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// 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
import (
"bytes"
"context"
"crypto/rand"
"encoding/base64"
"encoding/gob"
"encoding/json"
"errors"
"fmt"
"iter"
"log/slog"
"maps"
"net/url"
"path/filepath"
"reflect"
"slices"
"sync"
"sync/atomic"
"time"
"github.com/google/jsonschema-go/jsonschema"
internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
"github.com/modelcontextprotocol/go-sdk/internal/util"
"github.com/modelcontextprotocol/go-sdk/jsonrpc"
"github.com/yosida95/uritemplate/v3"
)
// DefaultPageSize is the default for [ServerOptions.PageSize].
const DefaultPageSize = 1000
// A Server is an instance of an MCP server.
//
// Servers expose server-side MCP features, which can serve one or more MCP
// sessions by using [Server.Run].
type Server struct {
// fixed at creation
impl *Implementation
opts ServerOptions
mu sync.Mutex
prompts *featureSet[*serverPrompt]
tools *featureSet[*serverTool]
resources *featureSet[*serverResource]
resourceTemplates *featureSet[*serverResourceTemplate]
sessions []*ServerSession
sendingMethodHandler_ MethodHandler
receivingMethodHandler_ MethodHandler
resourceSubscriptions map[string]map[*ServerSession]bool // uri -> session -> bool
pendingNotifications map[string]*time.Timer // notification name -> timer for pending notification send
}
// ServerOptions is used to configure behavior of the server.
type ServerOptions struct {
// Optional instructions for connected clients.
Instructions string
// Logger may be set to a non-nil value to enable logging of server activity.
Logger *slog.Logger
// If non-nil, called when "notifications/initialized" is received.
InitializedHandler func(context.Context, *InitializedRequest)
// PageSize is the maximum number of items to return in a single page for
// list methods (e.g. ListTools).
//
// If zero, defaults to [DefaultPageSize].
PageSize int
// If non-nil, called when "notifications/roots/list_changed" is received.
RootsListChangedHandler func(context.Context, *RootsListChangedRequest)
// If non-nil, called when "notifications/progress" is received.
ProgressNotificationHandler func(context.Context, *ProgressNotificationServerRequest)
// If non-nil, called when "completion/complete" is received.
CompletionHandler func(context.Context, *CompleteRequest) (*CompleteResult, error)
// If non-zero, defines an interval for regular "ping" requests.
// If the peer fails to respond to pings originating from the keepalive check,
// the session is automatically closed.
KeepAlive time.Duration
// Function called when a client session subscribes to a resource.
SubscribeHandler func(context.Context, *SubscribeRequest) error
// Function called when a client session unsubscribes from a resource.
UnsubscribeHandler func(context.Context, *UnsubscribeRequest) error
// Capabilities optionally configures the server's default capabilities,
// before any capabilities are inferred from other configuration or server
// features.
//
// If Capabilities is nil, the default server capabilities are {"logging":{}},
// for historical reasons. Setting Capabilities to a non-nil value overrides
// this default. For example, setting Capabilities to `&ServerCapabilities{}`
// disables the logging capability.
//
// # Interaction with capability inference
//
// "tools", "prompts", and "resources" capabilities are automatically added when
// tools, prompts, or resources are added to the server (for example, via
// [Server.AddPrompt]), with default value `{"listChanged":true}`. Similarly,
// if the [ClientOptions.SubscribeHandler] or
// [ClientOptions.CompletionHandler] are set, the inferred capabilities are
// adjusted accordingly.
//
// Any non-nil field in Capabilities overrides the inferred value.
// For example:
//
// - To advertise the "tools" capability, even if no tools are added, set
// Capabilities.Tools to &ToolCapabilities{ListChanged:true}.
// - To disable tool list notifications, set Capabilities.Tools to
// &ToolCapabilities{}.
//
// Conversely, if Capabilities does not set a field (for example, if the
// Prompts field is nil), the inferred capability will be used.
Capabilities *ServerCapabilities
// If true, advertises the prompts capability during initialization,
// even if no prompts have been registered.
//
// Deprecated: Use Capabilities instead.
HasPrompts bool
// If true, advertises the resources capability during initialization,
// even if no resources have been registered.
//
// Deprecated: Use Capabilities instead.
HasResources bool
// If true, advertises the tools capability during initialization,
// even if no tools have been registered.
//
// Deprecated: Use Capabilities instead.
