Repository: modelcontextprotocol/go-sdk

Issue:
Bug: Marshaling Content omits medadata
**Describe the bug**
Content interface has wireContent method - which marshals bytes to struct implementing Content interface.

Meta property in wireContent, has JSON tag _meta.

```
type wireContent struct {
        ....
	Meta        Meta              `json:"_meta,omitempty"`
}
```

however, structs do not have json tag defined, so it expects meta, instead of _meta from byte array.

```
type ResourceLink struct {
	...
	Meta        Meta
}
```

We should add json tag for Meta in all structs implementing Content interface. So ResourceLink would become

```
type ResourceLink struct {
	...
	Meta        Meta              `json:"_meta,omitempty"`
}
```

Hints from issue comments:
@IAmSurajBobade: Looks like @findleyr has already added relevant TODO comment https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/content.go#L5.@findleyr: Sorry, @IAmSurajBobade, I don't understand. The content types have custom MarshalJSON methods, which appear to preserve metadata. Is this not working?@IAmSurajBobade: @findleyr, updated description of the issue. This is the issue, due to which https://github.com/modelcontextprotocol/go-sdk/pull/570#discussion_r2515018632 is not working.@findleyr: @IAmSurajBobade this is not a bug: the way this works is that each struct that has a Content field has a custom UnmarshalJSON method, which unmarshals using contentsFromWire.

It is not expected that marshaling directly into content types works as expected. The custom marshalling at the top level is required for selecting the correct content type.

Solution patch:
diff --git a/examples/server/everything/main.go b/examples/server/everything/main.go
index 0b81919..f255ab4 100644
--- a/examples/server/everything/main.go
+++ b/examples/server/everything/main.go
@@ -7,6 +7,7 @@ package main
 
 import (
 	"context"
+	"encoding/base64"
 	"flag"
 	"fmt"
 	"log"
@@ -50,19 +51,28 @@ func main() {
 		CompletionHandler: complete, // support completions by setting this handler
 	}
 
-	server := mcp.NewServer(&mcp.Implementation{Name: "everything"}, opts)
+	// Optionally add an icon to the server implementation.
+	icons, err := iconToBase64DataURL("./mcp.png")
+	if err != nil {
+		log.Fatalf("failed to read icon: %v", err)
+	}
+
+	server := mcp.NewServer(&mcp.Implementation{Name: "everything", WebsiteURL: "https://example.com", Icons: icons}, opts)
 
 	// Add tools that exercise different features of the protocol.
 	mcp.AddTool(server, &mcp.Tool{Name: "greet", Description: "say hi"}, contentTool)
-	mcp.AddTool(server, &mcp.Tool{Name: "greet (structured)"}, structuredTool) // returns structured output
-	mcp.AddTool(server, &mcp.Tool{Name: "ping"}, pingingTool)                  // performs a ping
-	mcp.AddTool(server, &mcp.Tool{Name: "log"}, loggingTool)                   // performs a log
-	mcp.AddTool(server, &mcp.Tool{Name: "sample"}, samplingTool)               // performs sampling
-	mcp.AddTool(server, &mcp.Tool{Name: "elicit"}, elicitingTool)              // performs elicitation
-	mcp.AddTool(server, &mcp.Tool{Name: "roots"}, rootsTool)                   // lists roots
+	mcp.AddTool(server, &mcp.Tool{Name: "greet (structured)"}, structuredTool)                                // returns structured output
+	mcp.AddTool(server, &mcp.Tool{Name: "greet (with Icons)", Icons: icons}, structuredTool)                  // tool with icons
+	mcp.AddTool(server, &mcp.Tool{Name: "greet (content with ResourceLink)"}, resourceLinkContentTool(icons)) // tool that returns content with a resource link
+	mcp.AddTool(server, &mcp.Tool{Name: "ping"}, pingingTool)                                                 // performs a ping
+	mcp.AddTool(server, &mcp.Tool{Name: "log"}, loggingTool)                                                  // performs a log
+	mcp.AddTool(server, &mcp.Tool{Name: "sample"}, samplingTool)                                              // performs sampling
+	mcp.AddTool(server, &mcp.Tool{Name: "elicit"}, elicitingTool)                                             // performs elicitation
+	mcp.AddTool(server, &mcp.Tool{Name: "roots"}, rootsTool)                                                  // lists roots
 
