What causes "failed to get `{key}`: {e}" in rust-lang/mdBook?

Change renderers to a TOML array: renderers = ["html"]

failed to get `{key}`: {e}

Error message

failed to get `{key}`: {e}

What it means

preprocessor_should_run() reads the `preprocessor.<name>.renderers` list via Config::get(); if that read errors (e.g. the value has the wrong type and cannot deserialize to Vec<String>, or get() was given an invalid key) the error is wrapped with this context.

Source

Thrown at crates/mdbook-driver/src/mdbook.rs:567

fn preprocessor_should_run(
    preprocessor: &dyn Preprocessor,
    renderer: &dyn Renderer,
    cfg: &Config,
) -> Result<bool> {
    // default preprocessors should be run by default (if supported)
    if cfg.build.use_default_preprocessors && is_default_preprocessor(preprocessor) {
        return preprocessor.supports_renderer(renderer.name());
    }

    let key = format!("preprocessor.{}.renderers", preprocessor.name());
    let renderer_name = renderer.name();

    match cfg.get::<Vec<String>>(&key) {
        Ok(Some(explicit_renderers)) => {
            Ok(explicit_renderers.iter().any(|name| name == renderer_name))
        }
        Ok(None) => preprocessor.supports_renderer(renderer_name),
        Err(e) => bail!("failed to get `{key}`: {e}"),
    }
}

View on GitHub (pinned to dc21064fc2)

Solutions

Change renderers to a TOML array: renderers = ["html"]
Remove the renderers key to fall back to the preprocessor's supports_renderer() method
Validate book.toml with `mdbook build` output or TOML linting

Example fix

// before (book.toml)
[preprocessor.links]
renderers = "html"
// after
[preprocessor.links]
renderers = ["html"]

Defensive patterns

Strategy: validation

Validate before calling

// validate book.toml before build
let raw = toml::from_str::<toml::Value>(&std::fs::read_to_string("book.toml")?)?;
if let Some(r) = raw.get("preprocessor").and_then(|p| p.get("links")).and_then(|l| l.get("renderers")) {
    assert!(r.is_array(), "preprocessor.links.renderers must be a TOML array");
}

Try / catch

match preprocessor_should_run(&pp, renderer, &cfg) {
    Ok(run) => /* ... */,
    Err(e) if e.to_string().contains("failed to get") => {
        eprintln!("check preprocessor.<name>.renderers is an array: {e}");
    }
    Err(e) => return Err(e),
}

Prevention

Always write renderers as a TOML array: renderers = ["html"]
Never quote booleans or arrays in book.toml
Run mdbook build once after config edits to validate types

When it happens

Trigger: book.toml containing `renderers = "html"` (a string instead of an array) or other non-list value under preprocessor.<name>.renderers; nested get() failures from the config layer.

Common situations: Hand-edited book.toml with a malformed renderers entry; copying config snippets where renderers is a single string.

Related errors

AI-assisted analysis of rust-lang/mdBook@dc21064fc2 (2026-09-01). Data as JSON: /api/errors/6e812fc118ea835f. Report an issue: GitHub.