diff --git a/mcp/content.go b/mcp/content.go
index 8ea7555..d77674b 100644
--- a/mcp/content.go
+++ b/mcp/content.go
@@ -15,6 +15,7 @@ import (
 //
 // ToWire converts content to its jsonrpc2 wire format.
 type Content interface {
+	// TODO: unexport this, and move the tests that use it to this package.
 	ToWire() protocol.Content
 }
 
@@ -29,64 +30,69 @@ func (c TextContent) ToWire() protocol.Content {
 
 // ImageContent contains base64-encoded image data.
 type ImageContent struct {
-	Data     string
-	MimeType string
+	Data     []byte // base64-encoded
+	MIMEType string
 }
 
 func (c ImageContent) ToWire() protocol.Content {
-	return protocol.Content{Type: "image", MIMEType: c.MimeType, Data: c.Data}
+	return protocol.Content{Type: "image", MIMEType: c.MIMEType, Data: c.Data}
 }
 
 // AudioContent contains base64-encoded audio data.
 type AudioContent struct {
-	Data     string
-	MimeType string
+	Data     []byte
+	MIMEType string
 }
 
 func (c AudioContent) ToWire() protocol.Content {
-	return protocol.Content{Type: "audio", MIMEType: c.MimeType, Data: c.Data}
+	return protocol.Content{Type: "audio", MIMEType: c.MIMEType, Data: c.Data}
 }
 
 // ResourceContent contains embedded resources.
 type ResourceContent struct {
-	Resource Resource
+	Resource EmbeddedResource
 }
 
 func (r ResourceContent) ToWire() protocol.Content {
-	res := r.Resource.ToWire()
+	res := r.Resource.toWire()
 	return protocol.Content{Type: "resource", Resource: &res}
 }
 
-type Resource interface {
-	ToWire() protocol.Resource
+type EmbeddedResource interface {
+	toWire() protocol.ResourceContents
 }
 
-type TextResource struct {
+// The {Text,Blob}ResourceContents types match the protocol definitions,
+// but we represent both as a single type on the wire.
+
+// A TextResourceContents is the contents of a text resource.
+type TextResourceContents struct {
 	URI      string
-	MimeType string
+	MIMEType string
 	Text     string
 }
 
-func (r TextResource) ToWire() protocol.Resource {
-	return protocol.Resource{
+func (r TextResourceContents) toWire() protocol.ResourceContents {
+	return protocol.ResourceContents{
 		URI:      r.URI,
-		MIMEType: r.MimeType,
+		MIMEType: r.MIMEType,
 		Text:     r.Text,
+		// Blob is nil, indicating this is a TextResourceContents.
 	}
 }
 
-type BlobResource struct {
+// A BlobResourceContents is the contents of a blob resource.
+type BlobResourceContents struct {
 	URI      string
-	MimeType string
-	Blob     string
+	MIMEType string
+	Blob     []byte
 }
 
-func (r BlobResource) ToWire() protocol.Resource {
-	blob := r.Blob
-	return protocol.Resource{
+func (r BlobResourceContents) toWire() protocol.ResourceContents {
+	return protocol.ResourceContents{
 		URI:      r.URI,
-		MIMEType: r.MimeType,
-		Blob:     &blob,
+		MIMEType: r.MIMEType,
+		Blob:     r.Blob,
 	}
 }
 
