diff --git a/.jules/sentinel.md b/.jules/sentinel.md
new file mode 100644
index 0000000..d46dfc1
--- /dev/null
+++ b/.jules/sentinel.md
@@ -0,0 +1,4 @@
+## 2025-06-12 - Prevent Path Traversal in Export Output Path
+**Vulnerability:** The `exportGherkin` command allowed specifying absolute paths or relative paths with traversal payloads (`../`) via the `--output` argument, writing generated files outside the isolated project root directory (`config.root`).
+**Learning:** `path.join` normalizes paths but does not restrict them to a specific boundary, allowing arbitrary writes on the filesystem wherever the agent/user has permissions.
+**Prevention:** Construct absolute paths using `path.resolve(root, target)` and enforce boundaries by confirming the target is within bounds utilizing `const rel = path.relative(root, target)` combined with `rel.startsWith("..")` and `path.isAbsolute(rel)`. Use dedicated errors like `INVALID_ARGUMENT` within the CLI to surface machine-parseable constraints rather than leaking stack traces.
diff --git a/src/export/gherkin.ts b/src/export/gherkin.ts
index 4710ea9..daa167d 100644
--- a/src/export/gherkin.ts
+++ b/src/export/gherkin.ts
@@ -1,7 +1,8 @@
 import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
-import { dirname, join } from "node:path";
+import { dirname, join, relative, resolve, isAbsolute } from "node:path";
 import { findUseCaseFile, readConfig, relativePath } from "../files.js";
 import { parseUseCaseMarkdown } from "../format/parse.js";
+import { VspecError } from "../errors.js";
 import type { ParsedUseCase } from "../domain/types.js";
 
 export function renderGherkin(useCase: ParsedUseCase): string {
@@ -39,7 +40,13 @@ export function exportGherkin(args: { key: string; output?: string; cwd?: string
   if (!source) throw new Error("KEY_NOT_FOUND");
   const text = renderGherkin(parseUseCaseMarkdown(readFileSync(source, "utf8")));
   const output = args.output ?? join("tests", `${args.key}.feature`);
-  const outputPath = join(config.root, output);
+  const outputPath = resolve(config.root, output);
+
+  const rel = relative(config.root, outputPath);
+  if (rel.startsWith("..") || isAbsolute(rel)) {
+    throw new VspecError("INVALID_ARGUMENT", "Output path must be inside the project root.");
+  }
+
   mkdirSync(dirname(outputPath), { recursive: true });
   writeFileSync(outputPath, text);
   return { key: args.key, text, path: relativePath(outputPath, config.root) };
diff --git a/tests/gherkin.test.ts b/tests/gherkin.test.ts
index 1a0c28e..e67dd91 100644
--- a/tests/gherkin.test.ts
+++ b/tests/gherkin.test.ts
@@ -1,7 +1,12 @@
 import { readFileSync } from "node:fs";
+import { mkdirSync, rmSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
+import { randomUUID } from "node:crypto";
+import { tmpdir } from "node:os";
 import { describe, expect, it } from "vitest";
-import { renderGherkin } from "../src/export/gherkin.js";
+import { exportGherkin, renderGherkin } from "../src/export/gherkin.js";
+import { initProject } from "../src/project.js";
+import { VspecError } from "../src/errors.js";
 import { parseUseCaseMarkdown } from "../src/format/parse.js";
 
 describe("gherkin export", () => {
@@ -10,4 +15,42 @@ describe("gherkin export", () => {
     const expected = readFileSync(join(import.meta.dirname, "fixtures/export/VSPEC-010.feature"), "utf8");
     expect(renderGherkin(useCase)).toBe(expected);
   });
+
+  it("prevents path traversal when exporting via CLI/API", () => {
+    const root = join(tmpdir(), `vspec-traversal-test-${randomUUID()}`);
+    mkdirSync(root, { recursive: true });
+    try {
+      initProject({ root, key: "TEST" });
+      mkdirSync(join(root, "specs/usecases"), { recursive: true });
+      writeFileSync(join(root, "specs/usecases/TEST-001-test.md"), `---
+vspec_format: 1
+type: usecase
+key: TEST-001
+title: Test
+scope: System
+level: SUMMARY
+primary_actor: system
+status: DRAFT
+priority: P1
+format: CASUAL
+---
+## Main Success Scenario
+1. system does a thing
+## Success Guarantee
+Something happens.
+## Minimal Guarantee
+Nothing happens.
+`);
+
+      expect(() => {
+        exportGherkin({
+          cwd: root,
+          key: "TEST-001",
+          output: "../../../../tmp/vspec-test-traversal-output.txt",
+        });
+      }).toThrowError(new VspecError("INVALID_ARGUMENT", "Output path must be inside the project root."));
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
 });