HasTools bool
// SchemaCache, if non-nil, caches JSON schemas to avoid repeated
// reflection. This is useful for stateless server deployments where
// a new [Server] is created for each request. See [SchemaCache] for
// trade-offs and usage guidance.
SchemaCache *SchemaCache
// GetSessionID provides the next session ID to use for an incoming request.
// If nil, a default randomly generated ID will be used.
//
// Session IDs should be globally unique across the scope of the server,
// which may span multiple processes in the case of distributed servers.
//
// As a special case, if GetSessionID returns the empty string, the
// Mcp-Session-Id header will not be set.
GetSessionID func() string
}
// NewServer creates a new MCP server. The resulting server has no features:
// add features using the various Server.AddXXX methods, and the [AddTool] function.
//
// The server can be connected to one or more MCP clients using [Server.Run].
//
// The first argument must not be nil.
//
// If non-nil, the provided options are used to configure the server.
func NewServer(impl *Implementation, options *ServerOptions) *Server {
if impl == nil {
panic("nil Implementation")
}
var opts ServerOptions
if options != nil {
opts = *options
}
options = nil // prevent reuse
if opts.PageSize < 0 {
panic(fmt.Errorf("invalid page size %d", opts.PageSize))
}
if opts.PageSize == 0 {
opts.PageSize = DefaultPageSize
}
if opts.SubscribeHandler != nil && opts.UnsubscribeHandler == nil {
panic("SubscribeHandler requires UnsubscribeHandler")
}
if opts.UnsubscribeHandler != nil && opts.SubscribeHandler == nil {
panic("UnsubscribeHandler requires SubscribeHandler")
}
if opts.GetSessionID == nil {
opts.GetSessionID = rand.Text
}
if opts.Logger == nil { // ensure we have a logger
opts.Logger = ensureLogger(nil)
}
return &Server{
impl: impl,
opts: opts,
prompts: newFeatureSet(func(p *serverPrompt) string { return p.prompt.Name }),
tools: newFeatureSet(func(t *serverTool) string { return t.tool.Name }),
resources: newFeatureSet(func(r *serverResource) string { return r.resource.URI }),
resourceTemplates: newFeatureSet(func(t *serverResourceTemplate) string { return t.resourceTemplate.URITemplate }),
sendingMethodHandler_: defaultSendingMethodHandler,
receivingMethodHandler_: defaultReceivingMethodHandler[*ServerSession],
resourceSubscriptions: make(map[string]map[*ServerSession]bool),
pendingNotifications: make(map[string]*time.Timer),
}
}
// AddPrompt adds a [Prompt] to the server, or replaces one with the same name.
func (s *Server) AddPrompt(p *Prompt, h PromptHandler) {
// Assume there was a change, since add replaces existing items.
// (It's possible an item was replaced with an identical one, but not worth checking.)
s.changeAndNotify(
notificationPromptListChanged,
func() bool { s.prompts.add(&serverPrompt{p, h}); return true })
}
// RemovePrompts removes the prompts with the given names.
// It is not an error to remove a nonexistent prompt.
func (s *Server) RemovePrompts(names ...string) {
s.changeAndNotify(notificationPromptListChanged, func() bool { return s.prompts.remove(names...) })
}
// AddTool adds a [Tool] to the server, or replaces one with the same name.
// The Tool argument must not be modified after this call.
//
// The tool's input schema must be non-nil and have the type "object". For a tool
// that takes no input, or one where any input is valid, set [Tool.InputSchema] to
// `{"type": "object"}`, using your preferred library or `json.RawMessage`.
//
// If present, [Tool.OutputSchema] must also have type "object".
//
// When the handler is invoked as part of a CallTool request, req.Params.Arguments
// will be a json.RawMessage.
//
// Unmarshaling the arguments and validating them against the input schema are the
// caller's responsibility.
//
// Validating the result against the output schema, if any, is the caller's responsibility.
//
// Setting the result's Content, StructuredContent and IsError fields are the caller's
// responsibility.
