diff --git a/apps/server/src/providers/claude-provider.ts b/apps/server/src/providers/claude-provider.ts
index dd32050c..aab9ec97 100644
--- a/apps/server/src/providers/claude-provider.ts
+++ b/apps/server/src/providers/claude-provider.ts
@@ -239,9 +239,9 @@ export class ClaudeProvider extends BaseProvider {
     if (Array.isArray(prompt)) {
       // Multi-part prompt (with images)
       promptPayload = (async function* () {
-        const multiPartPrompt = {
+        const multiPartPrompt: SDKUserMessage = {
           type: 'user' as const,
-          session_id: sdkSessionId || undefined,
+          session_id: sdkSessionId || '',
           message: {
             role: 'user' as const,
             content: prompt,