For a few days have the error "Couldn't find open server" when trying render project

[aleksei-khitev]

Description

From Saturday (October, 2, 2025) morning I cannot render my project due to error ERROR: Couldn't find open server..
Before that rendering process was Ok.
May be something changed, or some server is down.
It would be great if you could help ))

The full stacktrace is here:

ERROR: Couldn't find open server.
Chrome process error: [1007/170745.377:ERROR:chrome\app\chrome_main.cc:321] Missing headless user data directory.


Stack trace:
    at criClient (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:38997:15)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:38955:22
    at async Semaphore.runExclusive (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:38629:29)
    at async Object.createPngsFromHtml (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:39226:42)
    at async makePng (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:83642:70)
    at async makeDefault (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:83702:24)
    at async Object.cell (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:83725:20)
    at async Promise.all (index 0)
    at async Object.document (file:///C:/Users/alexk/AppData/Local/Programs/Quarto/bin/quarto.js:39507:25)
Error occurred during cleanup: NotFound: entity not found

[mcanouil]

@aleksei-khitev I'd be happy to help you troubleshoot this!
However, to provide meaningful assistance, I'll need a bit more information than only an error message out of context. Could you please share:

• The code (small reproducible document) that's causing the error (or the relevant portion).
• What you were trying to accomplish when the error occurred such as the command you executed.
• The version of Quarto CLI being used.
• The output of quarto check.

Providing these details from the start helps save everyone's time and allows me to give you a more accurate and useful solution straight away.

Wild guess, the issue is related to a Chrome update and involve the headless mode.

---

You can share a self-contained "working" (reproducible) Quarto document using the following syntax, i.e., using more backticks than you have in your document (usually four ````).
See https://quarto.org/bug-reports.html#small-is-beautiful-aim-for-a-single-document-with-10-lines.

If you have multiple files (and if it is absolutely required to have multiple files), please share as a Git repository.

R	Python
````qmd --- title: "Reproducible Quarto Document" format: html engine: knitr --- This is a reproducible Quarto document. {{< lipsum 1 >}} ```{r} x <- c(1, 2, 3, 4, 5) y <- c(1, 4, 9, 16, 25) plot(x, y) ``` {#fig-placeholder} {{< lipsum 1 >}} The end after @fig-placeholder. ````	````qmd --- title: "Reproducible Quarto Document" format: html engine: jupyter --- This is a reproducible Quarto document. {{< lipsum 1 >}} ```{python} import matplotlib.pyplot as plt x = [1, 2, 3, 4, 5] y = [1, 4, 9, 16, 25] plt.plot(x, y) plt.show() ``` {#fig-placeholder} {{< lipsum 1 >}} The end after @fig-placeholder. ````

Additionally and if not already given, please share the output of quarto check within a code block (i.e., using three backticks ```txt), see https://quarto.org/bug-reports.html#check.

[aleksei-khitev]

As far as I experimented, the issue is connected with format.docx.mermaid.theme.
Because I removed almost everything from _quarto.yml and if I remove this property then everything is rendered. If I paste this to the yml, then it fails.
For now, _quarto.yml looks like this:

project:
  output-dir: target
format:
  docx:
    mermaid:
        theme: neutral  

And the issue is reproducable in the project with included file. Because I placed mermaid code block in the root file (lesson_1) and everything was fine.
I uploaded another zip file https://drive.google.com/file/d/1K3buS30NJDah-8ZJBoFzKERAuRic6DOk/view?usp=sharing

[mcanouil]

Could you make making a single document, using only what's needed? No _quarto.yml.
Then try with/without using the include shortcode?

For instance, could the following be enough? Then try to remove pieces of the diagram if the example does reproduce the issue.

---
title: "Java"
subtitle: Занятие 1
format: docx
---

```{mermaid}
%%{init: {'theme':'base', 'themeVariables':{'primaryColor':'#FDF6E3', 'textColor':'#000'}}}%%
flowchart TD

    %% --- Сущности ---
    Value["Значение"]:::val
    Variable["Переменная"]:::var
    Memory["Яейки в памяти"]:::mem

    %% --- Действия ---
    Value -->|Присваивание| Variable
    Variable --- Memory

     %% --- Стили ---
     classDef var fill:#add8e6,stroke:#333,stroke-width:2px;
     classDef val fill:#ff4422,stroke:#333,stroke-width:2px;
     classDef mem fill:#90ee90,stroke:#333,stroke-width:2px;
```

(FYI, the above works perfectly fine in my case on macOS.)

Note that this is debugging workflow and that it is not limited to Quarto.
Stripping unnecessary elements to get a small reproducible example helps figuring out the issue or even the solution.
That's something you have to learn because time is limited and developers/maintainers cannot spend their time to do this process or they won't have the time to work on the actual software.

[aleksei-khitev]

Yes, this file reproduces the issue:

> quarto render .\lesson_1.qmd
ERROR: AssertionError:
Error occurred during cleanup: TypeError: Child process has already terminated

If I remove format: docx everything is rendered Ok

[mcanouil]

Then try to remove pieces of the diagram if the example does reproduce the issue.

[aleksei-khitev]

With docx format and without diagram everything renders fine.
Now the content of the file is

---
title: "Java"
subtitle: Занятие 1
format: docx
---

## Something