//
// Most users should use the top-level function [AddTool], which handles all these
// responsibilities.
func (s *Server) AddTool(t *Tool, h ToolHandler) {
if err := validateToolName(t.Name); err != nil {
s.opts.Logger.Error(fmt.Sprintf("AddTool: invalid tool name %q: %v", t.Name, err))
}
if t.InputSchema == nil {
// This prevents the tool author from forgetting to write a schema where
// one should be provided. If we papered over this by supplying the empty
// schema, then every input would be validated and the problem wouldn't be
// discovered until runtime, when the LLM sent bad data.
panic(fmt.Errorf("AddTool %q: missing input schema", t.Name))
}
if s, ok := t.InputSchema.(*jsonschema.Schema); ok {
if s.Type != "object" {
panic(fmt.Errorf(`AddTool %q: input schema must have type "object"`, t.Name))
}
} else {
var m map[string]any
if err := remarshal(t.InputSchema, &m); err != nil {
panic(fmt.Errorf("AddTool %q: can't marshal input schema to a JSON object: %v", t.Name, err))
}
if typ := m["type"]; typ != "object" {
panic(fmt.Errorf(`AddTool %q: input schema must have type "object" (got %v)`, t.Name, typ))
}
}
if t.OutputSchema != nil {
if s, ok := t.OutputSchema.(*jsonschema.Schema); ok {
if s.Type != "object" {
panic(fmt.Errorf(`AddTool %q: output schema must have type "object"`, t.Name))
}
} else {
var m map[string]any
if err := remarshal(t.OutputSchema, &m); err != nil {
panic(fmt.Errorf("AddTool %q: can't marshal output schema to a JSON object: %v", t.Name, err))
}
if typ := m["type"]; typ != "object" {
panic(fmt.Errorf(`AddTool %q: output schema must have type "object" (got %v)`, t.Name, typ))
}
}
}
st := &serverTool{tool: t, handler: h}
// Assume there was a change, since add replaces existing tools.
// (It's possible a tool was replaced with an identical one, but not worth checking.)
// TODO: Batch these changes by size and time? The typescript SDK doesn't.
// TODO: Surface notify error here? best not, in case we need to batch.
s.changeAndNotify(notificationToolListChanged, func() bool { s.tools.add(st); return true })
}
func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out], cache *SchemaCache) (*Tool, ToolHandler, error) {
tt := *t
// Special handling for an "any" input: treat as an empty object.
if reflect.TypeFor[In]() == reflect.TypeFor[any]() && t.InputSchema == nil {
tt.InputSchema = &jsonschema.Schema{Type: "object"}
}
var inputResolved *jsonschema.Resolved
if _, err := setSchema[In](&tt.InputSchema, &inputResolved, cache); err != nil {
return nil, nil, fmt.Errorf("input schema: %w", err)
}
// Handling for zero values:
//
// If Out is a pointer type and we've derived the output schema from its
// element type, use the zero value of its element type in place of a typed
// nil.
var (
elemZero any // only non-nil if Out is a pointer type
outputResolved *jsonschema.Resolved
)
if t.OutputSchema != nil || reflect.TypeFor[Out]() != reflect.TypeFor[any]() {
var err error
elemZero, err = setSchema[Out](&tt.OutputSchema, &outputResolved, cache)
if err != nil {
return nil, nil, fmt.Errorf("output schema: %v", err)
}
}
th := func(ctx context.Context, req *CallToolRequest) (*CallToolResult, error) {
var input json.RawMessage
if req.Params.Arguments != nil {
input = req.Params.Arguments
}
// Validate input and apply defaults.
var err error
input, err = applySchema(input, inputResolved)
if err != nil {
// TODO(#450): should this be considered a tool error? (and similar below)
return nil, fmt.Errorf("%w: validating \"arguments\": %v", jsonrpc2.ErrInvalidParams, err)
}
// Unmarshal and validate args.
var in In
if input != nil {
if err := internaljson.Unmarshal(input, &in); err != nil {
return nil, fmt.Errorf("%w: %v", jsonrpc2.ErrInvalidParams, err)
}
}
// Call typed handler.
res, out, err := h(ctx, req, in)
// Handle server errors appropriately:
// - If the handler returns a structured error (like jsonrpc.Error), return it directly
// - If the handler returns a regular error, wrap it in a CallToolResult with IsError=true
// - This allows tools to distinguish between protocol errors and tool execution errors
if err != nil {
// Check if this is already a structured JSON-RPC error
if wireErr, ok := err.(*jsonrpc.Error); ok {
return nil, wireErr
}
// For regular errors, embed them in the tool result as per MCP spec
var errRes CallToolResult
errRes.SetError(err)
return &errRes, nil
}
if res == nil {
res = &CallToolResult{}
}
// Marshal the output and put the RawMessage in the StructuredContent field.
var outval any = out
if elemZero != nil {
// Avoid typed nil, which will serialize as JSON null.
