@@ -19,6 +19,7 @@ import {
   loadGuardConfig,
   parseAllowlistText,
   parseGuardConfig,
+  parseInjectedScannedFiles,
   parseTsPruneLine,
   tsPruneSpawn,
   validateAllowlistEntry,
@@ -28,6 +29,19 @@ import {
 
 const repoRoot = join(import.meta.dir, "..");
 
+// tsc --listFilesOnly costs ~1.5s per spawn and the count is deterministic
+// for a clean tree. The floor tests and the end-to-end probe share one run
+// through this cache; each caller computes it on first use, so no test
+// depends on another having run.
+let cachedScannedFileCount: number | undefined;
+function liveScannedFileCount(): number {
+  cachedScannedFileCount ??= countScannedFiles(
+    repoRoot,
+    loadGuardConfig().tsconfig,
+  );
+  return cachedScannedFileCount;
+}
+
 // A probe dead export in one of the scoped files must fail the guard: the
 // exact-name exemptions cover only the remaining deferred-cleanup flags
 // (the usage-formatter flags were removed with their exports by the CL-6815
@@ -310,6 +324,17 @@ describe("pinned scan invocation", () => {
     expect(() => parseGuardConfig(null)).toThrow();
     expect(() => parseGuardConfig([])).toThrow();
   });
+
+  test("the scanned-files injection accepts no override or a positive integer", () => {
+    expect(parseInjectedScannedFiles(undefined)).toBeUndefined();
+    expect(parseInjectedScannedFiles("1217")).toBe(1217);
+  });
+
+  test("the scanned-files injection fails closed on a bogus value", () => {
+    for (const bad of ["", "0", "-5", "1.5", "abc"]) {
+      expect(() => parseInjectedScannedFiles(bad)).toThrow();
+    }
+  });
 });
 
 describe("scan coverage floor", () => {
@@ -321,14 +346,14 @@ describe("scan coverage floor", () => {
 
   test("the live program file count clears the checked-in floor", () => {
     const config = loadGuardConfig();
-    const scanned = countScannedFiles(repoRoot, config.tsconfig);
-    expect(scanned).toBeGreaterThanOrEqual(config.minScannedFiles);
+    expect(liveScannedFileCount()).toBeGreaterThanOrEqual(
+      config.minScannedFiles,
+    );
   }, 120_000);
 
   test("the checked-in floor stays tight to the live count", () => {
     const config = loadGuardConfig();
-    const scanned = countScannedFiles(repoRoot, config.tsconfig);
-    expect(scanned).toBeLessThan(config.minScannedFiles * 1.1);
+    expect(liveScannedFileCount()).toBeLessThan(config.minScannedFiles * 1.1);
   }, 120_000);
 });
 
@@ -337,24 +362,58 @@ describe("scan coverage floor", () => {
 // scripts/ tree, the real guard runs as a subprocess, and the run must exit
 // nonzero naming the probe. The probe lives only for the test (never
 // committed) so the keep-alive check cannot itself become a dead export.
+//
+// The guard normally scans the full project via ts-prune (~5s); the probe's
+// temp tsconfig includes only the probe file, so this run pays only ts-prune
+// startup while still exercising the same wired-up gate (config load,
+// allowlist validation, node-spawned ts-prune, output parsing, coverage
+// floor, exit code).
 describe("violation end to end", () => {
   test("a temp dead export fails the guard, which names it", () => {
     const probeFile = `dead-export-guard-probe-${process.pid}.ts`;
     const probeName = `deadExportGuardProbe${process.pid}`;
     const probePath = join(repoRoot, "scripts", probeFile);
+    const narrowTsconfigFile = `dead-export-guard-tsconfig-${process.pid}.json`;
+    const narrowTsconfigPath = join(repoRoot, "scripts", narrowTsconfigFile);
     writeFileSync(probePath, `export const ${probeName} = 1;\n`);
+    writeFileSync(
+      narrowTsconfigPath,
+      JSON.stringify({
+        compilerOptions: {
+          noEmit: true,
+          target: "ESNext",
+          module: "ESNext",
+          moduleResolution: "bundler",
+          skipLibCheck: true,
+        },
+        // Relative to the narrow tsconfig's own directory (scripts/).
+        files: [probeFile],
+      }),
+    );
     try {
       const ran = spawnSync(
         process.execPath,
         ["scripts/check-dead-exports.ts"],
-        { cwd: repoRoot, encoding: "utf8" },
+        {
+          cwd: repoRoot,
+          encoding: "utf8",
+          // The file-count subprocess is exercised by the floor tests above;
+          // inject the same deterministic value so this run pays only
+          // ts-prune.
+          env: {
+            ...process.env,
+            DEAD_EXPORT_GUARD_SCANNED_FILES: String(liveScannedFileCount()),
+            DEAD_EXPORT_GUARD_TSCONFIG: `scripts/${narrowTsconfigFile}`,
+          },
+        },
       );
       expect(ran.status).toBe(1);
       expect(`${ran.stdout}\n${ran.stderr}`).toContain(
         `${probeFile}: ${probeName}`,
       );
     } finally {
       rmSync(probePath, { force: true });
+      rmSync(narrowTsconfigPath, { force: true });
     }
   }, 120_000);
 });
 
