> ## Documentation Index
> Fetch the complete documentation index at: https://docs.neuraltrust.ai/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> These docs cover three products: TrustGate (AI agent gateway), TrustGuard (runtime security), and TrustTest (AI red teaming). Start from each product overview for the definition and How it works. Prefer the .md URL next to a page in /llms.txt when you need the full article. Use /llms-full.txt for a single-file dump of the site.

# Rate Limiter

> Cap how many requests an application, or each end user behind it, may make in a sliding window. Refuse or delay the rest.

Applies to **LLM and MCP**.

A client stuck in a retry loop, a script someone pointed at production, a tenant
that decides to backfill a year of data — to the provider they are all one flood
of requests billed to you, and a rate-limit error for everyone else sharing the
key. The rate limiter stops the flood at the gateway, per application, so one
caller's bug stays that caller's problem.

Counts requests in a sliding window and refuses the ones over the line. You set a
**limit**, a **window** (`30s`, `1m`, `1h`), and the **Retry-After** the refusal
carries — by default the window, which is when the allowance actually returns.
Refused responses carry the standard rate-limit headers so a well-behaved client
backs off on its own.

What the counter counts depends on the scope. Gateway-wide, one counter covers all
traffic. Targeted, each application gets its own. This is the only type that
offers **Throttle** mode: a request over the line waits its share of the window
and then goes through, which turns a burst into a queue instead of a wall of 429s.

**Group by header** splits the counter by a header value — `X-User-Id`, say — so
one policy gives every end user of an application their own allowance.

## Where to apply it

| Scope               | Typical use                                                     |
| ------------------- | --------------------------------------------------------------- |
| **Gateway-wide**    | An organisation-level ceiling nobody can opt out of.            |
| **Targeted**        | Per-tenant or per-application quotas.                           |
| **Group by header** | Per end user inside one application, without a policy per user. |

A refused request gets HTTP 429 with the standard rate-limit headers and the
Retry-After you set. Pair it with an [LLM Budget](/trustgate/policies/llm-budget)
when the concern is spend rather than volume.