--- /dev/null
+++ b/cmds/fleet/src/better_nix_eval.rs
@@ -0,0 +1,469 @@
+use std::ffi::{OsStr, OsString};
+use std::process::Stdio;
+use std::sync::{Arc, Mutex, OnceLock};
+
+use abort_on_drop::ChildTask;
+use anyhow::{anyhow, bail, ensure, Context, Result};
+use futures::StreamExt;
+use r2d2::{Pool, PooledConnection};
+use serde::de::DeserializeOwned;
+use serde::Deserialize;
+use tokio::io::AsyncWriteExt;
+use tokio::process::{ChildStdin, ChildStdout, Command};
+use tokio::sync::oneshot;
+use tokio_util::codec::{FramedRead, LinesCodec};
+use tracing::debug;
+
+use crate::command::{process_child_stderr, ErrorRecorder, ErrorRecorderT, NixHandler};
+
+const REPL_DELIMITER: &str = "\"FLEET_MAGIC_REPL_DELIMITER\"";
+// To synchronize stderr and stdout. It works, yet I hate this.
+// There is no other way to catch errors, because they are coming from different streams, and they are not synchronized in tokio.
+const ERROR_DELIMITER: &str = "FLEET_MAGIC_ERROR_DELIMITER";
+
+pub struct NixSessionInner {
+	full_delimiter: String,
+	#[allow(dead_code)]
+	stderr_handler: ChildTask<Result<()>>,
+	error_recorder: ErrorRecorderT,
+	read: FramedRead<ChildStdout, LinesCodec>,
+	stdin: ChildStdin,
+	string_wrapping: (String, String),
+	number_wrapping: (String, String),
+	error_delimiter: String,
+
+	next_id: u32,
+	free_list: Vec<u32>,
+}
+const TRAIN_STRING: &str = "\"TRAIN_STRING\"";
+const TRAIN_NUMBER: &str = "13141516";
+
+struct ErrorRecorderHandle {
+	handle: ErrorRecorderT,
+}
+impl ErrorRecorderHandle {}
+impl Drop for ErrorRecorderHandle {
+	fn drop(&mut self) {
+		let mut recorded = self.handle.lock().unwrap();
+		assert!(recorded.is_some(), "exclusive");
+		*recorded = None;
+	}
+}
+
+struct ErrorCollector {
+	collected: Arc<Mutex<Vec<String>>>,
+	delim: String,
+	got_delim: Option<oneshot::Sender<()>>,
+}
+impl ErrorRecorder for ErrorCollector {
+	fn push_message(&mut self, msg: &str) -> bool {
+		if msg == self.delim {
+			let _ = self.got_delim
+				.take()
+				.expect("error delim is only expected once")
+				.send(());
+			 return true;
+		}
+		let Some(msg) = msg.strip_prefix("@nix ") else {
+			return false;
+		};
+		#[derive(Deserialize)]
+		struct ErrorAction {
+			action: String,
+			level: u32,
+			msg: String,
+		}
+		let Ok(act) = serde_json::from_str::<ErrorAction>(msg) else {
+			return false;
+		};
+		if act.action != "msg" || act.level != 0 {
+			return false;
+		}
+		self.collected.lock().unwrap().push(act.msg);
+		true
+	}
+}
+
+impl NixSessionInner {
+	async fn new(flake: &OsStr, extra_args: impl IntoIterator<Item = &OsStr>) -> Result<Self> {
+		let mut cmd = Command::new("nix");
+		cmd.arg("repl")
+			.arg(flake)
+			.arg("--log-format")
+			.arg("internal-json");
+		for arg in extra_args {
+			cmd.arg(arg);
+		}
+		cmd.stdin(Stdio::piped());
+		cmd.stdout(Stdio::piped());
+		cmd.stderr(Stdio::piped());
+		let cmd = cmd.spawn()?;
+		let stdout = cmd.stdout.unwrap();
+		let stderr = cmd.stderr.unwrap();
+		let mut stdin = cmd.stdin.unwrap();
+		let error_recorder = ErrorRecorderT::default();
+		let err_recorder = error_recorder.clone();
+		let stderr_handler = abort_on_drop::ChildTask::from(tokio::spawn(async move {
+			let mut handler = NixHandler::default();
+			process_child_stderr(stderr, &mut handler, err_recorder).await
+		}));
+		// Standard repl hello doesn't work with internal-json logger
+		stdin.write_all(REPL_DELIMITER.as_bytes()).await?;
