fix(worktree): prevent branch prefix conflicts

electron/ipc/git.ts

@@ -218,6 +218,18 @@ async function localBranchExists(repoRoot: string, branch: string): Promise<bool
   }
 }
 
+async function findLocalBranchPrefixConflict(
+  repoRoot: string,
+  branchName: string,
+): Promise<string | null> {
+  const parts = branchName.split('/');
+  for (let i = 1; i < parts.length; i++) {
+    const prefix = parts.slice(0, i).join('/');
+    if (await localBranchExists(repoRoot, prefix)) return prefix;
+  }
+  return null;
+}
+
 async function detectMainBranchUncached(repoRoot: string): Promise<string> {
   // Try remote HEAD reference first
   const branch = await resolveOriginHead(repoRoot);
@@ -722,6 +734,14 @@ export async function createWorktree(
   }
 
   // Create fresh worktree with new branch
+  const conflictingBranch = await findLocalBranchPrefixConflict(repoRoot, branchName);
+  if (conflictingBranch) {
+    throw new Error(
+      `Cannot create branch "${branchName}" because local branch "${conflictingBranch}" already exists. ` +
+        `Choose a branch prefix other than "${conflictingBranch}" or "${conflictingBranch}/...".`,
+    );
+  }
+
   const worktreeArgs = ['worktree', 'add', '-b', branchName, worktreePath];
   if (baseBranch) worktreeArgs.push(baseBranch);
   await exec('git', worktreeArgs, { cwd: repoRoot });

src/components/BranchPrefixField.tsx

@@ -4,6 +4,7 @@ import { theme } from '../lib/theme';
 interface BranchPrefixFieldProps {
   branchPrefix: string;
   branchPreview: string;
+  error?: string;
   projectPath: string | undefined;
   onPrefixChange: (prefix: string) => void;
 }
@@ -26,7 +27,7 @@ export function BranchPrefixField(props: BranchPrefixFieldProps) {
           placeholder="task"
           style={{
             background: theme.bgInput,
-            border: `1px solid ${theme.border}`,
+            border: `1px solid ${props.error ? theme.error : theme.border}`,
             'border-radius': '6px',
             padding: '4px 8px',
             color: theme.fg,
@@ -37,6 +38,9 @@ export function BranchPrefixField(props: BranchPrefixFieldProps) {
           }}
         />
       </div>
+      <Show when={props.error}>
+        <div style={{ 'font-size': '12px', color: theme.error }}>{props.error}</div>
+      </Show>
       <Show when={props.branchPreview && props.projectPath}>
         <div
           style={{

src/components/NewTaskDialog.tsx

@@ -1,4 +1,4 @@
-import { createSignal, createEffect, createUniqueId, Show, onCleanup } from 'solid-js';
+import { createSignal, createEffect, createMemo, createUniqueId, Show, onCleanup } from 'solid-js';
 import { Dialog } from './Dialog';
 import { invoke } from '../lib/ipc';
 import { IPC } from '../../electron/ipc/channels';
@@ -19,7 +19,12 @@ import {
   setDockerImage,
 } from '../store/store';
 import type { GitIsolationMode } from '../store/types';
-import { toBranchName, sanitizeBranchPrefix } from '../lib/branch-name';
+import {
+  toBranchName,
+  sanitizeBranchPrefix,
+  findBranchPrefixConflict,
+  branchPrefixConflictError,
+} from '../lib/branch-name';
 import { SegmentedButtons } from './SegmentedButtons';
 import { autoTaskNameFromPrompt } from '../lib/clean-task-name';
 import { extractGitHubUrl } from '../lib/github-url';
@@ -466,6 +471,16 @@ export function NewTaskDialog(props: NewTaskDialogProps) {
     return n ? `${prefix}/${toBranchName(n)}` : '';
   };
 
+  const branchPrefixConflict = createMemo(() => {
+    if (gitIsolation() !== 'worktree') return null;
+    return findBranchPrefixConflict(branchPrefix(), branches());
+  });
+
+  const branchPrefixError = createMemo(() => {
+    const c = branchPrefixConflict();
+    return c ? branchPrefixConflictError(c) : '';
+  });
+
   const selectedProjectPath = () => {
     const pid = selectedProjectId();
     return pid ? getProjectPath(pid) : undefined;
@@ -492,7 +507,14 @@ export function NewTaskDialog(props: NewTaskDialogProps) {
     // for git projects — so a task can't be created with a stale or empty
     // base branch (e.g. after a failed branch fetch).
     const branchOk = isNonGitProject() || (!!baseBranch() && !branchesError());
-    return hasContent && !!selectedProjectId() && !loading() && !branchesLoading() && branchOk;
+    return (
+      hasContent &&
+      !!selectedProjectId() &&
+      !loading() &&
+      !branchesLoading() &&
+      branchOk &&
+      !branchPrefixConflict()
+    );
   };
 
   async function handleSubmit(e: Event) {
@@ -517,13 +539,19 @@ export function NewTaskDialog(props: NewTaskDialogProps) {
       return;
     }
 
-    setLoading(true);
-    setError('');
-
     const p = prompt().trim() || undefined;
     const isFromDrop = !!store.newTaskDropUrl;
     const prefix = sanitizeBranchPrefix(branchPrefix());
+    const prefixConflict = branchPrefixConflict();
+    if (prefixConflict) {
+      setError(branchPrefixConflictError(prefixConflict));
+      return;
+    }
     const ghUrl = (p ? extractGitHubUrl(p) : null) ?? store.newTaskDropUrl ?? undefined;
+
+    setLoading(true);
+    setError('');
+
     try {
       // Persist the branch prefix to the project for next time
       updateProject(projectId, { branchPrefix: prefix });
@@ -757,6 +785,7 @@ export function NewTaskDialog(props: NewTaskDialogProps) {
             <BranchPrefixField
               branchPrefix={branchPrefix()}
               branchPreview={branchPreview()}
+              error={branchPrefixError()}
               projectPath={selectedProjectPath()}
               onPrefixChange={setBranchPrefix}
             />

src/lib/branch-name.ts

@@ -14,3 +14,17 @@ export function sanitizeBranchPrefix(prefix: string): string {
     .filter((segment) => segment.length > 0);
   return parts.join('/') || 'task';
 }
+
+export function findBranchPrefixConflict(
+  branchPrefix: string,
+  existingBranches: string[],
+): string | null {
+  const prefix = sanitizeBranchPrefix(branchPrefix);
+  return (
+    existingBranches.find((branch) => branch === prefix || prefix.startsWith(`${branch}/`)) ?? null
+  );
+}
+
+export function branchPrefixConflictError(conflict: string): string {
+  return `Branch prefix conflicts with existing branch "${conflict}". Choose a prefix other than "${conflict}" or "${conflict}/...".`;
+}