Configuration

Executive Summary

TMCPServer has five request-visible configuration properties — Instructions, CacheTtlMs, CacheScope, RedactErrorDetails, and DualEra — all set between Create and serving. Configuration freezes together with the registries when the first session is created; a later write raises EMCPServer.

Server := TMCPServer.Create('my-server', '1.0.0');
Server.Instructions := 'Greets people. Use "greet" with a name.';
Server.CacheTtlMs := 60000;
Server.RedactErrorDetails := True;

Instructions

A free-text description of how to use your server, surfaced to clients via server/discover (and the classic initialize result). Clients typically place it in the model's context — write it for the model: name the tools, say when to use them, keep it short.

CacheTtlMs and CacheScope

The caching hints (ttlMs / cacheScope, SEP-2549) stamped on server/discover, list, and read results, telling clients how long the answer stays valid:

• CacheTtlMs — default 300000 (5 minutes, the spec's own tools/list example). Registries are frozen after startup, so registry metadata is honestly cacheable. Must not be negative.
• CacheScope — 'private' (default) or 'public'.

One exception is applied per-result: a resources/read served by a dynamic reader always advertises ttlMs: 0 (revalidate) — the library cannot know how fresh a callback's data stays.

RedactErrorDetails

False by default: an escaped handler or dispatch exception carries its message to the client — the in-band detail trusted local clients use to self-correct.

Set it to True for servers whose error messages could leak internals: the client instead receives a generic message with a correlation reference (... (ref e-000001)), and the full exception — class, message, site — is logged to stderr under that reference for you to match up. (Best-effort on Unix: the first stderr emission upgrades a default SIGPIPE disposition to ignore, so a closed stderr cannot kill the process; a host's own SIGPIPE handling is left untouched.)

DualEra

True by default: the server answers the classic initialize handshake (protocol revisions 2024-11-05, 2025-06-18, 2025-11-25) alongside stateless 2026-07-28 requests. This is not about supporting outdated software — the classic handshake is what current clients speak, Claude Code and Codex included (see the wire captures in Your first conversation). Era selection follows how each client opens; handlers are era-blind, so the spec's transition to the stateless revision costs you nothing either way.

Set DualEra := False for a strict modern-only server: initialize is rejected with a diagnostic naming the supported versions. The Streamable HTTP binding forces this mode — HTTP clients speak 2026-07-28.

Freeze semantics

All five properties, like the registries, are mutable only until the first session exists (RunMCPStdioServer and TMCPHTTPServer create sessions internally). Configure fully, then serve:

Create → Instructions/Cache*/Redact/DualEra → Register* → serve

A write after freeze raises EMCPServer: Server configuration is frozen after session creation.

The behaviours behind these switches (SEP-2549 cache fields, the dual-era compatibility matrix) implement spec revision 2026-07-28 — see Spec grounding.