diff --git a/bin/gstack-diff-scope b/bin/gstack-diff-scope
index 3ec81cf09..83f7ba620 100755
--- a/bin/gstack-diff-scope
+++ b/bin/gstack-diff-scope
@@ -34,6 +34,13 @@ _default_base() {
   echo "main"
 }
 BASE="${1:-$(_default_base)}"
+# A bare branch name reads its remote-tracking twin when there is one: /ship
+# merges against origin/<base>, and a linked worktree or a Capy clone never
+# moves the local branch, so a stale one widens the scope (#3650).
+case "$BASE" in
+  */*|*[~^:@]*) ;;
+  *) git rev-parse --verify -q "refs/remotes/origin/${BASE}^{commit}" >/dev/null 2>&1 && BASE="origin/${BASE}" ;;
+esac
 HEAD_REF=""
 if [ "${2:-}" = "--head" ]; then HEAD_REF="${3:-}"; fi
 
diff --git a/bin/gstack-gate-diff b/bin/gstack-gate-diff
index 0a1af715c..d09b9bf7c 100755
--- a/bin/gstack-gate-diff
+++ b/bin/gstack-gate-diff
@@ -148,7 +148,7 @@ function main(argv: string[]): number {
       process.stdout.write(`GATE_ERROR=no_base ref=${baseRef} fix=${noBaseFix(baseRef, root)}\n`);
       return 2;
     }
-    if (base.local) console.error(`gstack-gate-diff: ${base.local === 'unrelated' ? `local ${baseRef} shares no history with HEAD` : `no local branch ${baseRef}`}; base is ${base.ref}`);
+    if (base.local) console.error(`gstack-gate-diff: ${base.local === 'unrelated' ? `local ${baseRef} shares no history with HEAD` : base.local === 'differs' ? `local ${baseRef} is not ${base.ref}` : `no local branch ${baseRef}`}; base is ${base.ref}`);
     const mb = mergeBase(base.sha, head, root);
     if (!mb) {
       process.stdout.write(`GATE_ERROR=no_base ref=${baseRef} fix=${noBaseFix(baseRef, root)}\n`);
diff --git a/changelog.d/gsta246-gate-diff-origin-base.md b/changelog.d/gsta246-gate-diff-origin-base.md
new file mode 100644
index 000000000..7f68b21e3
--- /dev/null
+++ b/changelog.d/gsta246-gate-diff-origin-base.md
@@ -0,0 +1 @@
+- `gstack-gate-diff` and `gstack-diff-scope` now read a bare base name such as `main` as `origin/main` when that ref exists, the base /ship fetches and merges against. Before, a local `main` that was behind `origin/main` (a linked worktree, or a Capy clone, where local `main` never moves) set the merge base, so the gate scan and the specialist scope also read commits already on `origin/main`. One /ship run in #3650 listed a `.github/workflows/ci.yml` hunk from upstream and dispatched an extra performance reviewer. `gstack-gate-diff` says so on stderr (`local main is not origin/main; base is origin/main`). An explicit `origin/main` or commit is unchanged (#3650).
diff --git a/lib/gate-diff/git.ts b/lib/gate-diff/git.ts
index a9d4f07c0..ee60115b9 100644
--- a/lib/gate-diff/git.ts
+++ b/lib/gate-diff/git.ts
@@ -85,22 +85,23 @@ export function isShallow(cwd: string): boolean {
 
 /**
  * The base as a ref that resolves here and, when `head` is given, shares
- * history with it: the ref itself, else its remote-tracking branch
- * (`origin/main` for `main`). A clone that checked out only the feature
- * branch has no local `main`, and the merge-base against `origin/main` is the
- * same commit; a stale local `main` that shares no history with HEAD (a
- * fetched-over clone) yields to the twin too, so the fix line's fetch into the
- * tracking ref is a fix and not a loop.
+ * history with it: for a branch name its remote-tracking branch first
+ * (`origin/main` for `main`), the base /ship merges against, else the ref
+ * itself. A local `main` that is missing, behind (a linked worktree or a Capy
+ * clone never moves it, #3650) or shares no history with HEAD (a fetched-over
+ * clone) yields to the twin, so the fix line's fetch into the tracking ref is
+ * a fix and not a loop.
  */
-export function resolveBase(ref: string, cwd: string, head?: string): { ref: string; sha: string; local?: 'missing' | 'unrelated' } | null {
+export function resolveBase(ref: string, cwd: string, head?: string): { ref: string; sha: string; local?: 'missing' | 'unrelated' | 'differs' } | null {
   const related = (sha: string) => !head || mergeBase(sha, head, cwd) !== null;
   const direct = resolvesToCommit(ref, cwd);
-  if (direct && related(direct)) return { ref, sha: direct };
-  if (!isBranchName(ref)) return direct ? { ref, sha: direct } : null;
-  for (const remote of remoteNames(cwd)) {
-    const tracking = `${remote}/${ref}`;
-    const sha = resolvesToCommit(tracking, cwd);
-    if (sha && related(sha)) return { ref: tracking, sha, local: direct ? 'unrelated' : 'missing' };
+  if (isBranchName(ref)) {
+    for (const remote of remoteNames(cwd)) {
+      const tracking = `${remote}/${ref}`;
+      const sha = resolvesToCommit(tracking, cwd);
+      if (!sha || !related(sha)) continue;
+      return sha === direct ? { ref: tracking, sha } : { ref: tracking, sha, local: !direct ? 'missing' : related(direct) ? 'differs' : 'unrelated' };
+    }
   }
   return direct ? { ref, sha: direct } : null;
 }
diff --git a/test/diff-scope.test.ts b/test/diff-scope.test.ts
index e33469c1a..67c01b06a 100644
--- a/test/diff-scope.test.ts
+++ b/test/diff-scope.test.ts
@@ -330,6 +330,25 @@ describe('exit-code contract (#2526)', () => {
   });
 });
 
