feat(subagent): sibling delegations are concurrency-safe

One assistant message carrying several subagent calls now overlaps them
under the rolling pool (maxParallelToolCalls) instead of serializing each
foreground delegation behind an exclusive barrier; results still commit
in model order.

tool-subagent declares isConcurrencySafe: () => true for every call form:
children work in their own sessions, a run never mutates the parent
session, and sibling workspace coordination already belongs to the model
for background, continuable, and workflow children. The former
serial-execution pin flips to a parallel pin plus a gated genuine-overlap
test, and the authored subagent-parallel snapshot pins the assembled
transcript (tool/call, tool/call, tool/result, tool/result) with
interchangeable twin children, which replay race-free under the existing
first-call binding and createdAt harvest ordering.

The provider seam now requires concurrent starts and continuable
preparations for distinct children to isolate operation-local state,
cancellation, settlement, and cleanup; a continuable gate test proves a
cancelled preparation leaves no Agent or durable Session while its
sibling persists independently.

Closes deepseek-harness/deepseek-harness#1489

docs/subsystems/subagent.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/subsystems/subagent.md
-subagent.md: 42359215b2e21028e13734c2a4cd2488f86a07af
-subagent.zh.md: 0d1d4c42fa774867679ff248bfc99b0024b04257
+subagent.md: cf728d9f15fdaaf8199954e91b564167e8efe439
+subagent.zh.md: 1a8e2e5837b8ac88f7d7b7ca767fb7aa21391a9f

docs/subsystems/subagent.md

@@ -390,7 +390,10 @@ Each provider is a named child-agent transport, and multiple providers may coexi
 /**
  * One registered transport for running child agents. Providers are trusted
  * same-process implementations; callers treat descriptors and returned values
- * as borrowed immutable data.
+ * as borrowed immutable data. The service may call one provider concurrently
+ * for distinct children. Providers isolate operation-local mutable state; a
+ * shared capacity controller may delay an operation but must not couple its
+ * settlement or cleanup to a sibling.
  */
 interface SubagentProvider {
   /** Unique registry name (e.g. `spawn`, `fork`, `acp`). */
@@ -411,7 +414,8 @@ interface SubagentProvider {
    * initial turn. Before fulfillment, the provider owns setup and cleans any
    * unpublished partial resources before rejecting. Ownership transfers on
    * fulfillment; subsequent turn or infrastructure failure settles through
-   * the returned run.
+   * the returned run. Distinct starts may overlap; cancellation, failure,
+   * result settlement, and disposal remain independent for each run.
    */
   start(request: ResolvedSubagentStartRequest): Promise<SubagentRun>
   /**
@@ -426,6 +430,8 @@ interface SubagentProvider {
    * continuation manager owns identity reservation, composition, Agent
    * creation, prompt delivery, cold resume, ownership, and disposal, so a
    * provider never sees the child's Agent, handle, turns, or teardown.
+   * Distinct preparations may overlap; each follows its own signal and returns
+   * data belonging only to `request.sessionId`.
    */
   prepareContinuable?(request: ContinuableCreateRequest): Promise<ContinuableCreateSpec>
 }

docs/subsystems/subagent.zh.md

@@ -392,7 +392,10 @@ interface SubagentRun {
 /**
  * One registered transport for running child agents. Providers are trusted
  * same-process implementations; callers treat descriptors and returned values
- * as borrowed immutable data.
+ * as borrowed immutable data. The service may call one provider concurrently
+ * for distinct children. Providers isolate operation-local mutable state; a
+ * shared capacity controller may delay an operation but must not couple its
+ * settlement or cleanup to a sibling.
  */
 interface SubagentProvider {
   /** Unique registry name (e.g. `spawn`, `fork`, `acp`). */
@@ -413,7 +416,8 @@ interface SubagentProvider {
    * initial turn. Before fulfillment, the provider owns setup and cleans any
    * unpublished partial resources before rejecting. Ownership transfers on
    * fulfillment; subsequent turn or infrastructure failure settles through
-   * the returned run.
+   * the returned run. Distinct starts may overlap; cancellation, failure,
+   * result settlement, and disposal remain independent for each run.
    */
   start(request: ResolvedSubagentStartRequest): Promise<SubagentRun>
   /**
@@ -428,6 +432,8 @@ interface SubagentProvider {
    * continuation manager owns identity reservation, composition, Agent
    * creation, prompt delivery, cold resume, ownership, and disposal, so a
    * provider never sees the child's Agent, handle, turns, or teardown.
+   * Distinct preparations may overlap; each follows its own signal and returns
+   * data belonging only to `request.sessionId`.
    */
   prepareContinuable?(request: ContinuableCreateRequest): Promise<ContinuableCreateSpec>
 }