diff --git a/changelog.d/3436.md b/changelog.d/3436.md
new file mode 100644
index 000000000..559b283ce
--- /dev/null
+++ b/changelog.d/3436.md
@@ -0,0 +1,2 @@
+<!-- ### Fixed -->
+- **fix(git): ブランチ切替の dirty 確認が失敗・timeout したとき切替を止める** (#3436): `status --porcelain` が null（失敗・timeout）を返すと未コミット変更の上で checkout が進みえたため、GitTimeoutError（504）で拒否する。reset のデフォルトブランチ判定も HEAD を読めないときは拒否する。
diff --git a/src/lib/git/git-branches.ts b/src/lib/git/git-branches.ts
index b6d212a40..40a81f69c 100644
--- a/src/lib/git/git-branches.ts
+++ b/src/lib/git/git-branches.ts
@@ -320,7 +320,11 @@ export async function checkoutBranch(
   // checked_out_elsewhere guard above.
   if (!force) {
     const status = await execGitCommand(['status', '--porcelain'], worktreePath);
-    if (status !== null && status.length > 0) {
+    // Fail-closed: a failed/timed-out status (null) must not be read as clean.
+    if (status === null) {
+      throw new GitTimeoutError('Could not verify working tree state');
+    }
+    if (status.length > 0) {
       throw new GitDirtyError('Working tree has uncommitted changes');
     }
   }
diff --git a/src/lib/git/git-reset.ts b/src/lib/git/git-reset.ts
index 00de6fdf1..27b741821 100644
--- a/src/lib/git/git-reset.ts
+++ b/src/lib/git/git-reset.ts
@@ -14,7 +14,7 @@ import {
   type ConflictResult,
 } from './git-exec';
 import { getDefaultBranch } from './git-default-branch';
-import { GitResetDefaultBranchError } from './git-errors';
+import { GitResetDefaultBranchError, GitTimeoutError } from './git-errors';
 
 const logger = createLogger('git-reset');
 
@@ -38,7 +38,8 @@ export interface ResetOptions {
  */
 async function isDefaultBranchForReset(worktreePath: string): Promise<boolean> {
   const current = await execGitCommand(['rev-parse', '--abbrev-ref', 'HEAD'], worktreePath);
-  if (current === null) return false;
+  // Fail-closed: an unreadable HEAD must not be treated as "not the default branch".
+  if (current === null) throw new GitTimeoutError('Could not verify current branch');
   const currentBranch = current.trim();
 
   const def = await getDefaultBranch(worktreePath);
diff --git a/tests/unit/lib/git/git-branches.test.ts b/tests/unit/lib/git/git-branches.test.ts
index 9223fbab8..032aa3f24 100644
--- a/tests/unit/lib/git/git-branches.test.ts
+++ b/tests/unit/lib/git/git-branches.test.ts
@@ -51,6 +51,7 @@ import {
   GitBranchNotMergedError,
   GitBranchCheckedOutElsewhereError,
   GitDirtyError,
+  GitTimeoutError,
   GitCurrentBranchError,
   GitDefaultBranchError,
 } from '@/lib/git/git-errors';
@@ -427,6 +428,38 @@ describe('checkoutBranch (Issue #781)', () => {
   });
 });
 
+describe('checkoutBranch fail-closed status (Issue #3436)', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockExistsSync.mockReturnValue(false);
+  });
+
+  it('does not checkout when status --porcelain fails (non-force)', async () => {
+    const calls: string[][] = [];
+    mockExecFileAsync.mockImplementation(async (_file: string, args: string[]) => {
+      calls.push(args);
+      if (args.join(' ').includes('status --porcelain')) throw new Error('boom');
+      return { stdout: '' };
+    });
+
+    await expect(checkoutBranch('/repo', { branch: 'feature/x' })).rejects.toBeInstanceOf(
+      GitTimeoutError
+    );
+    expect(calls.find((a) => a[0] === 'switch')).toBeUndefined();
+  });
+
+  it('force checkout does not consult status', async () => {
+    const calls: string[][] = [];
+    mockExecFileAsync.mockImplementation(async (_file: string, args: string[]) => {
+      calls.push(args);
+      return { stdout: '' };
+    });
+
+    await checkoutBranch('/repo', { branch: 'feature/x', force: true });
+    expect(calls.find((a) => a.includes('status'))).toBeUndefined();
+  });
+});
+
 describe('createBranch (Issue #781)', () => {
   beforeEach(() => {
     vi.clearAllMocks();