DeepSeek API Compatibility Issue: JSON Format Requirement and Schema Mismatch

[MingGuangShao]

Problem Description

When using DeepSeek API (deepseek/deepseek-chat) with Stagehand, there are two related issues:

1. Missing "json" keyword error: When using DeepSeek API, if the prompt doesn't explicitly contain the word "json", the API returns an error: Prompt must contain the word 'json' in some form to use 'response_format' of type 'json_object'

2. Schema mismatch error: When adding "json" to the prompt to satisfy DeepSeek's requirement, the model returns a JSON structure that doesn't match Stagehand's expected schema. DeepSeek returns a format like:

   {
     "action": "fill",
     "element": {"id": "...", "role": "...", "name": "..."},
     "argument": "Cota",
     "twoStep": false
   }

   But Stagehand expects:

   {
     "elementId": "string (required)",
     "description": "string (required)",
     "method": "string (required)",
     "arguments": ["array (required)"]
   }

This creates a catch-22 situation: without "json" in the prompt, DeepSeek API rejects the request; with "json" in the prompt, the response format doesn't match Stagehand's schema.

Environment

• Stagehand version: @browserbasehq/stagehand@^3.0.0
• Node.js version: v22.2.0
• DeepSeek API endpoint: https://api.deepseek.com/v1
• Model: deepseek/deepseek-chat

Code to Reproduce

import "dotenv/config";
import { Stagehand } from "@browserbasehq/stagehand";
import { z } from "zod/v3";

async function main() {
  const stagehand = new Stagehand({
    env: "LOCAL",
    model: "deepseek/deepseek-chat"
  });

  await stagehand.init();
  const page = stagehand.context.pages()[0];

  await page.goto("https://www.baidu.com");

  await stagehand.act("fill the search input with the text 'hello'");
  await stagehand.act("click the search button");

  // Extract structured data
  const title = await stagehand.extract("extract the page title", z.string());

  console.log(title);
  await stagehand.close();
}

main().catch((err) => {
  console.error(err);
  process.exit(1);
});

Error Messages

Error 1: Missing "json" keyword

APICallError [AI_APICallError]: Prompt must contain the word 'json' in some form to use 'response_format' of type 'json_object'.
    at ...
  url: 'https://api.deepseek.com/v1/chat/completions',
  requestBodyValues: {
    model: 'deepseek-chat',
    response_format: { type: 'json_object' },
    messages: [ [Object], [Object] ],
    ...
  },
  statusCode: 400,
  responseBody: `{"error":{"message":"Prompt must contain the word 'json' in some form to use 'response_format' of type 'json_object'.","type":"invalid_request_error","param":null,"code":"invalid_request_error"}}`

Error 2: Schema validation failure (when "json" is added to prompt)

NoObjectGeneratedError [AI_NoObjectGeneratedError]: No object generated: response did not match schema.
  cause: _TypeValidationError [AI_TypeValidationError]: Type validation failed
  Error message: [
    {
      "code": "invalid_type",
      "expected": "string",
      "received": "undefined",
      "path": ["elementId"],
      "message": "Required"
    },
    {
      "code": "invalid_type",
      "expected": "string",
      "received": "undefined",
      "path": ["description"],
      "message": "Required"
    },
    {
      "code": "invalid_type",
      "expected": "string",
      "received": "undefined",
      "path": ["method"],
      "message": "Required"
    },
    {
      "code": "invalid_type",
      "expected": "array",
      "received": "undefined",
      "path": ["arguments"],
      "message": "Required"
    }
  ]
  value: {
    action: 'fill',
    element: { id: '...', role: 'textbox', name: '...' },
    argument: 'Cota',
    twoStep: false
  }

Questions

1. Is DeepSeek officially supported by Stagehand? If not, what would be needed to add support?

[JerryWu1234]

let me try this