Prompt Args

// src/prompt/mod.rs
pub type PromptArgs = HashMap<String, serde_json::Value>;

It is an alias — not a newtype. There is no compile-time enforcement of required keys.

use langchainx::prompt_args;

let args = prompt_args! {
    "input" => "Hello world",
    "context" => "Some context",
    "count" => 42,
};
// Expands to HashMap::from([("input".to_string(), json!("Hello world")), ...])

Values are converted via serde_json::json!. Any Serialize type works.

Each chain documents its required keys. These are checked only at runtime.

Use chain.get_input_keys() to inspect at runtime:

let keys = chain.get_input_keys();
assert!(keys.contains(&"input".to_string()));

Until JOB-254 is resolved, validate manually:

fn validate_args(chain: &dyn Chain, args: &PromptArgs) -> Result<(), ChainError> {
    for key in chain.get_input_keys() {
        if !args.contains_key(&key) {
            return Err(ChainError::MissingObject(
                format!("missing required input key: '{key}'")
            ));
        }
    }
    Ok(())
}

// Template uses "question" but args use "input"
let prompt = message_formatter![fmt_template!(
    HumanMessagePromptTemplate::new(template_fstring!("{question}", "question"))
)];

// WRONG
let args = prompt_args! { "input" => "What is Rust?" };
// chain.invoke(args) → runtime error: missing variable "question"

// CORRECT
let args = prompt_args! { "question" => "What is Rust?" };