Quarto CLI 1.9.37 breaks extension with message "TypeError: Cannot read properties of undefined (reading 'reduce') "

[barendgehrels]

Bug description

Brew upgrade did a quarto upgrade to 1.9.37 and since then I could not use the extension anymore.

I use version:

Identifier quarto.quarto
Version 1.131.0
Last Updated 45 minutes ago
Size 30.55MB
Marketplace Published 4 years ago
Last Released 3 weeks ago

First I downgraded the extension but that did not help.

The error trace (for both) is:

  - Extension Host stack trace:                                                                                  
  TypeError: Cannot read properties of undefined (reading 'reduce')                                              
      at mz (.../quarto.quarto-1.120.0/out/main.js:77:4264)                                                      
      at o4 (.../quarto.quarto-1.120.0/out/main.js:77:4201)                                                      
      at YIe (.../quarto.quarto-1.120.0/out/main.js:359:12019)                                                   
      at IX (.../quarto.quarto-1.120.0/out/main.js:359:11387)                                                    
      at Object.slideIndex (.../quarto.quarto-1.120.0/out/main.js:395:3705)                                      
      at AE (.../quarto.quarto-1.120.0/out/main.js:516:1441)                                                     
      at LH.renderFormat (.../quarto.quarto-1.120.0/out/main.js:479:32499)                                                              

and

2026-05-02 11:20:05.221 [error] TypeError: Cannot read properties of undefined (reading 'reduce')
	at _z (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:77:5464)
	at d4 (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:77:5401)
	at Cke (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:359:12030)
	at FX (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:359:11398)
	at Object.slideIndex (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:395:3705)
	at hE (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:495:1598)
	at vH.renderFormat (/Users/.../.vscode/extensions/quarto.quarto-1.131.0/out/main.js:427:32456) quarto.preview {"value":"quarto.quarto","_lower":"quarto.quarto"}

Steps to reproduce

Upgrade to 1.9.37 and try a preview in VS Code. Even a .qmd file with just # title will give this error

Workaround: downgrade to 1.8.27 fixes the problem (also with latest extension).

The "quarto inspect" output will be different for both versions.

Actual behavior

It gives a popup with Cannot read properties of undefined (reading 'reduce') and nothing more.

The stacktrace can be found in Output, "Extension Host"

Expected behavior

It should give the preview as before

Your environment

• IDE: VS Code
• OS: MacOS Tahoe 26.4.1

Version: 1.118.1 (Universal)
Commit: 034f571df509819cc10b0c8129f66ef77a542f0e
Date: 2026-04-29T17:36:44+03:00 (3 days ago)
Electron: 39.8.8
ElectronBuildId: 13870025
Chromium: 142.0.7444.265
Node.js: 22.22.1
V8: 14.2.231.22-electron.0
OS: Darwin arm64 25.4.0

[cscheid]

I can repro this, thanks. Looking into it right now.

Edit 1: I think VS code changed something from under us again; this wasn't happening a few days ago last I used it :(

Edit 2: To make matters more annoying, I can't repro this in VS Code's extension development host, but I can clearly repro it in our production extension.

[cscheid]

Ok, I can now repro this locally on my extension dev host.

But the story is very weird. I can tell that this is a bug that somehow existed for many years. Ultimately, it's this:

In quarto-cli, in src/project/types.ts, we have:

55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 144) export interface ProjectFiles {
55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 145)   input: string[];
55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 146)   resources?: string[];
55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 147)   config?: string[];
55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 148)   configResources?: string[];
55282d6af6 (JJ Allaire          2022-05-28 08:52:44 -0400 149) }

Note the last time this changed: 2022.

In the quarto extension, we effectively redeclare this type, but with config: string[]:

3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 101) export type QuartoProjectConfig = {
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 102)   dir: string;
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 103)   config: {
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 104)     project: {
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 105)       type: string;
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 106)       preview: {
18e821585 packages/quarto-core/src/metadata.ts (J.J. Allaire 2022-12-15 09:50:21 -0500 107)         serve: { /* */ };
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 108)       };
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 109)     };
98b365efe packages/quarto-core/src/metadata.ts (J.J. Allaire 2023-09-24 08:22:04 -0400 110)     format: Record<string,unknown> | string;
05cb9acde packages/quarto-core/src/metadata.ts (J.J. Allaire 2023-07-28 14:05:55 -0400 111)     [key: string]: unknown;
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 112)   };
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 113)   files: {
79ac89065 packages/quarto-core/src/metadata.ts (J.J. Allaire 2023-06-28 19:48:23 -0400 114)     input: string[];
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 115)     config: string[];
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 116)   };
3115fd400 apps/lsp/src/shared/metadata.ts      (J.J. Allaire 2022-12-14 14:24:20 -0500 117) };

Note the dates again, all from 2023 etc. But here, config: string[]. This is clearly going to cause some trouble.

Specifically, we eventually get to configHash() in packages/quarto-core/src/metadata.ts, and there we have it:

function configHash(config: QuartoProjectConfig) {
  return config.files.config.reduce((hash, file) => {
    return hash + fs.statSync(file).mtimeMs.toLocaleString();
  }, "");
}

This clearly needs to be (config.files.config ?? []).reduce.... What I still don't understand is how this we only got bit by this right now.

[cscheid]

We have a PR up #961 with a full explanation. Thanks for the report, I hope to merge this early on Monday when folks are back in the office.

[barendgehrels]

Cool, thanks for the quick fix!

[cscheid]

@juliasilge also isolated the problem to a combination of three factors:

Quarto 1.9.37 changed quarto inspect <file> so that even files outside any project now get a project block in the output
No _quarto.yml anywhere up the directory tree, so files.config stays absent
The .qmd has no YAML front matter

This specifically means that you should be able to work around this until Monday by adding

---
work: around
---

to the beginning of your document (or any other YAML front matter)

[barendgehrels]

Quarto 1.9.37 changed quarto inspect so that even files outside any project now get a project block in the output

Right, so that explains why downgrading helped.