diff --git a/.changeset/21532-mcp-server-info-version-default.md b/.changeset/21532-mcp-server-info-version-default.md
new file mode 100644
index 00000000000..5babccd242b
--- /dev/null
+++ b/.changeset/21532-mcp-server-info-version-default.md
@@ -0,0 +1,12 @@
+---
+'@objectstack/mcp': patch
+---
+
+The MCP server's `serverInfo.version` is the package version unless you set one, as `MCPServerPluginOptions.version` always documented ("Defaults to package version") (#21532).
+
+Clause-②: no
+
+- Before, `MCPServerPlugin` and `MCPServerRuntime` each defaulted to the literal `1.0.0`, so every deployment built without the option, `os serve`'s auto-registration included, answered `initialize` with `serverInfo.version` `1.0.0` whatever the installed `@objectstack/mcp` was. Both defaults now read the version from the package's own `package.json`, ESM and CJS alike.
+- An explicit `version` option (`MCPServerPluginOptions.version`, `MCPServerRuntimeConfig.version`) is still answered as given.
+- `new MCPServerPlugin().version`, the kernel plugin's own version, is the package version too, where it was `1.0.0`. Its declared type is now `string | undefined`: if the manifest cannot be read (a bundle with no `package.json` beside it), `serverInfo.version` says `unknown` and the plugin's own `version` is left unset, which both kernels accept, instead of a placeholder they would refuse.
+- Pass `version` yourself to keep reporting a fixed string.
diff --git a/packages/mcp/package.json b/packages/mcp/package.json
index 49e5d5c6b8a..2943fd02063 100644
--- a/packages/mcp/package.json
+++ b/packages/mcp/package.json
@@ -19,7 +19,7 @@
     }
   },
   "scripts": {
-    "build": "tsup --config ../../tsup.config.ts && node ../../scripts/check-dts-emitted.mjs",
+    "build": "tsup && node ../../scripts/check-dts-emitted.mjs",
     "check:test-typecheck": "tsx ../../scripts/check-test-typecheck.mts --self-test && tsx ../../scripts/check-test-typecheck.mts --package packages/mcp --project tsconfig.test.json",
     "gen:test-typecheck-debt": "tsx ../../scripts/check-test-typecheck.mts --update --package packages/mcp --project tsconfig.test.json",
     "test": "vitest run",
diff --git a/packages/mcp/src/__tests__/plugin.test.ts b/packages/mcp/src/__tests__/plugin.test.ts
index 03c78778cad..582b6b996e2 100644
--- a/packages/mcp/src/__tests__/plugin.test.ts
+++ b/packages/mcp/src/__tests__/plugin.test.ts
@@ -98,7 +98,7 @@ describe('MCPServerPlugin', () => {
     it('should have correct plugin metadata', () => {
       const plugin = new MCPServerPlugin();
       expect(plugin.name).toBe('com.objectstack.mcp');
-      expect(plugin.version).toBe('1.0.0');
+      // `version` is the package manifest's, pinned in `../mcp-server-info-version.test.ts`.
       expect(plugin.type).toBe('standard');
     });
   });
diff --git a/packages/mcp/src/mcp-server-info-version.test.ts b/packages/mcp/src/mcp-server-info-version.test.ts
new file mode 100644
index 00000000000..1f3c639b244
--- /dev/null
+++ b/packages/mcp/src/mcp-server-info-version.test.ts
@@ -0,0 +1,155 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * `serverInfo.version` — what an MCP server answers in `initialize`.
+ *
+ * `MCPServerPluginOptions.version` is published as "Defaults to package
+ * version". Two literal `'1.0.0'` defaults (the plugin's, and
+ * `MCPServerRuntime`'s) made every deployment built without the option answer
+ * `1.0.0` while the package was elsewhere, so the registry listing in
+ * `server.json` and every running server disagreed. Both now read the
+ * package's own manifest; an explicit `version` still overrides it.
+ *
+ * The expected value is read from `package.json` HERE, never written down: a
+ * literal in this file would rot at the next release, and would let a
+ * regression that re-introduces some other constant pass on the day the two
+ * happen to agree.
