fix: MCP 2025-11-25 elicitation enum schemas lose enum constraints during decoding

[lloydrichards]

Problem

Elicitation form schemas containing enum definitions can be decoded as the broader string-schema variant. Because the string variant accepts the same base shape, enum-specific fields such as enum, oneOf, and legacy enumNames can be stripped before the request is sent.

The external conformance scenarios expose the problem for MCP 2025-11-25:

• elicitation-sep1034-defaults
• elicitation-sep1330-enums

Observed behavior

A valid enum schema is projected as a plain string schema or loses enum-specific information, causing the elicitation adapter checks to fail.

Expected behavior

Valid single-select, multi-select, and legacy titled enum schemas should preserve their enum constraints and titles through decoding and protocol projection.

Regression coverage

Add schema and adapter tests covering:

• enum with enumNames;
• oneOf titled choices;
• multi-select enum values;
• defaults on enum-backed elicitation fields.

The encoded elicitation request should retain the original constraints.

[lloydrichards]

see #8060 (comment)

[lloydrichards]

fix: preserve elicitation enum variants -> Keeps standard, titled, and legacy enum forms intact when decoding elicitation schemas. (elicitation-sep1330-enums)

This one was the most suprising for me, but apparent the order of the Schema.Union resulted in a different JsonSchema being emitted since tries its members in order and accepts the first schema that successfully decodes the input.

@gcanti I get that this is mentioned in the docs, but maybe there should be a gotcha about having narrower overlapping struct schemas first since it can result in bugs like this.

for example:

import { Schema } from "effect"

const PlainString = Schema.Struct({
  type: Schema.Literal("string")
})

const StringEnum = Schema.Struct({
  type: Schema.Literal("string"),
  enum: Schema.Array(Schema.String)
})

const input = {
  type: "string",
  enum: ["red", "green"]
}

// Broad schema first
const BroadFirst = Schema.Union([
  PlainString,
  StringEnum
])

console.log(Schema.decodeUnknownSync(BroadFirst)(input))
// { type: "string" }
// `enum` was treated as an excess property and removed.

// Specific schema first
const SpecificFirst = Schema.Union([
  StringEnum,
  PlainString
])

console.log(Schema.decodeUnknownSync(SpecificFirst)(input))
// { type: "string", enum: ["red", "green"] }