diff --git a/internal/cloud/server-workbench-http.ts b/internal/cloud/server-workbench-http.ts
index b9b4b3eb..883f9a2e 100644
--- a/internal/cloud/server-workbench-http.ts
+++ b/internal/cloud/server-workbench-http.ts
@@ -270,6 +270,33 @@ function attachWorkbenchRealtimeOtelContext(request, fields = {}) {
   };
 }
 
+function attachWorkbenchReadModelDiagnosticOtel(request, fields = {}) {
+  const context = request?.hwlabHttpRequestContext;
+  if (!context) return;
+  const code = textValue(fields.code) || "workbench_read_model_not_found";
+  const route = textValue(fields.route) || context.route || "/v1/workbench";
+  context.route = route;
+  context.lastHttpError = {
+    code,
+    category: "workbench_read_model",
+    layer: "workbench.read_model"
+  };
+  context.otelAttributes = {
+    ...(context.otelAttributes ?? {}),
+    "workbench.route": route,
+    "workbench.read_model.route": route,
+    "workbench.read_model.result": code,
+    "workbench.read_model.count": Number.isFinite(Number(fields.count)) ? Math.trunc(Number(fields.count)) : null,
+    "workbench.session_id": fields.sessionId ?? null,
+    "workbench.trace_id": fields.traceId ?? null,
+    "workbench.turn_id": fields.turnId ?? null,
+    traceId: fields.traceId ?? null,
+    "error.code": code,
+    "error.category": "workbench_read_model",
+    "error.layer": "workbench.read_model"
+  };
+}
+
 function emitWorkbenchRealtimeAcceptedOtelSpan(request, env = process.env) {
   const context = request?.hwlabHttpRequestContext;
   if (!context?.traceId) return;
@@ -1237,7 +1264,11 @@ async function handleWorkbenchSessionDetail(request, response, options, actor, s
   const result = await queryFactsByRouteId(readModel, sessionId, { families: WORKBENCH_SESSION_DETAIL_FAMILIES });
   if (result.error) return sendJson(response, 503, workbenchProjectionStoreError(result.error));
   const session = visibleFactSessions(result.facts, actor)[0] ?? null;
-  if (!session) return sendJson(response, 404, workbenchError("workbench_session_not_found", "Workbench session is not visible to the current actor.", { sessionId }));
+  if (!session) {
+    const body = workbenchError("workbench_session_not_found", "Workbench session is not visible to the current actor.", { sessionId });
+    attachWorkbenchReadModelDiagnosticOtel(request, { route: "/v1/workbench/sessions/:id", code: "workbench_session_not_found", sessionId, count: result.count });
+    return sendJson(response, 404, body);
+  }
   sendJson(response, 200, {
     ok: true,
     status: "found",
@@ -1253,7 +1284,11 @@ async function handleWorkbenchMessagePage(request, response, url, options, actor
   const result = await queryFactsByRouteId(readModel, sessionId, { families: WORKBENCH_SESSION_MESSAGE_PAGE_FAMILIES });
   if (result.error) return sendJson(response, 503, workbenchProjectionStoreError(result.error));
   const session = visibleFactSessions(result.facts, actor)[0] ?? null;
-  if (!session) return sendJson(response, 404, workbenchError("workbench_session_not_found", "Workbench session is not visible to the current actor.", { sessionId }));
+  if (!session) {
+    const body = workbenchError("workbench_session_not_found", "Workbench session is not visible to the current actor.", { sessionId });
+    attachWorkbenchReadModelDiagnosticOtel(request, { route: "/v1/workbench/sessions/:id/messages", code: "workbench_session_not_found", sessionId, count: result.count });
+    return sendJson(response, 404, body);
+  }
   const messages = factMessagesForSession(session, result.facts);
   const limit = boundedLimit(url.searchParams.get("limit"));
   const offset = cursorOffset(url.searchParams.get("cursor") ?? url.searchParams.get("after"));
@@ -1298,6 +1333,7 @@ async function handleWorkbenchTurnSnapshot(request, response, url, options, acto
   const found = Boolean(session || turn);
   if (!found) {
     const body = workbenchError("workbench_turn_not_found", "Workbench turn is not visible to the current actor.", { turnId, traceId });
+    attachWorkbenchReadModelDiagnosticOtel(request, { route: "/v1/workbench/turns/:id", code: "workbench_turn_not_found", traceId, turnId, count: result.count });
     recordWorkbenchTurnReadMetric(options, url, 404, body, startedAt);
     return sendJson(response, 404, body);
   }
@@ -1351,8 +1387,10 @@ async function handleWorkbenchTraceEventPage(request, response, url, options, ac
     if (contextSession || contextTurn) {
       const projection = factProjectionForTrace(context.facts, traceId);
       const blocker = traceEventsReadModelBlocker("workbench_trace_metadata_missing", "Workbench trace metadata is missing from the trace events read model.", { traceId, projection, session: contextSession, turn: contextTurn, route: url.pathname });
+      attachWorkbenchReadModelDiagnosticOtel(request, { route: "/v1/workbench/traces/:id/events", code: "workbench_trace_metadata_missing", sessionId: factSessionId(contextSession), traceId, count: context.count });
       return sendJson(response, 404, workbenchTraceEventsReadModelError(blocker, { traceId, projection, session: contextSession, turn: contextTurn }));
     }
+    attachWorkbenchReadModelDiagnosticOtel(request, { route: "/v1/workbench/traces/:id/events", code: "workbench_trace_not_found", traceId, count: context.count });
     return sendJson(response, 404, workbenchError("workbench_trace_not_found", "Workbench trace is not visible to the current actor.", { traceId }));
   }
   const projection = factProjectionForTrace(metadataFacts, traceId);