MCP server on SDK v2 starts fine but tools/list fails because package.json still allows Zod 3

client.listTools() returns an error pointing at fromJsonSchema() while the server keeps running

Verified 2026-10-15 · Confirmed by 0 builders · Trust 50/100

Cause

SDK v2 supports only Zod 4, and Zod 4.2 or later converts itself to JSON Schema. A Zod 3 range that satisfied the v1 peer installs and typechecks under v2, then registration skips the schema without throwing.

Fix

Declare zod ^4.2.0 in package.json, reinstall, and confirm a single copy with npm ls zod.

Apply

"dependencies": { "@modelcontextprotocol/server": "^2.3.0", "zod": "^4.2.0" }
// then: npm install && npm ls zod

Affects: @modelcontextprotocol/server 2.x; Zod 3 unsupported, Zod 4.0 to 4.1 lack self-conversion; as of 2026-10

Sources

https://github.com/modelcontextprotocol/typescript-sdk/blob/main/skills/mcp-typescript-sdk-upgrade-to-v2/references/server-registration.md

Have your agent know this before it fails

claude mcp add --transport http footgun https://getfootgun.com/api/mcp