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feat/Issue-13: elicitation support (#188)

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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 2025-08-22 18:59:51 +05:30 • committed by GitHub
commit c058c6a93b
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6 changed files with 836 additions and 1 deletions
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171
mcp/client.go
View file

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

86
mcp/elicitation_example_test.go Normal file
View file

@ -0,0 +1,86 @@
// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
// Use of this source code is governed by an MIT-style
// license that can be found in the LICENSE file.
package mcp_test
import (
"context"
"fmt"
"log"
"github.com/google/jsonschema-go/jsonschema"
"github.com/modelcontextprotocol/go-sdk/mcp"
)
func Example_elicitation() {
ctx := context.Background()
clientTransport, serverTransport := mcp.NewInMemoryTransports()
// Create server
server := mcp.NewServer(&mcp.Implementation{Name: "config-server", Version: "v1.0.0"}, nil)
serverSession, err := server.Connect(ctx, serverTransport, nil)
if err != nil {
log.Fatal(err)
}
// Create client with elicitation handler
// Note: Never use elicitation for sensitive data like API keys or passwords
client := mcp.NewClient(&mcp.Implementation{Name: "config-client", Version: "v1.0.0"}, &mcp.ClientOptions{
ElicitationHandler: func(ctx context.Context, request *mcp.ElicitRequest) (*mcp.ElicitResult, error) {
fmt.Printf("Server requests: %s\n", request.Params.Message)
// In a real application, this would prompt the user for input
// Here we simulate user providing configuration data
return &mcp.ElicitResult{
Action: "accept",
Content: map[string]any{
"serverEndpoint": "https://api.example.com",
"maxRetries": float64(3),
"enableLogs": true,
},
}, nil
},
})
_, err = client.Connect(ctx, clientTransport, nil)
if err != nil {
log.Fatal(err)
}
// Server requests user configuration via elicitation
configSchema := &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"serverEndpoint": {Type: "string", Description: "Server endpoint URL"},
"maxRetries": {Type: "number", Minimum: ptr(1.0), Maximum: ptr(10.0)},
"enableLogs": {Type: "boolean", Description: "Enable debug logging"},
},
Required: []string{"serverEndpoint"},
}
result, err := serverSession.Elicit(ctx, &mcp.ElicitParams{
Message: "Please provide your configuration settings",
RequestedSchema: configSchema,
})
if err != nil {
log.Fatal(err)
}
if result.Action == "accept" {
fmt.Printf("Configuration received: Endpoint: %v, Max Retries: %.0f, Logs: %v\n",
result.Content["serverEndpoint"],
result.Content["maxRetries"],
result.Content["enableLogs"])
}
// Output:
// Server requests: Please provide your configuration settings
// Configuration received: Endpoint: https://api.example.com, Max Retries: 3, Logs: true
}
// ptr is a helper function to create pointers for schema constraints
func ptr[T any](v T) *T {
return &v
}

