diff --git a/src/cli.rs b/src/cli.rs
index c740ac3..6f956d9 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -298,8 +298,8 @@ pub enum Command {
 /// Arguments shared by start and restart.
 #[derive(Args, Debug, Clone, Default)]
 pub struct StartArgs {
-    /// Agent name, alias, or group:<name>
-    pub agent: String,
+    /// Agent name, alias, or group:<name> (defaults to the project's default agent)
+    pub agent: Option<String>,
     /// Run detached in the background; output goes to the agent log file
     #[arg(short = 'b', long, conflicts_with = "foreground", action = ArgAction::SetTrue)]
     pub background: bool,
diff --git a/src/commands/run_cmd.rs b/src/commands/run_cmd.rs
index ad6583b..aae8e4e 100644
--- a/src/commands/run_cmd.rs
+++ b/src/commands/run_cmd.rs
@@ -11,10 +11,25 @@ use std::ffi::OsString;
 use std::path::PathBuf;
 use std::process::Command;
 
+/// Resolve the start target: the explicit agent/alias/group when given,
+/// otherwise the default agent of the project containing cwd (issue #30).
+pub fn resolve_start_target(app: &App, cwd: &std::path::Path, agent: Option<&str>) -> Result<String> {
+    if let Some(a) = agent.filter(|s| !s.is_empty()) {
+        return Ok(a.to_string());
+    }
+    crate::projects::default_agent_for_dir(app, cwd).ok_or_else(|| {
+        anyhow!(
+            "no agent given — set default_agent in this project's profile ('am init' does it) or pass an agent name"
+        )
+    })
+}
+
 pub fn start(app: &App, opts: &StartArgs) -> Result<i32> {
+    let cwd = std::env::current_dir().unwrap_or_default();
+    let target = resolve_start_target(app, &cwd, opts.agent.as_deref())?;
     let extra_env = parse_env_list(&opts.env)?;
     let extra_args = parse_extra_args(&opts.args);
-    if let Some(group) = crate::catalog::Catalog::parse_group_selector(&opts.agent) {
+    if let Some(group) = crate::catalog::Catalog::parse_group_selector(&target) {
         let members = app.catalog.group_members(group);
         if members.is_empty() {
             bail!("unknown group '{group}'");
@@ -24,7 +39,7 @@ pub fn start(app: &App, opts: &StartArgs) -> Result<i32> {
         }
         return Ok(0);
     }
-    let agent = require_agent(app, &opts.agent)?;
+    let agent = require_agent(app, &target)?;
     start_one(app, agent, &extra_args, &extra_env, opts.notify, opts.background)
 }
 
@@ -197,7 +212,9 @@ fn stop_one(app: &App, agent: &AgentDef, force: bool, timeout: u64) -> Result<i3
 
 pub fn restart(app: &App, opts: &StartArgs, force: bool, timeout: Option<u64>) -> Result<i32> {
     let timeout = timeout.unwrap_or_else(|| app.config.settings.stop_timeout());
-    if let Some(group) = crate::catalog::Catalog::parse_group_selector(&opts.agent) {
+    let cwd = std::env::current_dir().unwrap_or_default();
+    let target = resolve_start_target(app, &cwd, opts.agent.as_deref())?;
+    if let Some(group) = crate::catalog::Catalog::parse_group_selector(&target) {
         let members = app.catalog.group_members(group);
         if members.is_empty() {
             bail!("unknown group '{group}'");
@@ -212,7 +229,7 @@ pub fn restart(app: &App, opts: &StartArgs, force: bool, timeout: Option<u64>) -
         }
         return Ok(0);
     }
-    let agent = require_agent(app, &opts.agent)?;
+    let agent = require_agent(app, &target)?;
     stop_one(app, agent, force, timeout)?;
     let extra_env = parse_env_list(&opts.env)?;
     let extra_args = parse_extra_args(&opts.args);
diff --git a/src/repl.rs b/src/repl.rs
index 99152a7..ca2f0f1 100644
--- a/src/repl.rs
+++ b/src/repl.rs
@@ -1283,7 +1283,7 @@ fn handle_line(
             agent: need("agent name")?,
         },
         "start" => Command::Start(StartArgs {
-            agent: need("agent name")?,
+            agent: rest.first().cloned(),
             background: flag("--background") || flag("-b"),
             ..Default::default()
         }),
@@ -1294,7 +1294,7 @@ fn handle_line(
         },
         "restart" => Command::Restart {
             start: StartArgs {
-                agent: need("agent name")?,
+                agent: rest.first().cloned(),
                 background: flag("--background") || flag("-b"),
                 ..Default::default()
             },
diff --git a/tests/instrumentation_test.rs b/tests/instrumentation_test.rs
index 9e3add6..74b7b26 100644
--- a/tests/instrumentation_test.rs
+++ b/tests/instrumentation_test.rs
@@ -100,7 +100,7 @@ fn start_and_stop_emit_a_matching_pair() {
     app.state.set(&entry).unwrap();
 
     let opts = StartArgs {
-        agent: "sleeper".to_string(),
+        agent: Some("sleeper".to_string()),
         background: true,
         ..Default::default()
     };
diff --git a/tests/projects_test.rs b/tests/projects_test.rs
index c2d1b8e..12ef7b2 100644
--- a/tests/projects_test.rs
+++ b/tests/projects_test.rs
@@ -75,5 +75,18 @@ projects:
     );
     // Hors du projet : pas de profil.
     assert!(projects::default_agent_for_dir(&app, &std::env::temp_dir()).is_none());
+
+    // am start contextuel (issue #30) : résolution pure sans changer de cwd.
+    use agent_manager::commands::run_cmd;
+    assert_eq!(
+        run_cmd::resolve_start_target(&app, &sub, None).unwrap(),
+        "claude-code"
+    );
+    assert_eq!(
+        run_cmd::resolve_start_target(&app, &sub, Some("aider")).unwrap(),
+        "aider"
+    );
+    assert!(run_cmd::resolve_start_target(&app, &std::env::temp_dir(), None).is_err());
+
     let _ = std::fs::remove_dir_all(&dir);
 }