perf: build era wire schemas lazily through memoized factories

.changeset/lazy-era-wire-schemas.md

@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+'@modelcontextprotocol/server-legacy': patch
+---
+
+Build protocol-revision wire schemas lazily on first validation instead of at import. Each revision's schema set is now constructed by a module-level memoized factory, so importing the client or server package no longer pays the construction cost of both frozen wire-schema graphs up front. Method membership in the revision registries stays static, the schemas themselves are unchanged, and registry lookups keep returning reference-identical schema objects.

packages/core-internal/src/wire/rev2025-11-25/codec.ts

@@ -31,9 +31,9 @@ import type * as z from 'zod/v4';
 import type { CallToolResult, Result } from '../../types/types';
 import type { DecodedResult, EnvelopeIssue, LiftedWireMaterial, OutboundEnvelopeMaterial, ValidateOutcome, WireCodec } from '../codec';
 import { appendTextFallbackForNonObject } from '../textFallback';
+import { buildSchemas2025 } from './buildSchemas';
 import { isNonObjectJsonSchemaRoot, wrapOutputSchemaForLegacy } from './legacyWrap';
 import { getNotificationSchema, getRequestSchema, getResultSchema, hasNotificationMethod2025, hasRequestMethod2025 } from './registry';
-import { CreateMessageResultSchema, CreateMessageResultWithToolsSchema } from './schemas';
 
 function isPlainObject(value: unknown): value is Record<string, unknown> {
     return value !== null && typeof value === 'object' && !Array.isArray(value);
@@ -75,9 +75,13 @@ export const rev2025Codec: WireCodec = {
     validateInputResponse: (): ValidateOutcome<never> => NOT_IN_ERA,
 
     // Arrow literals can't carry overload signatures; the cast is sound (the
-    // boolean dispatches to exactly the schema each overload names).
-    samplingResultVariant: ((hasTools: boolean, raw: unknown) =>
-        triState(hasTools ? CreateMessageResultWithToolsSchema : CreateMessageResultSchema, raw)) as WireCodec['samplingResultVariant'],
+    // boolean dispatches to exactly the schema each overload names). The
+    // schemas are pulled through the era's memo so the codec module itself
+    // stays construction-free at import time.
+    samplingResultVariant: ((hasTools: boolean, raw: unknown) => {
+        const s = buildSchemas2025();
+        return triState(hasTools ? s.CreateMessageResultWithToolsSchema : s.CreateMessageResultSchema, raw);
+    }) as WireCodec['samplingResultVariant'],
 
     // The 2025 era carries no per-request `_meta` envelope — legacy wire
     // bytes stay identical (the never-stamp guarantee, outbound-request half).