feat: add background_process tool for launching long-running processes

Adds a new tool that allows launching processes (like game servers) in the
background while g3 continues to operate. The process runs independently
with stdout/stderr captured to a log file.

Features:
- Named process tracking for easy reference
- Automatic log capture to logs/background_processes/
- Returns PID and log file path for use with shell tool
- Automatic cleanup on agent shutdown via Drop trait

Usage: Use shell tool to interact with the process:
- Read logs: tail -100 <logfile>
- Check status: ps -p <pid>
- Stop process: kill <pid>

Files:
- New: crates/g3-core/src/background_process.rs
- New: crates/g3-core/tests/background_process_demo_test.rs
- Modified: crates/g3-core/src/lib.rs (tool definition + handler)
- Modified: crates/g3-core/src/prompts.rs (documentation)

crates/g3-core/src/background_process.rs

@@ -0,0 +1,279 @@
+//! Background process management for long-running tasks like game servers.
+//!
+//! This module provides a way to launch processes in the background with:
+//! - Automatic log capture to files (stdout/stderr combined)
+//! - Named process tracking for easy reference
+//! - Process lifecycle management (start, stop via shell)
+//!
+//! The design is intentionally minimal - only one tool (`background_process`) is exposed.
+//! Users can use the regular `shell` tool to:
+//! - Read logs: `cat /path/to/logs.txt` or `tail -100 /path/to/logs.txt`
+//! - Stop processes: `kill <pid>` or `pkill -f <name>`
+//! - Check status: `ps aux | grep <name>`
+
+use std::collections::HashMap;
+use std::fs::{self, OpenOptions};
+use std::io::Write;
+use std::path::PathBuf;
+use std::process::{Child, Command, Stdio};
+use std::sync::{Arc, Mutex};
+use std::time::{SystemTime, UNIX_EPOCH};
+use tracing::debug;
+
+/// Information about a running background process
+#[derive(Debug, Clone)]
+pub struct ProcessInfo {
+    /// User-provided name for the process
+    pub name: String,
+    /// The command that was executed
+    pub command: String,
+    /// Process ID
+    pub pid: u32,
+    /// Path to the log file (combined stdout/stderr)
+    pub log_file: PathBuf,
+    /// Timestamp when the process was started
+    pub started_at: u64,
+    /// Working directory where the process was started
+    pub working_dir: PathBuf,
+}
+
+/// Manages background processes launched by the agent
+#[derive(Debug)]
+pub struct BackgroundProcessManager {
+    /// Map of process name -> process info
+    processes: Arc<Mutex<HashMap<String, ProcessInfo>>>,
+    /// Map of process name -> child handle (for cleanup)
+    children: Arc<Mutex<HashMap<String, Child>>>,
+    /// Directory where log files are stored
+    log_dir: PathBuf,
+}
+
+impl BackgroundProcessManager {
+    /// Create a new background process manager
+    pub fn new(log_dir: PathBuf) -> Self {
+        // Ensure log directory exists
+        if let Err(e) = fs::create_dir_all(&log_dir) {
+            debug!("Failed to create log directory {:?}: {}", log_dir, e);
+        }
+
+        Self {
+            processes: Arc::new(Mutex::new(HashMap::new())),
+            children: Arc::new(Mutex::new(HashMap::new())),
+            log_dir,
+        }
+    }
+
+    /// Start a new background process
+    ///
+    /// # Arguments
+    /// * `name` - A unique name for this process (used to reference it later)
