diff --git a/src/main.rs b/src/main.rs
index b9b430a..7bc86f9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -51,10 +51,13 @@ enum Commands {
         #[arg(long)]
         cache_dir: Option<String>,
     },
-    /// Detect agents and add stria MCP server entry
+    /// Detect agents and add stria MCP server entry; optionally add .stria/ to .gitignore
     Setup {
         #[arg(long, default_value = "false")]
         yes: bool,
+        /// Repo root for .gitignore update. Skips .gitignore if not set.
+        #[arg(long)]
+        repo: Option<String>,
     },
     /// Remove stria from agent configurations
     Remove {
@@ -143,7 +146,10 @@ fn main() {
                 eprintln!("Watch error: {}", e);
             }
         }
-        Commands::Setup { yes } => {
+        Commands::Setup { yes, repo } => {
+            if let Some(r) = &repo {
+                setup::add_to_gitignore(r);
+            }
             setup::run_setup(yes);
         }
         Commands::Remove { yes } => {
diff --git a/src/setup.rs b/src/setup.rs
index 9a8bead..91561b7 100644
--- a/src/setup.rs
+++ b/src/setup.rs
@@ -131,6 +131,48 @@ pub fn find_configured_agents() -> Vec<(&'static str, String)> {
     found
 }
 
+/// Add .stria/ to .gitignore for a repo.
+/// Returns true if .gitignore was modified.
+pub fn add_to_gitignore(repo_path: &str) -> bool {
+    let gitignore_path = std::path::Path::new(repo_path).join(".gitignore");
+    let stria_dir = ".stria";
+
+    // Read existing or start fresh
+    let mut content = if gitignore_path.exists() {
+        match std::fs::read_to_string(&gitignore_path) {
+            Ok(c) => c,
+            Err(_) => return false,
+        }
+    } else {
+        String::new()
+    };
+
+    // Check if .stria/ is already listed
+    for line in content.lines() {
+        if line.trim() == stria_dir {
+            return false; // already present
+        }
+    }
+
+    // Append
+    if !content.is_empty() && !content.ends_with('\n') {
+        content.push('\n');
+    }
+    content.push_str(stria_dir);
+    content.push('\n');
+
+    match std::fs::write(&gitignore_path, content) {
+        Ok(_) => {
+            eprintln!("  Added {} to .gitignore", stria_dir);
+            true
+        }
+        Err(e) => {
+            eprintln!("  Warning: could not write .gitignore: {}", e);
+            false
+        }
+    }
+}
+
 pub fn run_setup(yes: bool) {
     let found = detect_agents_without_stria();
     if found.is_empty() {