diff --git a/bin/gstack-diff-scope b/bin/gstack-diff-scope
index e7c6d4f1f..dad11e551 100755
--- a/bin/gstack-diff-scope
+++ b/bin/gstack-diff-scope
@@ -28,20 +28,25 @@ set -euo pipefail
 # Detect the repo's default branch when no arg is given (#703-class
 # platform-agnostic rule, lib/default-base.ts in bash): origin/HEAD unless it
 # names the checked-out branch (a PR flow's clone can point it at the PR
-# branch, GST-511) -> the PR's base (gh, bounded; skipped without a timeout
-# binary) -> origin/main -> origin/master -> main. An origin/HEAD skipped that
-# way leaves BASE empty when nothing past it resolves, and that fails closed.
+# branch, GST-511) -> the PR's base (gh, killed after 5 s) -> origin/main ->
+# origin/master -> main. An origin/HEAD skipped that way leaves BASE empty when
+# nothing past it resolves, and that fails closed.
 SKIPPED_HEAD=""
 _default_base() {
-  local cur head pr to
-  cur=$(git symbolic-ref --quiet --short HEAD 2>/dev/null || true)
+  local cur head pr out g w
+  cur=$(git symbolic-ref --quiet HEAD 2>/dev/null || true)
+  cur="${cur#refs/heads/}"
   head=$(git symbolic-ref --quiet refs/remotes/origin/HEAD 2>/dev/null || true)
   head="${head#refs/remotes/origin/}"
   if [ -n "$head" ] && [ "$head" != "$cur" ]; then BASE="$head"; return; fi
   SKIPPED_HEAD="$head"
-  to=$(command -v gtimeout 2>/dev/null || command -v timeout 2>/dev/null || true)
-  if [ -n "$cur" ] && [ -n "$to" ]; then
-    pr=$(GH_PROMPT_DISABLED=1 "$to" 5 gh pr view --json baseRefName -q .baseRefName </dev/null 2>/dev/null | head -n 1 || true)
+  if [ -n "$cur" ] && command -v gh >/dev/null 2>&1 && out=$(mktemp "${TMPDIR:-/tmp}/gstack-diff-scope.XXXXXX" 2>/dev/null); then
+    GH_PROMPT_DISABLED=1 gh pr view --json baseRefName -q .baseRefName </dev/null >"$out" 2>/dev/null & g=$!
+    { sleep 5; kill "$g"; } >/dev/null 2>&1 & w=$!
+    wait "$g" 2>/dev/null || true
+    kill "$w" 2>/dev/null || true
+    pr=$(head -n 1 "$out" 2>/dev/null || true)
+    rm -f "$out"
     if [ -n "$pr" ]; then BASE="$pr"; return; fi
   fi
   git rev-parse --verify -q origin/main >/dev/null 2>&1 && { BASE="main"; return; }
diff --git a/lib/default-base.ts b/lib/default-base.ts
index a4326bf7a..fdb9dde51 100644
--- a/lib/default-base.ts
+++ b/lib/default-base.ts
@@ -27,7 +27,7 @@ function read(cmd: string, args: string[], cwd: string, timeout: number): string
 }
 
 export function defaultBaseBranch(cwd: string, remotes: readonly string[] = ['origin']): DefaultBase {
-  const current = read('git', ['symbolic-ref', '--quiet', '--short', 'HEAD'], cwd, 30_000);
+  const current = read('git', ['symbolic-ref', '--quiet', 'HEAD'], cwd, 30_000).replace(/^refs\/heads\//, '');
   let skipped: DefaultBase['skipped'];
   for (const remote of remotes) {
     const prefix = `refs/remotes/${remote}/`;
diff --git a/test/diff-scope.test.ts b/test/diff-scope.test.ts
index 1731d5f6d..e273313e9 100644
--- a/test/diff-scope.test.ts
+++ b/test/diff-scope.test.ts
@@ -451,8 +451,9 @@ describe('uncommitted work is visible (#2299)', () => {
 
   // Value: protects=a clone whose origin/HEAD names the checked-out PR branch scopes the PR against origin/main, and with no
   //   other base fails closed naming origin/HEAD (GST-511); fails_when=the default base takes origin/HEAD's branch when it is the
-  //   checked-out one (every SCOPE_*=false, exit 0); why_new=no test pointed origin/HEAD at the feature branch; seam=a gh stub on
-  //   PATH; no_claim=remotes other than origin.
+  //   checked-out one (every SCOPE_*=false, exit 0), including when a tag shares the branch's name, or the default branch's own
+  //   uncommitted work stops being scoped; why_new=no test pointed origin/HEAD at the feature branch; seam=a gh stub on PATH;
+  //   no_claim=remotes other than origin.
   test('origin/HEAD naming the checked-out branch is skipped: the PR base, then origin/main, and none fails closed', () => {
     const dir = createRepo(['lib/server.ts']);
     const git = (...a: string[]) => spawnSync('git', a, { cwd: dir, stdio: 'pipe', timeout: 5000 });
@@ -469,6 +470,17 @@ describe('uncommitted work is visible (#2299)', () => {
     expect(found.status).toBe(0);
     expect(found.out).toContain('SCOPE_BACKEND=true');
     expect(scope('json', 'no-such-base').out).toContain('SCOPE_ERROR=no_base');
+    git('tag', 'feature/test');
+    expect(scope('fail').out).toContain('SCOPE_BACKEND=true');
+    git('tag', '-d', 'feature/test');
+    git('checkout', '-q', 'main');
+    git('symbolic-ref', 'refs/remotes/origin/HEAD', 'refs/remotes/origin/main');
+    mkdirSync(join(dir, 'lib'), { recursive: true });
+    writeFileSync(join(dir, 'lib', 'local.ts'), 'export const local = 1;\n');
+    expect(scope('fail').out).toContain('SCOPE_BACKEND=true');
+    rmSync(join(dir, 'lib', 'local.ts'));
+    git('checkout', '-q', 'feature/test');
+    git('symbolic-ref', 'refs/remotes/origin/HEAD', 'refs/remotes/origin/feature/test');
     git('update-ref', '-d', 'refs/remotes/origin/main');
     const none = scope('fail');
     expect(none.status).toBe(2);
diff --git a/test/gstack-ship-policy.test.ts b/test/gstack-ship-policy.test.ts
index 2ab777004..40635fa4f 100644
--- a/test/gstack-ship-policy.test.ts
+++ b/test/gstack-ship-policy.test.ts
@@ -335,6 +335,9 @@ describe('resolveBaseBranch: origin/HEAD naming the checked-out branch (GST-511)
         return { out: r.stdout ?? '', err: r.stderr ?? '' };
       };
       expect(base('fail')).toEqual({ out: 'main', err: '' });
+      git('tag', 'feat');
+      expect(base('fail')).toEqual({ out: 'main', err: '' });
+      git('tag', '-d', 'feat');
       expect(base('json', 'release').out).toBe('release');
       git('update-ref', '-d', 'refs/remotes/origin/main');
       const none = base('fail');