+		stdin.write_all(b"\n").await?;
+		stdin.flush().await?;
+		let mut read = FramedRead::new(stdout, LinesCodec::new());
+		let mut full_delimiter = None;
+		while let Some(line) = read.next().await {
+			let line = line?;
+			if line.contains(REPL_DELIMITER) {
+				debug!("discovered repl delimiter with added colors: {line}");
+				full_delimiter = Some(line.to_owned());
+				break;
+			}
+		}
+		let Some(full_delimiter) = full_delimiter else {
+			bail!("failed to discover delimiter");
+		};
+		let mut res = Self {
+			full_delimiter,
+			error_delimiter: "[[filled after training]]".to_owned(),
+			stderr_handler,
+			error_recorder,
+			read,
+			stdin,
+			string_wrapping: Default::default(),
+			number_wrapping: Default::default(),
+
+			next_id: 0,
+			free_list: vec![],
+		};
+		res.train().await?;
+		Ok(res)
+	}
+	async fn train(&mut self) -> Result<()> {
+		{
+			let full_string = self.execute_expression_raw(TRAIN_STRING).await?;
+			let string_offset = full_string.find(TRAIN_STRING).expect("contained");
+			let string_prefix = &full_string[..string_offset];
+			let string_suffix = &full_string[string_offset + TRAIN_STRING.len()..];
+			self.string_wrapping = (string_prefix.to_owned(), string_suffix.to_owned());
+		}
+		{
+			let full_number = self.execute_expression_raw(TRAIN_NUMBER).await?;
+			let number_offset = full_number.find(TRAIN_NUMBER).expect("contained");
+			let number_prefix = &full_number[..number_offset];
+			let number_suffix = &full_number[number_offset + TRAIN_NUMBER.len()..];
+			self.number_wrapping = (number_prefix.to_owned(), number_suffix.to_owned());
+		}
+		{
+			struct TrainingErrorCollector(Option<oneshot::Sender<String>>);
+			impl ErrorRecorder for TrainingErrorCollector {
+				fn push_message(&mut self, msg: &str) -> bool {
+					if msg.contains(ERROR_DELIMITER) {
+						let _ = self
+							.0
+							.take()
+							.expect("error delimiter is sent once")
+							.send(msg.to_owned());
+					}
+					true
+				}
+			}
+			let (tx, rx) = oneshot::channel();
+			let _handle = self.record_error(TrainingErrorCollector(Some(tx)));
+			self.send_command(ERROR_DELIMITER).await?;
+			self.send_command(REPL_DELIMITER).await?;
+			self.read_until_delimiter().await?;
+			let msg = rx.await?;
+			self.error_delimiter = msg;
+		}
+		Ok(())
+	}
+	fn record_error(&mut self, v: impl ErrorRecorder + 'static) -> ErrorRecorderHandle {
+		{
+			let mut recorder = self.error_recorder.lock().unwrap();
+			assert!(recorder.is_none(), "recorder is already started");
+			*recorder = Some(Box::new(v));
+		}
+		ErrorRecorderHandle {
+			handle: self.error_recorder.clone(),
+		}
+	}
+	async fn send_command(&mut self, cmd: impl AsRef<[u8]>) -> Result<()> {
+		self.stdin.write_all(cmd.as_ref()).await?;
+		self.stdin.write_all(b"\n").await?;
+		Ok(())
+	}
+	async fn read_until_delimiter(&mut self) -> Result<String> {
+		let mut out = String::new();
+		while let Some(line) = self.read.next().await {
+			let line = line?;
+			if line == self.full_delimiter {
+				return Ok(out);
+			}
+			if !out.is_empty() {
+				out.push('\n');
+			}
+			out.push_str(&line);
+		}
+		bail!("didn't reached delimiter");
+	}
+	async fn execute_expression_number(&mut self, expr: impl AsRef<[u8]>) -> Result<u64> {
+		let num = self.number_wrapping.clone();
+		let n = self.execute_expression_wrapping(expr, &num).await?;
+		Ok(n.parse::<u64>()?)
