chore: change StringFormat to enum

Author: ByteBaker

crates/rmcp/src/model/elicitation_schema.rs

@@ -61,6 +61,21 @@ pub enum PrimitiveSchema {
 // STRING SCHEMA
 // =============================================================================
 
+/// String format types allowed by the MCP specification.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[serde(rename_all = "kebab-case")]
+pub enum StringFormat {
+    /// Email address format
+    Email,
+    /// URI format
+    Uri,
+    /// Date format (YYYY-MM-DD)
+    Date,
+    /// Date-time format (ISO 8601)
+    DateTime,
+}
+
 /// Schema definition for string properties.
 ///
 /// Compliant with MCP 2025-06-18 specification for elicitation schemas.
@@ -92,7 +107,7 @@ pub struct StringSchema {
 
     /// String format - limited to: "email", "uri", "date", "date-time"
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub format: Option<Cow<'static, str>>,
+    pub format: Option<StringFormat>,
 }
 
 impl Default for StringSchema {
@@ -117,31 +132,31 @@ impl StringSchema {
     /// Create an email string schema
     pub fn email() -> Self {
         Self {
-            format: Some(Cow::Borrowed("email")),
+            format: Some(StringFormat::Email),
             ..Default::default()
         }
     }
 
     /// Create a URI string schema
     pub fn uri() -> Self {
         Self {
-            format: Some(Cow::Borrowed("uri")),
+            format: Some(StringFormat::Uri),
             ..Default::default()
         }
     }
 
     /// Create a date string schema
     pub fn date() -> Self {
         Self {
-            format: Some(Cow::Borrowed("date")),
+            format: Some(StringFormat::Date),
             ..Default::default()
         }
     }
 
     /// Create a date-time string schema
     pub fn date_time() -> Self {
         Self {
-            format: Some(Cow::Borrowed("date-time")),
+            format: Some(StringFormat::DateTime),
             ..Default::default()
         }
     }
@@ -189,8 +204,8 @@ impl StringSchema {
     }
 
     /// Set format (limited to: "email", "uri", "date", "date-time")
-    pub fn format(mut self, format: impl Into<Cow<'static, str>>) -> Self {
-        self.format = Some(format.into());
+    pub fn format(mut self, format: StringFormat) -> Self {
+        self.format = Some(format);
         self
     }
 }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -2198,6 +2198,31 @@
         }
       ]
     },
+    "StringFormat": {
+      "description": "String format types allowed by the MCP specification.",
+      "oneOf": [
+        {
+          "description": "Email address format",
+          "type": "string",
+          "const": "email"
+        },
+        {
+          "description": "URI format",
+          "type": "string",
+          "const": "uri"
+        },
+        {
+          "description": "Date format (YYYY-MM-DD)",
+          "type": "string",
+          "const": "date"
+        },
+        {
+          "description": "Date-time format (ISO 8601)",
+          "type": "string",
+          "const": "date-time"
+        }
+      ]
+    },
     "StringSchema": {
       "description": "Schema definition for string properties.\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nSupports only the fields allowed by the MCP spec:\n- format limited to: \"email\", \"uri\", \"date\", \"date-time\"",
       "type": "object",
@@ -2211,9 +2236,13 @@
         },
         "format": {
           "description": "String format - limited to: \"email\", \"uri\", \"date\", \"date-time\"",
-          "type": [
-            "string",
-            "null"
+          "anyOf": [
+            {
+              "$ref": "#/definitions/StringFormat"
+            },
+            {
+              "type": "null"
+            }
           ]
         },
         "maxLength": {

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -2198,6 +2198,31 @@
         }
       ]
     },
+    "StringFormat": {
+      "description": "String format types allowed by the MCP specification.",
+      "oneOf": [
+        {
+          "description": "Email address format",
+          "type": "string",
+          "const": "email"
+        },
+        {
+          "description": "URI format",
+          "type": "string",
+          "const": "uri"
+        },
+        {
+          "description": "Date format (YYYY-MM-DD)",
+          "type": "string",
+          "const": "date"
+        },
+        {
+          "description": "Date-time format (ISO 8601)",
+          "type": "string",
+          "const": "date-time"
+        }
+      ]
+    },
     "StringSchema": {
       "description": "Schema definition for string properties.\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nSupports only the fields allowed by the MCP spec:\n- format limited to: \"email\", \"uri\", \"date\", \"date-time\"",
       "type": "object",
@@ -2211,9 +2236,13 @@
         },
         "format": {
           "description": "String format - limited to: \"email\", \"uri\", \"date\", \"date-time\"",
-          "type": [
-            "string",
-            "null"
+          "anyOf": [
+            {
+              "$ref": "#/definitions/StringFormat"
+            },
+            {
+              "type": "null"
+            }
           ]
         },
         "maxLength": {