feat/Issue-13: elicitation support (#188)
fixes: #13 Implements elicitation functionality from MCP 2025-06-18 specification. Changes - Add ElicitParams and ElicitResult protocol types - Add ServerSession.Elicit() method and ClientOptions.ElicitationHandler - Schema validation enforces top-level properties only - Support for accept/decline/cancel actions with progress tokens Tests - Integration test in TestEndToEnd - Schema validation, error handling, and capability declaration tests
This commit is contained in:
Kartik Verma
GitHub
parent
62db914082
commit
c058c6a93b
1 changed files with 836 additions and 1 deletions
171
mcp/client.go
171
mcp/client.go
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@ -6,12 +6,14 @@ package mcp
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import (
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"context"
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"encoding/json"
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"fmt"
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"iter"
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"slices"
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"sync"
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"time"
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"github.com/google/jsonschema-go/jsonschema"
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"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
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"github.com/modelcontextprotocol/go-sdk/jsonrpc"
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)
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@ -56,6 +58,9 @@ type ClientOptions struct {
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// Handler for sampling.
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// Called when a server calls CreateMessage.
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CreateMessageHandler func(context.Context, *CreateMessageRequest) (*CreateMessageResult, error)
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// Handler for elicitation.
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// Called when a server requests user input via Elicit.
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ElicitationHandler func(context.Context, *ElicitRequest) (*ElicitResult, error)
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// Handlers for notifications from the server.
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ToolListChangedHandler func(context.Context, *ToolListChangedRequest)
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PromptListChangedHandler func(context.Context, *PromptListChangedRequest)
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@ -111,6 +116,9 @@ func (c *Client) capabilities() *ClientCapabilities {
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if c.opts.CreateMessageHandler != nil {
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caps.Sampling = &SamplingCapabilities{}
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}
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if c.opts.ElicitationHandler != nil {
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caps.Elicitation = &ElicitationCapabilities{}
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}
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return caps
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}
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@ -268,6 +276,168 @@ func (c *Client) createMessage(ctx context.Context, req *CreateMessageRequest) (
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return c.opts.CreateMessageHandler(ctx, req)
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}
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func (c *Client) elicit(ctx context.Context, req *ElicitRequest) (*ElicitResult, error) {
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if c.opts.ElicitationHandler == nil {
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// TODO: wrap or annotate this error? Pick a standard code?
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return nil, jsonrpc2.NewError(CodeUnsupportedMethod, "client does not support elicitation")
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}
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// Validate that the requested schema only contains top-level properties without nesting
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if err := validateElicitSchema(req.Params.RequestedSchema); err != nil {
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return nil, jsonrpc2.NewError(CodeInvalidParams, err.Error())
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}
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res, err := c.opts.ElicitationHandler(ctx, req)
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if err != nil {
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return nil, err
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}
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// Validate elicitation result content against requested schema
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if req.Params.RequestedSchema != nil && res.Content != nil {
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resolved, err := req.Params.RequestedSchema.Resolve(nil)
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if err != nil {
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return nil, jsonrpc2.NewError(CodeInvalidParams, fmt.Sprintf("failed to resolve requested schema: %v", err))
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}
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if err := resolved.Validate(res.Content); err != nil {
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return nil, jsonrpc2.NewError(CodeInvalidParams, fmt.Sprintf("elicitation result content does not match requested schema: %v", err))
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}
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}
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return res, nil
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}
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// validateElicitSchema validates that the schema conforms to MCP elicitation schema requirements.
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// Per the MCP specification, elicitation schemas are limited to flat objects with primitive properties only.
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func validateElicitSchema(schema *jsonschema.Schema) error {
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if schema == nil {
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return nil // nil schema is allowed
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}
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// The root schema must be of type "object" if specified
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if schema.Type != "" && schema.Type != "object" {
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return fmt.Errorf("elicit schema must be of type 'object', got %q", schema.Type)
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}
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// Check if the schema has properties
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if schema.Properties != nil {
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for propName, propSchema := range schema.Properties {
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if propSchema == nil {
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continue
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}
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if err := validateElicitProperty(propName, propSchema); err != nil {
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return err
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}
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}
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}
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return nil
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}
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// validateElicitProperty validates a single property in an elicitation schema.
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func validateElicitProperty(propName string, propSchema *jsonschema.Schema) error {
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// Check if this property has nested properties (not allowed)
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if len(propSchema.Properties) > 0 {
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return fmt.Errorf("elicit schema property %q contains nested properties, only primitive properties are allowed", propName)
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}
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// Validate based on the property type - only primitives are supported
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switch propSchema.Type {
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case "string":
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return validateElicitStringProperty(propName, propSchema)
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case "number", "integer":
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return validateElicitNumberProperty(propName, propSchema)
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case "boolean":
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return validateElicitBooleanProperty(propName, propSchema)
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default:
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return fmt.Errorf("elicit schema property %q has unsupported type %q, only string, number, integer, and boolean are allowed", propName, propSchema.Type)
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}
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}
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// validateElicitStringProperty validates string-type properties, including enums.
