diff --git a/.changeset/reranker-topn-validation.md b/.changeset/reranker-topn-validation.md
new file mode 100644
index 00000000..d3cdf702
--- /dev/null
+++ b/.changeset/reranker-topn-validation.md
@@ -0,0 +1,5 @@
+---
+"@chatcops/core": patch
+---
+
+Validate topN in cohereReranker and voyageReranker to immediately return an empty array for NaN.
diff --git a/packages/core/src/retrieval/cohere-reranker.ts b/packages/core/src/retrieval/cohere-reranker.ts
index b48f8eca..b5412fdc 100644
--- a/packages/core/src/retrieval/cohere-reranker.ts
+++ b/packages/core/src/retrieval/cohere-reranker.ts
@@ -24,7 +24,7 @@ export function cohereReranker(options: CohereRerankerOptions): Reranker {
   return {
     async rerank(query, chunks, topN, rerankOptions = {}) {
       const limit = Math.floor(topN);
-      if (chunks.length === 0 || limit <= 0) return [];
+      if (chunks.length === 0 || !(limit > 0)) return [];
 
       const response = await postWithRetries(
         'Cohere',
diff --git a/packages/core/src/retrieval/voyage-reranker.ts b/packages/core/src/retrieval/voyage-reranker.ts
index 657e3e79..b6c6b34d 100644
--- a/packages/core/src/retrieval/voyage-reranker.ts
+++ b/packages/core/src/retrieval/voyage-reranker.ts
@@ -24,7 +24,7 @@ export function voyageReranker(options: VoyageRerankerOptions): Reranker {
   return {
     async rerank(query, chunks, topN, rerankOptions = {}) {
       const limit = Math.floor(topN);
-      if (chunks.length === 0 || limit <= 0) return [];
+      if (chunks.length === 0 || !(limit > 0)) return [];
 
       const response = await postWithRetries(
         'Voyage',
diff --git a/packages/core/tests/retrieval/cohere-reranker.test.ts b/packages/core/tests/retrieval/cohere-reranker.test.ts
index f72a8c84..c96597aa 100644
--- a/packages/core/tests/retrieval/cohere-reranker.test.ts
+++ b/packages/core/tests/retrieval/cohere-reranker.test.ts
@@ -45,6 +45,21 @@ describe('cohereReranker', () => {
     expect(send).not.toHaveBeenCalled();
   });
 
+  it('returns no results and makes no request for NaN and -Infinity topN values', async () => {
+    const send = vi.fn(async () => json({ results: [] }));
+    const reranker = cohereReranker({ apiKey: 'k', fetch: send });
+    expect(await reranker.rerank('q', chunks, Number.NaN)).toEqual([]);
+    expect(await reranker.rerank('q', chunks, -Infinity)).toEqual([]);
+    expect(send).not.toHaveBeenCalled();
+  });
+
+  it('asks for no more than available documents when topN is Infinity', async () => {
+    const send = vi.fn(async () => json({ results: [] }));
+    const reranker = cohereReranker({ apiKey: 'k', fetch: send });
+    await reranker.rerank('q', chunks, Infinity);
+    expect(JSON.parse(String((send.mock.calls[0] as unknown as [string, RequestInit])[1].body))).toMatchObject({ top_n: 3 });
+  });
+
   it('surfaces client API errors without retrying', async () => {
     const send = vi.fn(async () => json({ message: 'Invalid model' }, 400));
     const reranker = cohereReranker({ apiKey: 'k', fetch: send });
diff --git a/packages/core/tests/retrieval/voyage-reranker.test.ts b/packages/core/tests/retrieval/voyage-reranker.test.ts
index aff13df0..220305ac 100644
--- a/packages/core/tests/retrieval/voyage-reranker.test.ts
+++ b/packages/core/tests/retrieval/voyage-reranker.test.ts
@@ -45,6 +45,21 @@ describe('voyageReranker', () => {
     expect(send).not.toHaveBeenCalled();
   });
 
+  it('returns no results and makes no request for NaN and -Infinity topN values', async () => {
+    const send = vi.fn(async () => json({ object: 'list', data: [] }));
+    const reranker = voyageReranker({ apiKey: 'k', fetch: send });
+    expect(await reranker.rerank('q', chunks, Number.NaN)).toEqual([]);
+    expect(await reranker.rerank('q', chunks, -Infinity)).toEqual([]);
+    expect(send).not.toHaveBeenCalled();
+  });
+
+  it('caps top_k to available documents when topN is Infinity', async () => {
+    const send = vi.fn(async () => json({ object: 'list', data: [] }));
+    const reranker = voyageReranker({ apiKey: 'k', fetch: send });
+    await reranker.rerank('q', chunks, Infinity);
+    expect(JSON.parse(String((send.mock.calls[0] as unknown as [string, RequestInit])[1].body))).toMatchObject({ top_k: 3 });
+  });
+
   it('surfaces client API errors without retrying', async () => {
     const send = vi.fn(async () => json({ detail: 'Provided model is not supported.' }, 400));
     const reranker = voyageReranker({ apiKey: 'k', fetch: send });