diff --git a/mcp/protocol.go b/mcp/protocol.go
index 3ca6cb5e..944b73db 100644
--- a/mcp/protocol.go
+++ b/mcp/protocol.go
@@ -78,6 +78,8 @@ type CallToolResultFor[Out any] struct {
 	IsError bool `json:"isError,omitempty"`
 }
 
+func (*CallToolResultFor[Out]) mcpResult() {}
+
 // UnmarshalJSON handles the unmarshalling of content into the Content
 // interface.
 func (x *CallToolResultFor[Out]) UnmarshalJSON(data []byte) error {
@@ -97,6 +99,7 @@ func (x *CallToolResultFor[Out]) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+func (x *CallToolParamsFor[Out]) mcpParams()             {}
 func (x *CallToolParamsFor[Out]) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CallToolParamsFor[Out]) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -114,6 +117,7 @@ type CancelledParams struct {
 	RequestID any `json:"requestId"`
 }
 
+func (x *CancelledParams) mcpParams()             {}
 func (x *CancelledParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CancelledParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -207,6 +211,8 @@ type CompleteParams struct {
 	Ref      *CompleteReference     `json:"ref"`
 }
 
+func (*CompleteParams) mcpParams() {}
+
 type CompletionResultDetails struct {
 	HasMore bool     `json:"hasMore,omitempty"`
 	Total   int      `json:"total,omitempty"`
@@ -221,6 +227,8 @@ type CompleteResult struct {
 	Completion CompletionResultDetails `json:"completion"`
 }
 
+func (*CompleteResult) mcpResult() {}
+
 type CreateMessageParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
@@ -245,6 +253,7 @@ type CreateMessageParams struct {
 	Temperature  float64 `json:"temperature,omitempty"`
 }
 
+func (x *CreateMessageParams) mcpParams()             {}
 func (x *CreateMessageParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *CreateMessageParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -264,6 +273,7 @@ type CreateMessageResult struct {
 	StopReason string `json:"stopReason,omitempty"`
 }
 
+func (*CreateMessageResult) mcpResult() {}
 func (r *CreateMessageResult) UnmarshalJSON(data []byte) error {
 	type result CreateMessageResult // avoid recursion
 	var wire struct {
@@ -291,6 +301,7 @@ type GetPromptParams struct {
 	Name string `json:"name"`
 }
 
+func (x *GetPromptParams) mcpParams()             {}
 func (x *GetPromptParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *GetPromptParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -304,6 +315,8 @@ type GetPromptResult struct {
 	Messages    []*PromptMessage `json:"messages"`
 }
 
+func (*GetPromptResult) mcpResult() {}
+
 type InitializeParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
@@ -315,6 +328,7 @@ type InitializeParams struct {
 	ProtocolVersion string `json:"protocolVersion"`
 }
 
+func (x *InitializeParams) mcpParams()             {}
 func (x *InitializeParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *InitializeParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -338,12 +352,15 @@ type InitializeResult struct {
 	ServerInfo      *Implementation `json:"serverInfo"`
 }
 
+func (*InitializeResult) mcpResult() {}
+
 type InitializedParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *InitializedParams) mcpParams()             {}
 func (x *InitializedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *InitializedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -356,6 +373,7 @@ type ListPromptsParams struct {
 	Cursor string `json:"cursor,omitempty"`
 }
 
+func (x *ListPromptsParams) mcpParams()             {}
 func (x *ListPromptsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListPromptsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListPromptsParams) cursorPtr() *string     { return &x.Cursor }
@@ -371,6 +389,7 @@ type ListPromptsResult struct {
 	Prompts    []*Prompt `json:"prompts"`
 }
 
+func (x *ListPromptsResult) mcpResult()             {}
 func (x *ListPromptsResult) nextCursorPtr() *string { return &x.NextCursor }
 
 type ListResourceTemplatesParams struct {
@@ -382,6 +401,7 @@ type ListResourceTemplatesParams struct {
 	Cursor string `json:"cursor,omitempty"`
 }
 
+func (x *ListResourceTemplatesParams) mcpParams()             {}
 func (x *ListResourceTemplatesParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListResourceTemplatesParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListResourceTemplatesParams) cursorPtr() *string     { return &x.Cursor }
@@ -397,6 +417,7 @@ type ListResourceTemplatesResult struct {
 	ResourceTemplates []*ResourceTemplate `json:"resourceTemplates"`
 }
 
+func (x *ListResourceTemplatesResult) mcpResult()             {}
 func (x *ListResourceTemplatesResult) nextCursorPtr() *string { return &x.NextCursor }
 
 type ListResourcesParams struct {
@@ -408,6 +429,7 @@ type ListResourcesParams struct {
 	Cursor string `json:"cursor,omitempty"`
 }
 
+func (x *ListResourcesParams) mcpParams()             {}
 func (x *ListResourcesParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListResourcesParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListResourcesParams) cursorPtr() *string     { return &x.Cursor }
@@ -423,6 +445,7 @@ type ListResourcesResult struct {
 	Resources  []*Resource `json:"resources"`
 }
 
+func (x *ListResourcesResult) mcpResult()             {}
 func (x *ListResourcesResult) nextCursorPtr() *string { return &x.NextCursor }
 
 type ListRootsParams struct {
@@ -431,6 +454,7 @@ type ListRootsParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *ListRootsParams) mcpParams()             {}
 func (x *ListRootsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListRootsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -444,6 +468,8 @@ type ListRootsResult struct {
 	Roots []*Root `json:"roots"`
 }
 
