Improve AI assignment error messages for invalid models

- Add try-catch around OpenAI API call to catch model errors
- Provide clearer error messages when model name is invalid
- Handle empty response cases with helpful feedback
- Suggest checking AI Configuration settings when model fails

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/server/services/ai-assignment.ts

@@ -136,7 +136,18 @@ async function processAssignmentBatch(
       maxTokens: 4000,
     })
 
-    const response = await openai.chat.completions.create(params)
+    let response
+    try {
+      response = await openai.chat.completions.create(params)
+    } catch (apiError) {
+      // Provide clearer error for model-related issues
+      const errorMsg = apiError instanceof Error ? apiError.message : String(apiError)
+      if (errorMsg.includes('model') || errorMsg.includes('does not exist')) {
+        throw new Error(`Invalid AI model "${model}". Please check the model name in Settings > AI Configuration.`)
+      }
+      throw apiError
+    }
+
     const usage = extractTokenUsage(response)
     tokensUsed = usage.totalTokens
 
@@ -157,7 +168,15 @@ async function processAssignmentBatch(
 
     const content = response.choices[0]?.message?.content
     if (!content) {
-      throw new Error('No response from AI')
+      // Check if response indicates an issue
+      const finishReason = response.choices[0]?.finish_reason
+      if (finishReason === 'content_filter') {
+        throw new Error('AI response was filtered. Try a different model or simplify the project descriptions.')
+      }
+      if (!response.choices || response.choices.length === 0) {
+        throw new Error(`No response from model "${model}". This model may not exist or may not be available. Please verify the model name.`)
+      }
+      throw new Error(`Empty response from AI model "${model}". The model may not support this type of request.`)
     }
 
     const parsed = JSON.parse(content) as {