The API needs none of this
render_chart is a thin wrapper over one HTTP endpoint. A free key and a POST to /api/v1/charts gets you the same render, alt text and findings from any language, today.
Model Context Protocol
Give your assistant a chart it can hand straight to a human
ChartRelay has an MCP server with one tool, render_chart. The model sends a small JSON spec and gets back durable PNG and SVG URLs, alt text, and a structural quality review it can act on before anyone sees the chart.
Not on npm, and the repository is private: the MCP server is available to beta users on request today. The HTTP API behind it is open to anyone with a free key.
Availability
You cannot install this yet, and here is the honest state of it
The rest of this page describes the tool accurately — what it sends, what comes back, what it costs you in quota. What it cannot do is hand you a copy, because there is nothing public to fetch.
The behaviour is testable now
The playground and the chart checker run the same renderer and the same review this tool calls, with no account, so you can judge the output before asking for anything.
What publication would change
Only distribution. The tool name, the spec it takes, the text it returns and the three environment variables are what they will be; a published package makes them installable in one line.
One tool
What render_chart gives the model
Every successful call returns a block of text the model can read and quote, plus an inline PNG so it can see what it made.
A durable PNG URL
Served from GET /api/v1/charts/[id]?format=png with a one-year immutable cache header. Paste it into chat, an email, or a Slack message.
A durable SVG URL
The same chart at ?format=svg, for web pages and documents. The SVG carries a <title> and <desc> from the chart's title and alt text.
Alt text
A sentence generated from the data: the peak, the low, and the first-to-last change for time-ordered charts. Pie charts get their largest shares instead.
A quality review
Structural findings with a severity and a concrete fix — missing title, too many pie slices, a crowded chart, weak colour contrast, over-long axis labels.
An inline PNG preview
The server fetches its own PNG and attaches it to the tool result, so the model can look at the chart rather than trust the spec. Turn it off with one environment variable.
Your usage so far
Each result ends with the renders used against this month's quota, so an agent working in a loop can tell how much room is left.
Chart rendered (id kR2pQ7mXa4Tb9F).
PNG: https://chartrelay.dev/api/v1/charts/kR2pQ7mXa4Tb9F?format=png
SVG: https://chartrelay.dev/api/v1/charts/kR2pQ7mXa4Tb9F?format=svg
Alt text: Monthly recurring revenue. MRR peaks at 33,900 for Sep and bottoms at 18,400 for Apr. It increased 84.2% from first to last point.
Quality review: no structural problems found. Still verify the data and the takeaway in the title.
Usage this month: 42/1000.Generated on this page by the same renderer the API uses; the id and host are illustrative. URLs point at whatever CHARTRELAY_API_URL you configure.
Setup
What the setup will look like
Two shapes, neither of which runs today. They are worth reading now because they are the whole integration: one entry in a config file, three environment variables, no other moving parts.
Create an API key
Register for a free account and copy the key that is shown once. It starts with
cr_live_. This part works today and is useful on its own — the same key calls the HTTP API.Get the server
The step that does not exist yet. Ask for beta access and you will be sent the server plus the setup for your client; when the package is published this becomes the
npxline below and nothing else changes.Point it at a ChartRelay instance
Set
CHARTRELAY_API_URLto the deployment you render against. The built-in default is a placeholder domain, so it is always worth setting explicitly.Add the entry to your client and restart it
Then ask the assistant for a chart. It calls render_chart and replies with the URLs, the alt text and the review.
{
"mcpServers": {
"chartrelay": {
"command": "npx",
"args": ["-y", "@chartrelay/mcp"],
"env": {
"CHARTRELAY_API_KEY": "cr_live_...",
"CHARTRELAY_API_URL": "https://chartrelay.dev"
}
}
}
}Not runnable yet: npx cannot resolve @chartrelay/mcp until it is published. Beta users get a working equivalent with their copy of the server.
claude mcp add chartrelay \
-e CHARTRELAY_API_KEY=cr_live_... \
-e CHARTRELAY_API_URL=https://chartrelay.dev \
-- npx -y @chartrelay/mcpThe same server, added to Claude Code. The API key is stored by your MCP client, never by this page.
Environment variables
These three are the whole configuration surface; the server reads nothing else.
| Variable | Default | What it does |
|---|---|---|
CHARTRELAY_API_KEY | Required — no default | Sent as a Bearer token on every render. Without it, the tool returns an error telling the model to create a key instead of failing silently. |
CHARTRELAY_API_URL | https://chartrelay.example | The ChartRelay instance to call. The built-in default is a placeholder domain, so set this explicitly — to this deployment or to your own. |
CHARTRELAY_INCLUDE_IMAGE | true | Set it to false to skip the inline PNG preview and return only the text block. Useful when you are paying for image tokens you do not need. |
Each render_chart call counts as one render against your plan's monthly quota, the same as a direct API call.
