feat(cli): auto-save LANGSMITH_DEPLOYMENT_NAME to .env after first prompt

When the user is prompted for a deployment name during `langgraph deploy`,
the value is now persisted to the local .env file so subsequent runs
skip the prompt and reuse the saved name.

libs/cli/langgraph_cli/cli.py

@@ -90,27 +90,62 @@ _API_KEY_ENV_NAMES = (
 _DEPLOYMENT_NAME_ENV = "LANGSMITH_DEPLOYMENT_NAME"
 
 
+def _resolve_env_file_path(
+    config_json: dict, config_path: pathlib.Path
+) -> pathlib.Path | None:
+    """Return the .env file path from config, or None if env is an inline dict."""
+    env_field = config_json.get("env")
+    if isinstance(env_field, dict) and env_field:
+        return None
+    if isinstance(env_field, str):
+        return (config_path.parent / env_field).resolve()
+    return pathlib.Path.cwd() / ".env"
+
+
 def _parse_env_from_config(
     config_json: dict, config_path: pathlib.Path
 ) -> dict[str, str]:
     """Resolve env vars from langgraph.json 'env' field or a .env fallback."""
-    env_field = config_json.get("env")
-    # validate_config_file will default env to {}
-    if isinstance(env_field, dict) and env_field:
-        return {str(k): str(v) for k, v in env_field.items()}
-    if isinstance(env_field, str):
-        env_path = (config_path.parent / env_field).resolve()
-        if not env_path.exists():
+    env_path = _resolve_env_file_path(config_json, config_path)
+    if env_path is None:
+        env_field = config_json.get("env")
+        if isinstance(env_field, dict) and env_field:
+            return {str(k): str(v) for k, v in env_field.items()}
+        return {}
+    if not env_path.exists():
+        env_field = config_json.get("env")
+        if isinstance(env_field, str):
             click.secho(
                 f"Warning: env file '{env_field}' specified in langgraph.json not found.",
                 fg="yellow",
             )
-            return {}
-    else:
-        env_path = pathlib.Path.cwd() / ".env"
+        return {}
     return {k: v for k, v in dotenv_values(env_path).items() if v is not None}
 
 
+def _save_to_dotenv(env_path: pathlib.Path, key: str, value: str) -> None:
+    """Append or update a key=value pair in a .env file."""
+    lines: list[str] = []
+    found = False
+    new_line = f"{key}={value}"
+    if env_path.exists():
+        text = env_path.read_text()
+        lines = text.splitlines(keepends=True)
+        for i, line in enumerate(lines):
+            stripped = line.lstrip()
+            if stripped.startswith(f"{key}=") or stripped.startswith(f"{key} ="):
+                lines[i] = new_line + "\n"
+                found = True
+                break
+    if found:
+        env_path.write_text("".join(lines))
+    else:
+        with open(env_path, "a") as f:
+            if lines and not lines[-1].endswith("\n"):
+                f.write("\n")
+            f.write(new_line + "\n")
+
+
 def _secrets_from_env(
     env_vars: dict[str, str],
 ) -> list[dict[str, str]]:
@@ -817,6 +852,13 @@ def _deploy(
     if not deployment_id and not name:
         default_name = _normalize_image_name(pathlib.Path.cwd().name)
         name = click.prompt("Deployment name", default=default_name)
+        env_file = _resolve_env_file_path(config_json, config)
+        if env_file is not None and name:
+            _save_to_dotenv(env_file, _DEPLOYMENT_NAME_ENV, name)
+            click.secho(
+                f"   Saved {_DEPLOYMENT_NAME_ENV}={name} to {env_file}",
+                fg="green",
+            )
 
     secrets = _secrets_from_env(env_vars)
 

libs/cli/tests/unit_tests/test_deploy_helpers.py

@@ -10,6 +10,8 @@ from langgraph_cli.cli import (
     _normalize_image_name,
     _normalize_image_tag,
     _parse_env_from_config,
+    _resolve_env_file_path,
+    _save_to_dotenv,
 )
 
 
@@ -132,3 +134,73 @@ class TestParseEnvFromConfig:
         assert result["GOOD"] == "value"
         # EMPTY= gives empty string, not None, so it should be present
         assert result["EMPTY"] == ""
+
+
+class TestResolveEnvFilePath:
+    def test_inline_dict_returns_none(self, tmp_path):
+        config_path = tmp_path / "langgraph.json"
+        config_path.touch()
+        assert _resolve_env_file_path({"env": {"FOO": "bar"}}, config_path) is None
+
+    def test_string_returns_resolved_path(self, tmp_path):
+        config_path = tmp_path / "langgraph.json"
+        config_path.touch()
+        result = _resolve_env_file_path({"env": "my.env"}, config_path)
+        assert result == (tmp_path / "my.env").resolve()
+
+    def test_no_env_field_returns_cwd_dotenv(self, tmp_path, monkeypatch):
+        monkeypatch.chdir(tmp_path)
+        config_path = tmp_path / "langgraph.json"
+        config_path.touch()
+        result = _resolve_env_file_path({}, config_path)
+        assert result == tmp_path / ".env"
+
+    def test_empty_dict_returns_cwd_dotenv(self, tmp_path, monkeypatch):
+        monkeypatch.chdir(tmp_path)
+        config_path = tmp_path / "langgraph.json"
+        config_path.touch()
+        result = _resolve_env_file_path({"env": {}}, config_path)
+        assert result == tmp_path / ".env"
+
+
+class TestSaveToDotenv:
+    def test_creates_new_file(self, tmp_path):
+        env_file = tmp_path / ".env"
+        _save_to_dotenv(env_file, "MY_KEY", "my_value")
+        assert env_file.read_text() == "MY_KEY=my_value\n"
+
+    def test_appends_to_existing_file(self, tmp_path):
+        env_file = tmp_path / ".env"
+        env_file.write_text("EXISTING=val\n")
+        _save_to_dotenv(env_file, "NEW_KEY", "new_val")
+        assert env_file.read_text() == "EXISTING=val\nNEW_KEY=new_val\n"
+
+    def test_appends_newline_if_missing(self, tmp_path):
+        env_file = tmp_path / ".env"
+        env_file.write_text("EXISTING=val")
+        _save_to_dotenv(env_file, "NEW_KEY", "new_val")
+        assert env_file.read_text() == "EXISTING=val\nNEW_KEY=new_val\n"
+
+    def test_updates_existing_key(self, tmp_path):
+        env_file = tmp_path / ".env"
+        env_file.write_text("KEY1=old\nKEY2=keep\n")
+        _save_to_dotenv(env_file, "KEY1", "new")
+        content = env_file.read_text()
+        assert "KEY1=new\n" in content
+        assert "KEY2=keep\n" in content
+        assert "KEY1=old" not in content
+
+    def test_updates_key_with_spaces(self, tmp_path):
+        env_file = tmp_path / ".env"
+        env_file.write_text("KEY1 = old\nKEY2=keep\n")
+        _save_to_dotenv(env_file, "KEY1", "new")
+        content = env_file.read_text()
+        assert "KEY1=new\n" in content
+        assert "KEY1 = old" not in content
+
+    def test_preserves_other_lines(self, tmp_path):
+        env_file = tmp_path / ".env"
+        env_file.write_text("# comment\nA=1\nB=2\n")
+        _save_to_dotenv(env_file, "C", "3")
+        content = env_file.read_text()
+        assert content == "# comment\nA=1\nB=2\nC=3\n"