add usage info to branch summary, compaction and tool result entries (#6671)

* add usage info to branch summary entries

* add usage to compaction entries

* allow custom tools to report llm usage in tool results

* allow observing and patching usage in tool_result hooks

* agent-harness: save usage in entries

for compaction, branch summaries and tool results

packages/agent/src/agent-loop.ts

@@ -737,6 +737,7 @@ async function finalizeExecutedToolCall(
 					...result,
 					content: afterResult.content ?? result.content,
 					details: afterResult.details ?? result.details,
+					usage: afterResult.usage ?? result.usage,
 					terminate: afterResult.terminate ?? result.terminate,
 				};
 				isError = afterResult.isError ?? isError;
@@ -780,6 +781,7 @@ function createToolResultMessage(finalized: FinalizedToolCallOutcome): ToolResul
 		// so the null never enters session history or provider payloads.
 		content: finalized.result.content ?? [],
 		details: finalized.result.details,
+		usage: finalized.result.usage,
 		...(finalized.result.addedToolNames?.length ? { addedToolNames: finalized.result.addedToolNames } : {}),
 		isError: finalized.isError,
 		timestamp: Date.now(),

packages/agent/src/harness/agent-harness.ts

@@ -32,6 +32,7 @@ import type {
 	AgentHarnessResources,
 	AgentHarnessStreamOptions,
 	AgentHarnessStreamOptionsPatch,
+	CompactResult,
 	ExecutionEnv,
 	NavigateTreeResult,
 	PendingSessionWrite,
@@ -434,9 +435,16 @@ export class AgentHarness<
 					content: result.content,
 					details: result.details,
 					isError,
+					usage: result.usage,
 				});
 				return patch
-					? { content: patch.content, details: patch.details, isError: patch.isError, terminate: patch.terminate }
+					? {
+							content: patch.content,
+							details: patch.details,
+							isError: patch.isError,
+							usage: patch.usage,
+							terminate: patch.terminate,
+						}
 					: undefined;
 			},
 			prepareNextTurn: async () => {
@@ -690,9 +698,7 @@ export class AgentHarness<
 		}
 	}
 
-	async compact(
+	async compact(customInstructions?: string): Promise<CompactResult> {
-		customInstructions?: string,
-	): Promise<{ summary: string; firstKeptEntryId: string; tokensBefore: number; details?: unknown }> {
 		if (this.phase !== "idle") throw new AgentHarnessError("busy", "compact() requires idle harness");
 		this.phase = "compaction";
 		try {
@@ -723,6 +729,7 @@ export class AgentHarness<
 				result.tokensBefore,
 				result.details,
 				provided !== undefined,
+				result.usage,
 			);
 			const entry = await this.session.getEntry(entryId);
 			if (entry?.type === "compaction") {
@@ -764,6 +771,7 @@ export class AgentHarness<
 			let summaryEntry: NavigateTreeResult["summaryEntry"];
 			let summaryText: string | undefined = hookResult?.summary?.summary;
 			let summaryDetails: unknown = hookResult?.summary?.details;
+			let summaryUsage = hookResult?.summary?.usage;
 			if (!summaryText && options?.summarize && entries.length > 0) {
 				const model = this.model;
 				if (!model) throw new AgentHarnessError("invalid_state", "No model set for branch summary");
@@ -779,6 +787,7 @@ export class AgentHarness<
 					throw new AgentHarnessError("branch_summary", branchSummary.error.message, branchSummary.error);
 				}
 				summaryText = branchSummary.value.summary;
+				summaryUsage = branchSummary.value.usage;
 				summaryDetails = {
 					readFiles: branchSummary.value.readFiles,
 					modifiedFiles: branchSummary.value.modifiedFiles,
@@ -798,7 +807,12 @@ export class AgentHarness<
 			const summaryId = await this.session.moveTo(
 				newLeafId,
 				summaryText
-					? { summary: summaryText, details: summaryDetails, fromHook: hookResult?.summary !== undefined }
+					? {
+							summary: summaryText,
+							details: summaryDetails,
+							usage: summaryUsage,
+							fromHook: hookResult?.summary !== undefined,
+						}
 					: undefined,
 			);
 			if (summaryId) {

packages/agent/src/harness/compaction/branch-summarization.ts

@@ -252,6 +252,7 @@ export async function generateBranchSummary(
 
 	return ok({
 		summary: summary || "No summary generated",
+		usage: response.usage,
 		readFiles,
 		modifiedFiles,
 	});

packages/agent/src/harness/compaction/compaction.ts

@@ -101,10 +101,35 @@ export interface CompactionResult<T = unknown> {
 	firstKeptEntryId: string;
 	/** Estimated context tokens before compaction. */
 	tokensBefore: number;
+	/** Usage from the LLM call(s) that generated this summary, if available. */
+	usage?: Usage;
 	/** Optional implementation-specific details stored with the compaction entry. */
 	details?: T;
 }
 
+function combineUsage(first: Usage, second: Usage): Usage {
+	return {
+		input: first.input + second.input,
+		output: first.output + second.output,
+		cacheRead: first.cacheRead + second.cacheRead,
+		cacheWrite: first.cacheWrite + second.cacheWrite,
+		...(first.cacheWrite1h !== undefined || second.cacheWrite1h !== undefined
+			? { cacheWrite1h: (first.cacheWrite1h ?? 0) + (second.cacheWrite1h ?? 0) }
+			: {}),
+		...(first.reasoning !== undefined || second.reasoning !== undefined
+			? { reasoning: (first.reasoning ?? 0) + (second.reasoning ?? 0) }
+			: {}),
+		totalTokens: first.totalTokens + second.totalTokens,
+		cost: {
+			input: first.cost.input + second.cost.input,
+			output: first.cost.output + second.cost.output,
+			cacheRead: first.cost.cacheRead + second.cost.cacheRead,
+			cacheWrite: first.cost.cacheWrite + second.cost.cacheWrite,
+			total: first.cost.total + second.cost.total,
+		},
+	};
+}
+
 /** Compaction thresholds and retention settings. */
 export interface CompactionSettings {
 	/** Enable automatic compaction decisions. */
@@ -474,7 +499,7 @@ export async function generateSummary(
 	customInstructions?: string,
 	previousSummary?: string,
 	thinkingLevel?: ThinkingLevel,
-): Promise<Result<string, CompactionError>> {
+): Promise<Result<{ text: string; usage: Usage }, CompactionError>> {
 	const maxTokens = Math.min(
 		Math.floor(0.8 * reserveTokens),
 		model.maxTokens > 0 ? model.maxTokens : Number.POSITIVE_INFINITY,
@@ -523,7 +548,7 @@ export async function generateSummary(
 
 	const textContent = contentText(response.content);
 
-	return ok(textContent);
+	return ok({ text: textContent, usage: response.usage });
 }
 
 /** Prepared inputs for a compaction run. */
@@ -656,22 +681,26 @@ export async function compact(
 	}
 
 	let summary: string;
+	let summaryUsage: Usage;
 
 	if (isSplitTurn && turnPrefixMessages.length > 0) {
-		const historyResult =
+		let historyText = "No prior history.";
-			messagesToSummarize.length > 0
+		let historyUsage: Usage | undefined;
-				? await generateSummary(
+		if (messagesToSummarize.length > 0) {
-						messagesToSummarize,
+			const historyResult = await generateSummary(
-						models,
+				messagesToSummarize,
-						model,
+				models,
-						settings.reserveTokens,
+				model,
-						signal,
+				settings.reserveTokens,
-						customInstructions,
+				signal,
-						previousSummary,
+				customInstructions,
-						thinkingLevel,
+				previousSummary,
-					)
+				thinkingLevel,
-				: ok<string, CompactionError>("No prior history.");
+			);
-		if (!historyResult.ok) return err(historyResult.error);
+			if (!historyResult.ok) return err(historyResult.error);
+			historyText = historyResult.value.text;
+			historyUsage = historyResult.value.usage;
+		}
 		const turnPrefixResult = await generateTurnPrefixSummary(
 			turnPrefixMessages,
 			models,
@@ -681,7 +710,10 @@ export async function compact(
 			thinkingLevel,
 		);
 		if (!turnPrefixResult.ok) return err(turnPrefixResult.error);
-		summary = `${historyResult.value}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult.value}`;
+		summary = `${historyText}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult.value.text}`;
+		summaryUsage = historyUsage
+			? combineUsage(historyUsage, turnPrefixResult.value.usage)
+			: turnPrefixResult.value.usage;
 	} else {
 		const summaryResult = await generateSummary(
 			messagesToSummarize,
@@ -694,7 +726,8 @@ export async function compact(
 			thinkingLevel,
 		);
 		if (!summaryResult.ok) return err(summaryResult.error);
-		summary = summaryResult.value;
+		summary = summaryResult.value.text;
+		summaryUsage = summaryResult.value.usage;
 	}
 
 	const { readFiles, modifiedFiles } = computeFileLists(fileOps);
@@ -704,6 +737,7 @@ export async function compact(
 		summary,
 		firstKeptEntryId,
 		tokensBefore,
+		usage: summaryUsage,
 		details: { readFiles, modifiedFiles } as CompactionDetails,
 	});
 }
@@ -714,7 +748,7 @@ async function generateTurnPrefixSummary(
 	reserveTokens: number,
 	signal?: AbortSignal,
 	thinkingLevel?: ThinkingLevel,
-): Promise<Result<string, CompactionError>> {
+): Promise<Result<{ text: string; usage: Usage }, CompactionError>> {
 	const maxTokens = Math.min(
 		Math.floor(0.5 * reserveTokens),
 		model.maxTokens > 0 ? model.maxTokens : Number.POSITIVE_INFINITY,
@@ -749,5 +783,8 @@ async function generateTurnPrefixSummary(
 		);
 	}
 
