diff --git a/crates/loopal-agent-client/src/client.rs b/crates/loopal-agent-client/src/client.rs
index 8e366fb..7048eba 100644
--- a/crates/loopal-agent-client/src/client.rs
+++ b/crates/loopal-agent-client/src/client.rs
@@ -1,6 +1,6 @@
 //! IPC client — wraps `Connection` with agent protocol methods.
 
-use std::path::Path;
+use std::path::PathBuf;
 use std::sync::Arc;
 
 use serde_json::Value;
@@ -12,6 +12,22 @@ use loopal_ipc::protocol::methods;
 use loopal_ipc::transport::Transport;
 use loopal_protocol::{AgentEvent, ControlCommand, Envelope, UserQuestionResponse};
 
+/// Parameters for `agent/start` IPC request.
+#[derive(Debug, Default)]
+pub struct StartAgentParams {
+    pub cwd: PathBuf,
+    pub model: Option<String>,
+    pub mode: Option<String>,
+    pub prompt: Option<String>,
+    pub permission_mode: Option<String>,
+    pub no_sandbox: bool,
+    pub resume: Option<String>,
+    pub lifecycle: Option<String>,
+    pub agent_type: Option<String>,
+    /// Nesting depth (0 = root). Propagated from parent.
+    pub depth: Option<u32>,
+}
+
 /// High-level agent IPC client.
 pub struct AgentClient {
     connection: Arc<Connection>,
@@ -44,29 +60,18 @@ impl AgentClient {
     }
 
     /// Send `agent/start` to begin the agent loop.
-    #[allow(clippy::too_many_arguments)]
-    pub async fn start_agent(
-        &self,
-        cwd: &Path,
-        model: Option<&str>,
-        mode: Option<&str>,
-        prompt: Option<&str>,
-        permission_mode: Option<&str>,
-        no_sandbox: bool,
-        resume: Option<&str>,
-        lifecycle: Option<&str>,
-        agent_type: Option<&str>,
-    ) -> anyhow::Result<String> {
+    pub async fn start_agent(&self, p: &StartAgentParams) -> anyhow::Result<String> {
         let params = serde_json::json!({
-            "cwd": cwd.to_string_lossy(),
-            "model": model,
-            "mode": mode,
-            "prompt": prompt,
-            "permission_mode": permission_mode,
-            "no_sandbox": no_sandbox,
-            "resume": resume,
-            "lifecycle": lifecycle,
-            "agent_type": agent_type,
+            "cwd": p.cwd.to_string_lossy(),
+            "model": p.model,
+            "mode": p.mode,
+            "prompt": p.prompt,
+            "permission_mode": p.permission_mode,
+            "no_sandbox": p.no_sandbox,
+            "resume": p.resume,
+            "lifecycle": p.lifecycle,
+            "agent_type": p.agent_type,
+            "depth": p.depth,
         });
         let result = self
             .connection
diff --git a/crates/loopal-agent-client/src/lib.rs b/crates/loopal-agent-client/src/lib.rs
index c6c4cf6..c3da1b2 100644
--- a/crates/loopal-agent-client/src/lib.rs
+++ b/crates/loopal-agent-client/src/lib.rs
@@ -9,5 +9,5 @@ mod client;
 mod process;
 
 pub use bridge::{BridgeHandles, start_bridge};
-pub use client::{AgentClient, AgentClientEvent};
+pub use client::{AgentClient, AgentClientEvent, StartAgentParams};
 pub use process::AgentProcess;
diff --git a/crates/loopal-agent-hub/src/dispatch/dispatch_handlers.rs b/crates/loopal-agent-hub/src/dispatch/dispatch_handlers.rs
index 4e97c54..f6652f1 100644
--- a/crates/loopal-agent-hub/src/dispatch/dispatch_handlers.rs
+++ b/crates/loopal-agent-hub/src/dispatch/dispatch_handlers.rs
@@ -161,6 +161,7 @@ pub async fn handle_spawn_agent(
     let prompt = params["prompt"].as_str().map(String::from);
     let permission_mode = params["permission_mode"].as_str().map(String::from);
     let agent_type = params["agent_type"].as_str().map(String::from);
+    let depth = params["depth"].as_u64().map(|v| v as u32);
 
