diff --git a/src/trpc/routers/architect.ts b/src/trpc/routers/architect.ts
index b923269..faa217e 100644
--- a/src/trpc/routers/architect.ts
+++ b/src/trpc/routers/architect.ts
@@ -245,6 +245,36 @@ export function architectProcedures(publicProcedure: ProcedureBuilder) {
           });
         }
 
+        // Auto-dismiss stale decompose agents for this phase
+        const allAgents = await agentManager.list();
+        const decomposeAgents = allAgents.filter(
+          (a) => a.mode === 'decompose' && !a.userDismissedAt,
+        );
+
+        // Look up tasks to find which phase each decompose agent targets
+        const activeForPhase: typeof decomposeAgents = [];
+        const staleForPhase: typeof decomposeAgents = [];
+        for (const agent of decomposeAgents) {
+          if (!agent.taskId) continue;
+          const agentTask = await taskRepo.findById(agent.taskId);
+          if (agentTask?.phaseId !== input.phaseId) continue;
+
+          if (['crashed', 'idle'].includes(agent.status)) {
+            staleForPhase.push(agent);
+          } else if (['running', 'waiting_for_input'].includes(agent.status)) {
+            activeForPhase.push(agent);
+          }
+        }
+        for (const stale of staleForPhase) {
+          await agentManager.dismiss(stale.id);
+        }
+        if (activeForPhase.length > 0) {
+          throw new TRPCError({
+            code: 'CONFLICT',
+            message: `A decompose agent is already running for phase "${phase.name}"`,
+          });
+        }
+
         const decomposeTaskName = input.taskName ?? `Decompose: ${phase.name}`;
         const task = await taskRepo.create({
           phaseId: phase.id,