540
mcp/mcp_test.go
View file

@ -7,6 +7,7 @@ package mcp
import (
"bytes"
"context"
"encoding/json"
"errors"
"fmt"
"io"
@ -132,6 +133,15 @@ func TestEndToEnd(t *testing.T) {
CreateMessageHandler: func(context.Context, *CreateMessageRequest) (*CreateMessageResult, error) {
return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
},
ElicitationHandler: func(ctx context.Context, req *ElicitRequest) (*ElicitResult, error) {
return &ElicitResult{
Action: "accept",
Content: map[string]any{
"name": "Test User",
"email": "test@example.com",
},
}, nil
},
ToolListChangedHandler: func(context.Context, *ToolListChangedRequest) {
notificationChans["tools"] <- 0
},
@ -474,6 +484,19 @@ func TestEndToEnd(t *testing.T) {
}
})
t.Run("elicitation", func(t *testing.T) {
result, err := ss.Elicit(ctx, &ElicitParams{
Message: "Please provide information",
})
if err != nil {
t.Fatal(err)
}
if result.Action != "accept" {
t.Errorf("got action %q, want %q", result.Action, "accept")
}
})
// Disconnect.
cs.Close()
clientWG.Wait()
@ -906,6 +929,518 @@ func TestKeepAlive(t *testing.T) {
}
}
func TestElicitationUnsupportedMethod(t *testing.T) {
ctx := context.Background()
ct, st := NewInMemoryTransports()
// Server
s := NewServer(testImpl, nil)
ss, err := s.Connect(ctx, st, nil)
if err != nil {
t.Fatal(err)
}
defer ss.Close()
// Client without ElicitationHandler
c := NewClient(testImpl, &ClientOptions{
CreateMessageHandler: func(context.Context, *CreateMessageRequest) (*CreateMessageResult, error) {
return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
},
})
cs, err := c.Connect(ctx, ct, nil)
if err != nil {
t.Fatal(err)
}
defer cs.Close()
// Test that elicitation fails when no handler is provided
_, err = ss.Elicit(ctx, &ElicitParams{
Message: "This should fail",
RequestedSchema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"test": {Type: "string"},
},
},
})
if err == nil {
t.Error("expected error when ElicitationHandler is not provided, got nil")
}
if code := errorCode(err); code != CodeUnsupportedMethod {
t.Errorf("got error code %d, want %d (CodeUnsupportedMethod)", code, CodeUnsupportedMethod)
}
if !strings.Contains(err.Error(), "does not support elicitation") {
t.Errorf("error should mention unsupported elicitation, got: %v", err)
}
}
func TestElicitationSchemaValidation(t *testing.T) {
ctx := context.Background()
ct, st := NewInMemoryTransports()
s := NewServer(testImpl, nil)
ss, err := s.Connect(ctx, st, nil)
if err != nil {
t.Fatal(err)
}
defer ss.Close()
c := NewClient(testImpl, &ClientOptions{
ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
return &ElicitResult{Action: "accept", Content: map[string]any{"test": "value"}}, nil
},
})
cs, err := c.Connect(ctx, ct, nil)
if err != nil {
t.Fatal(err)
}
defer cs.Close()
// Test valid schemas - these should not return errors
validSchemas := []struct {
name string
schema *jsonschema.Schema
}{
{
name: "nil schema",
schema: nil,
},
{
name: "empty object schema",
schema: &jsonschema.Schema{
Type: "object",
},
},
{
name: "simple string property",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string"},
},
},
},
{
name: "string with valid formats",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"email": {Type: "string", Format: "email"},
"website": {Type: "string", Format: "uri"},
"birthday": {Type: "string", Format: "date"},
"created": {Type: "string", Format: "date-time"},
},
},
},
{
name: "string with constraints",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string", MinLength: ptr(1), MaxLength: ptr(100)},
},
},
},
{
name: "number with constraints",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"age": {Type: "integer", Minimum: ptr(0.0), Maximum: ptr(150.0)},
"score": {Type: "number", Minimum: ptr(0.0), Maximum: ptr(100.0)},
},
},
},
{
name: "boolean with default",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"enabled": {Type: "boolean", Default: json.RawMessage("true")},
},
},
},
{
name: "string enum",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"status": {
Type: "string",
Enum: []any{
"active",
"inactive",
"pending",
},
},
},
},
},
{
name: "enum with matching enumNames",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"priority": {
Type: "string",
Enum: []any{
"high",
"medium",
"low",
},
Extra: map[string]any{
"enumNames": []interface{}{"High Priority", "Medium Priority", "Low Priority"},
},
},
},
},
},
}
for _, tc := range validSchemas {
t.Run("valid_"+tc.name, func(t *testing.T) {
_, err := ss.Elicit(ctx, &ElicitParams{
Message: "Test valid schema: " + tc.name,
RequestedSchema: tc.schema,
})
if err != nil {
t.Errorf("expected no error for valid schema %q, got: %v", tc.name, err)
}
})
}
// Test invalid schemas - these should return errors
invalidSchemas := []struct {
name string
schema *jsonschema.Schema
expectedError string
}{
{
name: "root schema non-object type",
schema: &jsonschema.Schema{
Type: "string",
},
expectedError: "elicit schema must be of type 'object', got \"string\"",
},
{
name: "nested object property",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"user": {
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string"},
},
},
},
},
expectedError: "elicit schema property \"user\" contains nested properties, only primitive properties are allowed",
},
{
name: "property with explicit object type",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"config": {Type: "object"},
},
},
expectedError: "elicit schema property \"config\" has unsupported type \"object\", only string, number, integer, and boolean are allowed",
},
{
name: "array property",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"tags": {Type: "array", Items: &jsonschema.Schema{Type: "string"}},
},
},
expectedError: "elicit schema property \"tags\" has unsupported type \"array\", only string, number, integer, and boolean are allowed",
},
{
name: "array without items",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"items": {Type: "array"},
},
},
expectedError: "elicit schema property \"items\" has unsupported type \"array\", only string, number, integer, and boolean are allowed",
},
{
name: "unsupported string format",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"phone": {Type: "string", Format: "phone"},
},
},
expectedError: "elicit schema property \"phone\" has unsupported format \"phone\", only email, uri, date, and date-time are allowed",
},
{
