feat(subagents): inherit MCP tools in child runtimes (#2422)

Harvested from #2377 with thanks to @buko.

Threads the parent MCP tool pool into child SubAgentRuntime construction and registers MCP-backed tools for child agents when MCP is enabled, while leaving the broader mention-browser/provider/config work for focused follow-ups.

Validation:
- cargo fmt --all -- --check
- CARGO_TARGET_DIR=/Volumes/VIXinSSD/codewhale-pr2377-target cargo test -p codewhale-tui tools::subagent
- CARGO_TARGET_DIR=/Volumes/VIXinSSD/codewhale-target/harvest-2377-recheck cargo test -p codewhale-tui tools::subagent --all-features

crates/tui/src/core/engine.rs

@@ -697,6 +697,12 @@ impl Engine {
                         continue;
                     };
 
+                    let mcp_pool = if self.config.features.enabled(Feature::Mcp) {
+                        self.ensure_mcp_pool().await.ok()
+                    } else {
+                        None
+                    };
+
                     let mut runtime = SubAgentRuntime::new(
                         client,
                         self.session.model.clone(),
@@ -714,6 +720,7 @@ impl Engine {
                     )
                     .with_max_spawn_depth(self.config.max_spawn_depth)
                     .with_step_api_timeout(self.config.subagent_api_timeout)
+                    .with_mcp_pool(mcp_pool)
                     .background_runtime();
                     let route = resolve_subagent_assignment_route(
                         &runtime,
@@ -1179,6 +1186,12 @@ impl Engine {
             None
         };
 
+        let mcp_pool = if self.config.features.enabled(Feature::Mcp) {
+            self.ensure_mcp_pool().await.ok()
+        } else {
+            None
+        };
+
         let tool_registry = match mode {
             AppMode::Agent | AppMode::Yolo => {
                 if self.config.features.enabled(Feature::Subagents) {
@@ -1199,6 +1212,7 @@ impl Engine {
                         )
                         .with_max_spawn_depth(self.config.max_spawn_depth)
                         .with_step_api_timeout(self.config.subagent_api_timeout)
+                        .with_mcp_pool(mcp_pool.clone())
                         .with_parent_completion_tx(self.tx_subagent_completion.clone());
                         if let Some(context) = fork_context_for_runtime.clone() {
                             rt = rt.with_fork_context(context);

crates/tui/src/tools/registry.rs

@@ -777,7 +777,6 @@ impl ToolRegistryBuilder {
     /// MCP tools are marked `defer_loading` by default (except discovery
     /// helpers) to keep the model-visible catalog compact.
     #[must_use]
-    #[allow(dead_code)]
     pub fn with_mcp_tools(
         mut self,
         mcp_pool: std::sync::Arc<tokio::sync::Mutex<crate::mcp::McpPool>>,

crates/tui/src/tools/subagent/mod.rs

@@ -786,6 +786,8 @@ pub struct SubAgentRuntime {
     pub parent_completion_tx: Option<mpsc::UnboundedSender<SubAgentCompletion>>,
     /// Snapshot of the request prefix visible to an opt-in forked child.
     pub fork_context: Option<SubAgentForkContext>,
+    /// The parent's MCP pool if available.
+    pub mcp_pool: Option<std::sync::Arc<tokio::sync::Mutex<crate::mcp::McpPool>>>,
     /// Per-step DeepSeek API timeout for the child's `create_message` call.
     /// Resolved from `[subagents] api_timeout_secs` (clamped to 1..=1800) at
     /// engine construction so a slow but legitimate model turn does not
@@ -825,10 +827,21 @@ impl SubAgentRuntime {
             mailbox: None,
             parent_completion_tx: None,
             fork_context: None,
+            mcp_pool: None,
             step_api_timeout: DEFAULT_STEP_API_TIMEOUT,
         }
     }
 
+    /// Attach an MCP pool so the subagent can execute MCP tools.
+    #[must_use]
+    pub fn with_mcp_pool(
+        mut self,
+        pool: Option<std::sync::Arc<tokio::sync::Mutex<crate::mcp::McpPool>>>,
+    ) -> Self {
+        self.mcp_pool = pool;
+        self
+    }
+
     /// Override the per-step DeepSeek API timeout (default
     /// `DEFAULT_STEP_API_TIMEOUT`). Called by the engine after reading
     /// `[subagents] api_timeout_secs`. Tests may use this to fail fast
@@ -959,6 +972,7 @@ impl SubAgentRuntime {
             mailbox: self.mailbox.clone(),
             parent_completion_tx: self.parent_completion_tx.clone(),
             fork_context: self.fork_context.clone(),
+            mcp_pool: self.mcp_pool.clone(),
             step_api_timeout: self.step_api_timeout,
         }
     }
@@ -4762,17 +4776,21 @@ impl SubAgentToolRegistry {
         // review, RLM, sub-agent management (so grandchildren can spawn),
         // plus per-child fresh todo/plan state.
         let context = runtime.context.clone();
-        let registry = ToolRegistryBuilder::new()
-            .with_full_agent_surface(
-                Some(runtime.client.clone()),
-                runtime.model.clone(),
-                runtime.manager.clone(),
-                runtime.clone(),
-                runtime.allow_shell,
-                todo_list,
-                plan_state,
-            )
-            .build(context);
+        let mut registry = ToolRegistryBuilder::new().with_full_agent_surface(
+            Some(runtime.client.clone()),
+            runtime.model.clone(),
+            runtime.manager.clone(),
+            runtime.clone(),
+            runtime.allow_shell,
+            todo_list,
+            plan_state,
+        );
+
+        if let Some(pool) = runtime.mcp_pool.as_ref() {
+            registry = registry.with_mcp_tools(std::sync::Arc::clone(pool));
+        }
+
+        let registry = registry.build(context);
 
         Self {
             allowed_tools: explicit_allowed_tools,

crates/tui/src/tools/subagent/tests.rs

@@ -1736,6 +1736,7 @@ fn stub_runtime() -> SubAgentRuntime {
         mailbox: None,
         parent_completion_tx: None,
         fork_context: None,
+        mcp_pool: None,
         step_api_timeout: DEFAULT_STEP_API_TIMEOUT,
     }
 }