     // Parent: use explicit "parent" field from params if present (cross-hub),
     // otherwise use from_agent (local spawn).
@@ -182,6 +183,7 @@ pub async fn handle_spawn_agent(
             parent,
             permission_mode,
             agent_type,
+            depth,
         )
         .await
     });
diff --git a/crates/loopal-agent-hub/src/spawn_manager.rs b/crates/loopal-agent-hub/src/spawn_manager.rs
index 78bd759..1d03458 100644
--- a/crates/loopal-agent-hub/src/spawn_manager.rs
+++ b/crates/loopal-agent-hub/src/spawn_manager.rs
@@ -22,6 +22,7 @@ pub async fn spawn_and_register(
     parent: Option<String>,
     permission_mode: Option<String>,
     agent_type: Option<String>,
+    depth: Option<u32>,
 ) -> Result<String, String> {
     info!(agent = %name, parent = ?parent, "spawn: forking process");
     let agent_proc = loopal_agent_client::AgentProcess::spawn(None)
@@ -36,18 +37,19 @@ pub async fn spawn_and_register(
         return Err(format!("agent initialize failed: {e}"));
     }
     info!(agent = %name, "spawn: starting agent");
+    let model_for_registry = model.clone();
     let session_id = match client
-        .start_agent(
-            std::path::Path::new(&cwd),
-            model.as_deref(),
-            Some("act"),
-            prompt.as_deref(),
-            permission_mode.as_deref(),
-            false,
-            None,
-            Some("ephemeral"), // sub-agents always exit on idle
-            agent_type.as_deref(),
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: std::path::PathBuf::from(&cwd),
+            model,
+            mode: Some("act".to_string()),
+            prompt,
+            permission_mode,
+            lifecycle: Some("ephemeral".to_string()), // sub-agents always exit on idle
+            agent_type,
+            depth,
+            ..Default::default()
+        })
         .await
     {
         Ok(sid) => Some(sid),
@@ -65,7 +67,7 @@ pub async fn spawn_and_register(
         conn,
         incoming_rx,
         parent.as_deref(),
-        model.as_deref(),
+        model_for_registry.as_deref(),
         session_id.as_deref(),
     )
     .await;
diff --git a/crates/loopal-agent-hub/tests/suite/e2e_bootstrap_test.rs b/crates/loopal-agent-hub/tests/suite/e2e_bootstrap_test.rs
index 4788d61..a867c56 100644
--- a/crates/loopal-agent-hub/tests/suite/e2e_bootstrap_test.rs
+++ b/crates/loopal-agent-hub/tests/suite/e2e_bootstrap_test.rs
@@ -50,17 +50,12 @@ async fn full_bootstrap_hub_to_agent_roundtrip() {
 
     let cwd = std::env::temp_dir();
     client
-        .start_agent(
-            &cwd,
-            None, // use default model
-            Some("act"),
-            None, // no initial prompt
-            None,
-            true, // no sandbox
-            None,
-            None, // lifecycle: default
-            None, // agent_type
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: cwd.clone(),
+            mode: Some("act".to_string()),
+            no_sandbox: true,
+            ..Default::default()
+        })
         .await
         .expect("start_agent should work");
 
diff --git a/crates/loopal-agent-server/src/agent_setup.rs b/crates/loopal-agent-server/src/agent_setup.rs
index 1778770..299c3c6 100644
--- a/crates/loopal-agent-server/src/agent_setup.rs
+++ b/crates/loopal-agent-server/src/agent_setup.rs
@@ -81,8 +81,7 @@ pub fn build_with_frontend(
         task_store: Arc::new(TaskStore::new(tasks_dir)),
         hub_connection,
         cwd: cwd.to_path_buf(),
-        depth: 0,
-        max_depth: 3,
+        depth: start.depth.unwrap_or(0),
         agent_name: "main".into(),
         parent_event_tx: Some(event_tx),
         cancel_token: None,
@@ -136,6 +135,7 @@ pub fn build_with_frontend(
         &config.memory,
         start.agent_type.as_deref(),
         features,
+        start.depth.unwrap_or(0),
     );
 
