diff --git a/tools/bg/standing-by-detector.test.ts b/tools/bg/standing-by-detector.test.ts
index cc9e7b3c5..1c8a2e8d7 100644
--- a/tools/bg/standing-by-detector.test.ts
+++ b/tools/bg/standing-by-detector.test.ts
@@ -1,24 +1,119 @@
 import { describe, expect, test } from "bun:test";
-import { DEFAULT_CONFIG, pollOnce, runDetector, type DetectorConfig } from "./standing-by-detector";
+import {
+  DEFAULT_CONFIG,
+  parseArgs,
+  parsePositiveMinutes,
+  pollOnce,
+  type Adapters,
+} from "./standing-by-detector";
 
-describe("standing-by-detector slice 1", () => {
+function fakeAdapters(nowIso: string, lastCommitIso: string | null): Adapters {
+  return {
+    now: () => new Date(nowIso),
+    lastCommitIso: () => lastCommitIso,
+  };
+}
+
+describe("standing-by-detector slice 2", () => {
   test("default config has sensible thresholds", () => {
     expect(DEFAULT_CONFIG.pollIntervalMin).toBe(5);
     expect(DEFAULT_CONFIG.idleThresholdMin).toBe(15);
     expect(DEFAULT_CONFIG.once).toBe(false);
   });
 
-  test("pollOnce returns a result with no-op detection", () => {
-    const result = pollOnce(DEFAULT_CONFIG);
-    expect(result.idleDetected).toBe(false);
-    expect(result.note).toContain("slice-1 skeleton");
+  test("pollOnce with adapters returns expected result shape (no daemon mode)", () => {
+    const result = pollOnce(
+      DEFAULT_CONFIG,
+      fakeAdapters("2026-05-13T18:00:00Z", "2026-05-13T17:58:00Z"),
+    );
     expect(result.pollAt).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+    expect(typeof result.idleDetected).toBe("boolean");
+    expect(result.idleMinutes).toBe(2);
+  });
+
+  describe("pollOnce with injected adapters", () => {
+    test("flags idle when last commit is older than threshold", () => {
+      const result = pollOnce(
+        { ...DEFAULT_CONFIG, idleThresholdMin: 15 },
+        fakeAdapters("2026-05-13T18:00:00Z", "2026-05-13T17:40:00Z"),
+      );
+      expect(result.idleDetected).toBe(true);
+      expect(result.idleMinutes).toBe(20);
+      expect(result.lastCommitAt).toBe("2026-05-13T17:40:00.000Z");
+      expect(result.note).toContain("Standing-by candidate");
+    });
+
+    test("does NOT flag idle when last commit is recent", () => {
+      const result = pollOnce(
+        { ...DEFAULT_CONFIG, idleThresholdMin: 15 },
+        fakeAdapters("2026-05-13T18:00:00Z", "2026-05-13T17:55:00Z"),
+      );
+      expect(result.idleDetected).toBe(false);
+      expect(result.idleMinutes).toBe(5);
+      expect(result.note).toContain("under threshold");
+    });
+
+    test("flags idle at exactly the threshold (inclusive)", () => {
+      const result = pollOnce(
+        { ...DEFAULT_CONFIG, idleThresholdMin: 15 },
+        fakeAdapters("2026-05-13T18:00:00Z", "2026-05-13T17:45:00Z"),
+      );
+      expect(result.idleDetected).toBe(true);
+      expect(result.idleMinutes).toBe(15);
+    });
+
+    test("handles null lastCommit (fresh repo / git unavailable)", () => {
+      const result = pollOnce(
+        DEFAULT_CONFIG,
+        fakeAdapters("2026-05-13T18:00:00Z", null),
+      );
+      expect(result.idleDetected).toBe(false);
+      expect(result.lastCommitAt).toBeNull();
+      expect(result.idleMinutes).toBeNull();
+      expect(result.note).toContain("no commit found");
+    });
+
+    test("clamps negative idleMinutes to zero (clock-skew safety)", () => {
+      const result = pollOnce(
+        DEFAULT_CONFIG,
+        fakeAdapters("2026-05-13T17:00:00Z", "2026-05-13T18:00:00Z"),
+      );
+      expect(result.idleMinutes).toBe(0);
+      expect(result.idleDetected).toBe(false);
+    });
   });
 