 	// Add a basic prompt.
 	server.AddPrompt(&mcp.Prompt{Name: "greet"}, prompt)
+	server.AddPrompt(&mcp.Prompt{Name: "greet (with Icons)", Icons: icons}, prompt) // greet prompt with icons
 
 	// Add an embedded resource.
 	server.AddResource(&mcp.Resource{
@@ -70,6 +80,20 @@ func main() {
 		MIMEType: "text/plain",
 		URI:      "embedded:info",
 	}, embeddedResource)
+	server.AddResource(&mcp.Resource{ // text resource with icons
+		Name:     "info (with Icons)",
+		MIMEType: "text/plain",
+		URI:      "embedded:info",
+		Icons:    icons,
+	}, embeddedResource)
+
+	// Add a resource template.
+	server.AddResourceTemplate(&mcp.ResourceTemplate{
+		Name:        "Resource template (with Icon)",
+		MIMEType:    "text/plain",
+		URITemplate: "http://example.com/~{resource_name}/",
+		Icons:       icons,
+	}, embeddedResource)
 
 	// Serve over stdio, or streamable HTTP if -http is set.
 	if *httpAddr != "" {
@@ -140,6 +164,23 @@ func contentTool(ctx context.Context, req *mcp.CallToolRequest, args args) (*mcp
 	}, nil, nil
 }
 
+// resourceLinkContentTool returns a ResourceLink content with icons.
+func resourceLinkContentTool(icons []mcp.Icon) func(ctx context.Context, req *mcp.CallToolRequest, args args) (*mcp.CallToolResult, any, error) {
+	return func(ctx context.Context, req *mcp.CallToolRequest, args args) (*mcp.CallToolResult, any, error) {
+		return &mcp.CallToolResult{
+			Content: []mcp.Content{
+				&mcp.ResourceLink{
+					Name:     "greeting",
+					Title:    "A friendly greeting",
+					MIMEType: "text/plain",
+					URI:      "data:text/plain,Hi%20" + url.PathEscape(args.Name),
+					Icons:    icons,
+				},
+			},
+		}, nil, nil
+	}
+}
+
 type result struct {
 	Message string `json:"message" jsonschema:"the message to convey"`
 }
@@ -222,3 +263,16 @@ func complete(ctx context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResul
 		},
 	}, nil
 }
+
+func iconToBase64DataURL(path string) ([]mcp.Icon, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return []mcp.Icon{{
+		Source:   "data:image/png;base64," + base64.StdEncoding.EncodeToString(data),
+		MIMEType: "image/png",
+		Sizes:    []string{"48x48"},
+		Theme:    "light", // or "dark" or empty
+	}}, nil
+}
diff --git a/examples/server/everything/mcp.png b/examples/server/everything/mcp.png
new file mode 100644
index 0000000..8e08571
Binary files /dev/null and b/examples/server/everything/mcp.png differ
diff --git a/mcp/content.go b/mcp/content.go
index e53cad1..fb1a0d1 100644
--- a/mcp/content.go
+++ b/mcp/content.go
@@ -130,6 +130,8 @@ type ResourceLink struct {
 	Size        *int64
 	Meta        Meta
 	Annotations *Annotations
+	// Icons for the resource link, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 func (c *ResourceLink) MarshalJSON() ([]byte, error) {
@@ -143,6 +145,7 @@ func (c *ResourceLink) MarshalJSON() ([]byte, error) {
 		Size:        c.Size,
 		Meta:        c.Meta,
 		Annotations: c.Annotations,
+		Icons:       c.Icons,
 	})
 }
 