     crate::prompt_post::append_runtime_sections(&mut system_prompt, &kernel);
diff --git a/crates/loopal-agent-server/src/memory_adapter.rs b/crates/loopal-agent-server/src/memory_adapter.rs
index a71e4d4..4067e3f 100644
--- a/crates/loopal-agent-server/src/memory_adapter.rs
+++ b/crates/loopal-agent-server/src/memory_adapter.rs
@@ -50,6 +50,7 @@ impl MemoryProcessor for ServerMemoryProcessor {
             permission_mode: None,
             target_hub: None,
             agent_type: None,
+            depth: self.shared.depth + 1,
         };
         spawn_agent(&self.shared, params).await?;
         info!("memory-maintainer agent spawned via Hub");
diff --git a/crates/loopal-agent-server/src/params.rs b/crates/loopal-agent-server/src/params.rs
index b69ef24..a5c04db 100644
--- a/crates/loopal-agent-server/src/params.rs
+++ b/crates/loopal-agent-server/src/params.rs
@@ -18,6 +18,8 @@ pub struct StartParams {
     pub lifecycle: loopal_runtime::LifecycleMode,
     /// Agent type for fragment selection (e.g. "explore", "plan").
     pub agent_type: Option<String>,
+    /// Nesting depth (0 = root). Propagated from parent via IPC.
+    pub depth: Option<u32>,
 }
 
 /// Build a Kernel from config (production path: MCP, tools).
diff --git a/crates/loopal-agent-server/src/session_start.rs b/crates/loopal-agent-server/src/session_start.rs
index 138e9c6..97b1eeb 100644
--- a/crates/loopal-agent-server/src/session_start.rs
+++ b/crates/loopal-agent-server/src/session_start.rs
@@ -64,6 +64,7 @@ pub(crate) async fn start_session(
         resume: params["resume"].as_str().map(String::from),
         lifecycle,
         agent_type: params["agent_type"].as_str().map(String::from),
+        depth: params["depth"].as_u64().map(|v| v as u32),
     };
 
     let mut config = load_config(&cwd)?;
diff --git a/crates/loopal-agent-server/tests/suite/hub_harness.rs b/crates/loopal-agent-server/tests/suite/hub_harness.rs
index a3c8431..75f0057 100644
--- a/crates/loopal-agent-server/tests/suite/hub_harness.rs
+++ b/crates/loopal-agent-server/tests/suite/hub_harness.rs
@@ -131,6 +131,7 @@ pub async fn build_hub_harness_with(
         resume: None,
         lifecycle: loopal_runtime::LifecycleMode::Persistent,
         agent_type: None,
+        depth: None,
     };
     let (hub_conn, _hub_peer) = loopal_ipc::duplex_pair();
     let hub_connection = std::sync::Arc::new(loopal_ipc::Connection::new(hub_conn));
diff --git a/crates/loopal-agent/src/shared.rs b/crates/loopal-agent/src/shared.rs
index 548e17a..e21200b 100644
--- a/crates/loopal-agent/src/shared.rs
+++ b/crates/loopal-agent/src/shared.rs
@@ -75,10 +75,9 @@ pub struct AgentShared {
     pub hub_connection: Arc<Connection>,
     /// Initial working directory. Immutable after construction.
     pub cwd: PathBuf,
-    /// Current nesting depth (0 = root agent).
+    /// Current nesting depth (0 = root agent). Propagated via IPC so agents
+    /// can sense how deep they are in the delegation chain.
     pub depth: u32,
-    /// Maximum allowed nesting depth.
-    pub max_depth: u32,
     /// Name of the current agent.
     pub agent_name: String,
     /// Event sender for forwarding sub-agent events up the chain.
diff --git a/crates/loopal-agent/src/spawn.rs b/crates/loopal-agent/src/spawn.rs
index 4e840f6..a598c1f 100644
--- a/crates/loopal-agent/src/spawn.rs
+++ b/crates/loopal-agent/src/spawn.rs
@@ -21,6 +21,8 @@ pub struct SpawnParams {
     pub target_hub: Option<String>,
     /// Agent type for fragment selection (e.g. "explore", "plan").
     pub agent_type: Option<String>,
+    /// Nesting depth of the child agent (parent depth + 1).
+    pub depth: u32,
 }
 
