diff --git a/clients/javascript/src/bidi/client.ts b/clients/javascript/src/bidi/client.ts
index 4cc0df9e..5bd422f7 100644
--- a/clients/javascript/src/bidi/client.ts
+++ b/clients/javascript/src/bidi/client.ts
@@ -39,7 +39,22 @@ export class BiDiClient {
     this.pendingCommands.delete(response.id);
 
     if (response.type === 'error' && response.error) {
-      pending.reject(new Error(`${response.error}: ${response.message}`));
+      // Handle both flat and nested error formats for compatibility
+      let errorCode: string;
+      let errorMessage: string;
+
+      if (typeof response.error === 'object' && response.error !== null) {
+        // Nested format: { error: { error: "code", message: "msg" } }
+        const errObj = response.error as { error?: string; message?: string };
+        errorCode = errObj.error || 'unknown error';
+        errorMessage = errObj.message || 'An error occurred';
+      } else {
+        // Flat format (BiDi spec): { error: "code", message: "msg" }
+        errorCode = String(response.error);
+        errorMessage = response.message || 'An error occurred';
+      }
+
+      pending.reject(new Error(`${errorCode}: ${errorMessage}`));
     } else {
       pending.resolve(response.result);
     }
diff --git a/clients/javascript/src/bidi/types.ts b/clients/javascript/src/bidi/types.ts
index 7732beef..8b2d7d20 100644
--- a/clients/javascript/src/bidi/types.ts
+++ b/clients/javascript/src/bidi/types.ts
@@ -10,7 +10,7 @@ export interface BiDiResponse {
   id: number;
   type: 'success' | 'error';
   result?: unknown;
-  error?: string;
+  error?: string | { error?: string; message?: string }; // Can be flat (spec) or nested (current impl)
   message?: string;
 }