Legal documents with certified electronic signature for Mexico
Your customers sign contracts and promissory notes from their phone, with a biometric handwritten signature or with their SAT e.firma. The document is sealed under NOM-151 by an authorized PSC.
Signatures recorded on a single version of the document.
Singula records who signed, with which signature type and at what moment, on the same version of the document. At close, an authorized PSC seals the document and issues the NOM-151 certificate.
Signature types
Each signer can use a different signature type in the same document.
Biometric handwritten signature
The signature is drawn on the phone screen, with a selfie and a liveness check. The evidence file records the stroke, the face and the moment of each signature.
SAT e.firma (FIEL)
The digital certificate the SAT issues to individuals and companies. It is used from the same signing link.
How it works
From a PDF to a sealed document, in four steps.
Upload the document
Upload the PDF or start from a saved template. You can also generate it from your own system.
Define signers
One or several, in sequential order when you need it. You assign each signer their signature type.
Collect signatures
Each signer gets their own link by email or WhatsApp and signs from their phone.
NOM-151 seal
With the last signature, an authorized PSC seals the document and issues the NOM-151 certificate.
Your document, sealed by an authorized PSC.
NOM-151 is the technical method recognized in Mexico to attest that a document existed exactly as is on a given date.
Hash of the signed PDF, tied to the document's exact content.
Certified timestamp (RFC 3161) issued by a recognized authority.
Certificate issued by a PSC authorized by the Ministry of Economy.
Certificate downloadable as a PDF, attached to the signed document.
Hash the signed PDF
We compute the SHA-256 hash of the document with all signatures embedded.
Request to PSC
We send the hash to an authorized PSC, which applies an RFC 3161 timestamp.
Certified record
You receive a PDF with the NOM-151 seal and its preservation certificate.
Sign with the SAT e.firma
The e.firma, formerly FIEL, is the certificate the SAT hands to every taxpayer. Singula accepts it as a signature type on any document.
Equivalent to a handwritten signature under the Código de Comercio.
Singula confirms with the SAT that the certificate is still valid before accepting the signature.
The validity of an RFC’s e.firma can be checked before sending the document, with the days it has left.
The signer uploads their .cer and .key for the moment of signing only.
Applies to B2B contracts, mandates and digital powers of attorney.
What you get at the end
Once the document is completed, the file is available to download and is sent to your system.
The PDF with every signature embedded.
API field: signed_pdf_url · stringThe proof issued by the PSC, downloadable as a separate PDF from the contract.
API field: constancia_nom151_url · stringThe technical and biometric evidence captured from each signer.
API field: audit_trail · objectIndicates whether the document is pending signatures or completed.
Example completed
API field: status · stringWho signed, and which signature type they used.
API field: signers · arrayThe identifier you use to look up the document.
Example doc_lzk9m4ab3f
API field: document_id · stringThe field-by-field detail lives in the documentation.
Where this lands in your operation
Six documents Mexican companies send out for signature most often.
Promissory notes and loans
Biometric signature and NOM-151 sealing, with the evidence file of each signer.
Lease agreements
Landlord and tenant sign the same document from their phones, each with their own link.
Service contracts
Several signers in sequential order, with a signature type per signer.
Agreements and minutes
High volume of internal signatures from saved templates.
Academic agreements
Enrollments and collaboration agreements, signed from the student's phone.
Informed consent
Biometric signature with a selfie and liveness check of the patient.
The evidence of every signature, in a downloadable file.
Each signature generates a package of technical and biometric evidence, downloadable as an annex to the document.
That evidence is admissible under article 90-bis of the Código de Comercio and 210-A of the Código Federal de Procedimientos Civiles.
From PDF to signed contract, entirely digital
The full flow: upload the document, assign signers, biometric handwritten signature, NOM-151 seal and download of the certified document.
Operational details
Order between signers
You define who goes first. The next one gets their link as soon as the previous signer is done.
Live tracking
The dashboard shows who opened the link and who already signed.
Reminders
They are sent at the frequency you set, until the signer responds.
Notice to your system
Your system gets a notice the moment the document is complete, download link included.
Link or embedded flow
You send the link over WhatsApp, or embed the signing flow in your application, under your brand.
Ready-made templates for the most common Mexican contracts
Each template is saved once and personalised with variables on every send. You can also upload your own.
Integrations
The signature is triggered from the CRM or from your own system.
The automatic notice to your system covers any other tool.
The legal framework this rests on
Everything runs through the API.
Every operation on this page runs as an API call. The full field-by-field reference is in the documentation.
curl -X POST https://api.singula.mx/app/document \
-H "Authorization: Bearer sk_live_..." \
-F "[email protected]" \
-F 'payload={"title": "Contrato de arrendamiento",
"env": "production",
"signers": [
{"name": "Ana Ruiz", "email": "[email protected]", "sign_order": 1},
{"name": "Luis Mora", "email": "[email protected]", "sign_order": 2}
]}'
→ { "data": { "document": { "id": "doc_lzk9m4ab3f", "status": "draft" } } }
# Enviar a firma
curl -X POST https://api.singula.mx/app/document/doc_lzk9m4ab3f/send \
-H "Authorization: Bearer sk_live_..."
→ {
"data": {
"signing_links": [
{ "signer_email": "[email protected]", "sign_order": 1,
"url": "https://app.singula.mx/sign/8f3ka9dq",
"expires_at": "2026-08-05T10:31:00Z" }
]
}
}Frequently asked questions
In Mexico, yes. The Código de Comercio establishes it in articles 89 through 95 bis.
No. They open the link they receive and sign with a finger on their phone screen.
Yes. The same document can carry a biometric signature from one party and SAT e.firma from another. The signature type is assigned per signer.
The dashboard shows it, and reminders keep being sent at the frequency you set. The document stays open until you cancel it or extend it.
Yes. The test environment is free of charge and reproduces the full flow, from sending the link to the sealed document.
Start signing legally in minutes
Build the signing flow in the test environment, free of charge, and send your first document out for signature.