Add task branching feature (#15)

4 files changed, 224 insertions, 3 deletions

diff --git a/makima/frontend/src/components/mesh/BranchTaskModal.tsx b/makima/frontend/src/components/mesh/BranchTaskModal.tsx
new file mode 100644
index 0000000..ade4c7d
--- /dev/null
+++ b/makima/frontend/src/components/mesh/BranchTaskModal.tsx
@@ -0,0 +1,132 @@
+import { useState } from "react";
+import type { TaskWithSubtasks } from "../../lib/api";
+
+interface BranchTaskModalProps {
+  task: TaskWithSubtasks;
+  onBranch: (taskId: string, message: string, name?: string) => Promise<void>;
+  onClose: () => void;
+}
+
+export function BranchTaskModal({
+  task,
+  onBranch,
+  onClose,
+}: BranchTaskModalProps) {
+  const [name, setName] = useState(`Branch of ${task.name}`);
+  const [message, setMessage] = useState("");
+  const [submitting, setSubmitting] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const handleSubmit = async () => {
+    if (!message.trim()) {
+      setError("Message is required");
+      return;
+    }
+
+    setSubmitting(true);
+    setError(null);
+
+    try {
+      await onBranch(task.id, message.trim(), name.trim() || undefined);
+      onClose();
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Failed to create branch");
+    } finally {
+      setSubmitting(false);
+    }
+  };
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/70 backdrop-blur-sm">
+      <div className="w-full max-w-lg mx-4 bg-[#0a1628] border border-[rgba(117,170,252,0.3)] shadow-2xl">
+        {/* Header */}
+        <div className="flex items-center justify-between p-4 border-b border-[rgba(117,170,252,0.2)]">
+          <div className="flex items-center gap-2">
+            <span className="text-purple-400 text-lg">*</span>
+            <h2 className="font-mono text-sm text-[#75aafc] uppercase tracking-wide">
+              Branch Task
+            </h2>
+          </div>
+          <button
+            onClick={onClose}
+            className="text-[#555] hover:text-[#9bc3ff] transition-colors"
+            aria-label="Close"
+          >
+            <span className="text-xl">&times;</span>
+          </button>
+        </div>
+
+        {/* Content */}
+        <div className="p-4 space-y-4">
+          {/* Source task info */}
+          <div className="bg-[#0d1b2d] border border-[rgba(117,170,252,0.15)] p-3">
+            <p className="font-mono text-xs text-[#9bc3ff] mb-1 uppercase">
+              Branching From
+            </p>
+            <p className="font-mono text-sm text-[#dbe7ff]">{task.name}</p>
+            <p className="font-mono text-[10px] text-[#555] mt-1">
+              Status: {task.status}
+            </p>
+          </div>
+
+          {/* Name input */}
+          <div className="space-y-2">
+            <label className="block font-mono text-xs text-[#9bc3ff] uppercase">
+              Branch Name
+            </label>
+            <input
+              type="text"
+              value={name}
+              onChange={(e) => setName(e.target.value)}
+              placeholder="Enter branch task name..."
+              className="w-full px-3 py-2 bg-[#0d1b2d] border border-[#3f6fb3] text-[#dbe7ff] font-mono text-sm focus:outline-none focus:border-[#75aafc]"
+            />
+          </div>
+
+          {/* Message input */}
+          <div className="space-y-2">
+            <label className="block font-mono text-xs text-[#9bc3ff] uppercase">
+              Message <span className="text-red-400">*</span>
+            </label>
+            <textarea
+              value={message}
+              onChange={(e) => setMessage(e.target.value)}
+              placeholder="Enter a message for the new task to continue with..."
+              rows={4}
+              className="w-full px-3 py-2 bg-[#0d1b2d] border border-[#3f6fb3] text-[#dbe7ff] font-mono text-sm focus:outline-none focus:border-[#75aafc] resize-none"
+              autoFocus
+            />
+            <p className="font-mono text-[10px] text-[#555]">
+              This message will be sent to the branched task when it starts.
+            </p>
+          </div>
+
+          {/* Error message */}
+          {error && (
+            <div className="bg-red-900/20 border border-red-500/30 p-3 text-red-400 font-mono text-sm">
+              {error}
+            </div>
+          )}
+
+          {/* Actions */}
+          <div className="flex gap-3 pt-4 border-t border-[rgba(117,170,252,0.2)]">
+            <button
+              onClick={onClose}
+              disabled={submitting}
+              className="flex-1 px-4 py-2.5 font-mono text-sm text-[#9bc3ff] border border-[rgba(117,170,252,0.25)] hover:border-[#3f6fb3] disabled:opacity-50 disabled:cursor-not-allowed transition-colors uppercase"
+            >
+              Cancel
+            </button>
+            <button
+              onClick={handleSubmit}
+              disabled={submitting || !message.trim()}
+              className="flex-1 px-4 py-2.5 font-mono text-sm text-[#dbe7ff] bg-purple-700/30 border border-purple-400/50 hover:bg-purple-700/40 hover:border-purple-400/70 disabled:opacity-50 disabled:cursor-not-allowed transition-colors uppercase"
+            >
+              {submitting ? "Creating..." : "Create Branch"}
+            </button>
+          </div>
+        </div>
+      </div>
+    </div>
+  );
+}
diff --git a/makima/frontend/src/components/mesh/TaskDetail.tsx b/makima/frontend/src/components/mesh/TaskDetail.tsx
index efe26a8..a74f394 100644
--- a/makima/frontend/src/components/mesh/TaskDetail.tsx
+++ b/makima/frontend/src/components/mesh/TaskDetail.tsx
@@ -6,6 +6,7 @@ import { OverlayDiffViewer } from "./OverlayDiffViewer";
 import { PRPreview } from "./PRPreview";
 import { InlineSubtaskEditor } from "./InlineSubtaskEditor";
 import { DirectoryInput } from "./DirectoryInput";
+import { BranchTaskModal } from "./BranchTaskModal";
 
