diff --git a/crates/rmcp/src/model/annotated.rs b/crates/rmcp/src/model/annotated.rs
index 06800d1f..ac3dbd3e 100644
--- a/crates/rmcp/src/model/annotated.rs
+++ b/crates/rmcp/src/model/annotated.rs
@@ -20,20 +20,23 @@ pub struct Annotations {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub priority: Option<f32>,
     #[serde(skip_serializing_if = "Option::is_none", rename = "lastModified")]
-    pub last_modified: Option<DateTime<Utc>>,
+    pub last_modified: Option<String>,
 }
 
 impl Annotations {
-    /// Creates a new Annotations instance specifically for resources
-    /// optional priority, and a timestamp (defaults to now if None)
+    /// Creates annotations for a resource with priority and an RFC 3339 timestamp.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `priority` is not in the inclusive range `0.0..=1.0`.
     pub fn for_resource(priority: f32, timestamp: DateTime<Utc>) -> Self {
         assert!(
             (0.0..=1.0).contains(&priority),
             "Priority {priority} must be between 0.0 and 1.0"
         );
-        Annotations {
+        Self {
             priority: Some(priority),
-            last_modified: Some(timestamp),
+            last_modified: Some(timestamp.to_rfc3339()),
             audience: None,
         }
     }
@@ -48,12 +51,69 @@ impl Annotations {
         self
     }
 
+    /// Sets `lastModified` from a typed timestamp, serialized as RFC 3339.
     pub fn with_timestamp(mut self, timestamp: DateTime<Utc>) -> Self {
-        self.last_modified = Some(timestamp);
+        self.last_modified = Some(timestamp.to_rfc3339());
         self
     }
 
+    /// Sets `lastModified` to the current time, serialized as RFC 3339.
     pub fn with_timestamp_now(self) -> Self {
         self.with_timestamp(Utc::now())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use chrono::TimeZone;
+    use serde_json::json;
+
+    use super::*;
+
+    fn annotations_of(value: serde_json::Value) -> Annotations {
+        serde_json::from_value::<Annotations>(value).unwrap()
+    }
+
+    #[test]
+    fn preserves_date_only_last_modified_verbatim() {
+        let annotations = annotations_of(json!({ "lastModified": "2025-01-12" }));
+        assert_eq!(annotations.last_modified.as_deref(), Some("2025-01-12"));
+    }
+
+    #[test]
+    fn preserves_rfc3339_last_modified_verbatim() {
+        let value = "2025-01-12T15:00:58Z";
+        let annotations = annotations_of(json!({ "lastModified": value }));
+        assert_eq!(annotations.last_modified.as_deref(), Some(value));
+    }
+
+    #[test]
+    fn missing_last_modified_is_none() {
+        let annotations = annotations_of(json!({}));
+        assert_eq!(annotations.last_modified, None);
+    }
+
+    #[test]
+    fn null_last_modified_is_none() {
+        let annotations = annotations_of(json!({ "lastModified": null }));
+        assert_eq!(annotations.last_modified, None);
+    }
+
+    #[test]
+    fn invalid_last_modified_string_is_preserved() {
+        let annotations = annotations_of(json!({ "lastModified": "not-a-date" }));
+        assert_eq!(annotations.last_modified.as_deref(), Some("not-a-date"));
+    }
+
+    #[test]
+    fn timestamp_round_trips_as_rfc3339_string() {
+        let timestamp = Utc.with_ymd_and_hms(2025, 1, 12, 15, 0, 58).unwrap();
+        let annotations = Annotations::default().with_timestamp(timestamp);
+
+        let value = serde_json::to_value(&annotations).unwrap();
+        assert_eq!(value["lastModified"], json!(timestamp.to_rfc3339()));
+
+        let round_tripped: Annotations = serde_json::from_value(value).unwrap();
+        assert_eq!(round_tripped, annotations);
+    }
+}
diff --git a/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json b/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json
index 3bfdb7c4..c88cee46 100644
--- a/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json
+++ b/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json
@@ -54,8 +54,7 @@
           "type": [
             "string",
             "null"
-          ],
-          "format": "date-time"
+          ]
         },
         "priority": {
           "type": [
diff --git a/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json b/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json
index 3bfdb7c4..c88cee46 100644
--- a/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json
+++ b/crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json
@@ -54,8 +54,7 @@
           "type": [
             "string",
             "null"
-          ],
-          "format": "date-time"
+          ]
         },
         "priority": {
           "type": [
diff --git a/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json b/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
index fcf821f5..9b37ed21 100644
--- a/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
+++ b/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
@@ -54,8 +54,7 @@
           "type": [
             "string",
             "null"
-          ],
-          "format": "date-time"
+          ]
         },
         "priority": {
           "type": [
diff --git a/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json b/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
index fcf821f5..9b37ed21 100644
--- a/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
+++ b/crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
@@ -54,8 +54,7 @@
           "type": [
             "string",
             "null"
-          ],
-          "format": "date-time"
+          ]
         },
         "priority": {
           "type": [