smolvm_protocol/

lib.rs

//! Protocol types for smolvm host-guest communication.
//!
//! This crate defines the wire protocol for vsock communication between
//! the smolvm host and the guest agent (smolvm-agent).
//!
//! # Protocol Overview
//!
//! Communication uses JSON-encoded messages over vsock. Each message is
//! prefixed with a 4-byte big-endian length header.
//!
//! ```text
//! +----------------+-------------------+
//! | Length (4 BE)  | JSON payload      |
//! +----------------+-------------------+
//! ```

#![deny(missing_docs)]

use serde::{Deserialize, Serialize};

pub mod forkpoint;
pub mod guest_env;
pub mod image_ref;
pub mod publish_socket;
pub mod retry;
pub mod secrets;

pub use image_ref::{image_repo, normalize_image_ref};
pub use secrets::{SecretRef, SecretSourceKind};

/// Serde helper for encoding `Vec<u8>` as a base64 string in JSON.
///
/// Without this, serde_json serializes `Vec<u8>` as a JSON array of numbers
/// (e.g., `[104,101,108,108,111]`), which inflates binary data by ~4x.
/// Base64 encoding reduces this to ~1.33x.
pub mod base64_bytes {
    use base64::{engine::general_purpose::STANDARD, Engine};
    use serde::{Deserialize, Deserializer, Serializer};

    /// Serialize `Vec<u8>` as a base64 string.
    pub fn serialize<S: Serializer>(data: &[u8], serializer: S) -> Result<S::Ok, S::Error> {
        serializer.serialize_str(&STANDARD.encode(data))
    }

    /// Deserialize a base64 string into `Vec<u8>`.
    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Vec<u8>, D::Error> {
        let s = String::deserialize(deserializer)?;
        STANDARD.decode(&s).map_err(serde::de::Error::custom)
    }
}

/// Protocol version.
pub const PROTOCOL_VERSION: u32 = 1;

/// virtiofs tag under which the host exposes the Rosetta 2 Linux runtime to the
/// guest. Shared host↔guest so the launcher's `krun_add_virtiofs` tag and the
/// guest agent's `mount -t virtiofs` source can't drift apart.
pub const ROSETTA_TAG: &str = "rosetta";

/// Guest mount point for the Rosetta 2 Linux runtime. The ptrace wrapper execs
/// `<ROSETTA_GUEST_PATH>/rosetta` (the translator), so this path is baked into
/// both the wrapper and the `binfmt_misc` registration.
pub const ROSETTA_GUEST_PATH: &str = "/mnt/rosetta";

/// Maximum frame size (32 MB - layer exports use chunked streaming).
pub const MAX_FRAME_SIZE: u32 = 32 * 1024 * 1024;

/// Chunk size for streaming layer data (~16 MB raw, ~21 MB as base64 JSON).
pub const LAYER_CHUNK_SIZE: usize = 16 * 1024 * 1024;

/// Files at or below this size are written with a single `FileWrite`
/// message. Larger files must stream via
/// `FileWriteBegin` + `FileWriteChunk` so no single frame approaches
/// [`MAX_FRAME_SIZE`] (base64 + JSON inflation is ~1.4x).
///
/// Chosen to keep the single-shot frame comfortably under the frame
/// limit while preserving the fast-path latency for small config
/// files / scripts / keys.
pub const FILE_WRITE_SINGLE_SHOT_MAX: usize = 1024 * 1024;

/// Payload bytes per streaming upload chunk. Deliberately small —
/// equal to [`FILE_WRITE_SINGLE_SHOT_MAX`] — so each chunk's encoded
/// frame (~1.4 MB) fits inside typical kernel Unix-socket send
/// buffers (`SO_SNDBUF` defaults on the order of 200–256 KiB but
/// can grow). Larger chunks would force `write_all` to spin waiting
/// for the agent to drain, and any latency spike trips the 10 s
/// write timeout with `EAGAIN` — exactly the failure David
/// reproduced before this fix landed.
///
/// Note: [`LAYER_CHUNK_SIZE`] is 16 MiB for agent→host (download)
/// streaming, which works because the host side of the socket has
/// more headroom than the guest side. Upload streaming is the
/// asymmetric case and needs a smaller chunk.
pub const FILE_WRITE_CHUNK_SIZE: usize = FILE_WRITE_SINGLE_SHOT_MAX;