-  test("runDetector with once: true exits after one iteration", async () => {
-    const config: DetectorConfig = { ...DEFAULT_CONFIG, once: true };
-    const results = await runDetector(config);
-    expect(results).toHaveLength(1);
-    expect(results[0]!.idleDetected).toBe(false);
+  describe("parsePositiveMinutes", () => {
+    test("accepts positive finite numbers", () => {
+      expect(parsePositiveMinutes("5", "--poll-min")).toBe(5);
+    });
+
+    test("rejects undefined / non-numeric / zero / negative / Infinity", () => {
+      expect(() => parsePositiveMinutes(undefined, "--poll-min")).toThrow(/requires a value/);
+      expect(() => parsePositiveMinutes("abc", "--poll-min")).toThrow(/positive finite/);
+      expect(() => parsePositiveMinutes("0", "--poll-min")).toThrow(/positive finite/);
+      expect(() => parsePositiveMinutes("-3", "--poll-min")).toThrow(/positive finite/);
+      expect(() => parsePositiveMinutes("Infinity", "--poll-min")).toThrow(/positive finite/);
+    });
+  });
+
+  describe("parseArgs", () => {
+    test("default config when no args", () => {
+      expect(parseArgs([])).toEqual(DEFAULT_CONFIG);
+    });
+
+    test("--once flag", () => {
+      expect(parseArgs(["--once"]).once).toBe(true);
+    });
+
+    test("--poll-min + --idle-min set values", () => {
+      const config = parseArgs(["--poll-min", "10", "--idle-min", "30"]);
+      expect(config.pollIntervalMin).toBe(10);
+      expect(config.idleThresholdMin).toBe(30);
+    });
+
+    test("rejects unknown flags fail-fast", () => {
+      expect(() => parseArgs(["--unknown"])).toThrow(/unknown flag/);
+    });
   });
 });
diff --git a/tools/bg/standing-by-detector.ts b/tools/bg/standing-by-detector.ts
index c0eb315c3..31d4f19c2 100644
--- a/tools/bg/standing-by-detector.ts
+++ b/tools/bg/standing-by-detector.ts
@@ -1,20 +1,21 @@
-// standing-by-detector.ts — B-0440 slice 1: skeleton + no-op poll loop
+// standing-by-detector.ts — B-0440 slice 2: commit-history poll via `git log`
 //
-// Background service that detects when an agent has been "Standing by" for N
-// consecutive autonomous-loop ticks without producing substrate. Future slices
-// add commit-history polling, PR-activity polling, and bus integration to
-// publish nudges via B-0400 protocol.
+// Background service that detects when an agent has been Standing by (idle)
+// by comparing the timestamp of the most recent commit on HEAD against a
+// configurable idle threshold (`idleThresholdMin`). When the gap exceeds
+// the threshold the detector flags the agent as a Standing-by candidate.
 //
-// This slice ships ONLY the skeleton: poll-loop scaffolding + configurable
-// thresholds + log output. No real detection yet.
+// PR-activity polling and bus-publish are still TBD (slices 3 + 4).
 //
 // Run: bun tools/bg/standing-by-detector.ts [--once] [--poll-min N] [--idle-min N]
-// Compose with: B-0440 (this row) + B-0400 (bus) + B-0441 (proactive notifier).
+// Compose with: B-0440 + B-0400 (bus) + B-0441 (proactive notifier).
+
+import { spawnSync } from "node:child_process";
 
 export type DetectorConfig = {
   /** How often to poll, in minutes */
   pollIntervalMin: number;
-  /** Idle threshold — if no activity in this many minutes, flag Standing-by */
+  /** Idle threshold — if no commit in this many minutes, flag Standing-by */
   idleThresholdMin: number;
   /** When true, run a single poll and exit (for testing / cron-driven mode) */
   once: boolean;
@@ -29,44 +30,87 @@ export const DEFAULT_CONFIG: DetectorConfig = {
 export type PollResult = {
   pollAt: string; // ISO-8601
   idleDetected: boolean;
+  lastCommitAt: string | null; // ISO-8601 of the most recent commit on HEAD, or null
+  idleMinutes: number | null;
   note: string;
 };
 