+ */
+
+import { readFileSync } from 'node:fs';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { Client } from '@modelcontextprotocol/sdk/client/index.js';
+import { InMemoryTransport } from '@modelcontextprotocol/sdk/inMemory.js';
+import { LiteKernel } from '@objectstack/core';
+
+import { MCPServerPlugin } from './plugin.js';
+import { MCPServerRuntime } from './mcp-server-runtime.js';
+
+const MANIFEST_VERSION: string = JSON.parse(
+  readFileSync(new URL('../package.json', import.meta.url), 'utf8'),
+).version;
+
+const OVERRIDE = '9.9.9-override.1';
+
+const INITIALIZE = {
+  jsonrpc: '2.0',
+  id: 0,
+  method: 'initialize',
+  params: { protocolVersion: '2025-03-26', capabilities: {}, clientInfo: { name: 'pin', version: '0' } },
+};
+
+/** `initialize` over the Streamable HTTP door (`POST /api/v1/mcp`). */
+async function serverInfoOverHttp(runtime: MCPServerRuntime): Promise<{ name: string; version: string }> {
+  const res = await runtime.handleHttpRequest(
+    new Request('http://localhost/api/v1/mcp', {
+      method: 'POST',
+      headers: { 'content-type': 'application/json', accept: 'application/json, text/event-stream' },
+      body: JSON.stringify(INITIALIZE),
+    }),
+    { parsedBody: INITIALIZE },
+  );
+  const json = (await res.json()) as { result: { serverInfo: { name: string; version: string } } };
+  return json.result.serverInfo;
+}
+
+/** `initialize` against the long-lived server (the stdio composition), over an in-memory pair. */
+async function serverVersionOnLongLivedServer(runtime: MCPServerRuntime): Promise<string | undefined> {
+  const [clientSide, serverSide] = InMemoryTransport.createLinkedPair();
+  const client = new Client({ name: 'pin', version: '0' });
+  await runtime.server.connect(serverSide);
+  await client.connect(clientSide);
+  try {
+    return client.getServerVersion()?.version;
+  } finally {
+    await client.close();
+  }
+}
+
+function pluginContext() {
+  const services = new Map<string, unknown>();
+  return {
+    services,
+    ctx: {
+      registerService: vi.fn((name: string, service: unknown) => {
+        services.set(name, service);
+      }),
+      getService: vi.fn((name: string) => {
+        if (!services.has(name)) throw new Error(`Service "${name}" not found`);
+        return services.get(name);
+      }),
+      logger: { info: vi.fn(), warn: vi.fn(), error: vi.fn(), debug: vi.fn() },
+    },
+  };
+}
+
+async function runtimeOf(plugin: MCPServerPlugin): Promise<MCPServerRuntime> {
+  const { ctx, services } = pluginContext();
+  await plugin.init(ctx as never);
+  return services.get('mcp') as MCPServerRuntime;
+}
+
+describe('serverInfo.version — the default is the package version', () => {
+  it('MCPServerRuntime built with no config answers the package version', async () => {
+    expect((await serverInfoOverHttp(new MCPServerRuntime())).version).toBe(MANIFEST_VERSION);
+  });
+
+  it('MCPServerPlugin built with no options answers the package version (the `os serve` auto-registration shape)', async () => {
+    const runtime = await runtimeOf(new MCPServerPlugin());
+    expect((await serverInfoOverHttp(runtime)).version).toBe(MANIFEST_VERSION);
+  });
+
+  it('the long-lived (stdio) server answers the package version too', async () => {
+    expect(await serverVersionOnLongLivedServer(new MCPServerRuntime())).toBe(MANIFEST_VERSION);
+    expect(await serverVersionOnLongLivedServer(await runtimeOf(new MCPServerPlugin()))).toBe(MANIFEST_VERSION);
+  });
+
+  it("the kernel plugin's own `version` is the same one value", () => {
+    expect(new MCPServerPlugin().version).toBe(MANIFEST_VERSION);
+  });
+});
+
+describe('serverInfo.version — an explicit override is answered as given', () => {
+  it('MCPServerRuntime config.version', async () => {
+    const runtime = new MCPServerRuntime({ version: OVERRIDE });
+    expect((await serverInfoOverHttp(runtime)).version).toBe(OVERRIDE);
+    expect(await serverVersionOnLongLivedServer(new MCPServerRuntime({ version: OVERRIDE }))).toBe(OVERRIDE);
+  });
+
+  it('MCPServerPlugin options.version', async () => {
+    const runtime = await runtimeOf(new MCPServerPlugin({ version: OVERRIDE }));
+    expect((await serverInfoOverHttp(runtime)).version).toBe(OVERRIDE);
+  });
+});
+
+describe('an unreadable manifest degrades to an honest answer, never to a plugin the kernel refuses', () => {
+  afterEach(() => {
+    vi.doUnmock('node:module');
+    vi.resetModules();
+  });
+
+  it("serverInfo.version says 'unknown' and the plugin still loads", async () => {
+    // A bundle with no `package.json` beside it: `createRequire(...)('../package.json')` throws.
