How to build a tool

In this tutorial you will build weather-tool from scratch — a WASM tool that fetches current conditions, a 5-day forecast, and air quality data using the free Open-Meteo API (no API key required). By the end you will have a working tool your agent can call like this:

“What’s the weather in Tokyo right now?”

The complete source code for this tool is available on GitHub:

weather-tool source

Browse the full implementation — lib.rs, Cargo.toml, and weather-tool.capabilities.json.

Prerequisites

If you don’t have Rust yet, install it from rustup.rs:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Then add the WASM target:

rustup target add wasm32-wasip2

1. Create the project

cargo new --lib weather-tool
cd weather-tool

Replace the generated Cargo.toml with:

Cargo.toml

[package]
name = "weather-tool"
version = "0.1.0"
edition = "2021"
description = "Weather information tool for IronClaw (WASM component)"

[lib]
crate-type = ["cdylib"]

[dependencies]
wit-bindgen = "=0.36"
serde = { version = "1", features = ["derive"] }
serde_json = "1"

[profile.release]
opt-level = "s"
lto = true
strip = true
codegen-units = 1

[workspace]

crate-type = ["cdylib"] tells Cargo to produce a dynamic library — the format WASM components require. [workspace] stops Cargo from merging this crate into a parent workspace.

2. Wire up the WIT interface

Every IronClaw tool is a WASM component that implements a WIT interface. The host provides HTTP, logging, and workspace capabilities; your tool exports execute, schema, and description. Replace src/lib.rs with the following skeleton:

src/lib.rs

wit_bindgen::generate!({
    world: "sandboxed-tool",
    path: "../../wit/tool.wit", // path relative to your Cargo.toml
});

use serde::{Deserialize, Serialize};

struct WeatherTool;

impl exports::near::agent::tool::Guest for WeatherTool {
    fn execute(req: exports::near::agent::tool::Request) -> exports::near::agent::tool::Response {
        match execute_inner(&req.params) {
            Ok(result) => exports::near::agent::tool::Response {
                output: Some(result),
                error: None,
            },
            Err(e) => exports::near::agent::tool::Response {
                output: None,
                error: Some(e),
            },
        }
    }

    fn schema() -> String {
        SCHEMA.to_string()
    }

    fn description() -> String {
        "Get weather information using Open-Meteo (no API key required). \
         Supports three actions: 'get_current' returns current weather conditions \
         for a city; 'get_forecast' returns a 5-day daily forecast; \
         'get_air_quality' returns air pollution data for given coordinates."
            .to_string()
    }
}

export!(WeatherTool);

execute_inner is where the real logic lives — you will fill it in next.

The wit/tool.wit file ships with IronClaw. If you are building inside the IronClaw repo (e.g. under tools-src/my-tool/), the path ../../wit/tool.wit is correct. If you are building in a standalone directory, copy wit/tool.wit from the repo root and adjust the path accordingly.

If your tool uses private credentials (API keys, OAuth tokens), you still keep the same WIT interface. Secret handling is declared in *.capabilities.json and injected by the host at runtime. Your WASM tool should not ask the model for secrets in params.

3. Define the Execute Logic

The tool will receive parameters provided by the LLM in JSON format, then execute the right logic based on those parameters and return a result also in JSON format.

src/lib.rs

#[derive(Debug, Deserialize)]
#[serde(tag = "action", rename_all = "snake_case")]
enum Action {
    GetCurrent(WeatherParams),
    GetForecast(WeatherParams),
    GetAirQuality(AirQualityParams),
}

#[derive(Debug, Deserialize)]
struct WeatherParams {
    city: String,
    #[serde(default)]
    country_code: Option<String>,
    #[serde(default)]
    units: Option<String>, // "metric" (default) or "imperial"
}

#[derive(Debug, Deserialize)]
struct AirQualityParams {
    lat: f64,
    lon: f64,
}

fn execute_inner(params: &str) -> Result<String, String> {
    let action: Action =
        serde_json::from_str(params).map_err(|e| format!("Invalid parameters: {e}"))?;

    match action {
        Action::GetCurrent(p)    => get_current(p),
        Action::GetForecast(p)   => get_forecast(p),
        Action::GetAirQuality(p) => get_air_quality(p),
    }
}

Remember to match the action names and parameter structure in the JSON schema you will define later. The LLM relies on that schema to know what JSON to send, so if your Rust code expects country_code but the schema calls it country, the LLM won’t know to include it and you’ll get errors at runtime.

