Start a new task from a closed task

api/openapi.yaml

@@ -972,6 +972,32 @@ paths:
         "404": { $ref: "#/components/responses/Error" }
         "409": { $ref: "#/components/responses/Error" }
 
+  /api/sessions/{sessionId}/continue:
+    parameters:
+      - { name: sessionId, in: path, required: true, schema: { type: string } }
+    post:
+      operationId: continueSession
+      description: >
+        Creates a new task from a merged or discarded task's agent session: its branch and worktree start from the
+        project's default branch, with an agent session that continues that session's conversation from its end (the
+        vendor's own session fork) and gets the message as its first turn, as sendMessage does. The new task has
+        forked_from and no forked_at. Refused while the session's task is active, for a session that isn't a task's
+        implementer, and for an Antigravity session.
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema: { $ref: "#/components/schemas/MessageCreate" }
+      responses:
+        "201":
+          description: The new task.
+          content:
+            application/json:
+              schema: { $ref: "#/components/schemas/Task" }
+        "400": { $ref: "#/components/responses/Error" }
+        "404": { $ref: "#/components/responses/Error" }
+        "409": { $ref: "#/components/responses/Error" }
+
   /api/tasks/{taskId}/sessions:
     parameters:
       - { name: taskId, in: path, required: true, schema: { type: string } }
@@ -1539,7 +1565,7 @@ components:
         merge_commit: { type: string, description: "merged: the squash commit on the default branch; empty when there was nothing to merge." }
         todo_item: { type: string, description: "The title of the TODO.md item the task was started from; absent for other tasks." }
         forked_from: { type: string, description: "The task this one was forked from; absent for other tasks." }
-        forked_at: { type: integer, description: "With forked_from: the n of the snapshot it was forked at." }
+        forked_at: { type: integer, description: "With forked_from: the n of the snapshot it was forked at; absent when it continues a closed task from its end." }
         merge_queued_at: { type: string, format: date-time, description: "When Merge was requested while another task in the project was merging; absent once the merge starts or when none is held." }
         workflow: { type: string, description: "For a workflow run's task: the name of the workflow it froze at creation (getTaskWorkflow has its copy). A run's task has no agent, worktree or branch (branch, worktree_path and base_ref are empty); its stage tasks are its subtasks, and it closes once they all have, as merged when any of them merged. Absent for other tasks." }
         workflow_stage: { type: string, description: "For a stage task of a workflow run: the name of the stage it runs; parent_task_id is the run's task. Absent for other tasks." }