diff --git a/packages/playwright-core/src/client/fetch.ts b/packages/playwright-core/src/client/fetch.ts
index 3bfeead355..307b92f3c3 100644
--- a/packages/playwright-core/src/client/fetch.ts
+++ b/packages/playwright-core/src/client/fetch.ts
@@ -168,13 +168,13 @@ export class APIRequestContext extends ChannelOwner<channels.APIRequestContextCh
       if (options.data !== undefined) {
         if (isString(options.data)) {
           if (isJsonContentType(headers))
-            jsonData = options.data;
+            jsonData = isJsonParsable(options.data) ? options.data : JSON.stringify(options.data);
           else
             postDataBuffer = Buffer.from(options.data, 'utf8');
         } else if (Buffer.isBuffer(options.data)) {
           postDataBuffer = options.data;
         } else if (typeof options.data === 'object' || typeof options.data === 'number' || typeof options.data === 'boolean') {
-          jsonData = options.data;
+          jsonData = JSON.stringify(options.data);
         } else {
           throw new Error(`Unexpected 'data' type`);
         }
@@ -230,6 +230,20 @@ export class APIRequestContext extends ChannelOwner<channels.APIRequestContextCh
   }
 }
 
+function isJsonParsable(value: any) {
+  if (typeof value !== 'string')
+    return false;
+  try {
+    JSON.parse(value);
+    return true;
+  } catch (e) {
+    if (e instanceof SyntaxError)
+      return false;
+    else
+      throw e;
+  }
+}
+
 export class APIResponse implements api.APIResponse {
   private readonly _initializer: channels.APIResponse;
   private readonly _headers: RawHeaders;
diff --git a/packages/playwright-core/src/protocol/validator.ts b/packages/playwright-core/src/protocol/validator.ts
index 4413dd6c30..f3e26cbba2 100644
--- a/packages/playwright-core/src/protocol/validator.ts
+++ b/packages/playwright-core/src/protocol/validator.ts
@@ -175,7 +175,7 @@ scheme.APIRequestContextFetchParams = tObject({
   method: tOptional(tString),
   headers: tOptional(tArray(tType('NameValue'))),
   postData: tOptional(tBinary),
-  jsonData: tOptional(tAny),
+  jsonData: tOptional(tString),
   formData: tOptional(tArray(tType('NameValue'))),
   multipartData: tOptional(tArray(tType('FormField'))),
   timeout: tOptional(tNumber),
diff --git a/packages/playwright-core/src/server/fetch.ts b/packages/playwright-core/src/server/fetch.ts
index f198984101..3a96a00aa0 100644
--- a/packages/playwright-core/src/server/fetch.ts
+++ b/packages/playwright-core/src/server/fetch.ts
@@ -671,26 +671,11 @@ function parseCookie(header: string): channels.NetworkCookie | null {
   return cookie;
 }
 
-function isJsonParsable(value: any) {
-  if (typeof value !== 'string')
-    return false;
-  try {
-    JSON.parse(value);
-    return true;
-  } catch (e) {
-    if (e instanceof SyntaxError)
-      return false;
-    else
-      throw e;
-  }
-}
-
 function serializePostData(params: channels.APIRequestContextFetchParams, headers: HeadersObject): Buffer | undefined {
   assert((params.postData ? 1 : 0) + (params.jsonData ? 1 : 0) + (params.formData ? 1 : 0) + (params.multipartData ? 1 : 0) <= 1, `Only one of 'data', 'form' or 'multipart' can be specified`);
   if (params.jsonData !== undefined) {
-    const json = isJsonParsable(params.jsonData) ? params.jsonData : JSON.stringify(params.jsonData);
     setHeader(headers, 'content-type', 'application/json', true);
-    return Buffer.from(json, 'utf8');
+    return Buffer.from(params.jsonData, 'utf8');
   } else if (params.formData) {
     const searchParams = new URLSearchParams();
     for (const { name, value } of params.formData)
diff --git a/packages/protocol/src/channels.ts b/packages/protocol/src/channels.ts
index 92fa7a4d9e..d544e2a7e1 100644
--- a/packages/protocol/src/channels.ts
+++ b/packages/protocol/src/channels.ts
@@ -317,7 +317,7 @@ export type APIRequestContextFetchParams = {
   method?: string,
   headers?: NameValue[],
   postData?: Binary,
-  jsonData?: any,
+  jsonData?: string,
   formData?: NameValue[],
   multipartData?: FormField[],
   timeout?: number,
@@ -330,7 +330,7 @@ export type APIRequestContextFetchOptions = {
   method?: string,
   headers?: NameValue[],
   postData?: Binary,
-  jsonData?: any,
+  jsonData?: string,
   formData?: NameValue[],
   multipartData?: FormField[],
   timeout?: number,
diff --git a/packages/protocol/src/protocol.yml b/packages/protocol/src/protocol.yml
index a58144939e..615ccd6d4d 100644
--- a/packages/protocol/src/protocol.yml
+++ b/packages/protocol/src/protocol.yml
@@ -288,7 +288,7 @@ APIRequestContext:
           type: array?
           items: NameValue
         postData: binary?
-        jsonData: json?
+        jsonData: string?
         formData:
           type: array?
           items: NameValue
diff --git a/tests/library/global-fetch.spec.ts b/tests/library/global-fetch.spec.ts
index 5f0e992a4a..af0d7f770d 100644
--- a/tests/library/global-fetch.spec.ts
+++ b/tests/library/global-fetch.spec.ts
@@ -430,3 +430,24 @@ it('should keep headers capitalization', async ({ playwright, server }) => {
   expect(serverRequest.rawHeaders).toContain('vaLUE');
   await request.dispose();
 });
+
+it('should serialize post data on the client', async ({ playwright, server }) => {
+  const request = await playwright.request.newContext();
+  const serverReq = server.waitForRequest('/empty.html');
+  let onStack: boolean = true;
+  const postReq = request.post(server.EMPTY_PAGE, {
+    data: {
+      toJSON() {
+        if (!onStack)
+          throw new Error('Should not be called on the server');
+        return { 'foo': 'bar' };
+      }
+    }
+  });
+  onStack = false;
+  await postReq;
+  const body = await (await serverReq).postBody;
+  expect(body.toString()).toBe('{"foo":"bar"}');
+  // expect(serverRequest.rawHeaders).toContain('vaLUE');
+  await request.dispose();
+});