Prevent merging a branch into itself

packages/app/src/git/policy.test.ts

@@ -6,6 +6,7 @@ import {
   actionsWithoutPrimary,
   buildGitActions,
   isCurrentBranchBase,
+  isCurrentBranchLocalMergeTarget,
   type BuildGitActionsInput,
 } from "./policy";
 import { deriveMergeCapability, type ForgeSpecificStatusFacts } from "./merge-capability";
@@ -77,6 +78,7 @@ function createInput(
     hasRemote: false,
     isPaseoOwnedWorktree: false,
     isOnBaseBranch: true,
+    isOnLocalMergeTarget: false,
     hasUncommittedChanges: false,
     baseRefAvailable: true,
     baseRefLabel: "main",
@@ -219,6 +221,30 @@ describe("git-actions-policy", () => {
     expect(isCurrentBranchBase("release", "release")).toBe(true);
     expect(isCurrentBranchBase("release", "origin/release")).toBe(true);
     expect(isCurrentBranchBase(null, "refs/heads/release")).toBe(false);
+    expect(isCurrentBranchLocalMergeTarget("release", "refs/heads/release")).toBe(true);
+    expect(isCurrentBranchLocalMergeTarget("feature", "refs/remotes/origin/release")).toBe(false);
+    expect(isCurrentBranchLocalMergeTarget("release", "refs/remotes/upstream/release")).toBe(false);
+  });
+
+  it("does not offer merging into the current local branch when the remote base shares its name", () => {
+    const actions = buildGitActions(
+      createInput({
+        isPaseoOwnedWorktree: true,
+        isOnBaseBranch: false,
+        isOnLocalMergeTarget: isCurrentBranchLocalMergeTarget(
+          "release",
+          "refs/remotes/origin/release",
+        ),
+        baseRefLabel: "release",
+        aheadCount: 1,
+        behindBaseCount: 1,
+        aheadOfOrigin: null,
+        behindOfOrigin: null,
+        shipDefault: "merge",
+      }),
+    );
+    expect(actions.primary?.id).toBe("merge-from-base");
+    expect(actionsWithoutPrimary(actions).map((action) => action.id)).not.toContain("merge-branch");
   });
 
   it("offers update from the same-named remote base when it is ahead", () => {

packages/app/src/git/policy.ts

@@ -61,6 +61,18 @@ export function isCurrentBranchBase(
   return currentBranch === baseRef.replace(/^origin\//, "");
 }
 
+export function isCurrentBranchLocalMergeTarget(
+  currentBranch: string | null | undefined,
+  baseRef: string | null | undefined,
+): boolean {
+  if (!currentBranch || !baseRef) return false;
+  const localTarget = baseRef
+    .replace(/^refs\/heads\//, "")
+    .replace(/^refs\/remotes\/origin\//, "")
+    .replace(/^origin\//, "");
+  return currentBranch === localTarget;
+}
+
 export interface BuildGitActionsInput {
   isGit: boolean;
   githubFeaturesEnabled: boolean;
@@ -79,6 +91,7 @@ export interface BuildGitActionsInput {
   hasRemote: boolean;
   isPaseoOwnedWorktree: boolean;
   isOnBaseBranch: boolean;
+  isOnLocalMergeTarget: boolean;
   hasUncommittedChanges: boolean;
   baseRefAvailable: boolean;
   baseRefLabel: string;
@@ -360,7 +373,7 @@ function getPrimaryActionId(input: BuildGitActionsInput): GitActionId | null {
   if (input.shipDefault === "pr" && canUsePullRequestActionAsShipDefault(input)) {
     return "pr";
   }
-  if (!input.isOnBaseBranch && input.aheadCount > 0) {
+  if (!input.isOnBaseBranch && !input.isOnLocalMergeTarget && input.aheadCount > 0) {
     return "merge-branch";
   }
   if (!input.isOnBaseBranch && canMergeFromBase(input)) {
@@ -391,7 +404,7 @@ function getPullRequestActionIds(filter: {
 function getFeatureActionIds(input: BuildGitActionsInput): GitActionId[] {
   return [
     "merge-from-base",
-    "merge-branch",
+    ...(!input.isOnLocalMergeTarget ? (["merge-branch"] as const) : []),
     ...getPullRequestActionIds({ roles: ["status", "direct", "auto"], input }),
   ];
 }