 /// Result returned from Hub after spawning.
@@ -48,6 +50,7 @@ pub async fn spawn_agent(
         "prompt": params.prompt,
         "permission_mode": params.permission_mode,
         "agent_type": params.agent_type,
+        "depth": params.depth,
     });
     if let Some(ref hub) = params.target_hub {
         request["target_hub"] = json!(hub);
diff --git a/crates/loopal-agent/src/tools/collaboration/agent.rs b/crates/loopal-agent/src/tools/collaboration/agent.rs
index e1831e2..529d90f 100644
--- a/crates/loopal-agent/src/tools/collaboration/agent.rs
+++ b/crates/loopal-agent/src/tools/collaboration/agent.rs
@@ -93,12 +93,6 @@ async fn action_spawn(
         .get("target_hub")
         .and_then(|v| v.as_str())
         .map(String::from);
-    if shared.depth >= shared.max_depth {
-        return Ok(ToolResult::error(format!(
-            "Maximum nesting depth ({}) reached",
-            shared.max_depth
-        )));
-    }
 
     let mut config = subagent_type
         .and_then(|t| load_agent_configs(&shared.cwd).remove(t))
@@ -131,6 +125,7 @@ async fn action_spawn(
             permission_mode: Some(perm_mode.to_string()),
             target_hub,
             agent_type: subagent_type.map(String::from),
+            depth: shared.depth + 1,
         },
     )
     .await;
diff --git a/crates/loopal-agent/tests/suite/bridge_chain_test.rs b/crates/loopal-agent/tests/suite/bridge_chain_test.rs
index 9e7df19..94f4ef0 100644
--- a/crates/loopal-agent/tests/suite/bridge_chain_test.rs
+++ b/crates/loopal-agent/tests/suite/bridge_chain_test.rs
@@ -40,17 +40,11 @@ async fn full_chain_sub_agent_result_delivered_to_parent() {
     let client = AgentClient::new(client_t);
     client.initialize().await.expect("initialize");
     client
-        .start_agent(
-            fixture.path(),
-            None,
-            None,
-            Some("research this project"),
-            None,
-            false,
-            None,
-            None,
-            None,
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: fixture.path().to_path_buf(),
+            prompt: Some("research this project".to_string()),
+            ..Default::default()
+        })
         .await
         .expect("start_agent");
 
diff --git a/crates/loopal-agent/tests/suite/bridge_child_test.rs b/crates/loopal-agent/tests/suite/bridge_child_test.rs
index 52350af..d5bc626 100644
--- a/crates/loopal-agent/tests/suite/bridge_child_test.rs
+++ b/crates/loopal-agent/tests/suite/bridge_child_test.rs
@@ -57,17 +57,11 @@ pub(crate) async fn start_bridge_client(
     let client = AgentClient::new(client_t);
     client.initialize().await.expect("initialize");
     client
-        .start_agent(
-            fixture.path(),
-            None,
-            None,
-            Some("work"),
-            None,
-            false,
-            None,
-            None,
-            None,
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: fixture.path().to_path_buf(),
+            prompt: Some("work".to_string()),
+            ..Default::default()
+        })
         .await
         .expect("start_agent");
 
@@ -148,17 +142,11 @@ async fn bridge_cancel_sends_shutdown() {
     let client = AgentClient::new(client_t);
     client.initialize().await.unwrap();
     client
-        .start_agent(
-            fixture.path(),
-            None,
-            None,
-            Some("slow task"),
-            None,
-            false,
-            None,
-            None,
-            None,
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: fixture.path().to_path_buf(),
+            prompt: Some("slow task".to_string()),
+            ..Default::default()
+        })
         .await
         .unwrap();
 
diff --git a/crates/loopal-context/src/system_prompt.rs b/crates/loopal-context/src/system_prompt.rs
index b22ec75..e37c7d1 100644
--- a/crates/loopal-context/src/system_prompt.rs
+++ b/crates/loopal-context/src/system_prompt.rs
@@ -15,6 +15,7 @@ pub fn build_system_prompt(
     memory: &str,
     agent_type: Option<&str>,
     features: Vec<String>,
+    agent_depth: u32,
 ) -> String {
     let mut registry = FragmentRegistry::new(system_fragments());
 
