Agent Skills: MDBook Alternative Backend Plugin Development

MDBook Alternative Backend Plugin Development

Overview

Guide for developing custom MDBook alternative backend (renderer) plugins in Rust.

This skill covers:

• Planning plugin requirements with the user
• Skeleton project setup and templating
• RenderContext handling and output generation
• Test-driven development workflow
• Documentation (ADR, data flow, schema, I/O, examples, user stories)
• Finding existing backends that may already solve the problem
• Creating GitHub repos in arustydev/* namespace
• Configuration and book.toml integration

This skill does NOT cover:

• Preprocessors (see mdbook-plugin-preprocessor skill)
• Using existing backends (configuring, not building)
• Writing book content
• General Rust development patterns

Prerequisites

• Rust toolchain installed (rustup)
• MDBook installed (cargo install mdbook)
• GitHub CLI (gh) for repo creation
• Familiarity with Rust and Cargo

Workflow

Step 1: Discover Existing Backends

Before building, check if a backend already exists.

# Search GitHub for existing backends
gh search repos mdbook-<keyword> --limit 20

# Check crates.io
cargo search mdbook-<keyword>

# Browse the wiki
open "https://github.com/rust-lang/mdBook/wiki/Third-party-plugins"

Existing backends include:

• mdbook-epub - EPUB generator
• mdbook-pdf - PDF via Chrome
• mdbook-typst - PDF/PNG/SVG via Typst
• mdbook-pandoc - Multiple formats via Pandoc
• mdbook-man - Man pages
• mdbook-texi - Texinfo format

If existing backend covers your use case, adopt it instead.

Step 2: Plan Plugin Functionality

Gather requirements before writing code.

2.1 Define the Problem

Ask these questions:

1. What output format is needed?
2. What features of the book should be preserved?
3. What styling/theming options are needed?
4. What external tools (if any) are required?

2.2 Document with ADR

Create an Architecture Decision Record:

# ADR-001: <Backend Name> Design

## Status
Proposed

## Context
<Why is this backend needed? What format does it produce?>

## Decision
<How will it transform the book? What libraries/tools will it use?>

## Consequences
<Trade-offs: performance, dependencies, format limitations>

2.3 Map Data Flow (Mermaid)

flowchart LR
    A[book.toml] --> B[mdbook build]
    B --> C[Load Book]
    C --> D[Run Preprocessors]
    D --> E[RenderContext JSON]
    E --> F[mdbook-foo stdin]
    F --> G[Process Chapters]
    G --> H[Generate Output]
    H --> I[Write to destination/]

2.4 Define Input/Output Schema

Input (received via stdin as JSON):

// RenderContext structure
pub struct RenderContext {
    /// Version of mdbook that invoked this backend
    pub version: String,
    /// The book's root directory
    pub root: PathBuf,
    /// The book structure with all chapters
    pub book: Book,
    /// Configuration from book.toml
    pub config: Config,
    /// Where to write output files
    pub destination: PathBuf,
}

Output: Files written to destination directory in your target format.

2.5 Write User Stories

## User Stories

### US-001: Basic Rendering
As a book author,
I want to run `mdbook build` and get <format> output,
So that I can distribute my book in <format>.

### US-002: Configuration
As a book author,
I want to configure output options in `book.toml`,
So that I can customize the generated output.

### US-003: Error Handling
As a book author,
I want clear error messages when rendering fails,
So that I can diagnose and fix issues.

Step 3: Create GitHub Repository

# Create repo with standard naming
gh repo create arustydev/mdbook-<foo> \
  --public \
  --description "MDBook backend for <format> output" \
  --clone

cd mdbook-<foo>

# Initialize Rust project
cargo init --name mdbook-<foo>

# Apply standard templates
just apply-gist lang_rust type=bin
just apply-gist github_labels_rust
just apply-gist common

Step 4: Set Up Project Structure

mdbook-<foo>/
├── Cargo.toml
├── src/
│   ├── main.rs          # CLI entry point
│   └── lib.rs           # Rendering logic
├── templates/           # Output templates (if needed)
├── tests/
│   ├── integration.rs   # Integration tests
│   └── fixtures/        # Test book fixtures
├── docs/
│   ├── adr/             # Architecture decisions
│   └── book.toml        # Example configuration
├── .github/
│   └── workflows/
│       └── ci.yml       # CI/CD pipeline
└── README.md