-	return ok(contentText(response.content));
+	return ok({
+		text: contentText(response.content),
+		usage: response.usage,
+	});
 }

packages/agent/src/harness/session/session.ts

@@ -1,4 +1,4 @@
-import type { ImageContent, TextContent } from "@earendil-works/pi-ai";
+import type { ImageContent, TextContent, Usage } from "@earendil-works/pi-ai";
 import type { AgentMessage } from "../../types.ts";
 import { createBranchSummaryMessage, createCompactionSummaryMessage, createCustomMessage } from "../messages.ts";
 import type {
@@ -247,6 +247,7 @@ export class Session<TMetadata extends SessionMetadata = SessionMetadata> {
 		tokensBefore: number,
 		details?: T,
 		fromHook?: boolean,
+		usage?: Usage,
 	): Promise<string> {
 		return this.appendTypedEntry({
 			type: "compaction",
@@ -257,6 +258,7 @@ export class Session<TMetadata extends SessionMetadata = SessionMetadata> {
 			firstKeptEntryId,
 			tokensBefore,
 			details,
+			usage,
 			fromHook,
 		} satisfies CompactionEntry<T>);
 	}
@@ -317,7 +319,7 @@ export class Session<TMetadata extends SessionMetadata = SessionMetadata> {
 
 	async moveTo(
 		entryId: string | null,
-		summary?: { summary: string; details?: unknown; fromHook?: boolean },
+		summary?: { summary: string; details?: unknown; usage?: Usage; fromHook?: boolean },
 	): Promise<string | undefined> {
 		if (entryId !== null && !(await this.storage.getEntry(entryId))) {
 			throw new SessionError("not_found", `Entry ${entryId} not found`);
@@ -332,6 +334,7 @@ export class Session<TMetadata extends SessionMetadata = SessionMetadata> {
 			fromId: entryId ?? "root",
 			summary: summary.summary,
 			details: summary.details,
+			usage: summary.usage,
 			fromHook: summary.fromHook,
 		} satisfies BranchSummaryEntry);
 	}

packages/agent/src/harness/types.ts

@@ -1,4 +1,12 @@
-import type { ImageContent, Model, Models, SimpleStreamOptions, TextContent, Transport } from "@earendil-works/pi-ai";
+import type {
+	ImageContent,
+	Model,
+	Models,
+	SimpleStreamOptions,
+	TextContent,
+	Transport,
+	Usage,
+} from "@earendil-works/pi-ai";
 import type { AgentEvent, AgentMessage, AgentTool, QueueMode, ThinkingLevel } from "../index.ts";
 import type { Session } from "./session/session.ts";
 
@@ -365,6 +373,7 @@ export interface CompactionEntry<T = unknown> extends SessionTreeEntryBase {
 	firstKeptEntryId: string;
 	tokensBefore: number;
 	details?: T;
+	usage?: Usage;
 	fromHook?: boolean;
 }
 
@@ -373,6 +382,7 @@ export interface BranchSummaryEntry<T = unknown> extends SessionTreeEntryBase {
 	fromId: string;
 	summary: string;
 	details?: T;
+	usage?: Usage;
 	fromHook?: boolean;
 }
 
@@ -572,6 +582,7 @@ export interface ToolResultEvent {
 	content: Array<TextContent | ImageContent>;
 	details: unknown;
 	isError: boolean;
+	usage?: Usage;
 }
 
 export interface SessionBeforeCompactEvent {
@@ -687,6 +698,7 @@ export interface ToolResultPatch {
 	content?: Array<TextContent | ImageContent>;
 	details?: unknown;
 	isError?: boolean;
+	usage?: Usage;
 	terminate?: boolean;
 }
 
@@ -697,7 +709,12 @@ export interface SessionBeforeCompactResult {
 
 export interface SessionBeforeTreeResult {
 	cancel?: boolean;
-	summary?: { summary: string; details?: unknown };
+	summary?: {
+		summary: string;
+		details?: unknown;
+		/** Usage from the LLM call that generated this summary, if available. */
+		usage?: Usage;
+	};
 	customInstructions?: string;
 	replaceInstructions?: boolean;
 	label?: string;
@@ -738,6 +755,8 @@ export interface CompactResult {
 	summary: string;
 	firstKeptEntryId: string;
 	tokensBefore: number;
+	/** Usage from the LLM call(s) that generated this summary, if available. */
+	usage?: Usage;
 	details?: unknown;
 }
 
@@ -793,6 +812,7 @@ export interface GenerateBranchSummaryOptions {
 
 export interface BranchSummaryResult {
 	summary: string;
+	usage?: Usage;
 	readFiles: string[];
 	modifiedFiles: string[];
 }

packages/agent/src/types.ts

@@ -11,6 +11,7 @@ import type {
 	TextContent,
 	Tool,
 	ToolResultMessage,
+	Usage,
 } from "@earendil-works/pi-ai";
 import type { Static, TSchema } from "typebox";
 
@@ -69,15 +70,18 @@ export interface BeforeToolCallResult {
  * - `content`: if provided, replaces the tool result content array in full
  * - `details`: if provided, replaces the tool result details value in full
  * - `isError`: if provided, replaces the tool result error flag
+ * - `usage`: if provided, replaces the tool result usage
  * - `terminate`: if provided, replaces the early-termination hint
  *
  * Omitted fields keep the original executed tool result values.
- * There is no deep merge for `content` or `details`.
+ * There is no deep merge for `content`, `details`, or `usage`.
  */
 export interface AfterToolCallResult {
 	content?: (TextContent | ImageContent)[];
 	details?: unknown;
 	isError?: boolean;
+	/** Usage from the final tool execution itself, if available. Not used for main LLM context accounting. */
+	usage?: Usage;
 	/**
 	 * Hint that the agent should stop after the current tool batch.
 	 * Early termination only happens when every finalized tool result in the batch sets this to true.
@@ -273,6 +277,7 @@ export interface AgentLoopConfig extends SimpleStreamOptions {
 	 * - `content` replaces the full content array
 	 * - `details` replaces the full details payload
 	 * - `isError` replaces the error flag
+	 * - `usage` replaces the tool result usage
 	 * - `terminate` replaces the early-termination hint
 	 *
 	 * Any omitted fields keep their original values. No deep merge is performed.
@@ -352,6 +357,8 @@ export interface AgentToolResult<T> {
 	content: (TextContent | ImageContent)[];
 	/** Arbitrary structured details for logs or UI rendering. */
 	details: T;
+	/** Usage from the final tool execution itself, if available. Not used for main LLM context accounting. */
+	usage?: Usage;
 	/** Names of tools introduced by this result and available from this transcript point onward. */
 	addedToolNames?: string[];
 	/**

packages/agent/test/agent-loop.test.ts

@@ -239,6 +239,23 @@ describe("agentLoop with AgentMessage", () => {
 	it("should handle tool calls and results", async () => {
 		const toolSchema = Type.Object({ value: Type.String() });
 		const executed: string[] = [];
+		const toolUsage = {
+			input: 1,
+			output: 2,
+			cacheRead: 3,
+			cacheWrite: 4,
+			totalTokens: 10,
+			cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+		};
+		const patchedToolUsage = {
+			input: 5,
+			output: 6,
+			cacheRead: 7,
+			cacheWrite: 8,
+			totalTokens: 26,
+			cost: { input: 0.5, output: 0.6, cacheRead: 0.7, cacheWrite: 0.8, total: 2.6 },
+		};
+		let observedToolUsage: typeof toolUsage | undefined;
 		const tool: AgentTool<typeof toolSchema, { value: string }> = {
 			name: "echo",
 			label: "Echo",
@@ -249,6 +266,7 @@ describe("agentLoop with AgentMessage", () => {
 				return {
 					content: [{ type: "text", text: `echoed: ${params.value}` }],
 					details: { value: params.value },
+					usage: toolUsage,
 				};
 			},
 		};
@@ -264,6 +282,10 @@ describe("agentLoop with AgentMessage", () => {
 		const config: AgentLoopConfig = {
 			model: createModel(),
 			convertToLlm: identityConverter,
+			afterToolCall: async ({ result }) => {
+				observedToolUsage = result.usage;
+				return { usage: patchedToolUsage };
+			},
 		};
 
 		let callIndex = 0;
@@ -305,6 +327,10 @@ describe("agentLoop with AgentMessage", () => {
 		if (toolEnd?.type === "tool_execution_end") {
 			expect(toolEnd.isError).toBe(false);
 		}
+		expect(observedToolUsage).toEqual(toolUsage);
+		const messages = await stream.result();
+		const toolResult = messages.find((message) => message.role === "toolResult");
+		expect(toolResult?.role === "toolResult" ? toolResult.usage : undefined).toEqual(patchedToolUsage);
 	});
 
 	it("should not execute tool calls from a length-truncated assistant message", async () => {

packages/agent/test/harness/agent-harness.test.ts

@@ -5,6 +5,7 @@ import {
 	fauxProvider,
 	fauxToolCall,
 	type RegisterFauxProviderOptions,
+	type Usage,
 } from "@earendil-works/pi-ai";
 import { getModel } from "@earendil-works/pi-ai/compat";
 import { describe, expect, it } from "vitest";
@@ -14,7 +15,7 @@ import { InMemorySessionStorage } from "../../src/harness/session/memory-storage
 import { Session } from "../../src/harness/session/session.ts";
 import type { PromptTemplate, Skill } from "../../src/harness/types.ts";
 import type { AgentMessage, AgentTool } from "../../src/types.ts";
-import { calculateTool } from "../utils/calculate.ts";
+import { calculateTool, createCalculateToolWithUsage } from "../utils/calculate.ts";
 import { getCurrentTimeTool } from "../utils/get-current-time.ts";
 
 interface AppSkill extends Skill {
@@ -60,6 +61,34 @@ function getReasoning(options: unknown): unknown {
 	return options.reasoning;
 }
 
