feat(subagents): show cumulative token spend and cost

extensions/shared/context-utilization.ts

@@ -1,5 +1,7 @@
 /** Compact, defensive context-window utilization formatting for child agents. */
 
+import type { SubagentSpend } from "../subagents/src/domain.ts";
+
 export interface ContextUtilization {
   /** Current conversation context occupancy; null while it is unknown after compaction. */
   tokens?: number | null;
@@ -45,3 +47,22 @@ export function formatContextUtilization(usage: ContextUtilization) {
   const percent = contextPercent(usage);
   return `${percent === undefined ? "?" : percent}%/${formatCompactTokens(capacity)}`;
 }
+
+/**
+ * Render cumulative spend as `"<in> in (<cached> cached) · <out> out"` with an
+ * optional `" · $<cost>"` tail (4 decimals below $1, 2 decimals otherwise).
+ * Returns `undefined` when spend is absent or all token fields are zero, so
+ * callers can omit the statistic entirely.
+ */
+export function formatSubagentSpend(spend?: SubagentSpend) {
+  if (!spend) return undefined;
+  const { input, output, cacheRead, cacheWrite, cost } = spend;
+  if (input === 0 && output === 0 && cacheRead === 0 && cacheWrite === 0)
+    return undefined;
+  const inTokens = input + cacheRead + cacheWrite;
+  const cachedPart =
+    cacheRead > 0 ? ` (${formatCompactTokens(cacheRead)} cached)` : "";
+  let text = `${formatCompactTokens(inTokens)} in${cachedPart} · ${formatCompactTokens(output)} out`;
+  if (cost > 0) text += ` · $${cost < 1 ? cost.toFixed(4) : cost.toFixed(2)}`;
+  return text;
+}

extensions/shared/web-observer-registry.ts

@@ -1,6 +1,7 @@
 import type {
   LiveToolState,
   SubagentSnapshot,
+  SubagentSpend,
   TranscriptItem,
   TranscriptPart,
 } from "../subagents/src/domain.ts";
@@ -62,6 +63,8 @@ export interface WebSubagentDetail extends WebSubagentActivity {
   readonly liveTools: readonly LiveToolState[];
   readonly finalText: string;
   readonly errorText?: string;
+  /** Cumulative token spend (and cost when known) for the child session. */
+  readonly spend?: SubagentSpend;
   readonly truncated: boolean;
   readonly omittedEntries: number;
 }
@@ -405,6 +408,7 @@ function boundedAgentDetail<Status extends WebWorkflowAgentActivity["status"]>(
     liveTools,
     finalText,
     ...(errorText !== undefined ? { errorText } : {}),
+    ...(source.spend !== undefined ? { spend: source.spend } : {}),
     truncated:
       truncated ||
       omittedEntries > 0 ||