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func validateElicitStringProperty(propName string, propSchema *jsonschema.Schema) error {
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// Handle enum validation (enums are a special case of strings)
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if len(propSchema.Enum) > 0 {
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// Enums must be string type (or untyped which defaults to string)
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if propSchema.Type != "" && propSchema.Type != "string" {
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return fmt.Errorf("elicit schema property %q has enum values but type is %q, enums are only supported for string type", propName, propSchema.Type)
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}
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// Enum values themselves are validated by the JSON schema library
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// Validate enumNames if present - must match enum length
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if propSchema.Extra != nil {
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if enumNamesRaw, exists := propSchema.Extra["enumNames"]; exists {
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// Type check enumNames - should be a slice
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if enumNamesSlice, ok := enumNamesRaw.([]interface{}); ok {
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if len(enumNamesSlice) != len(propSchema.Enum) {
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return fmt.Errorf("elicit schema property %q has %d enum values but %d enumNames, they must match", propName, len(propSchema.Enum), len(enumNamesSlice))
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}
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} else {
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return fmt.Errorf("elicit schema property %q has invalid enumNames type, must be an array", propName)
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}
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}
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}
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return nil
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}
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// Validate format if specified - only specific formats are allowed
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if propSchema.Format != "" {
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allowedFormats := map[string]bool{
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"email": true,
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"uri": true,
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"date": true,
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"date-time": true,
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}
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if !allowedFormats[propSchema.Format] {
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return fmt.Errorf("elicit schema property %q has unsupported format %q, only email, uri, date, and date-time are allowed", propName, propSchema.Format)
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}
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}
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// Validate minLength constraint if specified
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if propSchema.MinLength != nil {
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if *propSchema.MinLength < 0 {
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return fmt.Errorf("elicit schema property %q has invalid minLength %d, must be non-negative", propName, *propSchema.MinLength)
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}
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}
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// Validate maxLength constraint if specified
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if propSchema.MaxLength != nil {
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if *propSchema.MaxLength < 0 {
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return fmt.Errorf("elicit schema property %q has invalid maxLength %d, must be non-negative", propName, *propSchema.MaxLength)
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}
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// Check that maxLength >= minLength if both are specified
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if propSchema.MinLength != nil && *propSchema.MaxLength < *propSchema.MinLength {
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return fmt.Errorf("elicit schema property %q has maxLength %d less than minLength %d", propName, *propSchema.MaxLength, *propSchema.MinLength)
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}
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}
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return nil
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}
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// validateElicitNumberProperty validates number and integer-type properties.
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func validateElicitNumberProperty(propName string, propSchema *jsonschema.Schema) error {
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if propSchema.Minimum != nil && propSchema.Maximum != nil {
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if *propSchema.Maximum < *propSchema.Minimum {
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return fmt.Errorf("elicit schema property %q has maximum %g less than minimum %g", propName, *propSchema.Maximum, *propSchema.Minimum)
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}
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}
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return nil
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}
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// validateElicitBooleanProperty validates boolean-type properties.
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func validateElicitBooleanProperty(propName string, propSchema *jsonschema.Schema) error {
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// Validate default value if specified - must be a valid boolean
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if propSchema.Default != nil {
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var defaultValue bool
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if err := json.Unmarshal(propSchema.Default, &defaultValue); err != nil {
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return fmt.Errorf("elicit schema property %q has invalid default value, must be a boolean: %v", propName, err)
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}
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}
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return nil
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}
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// AddSendingMiddleware wraps the current sending method handler using the provided
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// middleware. Middleware is applied from right to left, so that the first one is
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// executed first.
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@ -308,6 +478,7 @@ var clientMethodInfos = map[string]methodInfo{
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methodPing: newClientMethodInfo(clientSessionMethod((*ClientSession).ping), missingParamsOK),
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methodListRoots: newClientMethodInfo(clientMethod((*Client).listRoots), missingParamsOK),
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methodCreateMessage: newClientMethodInfo(clientMethod((*Client).createMessage), 0),
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methodElicit: newClientMethodInfo(clientMethod((*Client).elicit), missingParamsOK),
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notificationCancelled: newClientMethodInfo(clientSessionMethod((*ClientSession).cancel), notification|missingParamsOK),
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notificationToolListChanged: newClientMethodInfo(clientMethod((*Client).callToolChangedHandler), notification|missingParamsOK),
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notificationPromptListChanged: newClientMethodInfo(clientMethod((*Client).callPromptChangedHandler), notification|missingParamsOK),
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