Phase B: MCP tools tri-state Option<Vec<String>>

McpStdioServerConfig.tools and McpHttpServerConfig.tools change from
Vec<String> to Option<Vec<String>> with #[serde(default,
skip_serializing_if = Option::is_none)].

Semantics now match the runtime contract and the other SDKs:
  - None  (field omitted on wire) -> expose ALL tools
  - Some(vec![])                  -> expose NO tools
  - Some(non-empty)               -> explicit allowlist

Previously an empty Vec was treated as 'no tools' on the wire but the
runtime treats omission as 'all tools', so there was no way to opt back
into 'all' once the field was set.

Adds four tri-state serde tests (serialize + deserialize for each of
stdio and http).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

rust/src/types.rs

@@ -736,7 +736,7 @@ impl CloudSessionOptions {
 /// servers.insert(
 ///     "playwright".to_string(),
 ///     McpServerConfig::Stdio(McpStdioServerConfig {
-///         tools: vec!["*".to_string()],
+///         tools: Some(vec!["*".to_string()]),
 ///         command: "npx".to_string(),
 ///         args: vec!["-y".to_string(), "@playwright/mcp".to_string()],
 ///         ..Default::default()
@@ -745,7 +745,7 @@ impl CloudSessionOptions {
 /// servers.insert(
 ///     "weather".to_string(),
 ///     McpServerConfig::Http(McpHttpServerConfig {
-///         tools: vec!["forecast".to_string()],
+///         tools: Some(vec!["forecast".to_string()]),
 ///         url: "https://example.com/mcp".to_string(),
 ///         ..Default::default()
 ///     }),
@@ -772,9 +772,13 @@ pub enum McpServerConfig {
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct McpStdioServerConfig {
-    /// Tools to expose from this server. `["*"]` exposes all; `[]` exposes none.
-    #[serde(default)]
-    pub tools: Vec<String>,
+    /// Tools to expose from this server.
+    ///
+    /// - `None` (field omitted on the wire) — expose **all** tools.
+    /// - `Some(vec![])` — expose **no** tools.
+    /// - `Some(vec!["a", ...])` — expose only the listed tools.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<String>>,
     /// Optional timeout in milliseconds for tool calls to this server.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub timeout: Option<i64>,
@@ -798,9 +802,13 @@ pub struct McpStdioServerConfig {
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct McpHttpServerConfig {
-    /// Tools to expose from this server. `["*"]` exposes all; `[]` exposes none.
-    #[serde(default)]
-    pub tools: Vec<String>,
+    /// Tools to expose from this server.
+    ///
+    /// - `None` (field omitted on the wire) — expose **all** tools.
+    /// - `Some(vec![])` — expose **no** tools.
+    /// - `Some(vec!["a", ...])` — expose only the listed tools.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tools: Option<Vec<String>>,
     /// Optional timeout in milliseconds for tool calls to this server.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub timeout: Option<i64>,

rust/tests/e2e/mcp_and_agents.rs

@@ -51,7 +51,7 @@ async fn accept_mcp_server_config_without_args() {
                 let mcp_servers = HashMap::from([(
                     "test-server".to_string(),
                     McpServerConfig::Stdio(McpStdioServerConfig {
-                        tools: vec!["*".to_string()],
+                        tools: Some(vec!["*".to_string()]),
                         command: "echo".to_string(),
                         ..McpStdioServerConfig::default()
                     }),
@@ -389,7 +389,7 @@ fn multiple_mcp_servers() -> HashMap<String, McpServerConfig> {
     servers.insert(
         "server2".to_string(),
         McpServerConfig::Stdio(McpStdioServerConfig {
-            tools: vec!["*".to_string()],
+            tools: Some(vec!["*".to_string()]),
             command: echo_command(),
             args: echo_args("server2"),
             ..McpStdioServerConfig::default()
@@ -402,7 +402,7 @@ fn test_mcp_servers(message: &str) -> HashMap<String, McpServerConfig> {
     HashMap::from([(
         "test-server".to_string(),
         McpServerConfig::Stdio(McpStdioServerConfig {
-            tools: vec!["*".to_string()],
+            tools: Some(vec!["*".to_string()]),
             command: echo_command(),
             args: echo_args(message),
             ..McpStdioServerConfig::default()

rust/tests/e2e/rpc_mcp_and_skills.rs

@@ -438,7 +438,7 @@ fn test_mcp_servers(message: &str) -> HashMap<String, McpServerConfig> {
     HashMap::from([(
         message.to_string(),
         McpServerConfig::Stdio(McpStdioServerConfig {
-            tools: vec!["*".to_string()],
+            tools: Some(vec!["*".to_string()]),
             command: echo_command(),
             args: echo_args(message),
             ..McpStdioServerConfig::default()

