Best e-signature APIs in 2026: free tiers and real costs compared

Every e-signature API looks cheap on its pricing page. The real number depends on how many documents you send, and whether you pay per seat, per envelope or per completed document. This guide puts nine APIs side by side at 30, 300 and 1,500 documents a month, so you can see the cost for your volume before you write any code.

I work at Portant, and we sell one of these APIs, so I rank ours first and say exactly where the others beat it. The prices are each vendor's own published list prices as of October 2026. They are arithmetic, not quotes: high-volume plans are often negotiated.

What to compare in an e-signature API

• The billing unit. Some APIs charge per envelope sent, some per completed document, and some add a seat fee. An envelope that is sent but never signed still counts on most per-envelope plans.
• The free test path. Most vendors give a free sandbox, but sandbox documents are watermarked and not legally binding. Only a few give real, binding documents for free.
• What you must build before the first send. Templates, an OAuth app, a go-live review: each one is a day of work before you see a signed PDF.
• Whether an AI agent can call it. If an assistant or agent will send the documents, an MCP server or an API that accepts plain text saves you a document-generation step.

What each API costs at three volumes

How we calculated it: DocuSeal is one $20 Pro seat plus $0.20 per completed document. BoldSign is $30 a month for 40 envelopes plus $0.75 for each extra one. SignWell's first 25 API documents each month are free with a card on file, then from $0.85 a document, falling with volume, so the larger figures are an upper limit. PandaDoc's API Developer plan is $40 a month for 40 documents. Docusign's Starter plan is $50 a month billed annually for 40 envelopes, and Dropbox Sign's API Essentials is $75 a month billed annually for 50 requests. "Higher plan" means the vendor's next tier, whose price at that volume is not published per document. signNow is not in the table because its API prices differed across its own pages.

1. Sign PDF API by Portant, best for AI agents and small volumes

REST and MCP  ·  API key, or none with email confirmation  ·  Free plan: 30 requests a month

The Sign PDF API is the same service as our MCP server, as plain REST. You POST a document to https://mcp.portant.co/v1/requests with the signer's name and email, Portant emails the signer a private link, and you download the signed PDF when the status reads signed. On Plus and Scale, a webhook tells your app when the document is signed, so you do not have to poll. The document is one of Markdown, a PDF URL, a base64 PDF, a Google Doc link, or an upload ID for large files. The full reference is in the OpenAPI file.

With an API key, a request sends at once. With no key at all, the request must name the sender, and Portant emails the sender to review and confirm before anything reaches the signer, which makes it safe to try from a script or an agent before you sign up.

Pros

• Free for 30 real, legally binding requests a month, and the lowest list price at 300 and 1,500
• Markdown in, typeset PDF out, so an agent does not need a document generator
• The same tools as an MCP server for Claude, ChatGPT and Cursor
• On Plus and Scale: up to five signers in a set order, a webhook when a document is signed, reminders, saved templates, and signed copies stored until you delete them

Cons

• One signer on Free; up to five signers in a set order need Plus or Scale
• No embedded signing in your own app and no bulk send
• Monthly caps, not overage billing: Free is 30 a month and 10 a day, Plus 300, Scale 1,500
• On Free, the stored signed PDF is deleted after 14 days; Plus and Scale keep it until you delete it, and you can download it from the account page

2. Docusign eSignature API, best for enterprise signing

REST  ·  OAuth (JWT or authorization code)  ·  Free plan: Developer sandbox only

Docusign's API is the most widely used in the category, with SDKs, embedded signing, templates, bulk send and the compliance record that procurement teams ask for. It is the safe default when your customers already expect a Docusign envelope.

Pros

• The broadest feature set and the most familiar signer experience
• A free developer account that does not expire
• An MCP server for AI assistants on the same account

Cons

• The entry plan is $50 a month billed annually for 40 envelopes, so small volumes pay the most per document
• Moving to production needs a paid plan and a go-live step

3. Dropbox Sign API, best for a familiar signer experience

REST  ·  API key or OAuth  ·  Free plan: Test mode only

The Dropbox Sign API (formerly HelloSign) has a free test mode whose requests are not legally binding. API Essentials is $75 a month billed annually for 50 signature requests a month, and embedded signing needs the Standard API plan or higher. Above 500 requests a month, Dropbox asks you to contact sales.

Pros

• A clean signer experience that many people already know
• Embedded signing and bulk send on Standard

Cons

• The highest entry price in this list for a small volume
• The Standard API price differs between sources, so ask Dropbox for a quote

4. BoldSign API, best for clear pay-as-you-go pricing

REST and MCP  ·  API key  ·  Free plan: Sandbox only

BoldSign publishes the plainest API price: $30 a month for 40 envelopes, then $0.75 for each extra envelope, with bulk send at $0.25 each. The sandbox needs no credit card; its documents are watermarked and deleted after 14 days. BoldSign also has hosted and local MCP servers, with regional servers in the EU, Canada and Australia.