// The single-shot threshold must be <= the chunk size. They can be equal (a
// 1 MiB file is a single shot; a 1 MiB + 1 byte file streams as two chunks),
// but SINGLE_SHOT > CHUNK would be incoherent — a file slightly over the shot
// threshold would have to stream as a single oversized chunk. Enforced at
// compile time so a bad edit fails the build rather than a test.
const _: () = assert!(FILE_WRITE_SINGLE_SHOT_MAX <= FILE_WRITE_CHUNK_SIZE);

/// Hard ceiling on a single file transfer in either direction.
///
/// On the write path: enforced at `FileWriteBegin` by the agent —
/// `total_size > FILE_TRANSFER_MAX_TOTAL` is rejected before any
/// staging file is created.
///
/// On the read path: enforced by the host's `read_file` loop —
/// after the first chunk that pushes the accumulated total past the
/// cap, the call bails with an error and the partial buffer is
/// dropped. This protects the host process from OOM if the guest
/// (compromised or merely buggy) streams unbounded data.
///
/// 4 GiB matches the order-of-magnitude of the default overlay disk and the
/// `gpu_vram_mib` cap. It is the compiled-in default. The host-side transfer
/// paths (read/export, including `pack create --from-vm`) raise it at runtime
/// via `SMOLVM_FILE_TRANSFER_MAX_BYTES` (see
/// `agent::client::file_transfer_max_total`) so a VM snapshot whose overlay
/// carries a large dependency tree (e.g. a torch + CUDA-wheels environment is
/// ~5 GiB) can be packed without lowering the DoS bound for everyone else. The
/// guest agent's write-path check runs inside the VM and cannot see that host
/// env var, so it always enforces this const. Callers that need to move larger
/// blobs routinely should stage via a virtiofs mount instead of `cp`.
pub const FILE_TRANSFER_MAX_TOTAL: u64 = 4 * 1024 * 1024 * 1024;

/// Filename of the virtiofs-visible marker the agent creates when it is
/// ready to accept vsock connections.
///
/// The host polls for this file through its virtiofs mount of the guest
/// rootfs. The agent writes it (and optionally a symlink from `/oldroot/`)
/// during deferred init, just before opening the vsock listener.
///
/// Both sides must agree on this name; keeping it here prevents silent drift.
pub const AGENT_READY_MARKER: &str = ".smolvm-ready";

/// Well-known vsock ports.
pub mod ports {
    /// Control channel for workload VMs.
    pub const WORKLOAD_CONTROL: u32 = 5000;
    /// Log streaming from workload VMs.
    pub const WORKLOAD_LOGS: u32 = 5001;
    /// Agent control port (for OCI operations and management).
    pub const AGENT_CONTROL: u32 = 6000;
    /// SSH agent forwarding (host SSH_AUTH_SOCK bridged to guest).
    pub const SSH_AGENT: u32 = 6001;
    /// DNS filtering proxy (guest forwards DNS queries to host for filtering).
    pub const DNS_FILTER: u32 = 6002;
    /// Docker socket bridge: the guest listens on this vsock port and proxies
    /// each connection to the in-guest `/var/run/docker.sock`, so the host can
    /// reach the guest's Docker daemon over a host-side Unix socket
    /// (`DOCKER_HOST=unix://…`). Inbound (host connects in), like the agent
    /// control channel — unlike the outbound SSH/DNS/CUDA bridges.
    pub const DOCKER: u32 = 6003;
    /// CUDA-over-vsock (experimental): guest CUDA client forwards Driver-API
    /// calls to a host CUDA server that runs them on the host NVIDIA GPU.
    pub const CUDA: u32 = 7000;

