shunt

Shunt Claude Code to any model.

shunt is a spec-compliant Claude Code LLM gateway: a transparent proxy that, for the models you map, diverts inference to another LLM provider at the inference layer. It routes by the request's model id — everything else passes through to Anthropic unchanged (the "shunt").

The name is the mechanism: an electrical/railway shunt diverts a selected part of the flow onto a parallel path. Here, a mapped model's inference is diverted to another provider while Claude Code's tools and skills stay intact.

Phase 1 target: OpenAI / Codex / ChatGPT — translate Anthropic Messages ⇄ the OpenAI Responses API, over an OpenAI API key or a reused ChatGPT (codex login) subscription.

Status: private, early. May be open-sourced later. See docs/running.md to build, configure, and connect Claude Code, and docs/implementation-plan.md for the design and milestones.

Why

Claude Code sends every turn to the Anthropic API. shunt sits in front (via ANTHROPIC_BASE_URL) and, for the models you map, diverts their inference to another provider (OpenAI, Codex/ChatGPT, …). Because routing happens at the HTTP/inference layer — not by handing the task off to a different CLI — the session keeps running inside Claude Code's harness: same tool loop, same preloaded skills, same bundled-script path resolution. Only token generation is outsourced.

Contrast with the alternative approach (handing a subagent_type off to another runtime like Codex CLI), which cuts higher in the stack and drops persona and preloaded skills.

Per-model, not per-agent — and not a global swap

Selectivity is driven by the model id on each request, which Claude Code already lets you choose per context: the /model picker for the main session, a subagent definition's model: frontmatter, CLAUDE_CODE_SUBAGENT_MODEL for all subagents, or ANTHROPIC_CUSTOM_MODEL_OPTION to add a custom entry to the picker. So "divert only this agent / this session" is decided in Claude Code, and shunt just honors the model id it receives — no fragile per-agent system-prompt fingerprinting. Unlike the global model-swap proxies below, the main session can stay on Claude while only the models you name divert.

Related work / prior art

Claude Code–specific routers & proxies

• musistudio/claude-code-router — the largest in this niche; use Claude Code as a foundation and decide how requests reach different models/providers.
• 1rgs/claude-code-proxy — run Claude Code on OpenAI models.
• fuergaosi233/claude-code-proxy — Claude Code → OpenAI API proxy.
• seifghazi/claude-code-proxy — captures/visualizes in-flight Claude Code requests, with optional per-agent routing to other providers (the direct inspiration for shunt's subagent-routing idea).
• luohy15/y-router — a simple proxy enabling Claude Code to work with OpenRouter.
• tingxifa/claude_proxy — Cloudflare Workers proxy translating Claude API requests to OpenAI format (Gemini, Groq, Ollama).
• badlogic/claude-bridge — use any model provider with Claude Code.
• jimmc414/claude_n_codex_api_proxy — cross-runtime router: proxies Anthropic or OpenAI API calls to the local Claude Code or Codex CLI (routes to the local CLI when the API key is all 9s, else the real cloud API). Note the inverse direction — routing cloud-API calls to local CLIs, rather than routing Claude Code agents out to cloud providers.
• insightflo/chatgpt-codex-proxy — Anthropic-compatible /v1/messages proxy that serves Claude Code inference from the ChatGPT Codex backend (uses a ChatGPT Plus/Pro subscription instead of an API key). Same inference-layer swap as shunt, targeting the Codex/GPT subscription backend while keeping Claude Code's UI and MCP tools.

General AI gateways (adjacent infrastructure — possible backends)

• BerriAI/litellm — SDK + proxy/AI gateway calling 100+ LLM APIs in OpenAI format, with cost tracking, guardrails, load balancing.
• Portkey-AI/gateway — fast AI gateway routing to 1,600+ LLMs with integrated guardrails.
• maximhq/bifrost — high-performance AI gateway with adaptive load balancing and 1000+ model support.
• mazori-ai/modelgate — open-source LLM gateway + MCP server (Go): RBAC/policy enforcement, multi-provider (OpenAI, Anthropic, Gemini, Bedrock, Azure, and local Ollama), an MCP gateway with semantic tool search, and semantic response caching.

How shunt differs

Most Claude Code proxies above route all traffic to one alternative provider (a global model swap). shunt's focus is selective, per-model diversion driven by the request's model id: keep the main session on Claude, and shunt only the models you name onto other providers — the switchboard/patchbay use case. Because Claude Code already lets you bind a model per context (main session, subagent model: frontmatter, CLAUDE_CODE_SUBAGENT_MODEL), that same selectivity reaches down to individual agents without shunt ever inspecting who the caller is.

Claude Code integration (official surface)

Claude Code exposes a first-class gateway contract behind ANTHROPIC_BASE_URL — shunt should implement this rather than the fragile "hash the subagent's system prompt" heuristic that earlier Claude Code proxies rely on.

• LLM Gateway Protocol — the API contract: endpoints, headers/body fields to forward vs consume, feature pass-through, and attribution. A running gateway serves the machine-readable spec at GET /protocol.
  • Model discovery — Claude Code queries GET /v1/models?limit=1000 at startup (opt-in via CLAUDE_CODE_ENABLE_GATEWAY_MODEL_DISCOVERY=1) and adds returned models to the /model picker. Constraint: entries whose id doesn't begin with claude/anthropic are ignored — non-Claude models must be aliased or added manually.
  • System prompt attribution block — Claude Code prepends a client-version + conversation fingerprint to the system prompt; stable for the conversation lifetime (v2.1.181+). shunt forwards it unchanged (never strips it — that's the developer's call via CLAUDE_CODE_ATTRIBUTION_HEADER=0).
• Add a custom model option — ANTHROPIC_CUSTOM_MODEL_OPTION adds a gateway-routed entry to the /model picker without replacing built-in aliases; the ID skips validation, so any string the gateway accepts works. This is the primary way to select a non-Claude model (e.g. gpt-5.2-codex), since discovery ignores ids that don't begin with claude/anthropic.

Design implication for shunt: be a spec-compliant Anthropic-Messages gateway (/v1/messages, /v1/models, correct header/attribution pass-through), route by the request's model id, and translate Anthropic Messages ⇄ the OpenAI Responses API for mapped models — no prompt-shape heuristics that break on every Claude Code prompt change. See docs/implementation-plan.md.

License

TBD (private for now).