4. Implement the Actions

We will now implement the three actions: get_current, get_forecast, and get_air_quality. Each action will call the appropriate Open-Meteo API endpoint, parse the response, and return a JSON string with the relevant information.

Handling authenticated APIs

If your API needs a secret (for example a bearer token), you do not inject it in these Rust functions manually.Instead you will declare them in the capabilities file and let the host inject them at runtime.Your Rust code just calls api_get(...) with the right URL and headers, and the host adds credentials automatically for allowlisted hosts.You can still check for the presence of secrets if you want to return a custom error message when credentials are missing:

if !near::agent::host::secret_exists("example_api_token") {
        return Err("Missing secret: example_api_token. Run: ironclaw tool auth <tool-name>".into());
}

Geocoding helper

Open-Meteo needs coordinates, not city names. Add a helper that calls the free geocoding API:

src/lib.rs

#[derive(Debug, Deserialize)]
struct GeoResult {
    latitude: f64,
    longitude: f64,
    name: String,
    country: String,
}

fn geocode(city: &str, country_code: Option<&str>) -> Result<GeoResult, String> {
    let mut url = format!(
        "https://geocoding-api.open-meteo.com/v1/search?name={}&count=1&language=en&format=json",
        url_encode(city)
    );
    if let Some(cc) = country_code {
        if !cc.is_empty() {
            url.push_str(&format!("&countryCode={}", url_encode(cc)));
        }
    }

    near::agent::host::log(
        near::agent::host::LogLevel::Info,
        &format!("Geocoding: {city}"),
    );

    let resp = api_get(&url)?;
    let data: serde_json::Value =
        serde_json::from_str(&resp).map_err(|e| format!("Failed to parse geocoding: {e}"))?;

    let results = data["results"]
        .as_array()
        .ok_or_else(|| format!("City not found: {city}"))?;

    if results.is_empty() {
        return Err(format!("City not found: {city}"));
    }

    let r = &results[0];
    Ok(GeoResult {
        latitude:  r["latitude"].as_f64().unwrap_or(0.0),
        longitude: r["longitude"].as_f64().unwrap_or(0.0),
        name:      r["name"].as_str().unwrap_or(city).to_string(),
        country:   r["country"].as_str().unwrap_or("").to_string(),
    })
}

near::agent::host::log emits a structured log line visible in ironclaw output. The host collects all log entries and flushes them after the call completes.

API helper

src/lib.rs

fn api_get(url: &str) -> Result<String, String> {
    let headers = serde_json::json!({
        "Accept": "application/json",
        "User-Agent": "IronClaw-Weather-Tool/0.1"
    }).to_string();

    let resp = near::agent::host::http_request("GET", url, &headers, None, None)
        .map_err(|e| format!("HTTP request failed: {e}"))?;

    if resp.status < 200 || resp.status >= 300 {
        return Err(format!("API error (HTTP {}): {}", resp.status,
            String::from_utf8_lossy(&resp.body)));
    }

    String::from_utf8(resp.body).map_err(|e| format!("Invalid UTF-8 response: {e}"))
}

fn url_encode(s: &str) -> String {
    let mut out = String::with_capacity(s.len() * 2);
    for b in s.bytes() {
        match b {
            b'A'..=b'Z' | b'a'..=b'z' | b'0'..=b'9' | b'-' | b'_' | b'.' | b'~' => {
                out.push(b as char);
            }
            b' ' => out.push_str("%20"),
            _ => {
                out.push('%');
                out.push(char::from(b"0123456789ABCDEF"[(b >> 4) as usize]));
                out.push(char::from(b"0123456789ABCDEF"[(b & 0xf) as usize]));
            }
        }
    }
    out
}

