# `AttestoMCP.Server.Presentation`
[🔗](https://github.com/XukuLLC/attesto_mcp_server/blob/v2.8.0/lib/attesto_mcp_server/presentation.ex#L1)

Request-scoped server guidance and tool presentation.

## Instructions

`:instructions` remains a static string. `:instructions_provider` is an
optional one-argument callback (a function, `{module, function}`, or
`{module, function, prefix_args}`) that receives the trusted request context
and returns:

  * `{:ok, instructions}` with a non-empty UTF-8 string of at most 65,536
    bytes;
  * `:omit` to send no instructions for this request; or
  * `{:error, reason}` to fail the request.

A configured provider supplies the instructions for modern `server/discover`
and legacy `initialize`; the static string is used only when no provider is
configured. A failing provider never falls back to the static string. An
error, exception, exit, throw, invalid value, or request timeout returns a
controlled internal error (`"instructions_provider_failure"`) and publishes
no instructions. A failed legacy `initialize` leaves the session
unnegotiated, so a later valid `initialize` can succeed.

## Tool presentation

`:tool_presentation` is an optional two-argument callback that receives an
already visible tool's public descriptor (string keys) and the trusted
request context. It returns `{:ok, overrides}`, `:default`, or
`{:error, reason}`. `overrides` may contain only:

  * `"title"`: a non-empty UTF-8 string of at most 1,024 bytes;
  * `"description"`: a non-blank UTF-8 string of at most 4,096 bytes;
  * `"icons"`: a complete icon list (see `AttestoMCP.Server.API`);
  * `"_meta"`: application keys merged over the registered `_meta`. Keys
    must follow the MCP `_meta` key grammar and must not use a reserved MCP
    prefix, `progressToken`, or trace-context names.

Atom keys are accepted for these four fields. Any other field, including
`name`, `inputSchema`, `outputSchema`, `annotations`, `execution`, a handler,
scopes, or an authorization callback, fails the list request with
`"tool_presentation_failure"`. No partial catalog is returned.

Scope requirements, `authorize` callbacks, and HTTP definition policies
decide availability first; hidden tools are never passed to the callback.
Presentation never changes registered definitions, and `tools/call` repeats
every eligibility check. Customized descriptors are part of the pagination
fingerprint, so a cursor stops working when the presentation it was issued
for changes.

Both callbacks run in the request worker under the request deadline. Their
output is treated as private: responses that include it use
`cacheScope: "private"` and `ttlMs: 0` unless a cache policy explicitly
supplies a TTL (see `AttestoMCP.Server.CachePolicy`). Guidance is not an
authorization mechanism.

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*Consult [api-reference.md](api-reference.md) for complete listing*