@@ -54,6 +55,7 @@ pub fn build_system_prompt(
         features,
         agent_name: None,
         agent_type: agent_type.map(String::from),
+        agent_depth,
     };
 
     builder.build(&ctx)
diff --git a/crates/loopal-context/tests/suite/system_prompt_agent_test.rs b/crates/loopal-context/tests/suite/system_prompt_agent_test.rs
index b39e46d..dda60ae 100644
--- a/crates/loopal-context/tests/suite/system_prompt_agent_test.rs
+++ b/crates/loopal-context/tests/suite/system_prompt_agent_test.rs
@@ -29,6 +29,7 @@ fn explore_subagent_full_prompt() {
         "",
         Some("explore"),
         vec![],
+        0,
     );
 
     // Explore-specific content present
@@ -62,7 +63,7 @@ fn explore_subagent_full_prompt() {
 
 #[test]
 fn root_agent_excludes_agent_fragments() {
-    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![]);
+    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![], 0);
     // No agent fragments in root prompt
     assert!(
         !result.contains("sub-agent named"),
@@ -78,7 +79,7 @@ fn root_agent_excludes_agent_fragments() {
 
 #[test]
 fn plan_subagent_gets_plan_fragment() {
-    let result = build_system_prompt("", &[], "act", "/work", "", "", Some("plan"), vec![]);
+    let result = build_system_prompt("", &[], "act", "/work", "", "", Some("plan"), vec![], 0);
     assert!(
         result.contains("software architect"),
         "plan fragment should be included"
@@ -95,7 +96,7 @@ fn plan_subagent_gets_plan_fragment() {
 
 #[test]
 fn general_subagent_gets_default_fragment() {
-    let result = build_system_prompt("", &[], "act", "/work", "", "", Some("general"), vec![]);
+    let result = build_system_prompt("", &[], "act", "/work", "", "", Some("general"), vec![], 0);
     // Default sub-agent fragment (fallback for unknown types)
     assert!(
         result.contains("sub-agent named"),
diff --git a/crates/loopal-context/tests/suite/system_prompt_test.rs b/crates/loopal-context/tests/suite/system_prompt_test.rs
index ce3c2e1..abe2ab5 100644
--- a/crates/loopal-context/tests/suite/system_prompt_test.rs
+++ b/crates/loopal-context/tests/suite/system_prompt_test.rs
@@ -3,7 +3,17 @@ use loopal_tool_api::ToolDefinition;
 
 #[test]
 fn includes_instructions() {
-    let result = build_system_prompt("You are helpful.", &[], "act", "/tmp", "", "", None, vec![]);
+    let result = build_system_prompt(
+        "You are helpful.",
+        &[],
+        "act",
+        "/tmp",
+        "",
+        "",
+        None,
+        vec![],
+        0,
+    );
     assert!(result.contains("You are helpful."));
 }
 
@@ -14,7 +24,7 @@ fn tool_schemas_not_in_system_prompt() {
         description: "Read a file".into(),
         input_schema: serde_json::json!({"type": "object"}),
     }];
-    let result = build_system_prompt("Base", &tools, "act", "/workspace", "", "", None, vec![]);
+    let result = build_system_prompt("Base", &tools, "act", "/workspace", "", "", None, vec![], 0);
     // Tool schemas should NOT appear in system prompt — they go via ChatParams.tools
     assert!(!result.contains("# Available Tools"));
     assert!(!result.contains("## read"));
@@ -24,7 +34,7 @@ fn tool_schemas_not_in_system_prompt() {
 