+function createUsage(input: number, output: number, cacheRead = 0, cacheWrite = 0): Usage {
+	return {
+		input,
+		output,
+		cacheRead,
+		cacheWrite,
+		totalTokens: input + output + cacheRead + cacheWrite,
+		cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+	};
+}
+
+function createUserMessage(text: string): AgentMessage {
+	return { role: "user", content: [{ type: "text", text }], timestamp: Date.now() };
+}
+
+function createAssistantMessage(text: string): AgentMessage {
+	return {
+		role: "assistant",
+		content: [{ type: "text", text }],
+		api: "faux",
+		provider: "faux",
+		model: "faux-1",
+		usage: createUsage(100, 50),
+		stopReason: "stop",
+		timestamp: Date.now(),
+	};
+}
+
 describe("AgentHarness", () => {
 	it("constructs directly and exposes queue modes", () => {
 		const session = new Session(new InMemorySessionStorage());
@@ -427,14 +456,18 @@ describe("AgentHarness", () => {
 				}),
 		]);
 		const session = new Session(new InMemorySessionStorage());
+		const toolUsage = createUsage(1, 2, 3, 4);
+		const patchedToolUsage = createUsage(5, 6, 7, 8);
+		const calculateToolWithUsage = createCalculateToolWithUsage(toolUsage);
 		const harness = new AgentHarness({
 			models,
 			env: new NodeExecutionEnv({ cwd: process.cwd() }),
 			session,
 			model: registration.getModel(),
-			tools: [calculateTool],
+			tools: [calculateToolWithUsage],
 		});
 		const seenToolCalls: Array<{ id: string; name: string; expression: unknown }> = [];
+		let seenToolUsage: Usage | undefined;
 		harness.on("tool_call", (event) => {
 			seenToolCalls.push({ id: event.toolCallId, name: event.toolName, expression: event.input.expression });
 			return undefined;
@@ -442,9 +475,11 @@ describe("AgentHarness", () => {
 		harness.on("tool_result", (event) => {
 			expect(event.toolCallId).toBe("call-1");
 			expect(event.toolName).toBe("calculate");
+			seenToolUsage = event.usage;
 			return {
 				content: [{ type: "text", text: "patched result" }],
 				details: { patched: true },
+				usage: patchedToolUsage,
 				terminate: true,
 			};
 		});
@@ -455,16 +490,109 @@ describe("AgentHarness", () => {
 			(entry) => entry.type === "message" && entry.message.role === "toolResult",
 		);
 		expect(seenToolCalls).toEqual([{ id: "call-1", name: "calculate", expression: "2 + 2" }]);
+		expect(seenToolUsage).toEqual(toolUsage);
 		expect(toolResult).toMatchObject({
 			type: "message",
 			message: {
 				role: "toolResult",
 				content: [{ type: "text", text: "patched result" }],
 				details: { patched: true },
+				usage: patchedToolUsage,
 			},
 		});
 	});
 
+	it("persists generated compaction usage", async () => {
+		const registration = newFaux();
+		registration.setResponses([fauxAssistantMessage("## Goal\nTest summary")]);
+		const session = new Session(new InMemorySessionStorage());
+		await session.appendMessage(createUserMessage("one"));
+		await session.appendMessage(createAssistantMessage("two"));
+		const harness = new AgentHarness({
+			models,
+			env: new NodeExecutionEnv({ cwd: process.cwd() }),
+			session,
+			model: registration.getModel(),
+		});
+
+		const result = await harness.compact();
+		const compaction = (await session.getEntries()).find((entry) => entry.type === "compaction");
+
+		expect(result.usage?.totalTokens).toBeGreaterThan(0);
+		expect(compaction?.type === "compaction" ? compaction.usage : undefined).toEqual(result.usage);
+	});
+
+	it("persists hook-provided compaction usage", async () => {
+		const registration = newFaux();
+		const usage = createUsage(5, 6, 7, 8);
+		const session = new Session(new InMemorySessionStorage());
+		await session.appendMessage(createUserMessage("one"));
+		await session.appendMessage(createAssistantMessage("two"));
+		const harness = new AgentHarness({
+			models,
+			env: new NodeExecutionEnv({ cwd: process.cwd() }),
+			session,
+			model: registration.getModel(),
+		});
+		harness.on("session_before_compact", (event) => ({
+			compaction: {
+				summary: "hook summary",
+				firstKeptEntryId: event.preparation.firstKeptEntryId,
+				tokensBefore: event.preparation.tokensBefore,
+				usage,
+			},
+		}));
+
+		const result = await harness.compact();
+		const compaction = (await session.getEntries()).find((entry) => entry.type === "compaction");
+
+		expect(result.usage).toEqual(usage);
+		expect(compaction?.type === "compaction" ? compaction.usage : undefined).toEqual(usage);
+	});
+
+	it("persists generated branch summary usage", async () => {
+		const registration = newFaux();
+		registration.setResponses([fauxAssistantMessage("## Goal\nBranch summary")]);
+		const session = new Session(new InMemorySessionStorage());
+		const targetId = await session.appendMessage(createUserMessage("first branch"));
+		await session.appendMessage(createAssistantMessage("first reply"));
+		await session.appendMessage(createUserMessage("abandoned work"));
+		await session.appendMessage(createAssistantMessage("abandoned reply"));
+		const harness = new AgentHarness({
+			models,
+			env: new NodeExecutionEnv({ cwd: process.cwd() }),
+			session,
+			model: registration.getModel(),
+		});
+
+		const result = await harness.navigateTree(targetId, { summarize: true });
+
+		expect(result.summaryEntry?.usage?.totalTokens).toBeGreaterThan(0);
+	});
+
+	it("persists hook-provided branch summary usage", async () => {
+		const registration = newFaux();
+		const usage = createUsage(13, 14, 15, 16);
+		const session = new Session(new InMemorySessionStorage());
+		const targetId = await session.appendMessage(createUserMessage("first branch"));
+		await session.appendMessage(createAssistantMessage("first reply"));
+		await session.appendMessage(createUserMessage("abandoned work"));
+		await session.appendMessage(createAssistantMessage("abandoned reply"));
+		const harness = new AgentHarness({
+			models,
+			env: new NodeExecutionEnv({ cwd: process.cwd() }),
+			session,
+			model: registration.getModel(),
+		});
+		harness.on("session_before_tree", () => ({
+			summary: { summary: "hook branch summary", usage },
+		}));
+
+		const result = await harness.navigateTree(targetId, { summarize: true });
+
+		expect(result.summaryEntry?.usage).toEqual(usage);
+	});
+
 	it("preserves app tool types for getters and update events", async () => {
 		const session = new Session(new InMemorySessionStorage());
 		const env = new NodeExecutionEnv({ cwd: process.cwd() });

packages/agent/test/harness/compaction.test.ts

@@ -6,6 +6,7 @@ import {
 	fauxProvider,
 	type Message,
 	type Model,
+	type Models,
 	type Usage,
 } from "@earendil-works/pi-ai";
 import { beforeEach, describe, expect, it } from "vitest";
@@ -142,6 +143,17 @@ function createFauxModel(reasoning: boolean, maxTokens = 8192): { faux: FauxProv
 	return { faux, model: faux.getModel() };
 }
 
+function createModelsWithSimpleResponses(responses: AssistantMessage[]): Models {
+	const remaining = [...responses];
+	const stub = Object.create(models) as Models;
+	stub.completeSimple = async () => {
+		const response = remaining.shift();
+		if (!response) throw new Error("No faux completeSimple response queued");
+		return response;
+	};
+	return stub;
+}
+
 describe("harness compaction", () => {
 	beforeEach(() => {
 		nextId = 0;
@@ -501,7 +513,12 @@ describe("harness compaction", () => {
 			await generateSummary(messages, models, model, 2000, undefined, "focus", "old summary"),
 		);
 
-		expect(summary).toContain("Test summary");
+		expect(summary.text).toContain("Test summary");
+		expect(summary.usage.input).toBeGreaterThan(0);
+		expect(summary.usage.output).toBeGreaterThan(0);
+		expect(summary.usage.totalTokens).toBe(
+			summary.usage.input + summary.usage.output + summary.usage.cacheRead + summary.usage.cacheWrite,
+		);
 		expect(promptText).toContain("<previous-summary>\nold summary\n</previous-summary>");
 		expect(promptText).toContain("Additional focus: focus");
 	});
@@ -578,6 +595,30 @@ describe("harness compaction", () => {
 		expect(invalidResult).toMatchObject({ ok: false, error: { code: "invalid_session" } });
 	});
 
