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Publish FastMCP v4.0.0b3 docs (#4842)

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---
title: caching
sidebarTitle: caching
---
# `fastmcp.server.caching`
Server-level cache hints for FastMCP (SEP-2549).
A FastMCP server opts every SDK-cacheable result it emits into client-side
caching by setting `cache_ttl` (seconds) and, optionally, `cache_scope` on the
`FastMCP` constructor. The hint is uniform by construction: one server-level
value applies to `tools/list`, `prompts/list`, `resources/list`,
`resources/templates/list`, `resources/read`, and `server/discover` alike — no
per-component surface and no aggregation.
FastMCP does not hand-set the wire fields. It passes the hint through to the SDK
low-level `Server(cache_hints=...)`, whose runner fills `ttlMs`/`cacheScope` on
every cacheable result via `apply_cache_hint`, leaving any field a handler set
explicitly untouched. Honoring is modern-only and opt-in on the client: a hinted
server is inert unless the client passes `cache=` and negotiates `2026-07-28`.
## Functions
### `build_cache_hints` <sup><a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/caching.py#L29" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>
```python
build_cache_hints(cache_ttl: int | None, cache_scope: CacheScope | None) -> dict[CacheableMethod, CacheHint] | None
```
Build the per-method `CacheHint` map for the SDK low-level server.
`cache_ttl` is in seconds and is converted to the wire's milliseconds. When
`cache_ttl` is `None` the server emits no hint, so its wire output is
identical to a server that never set one; a `cache_scope` given without a
`cache_ttl` is meaningless (the client gates caching on the presence of a
TTL) and is rejected rather than silently ignored.
Returns `None` when no hint is set, or a map applying the same hint to every
SDK-cacheable method otherwise.
**Raises:**
- `ValueError`: If `cache_ttl` is not positive, or if `cache_scope` is set
without `cache_ttl`.