Failed building the serialization schema for GET

[punkpeye]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

5.6.2

Plugin version

No response

Node.js version

24

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

26.0

Description

Unclear why this suddenly started to throw an error.

FastifyError: Failed building the serialization schema for GET: /api/mcp/v1/servers, due to error Zod schema of type `custom` at properties > zodOpenApiCreateSchema > properties > servers > items > properties > tools > items > properties > inputSchema cannot be represented in OpenAPI. Please assign it metadata with `.meta()`

Link to code that reproduces the bug

N/A

Expected Behavior

I should be able to use custom types.

const McpServerToolModelZodSchema = z
  .object({
    description: z.string().nullable(),
    inputSchema: z.custom<JSONSchema7Object>().meta({
       description: 'MCP server tool input schema',
    }),
    name: z.string(),
  })
  .meta({
    description: 'MCP server tool',
    ref: 'McpServerTool',
  });

[gurgunday]

Did it start happening after you updated Fastify?

[punkpeye]

Did it start happening after you updated Fastify?

Cannot confidently say if this was triggered by Fastify or Zod update since I've updated all dependencies in one go.

[mcollina]

Thanks for reporting!

Can you provide steps to reproduce? We often need a reproducible example, e.g. some code that allows someone else to recreate your problem by just copying and pasting it. If it involves more than a couple of different file, create a new repository on GitHub and add a link to that.