Alta de autónomo por API — Register an autónomo

The alta case is a live capability: Melso’s professionals register autónomos from the Melso app today. Creating the company with its setup, the extra details fields, partner payment and the webhooks are preview. See Availability.

Someone opens an account with you before they have registered as an autónomo. With one call you hand Melso the registration; Melso’s professionals file it and your product hears when the person can start invoicing.

What Melso does

Administration	Filing	Result
AEAT	Census declaration, modelo 036: the activity and its IAE heading, the VAT regime and the income tax method	The autónomo appears in the census with their DNI or NIE as tax ID
Social Security (TGSS)	Alta in the RETA, with the reduced flat-rate contribution (`tarifa_plana`) when the person asks and qualifies	The resolution of the alta and the monthly contribution charged to `iban`

The alta has no draft to approve: asking for it is the person’s decision. Melso’s professionals ask through the case for anything they need, and close it with the administrations’ receipts.

Before you start

Plan. The company’s plan must include the alta, or the case is sold as a one-off service. List case kinds says which, under payment.
Mandates. Melso acts before the AEAT and the TGSS under the person’s representation: AEAT census and TGSS affiliation. The person signs them on a hosted page; see Mandates. Signing is never a partner call.
Start date. Send the case before start_date. The registration must be in place before the activity starts.

The flow

Create the company with its setup

Create a company of kind freelancer with setup_request register_autonomo and what you already know in setup_details:

curl https://api.melso.ai/v1/companies \
  -H "Authorization: Bearer $MELSO_API_KEY" \
  -H "Idempotency-Key: cus_51907-setup" \
  -H "Content-Type: application/json" \
  -d '{
    "kind": "freelancer",
    "name": "Marta Gil Romero",
    "external_id": "cus_51907",
    "owner": {"name": "Marta Gil Romero", "email": "marta@example.com", "phone": "+34600000000", "locale": "es"},
    "fiscal_profile": {
      "tax_id": "12345678Z",
      "legal_name": "Marta Gil Romero",
      "address": "Calle de la Paz 4, 2.º B",
      "postal_code": "46003",
      "city": "València",
      "activity_type": "profesional",
      "activity_description": "Diseño gráfico para empresas"
    },
    "setup_request": "register_autonomo",
    "setup_details": {
      "start_date": "2026-12-01",
      "activity": "Diseño gráfico para empresas",
      "birth_date": "1994-05-17",
      "nuss": "281234567840",
      "clients": "Empresas en España y algún cliente de la UE",
      "tarifa_plana": "si",
      "iban": "ES9121000418450200051332"
    },
    "services": ["invoicing", "tax_returns"]
  }'

The response carries setup.case_id, the alta case, and setup.missing, what it still needs. When nothing is missing and the plan covers it, the case is already with Melso’s professionals.

Collect what is missing and the mandates

Send the person to Melso’s hosted onboarding with Create an onboarding link and steps ["setup", "mandates"]. They complete the missing details, sign the AEAT and TGSS mandates, and the case is sent. You receive mandate.signed, then mandate.activated per administration.If you collect every detail yourself, request the mandates with Request a mandate and send the person to its signing_url.

Answer Melso's questions

case.information_requested arrives when a professional needs something, for example a copy of the DNI or which IAE heading fits best. Read the open information_request with Get a case, upload any file with Upload a document, and answer every item with Answer an information request:

{
  "request_id": "0b6f3a2e-9c1d-4e8f-a7b5-2d4c6e8f0a1b",
  "answers": [
    {"item_id": "dni", "file_id": "7e1d2c3b-4a5f-4e6d-8c7b-9a0f1e2d3c4b"},
    {"item_id": "iae", "answer": "Diseño gráfico, sin local"}
  ]
}

The case goes back to review.

Hear that the autónomo is registered

The case reaches done with the administration’s external_reference, and its documents hold the census receipt and the RETA resolution. You receive case.completed, then company.registered: the fiscal profile’s registration_status is alta and registered_on is set. From here the company is an ordinary autónomo: invoices, expenses and quarterly returns.

A company you already have

For an autónomo who is already on your partner account, open the case directly with Open a case:

curl https://api.melso.ai/v1/companies/$COMPANY_ID/cases \
  -H "Authorization: Bearer $MELSO_API_KEY" \
  -H "Idempotency-Key: alta-cus_51907" \
  -H "Content-Type: application/json" \
  -d '{
    "kind": "alta",
    "details": {"start_date": "2026-12-01", "activity": "Diseño gráfico para empresas"},
    "note": "Quiere tarifa plana. Clientes: empresas en España.",
    "submit": true
  }'

start_date (required) and activity are the live contract today; send the other details in note until the full details ship.

Events, in order

Event	Meaning for your product
`mandate.signed`, `mandate.activated`	Melso can act before the AEAT and the TGSS.
`case.updated`	The case moved, for example from `draft` to `review`.
`case.information_requested`	Ask the person, then answer the request.
`case.completed`	Filed. The receipts are on the case.
`company.registered`	The autónomo is registered; show them as active.

company.registered carries the company:

{
  "id": "2f7a9c1e-5b3d-4e8f-a6c4-1d9b7e5f3a2c",
  "type": "company.registered",
  "created_at": "2026-11-27T11:20:05Z",
  "company_id": "9c4e2a7b-1f3d-4b6a-8e5c-0d2f4a6b8c1e",
  "data": {
    "id": "9c4e2a7b-1f3d-4b6a-8e5c-0d2f4a6b8c1e",
    "name": "Marta Gil Romero",
    "kind": "freelancer",
    "onboarded": true,
    "external_id": "cus_51907",
    "setup": {
      "request": "register_autonomo",
      "case_id": "5a1c3e7b-9d2f-4b8a-a6e4-c0f2d4b6a8e1",
      "missing": [],
      "registered_at": "2026-11-27T11:20:03Z"
    },
    "created_at": "2026-11-20T09:02:41Z"
  }
}

Who pays

When the alta is sold as a one-off service, the case carries payment_url and waits for the owner. Under a partner agreement you can pay it for them: setup_payer partner on Create a company, or payer partner on Open a case (preview).

Get started

Solutions

Guides

Companies and access

Invoicing

Bookkeeping

Taxes

Payroll and Social Security

Alta de autónomo por API — Register an autónomo

What Melso does

Before you start

The flow

A company you already have

Events, in order

Who pays

​What Melso does

​Before you start

​The flow

​A company you already have

​Events, in order

​Who pays

What Melso does

Before you start

The flow

A company you already have

Events, in order

Who pays