// Instead, use the zero value of the unpointered type.
var z Out
if any(out) == any(z) { // zero is only non-nil if Out is a pointer type
outval = elemZero
}
}
if outval != nil {
outbytes, err := json.Marshal(outval)
if err != nil {
return nil, fmt.Errorf("marshaling output: %w", err)
}
outJSON := json.RawMessage(outbytes)
// Validate the output JSON, and apply defaults.
//
// We validate against the JSON, rather than the output value, as
// some types may have custom JSON marshalling (issue #447).
outJSON, err = applySchema(outJSON, outputResolved)
if err != nil {
return nil, fmt.Errorf("validating tool output: %w", err)
}
res.StructuredContent = outJSON // avoid a second marshal over the wire
// If the Content field isn't being used, return the serialized JSON in a
// TextContent block, as the spec suggests:
// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content.
if res.Content == nil {
res.Content = []Content{&TextContent{
Text: string(outJSON),
}}
}
}
return res, nil
} // end of handler
return &tt, th, nil
}
// setSchema sets the schema and resolved schema corresponding to the type T.
//
// If sfield is nil, the schema is derived from T.
//
// Pointers are treated equivalently to non-pointers when deriving the schema.
// If an indirection occurred to derive the schema, a non-nil zero value is
// returned to be used in place of the typed nil zero value.
//
// Note that if sfield already holds a schema, zero will be nil even if T is a
// pointer: if the user provided the schema, they may have intentionally
// derived it from the pointer type, and handling of zero values is up to them.
//
// If cache is non-nil, schemas are cached to avoid repeated reflection.
//
// TODO(rfindley): we really shouldn't ever return 'null' results. Maybe we
// should have a jsonschema.Zero(schema) helper?
func setSchema[T any](sfield *any, rfield **jsonschema.Resolved, cache *SchemaCache) (zero any, err error) {
rt := reflect.TypeFor[T]()
if rt.Kind() == reflect.Pointer {
rt = rt.Elem()
zero = reflect.Zero(rt).Interface()
}
var internalSchema *jsonschema.Schema
if *sfield == nil {
// No schema provided: check cache, or generate via reflection.
if cache != nil {
if schema, resolved, ok := cache.getByType(rt); ok {
*sfield = schema
*rfield = resolved
return zero, nil
}
}
internalSchema, err = jsonschema.ForType(rt, &jsonschema.ForOptions{})
if err != nil {
return zero, err
}
*sfield = internalSchema
resolved, err := internalSchema.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
if err != nil {
return zero, err
}
*rfield = resolved
if cache != nil {
cache.setByType(rt, internalSchema, resolved)
}
return zero, nil
}
// Schema was provided: check cache by pointer, or resolve it.
if providedSchema, ok := (*sfield).(*jsonschema.Schema); ok {
if cache != nil {
if resolved, ok := cache.getBySchema(providedSchema); ok {
*rfield = resolved
return zero, nil
}
}
internalSchema = providedSchema
} else {
// Schema provided as different type (e.g., map): remarshal to *Schema.
if err := remarshal(*sfield, &internalSchema); err != nil {
return zero, err
}
}
resolved, err := internalSchema.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
if err != nil {
return zero, err
}
*rfield = resolved
if cache != nil {
if providedSchema, ok := (*sfield).(*jsonschema.Schema); ok {
cache.setBySchema(providedSchema, resolved)
}
}
return zero, nil
}
// AddTool adds a tool and typed tool handler to the server.
//
// If the tool's input schema is nil, it is set to the schema inferred from the
// In type parameter. Types are inferred from Go types, and property
// descriptions are read from the 'jsonschema' struct tag. Internally, the SDK
// uses the github.com/google/jsonschema-go package for inference and
// validation. The In type argument must be a map or a struct, so that its
// inferred JSON Schema has type "object", as required by the spec. As a
// special case, if the In type is 'any', the tool's input schema is set to an
// empty object schema value.
//
// If the tool's output schema is nil, and the Out type is not 'any', the
// output schema is set to the schema inferred from the Out type argument,
// which must also be a map or struct. If the Out type is 'any', the output
// schema is omitted.
//
// Unlike [Server.AddTool], AddTool does a lot automatically, and forces
// tools to conform to the MCP spec. See [ToolHandlerFor] for a detailed
// description of this automatic behavior.
func AddTool[In, Out any](s *Server, t *Tool, h ToolHandlerFor[In, Out]) {
tt, hh, err := toolForErr(t, h, s.opts.SchemaCache)
if err != nil {
panic(fmt.Sprintf("AddTool: tool %q: %v", t.Name, err))
}
s.AddTool(tt, hh)
}
// RemoveTools removes the tools with the given names.
