feat(rmcp): add optional _meta to CallToolResult, EmbeddedResource, and ResourceContents (#386)

* feat(rmcp): support optional _meta on EmbeddedResource and ResourceContents

- Add optional _meta to RawEmbeddedResource and ResourceContents (text/blob)
- Update constructors/usages to set meta: None by default
- Add tests to lock behavior (embedded text/blob + CallToolResult)
- Update JSON Schemas to include optional _meta fields

* style(rmcp): rustfmt after _meta changes and tests

crates/rmcp/src/model.rs

@@ -1267,6 +1267,9 @@ pub struct CallToolResult {
     /// Whether this result represents an error condition
     #[serde(skip_serializing_if = "Option::is_none")]
     pub is_error: Option<bool>,
+    /// Optional protocol-level metadata for this result
+    #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+    pub meta: Option<Meta>,
 }
 
 impl CallToolResult {
@@ -1276,6 +1279,7 @@ impl CallToolResult {
             content,
             structured_content: None,
             is_error: Some(false),
+            meta: None,
         }
     }
     /// Create an error tool result with unstructured content
@@ -1284,6 +1288,7 @@ impl CallToolResult {
             content,
             structured_content: None,
             is_error: Some(true),
+            meta: None,
         }
     }
     /// Create a successful tool result with structured content
@@ -1305,6 +1310,7 @@ impl CallToolResult {
             content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(false),
+            meta: None,
         }
     }
     /// Create an error tool result with structured content
@@ -1330,6 +1336,7 @@ impl CallToolResult {
             content: vec![Content::text(value.to_string())],
             structured_content: Some(value),
             is_error: Some(true),
+            meta: None,
         }
     }
 
@@ -1377,6 +1384,9 @@ impl<'de> Deserialize<'de> for CallToolResult {
             structured_content: Option<Value>,
             #[serde(skip_serializing_if = "Option::is_none")]
             is_error: Option<bool>,
+            /// Accept `_meta` during deserialization
+            #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+            meta: Option<Meta>,
         }
 
         let helper = CallToolResultHelper::deserialize(deserializer)?;
@@ -1384,6 +1394,7 @@ impl<'de> Deserialize<'de> for CallToolResult {
             content: helper.content.unwrap_or_default(),
             structured_content: helper.structured_content,
             is_error: helper.is_error,
+            meta: helper.meta,
         };
 
         // Validate mutual exclusivity

crates/rmcp/src/model/content.rs

@@ -27,6 +27,8 @@ pub type ImageContent = Annotated<RawImageContent>;
 #[serde(rename_all = "camelCase")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct RawEmbeddedResource {
+    #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+    pub meta: Option<super::Meta>,
     pub resource: ResourceContents,
 }
 pub type EmbeddedResource = Annotated<RawEmbeddedResource>;
@@ -88,15 +90,20 @@ impl RawContent {
     }
 
     pub fn resource(resource: ResourceContents) -> Self {
-        RawContent::Resource(RawEmbeddedResource { resource })
+        RawContent::Resource(RawEmbeddedResource {
+            meta: None,
+            resource,
+        })
     }
 
     pub fn embedded_text<S: Into<String>, T: Into<String>>(uri: S, content: T) -> Self {
         RawContent::Resource(RawEmbeddedResource {
+            meta: None,
             resource: ResourceContents::TextResourceContents {
                 uri: uri.into(),
                 mime_type: Some("text".to_string()),
                 text: content.into(),
+                meta: None,
             },
         })
     }

crates/rmcp/src/model/meta.rs

@@ -98,7 +98,8 @@ variant_extension! {
         PromptListChangedNotification
     }
 }
-#[derive(Debug, Serialize, Deserialize, Clone, Default)]
+#[derive(Debug, Serialize, Deserialize, Clone, Default, PartialEq)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[serde(transparent)]
 pub struct Meta(pub JsonObject);
 const PROGRESS_TOKEN_FIELD: &str = "progressToken";

crates/rmcp/src/model/prompt.rs

@@ -149,12 +149,14 @@ impl PromptMessage {
             uri,
             mime_type: Some(mime_type),
             text: text.unwrap_or_default(),
+            meta: None,
         };
 
         Self {
             role,
             content: PromptMessageContent::Resource {
                 resource: RawEmbeddedResource {
+                    meta: None,
                     resource: resource_contents,
                 }
                 .optional_annotate(annotations),

crates/rmcp/src/model/resource.rs

@@ -1,6 +1,6 @@
 use serde::{Deserialize, Serialize};
 
-use super::Annotated;
+use super::{Annotated, Meta};
 
 /// Represents a resource in the extension with metadata
 #[derive(Debug, Serialize, Deserialize, Clone, PartialEq)]
@@ -51,6 +51,8 @@ pub enum ResourceContents {
         #[serde(skip_serializing_if = "Option::is_none")]
         mime_type: Option<String>,
         text: String,
+        #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+        meta: Option<Meta>,
     },
     #[serde(rename_all = "camelCase")]
     BlobResourceContents {
@@ -58,6 +60,8 @@ pub enum ResourceContents {
         #[serde(skip_serializing_if = "Option::is_none")]
         mime_type: Option<String>,
         blob: String,
+        #[serde(rename = "_meta", skip_serializing_if = "Option::is_none")]
+        meta: Option<Meta>,
     },
 }
 
@@ -67,6 +71,7 @@ impl ResourceContents {
             uri: uri.into(),
             mime_type: Some("text".into()),
             text: text.into(),
+            meta: None,
         }
     }
 }
@@ -114,6 +119,7 @@ mod tests {
             uri: "file:///test.txt".to_string(),
             mime_type: Some("text/plain".to_string()),
             text: "Hello world".to_string(),
+            meta: None,
         };
 
         let json = serde_json::to_string(&text_contents).unwrap();