old versions of mdbook fail with obscure errors

[tomassedovic]

Running mdbook build or just build fails with:

Error: Unable to parse the input

Caused by:
    missing field `items` at line 1 column 127727
2026-01-19 17:56:15 [ERROR] (mdbook::utils): Error: The "goals" preprocessor exited unsuccessfully with exit status: 1 status
error: Recipe `build` failed on line 8 with exit code 101

The input refers to STDIN of the preprocessor and indeed, the JSON that's passed to it doesn't contain any fields called items (though it does have sub_items.

[tomassedovic]

It was introduced by 747ec1d

I've reverted just that commit and the issue no longer appears.

[lqd]

I’ll gladly take a PR doing that

[tomassedovic]

It's because I had an old version of the mdbook binary installed.

The fix is running:

$ cargo install --version 0.5.2 mdbook
$ cargo install --version 0.17.0 mdbook-mermaid

Still, this would be great to catch. It's very much not obvious what's going on there.

I'll leave this open to track that for now. The fact that we depend on two external tools and upgrading them can break local dev (CI is fine because it's installing them from scratch) is not ideal.

[lqd]

Ok, let’s just rename the issue and expand the description with the results of your investigation.

[Darksonn]

I've started hitting this. Reinstalling tools does not fix it as far as I can tell.

📝 Generating blog post for 2026-02 (milestone: 2025h2)
🌐 Loading issues from GitHub API for milestone: 2025h2
Error: Failed to load milestone issues for 2025h2: error: EOF while parsing a value at line 1 column 0
   --> /usr/local/google/home/aliceryhl/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/spanned-0.6.1/src/error.rs:130:23
    |
130 |                 span: Span::here(),
    |                       ^^^^^^^^^^^^^
    |
ERROR The "goals" preprocessor exited unsuccessfully with exit status: 1 status

[lqd]

Looks like an EOF in the GH API, could it be transient or are you hitting this all the time?

[Darksonn]

I mean, I'm hitting it today.