fn wmo_description(code: u32) -> String {
    match code {
        0  => "Clear sky",
        1  => "Mainly clear",
        2  => "Partly cloudy",
        3  => "Overcast",
        45 => "Fog",
        51 => "Light drizzle",
        61 => "Slight rain",
        63 => "Moderate rain",
        65 => "Heavy rain",
        71 => "Slight snow",
        73 => "Moderate snow",
        75 => "Heavy snow",
        80 => "Slight rain showers",
        95 => "Thunderstorm",
        _  => "Unknown",
    }.to_string()
}

fn european_aqi_label(aqi: u32) -> String {
    match aqi {
        0..=20  => "Good",
        21..=40 => "Fair",
        41..=60 => "Moderate",
        61..=80 => "Poor",
        81..=100 => "Very Poor",
        _        => "Extremely Poor",
    }.to_string()
}

Get current weather

fn get_current(params: WeatherParams) -> Result<String, String> {
    if params.city.is_empty() {
        return Err("'city' must not be empty".into());
    }

    let geo = geocode(&params.city, params.country_code.as_deref())?;
    let units     = params.units.as_deref().unwrap_or("metric");
    let temp_unit = if units == "imperial" { "fahrenheit" } else { "celsius" };
    let wind_unit = if units == "imperial" { "mph" } else { "ms" };

    let url = format!(
        "https://api.open-meteo.com/v1/forecast\
         ?latitude={}&longitude={}\
         &current=temperature_2m,apparent_temperature,relative_humidity_2m,\
         weather_code,wind_speed_10m\
         &temperature_unit={}&wind_speed_unit={}",
        geo.latitude, geo.longitude, temp_unit, wind_unit
    );

    let resp = api_get(&url)?;
    let data: serde_json::Value =
        serde_json::from_str(&resp).map_err(|e| format!("Failed to parse response: {e}"))?;

    let current = &data["current"];
    let output = CurrentWeatherOutput {
        city:        geo.name,
        country:     geo.country,
        temperature: current["temperature_2m"].as_f64().unwrap_or(0.0),
        feels_like:  current["apparent_temperature"].as_f64().unwrap_or(0.0),
        humidity:    current["relative_humidity_2m"].as_u64().unwrap_or(0) as u32,
        description: wmo_description(current["weather_code"].as_u64().unwrap_or(0) as u32),
        wind_speed:  current["wind_speed_10m"].as_f64().unwrap_or(0.0),
        units:       units.to_string(),
    };

    serde_json::to_string(&output).map_err(|e| format!("Serialization error: {e}"))
}

Get forecast

src/lib.rs

fn get_forecast(params: WeatherParams) -> Result<String, String> {
    if params.city.is_empty() {
        return Err("'city' must not be empty".into());
    }

    let geo       = geocode(&params.city, params.country_code.as_deref())?;
    let units     = params.units.as_deref().unwrap_or("metric");
    let temp_unit = if units == "imperial" { "fahrenheit" } else { "celsius" };
    let wind_unit = if units == "imperial" { "mph" } else { "ms" };

    let url = format!(
        "https://api.open-meteo.com/v1/forecast\
         ?latitude={}&longitude={}\
         &daily=temperature_2m_max,temperature_2m_min,weather_code,\
         precipitation_probability_max\
         &temperature_unit={}&wind_speed_unit={}&forecast_days=5",
        geo.latitude, geo.longitude, temp_unit, wind_unit
    );

    let resp  = api_get(&url)?;
    let data: serde_json::Value =
        serde_json::from_str(&resp).map_err(|e| format!("Failed to parse response: {e}"))?;

    let daily    = &data["daily"];
    let times    = daily["time"].as_array().cloned().unwrap_or_default();
    let temp_max = daily["temperature_2m_max"].as_array().cloned().unwrap_or_default();
    let temp_min = daily["temperature_2m_min"].as_array().cloned().unwrap_or_default();
    let codes    = daily["weather_code"].as_array().cloned().unwrap_or_default();
    let precip   = daily["precipitation_probability_max"].as_array().cloned().unwrap_or_default();

