Convert CommonMark to HTML

Turn CommonMark, the strictly-specified standard dialect of Markdown, into HTML, the markup language of the web, readable by every browser. FileChanger runs the document conversion in one call, no local tooling to install.

In the browser

Upload a file and download the result, no pipeline required. Open the converter.

One API call

POST the file with from=commonmark and to=html, get the converted bytes back. Jump to the code.

From an agent

FileChanger ships an MCP server, so agents can run this conversion themselves. See the docs for setup.

Convert CommonMark to HTML in code

The same call from any language: POST the file to /api/convert, the response body is the converted file.

curl -X POST https://filechanger.io/api/convert \
  -H "X-Api-Key: fc_your_api_key_here" \
  -F "file=@input.md" \
  -F "from=commonmark" \
  -F "to=html" \
  -o output.html

import requests

with open("input.md", "rb") as f:
    response = requests.post(
        "https://filechanger.io/api/convert",
        headers={"X-Api-Key": "fc_your_api_key_here"},
        files={"file": f},
        data={"from": "commonmark", "to": "html"},
    )

response.raise_for_status()
with open("output.html", "wb") as out:
    out.write(response.content)

const formData = new FormData();
formData.append("file", fileInput.files[0]);
formData.append("from", "commonmark");
formData.append("to", "html");

const response = await fetch("https://filechanger.io/api/convert", {
  method: "POST",
  headers: { "X-Api-Key": "fc_your_api_key_here" },
  body: formData,
});

const output = await response.blob();

import java.net.http.HttpRequest.BodyPublishers;
import java.net.http.HttpResponse.BodyHandlers;
import java.nio.file.Path;

String boundary = "----FormBoundary" + System.currentTimeMillis();
byte[] fileBytes = java.nio.file.Files.readAllBytes(Path.of("input.md"));

String body = "--" + boundary + "\r\n"
    + "Content-Disposition: form-data; name=\"file\"; filename=\"input.md\"\r\n"
    + "Content-Type: application/octet-stream\r\n\r\n"
    + new String(fileBytes) + "\r\n"
    + "--" + boundary + "\r\nContent-Disposition: form-data; name=\"from\"\r\n\r\ncommonmark\r\n"
    + "--" + boundary + "\r\nContent-Disposition: form-data; name=\"to\"\r\n\r\nhtml\r\n"
    + "--" + boundary + "--";

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://filechanger.io/api/convert"))
    .header("X-Api-Key", "fc_your_api_key_here")
    .header("Content-Type", "multipart/form-data; boundary=" + boundary)
    .POST(BodyPublishers.ofString(body))
    .build();

HttpResponse<byte[]> response = client.send(
    request, BodyHandlers.ofByteArray());
java.nio.file.Files.write(Path.of("output.html"), response.body());

import Network.HTTP.Client
import Network.HTTP.Client.MultipartFormData
import qualified Data.ByteString.Lazy as BL

main :: IO ()
main = do
  manager <- newManager defaultManagerSettings
  initialReq <- parseRequest "POST https://filechanger.io/api/convert"
  let req = initialReq
        { requestHeaders = [("X-Api-Key", "fc_your_api_key_here")] }
  request <- formDataBody
    [ partFileSource "file" "input.md"
    , partBS "from" "commonmark"
    , partBS "to" "html"
    ] req

  response <- httpLbs request manager
  BL.writeFile "output.html" (responseBody response)

About CommonMark

CommonMark is the rigorously specified standard dialect of Markdown, created to end the ambiguity between competing implementations. Platforms and parsers that need predictable, portable Markdown rendering build on the CommonMark spec.

About HTML

HTML is the markup language of the web, rendered natively by every browser on every device. It is both a publishing target (turn any document into a web page) and a source format (turn saved pages and web content into editable documents).

Frequently asked questions

How do I convert CommonMark to HTML?

Upload your file in the browser converter and pick HTML as the target, or POST it to /api/convert with from=commonmark and to=html. The response body is the converted file.

Is converting CommonMark to HTML free?

Yes. The free tier includes 2 GB of conversions per month, metered on input size. See pricing for the paid tiers.

Is there an API for CommonMark to HTML conversion?

Yes, this whole conversion is one POST to /api/convert (the code examples above show it in five languages), plus an MCP server so AI agents can run it directly.