parameter_missing 🔌 API

API Error: Missing required parameter (parameter_missing / "x" is required)

The API rejected the request because a field it requires — in the query string, path, headers or JSON body — wasn’t sent or was empty.

Seen on: REST API

Meaning

Every endpoint declares which inputs are mandatory. If one is absent, null or an empty string, the server stops before doing any work and returns a 400 (sometimes 422) naming the missing field.

The field is often there in your code but never reaches the wire: it was undefined and got dropped during JSON serialization, it was sent as a query parameter when the API expects it in the body, or a form encoding/Content-Type mismatch meant the server couldn’t read the body at all.

Common causes

  • The field was left out of the request
  • The value was undefined/null and silently dropped by JSON.stringify
  • Sent in the wrong place (query string instead of body, or body instead of header)
  • Body not parsed because Content-Type is missing or wrong
  • Field name misspelled or wrong case (customerId vs customer_id)

⚡ Quick fix

  1. Read the error body — it usually names the missing parameter
  2. Log the exact request the client sends and compare it with the API docs
  3. Add client-side validation so required fields can’t be empty

Detailed fix by platform

Node.js

  1. `// undefined values vanish from JSON — check before sending const body = { email, plan }; for (const [k, v] of Object.entries(body)) if (v == null
  2. v === '') throw new Error(missing ${k}); await fetch(url, { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(body) });`

Python

  1. python
    r = requests.post(url, json={"email": email, "plan": plan})  # json= sets Content-Type for you
print(r.status_code, r.json())

curl

  1. curl -X POST https://api.example.com/v1/users -H 'Content-Type: application/json' -d '{"email":"a@b.com","plan":"pro"}'

How to diagnose

  1. Error body — Which parameter does it name?
  2. Raw request — Is the field present in the bytes actually sent (proxy/devtools log)?
  3. Location — Does the docs say query, path, header or body?
  4. Spelling — Exact name and case match the docs?

🧠 Still stuck? Analyze your error

Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.