+    vi.resetModules();
+    vi.doMock('node:module', async (importOriginal) => {
+      const actual = await importOriginal<typeof import('node:module')>();
+      return {
+        ...actual,
+        createRequire: () => {
+          throw new Error('ENOENT: no manifest beside this bundle');
+        },
+      };
+    });
+    const { MCPServerPlugin: BlindPlugin } = await import('./plugin.js');
+    const plugin = new BlindPlugin();
+
+    // The wire answer is a free string: it says it does not know.
+    const { ctx, services } = pluginContext();
+    await plugin.init(ctx as never);
+    expect((await serverInfoOverHttp(services.get('mcp') as MCPServerRuntime)).version).toBe('unknown');
+
+    // The kernel plugin's `version` has a grammar (SemVer 2.0.0): a placeholder string there would
+    // be refused by both kernels. `use()` throws on a refused contract.
+    const kernel = new LiteKernel({ logger: { level: 'silent' } });
+    expect(() => kernel.use(plugin)).not.toThrow();
+  });
+});
diff --git a/packages/mcp/src/mcp-server-runtime.ts b/packages/mcp/src/mcp-server-runtime.ts
index 9f36a33611a..6a9c1c49323 100644
--- a/packages/mcp/src/mcp-server-runtime.ts
+++ b/packages/mcp/src/mcp-server-runtime.ts
@@ -18,6 +18,7 @@ import {
   METADATA_UNAVAILABLE_CODE,
   metadataPartialListingSentence,
 } from './metadata-completeness.js';
+import { DEFAULT_SERVER_VERSION } from './package-version.js';
 import { protocolStdout } from './protocol-stdout.js';
 import { renderSkillMarkdown, type RenderSkillOptions } from './skill-md.js';
 import {
@@ -34,7 +35,7 @@ import { z } from 'zod';
 export interface MCPServerRuntimeConfig {
   /** Human-readable server name. */
   name?: string;
-  /** Server version (semver). */
+  /** Server version (semver). Defaults to the `@objectstack/mcp` package version. */
   version?: string;
   /** Optional instructions describing how to use the server. */
   instructions?: string;
@@ -959,7 +960,7 @@ export class MCPServerRuntime {
   constructor(config: MCPServerRuntimeConfig = {}) {
     this.config = {
       name: 'objectstack',
-      version: '1.0.0',
+      version: DEFAULT_SERVER_VERSION,
       transport: 'stdio',
       ...config,
     };
diff --git a/packages/mcp/src/package-version.ts b/packages/mcp/src/package-version.ts
new file mode 100644
index 00000000000..7c1fdebcc79
--- /dev/null
+++ b/packages/mcp/src/package-version.ts
@@ -0,0 +1,57 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * The one place `@objectstack/mcp` learns its own version.
+ *
+ * `MCPServerPluginOptions.version` is published as "Defaults to package
+ * version", and the MCP `initialize` answer carries that string as
+ * `serverInfo.version`. Both defaults used to be the literal `'1.0.0'` (the
+ * plugin's own, and `MCPServerRuntime`'s), so every deployment that built the
+ * plugin without options answered `1.0.0` while the package was somewhere
+ * else entirely, and the registry listing in `server.json` disagreed with
+ * every running server. Every default now reads this one value; an explicit
+ * `version` option still overrides it.
+ *
+ * The read is the same shape `@objectstack/runtime`
+ * (`packages/runtime/src/runtime-version.ts`) and `@objectstack/metadata-protocol`
+ * (`packages/metadata-protocol/src/discovery-version.ts`) already use for their
+ * own manifests: `createRequire(import.meta.url)` against `../package.json`,
+ * which sits one directory above both `src/` (vitest) and the bundled
+ * `dist/index.{js,cjs}` (tsup, single entry, `splitting: false`), so one path
+ * resolves the same manifest from every shape. Its CJS half rests on
+ * `shims: true` in this package's `tsup.config.ts` (see the comment there).
+ */
+
+import { createRequire } from 'node:module';
+
+function readOwnVersion(): string | undefined {
+  try {
+    const require = createRequire(import.meta.url);
+    const pkg = require('../package.json') as { version?: unknown };
+    return typeof pkg.version === 'string' && pkg.version.length > 0 ? pkg.version : undefined;
+  } catch {
+    return undefined;
+  }
+}
+
+/**
+ * `@objectstack/mcp`'s own installed version, from its `package.json`;
+ * `undefined` when the manifest cannot be read (a bundle that no longer has the
+ * file beside it).
+ *
+ * This is what the kernel plugin's own `version` takes, and the reason it stays
+ * `undefined` rather than becoming a placeholder string: both kernels refuse a
+ * plugin whose `version` is not SemVer 2.0.0, so any non-version placeholder
+ * would turn an unreadable manifest into an MCP plugin that never loads. An
+ * absent `version` is accepted, and the kernel supplies its own `0.0.0`.
