diff --git a/sdk/typescript/src/api.ts b/sdk/typescript/src/api.ts
index 4bb081be..16a83223 100644
--- a/sdk/typescript/src/api.ts
+++ b/sdk/typescript/src/api.ts
@@ -115,6 +115,10 @@ import {
   validatedGitEnvironment,
   validateMode,
 } from "./targets.js";
+import {
+  startHeadDriftMonitor,
+  type HeadDriftMonitor,
+} from "./target-drift.js";
 
 interface CodexThreadLike {
   readonly id: string | null;
@@ -425,6 +429,7 @@ export class CodexSecurity {
       id: string;
       options: WorkbenchCommandOptions;
     } | null = null;
+    let headDriftMonitor: HeadDriftMonitor | null = null;
     const workbench = this.#dependencies.runWorkbench ?? runWorkbench;
     try {
       const checkOpen = (): void => {
@@ -676,11 +681,11 @@ export class CodexSecurity {
         );
       }
       checkOpen();
+      const readRepositoryRevision =
+        this.#dependencies.repositoryRevision ?? repositoryRevision;
       const expectation: ScanExpectation = {
         repository: repo,
-        repositoryRevision: await (
-          this.#dependencies.repositoryRevision ?? repositoryRevision
-        )(repo, signal),
+        repositoryRevision: await readRepositoryRevision(repo, signal),
         target: normalized,
         mode,
         pluginVersion: runtime.plugin.version,
@@ -869,6 +874,27 @@ export class CodexSecurity {
         );
       }
       activeScan = { id: scanId, options: workbenchOptions };
+      const monitorsHeadDrift =
+        registeredRevision !== "unversioned" &&
+        (targetKind === "git_revision" ||
+          targetKind === "git_worktree" ||
+          (targetKind === "git_diff" && normalized.kind === "working_tree"));
+      if (monitorsHeadDrift) {
+        headDriftMonitor = startHeadDriftMonitor({
+          expectedRevision: registeredRevision,
+          readRevision: (revisionSignal) =>
+            readRepositoryRevision(repo, revisionSignal),
+          signal,
+          onDrift: () =>
+            notifyObserver(
+              "onWarning",
+              options.onWarning,
+              options.onObserverError,
+              "Repository HEAD changed while the scan was running; results remain bound to the original revision.",
+              { kind: "target_changed" },
+            ),
+        });
+      }
       checkOpen();
       const basePrompt = scanPrompt(
         normalized,
@@ -1241,6 +1267,7 @@ export class CodexSecurity {
       }
       throw failure;
     } finally {
+      headDriftMonitor?.stop();
       // Removing the temporary scan inputs is best effort. A throw here would replace the
       // outcome the try and catch blocks already produced, so these failures are reported
       // as warnings: a scan that failed has to say why it failed, not why its temporary
diff --git a/sdk/typescript/src/target-drift.ts b/sdk/typescript/src/target-drift.ts
new file mode 100644
index 00000000..b6251b9c
--- /dev/null
+++ b/sdk/typescript/src/target-drift.ts
@@ -0,0 +1,63 @@
+export interface HeadDriftMonitorOptions {
+  expectedRevision: string;
+  readRevision: (signal: AbortSignal) => Promise<string | null>;
+  signal: AbortSignal;
+  onDrift: () => void;
+  intervalMs?: number;
+}
+
+export interface HeadDriftMonitor {
+  readonly ready: Promise<void>;
+  check(): Promise<void>;
+  stop(): void;
+}
+
+const DEFAULT_HEAD_DRIFT_INTERVAL_MS = 1_000;
+
+export function startHeadDriftMonitor(
+  options: HeadDriftMonitorOptions,
+): HeadDriftMonitor {
+  let stopped = false;
+  let warned = false;
+  let checking = false;
+
+  const check = async (): Promise<void> => {
+    if (stopped || warned || options.signal.aborted || checking) return;
+    checking = true;
+    try {
+      const revision = await options.readRevision(options.signal);
+      if (
+        !stopped &&
+        !options.signal.aborted &&
+        revision !== null &&
+        revision !== options.expectedRevision &&
+        !warned
+      ) {
+        warned = true;
+        options.onDrift();
+      }
+    } catch {
+      // A transient inability to read HEAD should not interrupt a scan. The
+      // final target validation remains authoritative if the repository is
+      // unavailable or changes before completion.