+/** Adapter abstraction so tests can inject a deterministic clock + git-log result. */
+export type Adapters = {
+  now: () => Date;
+  lastCommitIso: () => string | null;
+};
+
+const REAL_ADAPTERS: Adapters = {
+  now: () => new Date(),
+  lastCommitIso: () => {
+    // eslint-disable-next-line sonarjs/no-os-command-from-path -- git invoked as explicit args array; no shell, no injection risk.
+    const result = spawnSync("git", ["log", "-1", "--format=%cI", "HEAD"], {
+      encoding: "utf8",
+      stdio: ["ignore", "pipe", "ignore"],
+    });
+    if (result.status !== 0 || !result.stdout) return null;
+    const trimmed = result.stdout.trim();
+    return trimmed.length > 0 ? trimmed : null;
+  },
+};
+
 /**
- * Single poll iteration. Slice 1 returns a no-op result with a placeholder
- * note. Future slices will implement actual commit-history + PR-activity
- * checks.
+ * Single poll iteration. Reads the most recent commit on HEAD and compares
+ * its timestamp against the configured idle threshold.
  */
-export function pollOnce(_config: DetectorConfig): PollResult {
+export function pollOnce(
+  config: DetectorConfig,
+  adapters: Adapters = REAL_ADAPTERS,
+): PollResult {
+  const pollAt = adapters.now();
+  const lastCommitIso = adapters.lastCommitIso();
+
+  if (lastCommitIso === null) {
+    return {
+      pollAt: pollAt.toISOString(),
+      idleDetected: false,
+      lastCommitAt: null,
+      idleMinutes: null,
+      note: "no commit found on HEAD (fresh repo or git unavailable); cannot evaluate idle threshold",
+    };
+  }
+
+  const lastCommit = new Date(lastCommitIso);
+  const idleMs = pollAt.getTime() - lastCommit.getTime();
+  const idleMinutes = Math.max(0, idleMs / 60_000);
+  const idleDetected = idleMinutes >= config.idleThresholdMin;
+
   return {
-    pollAt: new Date().toISOString(),
-    idleDetected: false,
-    note: "slice-1 skeleton — no detection yet; future slices add commit-history + PR-activity polls",
+    pollAt: pollAt.toISOString(),
+    idleDetected,
+    lastCommitAt: lastCommit.toISOString(),
+    idleMinutes,
+    note: idleDetected
+      ? `idle ${idleMinutes.toFixed(1)}min >= threshold ${config.idleThresholdMin}min — Standing-by candidate (future slice: publish bus nudge)`
+      : `last commit ${idleMinutes.toFixed(1)}min ago; under threshold ${config.idleThresholdMin}min`,
   };
 }
 
+/** Run a single poll iteration and return its result. */
+export function runOnce(config: DetectorConfig = DEFAULT_CONFIG): PollResult {
+  const result = pollOnce(config);
+  console.log(JSON.stringify(result));
+  return result;
+}
+
 /**
- * Run the poll loop. When `once: true`, runs exactly one iteration and
- * returns its result. Otherwise sleeps for pollIntervalMin between
- * iterations and runs forever; results are NOT accumulated (the daemon
- * cannot leak memory by retaining unbounded history).
+ * Run the detector as a daemon. Sleeps for pollIntervalMin between
+ * iterations and never returns; results are NOT accumulated.
  */
-export async function runDetector(config: DetectorConfig = DEFAULT_CONFIG): Promise<PollResult[]> {
-  if (config.once) {
-    const result = pollOnce(config);
-    console.log(JSON.stringify(result));
-    return [result];
-  }
-
-  // Daemon mode: emit each result but discard after logging — never accumulate.
+export async function runDaemon(config: DetectorConfig = DEFAULT_CONFIG): Promise<never> {
   while (true) {
-    const result = pollOnce(config);
-    console.log(JSON.stringify(result));
+    runOnce(config);
     await new Promise(resolve => setTimeout(resolve, config.pollIntervalMin * 60 * 1000));
   }
 }
 
-function parsePositiveMinutes(raw: string | undefined, name: string): number {
+export function parsePositiveMinutes(raw: string | undefined, name: string): number {
   if (raw === undefined) throw new Error(`${name} requires a value`);
   const n = Number(raw);
   if (!Number.isFinite(n) || n <= 0) {
@@ -75,16 +119,23 @@ function parsePositiveMinutes(raw: string | undefined, name: string): number {
   return n;
 }
 
-function parseArgs(argv: string[]): DetectorConfig {
+const KNOWN_FLAGS = new Set(["--once", "--poll-min", "--idle-min"]);
+
+export function parseArgs(argv: string[]): DetectorConfig {
   const config: DetectorConfig = { ...DEFAULT_CONFIG };
 
   for (let i = 0; i < argv.length; i++) {
     const arg = argv[i];
-    if (arg === "--once") config.once = true;
-    else if (arg === "--poll-min") {
+    if (arg === "--once") {
+      config.once = true;
+    } else if (arg === "--poll-min") {
       config.pollIntervalMin = parsePositiveMinutes(argv[++i], "--poll-min");
     } else if (arg === "--idle-min") {
       config.idleThresholdMin = parsePositiveMinutes(argv[++i], "--idle-min");
+    } else if (KNOWN_FLAGS.has(arg)) {
+      throw new Error(`internal: known flag ${arg} not handled`);
+    } else {
+      throw new Error(`unknown flag: ${arg}; known flags: ${[...KNOWN_FLAGS].join(", ")}`);
     }
   }
 
@@ -94,5 +145,9 @@ function parseArgs(argv: string[]): DetectorConfig {
 // CLI entry — only fires when invoked directly, not when imported by tests.
 if (import.meta.main) {
   const config = parseArgs(process.argv.slice(2));
-  await runDetector(config);
+  if (config.once) {
+    runOnce(config);
+  } else {
+    await runDaemon(config);
+  }
 }