The "{}" preprocessor exited unsuccessfully with {} status

Error message

The "{}" preprocessor exited unsuccessfully with {} status

What it means

mdbook-driver runs a builtin preprocessor as a child process and calls `ensure!(output.status.success(), ...)` after waiting for it. When the child exits with a non-zero status, the ensure condition fails and this anyhow error is returned, embedding the preprocessor name and the exit status. It is the generic 'backend preprocessor crashed' error.

Source

Thrown at crates/mdbook-driver/src/builtin_preprocessors/cmd.rs:99

                )?;
                // This should normally not be reached, since the validation
                // for NotFound should have already happened when running the
                // "supports" command.
                return Ok(book);
            }
        };

        self.write_input_to_child(&mut child, &book, ctx);

        let output = child.wait_with_output().with_context(|| {
            format!(
                "Error waiting for the \"{}\" preprocessor to complete",
                self.name
            )
        })?;

        trace!("{} exited with output: {:?}", self.cmd, output);
        ensure!(
            output.status.success(),
            format!(
                "The \"{}\" preprocessor exited unsuccessfully with {} status",
                self.name, output.status
            )
        );

        serde_json::from_slice(&output.stdout).with_context(|| {
            format!(
                "Unable to parse the preprocessed book from \"{}\" processor",
                self.name
            )
        })
    }

    fn supports_renderer(&self, renderer: &str) -> Result<bool> {
        debug!(
            "Checking if the \"{}\" preprocessor supports \"{}\"",

View on GitHub (pinned to dc21064fc2)

Solutions

Run the preprocessor command standalone on a sample input to see its real error output.
Check book.toml [preprocessor.X] command for typos and correct working directory assumptions.
Fix the preprocessor so it exits 0 on success; make sure it reads/writes the (input, output) JSON files correctly.
Run `mdbook build` with `RUST_LOG=trace` to see the child's captured output (traced just before the ensure).

Example fix

# before
defective_preproc.py  # raises on error -> exit 1
# after
defective_preproc.py || { echo "preproc failed" >&2; cat debug-log; }  # debug, then fix script to exit 0 on success

Defensive patterns

Strategy: try-catch

Try / catch

match mdbook_driver::process(book).build(...) {
    Err(e) if e.to_string().contains("exited unsuccessfully") => {
        eprintln!("preprocessor failed: {e:#}"); // inspect the named preprocessor
    }
    Err(e) => eprintln!("build failed: {e:#}"),
    Ok(_) => {}
}

Prevention

Test each preprocessor command standalone before wiring it into book.toml.
Make preprocessors exit 0 on success and log errors to stderr.
Pin/verify preprocessor binaries exist on PATH in CI.
Run builds with RUST_LOG=trace to capture preprocessor output.

When it happens

Trigger: Any preprocessor command defined in book.toml ([preprocessor.<name>].command) exits with non-zero status during mdbook build/test/serve; e.g. the command binary is a failing script, or `mdbook build` runs `links`/`index` builtin preprocessors that return an error code.

Common situations: A custom preprocessor script exits 1 on bad markdown; preprocessor binary missing dependencies and aborting; preprocessor killed by signal (status shows signal, e.g. 'signal: 9'); typo'd command that runs but fails.

Related errors

AI-assisted analysis of rust-lang/mdBook@dc21064fc2 (2026-09-01). Data as JSON: /api/errors/59363112ce2fb0bc. Report an issue: GitHub.