+	}
+	async fn execute_expression_string(&mut self, expr: impl AsRef<[u8]>) -> Result<String> {
+		let num = self.string_wrapping.clone();
+		let n = self.execute_expression_wrapping(expr, &num).await?;
+		let str: String = serde_json::from_str(&n)?;
+		Ok(str)
+	}
+	async fn execute_expression_to_json<V: DeserializeOwned>(
+		&mut self,
+		expr: impl AsRef<[u8]>,
+	) -> Result<V> {
+		let mut fexpr = b"builtins.toJSON (".to_vec();
+		fexpr.extend_from_slice(expr.as_ref());
+		fexpr.push(b')');
+		let v = self.execute_expression_string(fexpr).await?;
+		Ok(serde_json::from_str(&v)?)
+	}
+	async fn execute_expression_wrapping(
+		&mut self,
+		expr: impl AsRef<[u8]>,
+		wrapping: &(String, String),
+	) -> Result<String> {
+		let collected = Arc::new(Mutex::new(vec![]));
+		let (etx, erx) = oneshot::channel();
+		let _collector = self.record_error(ErrorCollector{collected:collected.clone(), delim: self.error_delimiter.clone(), got_delim: Some(etx)});
+		let res = self.execute_expression_raw(expr).await?;
+		let _ = self.execute_expression_raw(ERROR_DELIMITER).await?;
+		let _ = erx.await;
+		if res.is_empty() {
+			let c = collected.lock().unwrap();
+			if c.is_empty() {
+				bail!("expected expression, got nothing")
+			}
+			bail!("{}", c.join("\n"));
+		}
+		drop(_collector);
+		let Some(res) = res.strip_prefix(&wrapping.0) else {
+			bail!("invalid type")
+		};
+		let Some(res) = res.strip_suffix(&wrapping.1) else {
+			bail!("invalid type")
+		};
+		Ok(res.to_owned())
+	}
+	async fn execute_expression_empty(&mut self, expr: impl AsRef<[u8]>) -> Result<()> {
+		let collected = Arc::new(Mutex::new(vec![]));
+		let (etx, erx) = oneshot::channel();
+		let _collector = self.record_error(ErrorCollector{collected:collected.clone(), delim: self.error_delimiter.clone(), got_delim: Some(etx)});
+		let v = self.execute_expression_raw(expr).await?;
+		let _ = self.execute_expression_raw(ERROR_DELIMITER).await;
+		let _ = erx.await;
+
+		let c = collected.lock().unwrap();
+		if !c.is_empty() {
+			bail!("{}", c.join("\n"));
+		}
+		ensure!(v.is_empty(), "unexpected expression result");
+		Ok(())
+	}
+	async fn execute_expression_raw(&mut self, expr: impl AsRef<[u8]>) -> Result<String> {
+		self.send_command(expr).await?;
+		// It will be echoed
+		self.send_command(REPL_DELIMITER).await?;
+		self.read_until_delimiter().await
+	}
+	async fn execute_assign(&mut self, expr: impl AsRef<str>) -> Result<u32> {
+		let id = self.allocate_id();
+		self.execute_expression_empty(format!("sess_field_{id} = {}", expr.as_ref()))
+			.await?;
+		Ok(id)
+	}
+
+	/// Id should be immediately used
+	fn allocate_id(&mut self) -> u32 {
+		if let Some(free) = self.free_list.pop() {
+			free
+		} else {
+			let v = self.next_id;
+			self.next_id += 1;
+			v
+		}
+	}
+	// Nix has no way to deallocate variable, yet GC will correct everything not reachable.
