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serveAndOpen sends JS, CSS and JSON as application/octet-stream

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@tryingET is already working on this.

Since Oct 10, 2026.

  • #2414 by @tryingET — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
node.js, typescript
Domain
backend, cli

Research direction

Start in packages/cli/src/lib/serveAndOpen.ts at mimeTypeOfName() and its extname() lookup. Add regression checks through the actual server for JavaScript, CSS, JSON, source maps, unknown extensions, and the explicit HTML route. Done means recognized media types are returned for the listed extensions, unknown files keep the binary fallback, and HTML remains text/html.

Written by the indexing model from the issue text.

Description

Summary

The CLI's packaged serveAndOpen server sends JavaScript, CSS and JSON with
Content-Type: application/octet-stream. Its MIME table uses extension names without
a leading dot, but path.extname() returns dotted names. The HTML route supplies
text/html explicitly and is unaffected by this lookup.

Environment

  • Official npm @appland/appmap 3.204.0, unmodified packaged server/assets.
  • Node 26.9.0, Linux; loopback HTTP server. Source inspection finds the same
    lookup in the 3.196.1 release tag.
  • A separate Chromium 153.0.8010.47 observation also recorded these JS/CSS/JSON
    media types. This report concerns the MIME lookup, not a claim that MIME alone
    prevents script execution or explains other viewer failures.

Reproduction

In a disposable directory where the existing package is resolvable, save this as
mime-repro.cjs and run node mime-repro.cjs. It uses the genuine packaged server,
with no browser auto-open, response interception or installed-source modification.
The small JSON file is only a media-type fixture, not a rendering fixture.

const fs = require('node:fs');
const path = require('node:path');
const root = path.dirname(require.resolve('@appland/appmap/package.json'));
const serve = require(path.join(root, 'built/lib/serveAndOpen.js')).default;
fs.writeFileSync('mime-fixture.json', '{}\n');
serve('sequenceDiagram.html', {}, true, async (url) => {
  const base = new URL(url);
  base.hostname = '127.0.0.1';
  try {
    for (const name of ['/', '/sequenceDiagram.js', '/sequenceDiagram.css',
                        '/resource?mime-fixture.json']) {
      const response = await fetch(new URL(name, base));
      console.log(name, response.status, response.headers.get('content-type'));
      await response.arrayBuffer();
    }
    process.exit(0);
  } catch (error) {
    console.error(error);
    process.exit(1);
  }
}, false);

Expected and actual

Observed on 3.204.0:

/                               200 text/html
/sequenceDiagram.js             200 application/octet-stream
/sequenceDiagram.css            200 application/octet-stream
/resource?mime-fixture.json      200 application/octet-stream

Expected: a recognized JavaScript media type for .js, text/css for .css, and
application/json for .json. Unknown extensions can retain the existing binary
fallback. The explicit HTML response should remain text/html.

Source analysis

In the 3.204.0 source,
mimeTypeOfName() indexes a table whose keys are js, css, json and map with
extname(filename). For sequenceDiagram.js, the returned key is .js, so the
lookup misses and selects the fallback. The resource route uses the same function.
The .map case has the same static mismatch; the observations above cover
JS/CSS/JSON rather than claiming an executed source-map check.

The same lookup is present in
3.196.1.

Suggested fix

Use dotted table keys while retaining the binary fallback for unknown extensions.
Add regression checks through the actual server for JS, CSS, JSON, source maps,
unknown extensions and the explicit HTML route. These are proposed owner-side
changes, not a workaround applied to the published artifact.

Related

#1597 and
#2362 concern absent packaged HTML;
#1399 concerns double-sent headers.
They appear related to serving/publication, but not this extension-key mismatch.
The shipped compiled viewer assets are present; this report does not claim all UI
assets are missing or that correcting MIME will fix an independent runtime exception.

Dominant language
TypeScript
Stars
55
Forks
18
Avg merge
4d 8h
Merged PRs (30d)
3

Getting set up

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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