diff --git a/plugins/yarn/plugin.yaml b/plugins/yarn/plugin.yaml
new file mode 100644
index 00000000..3c103c49
--- /dev/null
+++ b/plugins/yarn/plugin.yaml
@@ -0,0 +1,7 @@
+name: yarn
+version: 1.0.0
+type: python
+main: src/plugin.py
+capabilities:
+  - config_provider
+
diff --git a/plugins/yarn/src/plugin.py b/plugins/yarn/src/plugin.py
new file mode 100644
index 00000000..26a51d48
--- /dev/null
+++ b/plugins/yarn/src/plugin.py
@@ -0,0 +1,339 @@
+import json
+import os
+import shutil
+import sys
+import uuid
+
+def log(msg):
+    sys.stderr.write(f"[yarn-plugin] {msg}\n")
+    sys.stderr.flush()
+
+def _safe_bool(value):
+    return bool(value)
+
+def _yaml_quote_string(value: str) -> str:
+    # Minimal YAML quoting: quote if it contains special chars or starts with certain tokens.
+    s = value
+    if s == "" or any(ch in s for ch in [":", "#", "{", "}", "[", "]", "&", "*", "!", "|", ">", "?", "-", "@", ",", "\n", "\r", "\t"]) or s.startswith(("{", "[", "*", "&", "!", "|", ">", "-", "?", "@")):
+        escaped = s.replace("\\", "\\\\").replace('"', '\\"')
+        return f'"{escaped}"'
+    return s
+
+def _to_yaml_value(value):
+    if isinstance(value, bool):
+        return "true" if value else "false"
+    if isinstance(value, int):
+        return str(value)
+    if isinstance(value, dict):
+        # Emit inline mapping
+        parts = []
+        for k, v in value.items():
+            parts.append(f"{k}: {_to_yaml_value(v)}")
+        return "{" + ", ".join(parts) + "}"
+    return _yaml_quote_string(str(value))
+
+def _yarnrcyml_dump(settings: dict) -> str:
+    # Deterministic ordering for stable writes.
+    lines = []
+    for key in sorted(settings.keys()):
+        val = settings[key]
+        # Yarn Berry supports nested objects
+        if isinstance(val, dict):
+            lines.append(f"{key}:")
+            for arch_key in sorted(val.keys()):
+                lines.append(f"  {arch_key}: {_to_yaml_value(val[arch_key])}")
+        else:
+            lines.append(f"{key}: {_to_yaml_value(val)}")
+    return "\n".join(lines) + "\n"
+
+def _yarnrc_dump_classic(settings: dict) -> str:
+    # Yarn classic .yarnrc uses `key value` per line.
+    def dump_value(v):
+        if isinstance(v, bool):
+            return "true" if v else "false"
+        if isinstance(v, int):
+            return str(v)
+        if isinstance(v, dict):
+            return json.dumps(v, ensure_ascii=False, separators=(",", ":"))
+        return str(v)
+
+    lines = []
+    for key in sorted(settings.keys()):
+        lines.append(f"{key} {dump_value(settings[key])}")
+    return "\n".join(lines) + "\n"
+
+def _parse_existing_kv_lines(lines: str) -> dict:
+    result = {}
+    for raw in lines.splitlines():
+        s = raw.strip()
+        if not s or s.startswith("#") or s.startswith(";"):
+            continue
+        if "=" in s:
+            k, v = s.split("=", 1)
+            result[k.strip()] = v.strip()
+            continue
+        parts = s.split(None, 1)
+        if len(parts) == 2:
+            k, v = parts
+            result[k.strip()] = v.strip()
+    return result
+
+def _get_user_home():
+    return os.path.expanduser("~")
+
+def get_yarnrc_paths():
+    home = _get_user_home()
+    return {
+        "berry": os.path.join(home, ".yarnrc.yml"),
+        "classic": os.path.join(home, ".yarnrc"),
+    }
+
+def log_error(error_msg):
+    log(error_msg)
+
+def _atomic_write(path: str, content: str):
+    dir_path = os.path.dirname(path) or "."
