Constituir una SL por API — Incorporate an S.L.

The incorporation case (constitucion_sl) is a live capability: Melso’s professionals incorporate S.L.s requested from the Melso app and its MCP server today. Creating the company with its setup from your platform, partner payment, attested approval and the company.registered event are preview. See Availability.

Founders who open an account with you before their company exists can incorporate it from your product. You send what the founders decided once; Melso’s professionals do the rest and come back only when someone has to decide, provide a document or sign.

What is included

Melso reserves the name at the Registro Mercantil Central, drafts the bylaws and the deed, arranges the notary signing, registers the company with the Registro Mercantil and obtains its tax ID and census registration. Notary and Registro Mercantil fees, the name reservation and the BORME publication are included. The share capital, digital certificates and the partners’ powers of attorney are not.

Who does what

Step	Who	Through the API
Name reservation at the Registro Mercantil Central	Melso, from `names`	`case.updated`
Capital deposit certificate	The bank where the company’s account is opened	Answer an information request
Bylaws and deed draft	Melso	`case.approval_requested`
Approving the deed draft	The owner	Attest a case approval, or the approval link
Signing the deed before the notary	The partners and the administrators, in person	Never a call
Registro Mercantil, tax ID and census	Melso	`case.updated`, then `case.completed`

A company in incorporation

The S.L. is a company of its own from the moment it is asked for. Until the Registro grants its legal name and tax ID it holds only its incorporation case: no books, invoices or taxes. The request, its payment, the conversation and the deed all belong to it.

The flow

Create the company with its setup

Create a company of kind company with setup_request incorporate_sl. It has no tax ID yet, so leave fiscal_profile out. setup_details follow CaseDetailsConstitucionSL:

curl https://api.melso.ai/v1/companies \
  -H "Authorization: Bearer $MELSO_API_KEY" \
  -H "Idempotency-Key: cus_52011-setup" \
  -H "Content-Type: application/json" \
  -d '{
    "kind": "company",
    "name": "Estudio Norte",
    "external_id": "cus_52011",
    "owner": {"name": "Pablo Ruiz Ortega", "email": "pablo@example.com", "phone": "+34600000001", "locale": "es"},
    "setup_request": "incorporate_sl",
    "setup_payer": "partner",
    "setup_details": {
      "name_ending": "sl",
      "names": ["Estudio Norte", "Norte Diseño", "Estudio Norte Creativo"],
      "activity": "Diseño gráfico y comunicación visual para empresas",
      "capital": 3000,
      "share_value": 1,
      "partners": [
        {"type": "person", "name": "Pablo Ruiz Ortega", "tax_id": "12345678Z", "share_percent": 60, "administrator": true, "has_certificate": true},
        {"type": "person", "name": "Ana Vidal Soler", "tax_id": "87654321X", "share_percent": 40, "administrator": false, "has_certificate": false}
      ],
      "registered_office": {"street_type": "Calle", "street": "Alcalá", "number": "120", "postal_code": "28009", "municipality": "Madrid", "province": "Madrid"}
    },
    "services": ["invoicing", "bookkeeping", "tax_returns"]
  }'

The stakes add up to exactly 100 and capital is a multiple of share_value. The response carries setup.case_id, the constitucion_sl case. The incorporation is sold once: with setup_payer partner your partner account pays it and the case goes straight to Melso. With owner, the case carries payment_url and starts once the owner pays.

Send the capital deposit certificate

For a contribution in money, the capital is deposited in an account in the company’s name before the deed is signed, and the bank certifies it. Melso asks for the certificate with case.information_requested, as a document item. If your product holds that account, send it without waiting for the person: upload it and answer the item with its file_id.

{
  "request_id": "3d5f7a9c-1b2e-4c6d-8e0f-2a4b6c8d0e1f",
  "answers": [
    {"item_id": "capital_deposit_certificate", "file_id": "8a6c4e2f-0b1d-4f3a-9c5e-7d9f1b3a5c7e"}
  ]
}

The same request may ask for the partners’ identity documents.

Get the deed draft approved

When the bylaws and the deed are drafted you receive case.approval_requested. Get the case: the drafts are in documents and the version is current_version. The owner approves that exact version, either:

in your product: show the drafts, collect the approval with your own authentication, and attest it:

{
  "expected_version": 2,
  "approver": {"name": "Pablo Ruiz Ortega", "email": "pablo@example.com", "role": "owner", "tax_id": "12345678Z", "external_user_id": "usr_3k9d2"},
  "evidence": {
    "approved_at": "2026-11-20T16:03:12Z",
    "ip_address": "203.0.113.24",
    "user_agent": "Mozilla/5.0",
    "authentication": "strong_customer_authentication",
    "shown_document_sha256": "4b1e7c9a2d5f8e0b3c6a9d2f5e8b1c4a7d0e3f6b9c2a5d8e1f4b7c0a3d6e9f2b"
  }
}

or on Melso’s page: send the owner the approval link.

The case moves to filing and you receive case.approved. To change the draft instead, add a message with changes true; Melso sends a new version and the earlier one can no longer be approved.

Sign before the notary

Melso arranges the notary and writes through the case with the date. The partners and the administrators sign the deed in person. That signature is theirs: no API call stands in for it.

Hear that the company exists

Melso registers the deed with the Registro Mercantil, obtains the tax ID and registers the company in the census. The case reaches done with the external_reference of the registration, and its documents hold the registered deed. You receive case.completed, then company.registered: the company has the legal name and tax ID the Registro granted.The company then becomes an ordinary empresa once Melso’s team turns its tax service on: mandates, invoicing setup and its tax calendar. If an administrator must register as an autónomo, open an alta for them as in Register an autónomo.

Events, in order

Event	Meaning for your product
`case.updated`	The case moved: name reserved, notary arranged, deed registered.
`case.information_requested`	A document or an answer is needed: the capital certificate, an identity document.
`case.approval_requested`	A deed draft to approve, at `current_version`.
`case.approved`	The owner approved it; Melso proceeds to the notary and the registries.
`case.completed`	Incorporated. The registered deed is on the case.
`company.registered`	The company has its tax ID; show it as active.

Events can arrive out of order. Read the case again with Get a case before you act on one.

What an approval is, and is not

An attested approval is your statement that the owner approved one exact version, with the evidence. It requires a partner agreement that defines who may approve and how you authenticate them. It does not sign the deed, and a version that changed after the person saw it is refused. See Approvals and filing.

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Payroll and Social Security

Constituir una SL por API — Incorporate an S.L.

What is included

Who does what

A company in incorporation

The flow

Events, in order

What an approval is, and is not

​What is included

​Who does what

​A company in incorporation

​The flow

​Events, in order

​What an approval is, and is not

What is included

Who does what

A company in incorporation

The flow

Events, in order

What an approval is, and is not