+	// async fn free_id(&mut self, id: u32) -> Result<()> {
+	// 	self.execute_expression_empty(format!("sess_field_{id} = null"))
+	// 		.await?;
+	// 	self.free_list.push(id);
+	// 	Ok(())
+	// }
+}
+
+#[derive(Clone)]
+pub struct NixSession(Arc<tokio::sync::Mutex<PooledConnection<NixSessionPoolInner>>>);
+
+#[derive(Clone, Debug)]
+enum Index {
+	String(String),
+	// Idx(u32),
+}
+pub struct Field {
+	full_path: Vec<Index>,
+	session: NixSession,
+	value: Option<u32>,
+}
+impl Field {
+	fn root(session: NixSession) -> Self {
+		Self {
+			full_path: vec![],
+			session,
+			value: None,
+		}
+	}
+	pub async fn field(session: NixSession, field: &str) -> Result<Self> {
+		Self::root(session).get_field_deep([field]).await
+	}
+	pub async fn get_field(&self, name: &str) -> Result<Self> {
+		self.get_field_deep([name]).await
+	}
+	pub async fn get_field_deep<'a>(
+		&self,
+		name: impl IntoIterator<Item = &'a str>,
+	) -> Result<Self> {
+		let mut iter = name.into_iter();
+
+		let mut full_path = self.full_path.clone();
+		let mut query = if let Some(id) = self.value {
+			format!("sess_field_{id}")
+		} else {
+			let first = iter.next().expect("name not empty");
+			ensure!(
+				!(first.contains('.') | first.contains(' ')),
+				"bad name for root query: {first}"
+			);
+			full_path.push(Index::String(first.to_string()));
+			first.to_string()
+		};
+		for v in iter {
+			full_path.push(Index::String(v.to_string()));
+			// Escape
+			let escaped = nixlike::serialize(v)?;
+			let escaped = escaped.trim();
+			query.push('.');
+			query.push_str(escaped);
+		}
+
+		let vid = self
+			.session
+			.0
+			.lock()
+			.await
+			.execute_assign(&query)
+			.await
+			.with_context(|| format!("full path: {:?}", full_path))?;
+		Ok(Self {
+			full_path,
+			session: self.session.clone(),
+			value: Some(vid),
+		})
+	}
+	pub async fn as_json<V: DeserializeOwned>(&self) -> Result<V> {
+		let id = self.value.expect("can't serialize root field");
+		self.session
+			.0
+			.lock()
+			.await
+			.execute_expression_to_json(&format!("sess_field_{id}"))
+			.await
+			.with_context(|| format!("full path: {:?}", self.full_path))
+	}
+	pub async fn list_fields(&self) -> Result<Vec<String>> {
+		let id = self.value.expect("can't list root fields");
+		self.session
+			.0
+			.lock()
+			.await
+			.execute_expression_to_json(&format!("builtins.attrNames sess_field_{id}"))
+			.await
+			.with_context(|| format!("full path: {:?}", self.full_path))
+	}
+}
+impl Drop for Field {
+	fn drop(&mut self) {
+		if let Some(id) = self.value {
+			if let Ok(mut lock) = self.session.0.try_lock() {
+				lock.free_list.push(id)
+			}
+			// Leaked
+		}
+	}
+}
+struct NixSessionPoolInner {
+	flake: OsString,
+	nix_args: Vec<OsString>,
+}
+
+#[derive(Debug)]
+pub struct NixPoolError(anyhow::Error);
+impl From<anyhow::Error> for NixPoolError {
+	fn from(value: anyhow::Error) -> Self {
+		Self(value)
+	}
+}
+impl std::error::Error for NixPoolError {}
+impl std::fmt::Display for NixPoolError {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		self.0.fmt(f)
+	}
+}
+impl r2d2::ManageConnection for NixSessionPoolInner {
+	type Connection = NixSessionInner;
+	type Error = NixPoolError;
+	fn connect(&self) -> std::result::Result<Self::Connection, Self::Error> {
+		let _v = TOKIO_RUNTIME
+			.get()
+			.expect("missed tokio runtime init!")
+			.enter();
+		Ok(futures::executor::block_on(NixSessionInner::new(
+			self.flake.as_os_str(),
+			self.nix_args.iter().map(OsString::as_os_str),
+		))?)
+	}
+
+	fn is_valid(&self, conn: &mut Self::Connection) -> std::result::Result<(), Self::Error> {
+		let _v = TOKIO_RUNTIME
+			.get()
+			.expect("missed tokio runtime init!")
+			.enter();
+		let res = futures::executor::block_on(conn.execute_expression_number("2 + 2"))?;
+		if res != 4 {
+			return Err(anyhow!("sanity check failed").into());
+		};
+		Ok(())
+	}
+
+	fn has_broken(&self, _conn: &mut Self::Connection) -> bool {
+		false
+	}
+}
+pub struct NixSessionPool(Pool<NixSessionPoolInner>);
+impl NixSessionPool {
+	pub async fn new(flake: OsString, nix_args: Vec<OsString>) -> Result<Self> {
+		let inner = tokio::task::block_in_place(|| {
+			r2d2::Builder::<NixSessionPoolInner>::new()
+				.min_idle(Some(0))
+				.build(NixSessionPoolInner { flake, nix_args })
+		})?;
+		Ok(Self(inner))
+	}
+	pub async fn get(&self) -> Result<NixSession> {
+		let v = tokio::task::block_in_place(|| self.0.get())?;
+		Ok(NixSession(Arc::new(tokio::sync::Mutex::new(v))))
+	}
+}
+
+pub static TOKIO_RUNTIME: OnceLock<tokio::runtime::Handle> = OnceLock::new();
--- /dev/null
+++ b/cmds/fleet/src/nix_eval.rs
@@ -0,0 +1,256 @@
+//! Calling nix eval for everything is slow, it is not easy to link nix evaluator itself,
+//! and tvix-nix doesn't have proper flake support. Fleets solution: automating nix repl calls.