Step 5: Configure Cargo.toml

[package]
name = "mdbook-<foo>"
version = "0.1.0"
edition = "2021"
description = "MDBook backend for <format> output"
license = "MIT"
repository = "https://github.com/arustydev/mdbook-<foo>"
keywords = ["mdbook", "backend", "renderer"]
categories = ["command-line-utilities", "text-processing"]

[dependencies]
mdbook-renderer = "0.2"
serde = { version = "1", features = ["derive"] }
serde_json = "1"
anyhow = "1"
log = "0.4"
env_logger = "0.11"
semver = "1"               # For version compatibility checks

[dev-dependencies]
pretty_assertions = "1"
tempfile = "3"

Step 6: Implement with TDD

6.1 Write Failing Test First

// tests/integration.rs
use mdbook_foo::render_book;
use tempfile::TempDir;

#[test]
fn test_basic_render() {
    let book_json = include_str!("fixtures/simple-book.json");
    let ctx: RenderContext = serde_json::from_str(book_json).unwrap();

    let output_dir = TempDir::new().unwrap();
    let result = render_book(&ctx, output_dir.path());

    assert!(result.is_ok());
    assert!(output_dir.path().join("output.foo").exists());
}

#[test]
fn test_empty_book() {
    // Test handling of books with no chapters
}

#[test]
fn test_configuration_options() {
    // Test custom configuration from book.toml
}

6.2 Implement RenderContext Handling

// src/lib.rs
use mdbook_renderer::RenderContext;
use anyhow::Result;
use std::path::Path;
use std::fs;

/// Backend configuration from book.toml
#[derive(Debug, Default, serde::Deserialize)]
#[serde(default, rename_all = "kebab-case")]
pub struct FooConfig {
    pub output_file: Option<String>,
    pub option1: bool,
    pub option2: String,
}

pub fn render_book(ctx: &RenderContext, destination: &Path) -> Result<()> {
    // Check version compatibility
    check_version(&ctx.version)?;

    // Get backend config
    let config: FooConfig = ctx.config
        .get_deserialized_opt("output.foo")?
        .unwrap_or_default();

    // Ensure destination exists
    fs::create_dir_all(destination)?;

    // Process each chapter
    for item in ctx.book.iter() {
        if let BookItem::Chapter(chapter) = item {
            process_chapter(chapter, &config, destination)?;
        }
    }

    Ok(())
}

fn check_version(version: &str) -> Result<()> {
    let version = semver::Version::parse(version)?;
    let min_version = semver::Version::new(0, 4, 0);

    if version < min_version {
        log::warn!(
            "mdbook version {} may not be compatible (minimum: {})",
            version,
            min_version
        );
    }
    Ok(())
}

fn process_chapter(
    chapter: &Chapter,
    config: &FooConfig,
    destination: &Path,
) -> Result<()> {
    // Transform chapter content to target format
    let output = transform_content(&chapter.content, config)?;

    // Write to destination
    let output_path = destination.join(format!("{}.foo", chapter.name));
    fs::write(output_path, output)?;

    Ok(())
}

fn transform_content(content: &str, config: &FooConfig) -> Result<String> {
    // TODO: Implement content transformation
    Ok(content.to_string())
}

6.3 Implement CLI Entry Point

// src/main.rs
use mdbook_renderer::RenderContext;
use mdbook_foo::render_book;
use std::io;
use std::process;

fn main() {
    env_logger::init();

    // Read RenderContext from stdin
    let mut stdin = io::stdin();
    let ctx = match RenderContext::from_json(&mut stdin) {
        Ok(ctx) => ctx,
        Err(e) => {
            eprintln!("Error parsing RenderContext: {}", e);
            process::exit(1);
        }
    };

    // Render the book
    if let Err(e) = render_book(&ctx, &ctx.destination) {
        eprintln!("Error rendering book: {}", e);
        process::exit(1);
    }
}

