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[FEATURE]: Detect Mermaid diagrams in untagged fences, not only ```mermaid #43304
Description
Feature hasn't been suggested before.
- I checked Render Mermaid fences as diagrams in chat #39816 (closed) and V2 TUI: add native Mermaid and LaTeX rendering #40508 (open). Both cover rendering Mermaid; neither mentions detection of untagged fences, which is what this asks for.
Describe the enhancement you want to request
decorateMermaid (from the merged, unreleased #43024) only acts on tagged fences:
if (!code.classList.contains("language-mermaid")) {
clearMermaid(wrapper)
return
}Correct by the letter — but models often don't tag the fence, even when explicitly asked for a Mermaid diagram.
Measured across the stored sessions of one instance ([email protected], Nemotron 3 Ultra 550B):
| fence | count |
|---|---|
``` (untagged) |
50 |
```text |
26 |
```mermaid |
22 |
```bash |
2 |
Not every untagged block is a diagram. But the diagrams that were there arrived untagged more often than tagged, and the user just sees a code block — with no way to tell whether the feature is missing or the tag is.
The mechanism is already in the PR. renderMermaidSvg does:
if (!(await mermaid.parse(source, { suppressErrors: true }))) returnThat alone identifies a diagram; non-diagram code doesn't parse. So the class check could be relaxed rather than replaced — attempt untagged (and text) blocks too and let parse decide, keeping the fast path for tagged ones. Cost is one suppressed parse per candidate, and it can be limited to complete blocks, which the code already distinguishes, so streaming is unaffected.
I first tried matching the first word against Mermaid's diagram keywords. It works, but ages badly — every new diagram type has to be added by hand. parse always knows.
Caveat: I haven't run your build; #43024 is merged but not in a published release (latest 1.18.18, 2026-08-13). This comes from reading the diff. The measurement is from our own sessions and stands on its own.
Activity
github-actions commentedon Aug 18, 2026
This issue doesn't fully meet our contributing guidelines.
What needs to be fixed:
Please edit this issue to address the above within 2 hours, or it will be automatically closed.
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[-]Mermaid rendering only triggers on ```mermaid-tagged fences — models often emit untagged blocks[/-][+][FEATURE]: Detect Mermaid diagrams in untagged fences, not only ```mermaid[/+]chronoflux-germany commentedon Aug 18, 2026
Rewritten using the Feature Request template rather than Bug Report, deliberately: the Bug Report template asks for version, steps to reproduce, OS and terminal, and I can't fill those honestly — #43024 isn't in a published release, so there is no shipped behaviour to reproduce. This is a suggestion about detection scope, not a defect report.
Also checked the two issues you linked before ticking "hasn't been suggested before": #39816 and #40508 both cover rendering; neither addresses untagged fences.
(Minor: the guidance mentions three templates including "Question", but
.github/ISSUE_TEMPLATE/currently holds onlybug-report.ymlandfeature-request.yml.)github-actions commentedon Aug 18, 2026
Thanks for updating the issue! This looks good now — the feature request is clear, well-documented with real measurements, and follows the template. Removing the compliance label.
github-actions commentedon Oct 18, 2026
To stay organized issues are automatically closed after 60 days of no activity. If the issue is still relevant please open a new one.