+    } finally {
+      checking = false;
+    }
+  };
+
+  const timer = setInterval(() => {
+    void check();
+  }, options.intervalMs ?? DEFAULT_HEAD_DRIFT_INTERVAL_MS);
+  timer.unref();
+  const ready = check();
+
+  return {
+    ready,
+    check,
+    stop: () => {
+      if (stopped) return;
+      stopped = true;
+      clearInterval(timer);
+    },
+  };
+}
diff --git a/sdk/typescript/tests-ts/api.test.ts b/sdk/typescript/tests-ts/api.test.ts
index 03b373af..64637ebb 100644
--- a/sdk/typescript/tests-ts/api.test.ts
+++ b/sdk/typescript/tests-ts/api.test.ts
@@ -1942,6 +1942,61 @@ describe("CodexSecurity orchestration", () => {
     await client.close();
   });
 
+  test("warns before completion when the registered repository HEAD drifts", async () => {
+    const root = await temporaryDirectory();
+    const repository = join(root, "repository");
+    const codexHome = join(root, "codex-home");
+    const scanDir = join(root, "scan");
+    await mkdir(repository);
+    await mkdir(codexHome);
+    await mkdir(scanDir, { mode: 0o700 });
+    let revisionReads = 0;
+    const warnings: string[] = [];
+    const warningDetails: Array<{ kind: "target_changed" } | undefined> = [];
+
+    const client = new TestClient(
+      {},
+      {
+        environment: { PATH: "/usr/bin", OPENAI_API_KEY: "" },
+        prepareRuntime: async () => ({
+          ...preparedRuntime(codexHome),
+          environment: { CODEX_HOME: codexHome, PATH: "/usr/bin" },
+        }),
+        resolvePluginPython: async () => "/managed/python",
+        prepareOutputDir: async () => scanDir,
+        repositoryRevision: async () => {
+          revisionReads += 1;
+          return revisionReads === 1 ? "before" : "after";
+        },
+        createCodex: () => ({
+          startThread: () => ({
+            id: null,
+            async runStreamed() {
+              await copyCompletedScan(root);
+              return { events: completedEvents() };
+            },
+          }),
+        }),
+      },
+    );
+
+    try {
+      await client.run(repository, {
+        onWarning: (warning, details) => {
+          warnings.push(warning);
+          warningDetails.push(details);
+        },
+      });
+      expect(warnings).toContain(
+        "Repository HEAD changed while the scan was running; results remain bound to the original revision.",
+      );
+      expect(warningDetails).toContainEqual({ kind: "target_changed" });
+      expect(revisionReads).toBeGreaterThanOrEqual(2);
+    } finally {
+      await client.close();
+    }
+  });
+
   test("passes the workbench snapshot contract to dirty Git scans", async () => {
     const root = await temporaryDirectory();
     const repository = join(root, "repository");
diff --git a/sdk/typescript/tests-ts/target-drift.test.ts b/sdk/typescript/tests-ts/target-drift.test.ts
new file mode 100644
index 00000000..1e6fe87e
--- /dev/null
+++ b/sdk/typescript/tests-ts/target-drift.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, test } from "bun:test";
+import { startHeadDriftMonitor } from "../src/target-drift.js";
+
+describe("head drift monitor", () => {
+  test("warns once after the repository revision changes", async () => {
+    const controller = new AbortController();
+    let revision = "before";
+    const warnings: string[] = [];
+    const monitor = startHeadDriftMonitor({
+      expectedRevision: "before",
+      readRevision: async () => revision,
+      signal: controller.signal,
+      onDrift: () => warnings.push("changed"),
+      intervalMs: 60_000,
+    });
+
+    try {
+      await monitor.ready;
+      expect(warnings).toEqual([]);
+
+      revision = "after";
+      await monitor.check();
+      await monitor.check();
+
+      expect(warnings).toEqual(["changed"]);
+    } finally {
+      monitor.stop();
+    }
+  });
+
+  test("ignores an unavailable revision and stops cleanly", async () => {
+    const controller = new AbortController();
+    const warnings: string[] = [];
+    const monitor = startHeadDriftMonitor({
+      expectedRevision: "before",
+      readRevision: async () => null,
+      signal: controller.signal,
+      onDrift: () => warnings.push("changed"),
+      intervalMs: 60_000,
+    });
+
+    await monitor.ready;
+    monitor.stop();
+    await monitor.check();
+
+    expect(warnings).toEqual([]);
+  });
+});