fix: Resolve TypeScript strict null check errors for Railway build

Fixes build failures caused by noUncheckedIndexedAccess TypeScript setting:

llm-navigator.service.ts:
- Add null checks for array index access in meaningfulParts
- Add null guard for line iteration in parseLLMResponse
- Fix array destructuring for split() results with proper fallbacks
- Fix singleOption access after length check
- Fix fallbackOption access with non-null assertions

llm-conversational-classifier.service.ts:
- Add null coalescing for split() array access
- Add null guards for userDecisions array access
- Fix lastDecision possibly undefined errors

Also includes pre-filter optimization for cross-chapter products to reduce
LLM token usage at root level.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: AryanBV

backend/src/services/llm-conversational-classifier.service.ts

@@ -267,10 +267,10 @@ async function continueConversation(
       // Parse the code from the answer (remove label if present)
       let selectedCode = selectedAnswer;
       if (selectedAnswer.includes('::')) {
-        selectedCode = selectedAnswer.split('::')[0].trim();
+        selectedCode = selectedAnswer.split('::')[0]?.trim() ?? selectedAnswer;
       } else if (selectedAnswer.includes(':')) {
         // Legacy format "CODE: label"
-        selectedCode = selectedAnswer.split(':')[0].trim();
+        selectedCode = selectedAnswer.split(':')[0]?.trim() ?? selectedAnswer;
       }
 
       logger.info(`[LLM-CLASSIFIER] User selected: ${selectedAnswer} -> code: ${selectedCode}`);
@@ -288,7 +288,7 @@ async function continueConversation(
         // Record user's decision for better reasoning
         if (state.pendingQuestionText && state.pendingQuestionOptions) {
           const selectedLabel = selectedAnswer.includes('::')
-            ? selectedAnswer.split('::')[1].trim()
+            ? (selectedAnswer.split('::')[1]?.trim() ?? codeDetails.description)
             : codeDetails.description;
 
           const alternatives = state.pendingQuestionOptions
@@ -625,10 +625,12 @@ async function buildSmartAlternatives(
       // No parent - return last question's alternatives only
       if (userDecisions.length > 0) {
         const lastDecision = userDecisions[userDecisions.length - 1];
-        return lastDecision.alternatives.map(alt => ({
-          code: alt.code,
-          description: alt.label
-        }));
+        if (lastDecision) {
+          return lastDecision.alternatives.map(alt => ({
+            code: alt.code,
+            description: alt.label
+          }));
+        }
       }
       return [];
     }
@@ -661,10 +663,12 @@ async function buildSmartAlternatives(
     // Fallback to last question's alternatives
     if (userDecisions.length > 0) {
       const lastDecision = userDecisions[userDecisions.length - 1];
-      return lastDecision.alternatives.map(alt => ({
-        code: alt.code,
-        description: alt.label
-      }));
+      if (lastDecision) {
+        return lastDecision.alternatives.map(alt => ({
+          code: alt.code,
+          description: alt.label
+        }));
+      }
     }
     return [];
   }
@@ -799,9 +803,9 @@ export async function skipToClassification(
               // Parse code from "CODE::LABEL" format if needed
               let code = selectedCode;
               if (selectedCode.includes('::')) {
-                code = selectedCode.split('::')[0].trim();
+                code = selectedCode.split('::')[0]?.trim() ?? selectedCode;
               } else if (selectedCode.includes(':')) {
-                code = selectedCode.split(':')[0].trim();
+                code = selectedCode.split(':')[0]?.trim() ?? selectedCode;
               }
 
               const codeDetails = await validateCode(code);

backend/src/services/llm-navigator.service.ts

@@ -90,7 +90,7 @@ async function buildUserFriendlyReasoning(
     );
 
     if (meaningfulParts.length >= 2) {
-      const mainCategory = meaningfulParts[0].description;
+      const mainCategory = meaningfulParts[0]!.description;
       const subParts = meaningfulParts.slice(1).map(p => p.description);
 
       let reasoning = `"${productName}" is classified under ${mainCategory}`;
@@ -102,7 +102,7 @@ async function buildUserFriendlyReasoning(
       }
       return reasoning;
     } else if (meaningfulParts.length === 1) {
-      let reasoning = `"${productName}" falls under ${meaningfulParts[0].description}`;
+      let reasoning = `"${productName}" falls under ${meaningfulParts[0]!.description}`;
       if (context) {
         reasoning += `. ${context}`;
       }
@@ -511,6 +511,38 @@ const CROSS_CHAPTER_PRODUCTS: Record<string, string[]> = {
   'seafood': ['03', '16'],
 };
 