name: "unsupported type",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"data": {Type: "null"},
},
},
expectedError: "elicit schema property \"data\" has unsupported type \"null\"",
},
{
name: "string with invalid minLength",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string", MinLength: ptr(-1)},
},
},
expectedError: "elicit schema property \"name\" has invalid minLength -1, must be non-negative",
},
{
name: "string with invalid maxLength",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string", MaxLength: ptr(-5)},
},
},
expectedError: "elicit schema property \"name\" has invalid maxLength -5, must be non-negative",
},
{
name: "string with maxLength less than minLength",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"name": {Type: "string", MinLength: ptr(10), MaxLength: ptr(5)},
},
},
expectedError: "elicit schema property \"name\" has maxLength 5 less than minLength 10",
},
{
name: "number with maximum less than minimum",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"score": {Type: "number", Minimum: ptr(100.0), Maximum: ptr(50.0)},
},
},
expectedError: "elicit schema property \"score\" has maximum 50 less than minimum 100",
},
{
name: "boolean with invalid default",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"enabled": {Type: "boolean", Default: json.RawMessage(`"not-a-boolean"`)},
},
},
expectedError: "elicit schema property \"enabled\" has invalid default value, must be a boolean",
},
{
name: "enum with mismatched enumNames length",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"priority": {
Type: "string",
Enum: []any{
"high",
"medium",
"low",
},
Extra: map[string]any{
"enumNames": []interface{}{"High Priority", "Medium Priority"}, // Only 2 names for 3 values
},
},
},
},
expectedError: "elicit schema property \"priority\" has 3 enum values but 2 enumNames, they must match",
},
{
name: "enum with invalid enumNames type",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"status": {
Type: "string",
Enum: []any{
"active",
"inactive",
},
Extra: map[string]any{
"enumNames": "not an array", // Should be array
},
},
},
},
expectedError: "elicit schema property \"status\" has invalid enumNames type, must be an array",
},
{
name: "enum without explicit type",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"priority": {
Enum: []any{
"high",
"medium",
"low",
},
},
},
},
expectedError: "elicit schema property \"priority\" has unsupported type \"\", only string, number, integer, and boolean are allowed",
},
{
name: "untyped property",
schema: &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"data": {},
},
},
expectedError: "elicit schema property \"data\" has unsupported type \"\", only string, number, integer, and boolean are allowed",
},
}
for _, tc := range invalidSchemas {
t.Run("invalid_"+tc.name, func(t *testing.T) {
_, err := ss.Elicit(ctx, &ElicitParams{
Message: "Test invalid schema: " + tc.name,
RequestedSchema: tc.schema,
})
if err == nil {
t.Errorf("expected error for invalid schema %q, got nil", tc.name)
return
}
if code := errorCode(err); code != CodeInvalidParams {
t.Errorf("got error code %d, want %d (CodeInvalidParams)", code, CodeInvalidParams)
}
if !strings.Contains(err.Error(), tc.expectedError) {
t.Errorf("error message %q does not contain expected text %q", err.Error(), tc.expectedError)
}
})
}
}
func TestElicitationProgressToken(t *testing.T) {
ctx := context.Background()
ct, st := NewInMemoryTransports()
s := NewServer(testImpl, nil)
ss, err := s.Connect(ctx, st, nil)
if err != nil {
t.Fatal(err)
}
defer ss.Close()
c := NewClient(testImpl, &ClientOptions{
ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
return &ElicitResult{Action: "accept"}, nil
},
})
cs, err := c.Connect(ctx, ct, nil)
if err != nil {
t.Fatal(err)
}
defer cs.Close()
params := &ElicitParams{
Message: "Test progress token",
Meta: Meta{},
}
params.SetProgressToken("test-token")
if token := params.GetProgressToken(); token != "test-token" {
t.Errorf("got progress token %v, want %q", token, "test-token")
}
_, err = ss.Elicit(ctx, params)
if err != nil {
t.Fatal(err)
}
}
func TestElicitationCapabilityDeclaration(t *testing.T) {
ctx := context.Background()
t.Run("with_handler", func(t *testing.T) {
ct, st := NewInMemoryTransports()
// Client with ElicitationHandler should declare capability
c := NewClient(testImpl, &ClientOptions{
ElicitationHandler: func(context.Context, *ElicitRequest) (*ElicitResult, error) {
return &ElicitResult{Action: "cancel"}, nil
},
})
s := NewServer(testImpl, nil)
ss, err := s.Connect(ctx, st, nil)
if err != nil {
t.Fatal(err)
}
defer ss.Close()
cs, err := c.Connect(ctx, ct, nil)
if err != nil {
t.Fatal(err)
}
defer cs.Close()
// The client should have declared elicitation capability during initialization
// We can verify this worked by successfully making an elicitation call
result, err := ss.Elicit(ctx, &ElicitParams{
Message: "Test capability",
RequestedSchema: &jsonschema.Schema{Type: "object"},
})
if err != nil {
t.Errorf("elicitation should work when capability is declared, got error: %v", err)
}
if result.Action != "cancel" {
t.Errorf("got action %q, want %q", result.Action, "cancel")
}
})
t.Run("without_handler", func(t *testing.T) {
ct, st := NewInMemoryTransports()
// Client without ElicitationHandler should not declare capability
c := NewClient(testImpl, &ClientOptions{
CreateMessageHandler: func(context.Context, *CreateMessageRequest) (*CreateMessageResult, error) {
return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
},
})
s := NewServer(testImpl, nil)
ss, err := s.Connect(ctx, st, nil)
if err != nil {
t.Fatal(err)
}
defer ss.Close()
cs, err := c.Connect(ctx, ct, nil)
if err != nil {
t.Fatal(err)
}
defer cs.Close()
// Elicitation should fail with UnsupportedMethod
_, err = ss.Elicit(ctx, &ElicitParams{
Message: "This should fail",
RequestedSchema: &jsonschema.Schema{Type: "object"},
})
if err == nil {
t.Error("expected UnsupportedMethod error when no capability declared")
}
if code := errorCode(err); code != CodeUnsupportedMethod {
t.Errorf("got error code %d, want %d (CodeUnsupportedMethod)", code, CodeUnsupportedMethod)
}
})
}
func TestKeepAliveFailure(t *testing.T) {
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
@ -1185,3 +1720,8 @@ func TestPointerArgEquivalence(t *testing.T) {
})
}
}
// ptr is a helper function to create pointers for schema constraints
func ptr[T any](v T) *T {
return &v
}