// It is not an error to remove a nonexistent tool.
func (s *Server) RemoveTools(names ...string) {
s.changeAndNotify(notificationToolListChanged, func() bool { return s.tools.remove(names...) })
}
// AddResource adds a [Resource] to the server, or replaces one with the same URI.
// AddResource panics if the resource URI is invalid or not absolute (has an empty scheme).
func (s *Server) AddResource(r *Resource, h ResourceHandler) {
s.changeAndNotify(notificationResourceListChanged,
func() bool {
if _, err := url.Parse(r.URI); err != nil {
panic(err) // url.Parse includes the URI in the error
}
s.resources.add(&serverResource{r, h})
return true
})
}
// RemoveResources removes the resources with the given URIs.
// It is not an error to remove a nonexistent resource.
func (s *Server) RemoveResources(uris ...string) {
s.changeAndNotify(notificationResourceListChanged, func() bool { return s.resources.remove(uris...) })
}
// AddResourceTemplate adds a [ResourceTemplate] to the server, or replaces one with the same URI.
// AddResourceTemplate panics if a URI template is invalid or not absolute (has an empty scheme).
func (s *Server) AddResourceTemplate(t *ResourceTemplate, h ResourceHandler) {
s.changeAndNotify(notificationResourceListChanged,
func() bool {
// Validate the URI template syntax
_, err := uritemplate.New(t.URITemplate)
if err != nil {
panic(fmt.Errorf("URI template %q is invalid: %w", t.URITemplate, err))
}
s.resourceTemplates.add(&serverResourceTemplate{t, h})
return true
})
}
// RemoveResourceTemplates removes the resource templates with the given URI templates.
// It is not an error to remove a nonexistent resource.
func (s *Server) RemoveResourceTemplates(uriTemplates ...string) {
s.changeAndNotify(notificationResourceListChanged, func() bool { return s.resourceTemplates.remove(uriTemplates...) })
}
func (s *Server) capabilities() *ServerCapabilities {
s.mu.Lock()
defer s.mu.Unlock()
// Start with user-provided capabilities as defaults, or use SDK defaults.
var caps *ServerCapabilities
if s.opts.Capabilities != nil {
// Deep copy the user-provided capabilities to avoid mutation.
caps = s.opts.Capabilities.clone()
} else {
// SDK defaults: only logging capability.
caps = &ServerCapabilities{
Logging: &LoggingCapabilities{},
}
}
// Augment with tools capability if tools exist or legacy HasTools is set.
if s.opts.HasTools || s.tools.len() > 0 {
if caps.Tools == nil {
caps.Tools = &ToolCapabilities{ListChanged: true}
}
}
// Augment with prompts capability if prompts exist or legacy HasPrompts is set.
if s.opts.HasPrompts || s.prompts.len() > 0 {
if caps.Prompts == nil {
caps.Prompts = &PromptCapabilities{ListChanged: true}
}
}
// Augment with resources capability if resources/templates exist or legacy HasResources is set.
if s.opts.HasResources || s.resources.len() > 0 || s.resourceTemplates.len() > 0 {
if caps.Resources == nil {
caps.Resources = &ResourceCapabilities{ListChanged: true}
}
if s.opts.SubscribeHandler != nil {
caps.Resources.Subscribe = true
}
}
// Augment with completions capability if handler is set.
if s.opts.CompletionHandler != nil {
if caps.Completions == nil {
caps.Completions = &CompletionCapabilities{}
}
}
return caps
}
func (s *Server) complete(ctx context.Context, req *CompleteRequest) (*CompleteResult, error) {
if s.opts.CompletionHandler == nil {
return nil, jsonrpc2.ErrMethodNotFound
}
return s.opts.CompletionHandler(ctx, req)
}
// Map from notification name to its corresponding params. The params have no fields,
// so a single struct can be reused.
var changeNotificationParams = map[string]Params{
notificationToolListChanged: &ToolListChangedParams{},
notificationPromptListChanged: &PromptListChangedParams{},
notificationResourceListChanged: &ResourceListChangedParams{},
}
// How long to wait before sending a change notification.
const notificationDelay = 10 * time.Millisecond
// changeAndNotify is called when a feature is added or removed.
// It calls change, which should do the work and report whether a change actually occurred.
// If there was a change, it sets a timer to send a notification.
