Consider type coercion for tool arguments

[domdomegg]

Problem

Models occasionally send tool arguments with incorrect JSON types - for example, numbers serialized as strings ({"thoughtNumber": "1"} instead of {"thoughtNumber": 1}). This causes Zod validation to reject otherwise valid requests.

This issue affects many MCP servers. For example, modelcontextprotocol/servers#2812 added a specific workaround to one part of the sequential-thinking server, but the maintainers noted this feels like the wrong layer to fix it.

The "proper" fix is probably smarter models, or as a consistent layer in the SDKs. This would avoid server authors either having to implement ad-hoc coercion logic OR suffer from these problems.

Proposal

The SDK could provide type coercion for tool arguments, either:

1. Built-in coercion - automatically coerce string→number, string→boolean etc. when the schema expects a different primitive type (e.g. similar to https://ajv.js.org/coercion.html)
2. Composable helper - export a coerceToolArgs(schema, args) utility that servers can use
3. Documentation - guidance on how to handle this with Zod's .coerce or .preprocess (but doing so safely so that e.g. "thing" doesn't become true when coerced to bool)

Example

// Current: fails validation
const args1 = { thoughtNumber: "1" }  // model sent number string
const args2 = { thoughtNumber: "one" }  // model sent string
const schema = z.object({ thoughtNumber: z.number() })
schema.parse(args1)  // ZodError: Expected number, received string
schema.parse(args2)  // ZodError: Expected number, received string

// With coercion: works
const coercedSchema = z.object({ thoughtNumber: z.coerce.number() })
coercedSchema.parse(args1)  // { thoughtNumber: 1 }
coercedSchema.parse(args2)  // ZodError: Expected number, received string