+    /// * `command` - The shell command to execute
+    /// * `working_dir` - The directory to run the command in
+    ///
+    /// # Returns
+    /// ProcessInfo on success, or an error message
+    pub fn start(
+        &self,
+        name: &str,
+        command: &str,
+        working_dir: &PathBuf,
+    ) -> Result<ProcessInfo, String> {
+        // Check if a process with this name already exists
+        {
+            let processes = self.processes.lock().unwrap();
+            if processes.contains_key(name) {
+                return Err(format!(
+                    "A process named '{}' is already running. Stop it first or use a different name.",
+                    name
+                ));
+            }
+        }
+
+        // Create log file with timestamp
+        let timestamp = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .as_secs();
+        let log_filename = format!("{}_{}.log", name, timestamp);
+        let log_file = self.log_dir.join(&log_filename);
+
+        // Open log file for writing
+        let log_handle = OpenOptions::new()
+            .create(true)
+            .write(true)
+            .truncate(true)
+            .open(&log_file)
+            .map_err(|e| format!("Failed to create log file: {}", e))?;
+
+        // Write header to log file
+        {
+            let mut file = &log_handle;
+            writeln!(file, "=== Background Process Log ===").ok();
+            writeln!(file, "Name: {}", name).ok();
+            writeln!(file, "Command: {}", command).ok();
+            writeln!(file, "Working Directory: {:?}", working_dir).ok();
+            writeln!(file, "Started: {}", timestamp).ok();
+            writeln!(file, "================================\n").ok();
+        }
+
+        // Clone the file handle for stderr
+        let log_handle_stderr = log_handle
+            .try_clone()
+            .map_err(|e| format!("Failed to clone log file handle: {}", e))?;
+
+        // Spawn the process
+        let child = Command::new("bash")
+            .arg("-c")
+            .arg(command)
+            .current_dir(working_dir)
+            .stdout(Stdio::from(log_handle))
+            .stderr(Stdio::from(log_handle_stderr))
+            .spawn()
+            .map_err(|e| format!("Failed to spawn process: {}", e))?;
+
+        let pid = child.id();
+
+        let info = ProcessInfo {
+            name: name.to_string(),
+            command: command.to_string(),
+            pid,
+            log_file: log_file.clone(),
+            started_at: timestamp,
+            working_dir: working_dir.clone(),
+        };
+
+        // Store process info and child handle
+        {
+            let mut processes = self.processes.lock().unwrap();
+            processes.insert(name.to_string(), info.clone());
+        }
+        {
+            let mut children = self.children.lock().unwrap();
+            children.insert(name.to_string(), child);
+        }
+
+        debug!(
+            "Started background process '{}' (PID: {}) with logs at {:?}",
+            name, pid, log_file
+        );
+
+        Ok(info)
+    }
+
+    /// List all tracked background processes
+    pub fn list(&self) -> Vec<ProcessInfo> {
+        let processes = self.processes.lock().unwrap();
+        processes.values().cloned().collect()
+    }
+
+    /// Get info about a specific process by name