rust/tests/session_test.rs

@@ -544,7 +544,7 @@ fn mcp_server_config_roundtrips_through_tagged_enum() {
         args: vec!["server.js".to_string()],
         env: HashMap::new(),
         cwd: None,
-        tools: vec!["*".to_string()],
+        tools: Some(vec!["*".to_string()]),
         timeout: None,
     });
     let json = serde_json::to_value(&stdio).unwrap();
@@ -566,6 +566,109 @@ fn mcp_server_config_roundtrips_through_tagged_enum() {
     assert_eq!(cfg_json["github"]["type"], "stdio");
 }
 
+#[test]
+fn mcp_stdio_tools_tri_state_serializes_correctly() {
+    use github_copilot_sdk::McpStdioServerConfig;
+
+    // None → field omitted (= "expose all tools")
+    let cfg = McpStdioServerConfig {
+        command: "echo".into(),
+        tools: None,
+        ..Default::default()
+    };
+    let json = serde_json::to_value(&cfg).unwrap();
+    assert!(
+        json.get("tools").is_none(),
+        "tools=None must be omitted on the wire; got {json}"
+    );
+
+    // Some(empty) → field present as []
+    let cfg = McpStdioServerConfig {
+        command: "echo".into(),
+        tools: Some(vec![]),
+        ..Default::default()
+    };
+    let json = serde_json::to_value(&cfg).unwrap();
+    assert_eq!(json["tools"], serde_json::json!([]));
+
+    // Some(non-empty) → field present as the explicit list
+    let cfg = McpStdioServerConfig {
+        command: "echo".into(),
+        tools: Some(vec!["a".into(), "b".into()]),
+        ..Default::default()
+    };
+    let json = serde_json::to_value(&cfg).unwrap();
+    assert_eq!(json["tools"], serde_json::json!(["a", "b"]));
+}
+
+#[test]
+fn mcp_stdio_tools_tri_state_deserializes_correctly() {
+    use github_copilot_sdk::McpStdioServerConfig;
+
+    // Missing field → None
+    let cfg: McpStdioServerConfig =
+        serde_json::from_value(serde_json::json!({ "command": "echo" })).unwrap();
+    assert_eq!(cfg.tools, None);
+
+    // Empty list → Some(empty)
+    let cfg: McpStdioServerConfig =
+        serde_json::from_value(serde_json::json!({ "command": "echo", "tools": [] })).unwrap();
+    assert_eq!(cfg.tools, Some(vec![]));
+
+    // Non-empty list → Some(list)
+    let cfg: McpStdioServerConfig =
+        serde_json::from_value(serde_json::json!({ "command": "echo", "tools": ["x"] })).unwrap();
+    assert_eq!(cfg.tools, Some(vec!["x".to_string()]));
+}
+
+#[test]
+fn mcp_http_tools_tri_state_serializes_correctly() {
+    use github_copilot_sdk::McpHttpServerConfig;
+
+    let cfg = McpHttpServerConfig {
+        url: "https://example.com".into(),
+        tools: None,
+        ..Default::default()
+    };
+    assert!(
+        serde_json::to_value(&cfg).unwrap().get("tools").is_none(),
+        "tools=None must be omitted on the wire"
+    );
+
+    let cfg = McpHttpServerConfig {
+        url: "https://example.com".into(),
+        tools: Some(vec![]),
+        ..Default::default()
+    };
+    assert_eq!(
+        serde_json::to_value(&cfg).unwrap()["tools"],
+        serde_json::json!([])
+    );
+
+    let cfg = McpHttpServerConfig {
+        url: "https://example.com".into(),
+        tools: Some(vec!["a".into()]),
+        ..Default::default()
+    };
+    assert_eq!(
+        serde_json::to_value(&cfg).unwrap()["tools"],
+        serde_json::json!(["a"])
+    );
+}
+
+#[test]
+fn mcp_http_tools_tri_state_deserializes_correctly() {
+    use github_copilot_sdk::McpHttpServerConfig;
+
+    let cfg: McpHttpServerConfig =
+        serde_json::from_value(serde_json::json!({ "url": "https://e.com" })).unwrap();
+    assert_eq!(cfg.tools, None);
+
+    let cfg: McpHttpServerConfig =
+        serde_json::from_value(serde_json::json!({ "url": "https://e.com", "tools": [] })).unwrap();
+    assert_eq!(cfg.tools, Some(vec![]));
+}
+
 #[test]
 fn permission_request_data_extracts_typed_kind() {
     use github_copilot_sdk::{PermissionRequestData, PermissionRequestKind};