diff --git a/internal/cloud/code-agent-agentrun-adapter.ts b/internal/cloud/code-agent-agentrun-adapter.ts
index d7bd6857..3525a33a 100644
--- a/internal/cloud/code-agent-agentrun-adapter.ts
+++ b/internal/cloud/code-agent-agentrun-adapter.ts
@@ -2937,12 +2937,18 @@ function withAgentRunRunnerJobCount(mapping = {}) {
   return mapping;
 }
 
-function hwlabSessionIdForParams(params = {}, traceId) {
-  return safeSessionId(params.sessionId) || agentRunSessionId(traceId);
+function hwlabSessionIdForParams(params = {}) {
+  return safeSessionId(params.sessionId) || null;
 }
 
 function scopedAgentRunSessionIdForParams(params = {}, traceId, backendProfile) {
   const baseSessionId = hwlabSessionIdForParams(params, traceId);
+  if (!safeSessionId(baseSessionId)) {
+    throw adapterError(
+      "agentrun_session_id_required",
+      "AgentRun persistent session/PVC requires params.sessionId; refusing to derive sessionRef.sessionId from traceId because that would create a new PVC per turn."
+    );
+  }
   const profile = agentRunSessionProfileToken(backendProfile);
   const base = String(baseSessionId).replace(/^ses_/u, "").replace(/[^A-Za-z0-9_]+/gu, "_").replace(/^_+|_+$/gu, "") || "session";
   return `ses_agentrun_${profile}_${base}`;