packages/agent-hermes/__tests__/session-detail.test.ts

@@ -140,7 +140,9 @@ function createTestDb(dbPath: string): TestDb {
   db.exec(`CREATE TABLE sessions (
     id TEXT PRIMARY KEY,
     model TEXT NOT NULL,
-    started_at INTEGER NOT NULL
+    started_at INTEGER NOT NULL,
+    input_tokens INTEGER DEFAULT 0,
+    output_tokens INTEGER DEFAULT 0
   )`);
   db.exec(`CREATE TABLE messages (
     id INTEGER PRIMARY KEY AUTOINCREMENT,

packages/agent-hermes/__tests__/usage-delta.test.ts

@@ -0,0 +1,110 @@
+import { describe, expect, test } from "vitest";
+import { computeUsageDelta, snapshotUsage } from "../src/hermes.js";
+import type { HermesSessionJson } from "../src/types.js";
+
+function makeSession(overrides: Partial<HermesSessionJson> = {}): HermesSessionJson {
+  return {
+    session_id: "test-session",
+    model: "test-model",
+    session_start: "2026-01-01T00:00:00Z",
+    messages: [],
+    inputTokens: 0,
+    outputTokens: 0,
+    ...overrides,
+  };
+}
+
+describe("snapshotUsage", () => {
+  test("returns zero snapshot for null session", () => {
+    const result = snapshotUsage(null);
+    expect(result).toEqual({ turns: 0, inputTokens: 0, outputTokens: 0 });
+  });
+
+  test("returns zero snapshot for empty session", () => {
+    const result = snapshotUsage(makeSession());
+    expect(result).toEqual({ turns: 0, inputTokens: 0, outputTokens: 0 });
+  });
+
+  test("counts assistant messages as turns", () => {
+    const result = snapshotUsage(
+      makeSession({
+        messages: [
+          { role: "user", content: "hello", reasoning: null, tool_calls: null },
+          { role: "assistant", content: "hi", reasoning: null, tool_calls: null },
+          { role: "user", content: "do X", reasoning: null, tool_calls: null },
+          { role: "tool", content: "result", reasoning: null, tool_calls: null },
+          { role: "assistant", content: "done", reasoning: null, tool_calls: null },
+        ],
+        inputTokens: 1000,
+        outputTokens: 500,
+      }),
+    );
+    expect(result).toEqual({ turns: 2, inputTokens: 1000, outputTokens: 500 });
+  });
+
+  test("ignores non-assistant messages for turn count", () => {
+    const result = snapshotUsage(
+      makeSession({
+        messages: [
+          { role: "user", content: "hello", reasoning: null, tool_calls: null },
+          { role: "tool", content: "result", reasoning: null, tool_calls: null },
+        ],
+      }),
+    );
+    expect(result.turns).toBe(0);
+  });
+});
+
+describe("computeUsageDelta", () => {
+  test("first visit: before is zero, after has all values", () => {
+    const before = { turns: 0, inputTokens: 0, outputTokens: 0 };
+    const after = { turns: 3, inputTokens: 5000, outputTokens: 2000 };
+    const result = computeUsageDelta(before, after, 12.5);
+    expect(result).toEqual({
+      turns: 3,
+      inputTokens: 5000,
+      outputTokens: 2000,
+      duration: 13,
+    });
+  });
+
+  test("re-entry: computes delta correctly", () => {
+    const before = { turns: 2, inputTokens: 3000, outputTokens: 1000 };
+    const after = { turns: 4, inputTokens: 8000, outputTokens: 3500 };
+    const result = computeUsageDelta(before, after, 7.3);
+    expect(result).toEqual({
+      turns: 2,
+      inputTokens: 5000,
+      outputTokens: 2500,
+      duration: 7,
+    });
+  });
+
+  test("floors negative deltas at 0 (defensive)", () => {
+    const before = { turns: 5, inputTokens: 10000, outputTokens: 5000 };
+    const after = { turns: 3, inputTokens: 8000, outputTokens: 4000 };
+    const result = computeUsageDelta(before, after, 1.0);
+    // turns would be negative (-2), floored to 0, then || 1 gives 1
+    expect(result.turns).toBe(1);
+    expect(result.inputTokens).toBe(0);
+    expect(result.outputTokens).toBe(0);
+  });
+
+  test("zero turns delta defaults to 1 (at least one turn happened)", () => {
+    const before = { turns: 3, inputTokens: 1000, outputTokens: 500 };
+    const after = { turns: 3, inputTokens: 2000, outputTokens: 1000 };
+    const result = computeUsageDelta(before, after, 5.0);
+    // turns delta is 0, || 1 gives 1
+    expect(result.turns).toBe(1);
+    expect(result.inputTokens).toBe(1000);
+    expect(result.outputTokens).toBe(500);
+  });
+
+  test("duration is rounded", () => {
+    const before = { turns: 0, inputTokens: 0, outputTokens: 0 };
+    const after = { turns: 1, inputTokens: 100, outputTokens: 50 };
+    expect(computeUsageDelta(before, after, 3.7).duration).toBe(4);
+    expect(computeUsageDelta(before, after, 3.2).duration).toBe(3);
+    expect(computeUsageDelta(before, after, 0.0).duration).toBe(0);
+  });
+});

