feat: add default value support to elicitation schemas (SEP-1034)

crates/rmcp/src/model/elicitation_schema.rs

@@ -108,6 +108,10 @@ pub struct StringSchema {
     /// String format - limited to: "email", "uri", "date", "date-time"
     #[serde(skip_serializing_if = "Option::is_none")]
     pub format: Option<StringFormat>,
+
+    /// Default value
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub default: Option<Cow<'static, str>>,
 }
 
 impl Default for StringSchema {
@@ -119,6 +123,7 @@ impl Default for StringSchema {
             min_length: None,
             max_length: None,
             format: None,
+            default: None,
         }
     }
 }
@@ -208,6 +213,12 @@ impl StringSchema {
         self.format = Some(format);
         self
     }
+
+    /// Set default value
+    pub fn with_default(mut self, default: impl Into<Cow<'static, str>>) -> Self {
+        self.default = Some(default.into());
+        self
+    }
 }
 
 // =============================================================================
@@ -241,6 +252,10 @@ pub struct NumberSchema {
     /// Maximum value (inclusive)
     #[serde(skip_serializing_if = "Option::is_none")]
     pub maximum: Option<f64>,
+
+    /// Default value
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub default: Option<f64>,
 }
 
 impl Default for NumberSchema {
@@ -251,6 +266,7 @@ impl Default for NumberSchema {
             description: None,
             minimum: None,
             maximum: None,
+            default: None,
         }
     }
 }
@@ -302,6 +318,12 @@ impl NumberSchema {
         self.description = Some(description.into());
         self
     }
+
+    /// Set default value
+    pub fn with_default(mut self, default: f64) -> Self {
+        self.default = Some(default);
+        self
+    }
 }
 
 // =============================================================================
@@ -491,6 +513,10 @@ pub struct EnumSchema {
     /// Human-readable description
     #[serde(skip_serializing_if = "Option::is_none")]
     pub description: Option<Cow<'static, str>>,
+
+    /// Default value
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub default: Option<String>,
 }
 
 impl EnumSchema {
@@ -502,6 +528,7 @@ impl EnumSchema {
             enum_names: None,
             title: None,
             description: None,
+            default: None,
         }
     }
 
@@ -522,6 +549,12 @@ impl EnumSchema {
         self.description = Some(description.into());
         self
     }
+
+    /// Set default value
+    pub fn with_default(mut self, default: String) -> Self {
+        self.default = Some(default);
+        self
+    }
 }
 
 // =============================================================================
@@ -1177,4 +1210,114 @@ mod tests {
         assert!(result.is_err());
         assert_eq!(result.unwrap_err(), "minimum must be <= maximum");
     }
+
+    #[test]
+    fn test_string_schema_with_default() {
+        let schema = StringSchema::new()
+            .with_default("example@test.com")
+            .description("Email with default");
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "string");
+        assert_eq!(json["default"], "example@test.com");
+        assert_eq!(json["description"], "Email with default");
+    }
+
+    #[test]
+    fn test_string_schema_without_default() {
+        let schema = StringSchema::new().description("Email without default");
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "string");
+        assert!(json.get("default").is_none());
+    }
+
+    #[test]
+    fn test_number_schema_with_default() {
+        let schema = NumberSchema::new()
+            .range(0.0, 100.0)
+            .with_default(50.0)
+            .description("Percentage with default");
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "number");
+        assert_eq!(json["default"], 50.0);
+        assert_eq!(json["minimum"], 0.0);
+        assert_eq!(json["maximum"], 100.0);
+    }
+
+    #[test]
+    fn test_number_schema_without_default() {
+        let schema = NumberSchema::new().range(0.0, 100.0);
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "number");
+        assert!(json.get("default").is_none());
+    }
+
+    #[test]
+    fn test_enum_schema_with_default() {
+        let schema = EnumSchema::new(vec!["US".to_string(), "UK".to_string(), "CA".to_string()])
+            .with_default("US".to_string())
+            .description("Country with default");
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "string");
+        assert_eq!(json["enum"], json!(["US", "UK", "CA"]));
+        assert_eq!(json["default"], "US");
+    }
+
+    #[test]
+    fn test_enum_schema_without_default() {
+        let schema = EnumSchema::new(vec!["US".to_string(), "UK".to_string()]);
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "string");
+        assert!(json.get("default").is_none());
+    }
+
+    #[test]
+    fn test_elicitation_schema_with_defaults() {
+        let schema = ElicitationSchema::builder()
+            .required_string_with("name", |s| s.length(1, 100))
+            .property(
+                "email",
+                PrimitiveSchema::String(StringSchema::email().with_default("user@example.com")),
+            )
+            .property(
+                "age",
+                PrimitiveSchema::Number(NumberSchema::new().range(0.0, 150.0).with_default(25.0)),
+            )
+            .property(
+                "country",
+                PrimitiveSchema::Enum(
+                    EnumSchema::new(vec!["US".to_string(), "UK".to_string()])
+                        .with_default("US".to_string()),
+                ),
+            )
+            .description("User registration with defaults")
+            .build()
+            .unwrap();
+
+        let json = serde_json::to_value(&schema).unwrap();
+
+        assert_eq!(json["type"], "object");
+        assert_eq!(json["properties"]["email"]["default"], "user@example.com");
+        assert_eq!(json["properties"]["age"]["default"], 25.0);
+        assert_eq!(json["properties"]["country"]["default"], "US");
+        assert!(json["properties"]["name"].get("default").is_none());
+    }
+
+    #[test]
+    fn test_default_serialization_roundtrip() {
+        let original = StringSchema::new()
+            .with_default("test")
+            .description("Test schema");
+
+        let json = serde_json::to_value(&original).unwrap();
+        let deserialized: StringSchema = serde_json::from_value(json).unwrap();
+
+        assert_eq!(original, deserialized);
+        assert_eq!(deserialized.default, Some(Cow::Borrowed("test")));
+    }
 }