+	it("combines usage for split-turn compaction summaries", async () => {
+		const messages: AgentMessage[] = [createUserMessage("Summarize this.")];
+		const { model } = createFauxModel(false);
+		const historyUsage = createMockUsage(1, 2, 3, 4);
+		const turnPrefixUsage = createMockUsage(5, 6, 7, 8);
+		const usageModels = createModelsWithSimpleResponses([
+			{ ...fauxAssistantMessage("history summary"), usage: historyUsage },
+			{ ...fauxAssistantMessage("turn prefix summary"), usage: turnPrefixUsage },
+		]);
+		const preparation: CompactionPreparation = {
+			firstKeptEntryId: "entry-keep",
+			messagesToSummarize: messages,
+			turnPrefixMessages: messages,
+			isSplitTurn: true,
+			tokensBefore: 100,
+			fileOps: { read: new Set(), written: new Set(), edited: new Set() },
+			settings: { enabled: true, reserveTokens: 2000, keepRecentTokens: 20 },
+		};
+
+		const result = getOrThrow(await compact(preparation, usageModels, model));
+
+		expect(result.usage).toEqual(createMockUsage(6, 8, 10, 12));
+	});
+
 	it("passes reasoning through turn-prefix summaries when enabled", async () => {
 		const messages: AgentMessage[] = [createUserMessage("Summarize this.")];
 		const seenOptions: Array<Record<string, unknown> | undefined> = [];
@@ -646,6 +687,7 @@ describe("harness compaction", () => {
 		const result = getOrThrow(await compact(preparation!, models, model));
 		expect(result.summary.length).toBeGreaterThan(0);
 		expect(result.firstKeptEntryId).toBeTruthy();
+		expect(result.usage?.totalTokens).toBeGreaterThan(0);
 		expect(result.details).toBeDefined();
 	});
 });

packages/agent/test/harness/session.test.ts

@@ -86,6 +86,49 @@ async function runSessionSuite(
 			expect(context.messages[1]?.role).toBe("branchSummary");
 		});
 
+		it("persists compaction usage", async () => {
+			const session = new Session(await createStorage());
+			const firstKeptEntryId = await session.appendMessage(createUserMessage("one"));
+			const usage = {
+				input: 1,
+				output: 2,
+				cacheRead: 3,
+				cacheWrite: 4,
+				totalTokens: 10,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			};
+
+			const compactionId = await session.appendCompaction(
+				"summary",
+				firstKeptEntryId,
+				1234,
+				undefined,
+				false,
+				usage,
+			);
+
+			const compactionEntry = await session.getEntry(compactionId);
+			expect(compactionEntry?.type === "compaction" ? compactionEntry.usage : undefined).toEqual(usage);
+		});
+
+		it("persists branch summary usage", async () => {
+			const session = new Session(await createStorage());
+			const user1 = await session.appendMessage(createUserMessage("one"));
+			const usage = {
+				input: 1,
+				output: 2,
+				cacheRead: 3,
+				cacheWrite: 4,
+				totalTokens: 10,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			};
+
+			const summaryId = await session.moveTo(user1, { summary: "summary text", usage });
+
+			const summaryEntry = await session.getEntry(summaryId!);
+			expect(summaryEntry?.type === "branch_summary" ? summaryEntry.usage : undefined).toEqual(usage);
+		});
+
 		it("supports custom message entries in context", async () => {
 			const session = new Session(await createStorage());
 			await session.appendMessage(createUserMessage("one"));

packages/agent/test/utils/calculate.ts

@@ -1,3 +1,4 @@
+import type { Usage } from "@earendil-works/pi-ai";
 import { type Static, Type } from "typebox";
 import type { AgentTool, AgentToolResult } from "../../src/types.ts";
 
@@ -30,3 +31,10 @@ export const calculateTool: AgentTool<typeof calculateSchema, undefined> = {
 		return calculate(args.expression);
 	},
 };
+
+export function createCalculateToolWithUsage(usage: Usage): AgentTool<typeof calculateSchema, undefined> {
+	return {
+		...calculateTool,
+		execute: async (_toolCallId: string, args: CalculateParams) => ({ ...calculate(args.expression), usage }),
+	};
+}

packages/ai/src/types.ts

@@ -408,6 +408,8 @@ export interface ToolResultMessage<TDetails = any> {
 	toolName: string;
 	content: (TextContent | ImageContent)[]; // Supports text and images
 	details?: TDetails;
+	/** Usage from the tool execution itself, if available. Not part of main LLM context accounting. */
+	usage?: Usage;
 	/**
 	 * Names from `Context.tools` that became available after this result.
 	 * Providers with native deferred tool loading use this as the load point;

packages/coding-agent/src/core/agent-session.ts

@@ -32,6 +32,7 @@ import type {
 	Model,
 	ProviderHeaders,
 	TextContent,
+	Usage,
 } from "@earendil-works/pi-ai/compat";
 import {
 	clampThinkingLevel,
@@ -104,6 +105,7 @@ import { type BuildSystemPromptOptions, buildSystemPrompt } from "./system-promp
 import { type BashOperations, createLocalBashOperations } from "./tools/bash.ts";
 import { createAllToolDefinitions } from "./tools/index.ts";
 import { createToolDefinitionFromAgentTool } from "./tools/tool-definition-wrapper.ts";
+import { addUsageToTotals, createUsageTotals } from "./usage-totals.ts";
 
 // ============================================================================
 // Skill Block Parsing
@@ -482,6 +484,7 @@ export class AgentSession {
 				content: result.content,
 				details: result.details,
 				isError,
+				usage: result.usage,
 			});
 
 			if (!hookResult) {
@@ -492,6 +495,7 @@ export class AgentSession {
 				content: hookResult.content,
 				details: hookResult.details,
 				isError: hookResult.isError ?? isError,
+				usage: hookResult.usage,
 			};
 		};
 	}
@@ -1812,6 +1816,7 @@ export class AgentSession {
 			let summary: string;
 			let firstKeptEntryId: string;
 			let tokensBefore: number;
+			let usage: Usage | undefined;
 			let details: unknown;
 
 			if (extensionCompaction) {
@@ -1819,6 +1824,7 @@ export class AgentSession {
 				summary = extensionCompaction.summary;
 				firstKeptEntryId = extensionCompaction.firstKeptEntryId;
 				tokensBefore = extensionCompaction.tokensBefore;
+				usage = extensionCompaction.usage;
 				details = extensionCompaction.details;
 			} else {
 				// Generate compaction result
@@ -1836,6 +1842,7 @@ export class AgentSession {
 				summary = result.summary;
 				firstKeptEntryId = result.firstKeptEntryId;
 				tokensBefore = result.tokensBefore;
+				usage = result.usage;
 				details = result.details;
 			}
 
@@ -1843,7 +1850,7 @@ export class AgentSession {
 				throw new Error("Compaction cancelled");
 			}
 
-			this.sessionManager.appendCompaction(summary, firstKeptEntryId, tokensBefore, details, fromExtension);
+			this.sessionManager.appendCompaction(summary, firstKeptEntryId, tokensBefore, details, fromExtension, usage);
 			const newEntries = this.sessionManager.getEntries();
 			const sessionContext = this.sessionManager.buildSessionContext();
 			this.agent.state.messages = sessionContext.messages;
@@ -1869,6 +1876,7 @@ export class AgentSession {
 				firstKeptEntryId,
 				tokensBefore,
 				estimatedTokensAfter,
+				usage,
 				details,
 			};
 			this._emit({
@@ -2084,6 +2092,7 @@ export class AgentSession {
 			let summary: string;
 			let firstKeptEntryId: string;
 			let tokensBefore: number;
+			let usage: Usage | undefined;
 			let details: unknown;
 
 			if (extensionCompaction) {
@@ -2091,6 +2100,7 @@ export class AgentSession {
 				summary = extensionCompaction.summary;
 				firstKeptEntryId = extensionCompaction.firstKeptEntryId;
 				tokensBefore = extensionCompaction.tokensBefore;
+				usage = extensionCompaction.usage;
 				details = extensionCompaction.details;
 			} else {
 				// Generate compaction result
@@ -2108,6 +2118,7 @@ export class AgentSession {
 				summary = compactResult.summary;
 				firstKeptEntryId = compactResult.firstKeptEntryId;
 				tokensBefore = compactResult.tokensBefore;
+				usage = compactResult.usage;
 				details = compactResult.details;
 			}
 
@@ -2122,7 +2133,7 @@ export class AgentSession {
 				return false;
 			}
 
-			this.sessionManager.appendCompaction(summary, firstKeptEntryId, tokensBefore, details, fromExtension);
+			this.sessionManager.appendCompaction(summary, firstKeptEntryId, tokensBefore, details, fromExtension, usage);
 			const newEntries = this.sessionManager.getEntries();
 			const sessionContext = this.sessionManager.buildSessionContext();
 			this.agent.state.messages = sessionContext.messages;
@@ -2148,6 +2159,7 @@ export class AgentSession {
 				firstKeptEntryId,
 				tokensBefore,
 				estimatedTokensAfter,
+				usage,
 				details,
 			};
 			this._emit({ type: "compaction_end", reason, result, aborted: false, willRetry });
@@ -2872,7 +2884,7 @@ export class AgentSession {
 		this._branchSummaryAbortController = new AbortController();
 
 		try {
-			let extensionSummary: { summary: string; details?: unknown } | undefined;
+			let extensionSummary: { summary: string; details?: unknown; usage?: Usage } | undefined;
 			let fromExtension = false;
 
