Resend vs Postmark

On the 29 checks both could be read on, Resend is 2 ahead of Postmark.

ResendyouPostmark

Resend96/100
Our scan, 9 Nov 2026, 04:00 UTC.
Find 100 out of 100, Read 100 out of 100, Act 90 out of 100. Weakest step: Act.
Postmark94/100
Our scan, 9 Nov 2026, 04:00 UTC.
Find 100 out of 100, Read 94 out of 100, Act 89 out of 100. Weakest step: Act.

These are public files we could read. They don't rank the products or show what an agent can do with them.

How close · on the 29 shared checks

Resend 96Postmark 94

Dashed: Resend after 1 fix (99). Our maths on these checks, not a forecast.

1 check passes on Postmark and fails on Resend. Resend passes 3 checks that fail on Postmark.

Fix these first

Most points for the least effort. What you tick goes into the prompts below, one for each team with a job to do.

Errors have a schema Engineering

If this passes, Resend's full score goes from 96 to 99. That's our checks, not a forecast of what agents do. Effort: hours, our estimate.

The pages marketing owns pass. What's left sits with engineering and product.

Passes on Postmark, fails our check on Resend

Most points for the least effort first, each next to the real file that passes.

FailAct step+4 pointsEffort: hours, our estimate
Fix 1: Your API spec doesn't say what an error looks like
Do this: In your OpenAPI spec, give each operation a 4xx and a default response with an error schema: a stable code or type and a message, ideally as application/problem+json (RFC 9457), defined once under components and referenced with $ref.
Errors have a schema: your API spec says what an error looks like: a code an agent can act on, and a message.
FailWhat we read on Resend (1)
- https://raw.githubusercontent.com/resend/resend-openapi/main/resend.jsonHTTP 2000 of 142 operations declare an error response with a schema
PassWhat passes on Postmark, from our 9 Nov 2026 scan
- https://postmarkapp.com/swagger/server.ymlHTTP 20043 of 43 operations declare an error response with a schema
View their file ↗
Why it might matter
Without a documented error shape, an agent may have to guess from the message why a call failed, and retry or give up wrongly.
Our call · Worth doing.
If agents call your API, a stable error code tells them what went wrong without reading prose.
Our judgement.
Report a mistake
How we test this
- In the OpenAPI spec AR-ACT-02 finds, we count the operations that declare a 4xx, 5xx or default response with a schema, directly or through a $ref.
- The credit is the share that do.
- We note when they use application/problem+json (RFC 9457), the standard shape.
- With no spec, it isn't counted.
- The IETF's draft agent-friendly API profile, is-agentic and Alpic check it.

Where you're ahead

Passes on Resend, fails our check on Postmark

Docs in Markdown
Fails our check on Postmark.
Every operation has an operationId
Fails our check on Postmark.
Install shown for four languages
Fails our check on Postmark.

Nobody here passes our check yet

A chance to be first, if it's worth doing for you.

Tools for browser agents
We found something close on Postmark. The full result has a dated note.
Don't do it for the score. It's still a proposed standard, and almost nobody on our boards passes our check, though Bird, Postmark and CrewAI load it from a script our check doesn't read. Build it only if agents need to operate forms on your site. Our judgement.

Check by check

Check	Resend	Postmark
Docs in Markdown	PassPass	FailFails our check
Every operation has an operationId	PassPass	FailFails our check
Errors have a schema	FailFails our check	PassPass
Install shown for four languages	PassPass	FailFails our check

31 checks in all, 4 differ. 2 checks count for only some of you (Couldn't check or Not counted where it doesn't), so your totals can differ even where no check does. * We found something close: the full result has a dated note.

Track it weekly, or add it for your coding agent

Add it for your coding agent

Adds a skill to your project. Ask your agent to use it, and it scans your live site, reads the real file behind each gap and helps write your own. It only publishes if you ask.

npx legible-cli skill

If Windows PowerShell blocks npx, type npx.cmd instead.

Get warned when something changes

A GitHub Action that runs in your own repo, once a week. It fails, naming the check, if something the first site passes stops passing or a rival starts passing something it fails today. Each alert links back to the result. Add a Slack webhook as a repo secret named SLACK_WEBHOOK_URL, and a change also posts to your channel; nothing else is sent to us. Anything we can't be sure of only warns. GitHub notifies whoever created the job, depending on their settings. In a public repo, it turns scheduled jobs off after 60 days with no activity.

.github/workflows/legible.yml

A fail means our check didn't pass on what we read, which doesn't prove the thing isn't there. How we check. Why another scanner gives a different number.