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internal/mcp: document content constructors

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Also, make sure that NewBlobResourceContents creates a Blob.

Change-Id: I2f0fe2ac6410d6b155deb2c8643a40bc73e25cd4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672418
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Jonathan Amsterdam 2025-05-13 18:17:05 -04:00
commit 7b100d7b30
1 changed files with 31 additions and 27 deletions
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58
mcp/content.go
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@ -10,16 +10,16 @@ import (
"fmt"
)
// The []byte fields below are marked omitzero, not omitempty:
// we want to marshal an empty byte slice.
// TODO(jba): figure out how to fix this for 1.23.
// Content is the wire format for content.
// 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.
// Use [NewTextContent], [NewImageContent], [NewAudioContent] or [NewResourceContent]
// to create one.
//
// The Type field must be one of "text", "image", "audio" or "resource". The
// constructors above populate this field appropriately.
// Although at most one of Text, Data, and Resource should be non-zero, consumers of Content
// use the Type field to determine which value to use; values in the other fields are ignored.
type Content struct {
Type string `json:"type"`
Text string `json:"text,omitempty"`
@ -44,42 +44,47 @@ func (c *Content) UnmarshalJSON(data []byte) error {
return nil
}
// NewTextContent creates a [Content] with text.
func NewTextContent(text string) Content {
return Content{Type: "text", Text: text}
}
// NewImageContent creates a [Content] with image data.
func NewImageContent(data []byte, mimeType string) Content {
return Content{Type: "image", Data: data, MIMEType: mimeType}
}
// NewAudioContent creates a [Content] with audio data.
func NewAudioContent(data []byte, mimeType string) Content {
return Content{Type: "audio", Data: data, MIMEType: mimeType}
}
func NewResourceContent(resource ResourceContents) Content {
return Content{Type: "resource", Resource: &resource}
// NewResourceContent creates a [Content] with an embedded resource.
func NewResourceContent(resource *ResourceContents) Content {
return Content{Type: "resource", Resource: resource}
}
// A ResourceContents is either a TextResourceContents or a BlobResourceContents.
// ResourceContents represents the union of the spec's {Text,Blob}ResourceContents types.
// 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.
// A ResourceContents is either a TextResourceContents or a BlobResourceContents.
// Use [NewTextResourceContents] or [NextBlobResourceContents] to create one.
type ResourceContents struct {
URI string `json:"uri,"`
URI string `json:"uri"` // resource location; must not be empty
MIMEType string `json:"mimeType,omitempty"`
Text string `json:"text"`
Blob []byte `json:"blob,omitempty"`
Blob []byte `json:"blob,omitempty"` // if nil, then text; else blob
}
func (r ResourceContents) MarshalJSON() ([]byte, error) {
// If we could assume Go 1.24, we could use omitzero for Blob and avoid this method.
if r.URI == "" {
return nil, errors.New("ResourceContents missing URI")
}
if r.Blob == nil {
// Text. Marshal normally.
type wireResourceContents ResourceContents
type wireResourceContents ResourceContents // (lacks MarshalJSON method)
return json.Marshal((wireResourceContents)(r))
}
// Blob.
@ -99,8 +104,9 @@ func (r ResourceContents) MarshalJSON() ([]byte, error) {
return json.Marshal(br)
}
func NewTextResourceContents(uri, mimeType, text string) ResourceContents {
return ResourceContents{
// NewTextResourceContents returns a [ResourceContents] containing text.
func NewTextResourceContents(uri, mimeType, text string) *ResourceContents {
return &ResourceContents{
URI: uri,
MIMEType: mimeType,
Text: text,
@ -108,15 +114,13 @@ func NewTextResourceContents(uri, mimeType, text string) ResourceContents {
}
}
// A BlobResourceContents is the contents of a blob resource.
type BlobResourceContents struct {
URI string
MIMEType string
Blob []byte
}
func NewBlobResourceContents(uri, mimeType string, blob []byte) ResourceContents {
return ResourceContents{
// NewTextResourceContents returns a [ResourceContents] containing a byte slice.
func NewBlobResourceContents(uri, mimeType string, blob []byte) *ResourceContents {
// The only way to distinguish text from blob is a non-nil Blob field.
if blob == nil {
blob = []byte{}
}
return &ResourceContents{
URI: uri,
MIMEType: mimeType,
Blob: blob,