34
mcp/protocol.go
View file

@ -968,7 +968,39 @@ func (*ResourceUpdatedNotificationParams) isParams() {}
// TODO(jba): add CompleteRequest and related types.
// TODO(jba): add ElicitRequest and related types.
// A request from the server to elicit additional information from the user via the client.
type ElicitParams struct {
// This property is reserved by the protocol to allow clients and servers to
// attach additional metadata to their responses.
Meta `json:"_meta,omitempty"`
// The message to present to the user.
Message string `json:"message"`
// A restricted subset of JSON Schema.
// Only top-level properties are allowed, without nesting.
RequestedSchema *jsonschema.Schema `json:"requestedSchema"`
}
func (x *ElicitParams) isParams() {}
func (x *ElicitParams) GetProgressToken() any { return getProgressToken(x) }
func (x *ElicitParams) SetProgressToken(t any) { setProgressToken(x, t) }
// The client's response to an elicitation/create request from the server.
type ElicitResult struct {
// This property is reserved by the protocol to allow clients and servers to
// attach additional metadata to their responses.
Meta `json:"_meta,omitempty"`
// The user action in response to the elicitation.
// - "accept": User submitted the form/confirmed the action
// - "decline": User explicitly declined the action
// - "cancel": User dismissed without making an explicit choice
Action string `json:"action"`
// The submitted form data, only present when action is "accept".
// Contains values matching the requested schema.
Content map[string]any `json:"content,omitempty"`
}
func (*ElicitResult) isResult() {}
// An Implementation describes the name and version of an MCP implementation, with an optional
// title for UI representation.

1
mcp/requests.go
View file

@ -24,6 +24,7 @@ type (
type (
CreateMessageRequest = ClientRequest[*CreateMessageParams]
ElicitRequest = ClientRequest[*ElicitParams]
InitializedClientRequest = ClientRequest[*InitializedParams]
InitializeRequest = ClientRequest[*InitializeParams]
ListRootsRequest = ClientRequest[*ListRootsParams]

5
mcp/server.go
View file

@ -834,6 +834,11 @@ func (ss *ServerSession) CreateMessage(ctx context.Context, params *CreateMessag
return handleSend[*CreateMessageResult](ctx, methodCreateMessage, newServerRequest(ss, orZero[Params](params)))
}
// Elicit sends an elicitation request to the client asking for user input.
func (ss *ServerSession) Elicit(ctx context.Context, params *ElicitParams) (*ElicitResult, error) {
return handleSend[*ElicitResult](ctx, methodElicit, newServerRequest(ss, orZero[Params](params)))
}
// Log sends a log message to the client.
// The message is not sent if the client has not called SetLevel, or if its level
// is below that of the last SetLevel.