3 files changed, 117 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/events.ts b/packages/kernel/src/runtime/events.ts
index 300e711..b194577 100644
--- a/packages/kernel/src/runtime/events.ts
+++ b/packages/kernel/src/runtime/events.ts
@@ -127,16 +127,29 @@ export function doneEvent(
 	reason: string,
 	durationMs?: number,
 	usage?: Usage,
+	contextSize?: number,
 ): AgentEvent {
+	if (durationMs !== undefined && usage !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, durationMs, usage, contextSize };
+	}
 	if (durationMs !== undefined && usage !== undefined) {
 		return { type: "done", conversationId, turnId, reason, durationMs, usage };
 	}
+	if (durationMs !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, durationMs, contextSize };
+	}
+	if (usage !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, usage, contextSize };
+	}
 	if (durationMs !== undefined) {
 		return { type: "done", conversationId, turnId, reason, durationMs };
 	}
 	if (usage !== undefined) {
 		return { type: "done", conversationId, turnId, reason, usage };
 	}
+	if (contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, contextSize };
+	}
 	return { type: "done", conversationId, turnId, reason };
 }
 
diff --git a/packages/kernel/src/runtime/run-turn.test.ts b/packages/kernel/src/runtime/run-turn.test.ts
index fa2aba4..dcaea7f 100644
--- a/packages/kernel/src/runtime/run-turn.test.ts
+++ b/packages/kernel/src/runtime/run-turn.test.ts
@@ -2480,4 +2480,101 @@ describe("runTurn", () => {
 			}
 		});
 	});
+
+	describe("contextSize", () => {
+		it("single-step turn: contextSize equals step inputTokens + outputTokens", async () => {
+			const provider = createFakeProvider([
+				[
+					{ type: "text-delta", delta: "Hello" },
+					{ type: "usage", usage: { inputTokens: 100, outputTokens: 50 } },
+					{ type: "finish", reason: "stop" },
+				],
+			]);
+
+			const { events, emit } = createCollectingEmit();
+
+			await runTurn({
+				provider,
+				messages: [userMessage],
+				tools: [],
+				dispatch: { maxConcurrent: 1, eager: false },
+				conversationId: "conv-1",
+				turnId: "turn-1",
+				emit,
+			});
+
+			const doneEvt = events.find((e) => e.type === "done");
+			expect(doneEvt).toBeDefined();
+			if (doneEvt?.type === "done") {
+				expect(doneEvt.contextSize).toBe(150);
+			}
+		});
+
+		it("multi-step turn: contextSize equals ONLY the last step's inputTokens + outputTokens", async () => {
+			const tool = createFakeTool("echo", async () => ({ content: "echoed" }));
+
+			const provider = createFakeProvider([
+				[
+					{ type: "tool-call", toolCallId: "tc1", toolName: "echo", input: {} },
+					{ type: "usage", usage: { inputTokens: 100, outputTokens: 20 } },
+					{ type: "finish", reason: "tool-calls" },
+				],
+				[
+					{ type: "text-delta", delta: "done" },
+					{ type: "usage", usage: { inputTokens: 300, outputTokens: 80 } },
+					{ type: "finish", reason: "stop" },
+				],
+			]);
+
+			const { events, emit } = createCollectingEmit();
+
+			await runTurn({
+				provider,
+				messages: [userMessage],
+				tools: [tool],
+				dispatch: { maxConcurrent: 1, eager: false },
+				conversationId: "conv-1",
+				turnId: "turn-1",
+				emit,
+			});
+
+			const doneEvt = events.find((e) => e.type === "done");
+			expect(doneEvt).toBeDefined();
+			if (doneEvt?.type === "done") {
+				expect(doneEvt.contextSize).toBe(380);
+				expect(doneEvt.usage).toBeDefined();
+				if (doneEvt.usage !== undefined) {
+					expect(doneEvt.contextSize).not.toBe(doneEvt.usage.inputTokens);
+				}
+			}
+		});
+
+		it("no usage reported: contextSize is undefined", async () => {
+			const provider = createFakeProvider([
+				[
+					{ type: "text-delta", delta: "Hello" },
+					{ type: "finish", reason: "stop" },
+				],
+			]);
+
+			const { events, emit } = createCollectingEmit();
+
+			await runTurn({
+				provider,
+				messages: [userMessage],
+				tools: [],
+				dispatch: { maxConcurrent: 1, eager: false },
+				conversationId: "conv-1",
+				turnId: "turn-1",
+				emit,
+			});
+
+			const doneEvt = events.find((e) => e.type === "done");
+			expect(doneEvt).toBeDefined();
+			if (doneEvt?.type === "done") {
+				expect(doneEvt.contextSize).toBeUndefined();
+				expect(doneEvt.usage).toBeUndefined();
+			}
+		});
+	});
 });
diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index b50e8ee..bf57854 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -449,6 +449,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 	const messages: ChatMessage[] = [...input.messages];
 	const resultMessages: ChatMessage[] = [];
 	let totalUsage = zeroUsage();
+	let lastStepUsage: Usage | undefined;
 	let finishReason = "stop";
 
 	const toolMap = new Map<string, ToolContract>();
@@ -513,6 +514,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 			});
 
 			totalUsage = addUsage(totalUsage, stepResult.usage);
+			lastStepUsage = stepResult.usage;
 
 			if (stepResult.assistantMessage !== undefined) {
 				messages.push(stepResult.assistantMessage);
@@ -571,6 +573,10 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 		totalUsage.outputTokens > 0 ||
 		totalUsage.cacheReadTokens !== undefined ||
 		totalUsage.cacheWriteTokens !== undefined;
+	const contextSize =
+		hasUsage && lastStepUsage !== undefined
+			? lastStepUsage.inputTokens + lastStepUsage.outputTokens
+			: undefined;
 	input.emit(
 		doneEvent(
 			conversationId,
@@ -578,6 +584,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 			finishReason,
 			turnDurationMs,
 			hasUsage ? totalUsage : undefined,
+			contextSize,
 		),
 	);