 			// Emit session_before_tree event
@@ -2907,6 +2919,7 @@ export class AgentSession {
 			// Run default summarizer if needed
 			let summaryText: string | undefined;
 			let summaryDetails: unknown;
+			let summaryUsage: Usage | undefined;
 			if (options.summarize && entriesToSummarize.length > 0 && !extensionSummary) {
 				const model = this.model!;
 				const { apiKey, headers, env } = await this._getSummarizationRequestAuth(model);
@@ -2929,6 +2942,7 @@ export class AgentSession {
 					throw new Error(result.error);
 				}
 				summaryText = result.summary;
+				summaryUsage = result.usage;
 				summaryDetails = {
 					readFiles: result.readFiles || [],
 					modifiedFiles: result.modifiedFiles || [],
@@ -2936,6 +2950,7 @@ export class AgentSession {
 			} else if (extensionSummary) {
 				summaryText = extensionSummary.summary;
 				summaryDetails = extensionSummary.details;
+				summaryUsage = extensionSummary.usage;
 			}
 
 			// Determine the new leaf position based on target type
@@ -2965,6 +2980,7 @@ export class AgentSession {
 					summaryText,
 					summaryDetails,
 					fromExtension,
+					summaryUsage,
 				);
 				summaryEntry = this.sessionManager.getEntry(summaryId) as BranchSummaryEntry;
 
@@ -3037,13 +3053,12 @@ export class AgentSession {
 		let toolResults = 0;
 		let totalMessages = 0;
 		let toolCalls = 0;
-		let totalInput = 0;
+		const usageTotals = createUsageTotals();
-		let totalOutput = 0;
-		let totalCacheRead = 0;
-		let totalCacheWrite = 0;
-		let totalCost = 0;
 
 		for (const entry of this.sessionManager.getEntries()) {
+			if ((entry.type === "branch_summary" || entry.type === "compaction") && entry.usage) {
+				addUsageToTotals(usageTotals, entry.usage);
+			}
 			if (entry.type !== "message") continue;
 			totalMessages++;
 			const message = entry.message;
@@ -3051,18 +3066,16 @@ export class AgentSession {
 				userMessages++;
 			} else if (message.role === "toolResult") {
 				toolResults++;
+				if (message.usage) {
+					addUsageToTotals(usageTotals, message.usage);
+				}
 			} else if (message.role === "assistant") {
 				assistantMessages++;
 				const assistantMsg = message as AssistantMessage;
 				if (Array.isArray(assistantMsg.content)) {
 					toolCalls += assistantMsg.content.filter((c) => c.type === "toolCall").length;
 				}
-				const usage = assistantMsg.usage;
+				addUsageToTotals(usageTotals, assistantMsg.usage);
-				totalInput += usage.input;
-				totalOutput += usage.output;
-				totalCacheRead += usage.cacheRead;
-				totalCacheWrite += usage.cacheWrite;
-				totalCost += usage.cost.total;
 			}
 		}
 
@@ -3075,13 +3088,13 @@ export class AgentSession {
 			toolResults,
 			totalMessages,
 			tokens: {
-				input: totalInput,
+				input: usageTotals.input,
-				output: totalOutput,
+				output: usageTotals.output,
-				cacheRead: totalCacheRead,
+				cacheRead: usageTotals.cacheRead,
-				cacheWrite: totalCacheWrite,
+				cacheWrite: usageTotals.cacheWrite,
-				total: totalInput + totalOutput + totalCacheRead + totalCacheWrite,
+				total: usageTotals.input + usageTotals.output + usageTotals.cacheRead + usageTotals.cacheWrite,
 			},
-			cost: totalCost,
+			cost: usageTotals.cost,
 			contextUsage: this.getContextUsage(),
 		};
 	}

packages/coding-agent/src/core/compaction/branch-summarization.ts

@@ -7,7 +7,7 @@
 
 import type { AgentMessage, StreamFn } from "@earendil-works/pi-agent-core";
 import { contentText } from "@earendil-works/pi-ai";
-import type { Model, SimpleStreamOptions } from "@earendil-works/pi-ai/compat";
+import type { Model, SimpleStreamOptions, Usage } from "@earendil-works/pi-ai/compat";
 import { completeSimple } from "@earendil-works/pi-ai/compat";
 import {
 	convertToLlm,
@@ -33,6 +33,7 @@ import {
 
 export interface BranchSummaryResult {
 	summary?: string;
+	usage?: Usage;
 	readFiles?: string[];
 	modifiedFiles?: string[];
 	aborted?: boolean;
@@ -363,6 +364,7 @@ export async function generateBranchSummary(
 
 	return {
 		summary: summary || "No summary generated",
+		usage: response.usage,
 		readFiles,
 		modifiedFiles,
 	};

packages/coding-agent/src/core/compaction/compaction.ts

@@ -90,10 +90,35 @@ export interface CompactionResult<T = unknown> {
 	firstKeptEntryId: string;
 	tokensBefore: number;
 	estimatedTokensAfter?: number;
+	/** Usage from the LLM call(s) that generated this summary, if available */
+	usage?: Usage;
 	/** Extension-specific data (e.g., ArtifactIndex, version markers for structured compaction) */
 	details?: T;
 }
 
+function combineUsage(first: Usage, second: Usage): Usage {
+	return {
+		input: first.input + second.input,
+		output: first.output + second.output,
+		cacheRead: first.cacheRead + second.cacheRead,
+		cacheWrite: first.cacheWrite + second.cacheWrite,
+		...(first.cacheWrite1h !== undefined || second.cacheWrite1h !== undefined
+			? { cacheWrite1h: (first.cacheWrite1h ?? 0) + (second.cacheWrite1h ?? 0) }
+			: {}),
+		...(first.reasoning !== undefined || second.reasoning !== undefined
+			? { reasoning: (first.reasoning ?? 0) + (second.reasoning ?? 0) }
+			: {}),
+		totalTokens: first.totalTokens + second.totalTokens,
+		cost: {
+			input: first.cost.input + second.cost.input,
+			output: first.cost.output + second.cost.output,
+			cacheRead: first.cost.cacheRead + second.cost.cacheRead,
+			cacheWrite: first.cost.cacheWrite + second.cost.cacheWrite,
+			total: first.cost.total + second.cost.total,
+		},
+	};
+}
+
 // ============================================================================
 // Types
 // ============================================================================
@@ -556,7 +581,7 @@ export async function generateSummary(
 	thinkingLevel?: ThinkingLevel,
 	streamFn?: StreamFn,
 	env?: Record<string, string>,
-): Promise<string> {
+): Promise<{ text: string; usage: Usage }> {
 	const maxTokens = Math.min(
 		Math.floor(0.8 * reserveTokens),
 		model.maxTokens > 0 ? model.maxTokens : Number.POSITIVE_INFINITY,
@@ -603,7 +628,7 @@ export async function generateSummary(
 
 	const textContent = contentText(response.content);
 
-	return textContent;
+	return { text: textContent, usage: response.usage };
 }
 
 // ============================================================================
@@ -759,24 +784,28 @@ export async function compact(
 
 	// Generate summaries and merge into one
 	let summary: string;
+	let summaryUsage: Usage;
 
 	if (isSplitTurn && turnPrefixMessages.length > 0) {
-		const historyResult =
+		let historyText = "No prior history.";
-			messagesToSummarize.length > 0
+		let historyUsage: Usage | undefined;
-				? await generateSummary(
+		if (messagesToSummarize.length > 0) {
-						messagesToSummarize,
+			const historyResult = await generateSummary(
-						model,
+				messagesToSummarize,
-						settings.reserveTokens,
+				model,
-						apiKey,
+				settings.reserveTokens,
-						headers,
+				apiKey,
-						signal,
+				headers,
-						customInstructions,
+				signal,
-						previousSummary,
+				customInstructions,
-						thinkingLevel,
+				previousSummary,
-						streamFn,
+				thinkingLevel,
-						env,
+				streamFn,
-					)
+				env,
-				: "No prior history.";
+			);
+			historyText = historyResult.text;
+			historyUsage = historyResult.usage;
+		}
 		const turnPrefixResult = await generateTurnPrefixSummary(
 			turnPrefixMessages,
 			model,
@@ -789,10 +818,11 @@ export async function compact(
 			streamFn,
 		);
 		// Merge into single summary
-		summary = `${historyResult}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult}`;
+		summary = `${historyText}\n\n---\n\n**Turn Context (split turn):**\n\n${turnPrefixResult.text}`;
+		summaryUsage = historyUsage ? combineUsage(historyUsage, turnPrefixResult.usage) : turnPrefixResult.usage;
 	} else {
 		// Just generate history summary
-		summary = await generateSummary(
+		const result = await generateSummary(
 			messagesToSummarize,
 			model,
 			settings.reserveTokens,
@@ -805,6 +835,8 @@ export async function compact(
 			streamFn,
 			env,
 		);
+		summary = result.text;
+		summaryUsage = result.usage;
 	}
 
 	// Compute file lists and append to summary
@@ -819,6 +851,7 @@ export async function compact(
 		summary,
 		firstKeptEntryId,
 		tokensBefore,
+		usage: summaryUsage,
 		details: { readFiles, modifiedFiles } as CompactionDetails,
 	};
 }
@@ -836,7 +869,7 @@ async function generateTurnPrefixSummary(
 	signal?: AbortSignal,
 	thinkingLevel?: ThinkingLevel,
 	streamFn?: StreamFn,
-): Promise<string> {
+): Promise<{ text: string; usage: Usage }> {
 	const maxTokens = Math.min(
 		Math.floor(0.5 * reserveTokens),
 		model.maxTokens > 0 ? model.maxTokens : Number.POSITIVE_INFINITY,
@@ -863,5 +896,8 @@ async function generateTurnPrefixSummary(
 		throw new Error(`Turn prefix summarization failed: ${response.errorMessage || "Unknown error"}`);
 	}
 
