fix(desktop): validate branch name for git ref path conflicts

apps/desktop/src/lib/trpc/routers/workspaces/procedures/create.ts

@@ -6,6 +6,7 @@ import { track } from "main/lib/analytics";
 import { localDb } from "main/lib/local-db";
 import { workspaceInitManager } from "main/lib/workspace-init-manager";
 import { SUPERSET_DIR_NAME, WORKTREES_DIR_NAME } from "shared/constants";
+import { findBranchPathConflict } from "shared/utils/branch";
 import { z } from "zod";
 import { publicProcedure, router } from "../../..";
 import {
@@ -328,6 +329,23 @@ export const createCreateProcedures = () => {
 					branch = generateBranchName({ existingBranches, authorPrefix });
 				}
 
+				// Check for Git ref path conflicts when creating a new branch
+				// e.g., can't create "release/v61" if "release" exists (or vice versa)
+				if (!existingBranchName) {
+					const conflictingBranch = findBranchPathConflict(
+						branch,
+						existingBranches,
+					);
+					if (conflictingBranch) {
+						const suggestedName = branch.replace(/\//g, "-");
+						throw new Error(
+							`Cannot create branch "${branch}" because branch "${conflictingBranch}" already exists. ` +
+								`Git cannot have branches where one is a path prefix of another. ` +
+								`Try using "${suggestedName}" instead.`,
+						);
+					}
+				}
+
 				const worktreePath = join(
 					homedir(),
 					SUPERSET_DIR_NAME,

apps/desktop/src/shared/utils/branch.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import {
+	findBranchPathConflict,
 	sanitizeAuthorPrefix,
 	sanitizeBranchName,
 	sanitizeSegment,
@@ -86,3 +87,54 @@ describe("sanitizeBranchName", () => {
 		expect(sanitizeBranchName("///")).toBe("");
 	});
 });
+
+describe("findBranchPathConflict", () => {
+	test("detects conflict when new branch is child of existing", () => {
+		// Creating release/v61 when "release" exists
+		expect(findBranchPathConflict("release/v61", ["release", "main"])).toBe(
+			"release",
+		);
+	});
+
+	test("detects conflict when new branch is parent of existing", () => {
+		// Creating "release" when "release/v61" exists
+		expect(findBranchPathConflict("release", ["release/v61", "main"])).toBe(
+			"release/v61",
+		);
+	});
+
+	test("detects deep nested conflicts", () => {
+		// Creating feature/auth/oauth when feature/auth exists
+		expect(
+			findBranchPathConflict("feature/auth/oauth", ["feature/auth", "main"]),
+		).toBe("feature/auth");
+	});
+
+	test("returns null when no conflict exists", () => {
+		expect(findBranchPathConflict("feature/new", ["release", "main"])).toBe(
+			null,
+		);
+	});
+
+	test("returns null for sibling branches", () => {
+		// release-v61 is not a child of release
+		expect(findBranchPathConflict("release-v61", ["release", "main"])).toBe(
+			null,
+		);
+	});
+
+	test("handles case insensitive comparison", () => {
+		expect(findBranchPathConflict("Release/V61", ["release", "main"])).toBe(
+			"release",
+		);
+	});
+
+	test("returns null for empty existing branches", () => {
+		expect(findBranchPathConflict("release/v61", [])).toBe(null);
+	});
+
+	test("does not match exact same branch name", () => {
+		// Exact match is not a path conflict (it's a duplicate, handled elsewhere)
+		expect(findBranchPathConflict("release", ["release"])).toBe(null);
+	});
+});

apps/desktop/src/shared/utils/branch.ts

@@ -20,3 +20,41 @@ export function sanitizeBranchName(name: string): string {
 		.filter(Boolean)
 		.join("/");
 }
+
+/**
+ * Checks if a new branch name would conflict with existing branches due to
+ * Git's ref storage using file/directory structure.
+ *
+ * Git stores branches as files in .git/refs/heads/. This means:
+ * - If "release" exists as a branch (file at refs/heads/release), you cannot
+ *   create "release/v1" (which would need refs/heads/release/ as a directory)
+ * - If "release/v1" exists, you cannot create "release" (same conflict)
+ *
+ * @param newBranch - The branch name to create
+ * @param existingBranches - List of existing branch names
+ * @returns The conflicting branch name if found, null otherwise
+ */
+export function findBranchPathConflict(
+	newBranch: string,
+	existingBranches: string[],
+): string | null {
+	const newBranchLower = newBranch.toLowerCase();
+
+	for (const existing of existingBranches) {
+		const existingLower = existing.toLowerCase();
+
+		// Check if the new branch would be a "child" of an existing branch
+		// e.g., creating "release/v61" when "release" exists
+		if (newBranchLower.startsWith(`${existingLower}/`)) {
+			return existing;
+		}
+
+		// Check if the new branch would be a "parent" of an existing branch
+		// e.g., creating "release" when "release/v61" exists
+		if (existingLower.startsWith(`${newBranchLower}/`)) {
+			return existing;
+		}
+	}
+
+	return null;
+}