6.4 Run Tests and Iterate

# Run tests (expect failures initially)
cargo test

# Implement until tests pass
cargo test -- --nocapture

# Check with clippy
cargo clippy -- -D warnings

# Format code
cargo fmt

Step 7: Test with Real Book

Create a test book:

mkdir -p tests/fixtures/test-book/src
cat > tests/fixtures/test-book/book.toml << 'EOF'
[book]
title = "Test Book"

[output.foo]
output-file = "book.foo"
option1 = true
EOF

cat > tests/fixtures/test-book/src/SUMMARY.md << 'EOF'
# Summary

- [Chapter 1](chapter1.md)
EOF

cat > tests/fixtures/test-book/src/chapter1.md << 'EOF'
# Chapter 1

Test content for backend rendering.
EOF

Test manually:

# Build and install locally
cargo install --path .

# Test with book
cd tests/fixtures/test-book
mdbook build
ls book/foo/  # Check output directory

Step 8: Document Configuration

Add to README.md:

## Installation

```bash
cargo install mdbook-<foo>

Configuration

Add to your book.toml:

[output.foo]
output-file = "book.foo"
option1 = true
option2 = "value"

Options

| Option | Type | Default | Description | |--------|------|---------|-------------| | output-file | string | "output.foo" | Output filename | | option1 | bool | false | Enable feature 1 | | option2 | string | "" | Configuration value |

Disabling HTML Output

By default, if you add a custom backend, the HTML backend is disabled. To keep HTML output alongside your backend:

[output.html]

[output.foo]

### Step 9: Set Up CI/CD

```yaml
# .github/workflows/ci.yml
name: CI

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
      - run: cargo test
      - run: cargo clippy -- -D warnings
      - run: cargo fmt --check

  integration:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
      - run: cargo install mdbook
      - run: cargo install --path .
      - run: cd tests/fixtures/test-book && mdbook build
      - name: Verify output
        run: test -d tests/fixtures/test-book/book/foo

Examples

Example: Simple Text Output

fn transform_content(content: &str, _config: &FooConfig) -> Result<String> {
    // Strip markdown, output plain text
    Ok(content.lines()
        .filter(|l| !l.starts_with('#'))
        .collect::<Vec<_>>()
        .join("\n"))
}

Example: JSON Output

use serde::Serialize;

#[derive(Serialize)]
struct ChapterOutput {
    name: String,
    content: String,
    word_count: usize,
}

fn transform_chapter(chapter: &Chapter) -> Result<String> {
    let output = ChapterOutput {
        name: chapter.name.clone(),
        content: chapter.content.clone(),
        word_count: chapter.content.split_whitespace().count(),
    };
    Ok(serde_json::to_string_pretty(&output)?)
}

Example: Template-Based Output

use handlebars::Handlebars;

fn render_with_template(chapter: &Chapter, template: &str) -> Result<String> {
    let mut handlebars = Handlebars::new();
    handlebars.register_template_string("chapter", template)?;

    let data = serde_json::json!({
        "title": chapter.name,
        "content": chapter.content,
    });

    Ok(handlebars.render("chapter", &data)?)
}

Troubleshooting

Backend not running

Check book.toml has [output.foo] section and binary is in PATH.

JSON parse errors

Ensure stdin handling is correct. Debug with:

RUST_LOG=debug mdbook build 2>&1 | head -100

Output directory issues

Always use fs::create_dir_all() before writing:

fs::create_dir_all(&ctx.destination)?;

HTML output missing

When custom backends are configured, HTML is disabled by default. Add [output.html] to keep it:

[output.html]

[output.foo]

Key Differences from Preprocessors

| Aspect | Preprocessor | Backend | |--------|--------------|---------| | Purpose | Modify book content | Generate output format | | Config | [preprocessor.foo] | [output.foo] | | Input | [PreprocessorContext, Book] | RenderContext | | Output | Modified Book JSON | Files to destination | | Crate | mdbook-preprocessor | mdbook-renderer |

References

• MDBook Backend Docs
• mdbook-renderer crate
• Third-party Plugins Wiki
• Example: mdbook-epub
• Example: mdbook-typst