@@ -155,6 +158,7 @@ func (c *ResourceLink) fromWire(wire *wireContent) {
 	c.Size = wire.Size
 	c.Meta = wire.Meta
 	c.Annotations = wire.Annotations
+	c.Icons = wire.Icons
 }
 
 // EmbeddedResource contains embedded resources.
@@ -237,6 +241,7 @@ type wireContent struct {
 	Size        *int64            `json:"size,omitempty"`
 	Meta        Meta              `json:"_meta,omitempty"`
 	Annotations *Annotations      `json:"annotations,omitempty"`
+	Icons       []Icon            `json:"icons,omitempty"`
 }
 
 func contentsFromWire(wires []*wireContent, allow map[string]bool) ([]Content, error) {
diff --git a/mcp/protocol.go b/mcp/protocol.go
index 1312dfb..54ccdd9 100644
--- a/mcp/protocol.go
+++ b/mcp/protocol.go
@@ -658,6 +658,23 @@ type ProgressNotificationParams struct {
 
 func (*ProgressNotificationParams) isParams() {}
 
+// Icon provides visual identifiers for their resources, tools, prompts, and implementations
+// See [/specification/draft/basic/index#icons] for notes on icons
+//
+// TODO(iamsurajbobade): update specification url from draft.
+type Icon struct {
+	// Source is A URI pointing to the icon resource (required). This can be:
+	// - An HTTP/HTTPS URL pointing to an image file
+	// - A data URI with base64-encoded image data
+	Source string `json:"src"`
+	// Optional MIME type if the server's type is missing or generic
+	MIMEType string `json:"mimeType,omitempty"`
+	// Optional size specification (e.g., ["48x48"], ["any"] for scalable formats like SVG, or ["48x48", "96x96"] for multiple sizes)
+	Sizes []string `json:"sizes,omitempty"`
+	// Optional Theme of the icon, e.g., "light" or "dark"
+	Theme string `json:"theme,omitempty"`
+}
+
 // 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
@@ -673,6 +690,8 @@ type Prompt struct {
 	// Intended for UI and end-user contexts — optimized to be human-readable and
 	// easily understood, even by those unfamiliar with domain-specific terminology.
 	Title string `json:"title,omitempty"`
+	// Icons for the prompt, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 // Describes an argument that a prompt can accept.
@@ -782,6 +801,8 @@ type Resource struct {
 	Title string `json:"title,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
+	// Icons for the resource, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 type ResourceListChangedParams struct {
@@ -822,6 +843,8 @@ type ResourceTemplate struct {
 	// A URI template (according to RFC 6570) that can be used to construct resource
 	// URIs.
 	URITemplate string `json:"uriTemplate"`
+	// Icons for the resource template, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 // The sender or recipient of messages and data in a conversation.
@@ -948,6 +971,8 @@ type Tool struct {
 	// If not provided, Annotations.Title should be used for display if present,
 	// otherwise Name.
 	Title string `json:"title,omitempty"`
+	// Icons for the tool, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 // Additional properties describing a Tool to clients.
@@ -1090,6 +1115,10 @@ type Implementation struct {
 	// easily understood, even by those unfamiliar with domain-specific terminology.
 	Title   string `json:"title,omitempty"`
 	Version string `json:"version"`
+	// WebsiteURL for the server, if any.
+	WebsiteURL string `json:"websiteUrl,omitempty"`
+	// Icons for the Server, if any.
+	Icons []Icon `json:"icons,omitempty"`
 }
 
 // Present if the server supports argument autocompletion suggestions.