 #[test]
 fn includes_fragments() {
-    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![]);
+    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![], 0);
     // Core fragments should be present
     assert!(
         result.contains("Output Efficiency"),
@@ -49,6 +59,7 @@ fn cwd_available_in_subagent_prompt() {
         "",
         Some("general"),
         vec![],
+        0,
     );
     assert!(
         result.contains("/Users/dev/project"),
@@ -59,7 +70,17 @@ fn cwd_available_in_subagent_prompt() {
 #[test]
 fn includes_skills() {
     let skills = "# Available Skills\n- /commit: Generate a git commit message";
-    let result = build_system_prompt("Base", &[], "act", "/workspace", skills, "", None, vec![]);
+    let result = build_system_prompt(
+        "Base",
+        &[],
+        "act",
+        "/workspace",
+        skills,
+        "",
+        None,
+        vec![],
+        0,
+    );
     assert!(result.contains("Available Skills"));
     assert!(result.contains("/commit"));
 }
@@ -75,6 +96,7 @@ fn includes_memory() {
         "## Key Patterns\n- Use DI",
         None,
         vec![],
+        0,
     );
     assert!(result.contains("# Project Memory"));
     assert!(result.contains("Key Patterns"));
@@ -82,7 +104,7 @@ fn includes_memory() {
 
 #[test]
 fn empty_memory_no_section() {
-    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![]);
+    let result = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![], 0);
     assert!(!result.contains("Project Memory"));
 }
 
@@ -94,7 +116,7 @@ fn tool_conditional_fragments() {
         description: "Execute commands".into(),
         input_schema: serde_json::json!({"type": "object"}),
     }];
-    let result = build_system_prompt("Base", &tools, "act", "/workspace", "", "", None, vec![]);
+    let result = build_system_prompt("Base", &tools, "act", "/workspace", "", "", None, vec![], 0);
     assert!(
         result.contains("Bash Tool Guidelines"),
         "bash guidelines missing when Bash tool present"
@@ -102,7 +124,7 @@ fn tool_conditional_fragments() {
 
     // Without Bash tool → no bash guidelines
     let result_no_bash =
-        build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![]);
+        build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![], 0);
     assert!(
         !result_no_bash.contains("Bash Tool Guidelines"),
         "bash guidelines should not appear without Bash"
@@ -121,6 +143,7 @@ fn feature_conditional_fragments() {
         "",
         None,
         vec!["memory".into()],
+        0,
     );
     assert!(
         with_memory.contains("Memory System"),
@@ -128,7 +151,7 @@ fn feature_conditional_fragments() {
     );
 
     // Without "memory" feature → no memory guidance
-    let without = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![]);
+    let without = build_system_prompt("Base", &[], "act", "/workspace", "", "", None, vec![], 0);
     assert!(
         !without.contains("Memory System"),
         "memory guidance should not appear without memory feature"
@@ -144,6 +167,7 @@ fn feature_conditional_fragments() {
         "",
         None,
         vec!["hooks".into()],
+        0,
     );
     assert!(
         with_hooks.contains("hooks"),
@@ -160,6 +184,7 @@ fn feature_conditional_fragments() {
         "",
         None,
         vec!["style_explanatory".into()],
+        0,
     );
     assert!(
         with_style.contains("Explanatory"),
@@ -176,6 +201,7 @@ fn feature_conditional_fragments() {
         "",
         None,
         vec!["subagent".into()],
+        0,
     );
     assert!(
         with_subagent.contains("Sub-Agent Usage"),
@@ -229,11 +255,30 @@ fn report_token_usage() {
     let mem = "## Architecture\n- 17 Rust crates\n- 200-line limit";
     let skills = "# Available Skills\n- /commit: Git commit\n- /review-pr: Review PR";
 
