diff --git a/tools/bg/missed-substrate-detector.test.ts b/tools/bg/missed-substrate-detector.test.ts
new file mode 100644
index 000000000..6eca95e28
--- /dev/null
+++ b/tools/bg/missed-substrate-detector.test.ts
@@ -0,0 +1,76 @@
+import { describe, expect, test } from "bun:test";
+import {
+  DEFAULT_CONFIG,
+  parseArgs,
+  parsePositiveMinutes,
+  pollOnce,
+  runOnce,
+} from "./missed-substrate-detector";
+
+describe("missed-substrate-detector slice 1", () => {
+  test("default config has sensible poll interval", () => {
+    expect(DEFAULT_CONFIG.pollIntervalMin).toBe(5);
+    expect(DEFAULT_CONFIG.once).toBe(false);
+  });
+
+  test("pollOnce returns a result with no-op cascade scan", () => {
+    const result = pollOnce(DEFAULT_CONFIG);
+    expect(result.cascadesDetected).toBe(0);
+    expect(result.note).toContain("slice-1 skeleton");
+    expect(result.pollAt).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+  });
+
+  test("runOnce returns a single result without entering daemon mode", () => {
+    const result = runOnce(DEFAULT_CONFIG);
+    expect(result.cascadesDetected).toBe(0);
+    expect(result.pollAt).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+  });
+
+  describe("parsePositiveMinutes", () => {
+    test("accepts positive finite numbers", () => {
+      expect(parsePositiveMinutes("5", "--poll-min")).toBe(5);
+      expect(parsePositiveMinutes("0.5", "--poll-min")).toBe(0.5);
+    });
+
+    test("rejects undefined", () => {
+      expect(() => parsePositiveMinutes(undefined, "--poll-min")).toThrow(/requires a value/);
+    });
+
+    test("rejects non-numeric strings", () => {
+      expect(() => parsePositiveMinutes("abc", "--poll-min")).toThrow(/positive finite/);
+    });
+
+    test("rejects zero and negatives", () => {
+      expect(() => parsePositiveMinutes("0", "--poll-min")).toThrow(/positive finite/);
+      expect(() => parsePositiveMinutes("-3", "--poll-min")).toThrow(/positive finite/);
+    });
+
+    test("rejects Infinity / NaN", () => {
+      expect(() => parsePositiveMinutes("Infinity", "--poll-min")).toThrow(/positive finite/);
+      expect(() => parsePositiveMinutes("NaN", "--poll-min")).toThrow(/positive finite/);
+    });
+  });
+
+  describe("parseArgs", () => {
+    test("default config when no args", () => {
+      expect(parseArgs([])).toEqual(DEFAULT_CONFIG);
+    });
+
+    test("--once flag sets once: true", () => {
+      expect(parseArgs(["--once"]).once).toBe(true);
+    });
+
+    test("--poll-min sets the interval", () => {
+      expect(parseArgs(["--poll-min", "10"]).pollIntervalMin).toBe(10);
+    });
+
+    test("rejects unknown flags fail-fast", () => {
+      expect(() => parseArgs(["--unknown"])).toThrow(/unknown flag/);
+      expect(() => parseArgs(["--pollmin", "5"])).toThrow(/unknown flag/);
+    });
+
+    test("rejects invalid --poll-min value", () => {
+      expect(() => parseArgs(["--poll-min", "abc"])).toThrow(/positive finite/);
+    });
+  });
+});
diff --git a/tools/bg/missed-substrate-detector.ts b/tools/bg/missed-substrate-detector.ts
new file mode 100644
index 000000000..f025e3eb0
--- /dev/null
+++ b/tools/bg/missed-substrate-detector.ts
@@ -0,0 +1,105 @@
+// missed-substrate-detector.ts — B-0442 slice 1: skeleton + no-op poll loop
+//
+// Background service that detects branch-vs-merged-PR drift, e.g., commits
+// landing on a feature branch AFTER its parent PR squash-merged. The
+// canonical operational example: the substrate-recovery cascade from earlier
+// today (recovered via a follow-up PR). This service mechanizes detection
+// so the cascade is caught BEFORE branch deletion erases substrate.
+//
+// This slice ships ONLY the skeleton. Future slices add merged-PR state
+// fetch, branch-vs-squash comparison, cascade-detection bus publish, and
+// optional auto-recovery-PR opening.
+//
+// Run: bun tools/bg/missed-substrate-detector.ts [--once] [--poll-min N]
+// Compose with: B-0442 + B-0400 (bus) + B-0440 / B-0441 (companion services).
+
+export type DetectorConfig = {
+  /** How often to poll, in minutes */
+  pollIntervalMin: number;
+  /** When true, run a single poll and exit */
+  once: boolean;
+};
+
+export const DEFAULT_CONFIG: DetectorConfig = {
+  pollIntervalMin: 5,
+  once: false,
+};
+
+export type PollResult = {
+  pollAt: string; // ISO-8601
+  cascadesDetected: number;
+  note: string;
+};
+
+/**
+ * Single poll iteration. Slice 1 returns a no-op result. Future slices
+ * fetch recent merged PRs and compare branch HEAD against squash content.
+ */
+export function pollOnce(_config: DetectorConfig): PollResult {
+  return {
+    pollAt: new Date().toISOString(),
+    cascadesDetected: 0,
+    note: "slice-1 skeleton — no detection yet; future slices add merged-PR scan + branch-vs-squash compare + bus publish",
+  };
+}
+
+/**
+ * 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 detector as a daemon. Sleeps for pollIntervalMin between
+ * iterations and never returns; results are NOT accumulated (no memory
+ * growth). Caller is responsible for process termination (SIGTERM, etc.).
+ */
+export async function runDaemon(config: DetectorConfig = DEFAULT_CONFIG): Promise<never> {
+  while (true) {
+    runOnce(config);
+    await new Promise(resolve => setTimeout(resolve, config.pollIntervalMin * 60 * 1000));
+  }
+}
+
+export function parsePositiveMinutes(raw: string | undefined, name: string): number {
+  if (raw === undefined) throw new Error(`${name} requires a value`);
+  const n = Number(raw);
+  if (!Number.isFinite(n) || n <= 0) {
+    throw new Error(`${name} must be a positive finite number; got "${raw}"`);
+  }
+  return n;
+}
+
+const KNOWN_FLAGS = new Set(["--once", "--poll-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") {
+      config.pollIntervalMin = parsePositiveMinutes(argv[++i], "--poll-min");
+    } else if (KNOWN_FLAGS.has(arg)) {
+      // Defensive: should be unreachable given the explicit checks above.
+      throw new Error(`internal: known flag ${arg} not handled`);
+    } else {
+      throw new Error(`unknown flag: ${arg}; known flags: ${[...KNOWN_FLAGS].join(", ")}`);
+    }
+  }
+
+  return config;
+}
+
+if (import.meta.main) {
+  const config = parseArgs(process.argv.slice(2));
+  if (config.once) {
+    runOnce(config);
+  } else {
+    await runDaemon(config);
+  }
+}