diff --git a/CLAUDE.md b/CLAUDE.md
index 4682dc362f..4a15788098 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -70,7 +70,7 @@ The SDK separates internal code from the public API surface:
 - **`@modelcontextprotocol/core-internal`** (main entry, `packages/core-internal/src/index.ts`) — Internal barrel. Exports everything (including Zod schemas, Protocol class, stdio utils). Only consumed by sibling packages within the monorepo (`private: true`).
 - **`@modelcontextprotocol/core-internal/public`** (`packages/core-internal/src/exports/public/index.ts`) — Curated public API. Exports only TypeScript types, error classes, constants, and guards. Re-exported by client and server packages.
 - **`@modelcontextprotocol/client`** and **`@modelcontextprotocol/server`** (`packages/*/src/index.ts`) — Final public surface. Package-specific exports (named explicitly) plus re-exports from `core-internal/public`.
-- **`@modelcontextprotocol/core`** (`packages/core/src/index.ts`) — Public Zod-schema package. Re-exports **only** the `*Schema` Zod constants (MCP spec + OAuth/OpenID), bundled from `core-internal` at build time. The published home for raw runtime validation (`CallToolResultSchema.parse(...)`); runtime-neutral (`zod` is its only dependency). Not consumed by the sibling packages — `client`/`server` keep their own bundled schema copies and stay Zod-free in their public surface.
+- **`@modelcontextprotocol/core`** (`packages/core/src/index.ts`) — Public Zod-schema package. Re-exports **only** the `*Schema` Zod constants (MCP spec + OAuth/OpenID), bundled from `core-internal` at build time. The published home for raw runtime validation (`CallToolResultSchema.parse(...)`); runtime-neutral (`zod` is its only dependency). Consumed by the sibling packages at runtime: `client`/`server`/`server-legacy` bundles resolve the schema modules from this package (see the `externalize-core-schemas` plugin in their tsdown configs) so one evaluated schema-graph copy is shared per application; their public surfaces stay Zod-free.
 
 When modifying exports:
 - Use explicit named exports, not `export *`, in package `index.ts` files and `core-internal/public`.
diff --git a/packages/client/package.json b/packages/client/package.json
index f96b681bc7..69b28f3c61 100644
--- a/packages/client/package.json
+++ b/packages/client/package.json
@@ -133,6 +133,7 @@
         "test:watch": "vitest"
     },
     "dependencies": {
+        "@modelcontextprotocol/core": "workspace:^",
         "cross-spawn": "catalog:runtimeClientOnly",
         "eventsource": "catalog:runtimeClientOnly",
         "eventsource-parser": "catalog:runtimeClientOnly",
diff --git a/packages/client/tsdown.config.ts b/packages/client/tsdown.config.ts
index 948cf95c88..9b5f37176e 100644
--- a/packages/client/tsdown.config.ts
+++ b/packages/client/tsdown.config.ts
@@ -1,7 +1,33 @@
 import { defineConfig } from 'tsdown';
 
