diff --git a/internal/cloud/server-workbench-http.ts b/internal/cloud/server-workbench-http.ts
index 7379ed97..087bd100 100644
--- a/internal/cloud/server-workbench-http.ts
+++ b/internal/cloud/server-workbench-http.ts
@@ -582,14 +582,16 @@ async function queryFactsForSessionSummaries(readModel, sessions = []) {
   const sessionIds = uniqueText(sessions.map(factSessionId));
   const traceIds = uniqueText(sessions.map(factLastTraceId));
   if (sessionIds.length === 0 && traceIds.length === 0) return { facts: emptyFactSet(), count: 0, durable: false, persistence: null };
-  const bySessionPromise = sessionIds.length > 0
-    ? readModel.queryFacts({ sessionIds, families: ["messages", "turns"] })
-    : Promise.resolve({ facts: emptyFactSet(), count: 0, durable: false, persistence: null });
-  const byTracePromise = traceIds.length > 0
-    ? readModel.queryFacts({ traceIds, families: ["checkpoints"] })
-    : Promise.resolve({ facts: emptyFactSet(), count: 0, durable: false, persistence: null });
-  const [bySession, byTrace] = await Promise.all([bySessionPromise, byTracePromise]);
+  // Session list summary is hot and is also sampled by the two-page web-probe
+  // observer. Avoid parallel fan-out here so one list request cannot consume
+  // multiple durable-read connections while another page is refreshing.
+  const bySession = sessionIds.length > 0
+    ? await readModel.queryFacts({ sessionIds, families: ["messages", "turns"] })
+    : { facts: emptyFactSet(), count: 0, durable: false, persistence: null };
   if (bySession.error) return bySession;
+  const byTrace = traceIds.length > 0
+    ? await readModel.queryFacts({ traceIds, families: ["checkpoints"] })
+    : { facts: emptyFactSet(), count: 0, durable: false, persistence: null };
   if (byTrace.error) return byTrace;
   const facts = mergeFactSets(bySession.facts, byTrace.facts);
   return { facts, count: Object.values(facts).reduce((sum, rows) => sum + rows.length, 0), durable: bySession.durable ?? byTrace.durable, persistence: bySession.persistence ?? byTrace.persistence ?? null };
diff --git a/internal/db/runtime-store.ts b/internal/db/runtime-store.ts
index 51d52930..68b03ca4 100644
--- a/internal/db/runtime-store.ts
+++ b/internal/db/runtime-store.ts
@@ -1342,14 +1342,18 @@ export class PostgresCloudRuntimeStore {
   async queryWorkbenchFacts(params = {}) {
     await this.assertReadyForDurableReads("workbench.facts.query");
     const families = workbenchFactFamilySet(params);
-    const entries = await Promise.all([
-      ["sessions", families.has("sessions") ? this.queryWorkbenchFactRows("workbench.sessions.query", "workbench_sessions", "session_json", params, { sessionId: "session_id", ownerUserId: "owner_user_id", projectId: "project_id", conversationId: "conversation_id", threadId: "thread_id", traceId: "last_trace_id", lastTraceId: "last_trace_id", status: "status" }) : Promise.resolve([])],
-      ["messages", families.has("messages") ? this.queryWorkbenchFactRows("workbench.messages.query", "workbench_messages", "message_json", params, { messageId: "message_id", sessionId: "session_id", turnId: "turn_id", traceId: "trace_id", role: "role", status: "status" }) : Promise.resolve([])],
-      ["parts", families.has("parts") ? this.queryWorkbenchFactRows("workbench.parts.query", "workbench_parts", "part_json", params, { partId: "part_id", messageId: "message_id", sessionId: "session_id", turnId: "turn_id", traceId: "trace_id", partType: "part_type", status: "status" }) : Promise.resolve([])],
-      ["turns", families.has("turns") ? this.queryWorkbenchFactRows("workbench.turns.query", "workbench_turns", "turn_json", params, { turnId: "turn_id", sessionId: "session_id", traceId: "trace_id", messageId: "message_id", status: "status" }) : Promise.resolve([])],
-      ["traceEvents", families.has("traceEvents") ? this.queryWorkbenchFactRows("workbench.trace-events.query", "workbench_trace_events", "event_json", params, { id: "id", traceId: "trace_id", sessionId: "session_id", turnId: "turn_id", messageId: "message_id", eventType: "event_type" }) : Promise.resolve([])],
-      ["checkpoints", families.has("checkpoints") ? this.queryWorkbenchFactRows("workbench.projection-checkpoints.query", "workbench_projection_checkpoints", "checkpoint_json", params, { traceId: "trace_id", sessionId: "session_id", turnId: "turn_id", runId: "run_id", commandId: "command_id", projectionStatus: "projection_status", projectionHealth: "projection_health" }) : Promise.resolve([])]
-    ].map(async ([family, rowsPromise]) => [family, await rowsPromise]));
+    // A single Workbench read can request several fact families. Running those
+    // SQL reads in parallel lets one HTTP request occupy the whole Postgres
+    // pool, which makes the control+observer pages amplify into 503s under
+    // periodic refresh. Keep this read model bounded to one connection per
+    // request; the page-level latency is preferable to pool starvation.
+    const entries = [];
+    entries.push(["sessions", families.has("sessions") ? await this.queryWorkbenchFactRows("workbench.sessions.query", "workbench_sessions", "session_json", params, { sessionId: "session_id", ownerUserId: "owner_user_id", projectId: "project_id", conversationId: "conversation_id", threadId: "thread_id", traceId: "last_trace_id", lastTraceId: "last_trace_id", status: "status" }) : []]);
+    entries.push(["messages", families.has("messages") ? await this.queryWorkbenchFactRows("workbench.messages.query", "workbench_messages", "message_json", params, { messageId: "message_id", sessionId: "session_id", turnId: "turn_id", traceId: "trace_id", role: "role", status: "status" }) : []]);
+    entries.push(["parts", families.has("parts") ? await this.queryWorkbenchFactRows("workbench.parts.query", "workbench_parts", "part_json", params, { partId: "part_id", messageId: "message_id", sessionId: "session_id", turnId: "turn_id", traceId: "trace_id", partType: "part_type", status: "status" }) : []]);
+    entries.push(["turns", families.has("turns") ? await this.queryWorkbenchFactRows("workbench.turns.query", "workbench_turns", "turn_json", params, { turnId: "turn_id", sessionId: "session_id", traceId: "trace_id", messageId: "message_id", status: "status" }) : []]);
+    entries.push(["traceEvents", families.has("traceEvents") ? await this.queryWorkbenchFactRows("workbench.trace-events.query", "workbench_trace_events", "event_json", params, { id: "id", traceId: "trace_id", sessionId: "session_id", turnId: "turn_id", messageId: "message_id", eventType: "event_type" }) : []]);
+    entries.push(["checkpoints", families.has("checkpoints") ? await this.queryWorkbenchFactRows("workbench.projection-checkpoints.query", "workbench_projection_checkpoints", "checkpoint_json", params, { traceId: "trace_id", sessionId: "session_id", turnId: "turn_id", runId: "run_id", commandId: "command_id", projectionStatus: "projection_status", projectionHealth: "projection_health" }) : []]);
     const facts = Object.fromEntries(entries);
     return withPersistence({ facts, count: Object.values(facts).reduce((sum, rows) => sum + rows.length, 0) }, this.summary());
   }