extensions/subagents/index.ts

@@ -72,7 +72,10 @@ import {
 } from "../shared/child-session.ts";
 import { onSetupApply } from "../shared/setup-apply.ts";
 import { completionOwnerFor } from "../shared/completion-inbox.ts";
-import { formatContextUtilization } from "../shared/context-utilization.ts";
+import {
+  formatContextUtilization,
+  formatSubagentSpend,
+} from "../shared/context-utilization.ts";
 import {
   registerEditorLayer,
   removeEditorLayer,
@@ -122,6 +125,7 @@ import {
   formatElapsed,
   latestText,
   type SubagentSnapshot,
+  type SubagentSpend,
 } from "./src/domain.ts";
 import {
   restoreSubagentIdCounters,
@@ -205,6 +209,7 @@ interface SubagentResultDetails {
   readonly outcome?: SubagentSnapshot["outcome"];
   readonly worktreeBranch?: string;
   readonly elapsed?: string;
+  readonly spend?: SubagentSpend;
   readonly artifactSaveFailed?: boolean;
   readonly fullResultSaved?: boolean;
   readonly structured?: unknown;
@@ -218,6 +223,7 @@ interface SubagentResultDetails {
     readonly outcome?: SubagentSnapshot["outcome"];
     readonly worktreeBranch?: string;
     readonly elapsed?: string;
+    readonly spend?: SubagentSpend;
     readonly artifactSaveFailed?: boolean;
     readonly fullResultSaved?: boolean;
     readonly structured?: unknown;
@@ -246,6 +252,7 @@ function describeSubagent(snap: SubagentSnapshot) {
   const details = [
     `${snap.backend}: ${snap.meta.modelLabel ?? "?"}`,
     formatContextUtilization(snap.usage),
+    formatSubagentSpend(snap.spend),
     formatElapsed(snap),
     snap.cwd,
   ].filter(Boolean);
@@ -399,6 +406,7 @@ export function createSubagentResultDispatcher(
               ? { worktreeBranch: snaps[0]!.worktreeBranch }
               : {}),
             elapsed: formatElapsed(snaps[0]!),
+            ...(snaps[0]!.spend ? { spend: snaps[0]!.spend } : {}),
             ...(projections[0]!.artifactPath ? { fullResultSaved: true } : {}),
             ...(projections[0]!.artifactSaveFailed
               ? { artifactSaveFailed: true }
@@ -423,6 +431,7 @@ export function createSubagentResultDispatcher(
                 ? { worktreeBranch: snap.worktreeBranch }
                 : {}),
               elapsed: formatElapsed(snap),
+              ...(snap.spend ? { spend: snap.spend } : {}),
               ...(projections[index]!.artifactPath
                 ? { fullResultSaved: true }
                 : {}),
@@ -476,6 +485,7 @@ function renderSubagentResult(
             outcome: details.outcome,
             worktreeBranch: details.worktreeBranch,
             elapsed: details.elapsed,
+            spend: details.spend,
             artifactSaveFailed: details.artifactSaveFailed,
             fullResultSaved: details.fullResultSaved,
           },
@@ -507,13 +517,23 @@ function renderSubagentResult(
     : displayContent.split("\n").slice(1).join("\n").trim();
   const md = new Markdown(body, 0, 0, getMarkdownTheme());
   const container = new Text(header, 0, 0);
+  const spendLines = results.flatMap((result) => {
+    const text = formatSubagentSpend(result.spend);
+    if (!text) return [];
+    return [batched ? `${sanitizeText(result.id)} · ${text}` : text];
+  });
+  const spendText = spendLines.length
+    ? new Text(theme.fg("dim", spendLines.join("\n")), 0, 0)
+    : undefined;
   return {
     render: (width: number) => [
       ...container.render(width),
+      ...(spendText?.render(width) ?? []),
       ...md.render(width),
     ],
     invalidate: () => {
       container.invalidate();
+      spendText?.invalidate();
       md.invalidate();
     },
   };
@@ -1384,6 +1404,7 @@ export default function (
                 ? { worktreeBranch: snap.worktreeBranch }
                 : {}),
               ...(snap ? { elapsed: formatElapsed(snap) } : {}),
+              ...(snap?.spend ? { spend: snap.spend } : {}),
               ...(fullResultsSaved.has(id) ? { fullResultSaved: true } : {}),
               ...(artifactSaveFailures.has(id)
                 ? { artifactSaveFailed: true }

extensions/subagents/navigation.ts

@@ -12,7 +12,10 @@ import {
 import { spinnerFrame } from "../shared/spinner.ts";
 import { sanitizeTerminalText } from "../shared/terminal-text.ts";
 import { formatElapsed, type SubagentSnapshot } from "./src/domain.ts";
-import { contextPercent } from "../shared/context-utilization.ts";
+import {
+  contextPercent,
+  formatSubagentSpend,
+} from "../shared/context-utilization.ts";
 
 export interface SubagentStripEntry {
   snapshot: SubagentSnapshot;
@@ -170,6 +173,7 @@ export class SubagentStripWidget {
         ...activity,
         formatElapsed(snapshot),
         percent === undefined ? undefined : `${percent}% ctx`,
+        formatSubagentSpend(snapshot.spend),
       ],
       this.strip.focused ? "enter open · ↑ back" : "↓ to manage",
       single || snapshot.status === "running"

