fix(rerank): truncate documents exceeding 2048-token context size

node-llama-cpp throws a hard error when any document + query + template
overhead exceeds the ranking context size. Truncate oversized documents
using the rerank model's tokenizer before passing them to rankAll().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/llm.ts

@@ -1022,6 +1022,9 @@ export class LlamaCpp implements LLM {
     }
   }
 
+  // Qwen3 reranker chat template overhead (system prompt, tags, separators)
+  private static readonly RERANK_TEMPLATE_OVERHEAD = 200;
+
   async rerank(
     query: string,
     documents: RerankDocument[],
@@ -1031,15 +1034,28 @@ export class LlamaCpp implements LLM {
     this.touchActivity();
 
     const contexts = await this.ensureRerankContexts();
+    const model = await this.ensureRerankModel();
+
+    // Truncate documents that would exceed the rerank context size.
+    // Budget = contextSize - template overhead - query tokens
+    const queryTokens = model.tokenize(query).length;
+    const maxDocTokens = LlamaCpp.RERANK_CONTEXT_SIZE - LlamaCpp.RERANK_TEMPLATE_OVERHEAD - queryTokens;
+
+    const truncatedDocs = documents.map((doc) => {
+      const tokens = model.tokenize(doc.text);
+      if (tokens.length <= maxDocTokens) return doc;
+      const truncatedText = model.detokenize(tokens.slice(0, maxDocTokens));
+      return { ...doc, text: truncatedText };
+    });
 
     // Build a map from document text to original indices (for lookup after sorting)
     const textToDoc = new Map<string, { file: string; index: number }>();
-    documents.forEach((doc, index) => {
+    truncatedDocs.forEach((doc, index) => {
       textToDoc.set(doc.text, { file: doc.file, index });
     });
 
     // Extract just the text for ranking
-    const texts = documents.map((doc) => doc.text);
+    const texts = truncatedDocs.map((doc) => doc.text);
 
     // Split documents across contexts for parallel evaluation.
     // Each context has its own sequence with a lock, so parallelism comes

test/llm.test.ts

@@ -365,6 +365,45 @@ describe.skipIf(!!process.env.CI)("LlamaCpp Integration", () => {
       // Log timing for monitoring batch performance
       console.log(`Batch rerank of 10 docs took ${elapsed}ms`);
     });
+
+    test("truncates and reranks document exceeding 2048 token context size", async () => {
+      // The reranker context is created with contextSize=2048. Documents that
+      // exceed the token budget (contextSize - template overhead - query tokens)
+      // should be silently truncated rather than crashing.
+      const paragraph = "The quick brown fox jumps over the lazy dog near the riverbank. " +
+        "Authentication tokens must be validated on every request to ensure security. " +
+        "Database queries should use prepared statements to prevent SQL injection attacks. " +
+        "The deployment pipeline includes linting, testing, building, and publishing stages. ";
+      // ~320 chars per paragraph, repeat 40 times = ~12800 chars ≈ 3200 tokens
+      const longText = paragraph.repeat(40);
+
+      const query = "How do I configure authentication?";
+      const documents: RerankDocument[] = [
+        { file: "short-relevant.md", text: "Authentication can be configured by setting AUTH_SECRET." },
+        { file: "long-doc.md", text: longText },
+        { file: "short-irrelevant.md", text: "The weather is sunny today." },
+      ];
+
+      console.log(`Long doc length: ${longText.length} chars (~${Math.round(longText.length / 4)} tokens)`);
+
+      const result = await llm.rerank(query, documents);
+
+      // Should return all 3 documents without crashing
+      expect(result.results).toHaveLength(3);
+
+      // All scores should be valid numbers in [0, 1]
+      for (const doc of result.results) {
+        expect(doc.score).toBeGreaterThanOrEqual(0);
+        expect(doc.score).toBeLessThanOrEqual(1);
+        expect(Number.isNaN(doc.score)).toBe(false);
+      }
+
+      // The short, directly relevant doc should still rank highest
+      console.log("Rerank results for long doc test:");
+      for (const doc of result.results) {
+        console.log(`  ${doc.file}: ${doc.score.toFixed(4)}`);
+      }
+    });
   });
 
   describe("expandQuery", () => {