-    let bare = build_system_prompt("", &[], "act", "/project", "", "", None, vec![]);
-    let with_tools = build_system_prompt("", &tools, "act", "/project", "", "", None, vec![]);
-    let full_act = build_system_prompt(instr, &tools, "act", "/project", skills, mem, None, vec![]);
-    let full_plan =
-        build_system_prompt(instr, &tools, "plan", "/project", skills, mem, None, vec![]);
+    let bare = build_system_prompt("", &[], "act", "/project", "", "", None, vec![], 0);
+    let with_tools = build_system_prompt("", &tools, "act", "/project", "", "", None, vec![], 0);
+    let full_act = build_system_prompt(
+        instr,
+        &tools,
+        "act",
+        "/project",
+        skills,
+        mem,
+        None,
+        vec![],
+        0,
+    );
+    let full_plan = build_system_prompt(
+        instr,
+        &tools,
+        "plan",
+        "/project",
+        skills,
+        mem,
+        None,
+        vec![],
+        0,
+    );
 
     let t_bare = estimate_tokens(&bare);
     let t_tools = estimate_tokens(&with_tools);
diff --git a/crates/loopal-meta-hub/tests/e2e/cluster_harness.rs b/crates/loopal-meta-hub/tests/e2e/cluster_harness.rs
index 475b7ab..6b989bf 100644
--- a/crates/loopal-meta-hub/tests/e2e/cluster_harness.rs
+++ b/crates/loopal-meta-hub/tests/e2e/cluster_harness.rs
@@ -106,7 +106,12 @@ impl HubHandle {
         client.initialize().await.expect("initialize");
         let cwd = std::env::temp_dir();
         client
-            .start_agent(&cwd, None, Some("act"), None, None, true, None, None, None)
+            .start_agent(&loopal_agent_client::StartAgentParams {
+                cwd: cwd.clone(),
+                mode: Some("act".to_string()),
+                no_sandbox: true,
+                ..Default::default()
+            })
             .await
             .expect("start_agent");
 
diff --git a/crates/loopal-prompt-system/prompts/tools/agent-guidelines.md b/crates/loopal-prompt-system/prompts/tools/agent-guidelines.md
index 7b6694b..ff5207d 100644
--- a/crates/loopal-prompt-system/prompts/tools/agent-guidelines.md
+++ b/crates/loopal-prompt-system/prompts/tools/agent-guidelines.md
@@ -33,6 +33,16 @@ Scale concurrency to task complexity:
 - **Avoid redundant exploration.** Don't split one search across many explore agents — give one agent a comprehensive, well-scoped prompt instead. Reserve multiple explore agents for genuinely separate areas of the codebase.
 - **Consider the cost.** A complex multi-area refactoring may justify several parallel agents; a focused bug investigation rarely does. Match the parallelism to the real breadth of the work.
 
+## Delegation Depth
+
+Your current depth in the agent tree is **{{ agent_depth }}** (0 = root, 1 = first-level sub-agent, etc.). Every sub-agent you spawn can itself spawn further sub-agents. This makes cascading delegation a real risk — if every agent subdivides its work, agent count grows exponentially and each individual agent ends up doing very little useful work.
+
+**The deeper you are, the more you should prefer doing the work yourself:**
+- At depth 0 (root): spawning sub-agents for broad tasks is natural.
+- At depth 1+: you were created to handle a specific scope. Consider whether your tools (Glob, Grep, Read, Edit, Bash) are sufficient before spawning children.
+- Only delegate further if your assigned scope genuinely contains multiple independent sub-problems that would each benefit from a separate context.
+- Give children **narrowly scoped, concrete tasks** — not vague directives that they would need to subdivide again.
+
 ## Agent Types
 
 - **explore**: READ-ONLY. Fast at finding files, searching code, reading content. Cannot modify anything.
diff --git a/crates/loopal-prompt/src/context.rs b/crates/loopal-prompt/src/context.rs
index c3b049a..b20e78c 100644
--- a/crates/loopal-prompt/src/context.rs
+++ b/crates/loopal-prompt/src/context.rs
@@ -40,6 +40,8 @@ pub struct PromptContext {
     /// Agent type for fragment selection. Some(_) implies this is a sub-agent.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub agent_type: Option<String>,
+    /// Nesting depth (0 = root). Available as `{{ agent_depth }}` in templates.
+    pub agent_depth: u32,
 }
 