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -667,6 +667,13 @@
       "description": "Schema definition for enum properties.\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nEnums must have string type and can optionally include human-readable names.",
       "type": "object",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "description": "Human-readable description",
           "type": [
@@ -1320,6 +1327,14 @@
       "description": "Schema definition for number properties (floating-point).\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nSupports only the fields allowed by the MCP spec.",
       "type": "object",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "number",
+            "null"
+          ],
+          "format": "double"
+        },
         "description": {
           "description": "Human-readable description",
           "type": [
@@ -2227,6 +2242,13 @@
       "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",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "description": "Human-readable description",
           "type": [

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json

@@ -667,6 +667,13 @@
       "description": "Schema definition for enum properties.\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nEnums must have string type and can optionally include human-readable names.",
       "type": "object",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "description": "Human-readable description",
           "type": [
@@ -1320,6 +1327,14 @@
       "description": "Schema definition for number properties (floating-point).\n\nCompliant with MCP 2025-06-18 specification for elicitation schemas.\nSupports only the fields allowed by the MCP spec.",
       "type": "object",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "number",
+            "null"
+          ],
+          "format": "double"
+        },
         "description": {
           "description": "Human-readable description",
           "type": [
@@ -2227,6 +2242,13 @@
       "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",
       "properties": {
+        "default": {
+          "description": "Default value",
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "description": {
           "description": "Human-readable description",
           "type": [

examples/servers/src/elicitation_stdio.rs

@@ -1,6 +1,6 @@
 //! Simple MCP Server with Elicitation
 //!
-//! Demonstrates user name collection via elicitation
+//! Demonstrates user name collection via elicitation and default values
 
 use std::sync::Arc;
 
@@ -106,6 +106,88 @@ impl ElicitationServer {
             "User name reset. Next greeting will ask for name again.".to_string(),
         )]))
     }
+
+    #[tool(description = "Reply to email with default values demonstration")]
+    async fn reply_email(
+        &self,
+        context: RequestContext<RoleServer>,
+    ) -> Result<CallToolResult, McpError> {
+        // Build schema with default values for email reply
+        let schema = ElicitationSchema::builder()
+            .string_property("recipient", |s| {
+                s.format(StringFormat::Email)
+                    .with_default("sender@example.com")
+                    .description("Email recipient")
+            })
+            .string_property("cc", |s| {
+                s.format(StringFormat::Email)
+                    .with_default("team@example.com")
+                    .description("CC recipients")
+            })
+            .property(
+                "priority",
+                PrimitiveSchema::Enum(
+                    EnumSchema::new(vec![
+                        "low".to_string(),
+                        "normal".to_string(),
+                        "high".to_string(),
+                    ])
+                    .with_default("normal".to_string())
+                    .description("Email priority"),
+                ),
+            )
+            .number_property("confidence", |n| {
+                n.range(0.0, 1.0)
+                    .with_default(0.8)
+                    .description("Reply confidence score")
+            })
+            .required_string("subject")
+            .required_string("body")
+            .description("Email reply configuration with defaults")
+            .build()
+            .map_err(|e| McpError::internal_error(format!("Schema build error: {}", e), None))?;
+
+        // Request email details with pre-filled defaults
+        let response = context
+            .peer
+            .create_elicitation(CreateElicitationRequestParam {
+                message: "Configure email reply".to_string(),
+                requested_schema: schema,
+            })
+            .await
+            .map_err(|e| McpError::internal_error(format!("Elicitation error: {}", e), None))?;
+
+        match response.action {
+            ElicitationAction::Accept => {
+                if let Some(reply_data) = response.content {
+                    let recipient = reply_data
+                        .get("recipient")
+                        .and_then(|v| v.as_str())
+                        .unwrap_or("unknown");
+                    let subject = reply_data
+                        .get("subject")
+                        .and_then(|v| v.as_str())
+                        .unwrap_or("No subject");
+                    let priority = reply_data
+                        .get("priority")
+                        .and_then(|v| v.as_str())
+                        .unwrap_or("normal");
+
+                    Ok(CallToolResult::success(vec![Content::text(format!(
+                        "Email reply configured:\nTo: {}\nSubject: {}\nPriority: {}\n\nDefaults were used for pre-filling the form!",
+                        recipient, subject, priority
+                    ))]))
+                } else {
+                    Ok(CallToolResult::success(vec![Content::text(
+                        "Email accepted but no content provided".to_string(),
+                    )]))
+                }
+            }
+            ElicitationAction::Decline | ElicitationAction::Cancel => Ok(CallToolResult::success(
+                vec![Content::text("Email reply cancelled".to_string())],
+            )),
+        }
+    }
 }
 
 #[tool_handler]