCharacters: report.resultSizeInCharacters,
 		});
 	}
 
diff --git a/src/vs/platform/agentHost/node/agentHostToolCallTracker.ts b/src/vs/platform/agentHost/node/agentHostToolCallTracker.ts
index 76b30e5f104447..1a6ffb15e13624 100644
--- a/src/vs/platform/agentHost/node/agentHostToolCallTracker.ts
+++ b/src/vs/platform/agentHost/node/agentHostToolCallTracker.ts
@@ -143,6 +143,7 @@ export class AgentHostToolCallTracker extends Disposable {
 		this._toolCalls.delete(key);
 		const resultBucket = deriveToolInvokedResult(result);
 		const totalTimeMs = timing.lifecycleStopWatch.elapsed();
+		const resultSizeInCharacters = JSON.stringify(result).length;
 
 		this._reporter.toolInvoked({
 			provider: timing.provider,
@@ -151,6 +152,7 @@ export class AgentHostToolCallTracker extends Disposable {
 			toolSourceKind: timing.toolSourceKind,
 			result: resultBucket,
 			invocationTimeMs: timing.invocationStopWatch?.elapsed(),
+			resultSizeInCharacters,
 		});
 
 		const stalled = this._stalledToolCalls.get(key);
diff --git a/src/vs/platform/agentHost/test/node/agentHostToolCallTelemetry.test.ts b/src/vs/platform/agentHost/test/node/agentHostToolCallTelemetry.test.ts
index 5c0370a51f2553..051aa818a208b2 100644
--- a/src/vs/platform/agentHost/test/node/agentHostToolCallTelemetry.test.ts
+++ b/src/vs/platform/agentHost/test/node/agentHostToolCallTelemetry.test.ts
@@ -17,7 +17,7 @@ import { ITelemetryService, TelemetryLevel } from '../../../telemetry/common/tel
 import { AgentSession, IAgent } from '../../common/agentService.js';
 import { SessionInputRequestKind } from '../../common/state/protocol/state.js';
 import { ActionType, type ChatAction } from '../../common/state/sessionActions.js';
-import { buildDefaultChatUri, MessageKind, SessionStatus, ToolCallConfirmationReason, ToolCallContributorKind, ToolCallStatus, type ToolCallContributor, type ToolCallResult } from '../../common/state/sessionState.js';
+import { buildDefaultChatUri, MessageKind, SessionStatus, ToolCallConfirmationReason, ToolCallContributorKind, ToolCallStatus, ToolResultContentType, type ToolCallContributor, type ToolCallResult } from '../../common/state/sessionState.js';
 import { IAgentHostCheckpointService, NULL_CHECKPOINT_SERVICE } from '../../common/agentHostCheckpointService.js';
 import { IAgentHostTerminalManager } from '../../node/agentHostTerminalManager.js';
 import { AgentHostLocalTurns } from '../../node/agentHostLocalTurns.js';
@@ -235,6 +235,7 @@ suite('AgentSideEffects — tool call telemetry', () => {
 				toolSourceKind: 'agentHost',
 				provider: 'mock',
 				invocationTimeMs: true,
+				resultSizeInCharacters: 41,
 			},
 		}]);
 	});
@@ -256,6 +257,7 @@ suite('AgentSideEffects — tool call telemetry', () => {
 				toolSourceKind: 'mcp',
 				provider: 'mock',
 				invocationTimeMs: undefined,
+				resultSizeInCharacters: 90,
 			},
 		}]);
 	});
@@ -284,10 +286,25 @@ suite('AgentSideEffects — tool call telemetry', () => {
 				toolSourceKind: 'client',
 				provider: 'mock',
 				invocationTimeMs: true,
+				resultSizeInCharacters: 47,
 			},
 		}]);
 	});
 
+	test('includes result content in the serialized result size', () => {
+		setupSession();
+		startTurn('turn-1');
+
+		toolStart('turn-1', 'tc-read', 'read_file');
+		toolComplete('turn-1', 'tc-read', {
+			success: true,
+			pastTenseMessage: 'read files',
+			content: [{ type: ToolResultContentType.Text, text: 'alpha\nbeta' }],
+		});
+
+		assert.deepStrictEqual(toolEvents()[0].data.resultSizeInCharacters, 97);
+	});
+
 	test('only accepts contributor refinements that preserve execution ownership', async () => {
 		setupSession();
 		startTurn('turn-1');
diff --git a/src/vs/platform/telemetry/common/languageModelToolTelemetry.ts b/src/vs/platform/telemetry/common/languageModelToolTelemetry.ts
index fc5cb83fe088d7..b4e84c7c71e5a9 100644
--- a/src/vs/platform/telemetry/common/languageModelToolTelemetry.ts
+++ b/src/vs/platform/telemetry/common/languageModelToolTelemetry.ts
@@ -22,6 +22,7 @@ export type LanguageModelToolInvokedEvent = LanguageModelToolTelemetryData & {
 	prepareTimeMs?: number;
 	invocationTimeMs?: number;
 	provider?: string;
+	resultSizeInCharacters?: number;
 };
 
 export type LanguageModelToolInvokedClassification = LanguageModelToolTelemetryClassification & {
@@ -29,6 +30,7 @@ export type LanguageModelToolInvokedClassification = LanguageModelToolTelemetryC
 	prepareTimeMs?: { classification: 'SystemMetaData'; purpose: 'PerformanceAndHealth'; comment: 'Time spent in prepareToolInvocation method in milliseconds.' };
 	invocationTimeMs?: { classification: 'SystemMetaData'; purpose: 'PerformanceAndHealth'; comment: 'Time spent in tool invoke method in milliseconds.' };
 	provider?: { classification: 'SystemMetaData'; purpose: 'FeatureInsight'; comment: 'The agent host provider that invoked the tool (e.g. copilotcli, claude, codex), if applicable.' };
+	resultSizeInCharacters?: { classification: 'SystemMetaData'; purpose: 'PerformanceAndHealth'; isMeasurement: true; comment: 'Length of the serialized Agent Host tool result in UTF-16 code units, if applicable.' };
 	owner: 'roblourens';
 	comment: 'Provides insight into the usage of language model tools.';
 };