Self-correction
The quality review is written for a model to act on
Every finding carries a severity, a plain description of the problem, and a suggested fix. The tool description tells the model to revise the spec and call again when the review reports issues, so a bad chart can be caught before it reaches a person.
// First attempt: a pie chart with eleven slices.
Quality review found 1 issue(s). Consider fixing and re-rendering:
- [warning] Too many slices make the chart hard to scan. Fix: Keep the
largest six categories and group the rest as Other.
// The model groups the tail into "Other" and calls render_chart again.
Quality review: no structural problems found. Still verify the data and
the takeaway in the title.Both messages are the exact strings in the quality review. Nothing here is automatic: the model chooses whether to re-render.
Errors
A pie chart with more than one series, or with negative values — cases the chart type genuinely cannot represent. The API refuses these rather than storing a chart, so the tool reports an error instead of handing the model a URL.
Warnings
A missing or placeholder title, more than seven pie slices, more than forty points on a non-pie chart, more than seven series, or a series colour under 3:1 against the background.
Tips
Category labels over twelve characters, which get shortened on the axis. When nothing else fires, a single tip says no structural problems were found.
What the agent can and cannot do
Worth reading before you let a model publish charts on your behalf.
It can
Draw five chart types
Bar, line, area, pie, and scatter. Colours, light or dark theme, legend on or off, and values printed on the marks are all part of the spec.
Handle real-sized data
Up to 12 series of up to 100 points each, at any size from 320x240 to 2400x1600 pixels.
Share the result with anyone
GET /api/v1/charts/[id] needs no key, so the PNG and SVG URLs work in an email or a public page. Renders are immutable: the same id always returns the same image.
Fix its own chart
Read the quality review, adjust the spec, and call render_chart again — no human in the loop for a mis-shaped chart.
It cannot
Tell you whether the data is true
The review inspects the chart's structure — series counts, slice counts, density, contrast, labels. It never sees your source system and cannot check that a number is right.
Plot an x/y relationship
Scatter places each value as a dot at its category label, in input order. There is no numeric x-axis, so it will not show a correlation.
Crop the value axis
The axis always includes zero. That rules out the exaggerated-difference chart, and it also rules out zooming in on a narrow range on purpose.
Change or delete a render
The v1 API is POST to create and GET to read. There is no edit and no delete, and no way to make a published URL private after the fact.
Questions
Can I install it from npm?
No. @chartrelay/mcp has not been published, so npx cannot fetch it, and the repository it lives in is private, so there is nothing to clone either. Publication is intended but not scheduled here, and this page will stop saying all this when it happens.
So how do I actually get it?
By asking. Email support@chartrelay.dev with the MCP client you use. The beta is small and hand-run, so expect a real answer rather than an automatic download — including a no if it is full.
Which MCP clients will it work with?
Any client that can launch a stdio MCP server, which includes Claude Desktop and Claude Code. The server uses the official @modelcontextprotocol/sdk and exposes a single tool, so there is nothing client-specific in it.
Do I need the MCP server to use ChartRelay?
No, and this is the important part: the MCP server is a thin wrapper over one HTTP endpoint. Post to /api/v1/charts with a Bearer key instead — the API docs cover the request and response in full, and nothing on this page is available there that is not available over HTTP.
What happens when I run out of renders?
The tool returns an error the model can read, saying the monthly limit was reached, with a link to the pricing page. It does not silently return an old chart, and a server error is never reported as a quota problem.
Does the model see the chart, or only a link?
Both, by default. The tool result carries the text block and an inline PNG of the chart. Set CHARTRELAY_INCLUDE_IMAGE to false if you only want the text.
Can I point it at my own deployment?
Not by running ChartRelay yourself: ChartRelay is not offered for self-hosting today, so there is no deployment of your own for it to talk to. What CHARTRELAY_API_URL gives you is a configurable target rather than a host compiled into the server — it takes any base URL, so the same server can be aimed at whichever ChartRelay deployment you have access to, which today means this hosted API.
Start with the API, ask about the MCP server
A free key works today and gives you the same renders, alt text and review the tool returns. The MCP wrapper is the part you have to ask for.
250 renders a month on the free plan once your email is confirmed (0 before). No card required.