+/**
+ * core-internal's two schema source modules (types/schemas.ts + shared/auth.ts) are published
+ * verbatim as @modelcontextprotocol/core (see packages/core/tsdown.config.ts). Resolving them to
+ * that package at runtime — instead of inlining yet another copy — keeps ONE evaluated copy of
+ * the spec + OAuth Zod schema graph per application, however many SDK packages it uses. Safe
+ * because core exports every name that crosses this boundary at runtime: SDK-internal helper
+ * schemas live in separate core-internal modules (e.g. types/listChangedOptions.ts) and remain
+ * bundled, and the remaining internal-only exports of the two modules are imported type-only.
+ * Type declarations still inline the types via the dts `paths` mapping below — only runtime
+ * duplication costs.
+ */
+const CORE_SCHEMA_MODULES = /[\\/]core-internal[\\/]src[\\/](?:types[\\/]schemas|shared[\\/]auth)\.ts$/;
+
 export default defineConfig({
     failOnWarn: 'ci-only',
+    plugins: [
+        {
+            name: 'externalize-core-schemas',
+            async resolveId(source, importer, options) {
+                if (importer === undefined) return null;
+                const resolved = await this.resolve(source, importer, options);
+                if (resolved !== null && CORE_SCHEMA_MODULES.test(resolved.id)) {
+                    return { id: '@modelcontextprotocol/core', external: true };
+                }
+                return null;
+            }
+        }
+    ],
     entry: [
         'src/index.ts',
         'src/stdio.ts',
diff --git a/packages/core-internal/src/types/index.ts b/packages/core-internal/src/types/index.ts
index 46dd118f11..350ca12c09 100644
--- a/packages/core-internal/src/types/index.ts
+++ b/packages/core-internal/src/types/index.ts
@@ -4,6 +4,7 @@ export * from './constants';
 export * from './enums';
 export * from './errors';
 export * from './guards';
+export * from './listChangedOptions';
 export * from './schemas';
 export * from './specTypeSchema';
 export * from './types';
diff --git a/packages/core-internal/src/types/listChangedOptions.ts b/packages/core-internal/src/types/listChangedOptions.ts
new file mode 100644
index 0000000000..441b7a6bc1
--- /dev/null
+++ b/packages/core-internal/src/types/listChangedOptions.ts
@@ -0,0 +1,31 @@
+import * as z from 'zod/v4';
+
+/**
+ * Base schema for list changed subscription options (without callback).
+ * Used internally for Zod validation of `autoRefresh` and `debounceMs`.
+ *
+ * Lives outside `schemas.ts` deliberately: it is an SDK option schema, not a spec
+ * schema, so it is not part of `@modelcontextprotocol/core`'s public surface —
+ * and `schemas.ts` is resolved from that package at runtime by the client/server
+ * bundles, which can only see the names core actually exports.
+ */
+export const ListChangedOptionsBaseSchema = z.object({
+    /**
+     * If `true`, the list will be refreshed automatically when a list changed notification is received.
+     * The callback will be called with the updated list.
+     *
+     * If `false`, the callback will be called with `null` items, allowing manual refresh.
+     *
+     * @default true
+     */
+    autoRefresh: z.boolean().default(true),
+    /**
+     * Debounce time in milliseconds for list changed notification processing.
+     *
+     * Multiple notifications received within this timeframe will only trigger one refresh.
+     * Set to `0` to disable debouncing.
+     *
+     * @default 300
+     */
+    debounceMs: z.number().int().nonnegative().default(300)
+});
diff --git a/packages/core-internal/src/types/schemas.ts b/packages/core-internal/src/types/schemas.ts
index 21960a6176..2e4908bffd 100644
--- a/packages/core-internal/src/types/schemas.ts
+++ b/packages/core-internal/src/types/schemas.ts
@@ -1452,31 +1452,6 @@ export const ToolListChangedNotificationSchema = NotificationSchema.extend({
     params: NotificationsParamsSchema.optional()
 });
 
