arc-dyn-llm

name	arc-dyn-llm
description	JOB-251 — Why Arc<dyn LLM> over LLMClone, migration pattern, and correct LLM ownership model.

The LLMClone supertrait is an anti-pattern. Prefer Arc for shared ownership — Arc is Clone by definition, no extra trait machinery needed. ## Current Problem (JOB-251)

LLMClone exists solely because dyn LLM is not Clone:

// CURRENT — anti-pattern
pub trait LLMClone {
    fn clone_box(&self) -> Box<dyn LLM>;
}
pub trait LLM: Sync + Send + LLMClone { ... }

// Used only in two places:
let stuff_chain = StuffDocumentBuilder::new().llm(llm.clone_box());
let condense_chain = CondenseQuestionGeneratorChain::new(llm.clone_box());

clone_box() allocates a new Box on every call. Every backend must also derive(Clone).

## Preferred Pattern: Arc

use std::sync::Arc;
use langchainx::language_models::llm::LLM;

// Cheap clone — just an atomic ref count increment
let llm: Arc<dyn LLM> = Arc::new(Claude::new());
let llm2 = Arc::clone(&llm);  // no Box allocation, no clone_box()

// Both chains share the same LLM instance
let stuff_chain = StuffDocumentBuilder::new().llm(Arc::clone(&llm));
let condense_chain = CondenseQuestionGeneratorChain::new(Arc::clone(&llm));

## Current Workaround (until JOB-251 is resolved)

clone_box() still works today. If you must clone an LLM to pass to two builders:

// Acceptable today — will be removed when JOB-251 is implemented
let llm: Box<dyn LLM> = Box::new(OpenAI::default());
let stuff = StuffDocumentBuilder::new().llm(llm.clone_box()).build()?;
let condense = CondenseQuestionGeneratorChain::new(llm.clone_box());

Do NOT add new code that introduces new clone_box() call sites. Prefer Arc.

## Migration Checklist (implementing JOB-251)

1. Remove LLMClone supertrait from LLM definition
2. Remove blanket impl<T: LLM + Clone> LLMClone for T
3. Change Box<dyn LLM> fields → Arc<dyn LLM> in all chain structs
4. Change From<L> for Box<dyn LLM> → From<L> for Arc<dyn LLM>
5. Replace all llm.clone_box() call sites with Arc::clone(&llm)
6. Update builder .llm() method signatures to accept impl Into<Arc<dyn LLM>>