% sections/related_work.tex

\section{Related Work}
\label{sec:related_work}

\subsection{LLM Routing and Model Selection}

\noindent\textbf{Binary routing.}
RouteLLM~\cite{ong2024routellm} pioneered preference-data-driven routing between a strong and weak model, training BERT, MLP, and causal LLM classifiers to estimate query difficulty.
Our work extends this to multi-model, multi-signal routing with per-decision plugin chains.

\noindent\textbf{Contrastive selection.}
RouterDC~\cite{chen2024routerdc} learns shared query-model embeddings via dual contrastive learning.
We integrate RouterDC as one of thirteen selection algorithms and extend it with signal-conditioned features (domain category, complexity).

\noindent\textbf{Cascading.}
AutoMix~\cite{aggarwal2023automix} formulates model cascading as a POMDP with self-verification.
We integrate AutoMix within our plugin-aware framework, where safety checks and caching can prevent unnecessary escalation.

\noindent\textbf{Benchmarking.}
RouterBench~\cite{hu2024routerbench} proposed a benchmark for multi-LLM routing with hybrid scoring.
Our Hybrid selector builds on this approach.

\noindent\textbf{RL-based routing.}
Router-R1~\cite{zhang2025routerr1} formulates multi-LLM routing as a sequential decision process, using rule-based reward RL for multi-round routing and aggregation.
GMTRouter~\cite{xie2025gmtrouter} uses graph-based learning for personalized multi-turn interactions.
We integrate both within the unified selection interface and extend them with the ReMoM multi-round reasoning strategy.

A key distinction of our work is that prior approaches address model selection in isolation, while we embed selection within a composable signal orchestration framework that also handles signal extraction, safety enforcement, caching, context augmentation, and multi-provider routing---enabling the same selection algorithms to serve fundamentally different deployment scenarios through configuration.

\subsection{Multi-Provider Backend Routing}

Commercial LLM gateway products (OpenRouter, AWS Bedrock, Azure AI Studio) provide multi-provider access but lack the composable signal-driven routing that enables differentiated policies per routing decision.
API management platforms (Kong, Apigee) offer gateway functionality but are not designed for semantic analysis of LLM requests.
Our system uniquely combines semantic model selection with multi-provider protocol abstraction, a pluggable authorization factory, and full OpenAI Responses API support for stateful conversations within the same composable framework.

\subsection{Mixture-of-Experts vs.\ Mixture-of-Models}

Sparse Mixture-of-Experts (MoE)~\cite{shazeer2017moe,jiang2024mixtral} routes \emph{tokens} to specialized sub-networks \emph{within} a single model architecture.
Our system operates at the \emph{request level}, routing entire requests across \emph{different model deployments}---a Mixture-of-Models (MoM) approach.
The two paradigms are complementary: our router can route to MoE models as backends.

\subsection{LLM Safety}

Prompt injection defenses range from perplexity filtering and input preprocessing~\cite{jain2023baseline} to fine-tuned safety classifiers~\cite{inan2023llamaguard}.
PII detection systems~\cite{lison2021anonymisation} identify sensitive information using rule-based and ML approaches.
Our safety subsystem integrates both within the routing pipeline with per-decision thresholds and policies, using LoRA adapters for memory-efficient multi-task classification.

\subsection{Hallucination Detection}

SelfCheckGPT~\cite{manakul2023selfcheckgpt} detects hallucinations via multi-sample consistency.
FActScore~\cite{min2023factscore} evaluates factual precision at the atomic fact level.
HaluGate differs in three respects:
(1)~a gating Sentinel that skips verification for non-factual queries;
(2)~token-level span identification rather than sentence-level scoring;
(3)~NLI-based explanation distinguishing contradiction from neutral unsupported content.

\subsection{Semantic Caching and RAG}

Semantic caching for LLMs uses embedding similarity to match incoming queries against previously seen requests, avoiding redundant model invocations.
Our cache extends this with per-decision policies, multiple backends, and integration with the safety pipeline (cache lookups occur \emph{after} safety checks but \emph{before} model invocation).
RAG integration~\cite{lewis2020rag,wang2021milvus} augments responses with retrieved context; our contribution is embedding RAG as a per-decision plugin within the routing framework.