    let entries = times.iter().enumerate().map(|(i, t)| ForecastEntry {
        date:        t.as_str().unwrap_or("").to_string(),
        temp_max:    temp_max.get(i).and_then(|v| v.as_f64()).unwrap_or(0.0),
        temp_min:    temp_min.get(i).and_then(|v| v.as_f64()).unwrap_or(0.0),
        description: wmo_description(codes.get(i).and_then(|v| v.as_u64()).unwrap_or(0) as u32),
        precipitation_probability_max: precip.get(i).and_then(|v| v.as_u64()).unwrap_or(0) as u32,
    }).collect();

    let output = ForecastOutput { city: geo.name, country: geo.country, units: units.to_string(), entries };
    serde_json::to_string(&output).map_err(|e| format!("Serialization error: {e}"))
}

Get air quality

src/lib.rs

fn get_air_quality(params: AirQualityParams) -> Result<String, String> {
    if params.lat < -90.0 || params.lat > 90.0 {
        return Err(format!("'lat' must be -90..90, got {}", params.lat));
    }
    if params.lon < -180.0 || params.lon > 180.0 {
        return Err(format!("'lon' must be -180..180, got {}", params.lon));
    }

    let url = format!(
        "https://air-quality-api.open-meteo.com/v1/air-quality\
         ?latitude={}&longitude={}\
         &current=pm10,pm2_5,european_aqi",
        params.lat, params.lon
    );

    let resp = api_get(&url)?;
    let data: serde_json::Value =
        serde_json::from_str(&resp).map_err(|e| format!("Failed to parse response: {e}"))?;

    let current = &data["current"];
    let aqi     = current["european_aqi"].as_u64().unwrap_or(0) as u32;

    let output = AirQualityOutput {
        lat:           params.lat,
        lon:           params.lon,
        european_aqi:  aqi,
        aqi_label:     european_aqi_label(aqi),
        pm2_5:         current["pm2_5"].as_f64().unwrap_or(0.0),
        pm10:          current["pm10"].as_f64().unwrap_or(0.0),
    };

    serde_json::to_string(&output).map_err(|e| format!("Serialization error: {e}"))
}

5. Define the JSON schema

The SCHEMA constant tells the LLM exactly what JSON to send. Use oneOf because the three actions have different required fields:

src/lib.rs

const SCHEMA: &str = r#"{
    "oneOf": [
        {
            "type": "object",
            "description": "Get current weather conditions for a city",
            "properties": {
                "action":       { "type": "string", "const": "get_current" },
                "city":         { "type": "string", "description": "City name, e.g. 'Tokyo'" },
                "country_code": { "type": "string", "description": "ISO 3166-1 alpha-2 code, e.g. 'JP'" },
                "units":        { "type": "string", "enum": ["metric", "imperial"] }
            },
            "required": ["action", "city"],
            "additionalProperties": false
        },
        {
            "type": "object",
            "description": "Get a 5-day daily weather forecast for a city",
            "properties": {
                "action":       { "type": "string", "const": "get_forecast" },
                "city":         { "type": "string" },
                "country_code": { "type": "string" },
                "units":        { "type": "string", "enum": ["metric", "imperial"] }
            },
            "required": ["action", "city"],
            "additionalProperties": false
        },
        {
            "type": "object",
            "description": "Get air quality data for a location by coordinates",
            "properties": {
                "action": { "type": "string", "const": "get_air_quality" },
                "lat":    { "type": "number", "description": "Latitude (-90 to 90)" },
                "lon":    { "type": "number", "description": "Longitude (-180 to 180)" }
            },
            "required": ["action", "lat", "lon"],
            "additionalProperties": false
        }
    ]
}"#;

6. Declare capabilities

Create weather-tool.capabilities.json next to Cargo.toml. This file is the sandbox allowlist — any host not listed here is blocked at runtime:

weather-tool.capabilities.json

{
  "version": "0.1.0",
  "wit_version": "0.3.0",
  "http": {
    "allowlist": [
      {
        "host": "geocoding-api.open-meteo.com",
        "path_prefix": "/v1/",
        "methods": ["GET"]
      },
      {
        "host": "api.open-meteo.com",
        "path_prefix": "/v1/",
        "methods": ["GET"]
      },
      {
        "host": "air-quality-api.open-meteo.com",
        "path_prefix": "/v1/",
        "methods": ["GET"]
      }
    ],
    "rate_limit": {
      "requests_per_minute": 60,
      "requests_per_hour": 500
    },
    "timeout_secs": 15
  }
}

The weather tool needs three hosts because get_current and get_forecast make two requests each: one to geocode the city name and one to fetch the weather data.