Test patch:
diff --git a/mcp/conformance_test.go b/mcp/conformance_test.go
index 3393efc..b2801db 100644
--- a/mcp/conformance_test.go
+++ b/mcp/conformance_test.go
@@ -111,6 +111,19 @@ func structuredTool(ctx context.Context, req *CallToolRequest, args *structuredI
 	return nil, &structuredOutput{"Ack " + args.In}, nil
 }
 
+func contentTool(ctx context.Context, req *CallToolRequest, args *structuredInput) (*CallToolResult, any, error) {
+	return &CallToolResult{
+		Content: []Content{
+			&ResourceLink{
+				Name:     "Example Resource Link with Icons",
+				MIMEType: "text/plain",
+				URI:      "https://example.com/resource/" + args.In,
+				Icons:    []Icon{iconObj},
+			},
+		},
+	}, nil, nil
+}
+
 type tomorrowInput struct {
 	Now time.Time
 }
@@ -135,12 +148,26 @@ func incTool(_ context.Context, _ *CallToolRequest, args incInput) (*CallToolRes
 	return nil, incOutput{args.X + 1}, nil
 }
 
+var iconObj = Icon{
+	Source:   "foobar",
+	MIMEType: "image/png",
+	Sizes:    []string{"48x48", "96x96"},
+	Theme:    "light",
+}
+
 // runServerTest runs the server conformance test.
 // It must be executed in a synctest bubble.
 func runServerTest(t *testing.T, test *conformanceTest) {
 	ctx := t.Context()
 	// Construct the server based on features listed in the test.
-	s := NewServer(&Implementation{Name: "testServer", Version: "v1.0.0"}, nil)
+	impl := &Implementation{Name: "testServer", Version: "v1.0.0"}
+
+	if test.name == "spec-sep-973-additional-metadata.txtar" {
+		impl.Icons = []Icon{iconObj}
+		impl.WebsiteURL = "https://github.com/modelcontextprotocol/go-sdk"
+	}
+
+	s := NewServer(impl, nil)
 	for _, tn := range test.tools {
 		switch tn {
 		case "greet":
@@ -148,6 +175,12 @@ func runServerTest(t *testing.T, test *conformanceTest) {
 				Name:        "greet",
 				Description: "say hi",
 			}, sayHi)
+		case "greetWithIcon":
+			AddTool(s, &Tool{
+				Name:        "greetWithIcon",
+				Description: "say hi",
+				Icons:       []Icon{iconObj},
+			}, sayHi)
 		case "structured":
 			AddTool(s, &Tool{Name: "structured"}, structuredTool)
 		case "tomorrow":
@@ -159,6 +192,12 @@ func runServerTest(t *testing.T, test *conformanceTest) {
 			}
 			inSchema.Properties["x"].Default = json.RawMessage(`6`)
 			AddTool(s, &Tool{Name: "inc", InputSchema: inSchema}, incTool)
+		case "contentTool":
+			AddTool(s, &Tool{
+				Name:        "contentTool",
+				Title:       "contentTool",
+				Description: "return resourceLink content with Icon",
+			}, contentTool)
 		default:
 			t.Fatalf("unknown tool %q", tn)
 		}
@@ -167,6 +206,13 @@ func runServerTest(t *testing.T, test *conformanceTest) {
 		switch pn {
 		case "code_review":
 			s.AddPrompt(codeReviewPrompt, codReviewPromptHandler)
+		case "code_reviewWithIcon":
+			s.AddPrompt(&Prompt{
+				Name:        "code_review",
+				Description: "do a code review",
+				Arguments:   []*PromptArgument{{Name: "Code", Required: true}},
+				Icons:       []Icon{iconObj},
+			}, codReviewPromptHandler)
 		default:
 			t.Fatalf("unknown prompt %q", pn)
 		}
@@ -177,6 +223,13 @@ func runServerTest(t *testing.T, test *conformanceTest) {
 			s.AddResource(resource1, readHandler)
 		case "info":
 			s.AddResource(resource3, handleEmbeddedResource)
+		case "infoWithIcon":
+			s.AddResource(&Resource{
+				Name:     "info",
+				MIMEType: "text/plain",
+				URI:      "embedded:info",
+				Icons:    []Icon{iconObj},
+			}, handleEmbeddedResource)
 		default:
 			t.Fatalf("unknown resource %q", rn)
 		}
diff --git a/mcp/content_test.go b/mcp/content_test.go
index 9366b0d..ae13c30 100644
--- a/mcp/content_test.go
+++ b/mcp/content_test.go
@@ -121,8 +121,9 @@ func TestContent(t *testing.T) {
 				Description: "This resource demonstrates all fields",
 				MIMEType:    "text/plain",
 				Meta:        mcp.Meta{"custom": "metadata"},
+				Icons:       []mcp.Icon{{Source: "foobar", MIMEType: "image/png", Sizes: []string{"48x48"}, Theme: "light"}},
 			},
-			`{"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"}}`,
+			`{"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"},"icons":[{"src":"foobar","mimeType":"image/png","sizes":["48x48"],"theme":"light"}]}`,
 		},
 	}
 