packages/agent-hermes/src/acp-client.ts

@@ -72,6 +72,11 @@ export class HermesAcpClient {
     return sessionId;
   }
 
+  /** Return the current session ID, or null if not connected. */
+  getSessionId(): string | null {
+    return this.sessionId;
+  }
+
   /** Send prompt and collect final assistant text from ACP stream chunks. */
   async prompt(text: string): Promise<AcpPromptResult> {
     if (this.sessionId === null) {

packages/agent-hermes/src/hermes.ts

@@ -12,9 +12,45 @@ import {
 import { HermesAcpClient } from "./acp-client.js";
 import { getCachedSessionId, setCachedSessionId } from "./session-cache.js";
 import { loadHermesSession, storeHermesSessionDetail } from "./session-detail.js";
+import type { HermesSessionJson } from "./types.js";
 
 const log = createLogger({ sink: { kind: "stderr" } });
 
+/** Snapshot of session metrics taken before and after a prompt call. */
+type UsageSnapshot = {
+  turns: number;
+  inputTokens: number;
+  outputTokens: number;
+};
+
+const ZERO_SNAPSHOT: UsageSnapshot = { turns: 0, inputTokens: 0, outputTokens: 0 };
+
+/** Extract usage metrics from a session. Returns zeros for null sessions. */
+export function snapshotUsage(session: HermesSessionJson | null): UsageSnapshot {
+  if (session === null) {
+    return ZERO_SNAPSHOT;
+  }
+  return {
+    turns: session.messages.filter((m) => m.role === "assistant").length,
+    inputTokens: session.inputTokens,
+    outputTokens: session.outputTokens,
+  };
+}
+
+/** Compute the delta between two snapshots (after minus before). Floors at 0. */
+export function computeUsageDelta(
+  before: UsageSnapshot,
+  after: UsageSnapshot,
+  durationSec: number,
+): Usage {
+  return {
+    turns: Math.max(0, after.turns - before.turns) || 1,
+    inputTokens: Math.max(0, after.inputTokens - before.inputTokens),
+    outputTokens: Math.max(0, after.outputTokens - before.outputTokens),
+    duration: Math.round(durationSec),
+  };
+}
+
 /** Assemble system prompt, task, and prior step outputs for Hermes. */
 export function buildHermesPrompt(ctx: AgentContext): string {
   const parts: string[] = [];
@@ -109,7 +145,11 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
     void client.close();
   });
 
-  async function runPrompt(ctx: AgentContext, useContinuation: boolean): Promise<AgentRunResult> {
+  async function runPrompt(
+    ctx: AgentContext,
+    useContinuation: boolean,
+    beforeSnapshot: UsageSnapshot,
+  ): Promise<AgentRunResult> {
     const effectiveCtx = useContinuation ? ctx : { ...ctx, isFirstVisit: true };
     const fullPrompt = buildHermesPrompt(effectiveCtx);
     const startMs = Date.now();
@@ -121,16 +161,9 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
       await setCachedSessionId(ctx.threadId, ctx.role, sessionId, ctx.storageRoot);
     }
 
-    const session = await loadHermesSession(sessionId);
-    const turns =
-      session !== null ? session.messages.filter((m) => m.role === "assistant").length : 1;
-
-    const usage: Usage = {
-      turns,
-      inputTokens: 0,
-      outputTokens: 0,
-      duration: Math.round(durationSec),
-    };
+    const afterSession = await loadHermesSession(sessionId);
+    const afterSnapshot = snapshotUsage(afterSession);
+    const usage = computeUsageDelta(beforeSnapshot, afterSnapshot, durationSec);
 
     return { output: text, detailHash, sessionId, assembledPrompt: fullPrompt, usage };
   }
@@ -139,8 +172,17 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
     const cwd = process.cwd();
     const attempt = await prepareSession(client, ctx, cwd, resumeDisabled);
 
