diff --git a/__tests__/query-params.test.ts b/__tests__/query-params.test.ts
new file mode 100644
index 0000000..2346f89
--- /dev/null
+++ b/__tests__/query-params.test.ts
@@ -0,0 +1,96 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { ApiClient } from '../src/ApiClient';
+
+const createApiClient = () =>
+  new ApiClient({
+    apiKey: 'test-api-key',
+    token: 'test-token',
+    baseUrl: 'https://example.com',
+    timeout: 3000,
+  });
+
+/**
+ * Sends a request with the given query params and returns the query string
+ * that was actually put on the wire.
+ */
+const queryStringFor = async (queryParams: Record<string, any>) => {
+  const fetchSpy = vi.spyOn(globalThis, 'fetch').mockResolvedValue(
+    new Response(JSON.stringify({}), {
+      status: 200,
+      headers: { 'Content-Type': 'application/json' },
+    }),
+  );
+
+  await createApiClient().sendRequest('GET', '/test', undefined, queryParams);
+
+  const requestedUrl = fetchSpy.mock.calls[0][0] as string;
+  return new URL(requestedUrl).search.replace(/^\?/, '');
+};
+
+describe('query param serialization', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it('serializes an array of objects as JSON', async () => {
+    const search = await queryStringFor({
+      sort: [{ field: 'created_at', direction: 1 }],
+    });
+
+    expect(search).toContain(
+      `sort=${encodeURIComponent('[{"field":"created_at","direction":1}]')}`,
+    );
+  });
+
+  it('keeps the param name for Date values', async () => {
+    const search = await queryStringFor({
+      start_time: new Date('2026-08-15T10:00:00.000Z'),
+    });
+
+    expect(search).toContain(
+      `start_time=${encodeURIComponent('2026-08-15T10:00:00.000Z')}`,
+    );
+  });
+
+  it('serializes an array of strings comma-separated', async () => {
+    const search = await queryStringFor({ ids: ['first', 'second'] });
+
+    expect(search).toContain(`ids=${encodeURIComponent('first,second')}`);
+  });
+
+  it('drops empty entries from an array of strings', async () => {
+    const search = await queryStringFor({
+      ids: ['first', null, undefined, 'second'],
+    });
+
+    expect(search).toContain(`ids=${encodeURIComponent('first,second')}`);
+  });
+
+  it('drops empty entries from an array of objects', async () => {
+    const search = await queryStringFor({
+      sort: [{ field: 'created_at', direction: 1 }, null],
+    });
+
+    expect(search).toContain(
+      `sort=${encodeURIComponent('[{"field":"created_at","direction":1}]')}`,
+    );
+  });
+
+  it('serializes a nested object as JSON', async () => {
+    const search = await queryStringFor({
+      payload: { sort: [{ field: 'created_at', direction: 1 }] },
+    });
+
+    expect(search).toContain(
+      `payload=${encodeURIComponent(
+        '{"sort":[{"field":"created_at","direction":1}]}',
+      )}`,
+    );
+  });
+
+  it('omits params with no value', async () => {
+    const search = await queryStringFor({ limit: undefined, id_gt: null });
+
+    expect(search).toBe('api_key=test-api-key');
+  });
+});
diff --git a/src/ApiClient.ts b/src/ApiClient.ts
index e8359e1..8a74dcd 100644
--- a/src/ApiClient.ts
+++ b/src/ApiClient.ts
@@ -2,6 +2,7 @@ import { randomUUID } from 'crypto';
 import { ApiConfig, RequestMetadata, StreamError } from './types';
 import { APIError } from './gen/models';
 import { getRateLimitFromResponseHeader } from './utils/rate-limit';
+import { isScalar, stringifyArrayParam } from './utils/query-params';
 
 export class ApiClient {
   private readonly dispatcher?: RequestInit['dispatcher'];
@@ -128,20 +129,17 @@ export class ApiClient {
     const newParams = [];
     for (const k in params) {
       const param = params[k];
+      if (param === null || param === undefined) continue;
       if (Array.isArray(param)) {
-        newParams.push(`${k}=${encodeURIComponent(param.join(','))}`);
+        newParams.push(
+          `${k}=${encodeURIComponent(stringifyArrayParam(param))}`,
+        );
       } else if (param instanceof Date) {
-        newParams.push(param.toISOString());
+        newParams.push(`${k}=${encodeURIComponent(param.toISOString())}`);
       } else if (typeof param === 'object') {
         newParams.push(`${k}=${encodeURIComponent(JSON.stringify(param))}`);
-      } else {
-        if (
-          typeof param === 'string' ||
-          typeof param === 'number' ||
-          typeof param === 'boolean'
-        ) {
-          newParams.push(`${k}=${encodeURIComponent(param)}`);
-        }
+      } else if (isScalar(param)) {
+        newParams.push(`${k}=${encodeURIComponent(param)}`);
       }
     }
 
diff --git a/src/utils/query-params.ts b/src/utils/query-params.ts
new file mode 100644
index 0000000..0f8cd71
--- /dev/null
+++ b/src/utils/query-params.ts
@@ -0,0 +1,21 @@
+export const isScalar = (value: any) => {
+  const type = typeof value;
+  return type === 'string' || type === 'number' || type === 'boolean';
+};
+
+/**
+ * Scalar arrays go on the wire comma-separated (`ids=a,b`), anything else has
+ * to be JSON or it stringifies to `[object Object]`. `null` and `undefined`
+ * entries are dropped so that a single empty value can't flip the format.
+ */
+export const stringifyArrayParam = (param: any[]) => {
+  const entries = [];
+  let allScalar = true;
+  for (const entry of param) {
+    if (entry == null) continue;
+    if (!isScalar(entry)) allScalar = false;
+    entries.push(entry);
+  }
+
+  return allScalar ? entries.join(',') : JSON.stringify(entries);
+};