-/**
- * Base schema for list changed subscription options (without callback).
- * Used internally for Zod validation of `autoRefresh` and `debounceMs`.
- */
-export const ListChangedOptionsBaseSchema = z.object({
-    /**
-     * If `true`, the list will be refreshed automatically when a list changed notification is received.
-     * The callback will be called with the updated list.
-     *
-     * If `false`, the callback will be called with `null` items, allowing manual refresh.
-     *
-     * @default true
-     */
-    autoRefresh: z.boolean().default(true),
-    /**
-     * Debounce time in milliseconds for list changed notification processing.
-     *
-     * Multiple notifications received within this timeframe will only trigger one refresh.
-     * Set to `0` to disable debouncing.
-     *
-     * @default 300
-     */
-    debounceMs: z.number().int().nonnegative().default(300)
-});
-
 /* Logging */
 /**
  * The severity of a log message.
diff --git a/packages/core-internal/src/types/specTypeSchema.ts b/packages/core-internal/src/types/specTypeSchema.ts
index f7a109cbd5..5cb02590d7 100644
--- a/packages/core-internal/src/types/specTypeSchema.ts
+++ b/packages/core-internal/src/types/specTypeSchema.ts
@@ -21,7 +21,7 @@ import * as schemas from './schemas';
  * Explicit allowlist of protocol Zod schemas that correspond to a public spec type in `types.ts`.
  *
  * This intentionally excludes internal helper schemas exported from `schemas.ts` that have no
- * matching public type (e.g. `ListChangedOptionsBaseSchema`, `BaseRequestParamsSchema`,
+ * matching public type (e.g. `BaseRequestParamsSchema`,
  * `NotificationsParamsSchema`, `ClientTasksCapabilitySchema`, `ServerTasksCapabilitySchema`).
  * Keeping the list explicit means new public spec types must be added here deliberately, and
  * internals never leak into `SpecTypeName`.
diff --git a/packages/core/test/coreSchemas.test.ts b/packages/core/test/coreSchemas.test.ts
index 85a0e08a09..fa7ac586af 100644
--- a/packages/core/test/coreSchemas.test.ts
+++ b/packages/core/test/coreSchemas.test.ts
@@ -37,7 +37,6 @@ describe('@modelcontextprotocol/core', () => {
         const SPEC_INTERNAL_HELPERS = [
             'BaseRequestParamsSchema',
             'ClientTasksCapabilitySchema',
-            'ListChangedOptionsBaseSchema',
             'NotificationsParamsSchema',
             'ServerTasksCapabilitySchema'
         ];
diff --git a/packages/core/test/distEntry.test.ts b/packages/core/test/distEntry.test.ts
new file mode 100644
index 0000000000..a3dc7f149b
--- /dev/null
+++ b/packages/core/test/distEntry.test.ts
@@ -0,0 +1,38 @@
+import { execFileSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { beforeAll, describe, expect, test } from 'vitest';
+
+const pkgDir = join(dirname(fileURLToPath(import.meta.url)), '..');
+const distEntry = join(pkgDir, 'dist', 'index.mjs');
+
+// client, server, and server-legacy resolve their schema modules from this
+// package's dist entry at runtime, so a missing or incomplete build breaks
+// every consumer of a built package (the repo's CI test job never runs a
+// standalone build — dist only exists if a test builds it, the same way
+// client's barrelClean.test.ts builds client). Build on demand and pin the
+// runtime surface the boundary depends on.
+describe('@modelcontextprotocol/core dist entry', () => {
+    beforeAll(() => {
+        if (!existsSync(distEntry)) {
+            execFileSync('pnpm', ['build'], { cwd: pkgDir, stdio: 'inherit' });
+        }
+    }, 120_000);
+
+    test('root entry resolves and exposes the runtime schema surface', async () => {
+        const mod = await import(distEntry);
+        for (const name of [
+            'CallToolResultSchema',
+            'InitializeRequestSchema',
+            'JSONRPCMessageSchema',
+            'OAuthMetadataSchema',
+            'OAuthProtectedResourceMetadataSchema'
+        ]) {
+            expect(mod[name], `core dist entry must export ${name}`).toBeDefined();
+        }
+        // The externalized boundary re-exports the whole spec schema surface;
+        // a partial build would show up as a collapsed export count.
+        expect(Object.keys(mod).length).toBeGreaterThan(150);
+    });
+});
diff --git a/packages/server-legacy/package.json b/packages/server-legacy/package.json
index acc880f6f6..36c5583aca 100644
--- a/packages/server-legacy/package.json
+++ b/packages/server-legacy/package.json
@@ -83,6 +83,7 @@
         "test:watch": "vitest"
     },
     "dependencies": {
+        "@modelcontextprotocol/core": "workspace:^",
         "zod": "catalog:runtimeShared",
         "raw-body": "catalog:runtimeServerOnly",
         "content-type": "catalog:runtimeShared",
diff --git a/packages/server-legacy/tsdown.config.ts b/packages/server-legacy/tsdown.config.ts
index b4c2881fd1..5da03a2c72 100644
--- a/packages/server-legacy/tsdown.config.ts
+++ b/packages/server-legacy/tsdown.config.ts
@@ -1,7 +1,33 @@
 import { defineConfig } from 'tsdown';
 