+    pub fn get(&self, name: &str) -> Option<ProcessInfo> {
+        let processes = self.processes.lock().unwrap();
+        processes.get(name).cloned()
+    }
+
+    /// Check if a process is still running
+    pub fn is_running(&self, name: &str) -> bool {
+        let mut children = self.children.lock().unwrap();
+        if let Some(child) = children.get_mut(name) {
+            match child.try_wait() {
+                Ok(Some(_)) => false, // Process has exited
+                Ok(None) => true,     // Still running
+                Err(_) => false,      // Error checking, assume not running
+            }
+        } else {
+            false
+        }
+    }
+
+    /// Remove a process from tracking (call after it has been killed)
+    pub fn remove(&self, name: &str) -> Option<ProcessInfo> {
+        let info = {
+            let mut processes = self.processes.lock().unwrap();
+            processes.remove(name)
+        };
+        {
+            let mut children = self.children.lock().unwrap();
+            children.remove(name);
+        }
+        info
+    }
+
+    /// Clean up all processes on shutdown
+    pub fn cleanup(&self) {
+        let mut children = self.children.lock().unwrap();
+        for (name, mut child) in children.drain() {
+            debug!("Cleaning up background process '{}'", name);
+            let _ = child.kill();
+        }
+    }
+}
+
+impl Drop for BackgroundProcessManager {
+    fn drop(&mut self) {
+        self.cleanup();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::thread;
+    use std::time::Duration;
+
+    #[test]
+    fn test_start_and_list_process() {
+        let temp_dir = std::env::temp_dir().join("g3_bg_test");
+        let _ = fs::remove_dir_all(&temp_dir);
+        fs::create_dir_all(&temp_dir).unwrap();
+
+        let manager = BackgroundProcessManager::new(temp_dir.clone());
+
+        // Start a simple process that sleeps
+        let result = manager.start("test_sleep", "sleep 10", &temp_dir);
+        assert!(result.is_ok());
+
+        let info = result.unwrap();
+        assert_eq!(info.name, "test_sleep");
+        assert!(info.pid > 0);
+        assert!(info.log_file.exists());
+
+        // List should contain our process
+        let list = manager.list();
+        assert_eq!(list.len(), 1);
+        assert_eq!(list[0].name, "test_sleep");
+
+        // Should be running
+        assert!(manager.is_running("test_sleep"));
+
+        // Cleanup
+        manager.cleanup();
+
+        // Give it a moment to clean up
+        thread::sleep(Duration::from_millis(100));
+
+        let _ = fs::remove_dir_all(&temp_dir);
+    }
+
+    #[test]
+    fn test_duplicate_name_rejected() {
+        let temp_dir = std::env::temp_dir().join("g3_bg_test_dup");
+        let _ = fs::remove_dir_all(&temp_dir);
+        fs::create_dir_all(&temp_dir).unwrap();
+
+        let manager = BackgroundProcessManager::new(temp_dir.clone());
+
+        // Start first process
+        let result1 = manager.start("my_game", "sleep 10", &temp_dir);
+        assert!(result1.is_ok());
+
+        // Try to start another with same name
+        let result2 = manager.start("my_game", "sleep 5", &temp_dir);
+        assert!(result2.is_err());
+        assert!(result2.unwrap_err().contains("already running"));
+
+        // Cleanup
+        manager.cleanup();
+        let _ = fs::remove_dir_all(&temp_dir);
+    }
+}

