Rig

Rig

Rig (crate rig-core) is a Rust library for building LLM-powered applications: provider clients, agents with tools, structured extractors, RAG pipelines, and streaming completions. This skill covers rig-core 0.40.0, workspace edition 2024, repo-pinned toolchain 1.94.0.

Setup

cargo add rig-core

Feature flags relevant to agent work:

• derive — enables the rig-derive macros (schema derivation for tool/extractor types)
• rmcp — enables MCP (Model Context Protocol) tool integration via the rmcp crate

[dependencies]
rig-core = { version = "0.40", features = ["rmcp"] }

derive and reqwest/rustls ship in rig-core's default feature set; add rmcp explicitly for MCP tools.

AgentBuilder quickstart

Every provider client exposes .agent(model), returning an AgentBuilder:

use rig_core::prelude::*;
use rig_core::providers::anthropic;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = anthropic::Client::from_env()?;

    let agent = client
        .agent(anthropic::completion::CLAUDE_SONNET_4_6)
        .preamble("Answer concisely.")
        .temperature(0.0)
        .build();

    let answer = agent.prompt("What is Rig?").await?;
    println!("{answer}");
    Ok(())
}

from_env() reads provider credentials from environment variables — no API key touches application code. .agent() takes impl Into<String> for the model argument: a provider module constant (anthropic::completion::CLAUDE_SONNET_4_6) or a raw string both work. See references/providers.md for the constants available per provider and the env-var contract each from_env() reads.

Hybrid multi-provider harness

A single Rust process commonly needs several models at once — a cheap model for extraction, a mid-tier model for the main loop, a strong model for review, one of them local. Rig provides the primitives (clients, AgentBuilder, the CompletionModel trait) — it does not ship a built-in multi-tier harness type. The tier-selection pattern in references/providers.md is guidance built on those primitives, not a first-party rig recipe.

The load-bearing constraint: CompletionModel is Clone-bound with no dyn-compatible variant (no Box<dyn CompletionModel>, no CompletionModelDyn). Route between tiers with enum dispatch over concrete Agent<M> types, generic functions bounded by M: CompletionModel, or concrete per-tier fields on your own harness struct — never a trait-object collection of agents. references/providers.md works through this with a worked example spanning Anthropic, OpenAI, Gemini, and a local ollama tier.

12-factor model configuration

Read model IDs from environment variables, never hardcode them in application code:

let model_id = std::env::var("AGENT_MODEL").unwrap_or_else(|_| {
    anthropic::completion::CLAUDE_HAIKU_4_5.to_string()
});
let agent = client.agent(model_id).build();

This applies per tier in a hybrid harness — AGENT_FAST_MODEL, AGENT_MAIN_MODEL, AGENT_REVIEW_MODEL, or equivalent names read at startup, not compiled in. Base URLs follow the same rule: ANTHROPIC_BASE_URL, OPENAI_BASE_URL, OLLAMA_API_BASE_URL are read by from_env() directly, or set explicitly via ClientBuilder::base_url() when a config value comes from somewhere other than the environment.

Reference map

• references/providers.md — per-provider client construction, env vars, model-ID strings vs constants, and the worked multi-provider/hybrid-tier harness example
• references/local-and-gateways.md — ollama (built-in), remote ollama, LM Studio, LiteLLM, Bifrost, and Lemonade Server via the OpenAI-compatible client + base URL, and the local-model typed tool-args gotcha
• references/agents-tools.md — full AgentBuilder surface, the Tool trait, extractors, and MCP integration via the rmcp feature
• references/rag-embeddings.md — EmbeddingModel, vector-store companion crates, and the RAG agent pattern
• references/streaming.md — streaming completions

Adversarial TDD

When implementing rig code under adversarial TDD, the rust plugin's rust-test-author/rust-implementer pair applies unchanged — load this skill into both workers. This skill is fully usable on its own without that pair.

Anti-fabrication

This skill follows core:anti-fabrication. Every client construction pattern, trait bound, and constant cited here is verified against the rig-core 0.40.0 source tree (see sources.md) — not inferred from training-data familiarity with an earlier rig version. Before asserting a trait signature or constant this skill and its references do not cover, check the installed rig-core version's source or docs.rs rather than guessing.