    /// Base vsock port for user-published Unix-socket bridges
    /// (`--expose-socket` / `--mount-socket`). Each published socket is assigned
    /// `PUBLISH_SOCKET_BASE + index`. Kept clear of the fixed ports above (and of
    /// CUDA at 7000) so a reasonable number of sockets never collides.
    pub const PUBLISH_SOCKET_BASE: u32 = 6100;

    /// Maximum number of user-published sockets per VM. Bounds the vsock-port
    /// window (`6100..6100+MAX`) below CUDA's 7000.
    pub const PUBLISH_SOCKET_MAX: usize = 64;
}

/// vsock CID constants.
pub mod cid {
    /// Host CID (always 2).
    pub const HOST: u32 = 2;
    /// Guest CID (always 3 for the first/only guest).
    pub const GUEST: u32 = 3;
    /// Any CID (for listening).
    pub const ANY: u32 = u32::MAX;
}

/// fsnotify event masks, mirroring the kernel's `FS_*` bits in
/// `include/linux/fsnotify_backend.h`. Shared by the host watcher (which maps a
/// host filesystem event to one of these) and the guest agent (which forwards
/// the raw bits to `/proc/smolvm-fsnotify`). Only the subset relevant to
/// file-watching tools is defined.
pub mod fsnotify_mask {
    /// File was modified.
    pub const FS_MODIFY: u32 = 0x0000_0002;
    /// Metadata changed (chmod/chown/utimes).
    pub const FS_ATTRIB: u32 = 0x0000_0004;
    /// Writable file was closed.
    pub const FS_CLOSE_WRITE: u32 = 0x0000_0008;
    /// File was moved away from the watched dir.
    pub const FS_MOVED_FROM: u32 = 0x0000_0040;
    /// File was moved into the watched dir.
    pub const FS_MOVED_TO: u32 = 0x0000_0080;
    /// Subfile was created.
    pub const FS_CREATE: u32 = 0x0000_0100;
    /// Subfile was deleted.
    pub const FS_DELETE: u32 = 0x0000_0200;
    /// Event occurred against a directory.
    pub const FS_ISDIR: u32 = 0x4000_0000;
}

/// A single host-originated filesystem change to replay into the guest.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct FsNotifyEvent {
    /// Guest-side absolute path the event occurred on (virtiofs staging path).
    pub path: String,
    /// `fsnotify_mask::FS_*` bitmask for the event.
    pub mask: u32,
}

// ============================================================================
// Agent Protocol (OCI Operations)
// ============================================================================

/// Agent request types (for image management and OCI operations).
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "method", rename_all = "snake_case")]
pub enum AgentRequest {
    /// Ping to check if agent is alive.
    Ping,

    /// Inject host-originated fsnotify events into the guest.
    ///
    /// virtiofs does not deliver host-side file changes to the guest as
    /// fsnotify/inotify events, so inotify-based hot-reload (Vite, webpack,
    /// nodemon) never fires when a mounted file is edited on the host. The host
    /// watches the mount source and sends the resulting events here; the agent
    /// writes them to `/proc/smolvm-fsnotify`, which fires the matching event on
    /// the guest inode so watchers on the (bind-mounted) container path wake up.
    /// Each `path` is a guest-side absolute path (the virtiofs staging path),
    /// `mask` an `fsnotify_mask::FS_*` bitmask.
    FsNotify {
        /// Host-originated filesystem changes to replay as guest fsnotify events.
        #[serde(default)]
        events: Vec<FsNotifyEvent>,
    },

    /// Pull an OCI image and extract layers.
    Pull {
        /// Image reference (e.g., "alpine:latest", "docker.io/library/ubuntu:22.04").
        image: String,
        /// OCI platform to pull (e.g., "linux/arm64", "linux/amd64").
        oci_platform: Option<String>,
        /// Optional registry authentication credentials.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        auth: Option<RegistryAuth>,
        /// Proxy URL applied to the registry client (sets HTTP_PROXY and HTTPS_PROXY).
        #[serde(default, skip_serializing_if = "Option::is_none")]
        proxy: Option<String>,
        /// Comma-separated NO_PROXY list of hosts/CIDRs that bypass the proxy.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        no_proxy: Option<String>,
    },

