diff --git a/src/app/SyncGithub.test.ts b/src/app/SyncGithub.test.ts
new file mode 100644
index 0000000..f4d2124
--- /dev/null
+++ b/src/app/SyncGithub.test.ts
@@ -0,0 +1,52 @@
+import { expect, test } from "bun:test";
+
+import type { CommitMetadataGroup } from "./SyncGithub";
+import { _is_master_base } from "./SyncGithub";
+
+function buildGroup(overrides: Partial<CommitMetadataGroup> = {}): CommitMetadataGroup {
+  return {
+    id: "group-1",
+    title: "title",
+    master_base: false,
+    base: "feature/base",
+    dirty: false,
+    commits: [],
+    pr: {
+      id: "pr-id",
+      number: 1,
+      state: "OPEN",
+      baseRefName: "master",
+      headRefName: "group-1",
+      commits: [] as any,
+      title: "t",
+      body: "",
+      url: "https://example.com/pr/1",
+      isDraft: false,
+    },
+    ...overrides,
+  } as CommitMetadataGroup;
+}
+
+test("treats master_base flag as master", () => {
+  const group = buildGroup({ master_base: true, base: "feature/base" });
+
+  const result = _is_master_base({ group, master_branch: "origin/master" });
+
+  expect(result).toBe(true);
+});
+
+test("treats group base matching master as master", () => {
+  const group = buildGroup({ base: "origin/master", master_base: false });
+
+  const result = _is_master_base({ group, master_branch: "origin/master" });
+
+  expect(result).toBe(true);
+});
+
+test("ignores pr base when group base differs", () => {
+  const group = buildGroup({ base: "feature/base", master_base: false });
+
+  const result = _is_master_base({ group, master_branch: "origin/master" });
+
+  expect(result).toBe(false);
+});
diff --git a/src/app/SyncGithub.tsx b/src/app/SyncGithub.tsx
index 9a0596b..3dec371 100644
--- a/src/app/SyncGithub.tsx
+++ b/src/app/SyncGithub.tsx
@@ -229,7 +229,7 @@ async function run() {
       // Unable to sync.
       // ```
       //
-      if (!is_master_base(group)) {
+      if (!is_master_base({ group, master_branch })) {
         await github.pr_edit({
           branch: group.id,
           base: master_branch,
@@ -247,7 +247,7 @@ async function run() {
     invariant(group.base, "group.base must exist");
 
     if (group.pr) {
-      if (!is_master_base(group)) {
+      if (!is_master_base({ group, master_branch })) {
         // ensure base matches pr in github
         await github.pr_edit({ branch: group.id, base: group.base });
       }
@@ -303,11 +303,7 @@ async function run() {
   }
 
   function is_master_base(group: CommitMetadataGroup) {
-    if (!group.pr) {
-      return false;
-    }
-
-    return group.master_base || `origin/${group.pr.baseRefName}` === master_branch;
+    return _is_master_base({ group, master_branch });
   }
 
   async function push_master_group(group: CommitMetadataGroup) {
@@ -351,4 +347,23 @@ async function run() {
   }
 }
 
-type CommitMetadataGroup = CommitMetadata.CommitRange["group_list"][number];
+export function _is_master_base(args: {
+  group: CommitMetadataGroup;
+  master_branch: string;
+}) {
+  const { group } = args;
+  const master_branch = args.master_branch;
+
+  if (!group.pr) {
+    return false;
+  }
+
+  // Treat as master-based only if we expect this group to live on master.
+  // Do not rely on the current PR base (it might be wrong and we want to fix it).
+  const master_branch_name = master_branch.replace(/^origin\//, "");
+  const group_base = group.base?.replace(/^origin\//, "");
+
+  return group.master_base || group_base === master_branch_name;
+}
+
+export type CommitMetadataGroup = CommitMetadata.CommitRange["group_list"][number];