crates/g3-core/src/lib.rs

@@ -1,3 +1,4 @@
+pub mod background_process;
 pub mod code_search;
 pub mod error_handling;
 pub mod feedback_extraction;
@@ -865,6 +866,7 @@ pub struct Agent<W: UiWriter> {
     requirements_sha: Option<String>,
     /// Working directory for tool execution (set by --codebase-fast-start)
     working_dir: Option<String>,
+    background_process_manager: std::sync::Arc<background_process::BackgroundProcessManager>,
 }
 
 impl<W: UiWriter> Agent<W> {
@@ -1178,6 +1180,10 @@ impl<W: UiWriter> Agent<W> {
             tool_call_count: 0,
             requirements_sha: None,
             working_dir: None,
+            background_process_manager: std::sync::Arc::new(
+                background_process::BackgroundProcessManager::new(
+                    paths::get_logs_dir().join("background_processes")
+                )),
         })
     }
 
@@ -2796,6 +2802,28 @@ impl<W: UiWriter> Agent<W> {
                     "required": ["command"]
                 }),
             },
+            Tool {
+                name: "background_process".to_string(),
+                description: "Launch a long-running process in the background (e.g., game servers, dev servers). The process runs independently and logs are captured to a file. Use the regular 'shell' tool to read logs (cat/tail), check status (ps), or stop the process (kill). Returns the PID and log file path.".to_string(),
+                input_schema: json!({
+                    "type": "object",
+                    "properties": {
+                        "name": {
+                            "type": "string",
+                            "description": "A unique name for this process (e.g., 'game_server', 'my_app'). Used to identify the process and its log file."
+                        },
+                        "command": {
+                            "type": "string",
+                            "description": "The shell command to execute in the background"
+                        },
+                        "working_dir": {
+                            "type": "string",
+                            "description": "Optional working directory. Defaults to current directory if not specified."
+                        }
+                    },
+                    "required": ["name", "command"]
+                }),
+            },
             Tool {
                 name: "read_file".to_string(),
                 description: "Read the contents of a file. For image files (png, jpg, jpeg, gif, bmp, tiff, webp), automatically extracts text using OCR. For text files, optionally read a specific character range.".to_string(),
@@ -4793,6 +4821,50 @@ impl<W: UiWriter> Agent<W> {
                     Ok("❌ Missing command argument".to_string())
                 }
             }
+            "background_process" => {
+                debug!("Processing background_process tool call");
+                let name = tool_call.args.get("name")
+                    .and_then(|v| v.as_str());
+                let name = match name {
+                    Some(n) => n,
+                    None => return Ok("❌ Missing 'name' argument".to_string()),
+                };
+
+                let command = tool_call.args.get("command")
+                    .and_then(|v| v.as_str());
+                let command = match command {
+                    Some(c) => c,
+                    None => return Ok("❌ Missing 'command' argument".to_string()),
+                };
+
+                // Use provided working_dir, or fall back to agent's working_dir, or current dir
+                let work_dir = tool_call.args.get("working_dir")
+                    .and_then(|v| v.as_str())
+                    .map(|s| std::path::PathBuf::from(shellexpand::tilde(s).as_ref()))
+                    .or_else(|| working_dir.map(std::path::PathBuf::from))
+                    .unwrap_or_else(|| std::env::current_dir().unwrap_or_default());
+
+                match self.background_process_manager.start(name, command, &work_dir) {
+                    Ok(info) => {
+                        Ok(format!(
+                            "✅ Background process '{}' started\n\n\
+                            **PID:** {}\n\
+                            **Log file:** {}\n\
+                            **Working dir:** {}\n\n\
+                            To interact with this process, use the shell tool:\n\
+                            - View logs: `tail -100 {}`\n\
+                            - Follow logs: `tail -f {}` (blocks until Ctrl+C)\n\
+                            - Check status: `ps -p {}`\n\
+                            - Stop process: `kill {}`",
+                            info.name, info.pid, 
+                            info.log_file.display(), info.working_dir.display(),
+                            info.log_file.display(), info.log_file.display(),
+                            info.pid, info.pid
+                        ))
+                    }
+                    Err(e) => Ok(format!("❌ Failed to start background process: {}", e)),
+                }
+            }
             "read_file" => {
                 debug!("Processing read_file tool call");
                 if let Some(file_path) = tool_call.args.get("file_path") {

crates/g3-core/src/prompts.rs

@@ -248,6 +248,12 @@ Short description for providers without native calling specs:
   - Format: {\"tool\": \"shell\", \"args\": {\"command\": \"your_command_here\"}
   - Example: {\"tool\": \"shell\", \"args\": {\"command\": \"ls ~/Downloads\"}
 
+- **background_process**: Launch a long-running process in the background (e.g., game servers, dev servers)
+  - Format: {\"tool\": \"background_process\", \"args\": {\"name\": \"unique_name\", \"command\": \"your_command\"}}
+  - Example: {\"tool\": \"background_process\", \"args\": {\"name\": \"game_server\", \"command\": \"./run.sh\"}}
+  - Returns PID and log file path. Use shell tool to read logs (`tail -100 <logfile>`), check status (`ps -p <pid>`), or stop (`kill <pid>`)
+  - Note: Process runs independently; logs are captured to a file for later inspection
+
 - **read_file**: Read the contents of a file (supports partial reads via start/end)
   - Format: {\"tool\": \"read_file\", \"args\": {\"file_path\": \"path/to/file\", \"start\": 0, \"end\": 100}
   - Example: {\"tool\": \"read_file\", \"args\": {\"file_path\": \"src/main.rs\"}