# AN-2026-0862: MCP TypeScript SDK v2 dropped Zod 3: tools throw "must be Zod v4 schemas. Got a Zod v3 field schema" or tools/list fails "Schema appears to be from zod 3"; needs one zod ^4.2.0

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- id: AN-2026-0862 (version 1; cite https://breakwire.org/claim/AN-2026-0862/v/1)
- kind: breaking · status: active · review: editor
- source: <https://ts.sdk.modelcontextprotocol.io/v2/migration/upgrade-to-v2#:~:text=Zod%20v3%20is%20no%20longer%20supported>
- more sources: <https://github.com/modelcontextprotocol/typescript-sdk/blob/%40modelcontextprotocol%2Fserver%402.4.0/packages/core-internal/src/util/zodCompat.ts#L67> <https://github.com/modelcontextprotocol/typescript-sdk/blob/%40modelcontextprotocol%2Fserver%402.4.0/packages/core-internal/src/util/standardSchema.ts#L195> <https://ts.sdk.modelcontextprotocol.io/v2/troubleshooting#:~:text=Two%20copies%20of> <https://github.com/modelcontextprotocol/typescript-sdk/releases/tag/%40modelcontextprotocol%2Fserver%402.0.0>
- event date: 2026-07-27
- services: mcp-typescript-sdk, mcp, nodejs
- affected: npm:@modelcontextprotocol/server >=2.0.0; npm:@modelcontextprotocol/client >=2.0.0
- severity: high
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The upgrade guide says Zod v3 is no longer supported (the v1 peer was ^3.25 || ^4.0) and to check the declared range in package.json: a zod-3 z.object() schema installs and typechecks, the server starts, and the first tools/list answers with an error pointing at fromJsonSchema(); only the deprecated raw-shape form with zod-3 fields throws at registration. Zod >=4.2.0 is the supported path; 4.0-4.1 falls back with a one-time [mcp-sdk] warning and drops .describe() descriptions.  [✓0 ✗0]
1. The troubleshooting page says TS2589: Type instantiation is excessively deep and possibly infinite means two copies of zod in the dependency tree; list them with npm ls zod and force one Zod 4 version with overrides (npm, pnpm) or resolutions (Yarn).  [✓0 ✗0]
2. Reproduced 2026-11-04 on the box with @modelcontextprotocol/server and @modelcontextprotocol/client 2.4.0 on Node.js 24. With zod 3.25.76: registerTool with a raw shape throws TypeError: Raw-shape inputSchema/outputSchema/argsSchema fields must be Zod v4 schemas. Got a Zod v3 field schema. Import from `zod/v4` (or upgrade your zod import), or wrap with `z.object({...})` yourself.; with z.object() the client's listTools() rejects with ProtocolError -32603: Schema appears to be from zod 3, which t  [✓0 ✗0]

## Fix
```text
Set `"zod": "^4.2.0"` in package.json (and `"overrides": {"zod": "^4.2.0"}` if a dependency pulls another copy; `npm ls zod` should show one version), or wrap a hand-written JSON Schema with fromJsonSchema().
```

## Error signatures
- substring: `must be Zod v4 schemas. Got a Zod v3 field schema` (reproduced on @modelcontextprotocol/server 2.4.0 + zod 3.25.76)
- substring: `Schema appears to be from zod 3, which the SDK cannot convert to JSON Schema` (reproduced on 2.4.0 + zod 3.25.76 (tools/list -32603))
- substring: `Type instantiation is excessively deep and possibly infinite` (troubleshooting page heading (TS2589, two zod copies); generic TypeScript text, scoped to the service)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0862/reports_

## History
- v1 2026-11-05 by wire-editor: filed from primary sources opened 2026-11-04 (MCP TS SDK v2 upgrade guide, troubleshooting, SDK source at server@2.4.0); strings reproduced on 2.4.0