+func (*ListRootsResult) mcpResult() {}
+
 type ListToolsParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
@@ -453,6 +479,7 @@ type ListToolsParams struct {
 	Cursor string `json:"cursor,omitempty"`
 }
 
+func (x *ListToolsParams) mcpParams()             {}
 func (x *ListToolsParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ListToolsParams) SetProgressToken(t any) { setProgressToken(x, t) }
 func (x *ListToolsParams) cursorPtr() *string     { return &x.Cursor }
@@ -468,6 +495,7 @@ type ListToolsResult struct {
 	Tools      []*Tool `json:"tools"`
 }
 
+func (x *ListToolsResult) mcpResult()             {}
 func (x *ListToolsResult) nextCursorPtr() *string { return &x.NextCursor }
 
 // The severity of a log message.
@@ -489,6 +517,7 @@ type LoggingMessageParams struct {
 	Logger string `json:"logger,omitempty"`
 }
 
+func (x *LoggingMessageParams) mcpParams()             {}
 func (x *LoggingMessageParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *LoggingMessageParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -550,6 +579,7 @@ type PingParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *PingParams) mcpParams()             {}
 func (x *PingParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *PingParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -569,6 +599,8 @@ type ProgressNotificationParams struct {
 	Total float64 `json:"total,omitempty"`
 }
 
+func (*ProgressNotificationParams) mcpParams() {}
+
 // A prompt or prompt template that the server offers.
 type Prompt struct {
 	// See [specification/2025-06-18/basic/index#general-fields] for notes on _meta
@@ -606,6 +638,7 @@ type PromptListChangedParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *PromptListChangedParams) mcpParams()             {}
 func (x *PromptListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *PromptListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -646,6 +679,7 @@ type ReadResourceParams struct {
 	URI string `json:"uri"`
 }
 
+func (x *ReadResourceParams) mcpParams()             {}
 func (x *ReadResourceParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ReadResourceParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -657,6 +691,8 @@ type ReadResourceResult struct {
 	Contents []*ResourceContents `json:"contents"`
 }
 
+func (*ReadResourceResult) mcpResult() {}
+
 // A known resource that the server is capable of reading.
 type Resource struct {
 	// See [specification/2025-06-18/basic/index#general-fields] for notes on _meta
@@ -697,6 +733,7 @@ type ResourceListChangedParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *ResourceListChangedParams) mcpParams()             {}
 func (x *ResourceListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ResourceListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -754,6 +791,7 @@ type RootsListChangedParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *RootsListChangedParams) mcpParams()             {}
 func (x *RootsListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *RootsListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -798,6 +836,7 @@ type SetLevelParams struct {
 	Level LoggingLevel `json:"level"`
 }
 
+func (x *SetLevelParams) mcpParams()             {}
 func (x *SetLevelParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *SetLevelParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -873,6 +912,7 @@ type ToolListChangedParams struct {
 	Meta `json:"_meta,omitempty"`
 }
 
+func (x *ToolListChangedParams) mcpParams()             {}
 func (x *ToolListChangedParams) GetProgressToken() any  { return getProgressToken(x) }
 func (x *ToolListChangedParams) SetProgressToken(t any) { setProgressToken(x, t) }
 
@@ -886,6 +926,8 @@ type SubscribeParams struct {
 	URI string `json:"uri"`
 }
 
+func (*SubscribeParams) mcpParams() {}
+
 // Sent from the client to request cancellation of resources/updated
 // notifications from the server. This should follow a previous
 // resources/subscribe request.
@@ -897,6 +939,8 @@ type UnsubscribeParams struct {
 	URI string `json:"uri"`
 }
 
+func (*UnsubscribeParams) mcpParams() {}
+
 // A notification from the server to the client, informing it that a resource
 // has changed and may need to be read again. This should only be sent if the
 // client previously sent a resources/subscribe request.
@@ -908,6 +952,8 @@ type ResourceUpdatedNotificationParams struct {
 	URI string `json:"uri"`
 }
 
+func (*ResourceUpdatedNotificationParams) mcpParams() {}
+
 // TODO(jba): add CompleteRequest and related types.
 
 // TODO(jba): add ElicitRequest and related types.
diff --git a/mcp/shared.go b/mcp/shared.go
index 319071f2..8d0ceb1c 100644
--- a/mcp/shared.go
+++ b/mcp/shared.go
@@ -335,6 +335,9 @@ func setProgressToken(p Params, pt any) {
 
 // Params is a parameter (input) type for an MCP call or notification.
 type Params interface {
+	// mcpParams discourages implementation of Params outside of this package.
+	mcpParams()
+
 	// GetMeta returns metadata from a value.
 	GetMeta() map[string]any
 	// SetMeta sets the metadata on a value.
@@ -356,6 +359,9 @@ type RequestParams interface {
 
 // Result is a result of an MCP call.
 type Result interface {
+	// mcpResult discourages implementation of Result outside of this package.
+	mcpResult()
+
 	// GetMeta returns metadata from a value.
 	GetMeta() map[string]any
 	// SetMeta sets the metadata on a value.
@@ -366,6 +372,7 @@ type Result interface {
 // Those methods cannot return nil, because jsonrpc2 cannot handle nils.
 type emptyResult struct{}
 
+func (*emptyResult) mcpResult()              {}
 func (*emptyResult) GetMeta() map[string]any { panic("should never be called") }
 func (*emptyResult) SetMeta(map[string]any)  { panic("should never be called") }