[mcp] Add dummy tools for previously called but currently missing tools. (#17853)

- [x] Add dummy tools for previously called but currently missing tools.
Currently supporting MCP tools only.

codex-rs/features/src/lib.rs

@@ -146,6 +146,8 @@ pub enum Feature {
     Apps,
     /// Enable the tool_search tool for apps.
     ToolSearch,
+    /// Expose placeholder tools for unavailable historical tool calls.
+    UnavailableDummyTools,
     /// Enable discoverable tool suggestions for apps.
     ToolSuggest,
     /// Enable plugins.
@@ -789,6 +791,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::UnavailableDummyTools,
+        key: "unavailable_dummy_tools",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::ToolSuggest,
         key: "tool_suggest",

codex-rs/features/src/tests.rs

@@ -133,6 +133,15 @@ fn tool_search_is_under_development_and_disabled_by_default() {
     assert_eq!(Feature::ToolSearch.default_enabled(), false);
 }
 
+#[test]
+fn unavailable_dummy_tools_is_under_development_and_disabled_by_default() {
+    assert_eq!(
+        Feature::UnavailableDummyTools.stage(),
+        Stage::UnderDevelopment
+    );
+    assert_eq!(Feature::UnavailableDummyTools.default_enabled(), false);
+}
+
 #[test]
 fn general_analytics_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::GeneralAnalytics.stage(), Stage::Stable);