@@ -19,6 +19,18 @@ import { fileURLToPath } from "node:url";
 // gate fails closed when the scanned file count drops below that config's
 // floor.
 
+// Test injection (same pattern as teardownDeadlineMs / maxScanBytes): a
+// subprocess test that needs the real gate without paying for the ~1.5s tsc
+// file-count run can pass the deterministic count itself. Production never
+// sets it, so the gate always computes the count in normal runs.
+const scannedFilesEnv = "DEAD_EXPORT_GUARD_SCANNED_FILES";
+
+// Test injection (same pattern as scannedFilesEnv): a subprocess test that
+// needs the wired-up gate without paying for the ~5s full-project ts-prune
+// run can point the pinned scan at a smaller tsconfig. Production never sets
+// it, so the gate always scans the pinned tsconfig in normal runs.
+const tsconfigEnv = "DEAD_EXPORT_GUARD_TSCONFIG";
+
 const here = dirname(fileURLToPath(import.meta.url));
 const repoRoot = dirname(here);
 const allowlistPath = join(here, "dead-export-allowlist.txt");
@@ -284,6 +296,22 @@ export function tsPruneSpawn(
   return { command: "node", args: [tsPruneBinPath, ...tsPruneArgs] };
 }
 
+// Parses the test-injection env override. Undefined means compute the count
+// as usual; anything else must be a positive integer or the gate fails
+// closed rather than trusting a bogus number.
+export function parseInjectedScannedFiles(
+  value: string | undefined,
+): number | undefined {
+  if (value === undefined) return undefined;
+  const parsed = Number(value);
+  if (!Number.isInteger(parsed) || parsed <= 0) {
+    throw new Error(
+      `${scannedFilesEnv} must be a positive integer when set (test injection only)`,
+    );
+  }
+  return parsed;
+}
+
 // Counts the TypeScript files the pinned tsconfig pulls into its program via
 // tsc --listFilesOnly: the same project ts-prune analyzes. A narrowed
 // tsconfig (or a moved scan root) shrinks this count, and the gate fails
@@ -329,6 +357,19 @@ function main(): void {
   } catch (err) {
     fail(`dead-export guard: invalid guard config: ${(err as Error).message}`);
   }
+  const overrideTsconfig = process.env[tsconfigEnv];
+  if (overrideTsconfig !== undefined) {
+    if (!existsSync(join(repoRoot, overrideTsconfig))) {
+      fail(
+        `dead-export guard: tsconfig override not found: ${overrideTsconfig}`,
+      );
+    }
+    config = {
+      ...config,
+      tsconfig: overrideTsconfig,
+      tsPruneArgs: ["-p", overrideTsconfig],
+    };
+  }
   const allowlistText = readFileSync(allowlistPath, "utf8");
   const allowlistProblems = [
     ...validateAllowlistText(allowlistText),
@@ -356,12 +397,17 @@ function main(): void {
   }
   const outcome = evaluateGuard(rules, String(pruned.stdout));
   let scannedFiles: number;
-  try {
-    scannedFiles = countScannedFiles(repoRoot, config.tsconfig);
-  } catch (err) {
-    fail(
-      `dead-export guard: could not count scanned files: ${(err as Error).message}`,
-    );
+  const injectedCount = parseInjectedScannedFiles(process.env[scannedFilesEnv]);
+  if (injectedCount !== undefined) {
+    scannedFiles = injectedCount;
+  } else {
+    try {
+      scannedFiles = countScannedFiles(repoRoot, config.tsconfig);
+    } catch (err) {
+      fail(
+        `dead-export guard: could not count scanned files: ${(err as Error).message}`,
+      );
+    }
   }
   console.log(
     `dead-export guard: ${outcome.dead} consumer-less exports, ${outcome.dead - outcome.violations.length} allowlisted, ${outcome.violations.length} violations, ${scannedFiles} scanned files (floor ${config.minScannedFiles})`,