    /// Query if an image exists locally.
    Query {
        /// Image reference.
        image: String,
    },

    /// List all cached images.
    ListImages,

    /// Run garbage collection on unused layers.
    GarbageCollect {
        /// If true, only report what would be deleted.
        dry_run: bool,
        /// If true, delete all image manifests and configs first,
        /// making all layers unreferenced so they get collected.
        #[serde(default)]
        purge_all: bool,
    },

    /// Prepare overlay rootfs for a workload.
    PrepareOverlay {
        /// Image reference.
        image: String,
        /// Unique workload ID for the overlay.
        workload_id: String,
    },

    /// Clean up overlay rootfs for a workload.
    CleanupOverlay {
        /// Workload ID to clean up.
        workload_id: String,
    },

    /// Format the storage disk (first-time setup).
    FormatStorage,

    /// Get storage disk status.
    StorageStatus,

    /// Test network connectivity directly from the agent (not via chroot).
    /// Used to debug TSI networking.
    NetworkTest {
        /// URL to test (e.g., "http://1.1.1.1")
        url: String,
    },

    /// Shutdown the agent.
    Shutdown,

    /// Export a layer as a tar archive.
    ///
    /// Used by `smolvm pack` to extract OCI layers for packaging.
    /// The agent streams the layer tar data back via LayerData responses.
    ExportLayer {
        /// Image digest (sha256:...).
        image_digest: String,
        /// Layer index (0-based).
        layer_index: usize,
    },

    /// Execute a command directly in the VM (not in a container).
    ///
    /// This runs the command in the agent's Alpine rootfs without any
    /// container isolation. Useful for VM-level operations and debugging.
    VmExec {
        /// Command and arguments.
        command: Vec<String>,
        /// Environment variables.
        #[serde(default)]
        env: Vec<(String, String)>,
        /// Working directory in the VM.
        workdir: Option<String>,
        /// Timeout in milliseconds.
        #[serde(default)]
        timeout_ms: Option<u64>,
        /// Interactive mode - stream I/O instead of buffering.
        #[serde(default)]
        interactive: bool,
        /// Allocate a pseudo-TTY for the command.
        #[serde(default)]
        tty: bool,
        /// Background mode - spawn and return PID immediately without waiting.
        #[serde(default)]
        background: bool,
        /// Data to pipe to the command's stdin.
        #[serde(default)]
        stdin_data: Option<String>,
    },

    /// Run a command in an image's rootfs.
    ///
    /// This prepares an overlay, chroots into it, and executes the command.
    /// Returns stdout, stderr, and exit code when the command completes.
    Run {
        /// Image reference (must be pulled first).
        image: String,
        /// Command and arguments.
        command: Vec<String>,
        /// Environment variables.
        #[serde(default)]
        env: Vec<(String, String)>,
        /// Working directory inside the rootfs.
        workdir: Option<String>,
        /// User inside the rootfs. If omitted, the OCI image default applies.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        user: Option<String>,
        /// Volume mounts to bind into the container.
        /// Each tuple is (virtiofs_tag, container_path, read_only).
        #[serde(default)]
        mounts: Vec<(String, String, bool)>,
        /// Timeout in milliseconds. If the command exceeds this duration,
        /// it will be killed and return exit code 124.
        #[serde(default)]
        timeout_ms: Option<u64>,
        /// Interactive mode - stream I/O instead of buffering.
        /// When true, output is streamed via Stdout/Stderr responses,
        /// and stdin can be sent via the Stdin request.
        #[serde(default)]
        interactive: bool,
        /// Allocate a pseudo-TTY for the command.
        /// Enables terminal features like colors, line editing, and signal handling.
        #[serde(default)]
        tty: bool,
        /// Detached mode — start the container and return immediately with the
        /// container ID. Only meaningful when `persistent_overlay_id` is set.
        /// Returns a `Completed` response with `stdout` containing the container ID.
        #[serde(default)]
        detached: bool,
        /// Run the workload as an unprivileged container: restricted capabilities,
        /// read-only cgroup, and no extra tmpfs. The default (false) is "VM-grade"
        /// — since the microVM is the isolation boundary, the workload gets a full
        /// capability set and the mounts an init system needs (so any image, incl.
        /// systemd, boots). Opt in for defense-in-depth when running untrusted code.
        #[serde(default)]
        unprivileged: bool,
        /// If set, use a persistent overlay that survives across exec sessions.
        /// The overlay is identified by this ID (typically the machine name)
        /// and reused on subsequent runs. If not set, an ephemeral overlay is
        /// created and destroyed after the run.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        persistent_overlay_id: Option<String>,
        /// Data to pipe to the command's stdin (non-interactive runs only).
        /// The pipe is closed after writing, so the command sees EOF.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        stdin_data: Option<String>,
        /// Spawn the container and return immediately with the crun PID.
        /// The container runs detached; stdout/stderr go to /dev/null.
        /// Incompatible with `interactive` and `tty`.
        #[serde(default)]
        background: bool,
    },