// This debounces change notifications: a single notification is sent after
// multiple changes occur in close proximity.
func (s *Server) changeAndNotify(notification string, change func() bool) {
s.mu.Lock()
defer s.mu.Unlock()
if change() && s.shouldSendListChangedNotification(notification) {
// Reset the outstanding delayed call, if any.
if t := s.pendingNotifications[notification]; t == nil {
s.pendingNotifications[notification] = time.AfterFunc(notificationDelay, func() { s.notifySessions(notification) })
} else {
t.Reset(notificationDelay)
}
}
}
// notifySessions sends the notification n to all existing sessions.
// It is called asynchronously by changeAndNotify.
func (s *Server) notifySessions(n string) {
s.mu.Lock()
sessions := slices.Clone(s.sessions)
s.pendingNotifications[n] = nil
s.mu.Unlock() // Don't hold the lock during notification: it causes deadlock.
notifySessions(sessions, n, changeNotificationParams[n], s.opts.Logger)
}
// shouldSendListChangedNotification checks if the server's capabilities allow
// sending the given list-changed notification.
func (s *Server) shouldSendListChangedNotification(notification string) bool {
// Get effective capabilities (considering user-provided defaults).
caps := s.opts.Capabilities
switch notification {
case notificationToolListChanged:
// If user didn't specify capabilities, default behavior sends notifications.
if caps == nil || caps.Tools == nil {
return true
}
return caps.Tools.ListChanged
case notificationPromptListChanged:
if caps == nil || caps.Prompts == nil {
return true
}
return caps.Prompts.ListChanged
case notificationResourceListChanged:
if caps == nil || caps.Resources == nil {
return true
}
return caps.Resources.ListChanged
default:
// Unknown notification, allow by default.
return true
}
}
// Sessions returns an iterator that yields the current set of server sessions.
//
// There is no guarantee that the iterator observes sessions that are added or
// removed during iteration.
func (s *Server) Sessions() iter.Seq[*ServerSession] {
s.mu.Lock()
clients := slices.Clone(s.sessions)
s.mu.Unlock()
return slices.Values(clients)
}
func (s *Server) listPrompts(_ context.Context, req *ListPromptsRequest) (*ListPromptsResult, error) {
s.mu.Lock()
defer s.mu.Unlock()
if req.Params == nil {
req.Params = &ListPromptsParams{}
}
return paginateList(s.prompts, s.opts.PageSize, req.Params, &ListPromptsResult{}, func(res *ListPromptsResult, prompts []*serverPrompt) {
res.Prompts = []*Prompt{} // avoid JSON null
for _, p := range prompts {
res.Prompts = append(res.Prompts, p.prompt)
}
})
}
func (s *Server) getPrompt(ctx context.Context, req *GetPromptRequest) (*GetPromptResult, error) {
s.mu.Lock()
prompt, ok := s.prompts.get(req.Params.Name)
s.mu.Unlock()
if !ok {
// Return a proper JSON-RPC error with the correct error code
return nil, &jsonrpc.Error{
Code: jsonrpc.CodeInvalidParams,
Message: fmt.Sprintf("unknown prompt %q", req.Params.Name),
}
}
return prompt.handler(ctx, req)
}
func (s *Server) listTools(_ context.Context, req *ListToolsRequest) (*ListToolsResult, error) {
s.mu.Lock()
defer s.mu.Unlock()
if req.Params == nil {
req.Params = &ListToolsParams{}
}
return paginateList(s.tools, s.opts.PageSize, req.Params, &ListToolsResult{}, func(res *ListToolsResult, tools []*serverTool) {
res.Tools = []*Tool{} // avoid JSON null
for _, t := range tools {
res.Tools = append(res.Tools, t.tool)
}
})
}
func (s *Server) callTool(ctx context.Context, req *CallToolRequest) (*CallToolResult, error) {
s.mu.Lock()
st, ok := s.tools.get(req.Params.Name)
s.mu.Unlock()
if !ok {
return nil, &jsonrpc.Error{
Code: jsonrpc.CodeInvalidParams,
Message: fmt.Sprintf("unknown tool %q", req.Params.Name),
}
}
res, err := st.handler(ctx, req)
if err == nil && res != nil && res.Content == nil {
res2 := *res
res2.Content = []Content{} // avoid "null"
res = &res2
}
return res, err
}
func (s *Server) listResources(_ context.Context, req *ListResourcesRequest) (*ListResourcesResult, error) {
s.mu.Lock()
defer s.mu.Unlock()
if req.Params == nil {
req.Params = &ListResourcesParams{}
}
return paginateList(s.resources, s.opts.PageSize, req.Params, &ListResourcesResult{}, func(res *ListResourcesResult, resources []*serverResource) {
res.Resources = []*Resource{} // avoid JSON null
for _, r := range resources {
res.Resources = append(res.Resources, r.resource)
}
})
}
func (s *Server) listResourceTemplates(_ context.Context, req *ListResourceTemplatesRequest) (*ListResourceTemplatesResult, error) {
s.mu.Lock()
defer s.mu.Unlock()
if req.Params == nil {
req.Params = &ListResourceTemplatesParams{}
}
return paginateList(s.resourceTemplates, s.opts.PageSize, req.Params, &ListResourceTemplatesResult{},
func(res *ListResourceTemplatesResult, rts []*serverResourceTemplate) {
res.ResourceTemplates = []*ResourceTemplate{} // avoid JSON null
for _, rt := range rts {
res.ResourceTemplates = append(res.ResourceTemplates, rt.resourceTemplate)
}
})
}
func (s *Server) readResource(ctx context.Context, req *ReadResourceRequest) (*ReadResourceResult, error) {
uri := req.Params.URI
// Look up the resource URI in the lists of resources and resource templates.