extensions/subagents/src/backends/pi.ts

@@ -55,6 +55,7 @@ import type {
   SpawnTask,
   SubagentEvent,
   SubagentMeta,
+  SubagentSpend,
   TranscriptPart,
 } from "../domain.ts";
 import { SendError, SpawnError } from "../domain.ts";
@@ -359,10 +360,26 @@ const makePiSession = (
 
     const emitUsage = () => {
       const usage = session.getContextUsage();
+      let spend: SubagentSpend | undefined;
+      try {
+        const stats = session.getSessionStats?.();
+        if (stats) {
+          spend = {
+            input: stats.tokens.input,
+            output: stats.tokens.output,
+            cacheRead: stats.tokens.cacheRead,
+            cacheWrite: stats.tokens.cacheWrite,
+            cost: stats.cost,
+          };
+        }
+      } catch {
+        // Session stats are observational; never fail a run over them.
+      }
       emit({
         _tag: "UsageChanged",
         tokens: usage?.tokens ?? undefined,
         contextWindow: activeModel()?.contextWindow ?? usage?.contextWindow,
+        ...(spend ? { spend } : {}),
       });
     };
 
@@ -402,6 +419,8 @@ const makePiSession = (
     const settle = (prompt = state.activePrompt) => {
       if (!prompt || state.activePrompt !== prompt || state.settled) return;
       state.settled = true;
+      // Final cumulative spend before the settled snapshot is read.
+      emitUsage();
       const last = prompt.lastAssistant;
       const partialText = prompt.liveText || prompt.finalText || undefined;
       const errorText =

extensions/subagents/src/domain.ts

@@ -216,11 +216,25 @@ export type SubagentEvent =
       readonly _tag: "UsageChanged";
       readonly tokens?: number;
       readonly contextWindow?: number;
+      readonly spend?: SubagentSpend;
     }
   | { readonly _tag: "MetaChanged"; readonly meta: Partial<SubagentMeta> }
   /** Non-fatal diagnostics. Fatal failures arrive as a RunSettled outcome. */
   | { readonly _tag: "BackendError"; readonly message: string };
 
+/**
+ * Cumulative token spend for a child session, mirroring Pi `SessionStats`.
+ * Unlike `usage` (context occupancy), these counters only grow and survive
+ * compaction; a resumed child keeps accumulating.
+ */
+export interface SubagentSpend {
+  readonly input: number;
+  readonly output: number;
+  readonly cacheRead: number;
+  readonly cacheWrite: number;
+  readonly cost: number;
+}
+
 // --- Snapshot ---------------------------------------------------------------
 
 /**
@@ -253,6 +267,8 @@ export interface SubagentSnapshot {
   readonly errorText?: string;
   readonly meta: SubagentMeta;
   readonly usage: { readonly tokens?: number; readonly contextWindow?: number };
+  /** Cumulative token spend (and cost when known) across the child session. */
+  readonly spend?: SubagentSpend;
   readonly transcript: ReadonlyArray<TranscriptItem>;
   /** Monotonic version bumped on every transcript mutation (see manager). */
   readonly transcriptVersion: number;

extensions/subagents/src/manager.ts

@@ -41,6 +41,7 @@ import type {
   SubagentOrigin,
   SubagentMeta,
   SubagentSnapshot,
+  SubagentSpend,
   SubagentStatus,
   TranscriptItem,
 } from "./domain.ts";
@@ -115,6 +116,7 @@ interface MutableSnapshot {
   errorText?: string;
   meta: SubagentMeta;
   usage: { tokens?: number; contextWindow?: number };
+  spend?: SubagentSpend;
   transcript: TranscriptItem[];
   /** Bumped on every transcript mutation; UI caches key on this. */
   transcriptVersion: number;
@@ -533,6 +535,7 @@ const makeManager = (config: SubagentManagerConfig = {}) =>
             tokens: event.tokens ?? s.usage.tokens,
             contextWindow: event.contextWindow ?? s.usage.contextWindow,
           };
+          s.spend = event.spend ?? s.spend;
           break;
         case "MetaChanged":
           s.meta = { ...s.meta, ...event.meta };

