diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index dda78942..872fb90c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -90,6 +90,16 @@ jobs:
         shell: pwsh
         run: pnpm build
 
+      # Hard gate for #25-class regressions: fails the run when the OpenTUI
+      # runtime chunk is missing its exports. Runs only on the Windows lane
+      # (which already produces a build); tsdown output is platform-shape-
+      # consistent, so one lane covers the build regression. Enforced now that
+      # #28 (the build fix that makes the chunk non-empty) has landed on main —
+      # the chunk is expected to carry its exports, so a miss is a real failure.
+      - name: Build-output sentinel (#25 regression guard)
+        if: matrix.os == 'windows-latest'
+        run: node scripts/check-build-output.mjs --enforce
+
       - name: Install with Windows script
         if: matrix.os == 'windows-latest'
         shell: pwsh
diff --git a/scripts/check-build-output.mjs b/scripts/check-build-output.mjs
new file mode 100644
index 00000000..eacde42e
--- /dev/null
+++ b/scripts/check-build-output.mjs
@@ -0,0 +1,123 @@
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+// Sentinel check that guards against #25-class regressions: if the bundler
+// (tsdown / rolldown) ever tree-shakes the OpenTUI runtime module's exports
+// away again, `step` fails at runtime with
+// "OpenTUI runtime did not export createLocalTuiClientApp()". Neither the
+// installer smoke test nor CI exercise the interactive TUI path, so an empty
+// chunk can ship undetected. This check surfaces it in CI logs instead.
+//
+// The check has two modes. Default is NON-BLOCKING: it logs a warning and
+// exits 0, which let it ship independently of #28 (the build fix that makes
+// the chunk non-empty) while that fix was still in flight. Now that #28 has
+// landed on `main`, CI runs this with `--enforce`, which makes a missing/empty
+// chunk fail the run — a real hard gate. The non-blocking default is kept for
+// local runs where you just want a heads-up without a non-zero exit.
+//
+// Run after `pnpm build`.
+
+export const REQUIRED_OPENTUI_RUNTIME_SYMBOLS = [
+  "createLocalTuiClientApp",
+  "LocalStepCliTuiApp",
+];
+
+const LOCAL_TUI_APP_CHUNK_PATTERN = /^local-tui-app.*\.js$/u;
+
+/**
+ * Recursively collect every `local-tui-app*.js` chunk under `dir`. Matches the
+ * post-#28 layout (`dist/runtime/local-tui-app.js`) and any hashed variant the
+ * bundler may emit. Source maps and the cjs build are skipped — the esm chunk
+ * is what the Node launcher resolves via the dynamic import.
+ */
+export function findLocalTuiAppChunks(dir) {
+  const matches = [];
+
+  if (!fs.existsSync(dir)) {
+    return matches;
+  }
+
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const fullPath = path.join(dir, entry.name);
+
+    if (entry.isDirectory()) {
+      matches.push(...findLocalTuiAppChunks(fullPath));
+      continue;
+    }
+
+    if (
+      entry.isFile() &&
+      LOCAL_TUI_APP_CHUNK_PATTERN.test(entry.name) &&
+      !entry.name.endsWith(".map")
+    ) {
+      matches.push(fullPath);
+    }
+  }
+
+  return matches;
+}
+
+/**
+ * Given a chunk's file contents, return the required symbols that are absent.
+ * Empty array means the chunk is complete.
+ */
+export function findMissingSymbols(
+  content,
+  required = REQUIRED_OPENTUI_RUNTIME_SYMBOLS,
+) {
+  return required.filter((symbol) => !content.includes(symbol));
+}
+
+// Only run the CLI entrypoint when executed directly (`node scripts/check-build-output.mjs`),
+// not when imported by tests.
