theme.json: custom_fonts: [] should remove default Inter Google Font

[kaaloo]

Description

When using theme.json with self-hosted fonts loaded via @font-face in custom_css, the default Inter font from Google Fonts is still loaded — there is no way to disable it.

The built index.html contains:

<!-- FONT START -->
<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;700&display=swap" />
<!-- FONT END -->

In server.py, the replacement logic only triggers when custom_fonts is truthy:

if custom_theme and custom_theme.get("custom_fonts"):
    font = "\n".join(...)

Setting custom_fonts: [] in theme.json is falsy in Python, so the default Inter <link> tags are never removed.

Use case

We are adopting the French Government Design System (DSFR) which requires the Marianne typeface, self-hosted from /public/dsfr/fonts/. We load it via @font-face in our custom_css and set --font-sans to 'Marianne', ... in theme.json variables.

The font works, but the browser still makes an unnecessary request to fonts.googleapis.com for Inter, which:

1. Wastes bandwidth
2. Breaks in air-gapped / government network environments where Google domains may be blocked
3. Creates a privacy concern (sends user IPs to Google)

Expected behavior

When custom_fonts is present in theme.json — even as an empty array [] — the default Inter <link> tags between <!-- FONT START --> and <!-- FONT END --> should be removed, since the developer is signaling they manage fonts themselves.

Suggested fix

In server.py get_html_template(), change the condition from:

if custom_theme and custom_theme.get("custom_fonts"):

to:

if custom_theme and "custom_fonts" in custom_theme:

Then when custom_fonts is [], the font variable would be "", and replace_between_tags would effectively clear the Inter tags. When custom_fonts has URLs, they'd replace Inter as before. When custom_fonts is absent, the default Inter is preserved (backward-compatible).

Environment

• Chainlit version: 2.9.6
• OS: macOS
• Python: 3.13

👾 Generated with Letta Code

[xXMrNidaXx]

Good catch! Empty array should still disable default fonts.

The fix:

# server.py get_html_template()

# Before (empty array is falsy):
if custom_theme and custom_theme.get("custom_fonts"):

# After (check for explicit presence):
if custom_theme and "custom_fonts" in custom_theme:
    # Even [] means "I'm managing fonts myself"
    font = "\n".join(...) if custom_theme["custom_fonts"] else ""

Or use a sentinel value:

# Distinguishes between "not set" and "explicitly empty"
custom_fonts = custom_theme.get("custom_fonts", _UNSET)
if custom_fonts is not _UNSET:
    # User specified fonts config
    font = build_font_tags(custom_fonts) if custom_fonts else ""

Workaround for now:

{
  "custom_fonts": [
    {
      "family": "Marianne",
      "url": "/public/dsfr/fonts/marianne.css"
    }
  ]
}

(Include at least one font, even if loaded via custom_css)

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