extensions/subagents/src/ui/takeover.ts

@@ -22,7 +22,10 @@ import { AgentSessionPage } from "../../../shared/agent-session-page.ts";
 import { hintLine, panelFrame } from "../../../shared/screen-chrome.ts";
 import { sanitizeTerminalText } from "../../../shared/terminal-text.ts";
 import { formatElapsed, type SubagentSnapshot } from "../domain.ts";
-import { formatContextUtilization } from "../../../shared/context-utilization.ts";
+import {
+  formatContextUtilization,
+  formatSubagentSpend,
+} from "../../../shared/context-utilization.ts";
 import { SPINNER_INTERVAL_MS, spinnerFrame } from "../../../shared/spinner.ts";
 import type { SubagentReadModel } from "../manager.ts";
 import { subagentTranscriptDocument } from "./transcript.ts";
@@ -359,13 +362,15 @@ export class SubagentDashboard implements Component {
       const prefix = ` ${glyph} `;
 
       const utilization = formatContextUtilization(snap.usage);
+      const spend = formatSubagentSpend(snap.spend);
       const metadata = [
         theme.fg("muted", snap.backend),
         theme.fg(
           "muted",
           sanitizeSubagentDisplayLine(snap.meta.modelLabel ?? "?") || "?",
         ),
         ...(utilization ? [theme.fg("muted", utilization)] : []),
+        ...(spend ? [theme.fg("dim", spend)] : []),
         theme.fg("muted", formatElapsed(snap)),
       ];
       const dot = theme.fg("dim", " · ");
@@ -448,6 +453,7 @@ export class TakeoverView implements Component, Focusable {
               options?.badge,
               snap.meta.modelLabel,
               formatContextUtilization(snap.usage),
+              formatSubagentSpend(snap.spend),
               formatElapsed(snap),
             ],
             errorText: snap.errorText,

extensions/subagents/src/ui/wait-result.ts

@@ -9,6 +9,8 @@ import {
   truncateToWidth,
 } from "@earendil-works/pi-tui";
 import { sanitizeText } from "../../../shared/agent-transcript.ts";
+import { formatSubagentSpend } from "../../../shared/context-utilization.ts";
+import type { SubagentSpend } from "../domain.ts";
 
 const MAX_STATUS_ROWS = 4;
 
@@ -19,6 +21,7 @@ export interface WaitResultItem {
   readonly outcome?: "completed" | "failed" | "interrupted";
   readonly worktreeBranch?: string;
   readonly elapsed?: string;
+  readonly spend?: SubagentSpend;
   readonly artifactSaveFailed?: boolean;
   readonly fullResultSaved?: boolean;
 }
@@ -105,6 +108,7 @@ export function buildWaitResultPreview(
       ? "uncertain"
       : singleLine(result.status ?? "settled");
     const elapsed = result.elapsed ? singleLine(result.elapsed) : "";
+    const spend = formatSubagentSpend(result.spend);
     const notices = [
       result.artifactSaveFailed ? "artifact not saved" : "",
       result.worktreeBranch
@@ -116,7 +120,7 @@ export function buildWaitResultPreview(
       ? theme.fg("warning", `${notices.join(" · ")} · `)
       : "";
     lines.push(
-      `  ${icon} ${attention}${theme.fg("accent", id)}${title ? theme.fg("muted", ` · ${title}`) : ""}${theme.fg("dim", ` · ${status}${elapsed ? ` · ${elapsed}` : ""}`)}`,
+      `  ${icon} ${attention}${theme.fg("accent", id)}${title ? theme.fg("muted", ` · ${title}`) : ""}${theme.fg("dim", ` · ${status}${elapsed ? ` · ${elapsed}` : ""}${spend ? ` · ${spend}` : ""}`)}`,
     );
   }
   if (results.length > statusRows.length) {