@@ -192,3 +193,53 @@ func TestEmbeddedResource(t *testing.T) {
 		}
 	}
 }
+
+// TestContentUnmarshal tests that unmarshaling JSON into various Content types
+// works correctly, including when the Content fields are initially nil.
+func TestContentUnmarshal(t *testing.T) {
+	valInt64 := int64(24)
+	tests := []struct {
+		name          string
+		json          string
+		content       mcp.Content
+		expectContent mcp.Content
+	}{
+		{
+			name:    "ResourceLink",
+			json:    `{"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"},"icons":[{"src":"foobar","mimeType":"image/png","sizes":["48x48"],"theme":"light"}], "size":24,"annotations":{"audience":["user","assistant"],"lastModified":"2025-01-12T15:00:58Z","priority":0.5}}`,
+			content: &mcp.ResourceLink{},
+			expectContent: &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"},
+				Size:        &valInt64,
+				Annotations: &mcp.Annotations{Audience: []mcp.Role{"user", "assistant"}, LastModified: "2025-01-12T15:00:58Z", Priority: 0.5},
+				Icons:       []mcp.Icon{{Source: "foobar", MIMEType: "image/png", Sizes: []string{"48x48"}, Theme: "light"}},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Test that unmarshaling doesn't panic on nil Content fields
+			defer func() {
+				if r := recover(); r != nil {
+					t.Errorf("UnmarshalJSON panicked: %v", r)
+				}
+			}()
+
+			err := json.Unmarshal([]byte(tt.json), tt.content)
+			if err != nil {
+				t.Errorf("UnmarshalJSON failed: %v", err)
+			}
+
+			// Verify that the Content field was properly populated
+			if cmp.Diff(tt.expectContent, tt.content) != "" {
+				t.Errorf("Content is not equal: %v", cmp.Diff(tt.expectContent, tt.content))
+			}
+		})
+	}
+}
diff --git a/mcp/testdata/conformance/server/spec-sep-973-additional-metadata.txtar b/mcp/testdata/conformance/server/spec-sep-973-additional-metadata.txtar
new file mode 100644
index 0000000..5441ab8
--- /dev/null
+++ b/mcp/testdata/conformance/server/spec-sep-973-additional-metadata.txtar
@@ -0,0 +1,216 @@
+Check behavior of server with Icons and websiteUrl metadata added as part of SEP-973 specification.
+
+check modelcontextprotocol/go-sdk/issues/552 for more details.
+
+Checks following:
+- Test setting websiteUrl, icons for mcp.Implementation
+- Test setting icons for mcp.Prompt
+- Test setting icons for mcp.Tool
+- Test setting icons for mcp.Resource
+- Test if tool call returning ResourceLink with icons works as expected
+
+
+-- tools --
+greetWithIcon
+contentTool
+
+-- prompts --
+code_reviewWithIcon
+
+-- resources --
+infoWithIcon
+
+-- client --
+{
+  "jsonrpc": "2.0",
+  "id": 1,
+  "method": "initialize",
+  "params": {
+    "protocolVersion": "2025-06-18",
+    "capabilities": {},
+    "clientInfo": { "name": "ExampleClient", "version": "1.0.0" }
+  }
+}
+{ "jsonrpc": "2.0", "method": "notifications/initialized" }
+{ "jsonrpc": "2.0", "id": 2, "method": "tools/list" }
+{ "jsonrpc": "2.0", "id": 3, "method": "resources/list" }