+    // Snapshot before prompt: for resumed sessions, captures cumulative state
+    // so we can compute the delta. For new sessions, this is ZERO_SNAPSHOT.
+    const currentSessionId = client.getSessionId();
+    const beforeSession =
+      attempt.resumed && currentSessionId !== null
+        ? await loadHermesSession(currentSessionId)
+        : null;
+    const beforeSnapshot = snapshotUsage(beforeSession);
+
     try {
-      return await runPrompt(ctx, attempt.useContinuation);
+      return await runPrompt(ctx, attempt.useContinuation, beforeSnapshot);
     } catch (error) {
       if (!attempt.resumed) {
         throw error;
@@ -150,7 +192,8 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
       log("8FQW2R6N", `continuation prompt failed, retrying with initial prompt: ${message}`);
       await client.close();
       await client.connect(cwd);
-      return runPrompt(ctx, false);
+      // Fresh session after retry — reset snapshot to zero
+      return runPrompt(ctx, false, ZERO_SNAPSHOT);
     }
   }
 
@@ -161,17 +204,20 @@ export function createHermesAgent(resumeDisabled: boolean): () => Promise<void>
   ): Promise<AgentRunResult> {
     // Client is already connected from runHermes — same ACP session,
     // so the agent sees the full conversation history (crucial for retries).
+    // Snapshot before the continuation prompt for delta computation.
+    const currentSessionId = client.getSessionId();
+    const beforeSession =
+      currentSessionId !== null ? await loadHermesSession(currentSessionId) : null;
+    const beforeSnapshot = snapshotUsage(beforeSession);
+
     const startMs = Date.now();
     const { text, sessionId } = await client.prompt(message);
     const durationSec = (Date.now() - startMs) / 1000;
     const { detailHash } = await storePromptResult(store, sessionId);
 
-    const usage: Usage = {
-      turns: 1,
-      inputTokens: 0,
-      outputTokens: 0,
-      duration: Math.round(durationSec),
-    };
+    const afterSession = await loadHermesSession(sessionId);
+    const afterSnapshot = snapshotUsage(afterSession);
+    const usage = computeUsageDelta(beforeSnapshot, afterSnapshot, durationSec);
 
     return { output: text, detailHash, sessionId, assembledPrompt: "", usage };
   }

packages/agent-hermes/src/index.ts

@@ -1,2 +1,7 @@
 export { HermesAcpClient } from "./acp-client.js";
-export { buildHermesPrompt, createHermesAgent } from "./hermes.js";
+export {
+  buildHermesPrompt,
+  computeUsageDelta,
+  createHermesAgent,
+  snapshotUsage,
+} from "./hermes.js";

packages/agent-hermes/src/session-detail.ts

@@ -106,7 +106,7 @@ function parseSessionJson(raw: unknown): HermesSessionJson | null {
       messages.push(msg);
     }
   }
-  return { session_id, model, session_start, messages };
+  return { session_id, model, session_start, messages, inputTokens: 0, outputTokens: 0 };
 }
 
 export function getHermesDbPath(): string {
@@ -117,6 +117,8 @@ type DbSessionRow = {
   id: string;
   model: string;
   started_at: number;
+  input_tokens: number;
+  output_tokens: number;
 };
 
 type DbMessageRow = {
@@ -156,7 +158,9 @@ export function loadHermesSessionFromDb(
   try {
     db = new DatabaseSync(resolvedPath, { readOnly: true });
     const session = db
-      .prepare("SELECT id, model, started_at FROM sessions WHERE id = ?")
+      .prepare(
+        "SELECT id, model, started_at, input_tokens, output_tokens FROM sessions WHERE id = ?",
+      )
       .get(sessionId) as DbSessionRow | null;
     if (session === null) {
       return null;
@@ -181,6 +185,8 @@ export function loadHermesSessionFromDb(
       model: session.model,
       session_start: new Date(session.started_at * 1000).toISOString(),
       messages,
+      inputTokens: session.input_tokens ?? 0,
+      outputTokens: session.output_tokens ?? 0,
     };
   } catch {
     return null;

packages/agent-hermes/src/types.ts

@@ -40,4 +40,6 @@ export type HermesSessionJson = {
   model: string;
   session_start: string;
   messages: HermesSessionMessage[];
+  inputTokens: number;
+  outputTokens: number;
 };