+/**
+ * PRE-FILTER root options BEFORE sending to LLM to reduce token usage
+ * For known cross-chapter products, only send relevant chapter headings
+ * This reduces prompts from 1125 options (~20K tokens) to 2-10 options (~500 tokens)
+ */
+function preFilterRootOptions(
+  userInput: string,
+  options: HierarchyOption[]
+): HierarchyOption[] {
+  const inputLower = userInput.toLowerCase();
+
+  // Check for cross-chapter products
+  for (const [product, validChapters] of Object.entries(CROSS_CHAPTER_PRODUCTS)) {
+    if (inputLower.includes(product)) {
+      // Filter to only include headings from valid chapters
+      const filtered = options.filter(opt => {
+        const chapterCode = opt.code.substring(0, 2);
+        return validChapters.includes(chapterCode);
+      });
+
+      if (filtered.length > 0) {
+        logger.info(`[LLM-NAV] Cross-chapter product "${product}" - filtered to chapters: ${validChapters.join(', ')}`);
+        return filtered;
+      }
+    }
+  }
+
+  // If no cross-chapter match, return original (will use full list)
+  // TODO: Add keyword-based filtering for other products to further reduce tokens
+  return options;
+}
+
 /**
  * Filter options to only include valid chapters for cross-chapter products
  * This prevents the LLM from hallucinating irrelevant chapters
@@ -572,6 +604,7 @@ function parseNavigationResponse(
 
   for (let i = 0; i < lines.length; i++) {
     const line = lines[i];
+    if (!line) continue;
 
     if (line.startsWith('ACTION:')) {
       action = line.replace('ACTION:', '').trim().toUpperCase();
@@ -658,12 +691,14 @@ function parseNavigationResponse(
       // New format: CODE|FRIENDLY_LABEL
       for (const optLine of optionCodes) {
         if (optLine.includes('|')) {
-          const [code, label] = optLine.split('|').map(s => s.trim());
-          const matchingOption = options.find(o => o.code === code || o.code.includes(code) || code.includes(o.code));
+          const parts = optLine.split('|').map(s => s.trim());
+          const optCode = parts[0] ?? '';
+          const label = parts[1] ?? '';
+          const matchingOption = options.find(o => o.code === optCode || o.code.includes(optCode) || optCode.includes(o.code));
           if (matchingOption) {
             finalOptions.push({
               code: matchingOption.code,
-              label: label || matchingOption.description.split(':')[0].trim(),
+              label: label || (matchingOption.description.split(':')[0]?.trim() ?? ''),
               description: matchingOption.description
             });
           }
@@ -776,10 +811,20 @@ export async function navigateHierarchy(
 
   try {
     // Get available options at current level
-    const options = currentCode
+    let options = currentCode
       ? await getChildrenForCode(currentCode)
       : await getAllChapters();
 
+    // PRE-FILTER at root level to reduce token usage
+    // For cross-chapter products, only send relevant chapters to LLM
+    if (currentCode === null) {
+      const filteredOptions = preFilterRootOptions(userInput, options);
+      if (filteredOptions.length > 0 && filteredOptions.length < options.length) {
+        logger.info(`[LLM-NAV] Pre-filtered from ${options.length} to ${filteredOptions.length} options`);
+        options = filteredOptions;
+      }
+    }
+
     logger.info(`[LLM-NAV] At ${currentCode || 'root'}, found ${options.length} options`);
 
     // If no options, we're at a leaf - return classification
@@ -1001,8 +1046,8 @@ export async function forceClassification(
     }
 
     // If only one option and it's not "Other", auto-select
-    if (options.length === 1 && !options[0].isOther) {
-      const singleOption = options[0];
+    const singleOption = options.length === 1 ? options[0] : undefined;
+    if (singleOption && !singleOption.isOther) {
       if (!singleOption.hasChildren) {
         return {
           type: 'classification',
@@ -1057,18 +1102,18 @@ export async function forceClassification(
     // Parse the code from response
     const codeMatch = content.match(/CODE:\s*([0-9.]+)/i);
     if (!codeMatch) {
-      // Fallback: pick the first non-Other option
-      const fallbackOption = options.find(o => !o.isOther) || options[0];
+      // Fallback: pick the first non-Other option (options is guaranteed non-empty at this point)
+      const fallbackOption = options.find(o => !o.isOther) ?? options[0]!;
       code = fallbackOption.code;
       logger.warn(`[LLM-NAV] Force mode: couldn't parse code, falling back to ${code}`);
     } else {
-      const selectedCode = codeMatch[1].trim();
+      const selectedCode = codeMatch[1]?.trim() ?? '';
       const matchedOption = options.find(o => o.code === selectedCode);
       if (matchedOption) {
         code = matchedOption.code;
       } else {
-        // Fallback
-        const fallbackOption = options.find(o => !o.isOther) || options[0];
+        // Fallback (options is guaranteed non-empty at this point)
+        const fallbackOption = options.find(o => !o.isOther) ?? options[0]!;
         code = fallbackOption.code;
       }
     }