+{ "jsonrpc": "2.0", "id": 4, "method": "prompts/list" }
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"method": "tools/call",
+	"params": {
+		"name": "contentTool",
+		"arguments": {
+			"In": "resource1"
+		}
+	}
+}
+
+-- server --
+{
+	"jsonrpc": "2.0",
+	"id": 1,
+	"result": {
+		"capabilities": {
+			"logging": {},
+			"prompts": {
+				"listChanged": true
+			},
+			"resources": {
+				"listChanged": true
+			},
+			"tools": {
+				"listChanged": true
+			}
+		},
+		"protocolVersion": "2025-06-18",
+		"serverInfo": {
+			"name": "testServer",
+			"version": "v1.0.0",
+			"websiteUrl": "https://github.com/modelcontextprotocol/go-sdk",
+			"icons": [
+				{
+					"src": "foobar",
+					"mimeType": "image/png",
+					"sizes": [
+						"48x48",
+						"96x96"
+					],
+					"theme": "light"
+				}
+			]
+		}
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 2,
+	"result": {
+		"tools": [
+			{
+				"description": "return resourceLink content with Icon",
+				"inputSchema": {
+					"type": "object",
+					"required": [
+						"In"
+					],
+					"properties": {
+						"In": {
+							"type": "string",
+							"description": "the input"
+						}
+					},
+					"additionalProperties": false
+				},
+				"name": "contentTool",
+				"title": "contentTool"
+			},
+			{
+				"description": "say hi",
+				"inputSchema": {
+					"type": "object",
+					"required": [
+						"Name"
+					],
+					"properties": {
+						"Name": {
+							"type": "string"
+						}
+					},
+					"additionalProperties": false
+				},
+				"name": "greetWithIcon",
+				"icons": [
+					{
+						"src": "foobar",
+						"mimeType": "image/png",
+						"sizes": [
+							"48x48",
+							"96x96"
+						],
+						"theme": "light"
+					}
+				]
+			}
+		]
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 3,
+	"result": {
+		"resources": [
+			{
+				"mimeType": "text/plain",
+				"name": "info",
+				"uri": "embedded:info",
+				"icons": [
+					{
+						"src": "foobar",
+						"mimeType": "image/png",
+						"sizes": [
+							"48x48",
+							"96x96"
+						],
+						"theme": "light"
+					}
+				]
+			}
+		]
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 4,
+	"result": {
+		"prompts": [
+			{
+				"arguments": [
+					{
+						"name": "Code",
+						"required": true
+					}
+				],
+				"description": "do a code review",
+				"name": "code_review",
+				"icons": [
+					{
+						"src": "foobar",
+						"mimeType": "image/png",
+						"sizes": [
+							"48x48",
+							"96x96"
+						],
+						"theme": "light"
+					}
+				]
+			}
+		]
+	}
+}
+{
+	"jsonrpc": "2.0",
+	"id": 5,
+	"result": {
+		"content": [
+			{
+				"type": "resource_link",
+				"mimeType": "text/plain",
+				"uri": "https://example.com/resource/resource1",
+				"name": "Example Resource Link with Icons",
+				"icons": [
+					{
+						"src": "foobar",
+						"mimeType": "image/png",
+						"sizes": [
+							"48x48",
+							"96x96"
+						],
+						"theme": "light"
+					}
+				]
+			}
+		]
+	}
+}