// This is a security check as well as an information lookup.
handler, mimeType, ok := s.lookupResourceHandler(uri)
if !ok {
// Don't expose the server configuration to the client.
// Treat an unregistered resource the same as a registered one that couldn't be found.
return nil, ResourceNotFoundError(uri)
}
res, err := handler(ctx, req)
if err != nil {
return nil, err
}
if res == nil || res.Contents == nil {
return nil, fmt.Errorf("reading resource %s: read handler returned nil information", uri)
}
// As a convenience, populate some fields.
for _, c := range res.Contents {
if c.URI == "" {
c.URI = uri
}
if c.MIMEType == "" {
c.MIMEType = mimeType
}
}
return res, nil
}
// lookupResourceHandler returns the resource handler and MIME type for the resource or
// resource template matching uri. If none, the last return value is false.
func (s *Server) lookupResourceHandler(uri string) (ResourceHandler, string, bool) {
s.mu.Lock()
defer s.mu.Unlock()
// Try resources first.
if r, ok := s.resources.get(uri); ok {
return r.handler, r.resource.MIMEType, true
}
// Look for matching template.
for rt := range s.resourceTemplates.all() {
if rt.Matches(uri) {
return rt.handler, rt.resourceTemplate.MIMEType, true
}
}
return nil, "", false
}
// fileResourceHandler returns a ReadResourceHandler that reads paths using dir as
// a base directory.
// It honors client roots and protects against path traversal attacks.
//
// The dir argument should be a filesystem path. It need not be absolute, but
// that is recommended to avoid a dependency on the current working directory (the
// check against client roots is done with an absolute path). If dir is not absolute
// and the current working directory is unavailable, fileResourceHandler panics.
//
// Lexical path traversal attacks, where the path has ".." elements that escape dir,
// are always caught. Go 1.24 and above also protects against symlink-based attacks,
// where symlinks under dir lead out of the tree.
func fileResourceHandler(dir string) ResourceHandler {
// Convert dir to an absolute path.
dirFilepath, err := filepath.Abs(dir)
if err != nil {
panic(err)
}
return func(ctx context.Context, req *ReadResourceRequest) (_ *ReadResourceResult, err error) {
defer util.Wrapf(&err, "reading resource %s", req.Params.URI)
// TODO(#25): use a memoizing API here.
rootRes, err := req.Session.ListRoots(ctx, nil)
if err != nil {
return nil, fmt.Errorf("listing roots: %w", err)
}
roots, err := fileRoots(rootRes.Roots)
if err != nil {
return nil, err
}
data, err := readFileResource(req.Params.URI, dirFilepath, roots)
if err != nil {
return nil, err
}
// TODO(jba): figure out mime type. Omit for now: Server.readResource will fill it in.
return &ReadResourceResult{Contents: []*ResourceContents{
{URI: req.Params.URI, Blob: data},
}}, nil
}
}
// ResourceUpdated sends a notification to all clients that have subscribed to the
// resource specified in params. This method is the primary way for a
// server author to signal that a resource has changed.
