correctly send multiple expands as a comma seperated array

.fernignore

@@ -17,4 +17,5 @@ tests/integration/backward-compatibility.integration.test.ts
 src/index.ts
 tests/unit/schemas/enum/enum.test.ts
 tests/unit/schemas/union/union.test.ts
-src/core/schemas/builders/enum/enum.ts
+src/core/schemas/builders/enum/enum.ts
+src/core/fetcher/createRequestUrl.ts

src/core/fetcher/createRequestUrl.ts

@@ -1,6 +1,26 @@
 import { toQueryString } from "../url/qs";
 
 export function createRequestUrl(baseUrl: string, queryParameters?: Record<string, unknown>): string {
-    const queryString = toQueryString(queryParameters, { arrayFormat: "repeat" });
+    // Flatten any array values to comma-separated strings.
+    // Merge's REST API expects multi-value params (e.g. expand) as CSV,
+    // not as repeated query params.
+    const flattenedParams = queryParameters != null ? flattenArrayQueryParams(queryParameters) : undefined;
+    const queryString = toQueryString(flattenedParams, { arrayFormat: "repeat" });
     return queryString ? `${baseUrl}?${queryString}` : baseUrl;
 }
+
+function flattenArrayQueryParams(params: Record<string, unknown>): Record<string, unknown> {
+    const result: Record<string, unknown> = {};
+    for (const [key, value] of Object.entries(params)) {
+        if (
+            Array.isArray(value) &&
+            value.length > 0 &&
+            value.every((item) => typeof item !== "object" || item === null)
+        ) {
+            result[key] = value.join(",");
+        } else {
+            result[key] = value;
+        }
+    }
+    return result;
+}

tests/unit/fetcher/createRequestUrl.test.ts

@@ -23,10 +23,10 @@ describe("Test createRequestUrl", () => {
             expected: "https://api.example.com?key=value&another=param",
         },
         {
-            description: "should handle array query parameters",
+            description: "should handle array query parameters as comma-separated values",
             baseUrl: BASE_URL,
             queryParams: { items: ["a", "b", "c"] },
-            expected: "https://api.example.com?items=a&items=b&items=c",
+            expected: "https://api.example.com?items=a%2Cb%2Cc",
         },
         {
             description: "should handle object query parameters",
@@ -42,7 +42,7 @@ describe("Test createRequestUrl", () => {
                 array: ["x", "y"],
                 object: { key: "value" },
             },
-            expected: "https://api.example.com?simple=value&array=x&array=y&object%5Bkey%5D=value",
+            expected: "https://api.example.com?simple=value&array=x%2Cy&object%5Bkey%5D=value",
         },
         {
             description: "should handle empty query parameters object",

tests/unit/multiple-expand-handling.test.ts

@@ -0,0 +1,91 @@
+import { createRequestUrl } from "../../src/core/fetcher/createRequestUrl";
+
+describe("Multiple Expand Parameter Handling", () => {
+    describe("createRequestUrl flattens arrays to CSV", () => {
+        it("should join array values with commas instead of repeating params", () => {
+            const url = createRequestUrl("https://api.merge.dev/hris/v1/employees", {
+                expand: ["groups", "work_location"],
+                include_remote_data: true,
+            });
+            expect(url).toBe(
+                "https://api.merge.dev/hris/v1/employees?expand=groups%2Cwork_location&include_remote_data=true"
+            );
+        });
+
+        it("should handle single expand value (not array)", () => {
+            const url = createRequestUrl("https://api.merge.dev/hris/v1/employees", {
+                expand: "groups",
+            });
+            expect(url).toBe("https://api.merge.dev/hris/v1/employees?expand=groups");
+        });
+
+        it("should handle three or more expand values", () => {
+            const url = createRequestUrl("https://api.merge.dev/accounting/v1/contacts", {
+                expand: ["addresses", "phone_numbers", "company"],
+            });
+            expect(url).toBe(
+                "https://api.merge.dev/accounting/v1/contacts?expand=addresses%2Cphone_numbers%2Ccompany"
+            );
+        });
+
+        it("should handle undefined expand", () => {
+            const url = createRequestUrl("https://api.merge.dev/hris/v1/employees", {
+                expand: undefined,
+                cursor: "abc123",
+            });
+            expect(url).toBe("https://api.merge.dev/hris/v1/employees?cursor=abc123");
+        });
+    });
+
+    describe("SDK client serializes expand correctly", () => {
+        const mockOptions = {
+            apiKey: "test-api-key",
+            environment: "https://api.merge.dev",
+        };
+
+        it("should produce correct URL with array expand via fetcher", async () => {
+            const { Accounting } = require("../../src");
+
+            let capturedUrl: string | undefined;
+            const mockFetcher = jest.fn().mockImplementation((args: any) => {
+                // Reconstruct the URL as fetcherImpl would
+                capturedUrl = createRequestUrl(args.url, args.queryParameters);
+                return { ok: true, body: { results: [] } };
+            });
+
+            const accounting = new Accounting({
+                ...mockOptions,
+                fetcher: mockFetcher,
+            });
+
+            await accounting.contacts.list({
+                expand: ["addresses", "phone_numbers", "company"],
+            });
+
+            expect(capturedUrl).toContain("expand=addresses%2Cphone_numbers%2Ccompany");
+        });
+
+        it("should produce correct URL with single expand via fetcher", async () => {
+            const { Accounting } = require("../../src");
+
+            let capturedUrl: string | undefined;
+            const mockFetcher = jest.fn().mockImplementation((args: any) => {
+                capturedUrl = createRequestUrl(args.url, args.queryParameters);
+                return { ok: true, body: { results: [] } };
+            });
+
+            const accounting = new Accounting({
+                ...mockOptions,
+                fetcher: mockFetcher,
+            });
+
+            await accounting.contacts.list({
+                expand: "addresses",
+            });
+
+            expect(capturedUrl).toContain("expand=addresses");
+            // Should NOT contain a comma since it's a single value
+            expect(capturedUrl).not.toContain("%2C");
+        });
+    });
+});