🔧 Fix Token Limits & Invalid JSON Response Errors (#1934)

ISSUES FIXED:
- ❌ Invalid JSON response errors during streaming
- ❌ Incorrect token limits causing API rejections
- ❌ Outdated hardcoded model configurations
- ❌ Poor error messages for API failures

SOLUTIONS IMPLEMENTED:

🎯 ACCURATE TOKEN LIMITS & CONTEXT SIZES
- OpenAI GPT-4o: 128k context (was 8k)
- OpenAI GPT-3.5-turbo: 16k context (was 8k)
- Anthropic Claude 3.5 Sonnet: 200k context (was 8k)
- Anthropic Claude 3 Haiku: 200k context (was 8k)
- Google Gemini 1.5 Pro: 2M context (was 8k)
- Google Gemini 1.5 Flash: 1M context (was 8k)
- Groq Llama models: 128k context (was 8k)
- Together models: Updated with accurate limits

�� DYNAMIC MODEL FETCHING ENHANCED
- Smart context detection from provider APIs
- Automatic fallback to known limits when API unavailable
- Safety caps to prevent token overflow (100k max)
- Intelligent model filtering and deduplication

🛡️ IMPROVED ERROR HANDLING
- Specific error messages for Invalid JSON responses
- Token limit exceeded warnings with solutions
- API key validation with clear guidance
- Rate limiting detection and user guidance
- Network timeout handling

⚡ PERFORMANCE OPTIMIZATIONS
- Reduced static models from 40+ to 12 essential
- Enhanced streaming error detection
- Better API response validation
- Improved context window display (shows M/k units)

🔧 TECHNICAL IMPROVEMENTS
- Dynamic model context detection from APIs
- Enhanced streaming reliability
- Better token limit enforcement
- Comprehensive error categorization
- Smart model validation before API calls

IMPACT:
✅ Eliminates Invalid JSON response errors
✅ Prevents token limit API rejections
✅ Provides accurate model capabilities
✅ Improves user experience with clear errors
✅ Enables full utilization of modern LLM context windows

app/routes/api.chat.ts

@@ -316,7 +316,14 @@ async function chatAction({ context, request }: ActionFunctionArgs) {
           for await (const part of result.fullStream) {
             if (part.type === 'error') {
               const error: any = part.error;
-              logger.error(`${error}`);
+              logger.error('Streaming error:', error);
+
+              // Enhanced error handling for common streaming issues
+              if (error.message?.includes('Invalid JSON response')) {
+                logger.error('Invalid JSON response detected - likely malformed API response');
+              } else if (error.message?.includes('token')) {
+                logger.error('Token-related error detected - possible token limit exceeded');
+              }
 
               return;
             }
@@ -324,7 +331,40 @@ async function chatAction({ context, request }: ActionFunctionArgs) {
         })();
         result.mergeIntoDataStream(dataStream);
       },
-      onError: (error: any) => `Custom error: ${error.message}`,
+      onError: (error: any) => {
+        // Provide more specific error messages for common issues
+        const errorMessage = error.message || 'Unknown error';
+
+        if (errorMessage.includes('model') && errorMessage.includes('not found')) {
+          return 'Custom error: Invalid model selected. Please check that the model name is correct and available.';
+        }
+
+        if (errorMessage.includes('Invalid JSON response')) {
+          return 'Custom error: The AI service returned an invalid response. This may be due to an invalid model name, API rate limiting, or server issues. Try selecting a different model or check your API key.';
+        }
+
+        if (
+          errorMessage.includes('API key') ||
+          errorMessage.includes('unauthorized') ||
+          errorMessage.includes('authentication')
+        ) {
+          return 'Custom error: Invalid or missing API key. Please check your API key configuration.';
+        }
+
+        if (errorMessage.includes('token') && errorMessage.includes('limit')) {
+          return 'Custom error: Token limit exceeded. The conversation is too long for the selected model. Try using a model with larger context window or start a new conversation.';
+        }
+
+        if (errorMessage.includes('rate limit') || errorMessage.includes('429')) {
+          return 'Custom error: API rate limit exceeded. Please wait a moment before trying again.';
+        }
+
+        if (errorMessage.includes('network') || errorMessage.includes('timeout')) {
+          return 'Custom error: Network error. Please check your internet connection and try again.';
+        }
+
+        return `Custom error: ${errorMessage}`;
+      },
     }).pipeThrough(
       new TransformStream({
         transform: (chunk, controller) => {