 interface TaskDetailProps {
   task: TaskWithSubtasks;
@@ -25,6 +26,8 @@ interface TaskDetailProps {
   viewingSubtaskId?: string | null;
   /** Navigate to view the contract */
   onViewContract?: (contractId: string) => void;
+  /** Branch the task to create a new task with same state */
+  onBranch?: (taskId: string, message: string, name?: string) => Promise<void>;
   // Optional advanced features
   overlayDiff?: string;
   changedFiles?: string[];
@@ -110,6 +113,7 @@ export function TaskDetail({
   onToggleSubtaskOutput,
   viewingSubtaskId,
   onViewContract,
+  onBranch,
   overlayDiff,
   changedFiles,
   onRequestDiff,
@@ -142,6 +146,8 @@ export function TaskDetail({
   const [isCloning, setIsCloning] = useState(false);
   const [cloneError, setCloneError] = useState<string | null>(null);
   const [cloneTargetDir, setCloneTargetDir] = useState("");
+  // Track branch modal state
+  const [showBranchModal, setShowBranchModal] = useState(false);
 
   // Check if task is running
   const isTaskRunning = task.status === "running" || task.status === "initializing" || task.status === "starting";
@@ -151,6 +157,8 @@ export function TaskDetail({
   const isSupervisor = task.isSupervisor === true;
   // Show continue for supervisors (always) or terminal states for other tasks
   const canContinue = isSupervisor || isTaskTerminal;
+  // Show branch button when task has run at least once (not pending)
+  const canBranch = onBranch && task.status !== "pending";
 
   // Determine which tasks to show: for supervisors, show contractTasks; for regular tasks, show subtasks
   const displayTasks = useMemo(() => {
@@ -380,6 +388,15 @@ export function TaskDetail({
                   Continue
                 </button>
               )}
+              {canBranch && (
+                <button
+                  onClick={() => setShowBranchModal(true)}
+                  className="px-3 py-1 font-mono text-xs text-purple-400 border border-purple-400/30 hover:border-purple-400/50 hover:bg-purple-400/10 transition-colors uppercase flex items-center gap-1"
+                >
+                  <span className="w-1.5 h-1.5 bg-purple-400 rounded-full" />
+                  Branch
+                </button>
+              )}
               <button
                 onClick={() => setIsEditing(true)}
                 className="px-3 py-1 font-mono text-xs text-[#9bc3ff] border border-[rgba(117,170,252,0.25)] hover:border-[#3f6fb3] transition-colors uppercase"
@@ -908,6 +925,15 @@ export function TaskDetail({
           </div>
         </div>
       )}
+
+      {/* Branch Task Modal */}
+      {showBranchModal && onBranch && (
+        <BranchTaskModal
+          task={task}
+          onBranch={onBranch}
+          onClose={() => setShowBranchModal(false)}
+        />
+      )}
     </div>
   );
 }
diff --git a/makima/frontend/src/lib/api.ts b/makima/frontend/src/lib/api.ts
index 14ec9f2..86ff06c 100644
--- a/makima/frontend/src/lib/api.ts
+++ b/makima/frontend/src/lib/api.ts
@@ -607,8 +607,8 @@ export interface TaskListResponse {
 }
 
 export interface CreateTaskRequest {
-  /** Contract this task belongs to (required) */
-  contractId: string;
+  /** Contract this task belongs to (optional - can be standalone) */
+  contractId?: string;
   name: string;
   description?: string;
   plan: string;
@@ -974,6 +974,49 @@ export async function listSubtasks(taskId: string): Promise<TaskListResponse> {
   return res.json();
 }
 
