diff --git a/src/compiler.rs b/src/compiler.rs
new file mode 100644
index 0000000..385b950
--- /dev/null
+++ b/src/compiler.rs
@@ -0,0 +1,95 @@
+use std::{fs, path::PathBuf, process, time::Instant};
+
+use serde::Deserialize;
+
+#[derive(Deserialize)]
+struct Config {
+    entry: String,
+    out_dir: String,
+}
+
+pub fn build() {
+    let start = Instant::now();
+
+    // 1. find project root
+    let root = find_project_root().unwrap_or_else(|| {
+        panic!("error: could not find project root (mist.json)");
+    });
+
+    println!("mistc build ({})", root.display());
+
+    // 2. load config
+    let config = load_config(&root);
+
+    let entry_path = root.join(&config.entry);
+    let out_dir = root.join(&config.out_dir);
+
+    println!("  → entry: {}", entry_path.display());
+
+    // 3. read entry file
+    let source = match fs::read_to_string(&entry_path) {
+        Ok(s) => s,
+        Err(e) => {
+            eprintln!("error: failed to read entry file\n  {}", e);
+            process::exit(1);
+        }
+    };
+
+    println!("  → parsing...");
+
+    let mut ast = match parser::parse(&source) {
+        Ok(ast) => ast,
+        Err(e) => {
+            eprintln!("error: parse failed\n{}", e);
+            process::exit(1);
+        }
+    };
+
+    println!("  → type checking...");
+
+    semantic::walk_ast(semantic::scope::Scope::from_top(&ast), &mut ast);
+
+    println!("  → generating Go code...");
+
+    let mut gc = codegen::GoCodegen::new();
+    let output = gc.generate(&ast);
+
+    // 4. ensure build dir
+    if let Err(e) = fs::create_dir_all(&out_dir) {
+        eprintln!("error: failed to create build dir\n  {}", e);
+        process::exit(1);
+    }
+
+    let out_file = out_dir.join("main.go");
+
+    if let Err(e) = fs::write(&out_file, output) {
+        eprintln!("error: failed to write output\n  {}", e);
+        process::exit(1);
+    }
+
+    let elapsed = start.elapsed();
+
+    println!("  ✓ built {}", out_file.display());
+    println!("build finished in {:.2?}", elapsed);
+}
+
+pub fn find_project_root() -> Option<PathBuf> {
+    let mut dir = std::env::current_dir().ok()?;
+
+    loop {
+        if dir.join("mist.json").exists() {
+            return Some(dir);
+        }
+
+        if !dir.pop() {
+            return None;
+        }
+    }
+}
+
+fn load_config(root: &std::path::Path) -> Config {
+    let content =
+        std::fs::read_to_string(root.join("mist.json")).expect("failed to read mist.json");
+
+    serde_json::from_str(&content).expect("invalid mist.json format")
+}
diff --git a/src/main.rs b/src/main.rs
index a9e6d3a..e2db46a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,5 @@
+pub mod compiler;
+
 use std::fs;
 use std::path::PathBuf;
 use std::process;
@@ -18,7 +20,7 @@ fn main() {
                 eprintln!("error: expected a file path\n  usage: mist build");
                 process::exit(1);
             }
-            cmd_build();
+            compiler::build();
         }
         "check" => {
             if args.len() < 3 {
@@ -48,10 +50,6 @@ fn main() {
     }
 }
 
-pub fn cmd_build() {
-    unimplemented!("build command is not implemented yet");
-}
-
 fn cmd_check(path: &str) {
     let source = read_ms_file(path);
     match parser::parse(&source) {