+//!
+//! Api is synchronous, yet it is good enough with pooling, and in environment without IFDs for using
+//! those blocking calls from async code.
+
+use std::borrow::Cow;
+use std::sync::{Arc, Mutex};
+use std::time::Instant;
+
+use anyhow::{anyhow, bail, ensure, Context, Result};
+use itertools::Itertools;
+use r2d2::PooledConnection;
+use rexpect::session::{PtyReplSession, PtySession};
+use serde::de::DeserializeOwned;
+use std::ffi::OsString;
+use tracing::info_span;
+
+fn parse_error(res: &str) -> Option<String> {
+	let res = if let Some(v) = res.strip_prefix("error: ") {
+		if let Some((first_line, next)) = v.split_once('\n') {
+			format!("{first_line}\n{}", unindent::unindent(next))
+		} else {
+			v.trim_start().to_owned()
+		}
+	} else if let Some(v) = res.strip_prefix("error:\n") {
+		let mut v = v.to_owned();
+		v.insert(0, '\n');
+		unindent::unindent(&v).trim_start().to_owned()
+	} else {
+		return None;
+	};
+	let res = res.trim_end();
+	Some(
+		res.replace('Â', "")
+			.split('\n')
+			.map(|l| l.strip_prefix("â\u{80}¦ ").unwrap_or(l))
+			.join("\n"),
+	)
+}
+pub struct NixSessionPool {
+	pub flake: OsString,
+	pub nix_args: Vec<OsString>,
+}
+
+#[derive(Debug)]
+pub struct NixPoolError(anyhow::Error);
+impl From<anyhow::Error> for NixPoolError {
+	fn from(value: anyhow::Error) -> Self {
+		Self(value)
+	}
+}
+impl std::error::Error for NixPoolError {}
+impl std::fmt::Display for NixPoolError {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		self.0.fmt(f)
+	}
+}
+
+impl r2d2::ManageConnection for NixSessionPool {
+	type Connection = NixSession;
+	type Error = NixPoolError;
+
+	fn connect(&self) -> std::result::Result<Self::Connection, Self::Error> {
+		Ok(NixSession::new(&self.flake, &self.nix_args, None)?)
+	}
+
+	fn is_valid(&self, conn: &mut Self::Connection) -> std::result::Result<(), Self::Error> {
+		let res = conn.expression_result("2 + 2")?;
+		if res != "4" {
+			return Err(anyhow!("basic expression failed").into());
+		}
+		Ok(())
+	}
+
+	fn has_broken(&self, conn: &mut Self::Connection) -> bool {
+		conn.finished
+	}
+}
+
+pub struct NixSession {
+	session: PtyReplSession,
+	next_id: u32,
+	free_list: Vec<u32>,
+	finished: bool,
+}
+impl NixSession {
+	fn new(flake: &OsString, args: &[OsString], timeout: Option<u64>) -> Result<Self> {
+		let mut cmd = std::process::Command::new("nix");
+		cmd.arg("repl");
+		cmd.arg(flake);
+		for arg in args {
+			cmd.arg(arg);
+		}
+		cmd.env("TERM", "dumb");
+		cmd.env("NO_COLOR", "1");
+		let pty_session = rexpect::session::spawn_command(cmd, timeout)?;
+		let mut repl = PtyReplSession {
+			prompt: "nix-repl> ".to_string(),
+			pty_session,
+			quit_command: Some(":q".to_string()),
+			echo_on: true,
+		};
+		repl.wait_for_prompt()?;
+		Ok(Self {
+			session: repl,
+			next_id: 0,
+			free_list: vec![],
+			finished: false,
+		})
+	}
+	fn expression_result(&mut self, cmd: &str) -> Result<String> {
+		dbg!(cmd);
+		self.session.send_line(cmd)?;
+		dbg!("waiting");
+		let result = self.session.wait_for_prompt()?;
+		let result = strip_ansi_escapes::strip_str(&result);
+		let result = result.trim();
+		dbg!(result);
+		Ok(result.to_owned())
+	}
+	fn json_result<V: DeserializeOwned>(&mut self, cmd: &str) -> Result<V> {
+		let v = match self.expression_result(&format!("builtins.toJSON ({cmd})")) {
+			Ok(v) => {
+				if let Some(e) = parse_error(&v) {
+					bail!("{e}")
+				}
+				v
+			}
+			Err(e) => {
+				self.finished = true;
+				bail!("{e}")
+			}
+		};
+		// Remove outer quoting
+		let v: String = serde_json::from_str(&v)?;
+		Ok(serde_json::from_str(&v)?)