+ */
+export const PACKAGE_VERSION: string | undefined = readOwnVersion();
+
+/**
+ * What an MCP server reports as `serverInfo.version` when the caller names none:
+ * the package version, or `'unknown'` when the manifest cannot be read —
+ * honest about not knowing, rather than a plausible-looking version a client
+ * could mistake for real identity. `serverInfo.version` is a free string on the
+ * wire, so unlike {@link PACKAGE_VERSION} it has no grammar to satisfy.
+ */
+export const DEFAULT_SERVER_VERSION: string = PACKAGE_VERSION ?? 'unknown';
diff --git a/packages/mcp/src/plugin.ts b/packages/mcp/src/plugin.ts
index 0092d820396..cd63b42fb96 100644
--- a/packages/mcp/src/plugin.ts
+++ b/packages/mcp/src/plugin.ts
@@ -23,6 +23,7 @@ import type { ExecutionContext } from '@objectstack/spec/kernel';
 import type { TenancyPosture } from '@objectstack/spec/security';
 import type { IAIService, IDataEngine, IMetadataService } from '@objectstack/spec/contracts';
 import { MCPServerRuntime } from './mcp-server-runtime.js';
+import { PACKAGE_VERSION, DEFAULT_SERVER_VERSION } from './package-version.js';
 import type { MCPServerRuntimeConfig, McpMergedMetadataRead } from './mcp-server-runtime.js';
 import type { ToolRegistry } from './types.js';
 import {
@@ -265,7 +266,7 @@ export class MCPServerPlugin implements Plugin {
    * kernel name this plugin when a consumer requires one before it inits.
    */
   providesServices = ['mcp'];
-  version = '1.0.0';
+  version = PACKAGE_VERSION;
   type = 'standard' as const;
   dependencies: string[] = [];
 
@@ -279,7 +280,7 @@ export class MCPServerPlugin implements Plugin {
   async init(ctx: PluginContext): Promise<void> {
     const config: MCPServerRuntimeConfig = {
       name: readEnvWithDeprecation('OS_MCP_SERVER_NAME', 'MCP_SERVER_NAME', { silent: true }) ?? this.options.name ?? 'objectstack',
-      version: this.options.version ?? '1.0.0',
+      version: this.options.version ?? DEFAULT_SERVER_VERSION,
       transport: (readEnvWithDeprecation('OS_MCP_SERVER_TRANSPORT', 'MCP_SERVER_TRANSPORT', { silent: true }) as 'stdio' | 'http') ?? this.options.transport ?? 'stdio',
       instructions: this.options.instructions,
       logger: ctx.logger,
diff --git a/packages/mcp/tsup.config.ts b/packages/mcp/tsup.config.ts
new file mode 100644
index 00000000000..e673f88214c
--- /dev/null
+++ b/packages/mcp/tsup.config.ts
@@ -0,0 +1,34 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { defineConfig } from 'tsup';
+
+import { dropSourcesContent } from '../../scripts/tsup-drop-sources-content.mjs';
+
+export default defineConfig({
+  entry: ['src/index.ts'],
+  splitting: false,
+  sourcemap: true,
+  clean: true,
+  dts: !process.env.OS_SKIP_DTS,
+  format: ['esm', 'cjs'],
+  target: 'es2020',
+  // LOAD-BEARING, and measured rather than assumed. `package-version.ts` reads
+  // this package's own `package.json` via `createRequire(import.meta.url)` —
+  // correct as written for the ESM output. The shared `tsup.config.ts` this
+  // package built with before has no `shims`, and there esbuild EMPTIES
+  // `import.meta` in the CJS bundle (`var import_meta = {}`), so
+  // `createRequire(undefined)` throws, the resolver's `catch` swallows it, and
+  // `require('@objectstack/mcp')` answers `serverInfo.version` `'unknown'`
+  // while the ESM build answers the manifest version: the two formats disagree
+  // and nothing fails at load. `shims: true` makes tsup rewrite `import.meta.url`
+  // in the CJS build to a real `__filename`-derived value (its
+  // `assets/cjs_shims.js`), so both formats resolve the SAME package.json.
+  // `packages/runtime/tsup.config.ts` and `packages/metadata-protocol/tsup.config.ts`
+  // carry it for the same reason. Do not drop this line while
+  // `package-version.ts` exists.
+  //
+  // Need-based injection — nothing else here references `__dirname` /
+  // `__filename`, so the ESM build's shim path is a no-op.
+  shims: true,
+  esbuildOptions: dropSourcesContent,
+});