Supported APIs

API compatibility layers, agent harness support, and example requests for the LLM Gateway.

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Overview

The LLM Gateway exposes one gateway URL, https://llm-gateway.aptible.com, and speaks several API dialects from it. Pick whichever matches how you’re already integrating:

• OpenAI-compatible — Chat Completions, Responses, and Embeddings, in both a strict /openai namespace and as un-namespaced defaults
• Anthropic-compatible — Messages, in a strict /anthropic namespace
• Agent harnesses — a dedicated, never-changing base URL per supported coding assistant or agent harness, under /harness/<name>

Every request authenticates with an LLM Key and selects a model by name in the request body — see Supported Models for the current model catalog.

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Authentication

Send your LLM Key as a bearer token:

Authorization: Bearer <YOUR_LLM_KEY>

Anthropic’s SDKs (and Claude Code) instead send an x-api-key header — the gateway accepts either:

x-api-key: <YOUR_LLM_KEY>

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OpenAI-compatible API

Use the /openai namespace for strict OpenAI compatibility:

Endpoint	Path
Chat Completions	POST /openai/v1/chat/completions
Responses	POST /openai/v1/responses
Embeddings	POST /openai/v1/embeddings

The same three endpoints are also available un-namespaced (/v1/chat/completions, /v1/responses, /v1/embeddings) as the gateway’s default dialect — use whichever your SDK or tooling expects.

For full request and response documentation, see OpenAI’s API reference.

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Anthropic-compatible API

Use the /anthropic namespace for Anthropic Messages:

Endpoint	Path
Messages	POST /anthropic/v1/messages
Count Tokens	POST /anthropic/v1/messages/count_tokens

For full request and response documentation, see Anthropic’s API reference.

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Agent harness support

Some coding assistants and agent harnesses need a single base URL they can be pointed at once and never have to edit again — even as the underlying API dialect they speak evolves. The LLM Gateway provides one under /harness/<name>, which resolves internally to the right compatibility endpoint:

Harness	Base URL
Claude Code	https://llm-gateway.aptible.com/harness/claude-code
Claude Desktop	https://llm-gateway.aptible.com/harness/claude-desktop
Grok Build	https://llm-gateway.aptible.com/harness/grok-build
Pi	https://llm-gateway.aptible.com/harness/pi

See the Claude Code guide for a full walkthrough of harness setup.

Don’t see your harness or coding assistant listed? Any tool that speaks OpenAI- or Anthropic-compatible APIs already works against the /openai or /anthropic endpoints above — a dedicated /harness route just saves you from ever having to change the base URL again. Contact us if you’d like a harness added.

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Example requests

Every example below uses the model field to select a model — see Supported Models for valid IDs.

Chat Completions

curl https://llm-gateway.aptible.com/openai/v1/chat/completions \
  -H "Authorization: Bearer $LLM_GATEWAY_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-5",
    "messages": [
      {"role": "user", "content": "Tell me a joke about cloud security"}
    ]
  }'

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Prompt caching

Prompt caching works through the LLM Gateway the same way it does with the provider directly: mark cacheable content with cache_control (Anthropic), or rely on the provider’s automatic caching (OpenAI).

The gateway serves each model from a pool of upstream providers. A provider’s cache only helps requests that reach that same provider, so to keep a conversation’s cache warm, send a session-id (or x-session-id) header that stays the same for the whole conversation:

session-id: <CONVERSATION_ID>

Requests that share a session ID are routed to the same upstream for up to an hour after the last request. Use a new, unique value, such as a UUID, for each conversation or agent run. Without a session ID, requests are spread across upstreams and see more cache misses.

Anthropic SDK (Python)

message = client.messages.create(
    model="claude-sonnet-5",
    max_tokens=1024,
    system=[{"type": "text", "text": LONG_SYSTEM_PROMPT, "cache_control": {"type": "ephemeral"}}],
    messages=messages,
    extra_headers={"session-id": conversation_id},
)

Claude Code, Codex CLI, OpenCode, and Pi already send their own session headers, which the gateway recognizes, so no configuration is needed for them.

A request that fails over to another provider during an outage starts with a cold cache on that provider.

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Automatic provider failover and client-side fallbacks

If a model is available from more than one provider, the gateway automatically fails over to another available provider during an outage — no client changes required. You can also specify your own fallback models in a request; see Supported Models for details on both.