+    os.makedirs(dir_path, exist_ok=True)
+    uid = uuid.uuid4().hex
+
+    backup_path = f"{path}.backup.{uid}"
+    if os.path.exists(path):
+        shutil.copy2(path, backup_path)
+
+    temp_path = f"{path}.tmp.{uid}"
+    try:
+        with open(temp_path, "w", encoding="utf-8", newline="\n") as f:
+            f.write(content)
+        os.replace(temp_path, path)
+    except Exception:
+        try:
+            if os.path.exists(temp_path):
+                os.remove(temp_path)
+        except OSError:
+            pass
+        raise
+
+def check_installed(args: dict) -> bool:
+    paths = get_yarnrc_paths()
+    found_yarn = (
+        shutil.which("yarn.cmd") is not None
+        or shutil.which("yarn.exe") is not None
+        or shutil.which("yarn") is not None
+    )
+
+    cfg_exists = os.path.exists(paths["berry"]) or os.path.exists(paths["classic"])
+    return bool(found_yarn or cfg_exists)
+
+
+YARN_SETTING_KEYS = {
+    "nodeLinker",
+    "packageManager",
+    "npmRegistryServer",
+    "npmAuthToken",
+    "caFilePath",
+    "enableImmutableInstalls",
+    "enableTelemetry",
+    "yarnPath",
+    "supportedArchitectures",
+    "pnpEnable",
+    "pnpFallbackMode",
+    "compressionLevel",
+}
+
+def _response(request_id: str, changed: bool, error: str = None) -> dict:
+    # JSON-RPC response format for this plugin:
+    # - On success: {requestId, changed}
+    # - On failure: {requestId, changed, error}
+    resp = {
+        "requestId": request_id,
+        "changed": changed,
+    }
+    if error is not None:
+        resp["error"] = error
+    return resp
+
+
+def _validate_and_normalize_settings(settings_raw: object):
+    if not isinstance(settings_raw, dict):
+        raise TypeError("settings must be an object")
+
+    settings = {}
+    for k, v in settings_raw.items():
+        if k not in YARN_SETTING_KEYS:
+            continue
+
+        if k in {"enableImmutableInstalls", "enableTelemetry", "pnpEnable"}:
+            settings[k] = _safe_bool(v)
+        elif k == "compressionLevel":
+            settings[k] = int(v)
+        elif k == "supportedArchitectures":
+            if isinstance(v, dict):
+                settings[k] = v
+            else:
+                raise TypeError("supportedArchitectures must be an object")
+        else:
+            settings[k] = str(v)
+
+    return settings
+
+def _apply_berry(berry_path: str, settings: dict, dry_run: bool, request_id: str) -> dict:
+    existing = {}
+    if os.path.exists(berry_path):
+        try:
+            with open(berry_path, "r", encoding="utf-8") as f:
+                for raw in f.read().splitlines():
+                    s = raw.strip()
+                    if not s or s.startswith("#"):
+                        continue
+                    if ":" in s:
+                        k, _ = s.split(":", 1)
+                        existing[k.strip()] = True
+        except Exception as e:
+            log(f"Warning: could not read existing {berry_path}: {e}")
+
+    merged = dict(existing)
+    for k, v in settings.items():
+        merged[k] = v
+
+    content = _yarnrcyml_dump(settings=merged)
+
+    if dry_run:
+        return {
+            "requestId": request_id,
+            "changed": True,
+        }
+
+    _atomic_write(berry_path, content)
+    return {
+        "requestId": request_id,
+        "changed": True,
+    }
+
+
+def _apply_classic(classic_path: str, settings: dict, dry_run: bool, request_id: str) -> dict:
+    existing = {}
+    if os.path.exists(classic_path):
+        try:
+            with open(classic_path, "r", encoding="utf-8") as f:
+                existing = _parse_existing_kv_lines(f.read())
+        except Exception as e:
+            log(f"Warning: could not read existing {classic_path}: {e}")
+
+    merged = dict(existing)
+    for k, v in settings.items():
+        merged[k] = v
+
+    content = _yarnrc_dump_classic(merged)
+
+    if dry_run:
+        return {
+            "requestId": request_id,
+            "changed": True,
+        }
+
+    _atomic_write(classic_path, content)
+    return {
+        "requestId": request_id,
+        "changed": True,
+    }
+
+
+def apply_config(args: dict, context: dict, request_id: str) -> dict:
+    dry_run = bool(context.get("dryRun", False))
+
+    try:
+        settings_raw = args.get("settings", {})
+        if not isinstance(settings_raw, dict):
+            raise TypeError("settings must be an object")
+
+        settings = _validate_and_normalize_settings(settings_raw)
+
+        if not settings:
+            return {
+                "requestId": request_id,
+                "changed": False,
+            }
+
+
+        paths = get_yarnrc_paths()
+        berry_exists = os.path.exists(paths["berry"])
+        classic_exists = os.path.exists(paths["classic"])
+
+        if berry_exists or (not classic_exists):
+            return _apply_berry(paths["berry"], settings, dry_run=dry_run, request_id=request_id)
+
+        return _apply_classic(paths["classic"], settings, dry_run=dry_run, request_id=request_id)
+
+    except Exception as e:
+        log_error(f"Failed to apply config: {e}")
+        return {
+            "requestId": request_id,
+            "changed": False,
+            "error": str(e),
+        }
+
+