packages/app/src/git/use-actions.tsx

@@ -13,6 +13,7 @@ import { type CheckoutPrStatusPayload, useCheckoutPrStatusQuery } from "@/git/us
 import {
   buildGitActions,
   isCurrentBranchBase,
+  isCurrentBranchLocalMergeTarget,
   narrowPullRequestState,
   type BuildGitActionsInput,
   type GitAction,
@@ -130,6 +131,7 @@ interface DerivedGitActionsState {
   hasRemote: boolean;
   isPaseoOwnedWorktree: boolean;
   isOnBaseBranch: boolean;
+  isOnLocalMergeTarget: boolean;
   shouldPromoteArchive: boolean;
 }
 
@@ -180,6 +182,7 @@ function deriveGitActionsState(args: DeriveGitActionsStateArgs): DerivedGitActio
     hasRemote: gitStatus?.hasRemote ?? false,
     isPaseoOwnedWorktree,
     isOnBaseBranch: isCurrentBranchBase(gitStatus?.currentBranch, baseRef),
+    isOnLocalMergeTarget: isCurrentBranchLocalMergeTarget(gitStatus?.currentBranch, baseRef),
     shouldPromoteArchive: computeShouldPromoteArchive({
       hasUncommittedChanges,
       postShipArchiveSuggested,
@@ -666,6 +669,7 @@ export function useGitActions({ serverId, cwd, icons }: UseGitActionsInput): Use
     hasRemote,
     isPaseoOwnedWorktree,
     isOnBaseBranch,
+    isOnLocalMergeTarget,
     shouldPromoteArchive,
   } = derived;
 
@@ -696,6 +700,7 @@ export function useGitActions({ serverId, cwd, icons }: UseGitActionsInput): Use
       hasRemote,
       isPaseoOwnedWorktree,
       isOnBaseBranch,
+      isOnLocalMergeTarget,
       hasUncommittedChanges,
       baseRefAvailable: Boolean(baseRef),
       baseRefLabel,
@@ -813,6 +818,7 @@ export function useGitActions({ serverId, cwd, icons }: UseGitActionsInput): Use
     behindBaseCount,
     isPaseoOwnedWorktree,
     isOnBaseBranch,
+    isOnLocalMergeTarget,
     githubFeaturesEnabled,
     forge,
     githubAutoMergeActionsEnabled,

packages/server/src/utils/checkout-git.test.ts

@@ -1971,6 +1971,13 @@ const x = 1;
     expect(status.currentBranch).toBe("feature/plain");
   });
 
+  it("rejects merging a branch into its own same-named remote base", async () => {
+    execFileSync("git", ["update-ref", "refs/remotes/origin/main", "HEAD"], { cwd: repoDir });
+    await expect(mergeToBase(repoDir, { baseRef: "refs/remotes/origin/main" })).rejects.toThrow(
+      "Cannot merge a branch into itself",
+    );
+  });
+
   it("merges the current branch into base from a worktree checkout", async () => {
     const worktree = await createLegacyWorktreeForTest({
       branchName: "main",

packages/server/src/utils/checkout-git.ts

@@ -3942,7 +3942,7 @@ export async function mergeToBase(
   const normalizedBaseRef = resolveMergeTargetBranch(baseRef);
   const currentWorktreeRoot = (await getWorktreeRoot(cwd, context)) ?? cwd;
   if (normalizedBaseRef === currentBranch) {
-    return currentWorktreeRoot;
+    throw new Error("Cannot merge a branch into itself");
   }
 
   const baseWorktree = await getWorktreePathForBranch(cwd, normalizedBaseRef);