-	return contentText(response.content);
+	return {
+		text: contentText(response.content),
+		usage: response.usage,
+	};
 }

packages/coding-agent/src/core/extensions/runner.ts

@@ -883,6 +883,10 @@ export class ExtensionRunner {
 						currentEvent.isError = handlerResult.isError;
 						modified = true;
 					}
+					if (handlerResult.usage !== undefined) {
+						currentEvent.usage = handlerResult.usage;
+						modified = true;
+					}
 				} catch (err) {
 					const message = err instanceof Error ? err.message : String(err);
 					const stack = err instanceof Error ? err.stack : undefined;
@@ -904,6 +908,7 @@ export class ExtensionRunner {
 			content: currentEvent.content,
 			details: currentEvent.details,
 			isError: currentEvent.isError,
+			usage: currentEvent.usage,
 		};
 	}
 

packages/coding-agent/src/core/extensions/types.ts

@@ -30,6 +30,7 @@ import type {
 	SimpleStreamOptions,
 	TextContent,
 	ToolResultMessage,
+	Usage,
 } from "@earendil-works/pi-ai";
 import type {
 	AutocompleteItem,
@@ -905,6 +906,8 @@ interface ToolResultEventBase {
 	input: Record<string, unknown>;
 	content: (TextContent | ImageContent)[];
 	isError: boolean;
+	/** Usage from the tool execution itself, if available. */
+	usage?: Usage;
 }
 
 export interface BashToolResultEvent extends ToolResultEventBase {
@@ -1072,6 +1075,7 @@ export interface ToolResultEventResult {
 	content?: (TextContent | ImageContent)[];
 	details?: unknown;
 	isError?: boolean;
+	usage?: Usage;
 }
 
 export interface MessageEndEventResult {
@@ -1104,6 +1108,7 @@ export interface SessionBeforeTreeResult {
 	summary?: {
 		summary: string;
 		details?: unknown;
+		usage?: Usage;
 	};
 	/** Override custom instructions for summarization */
 	customInstructions?: string;

packages/coding-agent/src/core/session-manager.ts

@@ -1,5 +1,5 @@
 import type { AgentMessage } from "@earendil-works/pi-agent-core";
-import { type ImageContent, type Message, type TextContent, uuidv7 } from "@earendil-works/pi-ai";
+import { type ImageContent, type Message, type TextContent, type Usage, uuidv7 } from "@earendil-works/pi-ai";
 import { randomUUID } from "crypto";
 import {
 	appendFileSync,
@@ -73,6 +73,8 @@ export interface CompactionEntry<T = unknown> extends SessionEntryBase {
 	tokensBefore: number;
 	/** Extension-specific data (e.g., ArtifactIndex, version markers for structured compaction) */
 	details?: T;
+	/** Usage from the LLM call(s) that generated this summary, if available */
+	usage?: Usage;
 	/** True if generated by an extension, undefined/false if pi-generated (backward compatible) */
 	fromHook?: boolean;
 }
@@ -83,6 +85,8 @@ export interface BranchSummaryEntry<T = unknown> extends SessionEntryBase {
 	summary: string;
 	/** Extension-specific data (not sent to LLM) */
 	details?: T;
+	/** Usage from the LLM call that generated this summary, if available */
+	usage?: Usage;
 	/** True if generated by an extension, false if pi-generated */
 	fromHook?: boolean;
 }
@@ -1096,6 +1100,7 @@ export class SessionManager {
 		tokensBefore: number,
 		details?: T,
 		fromHook?: boolean,
+		usage?: Usage,
 	): string {
 		const entry: CompactionEntry<T> = {
 			type: "compaction",
@@ -1106,6 +1111,7 @@ export class SessionManager {
 			firstKeptEntryId,
 			tokensBefore,
 			details,
+			usage,
 			fromHook,
 		};
 		this._appendEntry(entry);
@@ -1372,7 +1378,13 @@ export class SessionManager {
 	 * Same as branch(), but also appends a branch_summary entry that captures
 	 * context from the abandoned conversation path.
 	 */
-	branchWithSummary(branchFromId: string | null, summary: string, details?: unknown, fromHook?: boolean): string {
+	branchWithSummary(
+		branchFromId: string | null,
+		summary: string,
+		details?: unknown,
+		fromHook?: boolean,
+		usage?: Usage,
+	): string {
 		if (branchFromId !== null && !this.byId.has(branchFromId)) {
 			throw new Error(`Entry ${branchFromId} not found`);
 		}
@@ -1385,6 +1397,7 @@ export class SessionManager {
 			fromId: branchFromId ?? "root",
 			summary,
 			details,
+			usage,
 			fromHook,
 		};
 		this._appendEntry(entry);

packages/coding-agent/src/core/usage-totals.ts

@@ -0,0 +1,27 @@
+import type { Usage } from "@earendil-works/pi-ai/compat";
+
+export interface UsageTotals {
+	input: number;
+	output: number;
+	cacheRead: number;
+	cacheWrite: number;
+	cost: number;
+}
+
+export function createUsageTotals(): UsageTotals {
+	return {
+		input: 0,
+		output: 0,
+		cacheRead: 0,
+		cacheWrite: 0,
+		cost: 0,
+	};
+}
+
+export function addUsageToTotals(totals: UsageTotals, usage: Usage): void {
+	totals.input += usage.input;
+	totals.output += usage.output;
+	totals.cacheRead += usage.cacheRead;
+	totals.cacheWrite += usage.cacheWrite;
+	totals.cost += usage.cost.total;
+}

packages/coding-agent/src/modes/interactive/components/footer.ts

@@ -3,6 +3,7 @@ import { type Component, truncateToWidth, visibleWidth } from "@earendil-works/p
 import type { AgentSession } from "../../../core/agent-session.ts";
 import { areExperimentalFeaturesEnabled } from "../../../core/experimental.ts";
 import type { ReadonlyFooterDataProvider } from "../../../core/footer-data-provider.ts";
+import { addUsageToTotals, createUsageTotals } from "../../../core/usage-totals.ts";
 import { theme } from "../theme/theme.ts";
 
 /**
@@ -84,25 +85,21 @@ export class FooterComponent implements Component {
 		const state = this.session.state;
 
 		// Calculate cumulative usage from ALL session entries (not just post-compaction messages)
-		let totalInput = 0;
+		const usageTotals = createUsageTotals();
-		let totalOutput = 0;
-		let totalCacheRead = 0;
-		let totalCacheWrite = 0;
-		let totalCost = 0;
 		let latestCacheHitRate: number | undefined;
 
 		for (const entry of this.session.sessionManager.getEntries()) {
 			if (entry.type === "message" && entry.message.role === "assistant") {
-				totalInput += entry.message.usage.input;
+				addUsageToTotals(usageTotals, entry.message.usage);
-				totalOutput += entry.message.usage.output;
-				totalCacheRead += entry.message.usage.cacheRead;
-				totalCacheWrite += entry.message.usage.cacheWrite;
-				totalCost += entry.message.usage.cost.total;
 
 				const latestPromptTokens =
 					entry.message.usage.input + entry.message.usage.cacheRead + entry.message.usage.cacheWrite;
 				latestCacheHitRate =
 					latestPromptTokens > 0 ? (entry.message.usage.cacheRead / latestPromptTokens) * 100 : undefined;
+			} else if (entry.type === "message" && entry.message.role === "toolResult" && entry.message.usage) {
+				addUsageToTotals(usageTotals, entry.message.usage);
+			} else if ((entry.type === "branch_summary" || entry.type === "compaction") && entry.usage) {
+				addUsageToTotals(usageTotals, entry.usage);
 			}
 		}
 
@@ -130,19 +127,20 @@ export class FooterComponent implements Component {
 
 		// Build stats line
 		const statsParts = [];
-		if (totalInput) statsParts.push(`↑${formatTokens(totalInput)}`);
+		if (usageTotals.input) statsParts.push(`↑${formatTokens(usageTotals.input)}`);
-		if (totalOutput) statsParts.push(`↓${formatTokens(totalOutput)}`);
+		if (usageTotals.output) statsParts.push(`↓${formatTokens(usageTotals.output)}`);
-		if (totalCacheRead) statsParts.push(`R${formatTokens(totalCacheRead)}`);
+		if (usageTotals.cacheRead) statsParts.push(`R${formatTokens(usageTotals.cacheRead)}`);
-		if (totalCacheWrite) statsParts.push(`W${formatTokens(totalCacheWrite)}`);
+		if (usageTotals.cacheWrite) statsParts.push(`W${formatTokens(usageTotals.cacheWrite)}`);
-		if ((totalCacheRead > 0 || totalCacheWrite > 0) && latestCacheHitRate !== undefined) {
+		if ((usageTotals.cacheRead > 0 || usageTotals.cacheWrite > 0) && latestCacheHitRate !== undefined) {
 			statsParts.push(`CH${latestCacheHitRate.toFixed(1)}%`);
 		}
+
 		// Kimi Coding is subscription-backed despite using API-key authentication.
 		const usingSubscription = state.model
 			? state.model.provider === "kimi-coding" || this.session.modelRuntime.isUsingOAuth(state.model.provider)
 			: false;
-		if (totalCost || usingSubscription) {
+		if (usageTotals.cost || usingSubscription) {
-			const costStr = `$${totalCost.toFixed(3)}${usingSubscription ? " (sub)" : ""}`;
+			const costStr = `$${usageTotals.cost.toFixed(3)}${usingSubscription ? " (sub)" : ""}`;
 			statsParts.push(costStr);
 		}
 

packages/coding-agent/test/agent-session-stats.test.ts

@@ -1,5 +1,5 @@
 import { Agent } from "@earendil-works/pi-agent-core";
-import { type AssistantMessage, getModel, type Usage } from "@earendil-works/pi-ai/compat";
+import { type AssistantMessage, getModel, type ToolResultMessage, type Usage } from "@earendil-works/pi-ai/compat";
 import { describe, expect, it } from "vitest";
 import { AgentSession } from "../src/core/agent-session.ts";
 import { AuthStorage } from "../src/core/auth-storage.ts";
@@ -48,6 +48,18 @@ function createUserMessage(text: string, timestamp: number) {
 	};
 }
 
