perf(task): defer subagent launch setup

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- Moved subagent model-registry refresh and session-file opening off the launch critical path. Registry refresh now runs in the background while model resolution uses the registry's static/provider lookup, and `SessionManager.open` starts before prewalk/output-schema/session-option assembly so session creation waits only at its actual input boundary.
+
 ## [17.2.1] - 2026-07-30
 
 ### Added

packages/coding-agent/src/task/executor.ts

@@ -2604,7 +2604,12 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 			}
 			checkAbort();
 			if (!registryFromParent) {
-				await awaitAbortable(modelRegistry.refresh());
+				void modelRegistry.refresh().catch(err => {
+					logger.warn("Subagent model registry refresh failed", {
+						id,
+						error: err instanceof Error ? err.message : String(err),
+					});
+				});
 			} else {
 				logger.debug("runSubagent: reusing parent modelRegistry; skipping refresh");
 			}
@@ -2719,19 +2724,12 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 			}
 
 			const effectiveCwd = worktree ?? cwd;
-			const sessionManager = sessionFile
-				? await awaitAbortable(
-						SessionManager.open(sessionFile, undefined, undefined, {
-							initialCwd: effectiveCwd,
-							suppressBreadcrumb: true,
-						}),
-					)
-				: SessionManager.inMemory(effectiveCwd);
-			if (options.parentArtifactManager) {
-				sessionManager.adoptArtifactManager(options.parentArtifactManager);
-			}
-			sessionOpenedAt = performance.now();
-
+			const sessionManagerPromise = sessionFile
+				? SessionManager.open(sessionFile, undefined, undefined, {
+						initialCwd: effectiveCwd,
+						suppressBreadcrumb: true,
+					})
+				: Promise.resolve(SessionManager.inMemory(effectiveCwd));
 			const restrictToolNames = options.restrictToolNames === true;
 			const enableMCP = !restrictToolNames && (options.enableMCP ?? true);
 			const mcpManager = enableMCP ? options.mcpManager : undefined;
@@ -2851,6 +2849,12 @@ export async function runSubprocess(options: ExecutorOptions): Promise<SingleRes
 				},
 			});
 
+			const sessionManager = await awaitAbortable(sessionManagerPromise);
+			if (options.parentArtifactManager) {
+				sessionManager.adoptArtifactManager(options.parentArtifactManager);
+			}
+			sessionOpenedAt = performance.now();
+
 			const sessionPromise = createAgentSession(buildSubagentSessionOptions(sessionManager, null));
 			let session: AgentSession;
 			try {