Pros

• A published per-envelope price, so the bill is easy to forecast
• Regional hosting for data residency

Cons

• Overage adds up: 300 envelopes a month is about $225 at list price

5. DocuSeal API, best for high volume and self-hosting

REST and MCP  ·  API token  ·  Free plan: Open source edition

DocuSeal charges $0.20 per completed document for API and embedded signing, on top of at least one Pro seat at $20 a month. A submission with several files or signers counts once. The open-source edition is free to self-host, and self-hosted Pro is $20 per user a month with the same per-document fee.

Pros

• The lowest list price per document at volume, and you pay only for completed documents
• Self-hosting for full control of the data

Cons

• Template first: the send tool fails if the template has no fields
• The API needs at least one paid Pro seat

6. SignWell API, best for low-volume pay-as-you-go

REST and local MCP  ·  API key  ·  Free plan: 25 API documents a month with a card

SignWell's API is pay-as-you-go from $0.85 a document, falling to $0.20 or less at high volume. The first 25 API documents each month are free with a card on file, and a free developer account allows unlimited test documents.

Pros

• Very cheap at small volumes
• No monthly minimum

Cons

• We could not confirm the price at each volume step, so the larger figures in our table are an upper limit
• Its MCP server is local only

7. PandaDoc API, best for documents from templates with pricing tables

REST and MCP  ·  API key or OAuth  ·  Free plan: Sandbox only

PandaDoc's API Developer plan is $40 a month for 40 sent documents after a 14-day trial. The sandbox is free and watermarked, and it only sends to your own email domain. API credits come in packs, with no pay-as-you-go. The value is document creation: templates, pricing tables and a content library, not only signing.

Pros

• Create the document and get it signed in one API
• An MCP server and a ChatGPT app on the same account

Cons

• Credit packs, not a per-document price, above the Developer plan
• Signable documents need a template

8. Signbee, best for an agent-first free trial

REST and local MCP  ·  Optional API key  ·  Free plan: 5 documents a month

Signbee is an e-signature API for developers and agents, with Markdown and PDF inputs and a local MCP server. Without an API key, the sender confirms each send with an email code. It is free for 5 documents a month, but its own pages show two different paid pricing models, so check the live page before you budget.

Pros

• Markdown in, signed PDF out, with no account for the first send

Cons

• Two parties only
• Paid pricing that is not consistent across its own pages

9. Adobe Acrobat Sign API, best for companies already on Adobe enterprise plans

REST  ·  OAuth  ·  Free plan: Developer Edition

Adobe reserves API access for enterprise and developer tier accounts, and publishes no self-serve production API price. There is a free Developer Edition for building. If your company already licenses Acrobat Sign Solutions, the API comes with the conversation you have with Adobe sales.

Docusign API pricing, explained

Docusign's US developer pricing page lists three API plans, all billed annually:

The developer account is free, does not expire, and sends watermarked test envelopes that are not legally binding and are removed after 30 days. To send real envelopes you need a paid API plan and a go-live step. Prices on other Docusign country pages and on third-party sites differ, so treat the table as Docusign's US list price in October 2026.

How to choose

• If an AI agent or a script sends the documents, and five signers or fewer are enough, start with the Sign PDF API by Portant: free for 30 a month, and no template to build.
• If you send thousands of documents and can manage templates, DocuSeal has the lowest list price per completed document.
• If your customers expect a Docusign envelope, pay for the Docusign API.
• If you need embedded signing inside your own app, compare Dropbox Sign Standard, Docusign and DocuSeal.

To compare the MCP servers rather than the APIs, see our guide to the best e-signature MCP servers.

Frequently asked questions

Is there a free e-signature API?

Yes. The Sign PDF API by Portant is free for 30 legally binding requests a month with no credit card, and Signbee is free for 5 documents a month. SignWell gives 25 free API documents a month with a card on file. Most other vendors offer only a free sandbox whose documents are watermarked and not legally binding.

Is the Docusign API free?

Only for testing. Docusign's free developer account sends watermarked envelopes that are not legally binding. Production use needs a paid API plan, from $50 a month billed annually for 40 envelopes on Docusign's US price list.

What is the cheapest e-signature API?

For up to 30 documents a month, the Sign PDF API by Portant is free. At 300 and 1,500 documents a month, its paid plans are $12 and $39 a month. Among template-based APIs, DocuSeal's $0.20 per completed document plus a $20 Pro seat is the lowest list price at volume.

What is the difference between an e-signature API and an MCP server?

An API is for code you write: your app calls it with HTTP requests. An MCP server gives the same actions to an AI assistant such as Claude or ChatGPT, so a person can send a document by asking. Sign PDF, Docusign, PandaDoc, BoldSign and DocuSeal are among the vendors that offer both.

Are documents signed through an e-signature API legally binding?

Yes, when a real signer signs on a production account. Under the ESIGN Act and UETA in the US and eIDAS in the EU, a signature cannot be refused legal effect only because it is electronic. Sandbox and test-mode documents are not legally binding.