+/**
+ * core-internal's two schema source modules (types/schemas.ts + shared/auth.ts) are published
+ * verbatim as @modelcontextprotocol/core (see packages/core/tsdown.config.ts). Resolving them to
+ * that package at runtime — instead of inlining yet another copy — keeps ONE evaluated copy of
+ * the spec + OAuth Zod schema graph per application, however many SDK packages it uses. Safe
+ * because core exports every name that crosses this boundary at runtime: SDK-internal helper
+ * schemas live in separate core-internal modules (e.g. types/listChangedOptions.ts) and remain
+ * bundled, and the remaining internal-only exports of the two modules are imported type-only.
+ * Type declarations still inline the types via the dts `paths` mapping — only runtime
+ * duplication costs.
+ */
+const CORE_SCHEMA_MODULES = /[\\/]core-internal[\\/]src[\\/](?:types[\\/]schemas|shared[\\/]auth)\.ts$/;
+
 export default defineConfig({
     failOnWarn: 'ci-only',
+    plugins: [
+        {
+            name: 'externalize-core-schemas',
+            async resolveId(source, importer, options) {
+                if (importer === undefined) return null;
+                const resolved = await this.resolve(source, importer, options);
+                if (resolved !== null && CORE_SCHEMA_MODULES.test(resolved.id)) {
+                    return { id: '@modelcontextprotocol/core', external: true };
+                }
+                return null;
+            }
+        }
+    ],
     entry: ['src/index.ts', 'src/sse/index.ts', 'src/auth/index.ts'],
     format: ['esm', 'cjs'],
     fixedExtension: true,
diff --git a/packages/server/package.json b/packages/server/package.json
index d77e533db8..5e3a648857 100644
--- a/packages/server/package.json
+++ b/packages/server/package.json
@@ -133,6 +133,7 @@
         "test:watch": "vitest"
     },
     "dependencies": {
+        "@modelcontextprotocol/core": "workspace:^",
         "zod": "catalog:runtimeShared"
     },
     "devDependencies": {
diff --git a/packages/server/tsdown.config.ts b/packages/server/tsdown.config.ts
index adec842c2f..efd271d75e 100644
--- a/packages/server/tsdown.config.ts
+++ b/packages/server/tsdown.config.ts
@@ -1,7 +1,33 @@
 import { defineConfig } from 'tsdown';
 
+/**
+ * core-internal's two schema source modules (types/schemas.ts + shared/auth.ts) are published
+ * verbatim as @modelcontextprotocol/core (see packages/core/tsdown.config.ts). Resolving them to
+ * that package at runtime — instead of inlining yet another copy — keeps ONE evaluated copy of
+ * the spec + OAuth Zod schema graph per application, however many SDK packages it uses. Safe
+ * because core exports every name that crosses this boundary at runtime: SDK-internal helper
+ * schemas live in separate core-internal modules (e.g. types/listChangedOptions.ts) and remain
+ * bundled, and the remaining internal-only exports of the two modules are imported type-only.
+ * Type declarations still inline the types via the dts `paths` mapping below — only runtime
+ * duplication costs.
+ */
+const CORE_SCHEMA_MODULES = /[\\/]core-internal[\\/]src[\\/](?:types[\\/]schemas|shared[\\/]auth)\.ts$/;
+
 export default defineConfig({
     failOnWarn: 'ci-only',
+    plugins: [
+        {
+            name: 'externalize-core-schemas',
+            async resolveId(source, importer, options) {
+                if (importer === undefined) return null;
+                const resolved = await this.resolve(source, importer, options);
+                if (resolved !== null && CORE_SCHEMA_MODULES.test(resolved.id)) {
+                    return { id: '@modelcontextprotocol/core', external: true };
+                }
+                return null;
+            }
+        }
+    ],
     entry: [
         'src/index.ts',
         'src/stdio.ts',
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 82fdee06b0..58132a04fc 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -1290,6 +1290,9 @@ importers:
 
   packages/client:
     dependencies:
+      '@modelcontextprotocol/core':
+        specifier: workspace:^
+        version: link:../core
       cross-spawn:
         specifier: catalog:runtimeClientOnly
         version: 7.0.6
@@ -1778,6 +1781,9 @@ importers:
 
   packages/server:
     dependencies:
+      '@modelcontextprotocol/core':
+        specifier: workspace:^
+        version: link:../core
       zod:
         specifier: catalog:runtimeShared
         version: 4.3.6
@@ -1845,6 +1851,9 @@ importers:
 
   packages/server-legacy:
     dependencies:
+      '@modelcontextprotocol/core':
+        specifier: workspace:^
+        version: link:../core
       content-type:
         specifier: catalog:runtimeShared
         version: 1.0.5