diff --git a/scripts/secret-scan.mjs b/scripts/secret-scan.mjs
index 1bec995e7..388cc278c 100644
--- a/scripts/secret-scan.mjs
+++ b/scripts/secret-scan.mjs
@@ -8,6 +8,16 @@ const YELLOW = "\x1b[1;33m";
 const NC = "\x1b[0m";
 const IGNORE_FILE = ".secretscanignore";
 
+// Fast-mode contract (sf-mp4w2dij-xm6cwj AC4): the regex-only path is the
+// today-default fast path. SF_SECURITY_FAST=1 is the explicit opt-in for
+// callers that want to assert "regex-only, no LLM escalation, sub-100ms"
+// regardless of any future tiered reviewer landing in this script. When the
+// LLM-backed review hook (AC1) lands, the absence of this env var becomes
+// the trigger for escalation; setting it=1 keeps the offline / pre-commit
+// fast path. Today the variable changes only the trailing status line so
+// operators can verify the contract is observable.
+const FAST_MODE = process.env.SF_SECURITY_FAST === "1";
+
 const PATTERNS = [
 	{ label: "AWS Access Key", regex: /AKIA[0-9A-Z]{16}/g },
 	{
@@ -251,4 +261,10 @@ if (findings > 0) {
 	process.exit(1);
 }
 
-process.stdout.write("secret-scan: no secrets detected ✓\n");
+if (FAST_MODE) {
+	process.stdout.write(
+		"secret-scan: fast mode (regex-only, no LLM escalation) ✓\n",
+	);
+} else {
+	process.stdout.write("secret-scan: no secrets detected ✓\n");
+}
diff --git a/scripts/secret-scan.sh b/scripts/secret-scan.sh
index 1fb53d4a2..4ff252c75 100755
--- a/scripts/secret-scan.sh
+++ b/scripts/secret-scan.sh
@@ -215,6 +215,11 @@ if [[ $FINDINGS -gt 0 ]]; then
   echo -e "${RED}Commit blocked. Remove the secrets or add exceptions${NC}"
   echo -e "${RED}to .secretscanignore if these are false positives.${NC}"
   echo -e "${RED}━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━${NC}"
+elif [[ "${SF_SECURITY_FAST:-0}" == "1" ]]; then
+  # AC4 contract (sf-mp4w2dij-xm6cwj): SF_SECURITY_FAST=1 is the explicit
+  # opt-in for the regex-only fast path. When LLM escalation lands, this
+  # env var becomes the documented opt-out for offline/pre-commit callers.
+  echo "secret-scan: fast mode (regex-only, no LLM escalation) ✓"
 else
   echo "secret-scan: no secrets detected ✓"
 fi
diff --git a/src/tests/secret-scan.test.ts b/src/tests/secret-scan.test.ts
index c47bff422..87be06fc8 100644
--- a/src/tests/secret-scan.test.ts
+++ b/src/tests/secret-scan.test.ts
@@ -18,6 +18,7 @@ const scanScript = join(projectRoot, "scripts", "secret-scan.sh");
 function scanContent(
 	content: string,
 	filename = "test-file.ts",
+	extraEnv: Record<string, string> = {},
 ): { status: number; stdout: string; stderr: string } {
 	const dir = mkdtempSync(join(tmpdir(), "secret-scan-test-"));
 	try {
@@ -38,7 +39,7 @@ function scanContent(
 		const result = spawnSync("bash", [scanScript], {
 			cwd: dir,
 			encoding: "utf-8",
-			env: { ...process.env, TERM: "dumb" },
+			env: { ...process.env, TERM: "dumb", ...extraEnv },
 		});
 
 		return {
@@ -51,6 +52,32 @@ function scanContent(
 	}
 }
 
+// ── Fast-mode contract (sf-mp4w2dij-xm6cwj AC4) ─────────────────────
+
+test(
+	"SF_SECURITY_FAST=1 prints fast-mode status on clean scan",
+	{ skip: isWindows },
+	() => {
+		const result = scanContent("const x = 1;", "clean.ts", {
+			SF_SECURITY_FAST: "1",
+		});
+		assert.equal(result.status, 0, `should pass: ${result.stdout}`);
+		assert.match(result.stdout, /fast mode \(regex-only, no LLM escalation\)/);
+	},
+);
+
+test(
+	"SF_SECURITY_FAST unset prints default clean status",
+	{ skip: isWindows },
+	() => {
+		const result = scanContent("const x = 1;", "clean.ts");
+		assert.equal(result.status, 0, `should pass: ${result.stdout}`);
+		assert.match(result.stdout, /no secrets detected/);
+		// Must NOT carry the fast-mode marker when unset
+		assert.doesNotMatch(result.stdout, /fast mode/);
+	},
+);
+
 // ── Detection tests ──────────────────────────────────────────────────
 
 test("detects AWS access key", { skip: isWindows }, () => {