diff --git a/src/merkle/merkleClient.ts b/src/merkle/merkleClient.ts
index 057045b..f4f8012 100644
--- a/src/merkle/merkleClient.ts
+++ b/src/merkle/merkleClient.ts
@@ -7,11 +7,17 @@ import { joinUrl } from '../utils/url';
 const DEFAULT_MERKLE_REQUEST_TIMEOUT_MS = 15_000;
 
 /**
- * Build query string with repeated keys (cid=1&cid=2...).
+ * Build query string in rails-style bracket notation:
+ *   cid[0]=1&cid[1]=2&cid[2]=3
+ *
+ * The freezer service uses `serde_qs` on the server side, which by
+ * default expects this format, NOT the rack-style repeated bare keys
+ * (cid=1&cid=2&cid=3). Repeated bare keys produce
+ * "Multiple values for one key: cid" 400 errors.
  */
-const withRepeatedQuery = (url: string, key: string, values: Array<string | number>) => {
+const withIndexedQuery = (url: string, key: string, values: Array<string | number>) => {
   const search = new URLSearchParams();
-  for (const v of values) search.append(key, String(v));
+  values.forEach((v, i) => search.append(`${key}[${i}]`, String(v)));
   const qs = search.toString();
   return qs ? `${url}?${qs}` : url;
 };
@@ -116,7 +122,7 @@ export class MerkleClient {
     if (!Array.isArray(cids) || cids.length === 0) {
       throw new SdkError('MERKLE', 'Merkle proof requires at least one cid', { cids });
     }
-    const url = withRepeatedQuery(joinUrl(this.baseUrl, '/api/v1/merkle'), 'cid', cids);
+    const url = withIndexedQuery(joinUrl(this.baseUrl, '/api/v1/merkle'), 'cid', cids);
     this.debugEmit?.({ type: 'debug', payload: { scope: 'http:merkle', message: 'request', detail: { method: 'GET', url } } });
     let response: Response;
     const requestTimeoutMs =
diff --git a/tests/merkleClient.test.ts b/tests/merkleClient.test.ts
index 2a0f2c2..5fd3358 100644
--- a/tests/merkleClient.test.ts
+++ b/tests/merkleClient.test.ts
@@ -74,4 +74,35 @@ describe('MerkleClient.getProofByCids', () => {
       message: 'Merkle proof request failed',
     });
   });
+
+  // Regression: server uses serde_qs which expects `cid[N]=...`,
+  // not repeated bare `cid=...&cid=...`. The latter produces a 400
+  // "Multiple values for one key: cid". Verify the URL we send.
+  it('builds query in rails-style bracket notation, not repeated bare keys', async () => {
+    let capturedUrl = '';
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(async (url: string) => {
+        capturedUrl = url;
+        return new Response(
+          JSON.stringify({
+            proof: [
+              { path: [], leaf_index: '0' },
+              { path: [], leaf_index: '1' },
+              { path: [], leaf_index: '2' },
+            ],
+            merkle_root: '0x1',
+            latest_cid: 0,
+          }),
+          { status: 200, headers: { 'content-type': 'application/json' } },
+        );
+      }),
+    );
+    const client = new MerkleClient('https://merkle.example');
+    await client.getProofByCids([6067, 5055, 7792]);
+    expect(capturedUrl).toContain('cid%5B0%5D=6067');
+    expect(capturedUrl).toContain('cid%5B1%5D=5055');
+    expect(capturedUrl).toContain('cid%5B2%5D=7792');
+    expect(capturedUrl).not.toMatch(/[?&]cid=\d+&cid=\d+/);
+  });
 });
diff --git a/tests/merkleEngine.test.ts b/tests/merkleEngine.test.ts
index a50b763..1438bc6 100644
--- a/tests/merkleEngine.test.ts
+++ b/tests/merkleEngine.test.ts
@@ -32,7 +32,7 @@ describe('MerkleEngine', () => {
     expect(engine.currentMerkleRootIndex(33)).toBe(1);
   });
 
-  it('fetches remote proof with repeated cid query', async () => {
+  it('fetches remote proof using rails-style cid[N] query (server uses serde_qs)', async () => {
     const engine = new MerkleEngine(() => ({ merkleProofUrl: 'https://merkle.invalid' }), bridge);
     const fetchMock = vi.fn().mockResolvedValue({
       ok: true,
@@ -46,7 +46,7 @@ describe('MerkleEngine', () => {
 
     const res = await engine.getProofByCids({ chainId: 1, cids: [7], totalElements: 33n });
     expect(res.latest_cid).toBe(0);
-    expect(fetchMock).toHaveBeenCalledWith('https://merkle.invalid/api/v1/merkle?cid=7', expect.objectContaining({ signal: expect.anything() }));
+    expect(fetchMock).toHaveBeenCalledWith('https://merkle.invalid/api/v1/merkle?cid%5B0%5D=7', expect.objectContaining({ signal: expect.anything() }));
   });
 
   it('throws SdkError(MERKLE) when all utxos lack memos', async () => {