LiteLLM for Go. A single provider-agnostic client over Anthropic, OpenAI, Gemini, and the rest — with a router, fallback, cost tracking, and capability flags on top.

llmgate is the LLM gateway that vectorless-engine depends on, extracted into its own module so anything written in Go can use it. It is not a rewrite of LiteLLM. It sits on top of tmc/langchaingo — langchaingo handles every provider's wire protocol, llmgate wraps that behind a tiny Client interface and adds the production features langchaingo deliberately doesn't include.

The caller holds a Client. Middlewares (retry.New, budget.New, cache.New) wrap the client; a router.New can sit anywhere in the chain to fall over between providers. The private internal/adapter is the single seam where llmgate's interface meets langchaingo's provider implementations — one adapter serves all three providers.

Order matters. The outermost wrapper sees the call first; the innermost hits the network. Put cache.New below budget.New so cache hits don't burn budget. Put retry.New on top so retries run regardless of which inner layer tripped.

A call threads through the stack, optionally falls over to a backup provider, and comes back with Usage.{InputTokens, OutputTokens, TotalTokens, CostUSD} populated — no extra call, computed from a static price table.

The Go ecosystem has two extremes:

• Vendor SDKs (openai-go, anthropic-sdk-go) — great typing, no portability. Swap providers, rewrite your call site.
• Thin wrappers — portable, but you lose cost, retries, fallbacks, and capability introspection.

llmgate is the middle layer. One interface. Every provider behind it. All the production concerns — router, fallback on rate-limit, cost per call, capability flags — baked in rather than bolted on.

Early code. Interface is the stable surface; implementations evolve underneath. Roadmap lives in ROADMAP.md.

What's in now:

• Client interface with Complete + CountTokens

• Anthropic, OpenAI, Gemini — all backed by langchaingo/llms, in the provider/ subpackages

• A single internal adapter; add a provider = add a ~30-line file

• retry.New middleware for exp-backoff on transient errors

• Cost tracking via a static pricing table + Usage.CostUSD on every Response

• Capability flags (MaxContext, SupportsJSONMode, SupportsStreaming, SupportsTools, SupportsVision) with a capabilities.Capable interface

• router.New with per-provider fallback (router.OnRateLimit, router.OnTransient, or a custom router.FallbackPolicy)

• budget.New middleware with daily + total USD caps and UTC rollover

• cache.New middleware — in-memory LRU keyed on request shape, optional TTL

• Error classification (Classify, IsRateLimited, IsTransient, IsAuth) that the retry predicate and router policies use to decide what to retry or fall over on

• Mock client with call recording for tests

• Native tool calling (Request.Tools, Response.ToolCalls) across all three providers

Coming next:

• Streaming (Streamer is declared; no provider implements it yet)
• Native count_tokens via each provider's counting endpoint
• Anthropic prompt caching and OpenAI strict structured outputs, both of which need a native client rather than the langchaingo path

go get github.com/hallelx2/llmgate

package main

import (
    "context"
    "fmt"
    "os"

    "github.com/hallelx2/llmgate"
    "github.com/hallelx2/llmgate/middleware/retry"
    "github.com/hallelx2/llmgate/provider/anthropic"
)

func main() {
    client, err := anthropic.New(anthropic.Config{
        APIKey: os.Getenv("ANTHROPIC_API_KEY"),
        Model:  "claude-sonnet-4-5",
    })
    if err != nil {
        panic(err)
    }

    // Optional: wrap in exponential-backoff retry middleware.
    client = retry.New(retry.Config{MaxRetries: 3})(client)

    resp, err := client.Complete(context.Background(), llmgate.Request{
        Messages: []llmgate.Message{
            {Role: llmgate.RoleUser, Content: "In one sentence: what is vectorless retrieval?"},
        },
        MaxTokens: 256,
    })
    if err != nil {
        panic(err)
    }

    fmt.Println(resp.Content)
}

Every Response carries a Usage with the token breakdown split by billing tier, not just a prompt/completion pair:

resp.Usage.InputTokens      // uncached prompt tokens
resp.Usage.CacheWriteTokens // written to the prompt cache (1.25x on Anthropic)
resp.Usage.CacheReadTokens  // served from cache (0.1x Anthropic, 0.5x OpenAI, 0.25x Google)
resp.Usage.ReasoningTokens  // thinking tokens — a subset of OutputTokens
resp.Usage.CostUSD

The providers disagree about whether cached tokens are counted inside the prompt total — Anthropic reports them alongside, OpenAI and Google report them within — so llmgate normalizes to the disjoint form. InputTokens + CacheWriteTokens + CacheReadTokens is the whole prompt on every provider.

Three flags say how much to trust the number:

That distinction matters: a CostUSD of 0 with Priced true asserts the call was free, which it never is. When a provider reports no usage at all, llmgate estimates from a tokenizer and flags it rather than reporting zero.

Lookups resolve dated, prefixed, and gateway-qualified IDs to their base model, so none of these price at $0:

claude-sonnet-4-5-20250929        -> claude-sonnet-4-5
models/gemini-2.5-flash           -> gemini-2.5-flash
us.anthropic.claude-opus-4-1-v1:0 -> claude-opus-4-1
z-ai/glm-4.6                      -> glm-4.6

Rates are keyed by model ID alone, never by provider — a model served through a gateway that speaks another vendor's protocol (GLM over an Anthropic-compatible endpoint, say) still prices correctly.

The compiled-in table drifts as vendors change rates. No vendor publishes machine-readable pricing, so UseRemote layers a community aggregate over it:

stop, err := pricing.UseRemote(ctx, pricing.RemoteConfig{
    CacheDir: "/var/cache/llmgate", // survive restarts
    OnError:  func(src string, err error) { log.Warn("price refresh", "src", src, "err", err) },
})
defer stop()

Sources default to LiteLLM's price table then OpenRouter's model API, both public and unauthenticated.

This is opt-in and stays that way — importing pricing performs no network I/O. Resolution is Register overrides → remote snapshot → embedded table, and it fails open at every layer: lookups never block on the network, a failed fetch keeps the previous snapshot, and a snapshot whose rates have drifted more than 10x from the embedded values is rejected as corrupt rather than adopted. pricing.AsOf() reports the vintage of whatever is loaded.

• The interface is tiny. Complete, CountTokens, later Stream, later Capabilities. If it doesn't fit, it goes in a middleware, not the interface.
• Middleware over inheritance. Retries, caching, cost tracking, rate-limiting — all func(Client) Client wrappers. Compose them.
• No magic config. No viper, no auto-reload, no remote backends. Pass a struct, get a client.
• Provider-specific features are honoured where they matter. Anthropic prompt caching, OpenAI structured outputs, Gemini long context — opt in via config, not via a lowest-common-denominator API.
• Pure IO-bound code. Parallelism is always network-bound. errgroup + semaphore, no worker pools.

See ROADMAP.md for what is shipped and what is next.

Apache 2.0. See LICENSE.