ref(mcp-server): Shrink the stdio bundle's Zod and memory footprint #1761

Follow-up from #1731. BYK asked for @sentry/mcp-server to use valibot instead of Zod, like the CLI. We left it out of #1731 and track it here, together with the bundle size and memory use it affects.

Measurements

Taken from the esbuild metafile of packages/mcp-server/dist/sentry-mcp.cjs, built from the #1731 branch:

The 508 KB of Zod breaks down as:

• zod/v4/locales: 260 KB. Every error-message locale is included, but we only use English. The classic, mini, and core entry points all import locales/index.js.
• zod/v3: about 58 KB. It is loaded by @ai-sdk/provider-utils, zod-to-json-schema, and the v1 SDK's zod-compat.
• zod/v4 core and classic: about 190 KB.

Memory on Node 22: bare node uses about 45 MB RSS. The bundle uses about 137 MB RSS for --version and about 144 MB RSS (27 MB heap) when idle as a stdio server. Loading zod alone adds about 3.5 MB of heap and 80 ms.

Why moving to valibot alone won't fix this

@modelcontextprotocol/server 2.x depends on zod ^4.2.0 and loads zod/v4 internally. Replacing our schemas with valibot would still ship Zod core in the bundle. On top of that, the change would touch 96 non-test files in mcp-core and 6 in mcp-cloudflare, and valibot would need @valibot/to-json-schema to produce the tool JSON schemas the SDK requires.

Options, cheapest first

1. Stub zod/v4/locales down to en in packages/mcp-server/script/bundle.ts. This saves about 260 KB without changing any code.
2. Remove the zod/v3 and zod-to-json-schema paths and the v1 @modelcontextprotocol/sdk from the stdio bundle, if nothing in that graph needs them at runtime.
3. Lazy-load the tool modules so --version, --help, and startup don't parse the whole bundle.
4. Only if that isn't enough: migrate mcp-core schemas to Standard Schema / valibot. Measure first, because the SDK keeps Zod in the bundle regardless.

Done when

• The bundle size and idle RSS have been measured before and after each change.
• MAX_BUNDLE_BYTES in bundle.ts is lowered to the new size.

via BYK.

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