 impl PromptContext {
@@ -66,6 +68,7 @@ impl Default for PromptContext {
             features: Vec::new(),
             agent_name: None,
             agent_type: None,
+            agent_depth: 0,
         }
     }
 }
diff --git a/crates/loopal-runtime/tests/agent_loop/llm_test.rs b/crates/loopal-runtime/tests/agent_loop/llm_test.rs
index 9e2b58b..bf6d726 100644
--- a/crates/loopal-runtime/tests/agent_loop/llm_test.rs
+++ b/crates/loopal-runtime/tests/agent_loop/llm_test.rs
@@ -210,6 +210,7 @@ fn report_real_system_prompt_tokens() {
         "",
         None,
         vec![],
+        0,
     );
     runner.params.config.system_prompt = real_prompt.clone();
     let params = runner
@@ -228,6 +229,7 @@ fn report_real_system_prompt_tokens() {
         "",
         None,
         vec![],
+        0,
     );
     let fragment_tokens = loopal_context::estimate_tokens(&prompt_no_tools);
 
diff --git a/crates/loopal-test-support/src/agent_ctx.rs b/crates/loopal-test-support/src/agent_ctx.rs
index dd2de73..c139b9d 100644
--- a/crates/loopal-test-support/src/agent_ctx.rs
+++ b/crates/loopal-test-support/src/agent_ctx.rs
@@ -67,7 +67,6 @@ fn agent_tool_context_inner(
         hub_connection,
         cwd,
         depth: 0,
-        max_depth: 3,
         agent_name: "test".into(),
         parent_event_tx: None,
         cancel_token: None,
diff --git a/crates/loopal-test-support/src/ipc_harness.rs b/crates/loopal-test-support/src/ipc_harness.rs
index d945b2e..e8b5123 100644
--- a/crates/loopal-test-support/src/ipc_harness.rs
+++ b/crates/loopal-test-support/src/ipc_harness.rs
@@ -68,17 +68,11 @@ pub async fn build_ipc_harness(
     let client = loopal_agent_client::AgentClient::new(client_transport);
     client.initialize().await.expect("IPC initialize failed");
     client
-        .start_agent(
-            fixture.path(),
-            None,
-            None,
-            Some("hello"),
-            None,
-            false,
-            None,
-            None,
-            None,
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: fixture.path().to_path_buf(),
+            prompt: Some("hello".to_string()),
+            ..Default::default()
+        })
         .await
         .expect("agent/start failed");
 
diff --git a/crates/loopal-test-support/src/wiring.rs b/crates/loopal-test-support/src/wiring.rs
index 67c18e1..7b08dc1 100644
--- a/crates/loopal-test-support/src/wiring.rs
+++ b/crates/loopal-test-support/src/wiring.rs
@@ -97,7 +97,6 @@ pub(crate) async fn wire(builder: HarnessBuilder) -> (SpawnedHarness, AgentLoopR
         hub_connection,
         cwd,
         depth: 0,
-        max_depth: 3,
         agent_name: "main".to_string(),
         parent_event_tx: Some(event_tx),
         cancel_token: None,
diff --git a/src/bootstrap/hub_bootstrap.rs b/src/bootstrap/hub_bootstrap.rs
index 7a99453..ae167d5 100644
--- a/src/bootstrap/hub_bootstrap.rs
+++ b/src/bootstrap/hub_bootstrap.rs
@@ -60,17 +60,17 @@ pub async fn bootstrap_hub_and_agent(
         None // default: persistent (server decides based on prompt)
     };
     let root_session_id = client
-        .start_agent(
-            cwd,
-            Some(&config.settings.model),
-            Some(mode_str),
-            prompt.as_deref(),
-            cli.permission.as_deref(),
-            cli.no_sandbox,
-            resume,
-            lifecycle_str,
-            None, // root agent has no agent_type
-        )
+        .start_agent(&loopal_agent_client::StartAgentParams {
+            cwd: cwd.to_path_buf(),
+            model: Some(config.settings.model.clone()),
+            mode: Some(mode_str.to_string()),
+            prompt: prompt.clone(),
+            permission_mode: cli.permission.clone(),
+            no_sandbox: cli.no_sandbox,
+            resume: resume.map(String::from),
+            lifecycle: lifecycle_str.map(String::from),
+            ..Default::default()
+        })
         .await?;
 
     let (root_conn, incoming_rx) = client.into_parts();