diff --git a/src/commands/test.test.ts b/src/commands/test.test.ts
index 11ec49e..79342eb 100644
--- a/src/commands/test.test.ts
+++ b/src/commands/test.test.ts
@@ -10,7 +10,7 @@ import {
 } from 'node:fs';
 import { execFileSync, spawn } from 'node:child_process';
 import { tmpdir } from 'node:os';
-import { join } from 'node:path';
+import { join, relative } from 'node:path';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import type { Command } from 'commander';
 import type { RunResponse } from '../lib/runs.types.js';
@@ -3409,6 +3409,55 @@ describe('runScaffold', () => {
     await expect(runScaffold({ ...opts, force: true }, deps)).resolves.toBeDefined();
   });
 
+  // A trailing separator means "a directory", whether the path is relative
+  // (path.resolve drops the separator) or a native Windows path ending in `\`.
+  // Neither may be written as a FILE named after the intended directory.
+  it.each([
+    [
+      'relative path ending in /',
+      (dir: string) => `${relative(process.cwd(), join(dir, 'plans'))}/`,
+    ],
+    ['absolute path ending in \\', (dir: string) => `${join(dir, 'plans')}\\`],
+  ])('--out given a %s rejects and writes nothing', async (_label, outFor) => {
+    const dir = mkdtempSync(join(tmpdir(), 'cli-scaffold-sep-'));
+    await expect(
+      runScaffold(
+        {
+          profile: 'default',
+          output: 'text',
+          debug: false,
+          scaffoldType: 'frontend',
+          out: outFor(dir),
+          force: false,
+        },
+        { stdout: () => undefined, stderr: () => undefined, env: {} },
+      ),
+    ).rejects.toMatchObject({ code: 'VALIDATION_ERROR', exitCode: 5 });
+    expect(readdirSync(dir)).toEqual([]);
+    rmSync(dir, { recursive: true, force: true });
+  });
+
+  it('--out naming an existing directory with a trailing separator says it is a directory, not that it exists', async () => {
+    const dir = mkdtempSync(join(tmpdir(), 'cli-scaffold-sepdir-'));
+    await expect(
+      runScaffold(
+        {
+          profile: 'default',
+          output: 'text',
+          debug: false,
+          scaffoldType: 'frontend',
+          out: `${dir}/`,
+          force: false,
+        },
+        { stdout: () => undefined, stderr: () => undefined, env: {} },
+      ),
+    ).rejects.toMatchObject({
+      exitCode: 5,
+      nextAction: expect.stringContaining('must point to a file, not a directory'),
+    });
+    rmSync(dir, { recursive: true, force: true });
+  });
+
   it('--out pointing at an existing path (here a directory) rejects without --force', async () => {
     const dir = mkdtempSync(join(tmpdir(), 'cli-scaffold-dir-'));
     await expect(
diff --git a/src/commands/test.ts b/src/commands/test.ts
index 8bf5ce5..77bddcc 100644
--- a/src/commands/test.ts
+++ b/src/commands/test.ts
@@ -6237,6 +6237,8 @@ export async function runScaffold(
 
   if (opts.out !== undefined) {
     const resolved = isAbsolute(opts.out) ? opts.out : resolve(process.cwd(), opts.out);
+    // Before the overwrite check: `--out existing-dir/` is a directory, not a file that already exists.
+    assertOutIsFilePath(opts.out);
     // Never clobber silently: scaffolds are starting points the user edits, so
     // an accidental re-run must not erase their work. --force opts in.
     if (!opts.force && existsSync(resolved)) {
@@ -16461,6 +16463,16 @@ interface FileSink {
  * `resolved` directly means the real `--out` file is never truncated
  * up front, see `closeOutputFile` for the commit step.
  */
+/**
+ * Check the RAW path: `resolve()` drops a trailing separator, and on
+ * Windows the separator is usually `\`.
+ */
+function assertOutIsFilePath(rawPath: string): void {
+  if (/[\\/]$/.test(rawPath)) {
+    throw localValidationError('out', 'must point to a file, not a directory');
+  }
+}
+
 function openOutputFile(rawPath: string): FileSink {
   if (typeof rawPath !== 'string' || rawPath.length === 0) {
     throw localValidationError('out', 'must be a non-empty file path');
@@ -16469,9 +16481,7 @@ function openOutputFile(rawPath: string): FileSink {
   // Defensive: reject obviously-bad paths up front (a directory string
   // would fail later with EISDIR; that's a clearer 5/VALIDATION_ERROR
   // surface than letting it crash mid-write with TransportError).
-  if (resolved.endsWith('/')) {
-    throw localValidationError('out', 'must point to a file, not a directory');
-  }
+  assertOutIsFilePath(rawPath);
   // Validate the parent dir synchronously so a missing or non-directory
   // parent surfaces as exit 5 / VALIDATION_ERROR rather than exit 1 /
   // TRANSPORT_ERROR. Without this, an ENOENT/ENOTDIR fires asynchronously
diff --git a/src/lib/junit-report.test.ts b/src/lib/junit-report.test.ts
index 6ccdce6..6e30e02 100644
--- a/src/lib/junit-report.test.ts
+++ b/src/lib/junit-report.test.ts
@@ -1,6 +1,6 @@
-import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { mkdtempSync, readdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
-import { join } from 'node:path';
+import { join, relative } from 'node:path';
 import { describe, expect, it } from 'vitest';
 import { ApiError } from './errors.js';
 import {
@@ -402,6 +402,20 @@ describe('writeJUnitReportFile', () => {
     });
   });
 
+  it.each(['/', '\\'])(
+    'rejects a relative directory path ending in %s and writes nothing',
+    async sep => {
+      const dir = mkdtempSync(join(tmpdir(), 'junit-report-file-sep-'));
+      const target = `${relative(process.cwd(), join(dir, 'reports'))}${sep}`;
+      await expect(writeJUnitReportFile(target, '<testsuites/>')).rejects.toMatchObject({
+        code: 'VALIDATION_ERROR',
+        exitCode: 5,
+      });
+      expect(readdirSync(dir)).toEqual([]);
+      rmSync(dir, { recursive: true, force: true });
+    },
+  );
+
   it('rejects parent that is a file', async () => {
     const dir = mkdtempSync(join(tmpdir(), 'junit-report-file-parent-'));
     const parentFile = join(dir, 'not-a-dir');
diff --git a/src/lib/junit-report.ts b/src/lib/junit-report.ts
index f523cd3..b90c862 100644
--- a/src/lib/junit-report.ts
+++ b/src/lib/junit-report.ts
@@ -272,7 +272,8 @@ async function assertReportFileParent(rawPath: string): Promise<string> {
     throw localValidationError('report-file', 'must be a non-empty file path');
   }
   const resolved = isAbsolute(rawPath) ? rawPath : resolve(process.cwd(), rawPath);
-  if (resolved.endsWith('/') || resolved.endsWith('\\')) {
+  // Check the RAW path: `resolve()` drops a trailing separator.
+  if (/[\\/]$/.test(rawPath)) {
     throw localValidationError('report-file', 'must point to a file, not a directory');
   }