+const isMain =
+  process.argv[1] &&
+  fileURLToPath(import.meta.url) === path.resolve(process.argv[1]);
+if (isMain) {
+  const enforce = process.argv.slice(2).includes("--enforce");
+  const DIST_DIR = path.resolve("dist");
+
+  const report = (lines) => {
+    const prefix = enforce ? "✗" : "⚠";
+    for (const line of lines) {
+      console.error(`${prefix} ${line}`);
+    }
+  };
+
+  const chunks = findLocalTuiAppChunks(DIST_DIR);
+
+  if (chunks.length === 0) {
+    report([
+      "build output check: no local-tui-app chunk found under dist/",
+      "Run `pnpm build` first; if you already did, the tsdown entry for src/runtime/local-tui-app.ts may be missing.",
+    ]);
+    process.exit(enforce ? 1 : 0);
+  }
+
+  const failures = [];
+
+  for (const chunk of chunks) {
+    const missing = findMissingSymbols(fs.readFileSync(chunk, "utf8"));
+
+    if (missing.length > 0) {
+      failures.push({ chunk, missing });
+    }
+  }
+
+  if (failures.length > 0) {
+    report([
+      "build output check: OpenTUI runtime exports missing from built chunk(s)",
+      ...failures.map(
+        ({ chunk, missing }) =>
+          `  ${path.relative(DIST_DIR, chunk)}: missing ${missing.join(", ")}`,
+      ),
+      "This is the #25 regression — the bundler tree-shook the dynamic-import target. Verify the tsdown entry for local-tui-app is present.",
+    ]);
+    process.exit(enforce ? 1 : 0);
+  }
+
+  console.log(
+    `✓ build output check passed: local-tui-app chunk exports ${REQUIRED_OPENTUI_RUNTIME_SYMBOLS.join(", ")}`,
+  );
+}
diff --git a/scripts/check-build-output.test.ts b/scripts/check-build-output.test.ts
new file mode 100644
index 00000000..b6bcc75b
--- /dev/null
+++ b/scripts/check-build-output.test.ts
@@ -0,0 +1,97 @@
+import { describe, it, expect, afterEach } from "vitest";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import {
+  findLocalTuiAppChunks,
+  findMissingSymbols,
+  REQUIRED_OPENTUI_RUNTIME_SYMBOLS,
+} from "./check-build-output.mjs";
+
+const tmpDirs: string[] = [];
+
+afterEach(() => {
+  while (tmpDirs.length > 0) {
+    const dir = tmpDirs.pop()!;
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+function makeTmpDist(tree: Record<string, string>): string {
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), "step-build-check-"));
+  tmpDirs.push(root);
+  for (const [relPath, content] of Object.entries(tree)) {
+    const fullPath = path.join(root, relPath);
+    fs.mkdirSync(path.dirname(fullPath), { recursive: true });
+    fs.writeFileSync(fullPath, content);
+  }
+  return root;
+}
+
+describe("findLocalTuiAppChunks", () => {
+  it("finds the post-#28 layout dist/runtime/local-tui-app.js", () => {
+    const root = makeTmpDist({
+      "runtime/local-tui-app.js": "export {}",
+      "runtime/local-tui-app.js.map": "{}",
+      "runtime/local-opentui-entry.js": "export {}",
+    });
+
+    const chunks = findLocalTuiAppChunks(root);
+    expect(chunks).toEqual([path.join(root, "runtime", "local-tui-app.js")]);
+  });
+
+  it("finds hashed chunk variants emitted directly under dist/", () => {
+    const root = makeTmpDist({
+      "local-tui-app-dRVjt31N.js": "export {}",
+      "local-tui-app-dRVjt31N.js.map": "{}",
+    });
+
+    const chunks = findLocalTuiAppChunks(root);
+    expect(chunks).toEqual([path.join(root, "local-tui-app-dRVjt31N.js")]);
+  });
+
+  it("skips source maps and the cjs build", () => {
+    const root = makeTmpDist({
+      "runtime/local-tui-app.js": "export {}",
+      "runtime/local-tui-app.cjs": "export {}",
+      "runtime/local-tui-app.js.map": "{}",
+    });
+
+    const chunks = findLocalTuiAppChunks(root);
+    expect(chunks).toEqual([path.join(root, "runtime", "local-tui-app.js")]);
+  });
+
+  it("returns an empty list when dist/ is missing (instead of throwing)", () => {
+    expect(
+      findLocalTuiAppChunks(path.join(os.tmpdir(), "does-not-exist")),
+    ).toEqual([]);
+  });
+});
+
+describe("findMissingSymbols", () => {
+  it("returns no missing symbols for a chunk that exports everything", () => {
+    const content = `
+      export class LocalStepCliTuiApp {}
+      export async function createLocalTuiClientApp() {}
+    `;
+    expect(findMissingSymbols(content)).toEqual([]);
+  });
+
+  it("flags the #25 regression: an empty shell with only imports", () => {
+    // This is literally what main produced before #28: 3 import lines, no exports.
+    const emptyShell = [
+      'import "./local-session-target-DAN8-Cwd.js";',
+      'import "./local-tui-bootstrap-xGRRjkP0.js";',
+      'import process from "node:process";',
+    ].join("\n");
+
+    expect(findMissingSymbols(emptyShell).sort()).toEqual(
+      [...REQUIRED_OPENTUI_RUNTIME_SYMBOLS].sort(),
+    );
+  });
+
+  it("reports only the symbols that are actually absent", () => {
+    const partial = "export class LocalStepCliTuiApp {}";
+    expect(findMissingSymbols(partial)).toEqual(["createLocalTuiClientApp"]);
+  });
+});