func (s *Server) ResourceUpdated(ctx context.Context, params *ResourceUpdatedNotificationParams) error {
s.mu.Lock()
subscribedSessions := s.resourceSubscriptions[params.URI]
sessions := slices.Collect(maps.Keys(subscribedSessions))
s.mu.Unlock()
notifySessions(sessions, notificationResourceUpdated, params, s.opts.Logger)
s.opts.Logger.Info("resource updated notification sent", "uri", params.URI, "subscriber_count", len(sessions))
return nil
}
func (s *Server) subscribe(ctx context.Context, req *SubscribeRequest) (*emptyResult, error) {
if s.opts.SubscribeHandler == nil {
return nil, fmt.Errorf("%w: server does not support resource subscriptions", jsonrpc2.ErrMethodNotFound)
}
if err := s.opts.SubscribeHandler(ctx, req); err != nil {
return nil, err
}
s.mu.Lock()
defer s.mu.Unlock()
if s.resourceSubscriptions[req.Params.URI] == nil {
s.resourceSubscriptions[req.Params.URI] = make(map[*ServerSession]bool)
}
s.resourceSubscriptions[req.Params.URI][req.Session] = true
s.opts.Logger.Info("resource subscribed", "uri", req.Params.URI, "session_id", req.Session.ID())
return &emptyResult{}, nil
}
func (s *Server) unsubscribe(ctx context.Context, req *UnsubscribeRequest) (*emptyResult, error) {
if s.opts.UnsubscribeHandler == nil {
return nil, jsonrpc2.ErrMethodNotFound
}
if err := s.opts.UnsubscribeHandler(ctx, req); err != nil {
return nil, err
}
s.mu.Lock()
defer s.mu.Unlock()
if subscribedSessions, ok := s.resourceSubscriptions[req.Params.URI]; ok {
delete(subscribedSessions, req.Session)
if len(subscribedSessions) == 0 {
delete(s.resourceSubscriptions, req.Params.URI)
}
}
s.opts.Logger.Info("resource unsubscribed", "uri", req.Params.URI, "session_id", req.Session.ID())
return &emptyResult{}, nil
}
// Run runs the server over the given transport, which must be persistent.
//
// Run blocks until the client terminates the connection or the provided
// context is cancelled. If the context is cancelled, Run closes the connection.
//
// If tools have been added to the server before this call, then the server will
// advertise the capability for tools, including the ability to send list-changed notifications.
// If no tools have been added, the server will not have the tool capability.
// The same goes for other features like prompts and resources.
//
// Run is a convenience for servers that handle a single session (or one session at a time).
// It need not be called on servers that are used for multiple concurrent connections,
// as with [StreamableHTTPHandler].
func (s *Server) Run(ctx context.Context, t Transport) error {
s.opts.Logger.Info("server run start")
ss, err := s.Connect(ctx, t, nil)
if err != nil {
s.opts.Logger.Error("server connect failed", "error", err)
return err
}
ssClosed := make(chan error)
go func() {
ssClosed <- ss.Wait()
}()
select {
case <-ctx.Done():
ss.Close()
<-ssClosed // wait until waiting go routine above actually completes
s.opts.Logger.Error("server run cancelled", "error", ctx.Err())
return ctx.Err()
case err := <-ssClosed:
if err != nil {
s.opts.Logger.Error("server session ended with error", "error", err)
} else {
s.opts.Logger.Info("server session ended")
}
return err
}
}
// bind implements the binder[*ServerSession] interface, so that Servers can
// be connected using [connect].
func (s *Server) bind(mcpConn Connection, conn *jsonrpc2.Connection, state *ServerSessionState, onClose func()) *ServerSession {
assert(mcpConn != nil && conn != nil, "nil connection")
ss := &ServerSession{conn: conn, mcpConn: mcpConn, server: s, onClose: onClose}
if state != nil {
ss.state = *state
}
s.mu.Lock()
s.sessions = append(s.sessions, ss)
s.mu.Unlock()
s.opts.Logger.Info("server session connected", "session_id", ss.ID())
return ss
}
// disconnect implements the binder[*ServerSession] interface, so that
// Servers can be connected using [connect].
func (s *Server) disconnect(cc *ServerSession) {
s.mu.Lock()
defer s.mu.Unlock()
s.sessions = slices.DeleteFunc(s.sessions, func(cc2 *ServerSession) bool {
return cc2 == cc
})
for _, subscribedSessions := range s.resourceSubscriptions {
delete(subscribedSessions, cc)
}
s.opts.Logger.Info("server session disconnected", "session_id", cc.ID())
}
// ServerSessionOptions configures the server session.
type ServerSessionOptions struct {
State *ServerSessionState
onClose func() // used to clean up associated resources
}