@@ -97,22 +103,22 @@ func ContentFromWireContent(c protocol.Content) Content {
 	case "text":
 		return TextContent{Text: c.Text}
 	case "image":
-		return ImageContent{Data: c.Data, MimeType: c.MIMEType}
+		return ImageContent{Data: c.Data, MIMEType: c.MIMEType}
 	case "audio":
-		return AudioContent{Data: c.Data, MimeType: c.MIMEType}
+		return AudioContent{Data: c.Data, MIMEType: c.MIMEType}
 	case "resource":
 		r := ResourceContent{}
 		if c.Resource != nil {
 			if c.Resource.Blob != nil {
-				r.Resource = BlobResource{
+				r.Resource = BlobResourceContents{
 					URI:      c.Resource.URI,
-					MimeType: c.Resource.MIMEType,
-					Blob:     *c.Resource.Blob,
+					MIMEType: c.Resource.MIMEType,
+					Blob:     c.Resource.Blob,
 				}
 			} else {
-				r.Resource = TextResource{
+				r.Resource = TextResourceContents{
 					URI:      c.Resource.URI,
-					MimeType: c.Resource.MIMEType,
+					MIMEType: c.Resource.MIMEType,
 					Text:     c.Resource.Text,
 				}
 			}
diff --git a/mcp/content_test.go b/mcp/content_test.go
index 0ba41a0..4abf35e 100644
--- a/mcp/content_test.go
+++ b/mcp/content_test.go
@@ -19,24 +19,24 @@ func TestContent(t *testing.T) {
 	}{
 		{mcp.TextContent{Text: "hello"}, protocol.Content{Type: "text", Text: "hello"}},
 		{
-			mcp.ImageContent{Data: "a1b2c3", MimeType: "image/png"},
-			protocol.Content{Type: "image", Data: "a1b2c3", MIMEType: "image/png"},
+			mcp.ImageContent{Data: []byte("a1b2c3"), MIMEType: "image/png"},
+			protocol.Content{Type: "image", Data: []byte("a1b2c3"), MIMEType: "image/png"},
 		},
 		{
-			mcp.AudioContent{Data: "a1b2c3", MimeType: "audio/wav"},
-			protocol.Content{Type: "audio", Data: "a1b2c3", MIMEType: "audio/wav"},
+			mcp.AudioContent{Data: []byte("a1b2c3"), MIMEType: "audio/wav"},
+			protocol.Content{Type: "audio", Data: []byte("a1b2c3"), MIMEType: "audio/wav"},
 		},
 		{
 			mcp.ResourceContent{
-				Resource: mcp.TextResource{
+				Resource: mcp.TextResourceContents{
 					URI:      "file://foo",
-					MimeType: "text",
+					MIMEType: "text",
 					Text:     "abc",
 				},
 			},
 			protocol.Content{
 				Type: "resource",
-				Resource: &protocol.Resource{
+				Resource: &protocol.ResourceContents{
 					URI:      "file://foo",
 					MIMEType: "text",
 					Text:     "abc",
@@ -45,18 +45,18 @@ func TestContent(t *testing.T) {
 		},
 		{
 			mcp.ResourceContent{
-				Resource: mcp.BlobResource{
+				Resource: mcp.BlobResourceContents{
 					URI:      "file://foo",
-					MimeType: "text",
-					Blob:     "a1b2c3",
+					MIMEType: "text",
+					Blob:     []byte("a1b2c3"),
 				},
 			},
 			protocol.Content{
 				Type: "resource",
-				Resource: &protocol.Resource{
+				Resource: &protocol.ResourceContents{
 					URI:      "file://foo",
 					MIMEType: "text",
-					Blob:     ptr("a1b2c3"),
+					Blob:     []byte("a1b2c3"),
 				},
 			},
 		},
@@ -69,7 +69,3 @@ func TestContent(t *testing.T) {
 		}
 	}
 }
-
-func ptr[T any](t T) *T {
-	return &t
-}
diff --git a/mcp/protocol/content.go b/mcp/protocol/content.go
index 3b93f43..1202704 100644
--- a/mcp/protocol/content.go
+++ b/mcp/protocol/content.go
@@ -9,27 +9,35 @@ import (
 	"fmt"
 )
 
+// The []byte fields below are marked omitzero, not omitempty:
+// we want to marshal an empty byte slice.
+
 // Content is the wire format for content.
-//
-// The Type field distinguishes the type of the content.
-// At most one of Text, MIMEType, Data, and Resource is non-zero.
+// It represents the protocol types TextContent, ImageContent, AudioContent
+// 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.
 type Content struct {
-	Type     string    `json:"type"`
-	Text     string    `json:"text,omitempty"`
-	MIMEType string    `json:"mimeType,omitempty"`
-	Data     string    `json:"data,omitempty"`
-	Resource *Resource `json:"resource,omitempty"`
+	Type        string            `json:"type"`
+	Text        string            `json:"text,omitempty"`
+	MIMEType    string            `json:"mimeType,omitempty"`
+	Data        []byte            `json:"data,omitzero"`
+	Resource    *ResourceContents `json:"resource,omitempty"`
+	Annotations *Annotations      `json:"annotations,omitempty"`
 }
 
-// Resource is the wire format for embedded resources.
+// A ResourceContents is either a TextResourceContents or a BlobResourceContents.
+// See https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-03-26/schema.ts#L524-L551
+// for the inheritance structure.
+// If Blob is nil, this is a TextResourceContents; otherwise it's a BlobResourceContents.
 //
-// The URI field describes the resource location. At most one of Text and Blob
-// is non-zero.
-type Resource struct {
-	URI      string  `json:"uri,"`
-	MIMEType string  `json:"mimeType,omitempty"`
-	Text     string  `json:"text"`
-	Blob     *string `json:"blob"` // blob is a pointer to distinguish empty from missing data
+// The URI field describes the resource location.
+type ResourceContents struct {
+	URI      string `json:"uri,"`
+	MIMEType string `json:"mimeType,omitempty"`
+	Text     string `json:"text"`
+	Blob     []byte `json:"blob,omitzero"`
 }
 
 func (c *Content) UnmarshalJSON(data []byte) error {