prompt-args

name	prompt-args
description	JOB-254 — PromptArgs pitfalls, required key validation, and typed input patterns.

PromptArgs is HashMap. Missing keys cause runtime panics. Always match prompt_args! keys exactly to template_fstring! variable names. ## Type Definition

// 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.

## prompt_args! Macro

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.

## Chain Input Key Contracts

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

Chain	Required keys
LLMChain	whatever variables are in the prompt template
ConversationalChain	"input"
ConversationalRetrieverChain	"input"
AgentExecutor	"input" (hardcoded)

Use chain.get_input_keys() to inspect at runtime:

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

## Validating Keys Before Calling (current best practice)

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(())
}

## Common Mistake: Key Mismatch

// 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?" };