mcp: add ResourceLink type (#76)

This PR introduces the ResourceLink type to represent links to
resources. The new type includes the following fields: `uri`, `name`,
`title`, `description`, `mimeType`, `size`, and `_meta`.

This implementation conforms to the `ResourceLink` schema introduced in
the [MCP schema
2025-06-18](1cfdf1e7a8/schema/2025-06-18/schema.json (L1826)).

Note: `ResourceLink` extends from the `Resource` type in the MCP schema.
As such, it inherits support for optional fields like `title`, `_meta`,
etc. Thus, `URI`, `Name`, `Title`, `Description`, and `Size` are added
to the `wireContent`

For reference, the [TypeScript SDK test
suite](fb4d07f0ee/src/types.test.ts (L43))
provides examples demonstrating valid ResourceLink objects with all
optional fields populated.

https://modelcontextprotocol.io/specification/2025-06-18/server/tools#resource-links
- [ResourceLink JSON
Schema](1cfdf1e7a8/schema/2025-06-18/schema.json (L1826))

Signed-off-by: Xin Fu <xfu83@bloomberg.net>

Fixes #14.

mcp/content.go

@@ -10,10 +10,8 @@ import (
 	"fmt"
 )
 
-// A Content is a [TextContent], [ImageContent], [AudioContent] or
-// [EmbeddedResource].
-//
-// TODO(rfindley): add ResourceLink.
+// A Content is a [TextContent], [ImageContent], [AudioContent],
+// [ResourceLink], or [EmbeddedResource].
 type Content interface {
 	MarshalJSON() ([]byte, error)
 	fromWire(*wireContent)
@@ -91,6 +89,43 @@ func (c *AudioContent) fromWire(wire *wireContent) {
 	c.Annotations = wire.Annotations
 }
 
+// ResourceLink is a link to a resource
+type ResourceLink struct {
+	URI         string
+	Name        string
+	Title       string
+	Description string
+	MIMEType    string
+	Size        *int64
+	Meta        Meta
+	Annotations *Annotations
+}
+
+func (c *ResourceLink) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&wireContent{
+		Type:        "resource_link",
+		URI:         c.URI,
+		Name:        c.Name,
+		Title:       c.Title,
+		Description: c.Description,
+		MIMEType:    c.MIMEType,
+		Size:        c.Size,
+		Meta:        c.Meta,
+		Annotations: c.Annotations,
+	})
+}
+
+func (c *ResourceLink) fromWire(wire *wireContent) {
+	c.URI = wire.URI
+	c.Name = wire.Name
+	c.Title = wire.Title
+	c.Description = wire.Description
+	c.MIMEType = wire.MIMEType
+	c.Size = wire.Size
+	c.Meta = wire.Meta
+	c.Annotations = wire.Annotations
+}
+
 // EmbeddedResource contains embedded resources.
 type EmbeddedResource struct {
 	Resource    *ResourceContents
@@ -151,17 +186,22 @@ func (r ResourceContents) MarshalJSON() ([]byte, error) {
 }
 
 // wireContent is the wire format for content.
-// It represents the protocol types TextContent, ImageContent, AudioContent
-// and EmbeddedResource.
+// It represents the protocol types TextContent, ImageContent, AudioContent,
+// ResourceLink, and EmbeddedResource.
 // The Type field distinguishes them. In the protocol, each type has a constant
 // value for the field.
-// At most one of Text, Data, and Resource is non-zero.
+// At most one of Text, Data, Resource, and URI is non-zero.
 type wireContent struct {
 	Type        string            `json:"type"`
 	Text        string            `json:"text,omitempty"`
 	MIMEType    string            `json:"mimeType,omitempty"`
 	Data        []byte            `json:"data,omitempty"`
 	Resource    *ResourceContents `json:"resource,omitempty"`
+	URI         string            `json:"uri,omitempty"`
+	Name        string            `json:"name,omitempty"`
+	Title       string            `json:"title,omitempty"`
+	Description string            `json:"description,omitempty"`
+	Size        *int64            `json:"size,omitempty"`
 	Meta        Meta              `json:"_meta,omitempty"`
 	Annotations *Annotations      `json:"annotations,omitempty"`
 }
@@ -195,6 +235,10 @@ func contentFromWire(wire *wireContent, allow map[string]bool) (Content, error)
 		v := new(AudioContent)
 		v.fromWire(wire)
 		return v, nil
+	case "resource_link":
+		v := new(ResourceLink)
+		v.fromWire(wire)
+		return v, nil
 	case "resource":
 		v := new(EmbeddedResource)
 		v.fromWire(wire)

mcp/content_test.go

@@ -82,6 +82,24 @@ func TestContent(t *testing.T) {
 			},
 			`{"type":"resource","resource":{"uri":"file://foo","mimeType":"text","text":"abc"},"_meta":{"key":"value"},"annotations":{"priority":1}}`,
 		},
+		{
+			&mcp.ResourceLink{
+				URI:  "file:///path/to/file.txt",
+				Name: "file.txt",
+			},
+			`{"type":"resource_link","uri":"file:///path/to/file.txt","name":"file.txt"}`,
+		},
+		{
+			&mcp.ResourceLink{
+				URI:         "https://example.com/resource",
+				Name:        "Example Resource",
+				Title:       "A comprehensive example resource",
+				Description: "This resource demonstrates all fields",
+				MIMEType:    "text/plain",
+				Meta:        mcp.Meta{"custom": "metadata"},
+			},
+			`{"type":"resource_link","mimeType":"text/plain","uri":"https://example.com/resource","name":"Example Resource","title":"A comprehensive example resource","description":"This resource demonstrates all fields","_meta":{"custom":"metadata"}}`,
+		},
 	}
 
 	for _, test := range tests {