SEP-1330 Elicitation Enum Schema Improvements and Standards Compliance (modelcontextprotocol#1148)

* Replace enumNames with standard-compliant oneOf pattern

The enumNames property is not a valid JSON Schema keyword. Updated
EnumSchema to use the standard-compliant oneOf pattern with const
and title keywords for each option, as recommended by JSON Schema
specification.

* Refactor enum schemas to support 3 distinct types

- SingleSelectEnumSchema: Supports both plain enum[] and oneOf with titles
- MultiSelectEnumSchema: Array type for multi-selection with/without titles
- DeprecatedEnumSchema: Maintains backward compatibility with enumNames

This replaces the non-standard enumNames approach with JSON Schema-compliant
patterns using oneOf/const for titled enums, while preserving backward
compatibility and adding multi-select support.

* nit: address tiny comment about # maxItems

* Refactor enum schemas with typed variants and array support

* Update enum examples in elicitation docs with validated schemas

* replace deprecated with legacy

* nits

* improve comment

* reset docs/specification/2025-06-18/schema.mdx

* fix

* Improve docs and add changelog entry

* add another doc comment

* add another doc comment

* Update multi-select to align with SEP-1330

* Fix default values on new elicitation schemas

* Make individual Schemas interfaces rather than type aliases

* run prettier

* update types based on discussion to match validation

---------

Co-authored-by: Tapan Chugh <[email protected]>
Co-authored-by: Cliff Hall <[email protected]>
Co-authored-by: Claude <[email protected]>
Co-authored-by: Felix Weinberger <[email protected]>

Author: 5 people

docs/specification/draft/changelog.mdx

@@ -13,6 +13,7 @@ the previous revision, [2025-06-18](/specification/2025-06-18).
 2. Allow servers to expose icons as additional metadata for tools, resources, resource templates, and prompts ([SEP-973](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/973)).
 3. Enhance authorization flows with incremental scope consent via `WWW-Authenticate` ([SEP-835](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/835))
 4. Provide guidance on tool names ([SEP-986](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1603))
+5. Update `ElicitResult` and `EnumSchema` to use a more standards-based approach and support titled, untitled, single-select, and multi-select enums ([SEP-1330](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/1330)).
 
 ## Minor changes
 

docs/specification/draft/client/elicitation.mdx

@@ -265,14 +265,69 @@ The schema is restricted to these primitive types:
    ```
 
 4. **Enum Schema**
+
+   Single-select enum (without titles):
+
    ```json
    {
      "type": "string",
-     "title": "Display Name",
-     "description": "Description text",
-     "enum": ["option1", "option2", "option3"],
-     "enumNames": ["Option 1", "Option 2", "Option 3"],
-     "default": "option1"
+     "title": "Color Selection",
+     "description": "Choose your favorite color",
+     "enum": ["Red", "Green", "Blue"],
+     "default": "Red"
+   }
+   ```
+
+   Single-select enum (with titles):
+
+   ```json
+   {
+     "type": "string",
+     "title": "Color Selection",
+     "description": "Choose your favorite color",
+     "oneOf": [
+       { "const": "#FF0000", "title": "Red" },
+       { "const": "#00FF00", "title": "Green" },
+       { "const": "#0000FF", "title": "Blue" }
+     ],
+     "default": "#FF0000"
+   }
+   ```
+
+   Multi-select enum (without titles):
+
+   ```json
+   {
+     "type": "array",
+     "title": "Color Selection",
+     "description": "Choose your favorite colors",
+     "minItems": 1,
+     "maxItems": 2,
+     "items": {
+       "type": "string",
+       "enum": ["Red", "Green", "Blue"]
+     },
+     "default": ["Red", "Green"]
+   }
+   ```
+
+   Multi-select enum (with titles):
+
+   ```json
+   {
+     "type": "array",
+     "title": "Color Selection",
+     "description": "Choose your favorite colors",
+     "minItems": 1,
+     "maxItems": 2,
+     "items": {
+       "oneOf": [
+         { "const": "#FF0000", "title": "Red" },
+         { "const": "#00FF00", "title": "Green" },
+         { "const": "#0000FF", "title": "Blue" }
+       ]
+     },
+     "default": ["#FF0000", "#00FF00"]
    }
    ```
 
@@ -284,7 +339,7 @@ Clients can use this schema to:
 
 All primitive types support optional default values to provide sensible starting points. Clients that support defaults SHOULD pre-populate form fields with these values.
 
-Note that complex nested structures, arrays of objects, and other advanced JSON Schema features are intentionally not supported to simplify client user experience.
+Note that complex nested structures, arrays of objects (beyond enums), and other advanced JSON Schema features are intentionally not supported to simplify client user experience.
 
 ## Response Actions