Run each workflow stage as its own task, with fan-out and fan-in

api/openapi.yaml

@@ -933,6 +933,8 @@ components:
         forked_at: { type: integer, description: "With forked_from: the n of the snapshot it was forked at." }
         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: "The name of the workflow the task froze at creation (getTaskWorkflow has its copy); absent for a task without one." }
+        workflow_run: { type: string, description: "With workflow: the id every task of one run of the workflow shares." }
+        workflow_stage: { type: string, description: "With workflow: the name of the stage this task runs." }
 
     Todo:
       type: object
@@ -982,7 +984,6 @@ components:
       required: [title]
       properties:
         title: { type: string }
-        workflow: { type: string, description: "The name of a workflow for the task to freeze: it keeps a copy of the workflow's definition as it is now." }
 
     WorkflowSummary:
       type: object
@@ -994,8 +995,8 @@ components:
     WorkflowDefinition:
       type: object
       description: >
-        A workflow's file, which holds everything but its name: a sequence of stages run in order. Sent and returned
-        as YAML.
+        A workflow's file, which holds everything but its name: its stages, each run as its own task once the stages
+        it comes after have finished. Sent and returned as YAML.
       required: [stages]
       properties:
         description: { type: string, x-go-type-skip-optional-pointer: true }
@@ -1009,6 +1010,20 @@ components:
       required: [name, model, prompt]
       properties:
         name: { type: string, description: "One line of 1 to 100 characters, unique within the workflow." }
+        after:
+          type: array
+          items: { type: string }
+          description: >
+            The stages whose outputs this stage gets, by name; it starts once all of them have finished. Absent means
+            the previous stage in the list (for the first stage, none); empty means it starts from the brief.
+        base:
+          type: string
+          enum: [parent, main]
+          x-enum-varnames: [StageBaseParent, StageBaseMain]
+          description: >
+            Where the stage's task branches from. parent (the default): the commit its one after stage finished at, or
+            for a stage with none, where the workflow was started from. main: the project's default branch. Refused on
+            a stage with more than one after stage, which always branches from the default branch.
         model: { type: string, description: "The stage agent's model, which also picks its vendor: gpt-* models run on Codex, every other model on Claude Code." }
         prompt: { type: string, description: "The stage agent's instructions." }
 
@@ -1149,8 +1164,6 @@ components:
         created_at: { type: string, format: date-time }
         forked_from: { type: string, description: "A fork's first start resumes this vendor session, cut at forked_at, under a new vendor_session_id; absent for other sessions." }
         forked_at: { type: string, description: "With forked_from: the vendor's id of the last thing the fork keeps, a message for Claude Code, a turn for Codex." }
-        workflow_stage: { type: integer, description: "The stage (from 1) of the task's workflow this session runs; absent for a session outside a workflow." }
-
     SessionCreate:
       type: object
       required: [prompt]