+describe('a bare base name reads origin/<base> when it exists (#3650)', () => {
+  test('a local main behind origin/main does not widen the scope to upstream commits', () => {
+    const dir = createRepo(['README2.md']);
+    const git = (...args: string[]) => spawnSync('git', args, { cwd: dir, stdio: 'pipe', timeout: 5000 }).stdout.toString().trim();
+    const seed = git('rev-parse', 'main');
+    git('checkout', '-q', 'main');
+    writeFileSync(join(dir, 'service.py'), 'print(1)\n');
+    git('add', '.');
+    git('commit', '-qm', 'upstream backend change');
+    git('update-ref', 'refs/remotes/origin/main', 'HEAD');
+    git('checkout', '-q', 'feature/test');
+    git('rebase', '-q', 'origin/main');
+    git('branch', '-f', 'main', seed);
+    expect(git('diff', '--name-only', 'main...HEAD').split('\n').sort()).toEqual(['README2.md', 'service.py']);
+    const scope = runScope(dir);
+    expect([scope.SCOPE_BACKEND, scope.SCOPE_DOCS]).toEqual(['false', 'true']);
+  });
+});
+
 describe('uncommitted work is visible (#2299)', () => {
   test('uncommitted change on a branch with no commits sets the scope', () => {
     const dir = mkdtempSync(join(tmpdir(), 'diff-scope-dirty-'));
diff --git a/test/gate-diff-cli.test.ts b/test/gate-diff-cli.test.ts
index 3a8545e09..ce8aace18 100644
--- a/test/gate-diff-cli.test.ts
+++ b/test/gate-diff-cli.test.ts
@@ -153,6 +153,32 @@ describe('gstack-gate-diff arguments and exit contract', () => {
   // fetches from it; fails_when=remoteNames or resolveBase hard-code origin, or noBaseFix falls back to the literal 'origin'
   // while a real remote exists; why_new=every existing fixture has either an origin remote or none; seam=none;
   // no_claim=nothing about the order when origin and another remote both carry the branch.
+  // Value: protects=a bare base name scans from the same merge base /ship merges against (origin/<base>) when the local
+  // branch is related but behind (#3650); fails_when=resolveBase prefers any related local branch, so a stale local main
+  // widens the scan to commits already upstream; why_new=the stale-main test above covers only unrelated history;
+  // seam=none; no_claim=nothing about a local base that is ahead of its remote-tracking twin.
+  test('a related local main that is behind origin/main yields to origin/main (#3650)', () => {
+    const src = repo({ fixture: 'rh1-skip', commit: true });
+    const seed = gitIn(src, ['rev-parse', 'main']).trim();
+    gitIn(src, ['checkout', '-q', 'main']);
+    fs.mkdirSync(path.join(src, '.github', 'workflows'), { recursive: true });
+    fs.writeFileSync(path.join(src, '.github', 'workflows', 'ci.yml'), 'on: push\njobs: {}\n');
+    gitIn(src, ['add', '-A']);
+    gitIn(src, ['commit', '-q', '-m', 'upstream ci change']);
+    gitIn(src, ['checkout', '-q', 'feature']);
+    gitIn(src, ['rebase', '-q', 'main']);
+    const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-gate-diff-behind-'));
+    repos.push(dir);
+    gitIn(dir, ['clone', '-q', '--branch', 'feature', `file://${src}`, '.']);
+    gitIn(dir, ['branch', '-f', 'main', seed]);
+    const bare = run(dir, ['main']);
+    const remote = run(dir, ['origin/main']);
+    expect([bare.status, remote.status]).toEqual([0, 0]);
+    expect(summary(bare.stdout).candidate).toBe(summary(remote.stdout).candidate);
+    expect(bare.stdout).toBe(remote.stdout);
+    expect(bare.stderr).toContain('gstack-gate-diff: local main is not origin/main; base is origin/main');
+  });
+
   test('a clone whose only remote is upstream resolves the base to upstream/main, and the fix line fetches from upstream', () => {
     const src = repo({ fixture: 'rh1-skip', commit: true });
     const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-gate-diff-upstream-'));