debugging 'invalid_request' on TOKEN endpoint

Question

I have set up a new User Pool with an App Client:  
- no App client secret  
- Auth Flows Configuration ALLOW_USER_PASSWORD_AUTH and ALLOW_REFRESH_TOKEN_AUTH  
  
Under App Integration I have:  
- enabled Cognito User Pool  
- provided Callback URL(s)  
- enabled Authorization code grant  
- Allowed OAuth Scopes: email, opened   
  
In my app I call the LOGIN endpoint:  
`https://myapp.auth.ap-southeast-2.amazoncognito.com/login?client_id=${AUTH_CLIENT_ID}&state=${state}&response_type=code&redirect_uri=${AUTH_CALLBACK_URI}`  
  
When I get back the code from the redirect I call the TOKEN endpoint:  
  
    const config = {  
        headers: {  
            'Content-Type': 'application/x-www-form-urlencoded'  
        }  
    }  
  
    const data = {  
        "grant_type": "authorization_code",  
        "client_id": AUTH_CLIENT_ID,  
        "code": code,  
        "redirect_uri": AUTH_CALLBACK_URI  
    }  
       
    const response = await axios.post('https://hilltop-dev.auth.ap-southeast-2.amazoncognito.com/oauth2/token', data, config)  
  
From the above request, I get a 400 invalid_request response with no details.  
  
There are no logs I can find for Cognito with any more details.  
There are no CloudTrail events with any more details  
  
As far as I can tell after checking several times the request is valid.  
  
I've tried setting the same app but with a client_secret and Authorization basic base64 header, but get the same invalid_request response.  
  
What's wrong with this request? How do I debug this request without any info??

Answer

Figured out the POST body isn't JSON but a query string format:  
  
	// redirect the user to this URI  
	function getAuthUri() {  
		const state = crypto.randomBytes(20).toString('hex')  
		const uri = `${AUTH_DOMAIN}/login?client_id=${AUTH_CLIENT_ID}&state=${state}&response_type=code&redirect_uri=${AUTH_CALLBACK_URI}`  
		return uri  
	}  
  
	// after extracting the code from the authentication redirect (to AUTH_CALLBACK_URI), use this to get the access token  
	async function getAccessToken(code) {  
		const config = {  
			headers: {  
				'Authorization': 'Basic ' _ Buffer.from(AUTH_CLIENT_ID _ ':' + AUTH_CLIENT_SECRET).toString('base64'),  
				'Content-Type': 'application/x-www-form-urlencoded'  
			}  
		}  
  
		const body = `grant_type=authorization_code&client_id=${AUTH_CLIENT_ID}&code=${code}&redirect_uri=${AUTH_CALLBACK_URI}`  
  
		try {  
			const response = await axios.post(`${AUTH_DOMAIN}/oauth2/token`, body, config)  
			return response.data.access_token  
		}  
		catch (err) {  
			console.log(err.response.data.error)  
			throw err  
		}  
	}  
  
	// use the access token  
	async function getUser(accessToken) {  
		const config = {  
			headers: {  
				Authorization: `Bearer ${accessToken}`  
			}  
		}  
  
		const response = await axios.get(`${AUTH_DOMAIN}/oauth2/userInfo`, config)  
		const user = response.data  
		return user  
	}

debugging 'invalid_request' on TOKEN endpoint

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