+function createToolResultMessage(usage: Usage): ToolResultMessage {
+	return {
+		role: "toolResult",
+		toolCallId: "tool-call-1",
+		toolName: "test_tool",
+		content: [{ type: "text", text: "tool result" }],
+		usage,
+		isError: false,
+		timestamp: 1,
+	};
+}
+
 async function createSession() {
 	const settingsManager = SettingsManager.inMemory();
 	const sessionManager = SessionManager.inMemory();
@@ -143,6 +155,75 @@ describe("AgentSession.getSessionStats", () => {
 		}
 	});
 
+	it("includes branch summary usage in session totals", async () => {
+		const { session, sessionManager } = await createSession();
+
+		try {
+			sessionManager.branchWithSummary(null, "summary", undefined, false, {
+				input: 10,
+				output: 20,
+				cacheRead: 30,
+				cacheWrite: 40,
+				totalTokens: 100,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			});
+			syncAgentMessages(session, sessionManager);
+
+			const stats = session.getSessionStats();
+			expect(stats.tokens).toEqual({ input: 10, output: 20, cacheRead: 30, cacheWrite: 40, total: 100 });
+			expect(stats.cost).toBe(1);
+		} finally {
+			session.dispose();
+		}
+	});
+
+	it("includes compaction usage in session totals", async () => {
+		const { session, sessionManager } = await createSession();
+
+		try {
+			const firstKeptEntryId = sessionManager.appendMessage(createUserMessage("hello", 1));
+			sessionManager.appendCompaction("summary", firstKeptEntryId, 100, undefined, false, {
+				input: 10,
+				output: 20,
+				cacheRead: 30,
+				cacheWrite: 40,
+				totalTokens: 100,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			});
+			syncAgentMessages(session, sessionManager);
+
+			const stats = session.getSessionStats();
+			expect(stats.tokens).toEqual({ input: 10, output: 20, cacheRead: 30, cacheWrite: 40, total: 100 });
+			expect(stats.cost).toBe(1);
+		} finally {
+			session.dispose();
+		}
+	});
+
+	it("includes tool result usage in session totals", async () => {
+		const { session, sessionManager } = await createSession();
+
+		try {
+			sessionManager.appendMessage(
+				createToolResultMessage({
+					input: 10,
+					output: 20,
+					cacheRead: 30,
+					cacheWrite: 40,
+					totalTokens: 100,
+					cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+				}),
+			);
+			syncAgentMessages(session, sessionManager);
+
+			const stats = session.getSessionStats();
+			expect(stats.tokens).toEqual({ input: 10, output: 20, cacheRead: 30, cacheWrite: 40, total: 100 });
+			expect(stats.cost).toBe(1);
+		} finally {
+			session.dispose();
+		}
+	});
+
 	it("ignores zero-usage messages when checking for post-compaction context usage", async () => {
 		const { session, sessionManager } = await createSession();
 

packages/coding-agent/test/branch-summary-extensions.test.ts

@@ -0,0 +1,55 @@
+import type { Usage } from "@earendil-works/pi-ai/compat";
+import { afterEach, describe, expect, it } from "vitest";
+import { createHarness, type Harness } from "./suite/harness.ts";
+import { assistantMsg, userMsg } from "./utilities.ts";
+
+describe("Branch summary extensions", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	it("persists extension-provided summary usage in session totals", async () => {
+		const usage: Usage = {
+			input: 10,
+			output: 20,
+			cacheRead: 30,
+			cacheWrite: 40,
+			totalTokens: 100,
+			cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+		};
+		const harness = await createHarness({
+			extensionFactories: [
+				(pi) => {
+					pi.on("session_before_tree", () => ({
+						summary: {
+							summary: "Summary provided by extension",
+							usage,
+						},
+					}));
+				},
+			],
+		});
+		harnesses.push(harness);
+
+		const targetId = harness.sessionManager.appendMessage(userMsg("first branch"));
+		harness.sessionManager.appendMessage(assistantMsg("first reply"));
+		harness.sessionManager.appendMessage(userMsg("abandoned branch work"));
+		harness.sessionManager.appendMessage(assistantMsg("abandoned reply"));
+
+		const result = await harness.session.navigateTree(targetId, { summarize: true });
+		const summaryEntry = result.summaryEntry;
+
+		expect(summaryEntry?.type).toBe("branch_summary");
+		expect(summaryEntry?.fromHook).toBe(true);
+		expect(summaryEntry?.summary).toBe("Summary provided by extension");
+		expect(summaryEntry?.usage).toEqual(usage);
+
+		const stats = harness.session.getSessionStats();
+		expect(stats.tokens).toEqual({ input: 12, output: 22, cacheRead: 30, cacheWrite: 40, total: 104 });
+		expect(stats.cost).toBe(1);
+	});
+});

packages/coding-agent/test/compaction-summary-reasoning.test.ts

@@ -57,7 +57,7 @@ describe("generateSummary reasoning options", () => {
 	});
 
 	it("uses the provided thinking level for reasoning-capable models", async () => {
-		await generateSummary(
+		const result = await generateSummary(
 			messages,
 			createModel(true),
 			2000,
@@ -69,6 +69,9 @@ describe("generateSummary reasoning options", () => {
 			"medium",
 		);
 
+		expect(result.text).toBe("## Goal\nTest summary");
+		expect(result.usage).toEqual(mockSummaryResponse.usage);
+
 		expect(completeSimpleMock).toHaveBeenCalledTimes(1);
 		expect(completeSimpleMock.mock.calls[0][2]).toMatchObject({
 			reasoning: "medium",
@@ -127,8 +130,15 @@ describe("generateSummary reasoning options", () => {
 			settings: { enabled: true, reserveTokens: 500000, keepRecentTokens: 20000 },
 		};
 
-		await compact(preparation, createModel(false, 128000), "test-key");
+		const result = await compact(preparation, createModel(false, 128000), "test-key");
 
+		expect(result.usage).toEqual({
+			...mockSummaryResponse.usage,
+			input: 20,
+			output: 20,
+			totalTokens: 40,
+			cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+		});
 		expect(completeSimpleMock.mock.calls.map((call) => call[2]?.maxTokens)).toEqual([128000, 128000]);
 	});
 });

packages/coding-agent/test/footer-width.test.ts

@@ -21,20 +21,46 @@ function createSession(options: {
 	reasoning?: boolean;
 	thinkingLevel?: string;
 	usage?: AssistantUsage;
+	branchUsage?: AssistantUsage;
+	compactionUsage?: AssistantUsage;
+	toolUsage?: AssistantUsage;
 }): AgentSession {
 	const usage = options.usage;
-	const entries =
+	const entries: Array<Record<string, unknown>> = [];
-		usage === undefined
+
-			? []
+	if (usage !== undefined) {
-			: [
+		entries.push({
-					{
+			type: "message",
-						type: "message",
+			message: {
-						message: {
+				role: "assistant",
-							role: "assistant",
+				usage,
-							usage,
+			},
-						},
+		});
-					},
+	}
-				];
+
+	if (options.branchUsage !== undefined) {
+		entries.push({
+			type: "branch_summary",
+			usage: options.branchUsage,
+		});
+	}
+
+	if (options.compactionUsage !== undefined) {
+		entries.push({
+			type: "compaction",
+			usage: options.compactionUsage,
+		});
+	}
+
+	if (options.toolUsage !== undefined) {
+		entries.push({
+			type: "message",
+			message: {
+				role: "toolResult",
+				usage: options.toolUsage,
+			},
+		});
+	}
 
 	const session = {
 		state: {
@@ -125,6 +151,44 @@ describe("FooterComponent width handling", () => {
 		}
 	});
 
+	it("includes summary and tool result usage in the total cost", () => {
+		const session = createSession({
+			sessionName: "",
+			usage: {
+				input: 100,
+				output: 10,
+				cacheRead: 0,
+				cacheWrite: 0,
+				cost: { total: 0.5 },
+			},
+			branchUsage: {
+				input: 20,
+				output: 5,
+				cacheRead: 0,
+				cacheWrite: 0,
+				cost: { total: 0.25 },
+			},
+			compactionUsage: {
+				input: 5,
+				output: 2,
+				cacheRead: 0,
+				cacheWrite: 0,
+				cost: { total: 0.125 },
+			},
+			toolUsage: {
+				input: 15,
+				output: 3,
+				cacheRead: 0,
+				cacheWrite: 0,
+				cost: { total: 0.375 },
+			},
+		});
+		const footer = new FooterComponent(session, createFooterData(1));
+
+		const statsLine = stripAnsi(footer.render(120)[1]);
+		expect(statsLine).toContain("$1.250");
+	});
+
 	it("shows the latest cache hit rate when cache usage is present", () => {
 		const session = createSession({
 			sessionName: "",

packages/coding-agent/test/session-manager/tree-traversal.test.ts

@@ -71,7 +71,15 @@ describe("SessionManager append and tree traversal", () => {
 
 			const id1 = session.appendMessage(userMsg("1"));
 			const id2 = session.appendMessage(assistantMsg("2"));
-			const compactionId = session.appendCompaction("summary", id1, 1000);
+			const usage = {
+				input: 10,
+				output: 20,
+				cacheRead: 30,
+				cacheWrite: 40,
+				totalTokens: 100,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			};
+			const compactionId = session.appendCompaction("summary", id1, 1000, undefined, false, usage);
 			const _id3 = session.appendMessage(userMsg("3"));
 
 			const entries = session.getEntries();
@@ -83,6 +91,7 @@ describe("SessionManager append and tree traversal", () => {
 				expect(compactionEntry.summary).toBe("summary");
 				expect(compactionEntry.firstKeptEntryId).toBe(id1);
 				expect(compactionEntry.tokensBefore).toBe(1000);
+				expect(compactionEntry.usage).toEqual(usage);
 			}
 
 			expect(entries[3].parentId).toBe(compactionId);
@@ -319,7 +328,15 @@ describe("SessionManager append and tree traversal", () => {
 			const _id2 = session.appendMessage(assistantMsg("2"));
 			const _id3 = session.appendMessage(userMsg("3"));
 