7. Add secrets and auth (for tools that need credentials)

This weather tool uses Open-Meteo, so it does not need a secret. If your tool calls an API that needs a token, declare that in the capabilities file so IronClaw can inject it at request time. Example capability sections (pattern used in tools-src/* on the IronClaw repo):

weather-tool.capabilities.json

{
    "http": {
        "allowlist": [
            {
                "host": "api.example.com",
                "path_prefix": "/v1/",
                "methods": ["GET", "POST"]
            }
        ],
        "credentials": {
            "example_api_token": {
                "secret_name": "example_api_token",
                "location": { "type": "bearer" },
                "host_patterns": ["api.example.com"]
            }
        }
    },
    "secrets": {
        "allowed_names": ["example_api_token"]
    },
    "auth": {
        "secret_name": "example_api_token",
        "display_name": "Example API",
        "instructions": "Create an API token in your provider dashboard",
        "setup_url": "https://example.com/settings/api",
        "token_hint": "Starts with 'ex_'",
        "env_var": "EXAMPLE_API_TOKEN"
    }
}

How this works:

http.credentials maps a stored secret to where it should be injected (bearer, custom header, query param, or URL placeholder).
secrets.allowed_names lets the tool check secret presence with near::agent::host::secret_exists(...).
auth tells IronClaw how to collect credentials.

After installing the tool, run auth once:

ironclaw tool auth <tool-name>

Auth flow priority is:

Use auth.env_var if it is set in your environment.
Use OAuth if auth.oauth is configured.
Fall back to manual token entry using instructions and setup_url.

If your capabilities include setup.required_secrets (for example OAuth client id/client secret fields), run setup as well:

ironclaw tool setup <tool-name>

This keeps credentials outside agent-visible prompts and lets the host inject them only where allowlisted.

8. Build and install

cargo build --target wasm32-wasip2 --release

ironclaw tool install ./target/wasm32-wasip2/release/weather_tool.wasm \
  --capabilities ./weather-tool.capabilities.json \
  --name weather-tool

Verify it loaded:

ironclaw tool list

If your tool defines secret variables, authenticate now:

ironclaw tool auth <tool-name>

If your tool defines setup.required_secrets, run:

ironclaw tool setup <tool-name>

Try it out

Start IronClaw and ask your agent:

“What’s the weather in Buenos Aires?”
“Give me a 5-day forecast for London, GB in imperial units.”
“What’s the air quality at coordinates 35.6762, 139.6503?”

The agent resolves the right action from the schema and calls the tool automatically.

Capabilities

Channels

Extensions (Tools)

Tutorials

How to build a tool

weather-tool source

Prerequisites

1. Create the project

2. Wire up the WIT interface

3. Define the Execute Logic

4. Implement the Actions

Geocoding helper

API helper

Get current weather

Get forecast

Get air quality

5. Define the JSON schema

6. Declare capabilities

7. Add secrets and auth (for tools that need credentials)

8. Build and install

Try it out

Capabilities

Channels

Extensions (Tools)

Tutorials

weather-tool source

​Prerequisites

​1. Create the project

​2. Wire up the WIT interface

​3. Define the Execute Logic

​4. Implement the Actions

​Geocoding helper

​API helper

​Get current weather

​Get forecast

​Get air quality

​5. Define the JSON schema

​6. Declare capabilities

​7. Add secrets and auth (for tools that need credentials)

​8. Build and install

​Try it out

Prerequisites

1. Create the project

2. Wire up the WIT interface

3. Define the Execute Logic

4. Implement the Actions

Geocoding helper

API helper

Get current weather

Get forecast

Get air quality

5. Define the JSON schema

6. Declare capabilities

7. Add secrets and auth (for tools that need credentials)

8. Build and install

Try it out