+def main():
+    input_data = sys.stdin.read()
+
+    if not input_data:
+        response = {
+            "requestId": "unknown",
+            "changed": False,
+            "error": "No input provided on stdin",
+        }
+
+        sys.stdout.write(json.dumps(response) + "\n")
+        sys.stdout.flush()
+        return
+
+    try:
+        request = json.loads(input_data)
+    except Exception as e:
+        response = {
+            "requestId": "unknown",
+            "changed": False,
+            "error": f"Failed to parse JSON request: {str(e)}",
+        }
+
+        sys.stdout.write(json.dumps(response) + "\n")
+        sys.stdout.flush()
+        return
+
+    request_id = request.get("requestId") or "unknown"
+
+    command = request.get("command")
+    args = request.get("args", {})
+    context = request.get("context", {})
+
+    try:
+        if command == "check_installed":
+            installed = check_installed(args)
+            response = {
+                "requestId": request_id,
+                "installed": installed,
+            }
+
+        elif command == "apply":
+            response = apply_config(args, context, request_id)
+        else:
+            response = {
+                "requestId": request_id,
+                "changed": False,
+                "error": f"Unknown command: {command}",
+            }
+
+    except Exception as e:
+        response = {
+            "requestId": request_id,
+            "changed": False,
+            "error": f"Internal Script Error: {str(e)}",
+        }
+
+
+    sys.stdout.write(json.dumps(response) + "\n")
+    sys.stdout.flush()
+
+if __name__ == "__main__":
+    main()
+
diff --git a/plugins/yarn/test/test_yarn.py b/plugins/yarn/test/test_yarn.py
new file mode 100644
index 00000000..059b7207
--- /dev/null
+++ b/plugins/yarn/test/test_yarn.py
@@ -0,0 +1,121 @@
+import json
+import os
+import sys
+from io import StringIO
+from unittest.mock import patch
+
+test_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "src"))
+sys.path.append(test_dir)
+try:
+    from src.plugin import main
+finally:
+    sys.path.remove(test_dir)
+
+
+def run_plugin(input_dict):
+    input_str = json.dumps(input_dict)
+
+    old_stdin = sys.stdin
+    old_stdout = sys.stdout
+    sys.stdin = StringIO(input_str)
+    sys.stdout = StringIO()
+
+    try:
+        main()
+        output_str = sys.stdout.getvalue()
+        return json.loads(output_str)
+    finally:
+        sys.stdin = old_stdin
+        sys.stdout = old_stdout
+
+
+@patch("src.plugin._get_user_home")
+def test_check_installed_via_config(mock_home, tmp_path):
+    # Create berry config file
+    berry = tmp_path / ".yarnrc.yml"
+    berry.write_text("nodeLinker: node-modules\n", encoding="utf-8")
+    mock_home.return_value = str(tmp_path)
+
+    response = run_plugin({"requestId": "r1", "command": "check_installed"})
+    assert response["installed"] is True
+
+
+
+@patch("src.plugin._get_user_home")
+def test_apply_dry_run_berry(mock_home, tmp_path):
+    mock_home.return_value = str(tmp_path)
+    # No config exists; should attempt to create .yarnrc.yml on apply
+
+    request = {
+        "requestId": "r2",
+        "command": "apply",
+        "args": {
+            "settings": {
+                "nodeLinker": "node-modules",
+                "enableTelemetry": False,
+                "compressionLevel": 0,
+                "supportedArchitectures": {"os": "linux"},
+            }
+        },
+        "context": {"dryRun": True},
+    }
+
+    response = run_plugin(request)
+    assert response["success"] is True
+    assert response["changed"] is True
+
+    assert not (tmp_path / ".yarnrc.yml").exists()
+
+
+@patch("src.plugin._get_user_home")
+def test_apply_writes_berry_file_with_newline(mock_home, tmp_path):
+    mock_home.return_value = str(tmp_path)
+
+    request = {
+        "requestId": "r3",
+        "command": "apply",
+        "args": {
+            "settings": {
+                "nodeLinker": "node-modules",
+                "enableTelemetry": False,
+                "compressionLevel": 7,
+                "supportedArchitectures": {"cpu": "x64"},
+            }
+        },
+        "context": {"dryRun": False},
+    }
+
+    response = run_plugin(request)
+    assert response["changed"] is True
+
+    p = tmp_path / ".yarnrc.yml"
+    assert p.exists()
+    content = p.read_text(encoding="utf-8")
+    assert content.endswith("\n")
+    assert "nodeLinker:" in content
+    assert "enableTelemetry:" in content
+
+
+@patch("src.plugin._get_user_home")
+def test_apply_classic_prefers_classic_if_present(mock_home, tmp_path):
+    mock_home.return_value = str(tmp_path)
+
+    # Create classic file
+    (tmp_path / ".yarnrc").write_text("npmRegistryServer https://example.com\n", encoding="utf-8")
+
+    request = {
+        "requestId": "r4",
+        "command": "apply",
+        "args": {
+            "settings": {
+                "npmRegistryServer": "https://registry.yarnpkg.com",
+            }
+        },
+        "context": {"dryRun": False},
+    }
+
+    response = run_plugin(request)
+    assert response["changed"] is True
+
+    content = (tmp_path / ".yarnrc").read_text(encoding="utf-8")
+    assert "npmRegistryServer https://registry.yarnpkg.com" in content