-			const summaryId = session.branchWithSummary(id1, "Summary of abandoned work");
+			const usage = {
+				input: 10,
+				output: 20,
+				cacheRead: 30,
+				cacheWrite: 40,
+				totalTokens: 100,
+				cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+			};
+			const summaryId = session.branchWithSummary(id1, "Summary of abandoned work", undefined, false, usage);
 
 			expect(session.getLeafId()).toBe(summaryId);
 
@@ -329,6 +346,7 @@ describe("SessionManager append and tree traversal", () => {
 			expect(summaryEntry?.parentId).toBe(id1);
 			if (summaryEntry?.type === "branch_summary") {
 				expect(summaryEntry.summary).toBe("Summary of abandoned work");
+				expect(summaryEntry.usage).toEqual(usage);
 			}
 		});
 

packages/coding-agent/test/suite/agent-session-compaction.test.ts

@@ -97,6 +97,14 @@ describe("AgentSession compaction characterization", () => {
 	});
 
 	it("manually compacts using an extension-provided summary", async () => {
+		const summaryUsage = {
+			input: 10,
+			output: 20,
+			cacheRead: 30,
+			cacheWrite: 40,
+			totalTokens: 100,
+			cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+		};
 		const harness = await createHarness({
 			settings: { compaction: { keepRecentTokens: 1 } },
 			extensionFactories: [
@@ -106,6 +114,7 @@ describe("AgentSession compaction characterization", () => {
 							summary: "summary from extension",
 							firstKeptEntryId: event.preparation.firstKeptEntryId,
 							tokensBefore: event.preparation.tokensBefore,
+							usage: summaryUsage,
 							details: { source: "extension" },
 						},
 					}));
@@ -116,14 +125,26 @@ describe("AgentSession compaction characterization", () => {
 
 		await harness.session.prompt("one");
 		await harness.session.prompt("two");
+		const statsBefore = harness.session.getSessionStats();
 
 		const result = await harness.session.compact();
 		const compactionEntries = harness.sessionManager.getEntries().filter((entry) => entry.type === "compaction");
 		const estimatedTokensAfter = harness.session.messages.reduce((sum, message) => sum + estimateTokens(message), 0);
 
 		expect(result.summary).toBe("summary from extension");
+		expect(result.usage).toEqual(summaryUsage);
 		expect(result.estimatedTokensAfter).toBe(estimatedTokensAfter);
 		expect(compactionEntries).toHaveLength(1);
+		const compactionEntry = compactionEntries[0];
+		if (compactionEntry?.type === "compaction") {
+			expect(compactionEntry.usage).toEqual(summaryUsage);
+		}
+		const statsAfter = harness.session.getSessionStats();
+		expect(statsAfter.tokens.input).toBe(statsBefore.tokens.input + summaryUsage.input);
+		expect(statsAfter.tokens.output).toBe(statsBefore.tokens.output + summaryUsage.output);
+		expect(statsAfter.tokens.cacheRead).toBe(statsBefore.tokens.cacheRead + summaryUsage.cacheRead);
+		expect(statsAfter.tokens.cacheWrite).toBe(statsBefore.tokens.cacheWrite + summaryUsage.cacheWrite);
+		expect(statsAfter.cost).toBe(statsBefore.cost + summaryUsage.cost.total);
 		expect(harness.session.messages[0]?.role).toBe("compactionSummary");
 	});
 
@@ -154,6 +175,22 @@ describe("AgentSession compaction characterization", () => {
 		expect(getStreamCallCount()).toBe(1);
 	});
 
+	it("persists usage from pi-generated manual compaction", async () => {
+		const harness = await createHarness({ withConfiguredAuth: false });
+		harnesses.push(harness);
+		seedCompactableSession(harness);
+		useSummaryStreamFn(harness, "summary from custom stream");
+
+		const result = await harness.session.compact();
+
+		const compactionEntries = harness.sessionManager.getEntries().filter((entry) => entry.type === "compaction");
+		expect(result.usage).toEqual(createUsage(10));
+		expect(compactionEntries).toHaveLength(1);
+		expect(compactionEntries[0]?.type === "compaction" ? compactionEntries[0].usage : undefined).toEqual(
+			createUsage(10),
+		);
+	});
+
 	it("auto-compacts with a custom streamFn when registry auth is absent", async () => {
 		const harness = await createHarness({ withConfiguredAuth: false });
 		harnesses.push(harness);

packages/coding-agent/test/suite/agent-session-model-extension.test.ts

@@ -1,5 +1,5 @@
 import type { AgentTool, ThinkingLevel } from "@earendil-works/pi-agent-core";
-import { fauxAssistantMessage, fauxToolCall, type Model } from "@earendil-works/pi-ai";
+import { fauxAssistantMessage, fauxToolCall, type Model, type Usage } from "@earendil-works/pi-ai";
 import { Type } from "typebox";
 import { afterEach, describe, expect, it } from "vitest";
 import type { BuildSystemPromptOptions, ExtensionAPI } from "../../src/index.ts";
@@ -156,6 +156,23 @@ describe("AgentSession model and extension characterization", () => {
 	});
 
 	it("allows extension tool_result handlers to modify tool results", async () => {
+		const toolUsage: Usage = {
+			input: 1,
+			output: 2,
+			cacheRead: 3,
+			cacheWrite: 4,
+			totalTokens: 10,
+			cost: { input: 0.1, output: 0.2, cacheRead: 0.3, cacheWrite: 0.4, total: 1 },
+		};
+		const patchedToolUsage: Usage = {
+			input: 5,
+			output: 6,
+			cacheRead: 7,
+			cacheWrite: 8,
+			totalTokens: 26,
+			cost: { input: 0.5, output: 0.6, cacheRead: 0.7, cacheWrite: 0.8, total: 2.6 },
+		};
+		let observedToolUsage: Usage | undefined;
 		const echoTool: AgentTool = {
 			name: "echo",
 			label: "Echo",
@@ -163,17 +180,21 @@ describe("AgentSession model and extension characterization", () => {
 			parameters: Type.Object({ text: Type.String() }),
 			execute: async (_toolCallId, params) => {
 				const text = typeof params === "object" && params !== null && "text" in params ? String(params.text) : "";
-				return { content: [{ type: "text", text }], details: { text } };
+				return { content: [{ type: "text", text }], details: { text }, usage: toolUsage };
 			},
 		};
 		const harness = await createHarness({
 			tools: [echoTool],
 			extensionFactories: [
 				(pi) => {
-					pi.on("tool_result", async () => ({
+					pi.on("tool_result", async (event) => {
-						content: [{ type: "text", text: "patched result" }],
+						observedToolUsage = event.usage;
-						details: { patched: true },
+						return {
-					}));
+							content: [{ type: "text", text: "patched result" }],
+							details: { patched: true },
+							usage: patchedToolUsage,
+						};
+					});
 				},
 			],
 		});
@@ -196,9 +217,12 @@ describe("AgentSession model and extension characterization", () => {
 		await harness.session.prompt("hi");
 
 		expect(getAssistantTexts(harness)).toContain("patched result");
-		expect(
+		const toolResult = harness.session.messages.find(
-			harness.session.messages.find((message) => message.role === "toolResult" && message.details?.patched === true),
+			(message) => message.role === "toolResult" && message.details?.patched === true,
-		).toBeDefined();
+		);
+		expect(observedToolUsage).toEqual(toolUsage);
+		expect(toolResult).toBeDefined();
+		expect(toolResult?.role === "toolResult" ? toolResult.usage : undefined).toEqual(patchedToolUsage);
 	});
 
 	it("allows extension context handlers to modify messages before the LLM call", async () => {

packages/coding-agent/test/suite/regressions/6324-branch-summary-ambient-auth.test.ts

@@ -37,7 +37,7 @@ describe("issue #6324 branch summary ambient auth", () => {
 						cacheRead: 0,
 						cacheWrite: 0,
 						totalTokens: 2,
-						cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+						cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0.25 },
 					},
 					stopReason: "stop",
 					timestamp: Date.now(),
@@ -57,5 +57,6 @@ describe("issue #6324 branch summary ambient auth", () => {
 		expect(streamCallCount).toBe(1);
 		expect(result.summaryEntry?.type).toBe("branch_summary");
 		expect(result.summaryEntry?.summary).toContain("branch summary text");
+		expect(result.summaryEntry?.usage?.cost.total).toBe(0.25);
 	});
 });