+	}
+	/// Id should be immediately used
+	fn allocate_id(&mut self) -> u32 {
+		if let Some(free) = self.free_list.pop() {
+			free
+		} else {
+			let v = self.next_id;
+			self.next_id += 1;
+			v
+		}
+	}
+	fn allocate_result(&mut self, cmd: &str) -> Result<u32> {
+		let id = self.allocate_id();
+		match self.expression_result(&format!("sess_field_{id} = ({cmd})")) {
+			Ok(v) => {
+				if let Some(e) = parse_error(&v) {
+					self.free_list.push(id);
+					bail!("{e}")
+				}
+			}
+			Err(e) => {
+				self.finished = true;
+			}
+		}
+
+		Ok(id)
+	}
+	/// Nix has no way to deallocate variable, yet GC will correct everything not reachable.
+	fn free_id(&mut self, id: u32) {
+		if let Err(e) = self.expression_result(&format!("sess_field_{id} = null")) {
+			self.finished = true;
+		} else {
+			self.free_list.push(id)
+		}
+	}
+}
+
+#[derive(Clone, Debug)]
+enum Index {
+	String(String),
+	Idx(u32),
+}
+
+pub struct Field {
+	full_path: Vec<Index>,
+	session: Arc<Mutex<PooledConnection<NixSessionPool>>>,
+	value: Option<u32>,
+}
+impl Field {
+	pub fn root(conn: PooledConnection<NixSessionPool>) -> Self {
+		Self {
+			full_path: vec![],
+			session: Arc::new(Mutex::new(conn)),
+			value: None,
+		}
+	}
+	pub fn get_field_deep<'a>(&self, name: impl IntoIterator<Item = &'a str>) -> Result<Self> {
+		let mut iter = name.into_iter();
+
+		let mut full_path = self.full_path.clone();
+		let mut query = if let Some(id) = self.value {
+			format!("sess_field_{id}")
+		} else {
+			let first = iter.next().expect("name not empty");
+			ensure!(
+				!(first.contains('.') | first.contains(' ')),
+				"bad name for root query: {first}"
+			);
+			full_path.push(Index::String(first.to_string()));
+			first.to_string()
+		};
+		for v in iter {
+			full_path.push(Index::String(v.to_string()));
+			// Escape
+			let escaped = nixlike::serialize(v)?;
+			let escaped = escaped.trim();
+			query.push('.');
+			query.push_str(escaped);
+		}
+
+		let vid = self
+			.session
+			.lock()
+			.unwrap()
+			.allocate_result(&query)
+			.with_context(|| format!("full path: {:?}", full_path))?;
+		Ok(Self {
+			full_path,
+			session: self.session.clone(),
+			value: Some(vid),
+		})
+	}
+	pub fn get_field<'a>(&self, name: &str) -> Result<Self> {
+		self.get_field_deep([name])
+	}
+	pub fn as_json<V: DeserializeOwned>(&self) -> Result<V> {
+		let id = self.value.expect("can't serialize root field");
+		self.session
+			.lock()
+			.unwrap()
+			.json_result(&format!("sess_field_{id}"))
+			.with_context(|| format!("full path: {:?}", self.full_path))
+	}
+	pub fn list_fields(&self) -> Result<Vec<String>> {
+		let id = self.value.expect("can't list root fields");
+		self.session
+			.lock()
+			.unwrap()
+			.json_result(&format!("builtins.attrNames sess_field_{id}"))
+			.with_context(|| format!("full path: {:?}", self.full_path))
+	}
+}
+impl Drop for Field {
+	fn drop(&mut self) {
+		if let Some(id) = self.value {
+			self.session.lock().unwrap().free_id(id)
+		}
+	}
+}