+// =============================================================================
+// Task Branching
+// =============================================================================
+
+/** Request to branch a task */
+export interface BranchTaskRequest {
+  /** Message to send to the new branched task */
+  message: string;
+  /** Optional name for the new task (defaults to "Branch of {original task name}") */
+  name?: string;
+  /** Whether to include conversation history from the source task */
+  includeConversation?: boolean;
+}
+
+/** Response from branching a task */
+export interface BranchTaskResponse {
+  /** The newly created task */
+  task: Task;
+  /** Number of conversation messages copied to the new task */
+  messageCount: number;
+  /** ID of the daemon assigned to the new task (null if not yet assigned) */
+  daemonId: string | null;
+}
+
+/**
+ * Branch a task to create a new task with the same state.
+ * Copies the worktree and optionally the conversation history.
+ */
+export async function branchTask(
+  taskId: string,
+  request: BranchTaskRequest
+): Promise<BranchTaskResponse> {
+  const res = await authFetch(`${API_BASE}/api/v1/mesh/tasks/${taskId}/branch`, {
+    method: "POST",
+    body: JSON.stringify(request),
+  });
+  if (!res.ok) {
+    const errorText = await res.text();
+    throw new Error(`Failed to branch task: ${errorText || res.statusText}`);
+  }
+  return res.json();
+}
+
 export async function listTaskEvents(
   taskId: string
 ): Promise<TaskEventListResponse> {
diff --git a/makima/frontend/src/routes/mesh.tsx b/makima/frontend/src/routes/mesh.tsx
index 142cc54..453bdff 100644
--- a/makima/frontend/src/routes/mesh.tsx
+++ b/makima/frontend/src/routes/mesh.tsx
@@ -9,7 +9,7 @@ import { ContractCompleteQuestion } from "../components/mesh/ContractCompleteQue
 import { useTasks } from "../hooks/useTasks";
 import { useTaskSubscription, type TaskUpdateEvent, type TaskOutputEvent } from "../hooks/useTaskSubscription";
 import type { TaskWithSubtasks, MeshChatContext, ContractSummary, ContractWithRelations, DaemonDirectory, TaskSummary } from "../lib/api";
-import { startTask as startTaskApi, stopTask as stopTaskApi, getTaskOutput, listContracts, getContract, getDaemonDirectories, continueTask as continueTaskApi, resumeSupervisor } from "../lib/api";
+import { startTask as startTaskApi, stopTask as stopTaskApi, getTaskOutput, listContracts, getContract, getDaemonDirectories, continueTask as continueTaskApi, resumeSupervisor, branchTask } from "../lib/api";
 import { DirectoryInput } from "../components/mesh/DirectoryInput";
 import { useAuth } from "../contexts/AuthContext";
 import { useSupervisorQuestions } from "../contexts/SupervisorQuestionsContext";
@@ -461,6 +461,25 @@ export default function MeshPage() {
     [editTask, taskDetail]
   );
 
+  const handleBranch = useCallback(
+    async (taskId: string, message: string, name?: string) => {
+      try {
+        const result = await branchTask(taskId, {
+          message,
+          name,
+          includeConversation: true,
+        });
+        console.log(`[Mesh] Task branched, new task ID: ${result.task.id}`);
+        // Navigate to the new branched task
+        navigate(`/mesh/${result.task.id}`);
+      } catch (e) {
+        console.error("Failed to branch task:", e);
+        throw e; // Re-throw so the modal can display the error
+      }
+    },
+    [navigate]
+  );
+
   // Open contract selection modal
   const handleCreate = useCallback(async () => {
     if (creating || contractsLoading) return;
@@ -742,6 +761,7 @@ export default function MeshPage() {
                       onToggleSubtaskOutput={handleToggleSubtaskOutput}
                       viewingSubtaskId={viewingSubtaskId}
                       onViewContract={(contractId) => navigate(`/contracts/${contractId}`)}
+                      onBranch={handleBranch}
                       contractTasks={taskDetail.isSupervisor ? contractTasks : undefined}
                     />
                   </div>