diff --git a/src/config/api.ts b/src/config/api.ts
index d71a8a7..9c791b5 100644
--- a/src/config/api.ts
+++ b/src/config/api.ts
@@ -60,15 +60,6 @@ export function getWebSocketURL(path: string): string {
   return `${protocol}//${window.location.host}${path}`;
 }
 
-/**
- * Generic API response wrapper from squirrel-backend
- */
-export interface ApiResultResponse<T> {
-  errorCode: number;
-  errorMessage: string | null;
-  payload: T;
-}
-
 /**
  * Paged result wrapper for paginated endpoints
  */
diff --git a/src/contexts/LivePVContext.tsx b/src/contexts/LivePVContext.tsx
index 99575c6..5792708 100644
--- a/src/contexts/LivePVContext.tsx
+++ b/src/contexts/LivePVContext.tsx
@@ -64,40 +64,36 @@ export function LivePVProvider({
         throw new Error(`HTTP ${response.status}`);
       }
 
-      const data = await response.json();
-
-      if (data.errorCode === 0 && data.payload) {
-        const rawValues = data.payload as Record<
-          string,
-          {
-            value: unknown;
-            connected: boolean;
-            updated_at: number;
-            status?: string;
-            severity?: number;
-            units?: string;
-          }
-        >;
-
-        // Transform backend format to EpicsData format
-        setLiveValues((prev) => {
-          const next = new Map(prev);
-          Object.entries(rawValues).forEach(([pvName, rawValue]) => {
-            // Map backend fields to EpicsData fields
-            const epicsData: EpicsData = {
-              data: rawValue.value as EpicsData['data'],
-              severity: rawValue.severity,
-              units: rawValue.units,
-              timestamp: rawValue.updated_at ? new Date(rawValue.updated_at * 1000) : undefined,
-            };
-            next.set(pvName, epicsData);
-          });
-          return next;
+      const rawValues = (await response.json()) as Record<
+        string,
+        {
+          value: unknown;
+          connected: boolean;
+          updated_at: number;
+          status?: string;
+          severity?: number;
+          units?: string;
+        }
+      >;
+
+      // Transform backend format to EpicsData format
+      setLiveValues((prev) => {
+        const next = new Map(prev);
+        Object.entries(rawValues).forEach(([pvName, rawValue]) => {
+          // Map backend fields to EpicsData fields
+          const epicsData: EpicsData = {
+            data: rawValue.value as EpicsData['data'],
+            severity: rawValue.severity,
+            units: rawValue.units,
+            timestamp: rawValue.updated_at ? new Date(rawValue.updated_at * 1000) : undefined,
+          };
+          next.set(pvName, epicsData);
         });
-        setLastUpdate(new Date());
-        setIsConnected(true);
-        setError(null);
-      }
+        return next;
+      });
+      setLastUpdate(new Date());
+      setIsConnected(true);
+      setError(null);
     } catch (err) {
       setError(err instanceof Error ? err.message : 'Failed to fetch live values');
       setIsConnected(false);
diff --git a/src/services/apiClient.ts b/src/services/apiClient.ts
index 9436ff8..bc2bd26 100644
--- a/src/services/apiClient.ts
+++ b/src/services/apiClient.ts
@@ -2,7 +2,41 @@
  * Base API client for making HTTP requests to squirrel-backend
  */
 
-import { API_CONFIG, ApiKeyError, ApiResultResponse } from '../config/api';
+import { API_CONFIG, ApiKeyError } from '../config/api';
+
+interface ValidationErrorItem {
+  loc?: (string | number)[];
+  msg: string;
+  type?: string;
+}
+
+async function extractErrorMessage(response: Response): Promise<string> {
+  const fallback = `HTTP error! status: ${response.status}`;
+  let body: string;
+  try {
+    body = await response.text();
+  } catch {
+    return fallback;
+  }
+  if (!body) return fallback;
+
+  try {
+    const parsed = JSON.parse(body);
+    const { detail } = parsed;
+    if (typeof detail === 'string') return detail;
+    if (Array.isArray(detail)) {
+      return (detail as ValidationErrorItem[])
+        .map((d) => `${d.loc?.join('.') ?? ''}: ${d.msg}`)
+        .join('; ');
+    }
+  } catch {
+    // not JSON — fall through
+  }
+
+  // eslint-disable-next-line no-console
+  console.error('Server error response:', body);
+  return `${fallback}, details: ${body}`;
+}
 
 class APIClient {
   // eslint-disable-next-line class-methods-use-this
@@ -28,24 +62,14 @@ class APIClient {
         if (response.status === 401 || response.status === 403) {
           throw new ApiKeyError(response.status);
         }
-        // Try to get error details from response body
-        try {
-          const errorData = await response.text();
-          // eslint-disable-next-line no-console
-          console.error('Server error response:', errorData);
-          throw new Error(`HTTP error! status: ${response.status}, details: ${errorData}`);
-        } catch {
-          throw new Error(`HTTP error! status: ${response.status}`);
-        }
+        throw new Error(await extractErrorMessage(response));
       }
 
-      const data: ApiResultResponse<T> = await response.json();
-
-      if (data.errorCode !== 0) {
-        throw new Error(data.errorMessage || 'API error');
+      if (response.status === 204 || response.headers.get('content-length') === '0') {
+        return undefined as T;
       }
 
-      return data.payload;
+      return (await response.json()) as T;
     } catch (error) {
       clearTimeout(timeoutId);
       if (error instanceof Error && error.name === 'AbortError') {
diff --git a/src/services/heartbeatService.ts b/src/services/heartbeatService.ts
index bc54beb..0c88d69 100644
--- a/src/services/heartbeatService.ts
+++ b/src/services/heartbeatService.ts
@@ -65,9 +65,9 @@ class HeartbeatService {
       const data = await response.json();
 
       // Backend returns: { alive: boolean, timestamp: number | null, age_seconds: number | null }
-      const alive = data.payload?.alive ?? false;
-      const age = data.payload?.age_seconds ?? null;
-      const timestamp = data.payload?.timestamp ?? null;
+      const alive = data.alive ?? false;
+      const age = data.age_seconds ?? null;
+      const timestamp = data.timestamp ?? null;
 
       this.lastKnownState = { alive, age, timestamp };
       this.notifyCallbacks(alive, age);