feat: add explicit tracked subsession yielding

docs/config.html

@@ -525,24 +525,36 @@
               <h3><code>subsessions</code></h3>
               <p>
                 Boolean. Beta. Controls whether agents receive the tracked-subsession tools:
-                <code>spawn_subsession</code>, <code>list_subsessions</code>, <code>check_subsession</code>, and
-                <code>read_subsession</code>. Defaults to <code>false</code> and also requires <code>spawnSessions</code>
-                to be enabled.
+                <code>spawn_subsession</code>, <code>list_subsessions</code>, <code>check_subsession</code>,
+                <code>read_subsession</code>, and <code>yield_to_subsessions</code>. Defaults to <code>false</code> and also
+                requires <code>spawnSessions</code> to be enabled.
               </p>
               <p>
-                Tracked subsessions let an agent delegate work to child sessions, receive a notification when each child
-                stops working, and inspect their status and transcripts. Calling <code>spawn_subsession</code> returns
-                immediately. The parent can continue independent work while treating every child whose result it needs
-                as pending. Before producing work that depends on those results, the parent reaches a join point and
-                yields until every required child has sent a completion notice.
+                Tracked subsessions are join-oriented. Calling <code>spawn_subsession</code> returns immediately, so the
+                parent can continue independent work while the child runs. Work whose result the parent does not need to
+                join belongs in the fire-and-forget <code>spawn_session</code> tool instead.
               </p>
               <p>
-                A completion notice wakes an idle parent. If the parent is busy, the notice queues until the current
-                turn ends rather than interrupting in-flight work. For multiple required children, each notice resolves
-                one pending child; after processing it, the parent yields again if another required child is pending.
-                <code>list_subsessions</code>, <code>check_subsession</code>, and <code>read_subsession</code> provide
-                on-demand status and transcript inspection for deliberate progress checks or recovery. Completion
-                notifications, rather than polling these tools, are the normal synchronization mechanism.
+                At a join point, after finishing its independent work, the parent calls
+                <code>yield_to_subsessions</code> alone as the final action in its tool batch. Pi ends a tool batch early
+                only when every result in that batch is terminating. If any tracked child is still working, the action
+                ends the current agent run so the parent becomes idle. If none are working, it does not end the run and
+                clearly reports that there is nothing to wait for.
+              </p>
+              <p>
+                A completion notice automatically wakes an idle parent. If the parent is busy, the notice queues until
+                the current turn ends rather than interrupting in-flight work. When multiple children finish at
+                different times, the parent handles each completion and calls <code>yield_to_subsessions</code> again
+                while another child is still working.
+              </p>
+              <p>
+                <code>list_subsessions</code>, <code>check_subsession</code>, and <code>read_subsession</code> never yield
+                or change control flow and are not completion-polling mechanisms. They remain available for deliberate
+                inspection or recovery. While a child is working, agent-facing <code>check_subsession</code> and
+                <code>read_subsession</code> withhold partial output and transcript entries and instead direct the parent
+                to continue independent work or yield at the join point. Once the child is no longer working, its output
+                and transcript are available. Completion notifications, rather than polling inspection tools, are the
+                normal synchronization mechanism.
               </p>
               <p>
                 In <strong>Settings → Session daemon</strong>, these keys are saved on the selected machine. Restart the

docs/config.md

@@ -181,11 +181,15 @@ The per-request size limit is still controlled by `maxUploadBytes` / `PI_WEB_MAX
 
 `spawnSessions` controls whether agents receive the `spawn_session` tool. It defaults to `true`; set it to `false` if you do not want an agent to start independent PI WEB sessions.
 
-`subsessions` is beta and controls whether agents receive the tracked-subsession tools: `spawn_subsession`, `list_subsessions`, `check_subsession`, and `read_subsession`. It defaults to `false` and also requires `spawnSessions` to be enabled.
+`subsessions` is beta and controls whether agents receive the tracked-subsession tools: `spawn_subsession`, `list_subsessions`, `check_subsession`, `read_subsession`, and `yield_to_subsessions`. It defaults to `false` and also requires `spawnSessions` to be enabled.
 
-Tracked subsessions let an agent delegate work to child sessions, receive a notification when each child stops working, and inspect their status and transcripts. Calling `spawn_subsession` returns immediately. The parent can continue independent work while treating every child whose result it needs as pending. Before producing work that depends on those results, the parent reaches a join point and yields until every required child has sent a completion notice.
+Tracked subsessions are join-oriented. Calling `spawn_subsession` returns immediately, so the parent can continue independent work while the child runs. Work whose result the parent does not need to join belongs in the fire-and-forget `spawn_session` tool instead.
 
-A completion notice wakes an idle parent. If the parent is busy, the notice queues until the current turn ends rather than interrupting in-flight work. For multiple required children, each notice resolves one pending child; after processing it, the parent yields again if another required child is pending. `list_subsessions`, `check_subsession`, and `read_subsession` provide on-demand status and transcript inspection for deliberate progress checks or recovery. Completion notifications, rather than polling these tools, are the normal synchronization mechanism.
+At a join point, after finishing its independent work, the parent calls `yield_to_subsessions` alone as the final action in its tool batch. Pi ends a tool batch early only when every result in that batch is terminating. If any tracked child is still working, the action ends the current agent run so the parent becomes idle. If none are working, it does not end the run and clearly reports that there is nothing to wait for.
+
+A completion notice automatically wakes an idle parent. If the parent is busy, the notice queues until the current turn ends rather than interrupting in-flight work. When multiple children finish at different times, the parent handles each completion and calls `yield_to_subsessions` again while another child is still working.
+
+`list_subsessions`, `check_subsession`, and `read_subsession` never yield or change control flow and are not completion-polling mechanisms. They remain available for deliberate inspection or recovery. While a child is working, agent-facing `check_subsession` and `read_subsession` withhold partial output and transcript entries and instead direct the parent to continue independent work or yield at the join point. Once the child is no longer working, its output and transcript are available. Completion notifications, rather than polling inspection tools, are the normal synchronization mechanism.
 
 In **Settings → Session daemon**, these keys are saved on the selected machine. Restart the session daemon on that machine after changing them.