Hom Adapter Development

HOM Adapter Development

When to Use

Invoke this skill when:

• Adding a new AI harness adapter to crates/hom-adapters/src/
• Modifying an existing adapter's build_command, translate_input, parse_screen, or detect_completion
• Adding or modifying a sideband channel (HTTP, RPC, JSON-RPC)
• Changing the HarnessAdapter trait in hom-core

The Adapter Contract

Every adapter implements HarnessAdapter from crates/hom-core/src/traits.rs. This trait is the single integration point between HOM and a harness. Breaking it breaks everything.

Required Methods

Process: Adding a New Adapter

Step 1: Research the Harness

Before writing any code, answer these questions:

1. Binary name and install method — What command spawns it? Is it npm, cargo, go install?
2. CLI flags — How do you set model, working directory, output format?
3. Interactive mode — What does the prompt look like when waiting for input? (exact characters)
4. Structured output — Does it support JSON/JSONL output mode?
5. Completion signals — What appears on screen when a task finishes?
6. Error patterns — What do errors look like on the terminal?
7. Sideband — Does it expose an HTTP API, RPC interface, or ACP server?

Step 2: Write Tests First (TDD)

Create crates/hom-adapters/tests/<harness_name>_test.rs.

The snippet below is a template, not copy-paste-ready code. Replace placeholder names like MyAdapter, MyHarness, and helper builders with the real adapter and test helpers for your harness:

#[test]
fn test_build_command_default() {
    let adapter = MyAdapter::new();
    let config = HarnessConfig::new(HarnessType::MyHarness, PathBuf::from("/tmp"));
    let spec = adapter.build_command(&config);
    assert_eq!(spec.program, "my-harness-binary");
    assert!(spec.args.is_empty());
}

#[test]
fn test_build_command_with_model() {
    let adapter = MyAdapter::new();
    let config = HarnessConfig::new(HarnessType::MyHarness, PathBuf::from("/tmp"))
        .with_model("gpt-4");
    let spec = adapter.build_command(&config);
    assert!(spec.args.contains(&"--model".to_string()));
    assert!(spec.args.contains(&"gpt-4".to_string()));
}

#[test]
fn test_detect_completion_waiting() {
    let adapter = MyAdapter::new();
    let screen = make_screen_with_last_line("❯ ");
    assert!(matches!(adapter.detect_completion(&screen), CompletionStatus::WaitingForInput));
}

#[test]
fn test_translate_prompt() {
    let adapter = MyAdapter::new();
    let bytes = adapter.translate_input(&OrchestratorCommand::Prompt("hello".into()));
    assert_eq!(bytes, b"hello\n");
}

Run tests: cargo test -p hom-adapters. They must fail before you write the implementation.

Step 3: Implement the Adapter

Create crates/hom-adapters/src/<harness_name>.rs. Follow the pattern of existing adapters (see claude_code.rs as reference).

Step 4: Register in AdapterRegistry

Add to crates/hom-adapters/src/lib.rs:

1. pub mod <harness_name>;
2. Insert into AdapterRegistry::new()

Step 5: Add HarnessType Variant

In crates/hom-core/src/types.rs:

1. Add variant to HarnessType enum
2. Add display_name() match arm
3. Add default_binary() match arm
4. Add from_str_loose() match arm(s)

Step 6: Add Default Config

In config/default.toml, add a [harnesses.<name>] section.

Step 7: Verify

cargo check           # Must pass
cargo test -p hom-adapters  # All new tests green
cargo clippy          # No warnings

Red Flags — STOP and Rethink

• Adding a dependency to hom-core for one adapter — Adapter-specific deps go in hom-adapters only
• Hardcoding paths or API keys — Use HarnessConfig.env_vars and config.toml
• Modifying HarnessAdapter trait for one harness — The trait serves ALL 7 harnesses. Use sideband() for harness-specific features
• Skipping detect_completion tests — This is the #1 source of workflow hangs
• Parsing screen output with regex alone — Screen state includes colors, cursor position, line wrapping. Use ScreenSnapshot methods, not raw string matching

Checklist Before Committing

• HarnessType variant added with all match arms
• Adapter struct created implementing full HarnessAdapter trait
• Registered in AdapterRegistry::new()
• config/default.toml entry added
• from_str_loose() handles common aliases
• Tests for build_command, translate_input, detect_completion
• cargo check && cargo test -p hom-adapters && cargo clippy all pass