diff --git a/src/atlas-loader.js b/src/atlas-loader.js
index ed04885..2d1e8da 100644
--- a/src/atlas-loader.js
+++ b/src/atlas-loader.js
@@ -51,8 +51,8 @@ function loadAtlasDirs(dirs) {
       const spec = loadAtlasSpec(specPath);
       if (registry[spec.atlas_id]) {
         warnings.push(
-          `duplicate atlas_id '${spec.atlas_id}' at ${specPath}; ` +
-          `keeping earlier registration from ${registry[spec.atlas_id].specPath}`
+          `atlas_id '${spec.atlas_id}' is a duplicate; ` +
+          `${specPath} ignored, keeping earlier registration from ${registry[spec.atlas_id].specPath}`
         );
         continue;
       }
diff --git a/src/cli.js b/src/cli.js
index 3ed76cb..f1b037d 100644
--- a/src/cli.js
+++ b/src/cli.js
@@ -32,6 +32,7 @@ function run(argv, { out, err }) {
   const handlers = {
     encode: require('./commands/encode'),
     decode: require('./commands/decode'),
+    build: require('./commands/build'),
   };
   const handler = handlers[head];
   if (!handler) {
diff --git a/src/commands/build.js b/src/commands/build.js
new file mode 100644
index 0000000..ac47bc2
--- /dev/null
+++ b/src/commands/build.js
@@ -0,0 +1,94 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { parseDsl } = require('../dsl-parser');
+const { loadAtlasDirs } = require('../atlas-loader');
+const { buildMap } = require('../builder');
+
+const USAGE = 'usage: sporel-mapper build <spec.txt> <out.json> [--atlas-dir <path> ...]\n';
+
+function parseArgs(args) {
+  const positional = [];
+  const atlasDirs = [];
+  let i = 0;
+  while (i < args.length) {
+    const a = args[i];
+    if (a === '--atlas-dir') {
+      if (i + 1 >= args.length) {
+        throw new Error(`--atlas-dir requires a path`);
+      }
+      atlasDirs.push(args[i + 1]);
+      i += 2;
+    } else {
+      positional.push(a);
+      i += 1;
+    }
+  }
+  return { positional, atlasDirs };
+}
+
+function run(args, { out, err }) {
+  let parsed;
+  try {
+    parsed = parseArgs(args);
+  } catch (e) {
+    err.write(`error: ${e.message}\n${USAGE}`);
+    return 1;
+  }
+  if (parsed.positional.length !== 2) {
+    err.write(USAGE);
+    return 1;
+  }
+  const [specPath, outPath] = parsed.positional;
+
+  let dslText;
+  try {
+    dslText = fs.readFileSync(specPath, 'utf8');
+  } catch (e) {
+    err.write(`error: cannot read ${specPath}: ${e.message}\n`);
+    return 1;
+  }
+
+  let ast;
+  try {
+    ast = parseDsl(dslText);
+  } catch (e) {
+    const line = e.line || '?';
+    err.write(`${specPath}:${line}: ${e.message}\n`);
+    return 1;
+  }
+
+  let registry;
+  try {
+    const result = loadAtlasDirs(parsed.atlasDirs);
+    registry = result.registry;
+    for (const w of result.warnings) {
+      err.write(`warning: ${w}\n`);
+    }
+  } catch (e) {
+    err.write(`error: ${e.message}\n`);
+    return 1;
+  }
+
+  const defaultId = path.basename(outPath, path.extname(outPath));
+  let map;
+  try {
+    map = buildMap(ast, registry, { defaultId });
+  } catch (e) {
+    const line = e.line || '?';
+    err.write(`${specPath}:${line}: ${e.message}\n`);
+    return 1;
+  }
+
+  try {
+    fs.writeFileSync(outPath, JSON.stringify(map, null, 2) + '\n');
+  } catch (e) {
+    err.write(`error: cannot write ${outPath}: ${e.message}\n`);
+    return 1;
+  }
+
+  return 0;
+}
+
+module.exports = { run };
diff --git a/tests/build.test.js b/tests/build.test.js
new file mode 100644
index 0000000..8d07249
--- /dev/null
+++ b/tests/build.test.js
@@ -0,0 +1,98 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { run } = require('../src/cli');
+
+class Sink {
+  constructor() { this.chunks = []; }
+  write(s) { this.chunks.push(s); }
+  get text() { return this.chunks.join(''); }
+}
+
+const FIX_ATLAS = path.join(__dirname, 'fixtures', 'atlases');
+const FIX_DSL = path.join(__dirname, 'fixtures', 'dsl');
+
+function tmpOut(name) {
+  return path.join(os.tmpdir(), `${name}.json`);
+}
+
+test('build: minimal DSL → valid v2 JSON file', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('minimal');
+  try {
+    const code = run(['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+    assert.equal(code, 0, err.text);
+    const json = JSON.parse(fs.readFileSync(outPath, 'utf8'));
+    assert.equal(json.schema_version, 2);
+    assert.equal(json.id, 'minimal');           // derived from out filename
+    assert.deepEqual(json.size, { w: 4, h: 4 });
+    assert.deepEqual(json.atlases, ['atlas_a']);
+    assert.equal(json.layers.surface.tiles.length, 16);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});
+
+test('build: full DSL with id directive overrides filename-derived id', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('full');
+  try {
+    const code = run(['build', path.join(FIX_DSL, 'full.txt'), outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+    assert.equal(code, 0, err.text);
+    const json = JSON.parse(fs.readFileSync(outPath, 'utf8'));
+    assert.equal(json.id, 'full_demo');
+    assert.deepEqual(Object.keys(json.layers).sort(), ['surface', 'wall']);
+    assert.equal(json.roof.length, 16);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});
+
+test('build: out-of-bounds DSL reports file:line on stderr, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('err');
+  const inPath = path.join(FIX_DSL, 'error_oob.txt');
+  const code = run(['build', inPath, outPath, '--atlas-dir', FIX_ATLAS], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error_oob\.txt:4:/);
+  assert.match(err.text, /out of bounds/);
+  // Output file must NOT exist
+  assert.equal(fs.existsSync(outPath), false);
+});
+
+test('build: missing --atlas-dir for DSL that needs atlases → error', () => {
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('noatlas');
+  const code = run(['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', '/no/such/dir'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /error/i);
+  if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+});
+
+test('build: insufficient positional args prints usage, exit 1', () => {
+  const out = new Sink(); const err = new Sink();
+  const code = run(['build', 'only_one_arg.txt'], { out, err });
+  assert.equal(code, 1);
+  assert.match(err.text, /usage: sporel-mapper build/);
+});
+
+test('build: duplicate atlas across --atlas-dir paths warns on stderr but exits 0', () => {
+  // Use both fixture dirs (atlases + atlases-dup)
+  const FIX_DUP = path.join(__dirname, 'fixtures', 'atlases-dup');
+  const out = new Sink(); const err = new Sink();
+  const outPath = tmpOut('dup');
+  try {
+    const code = run(
+      ['build', path.join(FIX_DSL, 'minimal.txt'), outPath, '--atlas-dir', FIX_ATLAS, '--atlas-dir', FIX_DUP],
+      { out, err }
+    );
+    assert.equal(code, 0, err.text);
+    assert.match(err.text, /warning.*atlas_a.*duplicate/i);
+  } finally {
+    if (fs.existsSync(outPath)) fs.unlinkSync(outPath);
+  }
+});