feat: allow disabling delegation tools

Author: Lixtt

core/src/agent_api/capabilities.rs

@@ -18,6 +18,8 @@ use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
+const DISABLE_DELEGATION_TOOLS_ENV: &str = "A3S_CODE_DISABLE_DELEGATION_TOOLS";
+
 pub(super) struct SessionCapabilityInput<'a> {
     pub(super) code_config: &'a CodeConfig,
     pub(super) base_config: &'a AgentConfig,
@@ -163,6 +165,9 @@ fn register_task_capability(
     use crate::tools::register_task_with_mcp;
 
     let registry = AgentRegistry::new();
+    let disable_delegation_tools = disable_delegation_tools_from_env(
+        std::env::var(DISABLE_DELEGATION_TOOLS_ENV).ok().as_deref(),
+    );
     let built_in_agent_dirs = built_in_agent_dirs(workspace);
     for dir in code_config
         .agent_dirs
@@ -178,6 +183,10 @@ fn register_task_capability(
         registry.register_worker(worker.clone());
     }
 
+    if disable_delegation_tools {
+        return Arc::new(registry);
+    }
+
     let parent_context = ChildRunContext {
         security_provider: opts.security_provider.clone(),
         hook_engine: None,
@@ -203,6 +212,17 @@ fn register_task_capability(
     registry
 }
 
+fn disable_delegation_tools_from_env(value: Option<&str>) -> bool {
+    value
+        .map(|value| {
+            matches!(
+                value.trim().to_ascii_lowercase().as_str(),
+                "1" | "true" | "yes" | "on"
+            )
+        })
+        .unwrap_or(false)
+}
+
 fn built_in_agent_dirs(workspace: &Path) -> Vec<PathBuf> {
     let mut dirs = Vec::new();
     if let Some(home) = std::env::var_os("HOME").or_else(|| std::env::var_os("USERPROFILE")) {
@@ -265,6 +285,26 @@ fn group_mcp_tools_by_server(all_tools: Vec<(String, McpTool)>) -> HashMap<Strin
     by_server
 }
 
+#[cfg(test)]
+mod tests {
+    use super::disable_delegation_tools_from_env;
+
+    #[test]
+    fn disable_delegation_tools_accepts_common_truthy_values() {
+        for value in ["1", "true", "TRUE", " yes ", "on", "On"] {
+            assert!(disable_delegation_tools_from_env(Some(value)));
+        }
+    }
+
+    #[test]
+    fn disable_delegation_tools_rejects_empty_missing_and_falsey_values() {
+        for value in ["", "0", "false", "no", "off", "enabled"] {
+            assert!(!disable_delegation_tools_from_env(Some(value)));
+        }
+        assert!(!disable_delegation_tools_from_env(None));
+    }
+}
+
 fn build_context_providers(
     opts: &SessionOptions,
     workspace: &Path,