linear
diff --git a/‎src/git.test.ts‎
Lines changed: 24 additions & 0 deletions b/‎src/git.test.ts‎
Lines changed: 24 additions & 0 deletions
diff --git a/‎src/git.ts‎
Lines changed: 19 additions & 0 deletions b/‎src/git.ts‎
Lines changed: 19 additions & 0 deletions
@@ -13,6 +13,7 @@ import {
   getCommitContextsBetweenShas,
   getCommitParents,
   getRepoInfo,
+  isAncestor,
   normalizePathspec,
   parseRepoUrl,
   resolveFirstSyncBoundary,
@@ -911,6 +912,29 @@ describe("merge commit handling", () => {
     });
   });
 
+  describe("isAncestor", () => {
+    it("returns true when sha is an ancestor of headSha", () => {
+      expect(isAncestor(mergeRepo.commits.base, mergeRepo.commits.mergeCommit, mergeRepo.cwd)).toBe(true);
+    });
+
+    it("returns true for a sha equal to headSha", () => {
+      expect(isAncestor(mergeRepo.commits.mergeCommit, mergeRepo.commits.mergeCommit, mergeRepo.cwd)).toBe(true);
+    });
+
+    it("returns false when sha is not on headSha's history", () => {
+      // featureBranch is reachable from mergeCommit (parent #2), but mergeCommit
+      // is not reachable from featureBranch — that's the asymmetric case the
+      // walk relies on to skip side-branch candidates.
+      expect(isAncestor(mergeRepo.commits.mergeCommit, mergeRepo.commits.featureBranch, mergeRepo.cwd)).toBe(false);
+    });
+
+    it("returns false for an unknown sha", () => {
+      expect(isAncestor("0000000000000000000000000000000000000000", mergeRepo.commits.mergeCommit, mergeRepo.cwd)).toBe(
+        false,
+      );
+    });
+  });
+
   describe("getCommitContextsBetweenShas with merge commits", () => {
     it("should include merge commit when path filtering would exclude it", () => {
       // The merge node itself adds no file changes, so default simplification
 
@@ -165,6 +165,25 @@ export function commitExists(sha: string, cwd: string = process.cwd()): boolean
   }
 }
 
+/**
+ * True iff `sha` is reachable by walking parents from `headSha`.
+ *
+ * Used to verify that a candidate base SHA is actually on HEAD's history before
+ * we hand it to `git log <base>..<HEAD>` — a candidate from a side branch (e.g.
+ * a hotfix release) will scan a wrong range otherwise.
+ */
+export function isAncestor(sha: string, headSha: string, cwd: string = process.cwd()): boolean {
+  try {
+    execSync(`git merge-base --is-ancestor ${sha} ${headSha}`, {
+      cwd,
+      stdio: ["ignore", "ignore", "ignore"],
+    });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 const SHA_PATTERN = /^[0-9a-f]{7,40}$/i;
 
 /**