    /// Send stdin data to a running interactive command.
    Stdin {
        /// Input data to send to the command's stdin.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
    },

    /// Resize the PTY window (for TTY mode).
    Resize {
        /// New width in columns.
        cols: u16,
        /// New height in rows.
        rows: u16,
    },

    // ========================================================================
    // File I/O
    // ========================================================================
    /// Write a file inside the VM in a single message.
    ///
    /// Use only for files up to [`FILE_WRITE_SINGLE_SHOT_MAX`]. Larger
    /// files must stream via [`Self::FileWriteBegin`] +
    /// [`Self::FileWriteChunk`] to avoid exceeding [`MAX_FRAME_SIZE`]
    /// after base64 + JSON inflation.
    FileWrite {
        /// Absolute path in the VM filesystem.
        path: String,
        /// File contents.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
        /// File mode (e.g., 0o644). None = default (0644).
        #[serde(default)]
        mode: Option<u32>,
    },

    /// Open a streaming file upload session on this connection.
    ///
    /// Must be followed by one or more [`Self::FileWriteChunk`]
    /// requests. The final chunk sets `done: true` to finalize.
    /// Dropping the connection (or sending any non-chunk request)
    /// before `done` aborts the session and leaves no partial file
    /// at `path`.
    ///
    /// Sessions are per-connection — one session at a time.
    FileWriteBegin {
        /// Absolute path in the VM filesystem.
        path: String,
        /// File mode (e.g., 0o644). None = default (0644).
        #[serde(default)]
        mode: Option<u32>,
        /// Expected total size in bytes. Rejected if it exceeds
        /// [`FILE_TRANSFER_MAX_TOTAL`]. The agent uses this for an
        /// early-fail check only; the actual size written is the sum
        /// of chunk byte lengths.
        total_size: u64,
    },

    /// Append a chunk to the currently open streaming upload.
    /// If `done` is true, the agent fsyncs and atomically renames the
    /// staging file onto the target path.
    FileWriteChunk {
        /// Chunk bytes. Typically [`FILE_WRITE_CHUNK_SIZE`] except
        /// for the last chunk.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
        /// True on the final chunk; closes and renames the staging
        /// file. False on intermediate chunks.
        done: bool,
    },

    /// Read a file from the VM.
    FileRead {
        /// Absolute path in the VM filesystem.
        path: String,
    },

    /// Create (without starting) a Kubernetes pod container whose rootfs is a
    /// virtiofs-shared host directory (containerd snapshotter output) and whose
    /// process definition comes from the host's OCI config. The agent builds
    /// its crun bundle around the shared rootfs; nothing runs until
    /// `PodStart`. Part of the containerd shim v2 datapath
    /// (docs/kubernetes-runtime.md).
    PodCreate {
        /// Container ID (containerd task id).
        id: String,
        /// Rootfs path relative to the sandbox's shared virtiofs mount. The
        /// shim boots the sandbox VM with ONE shared dir and bind-mounts each
        /// container's rootfs under it (virtiofs shares are fixed at boot, but
        /// pod containers are created afterwards), so the guest resolves this
        /// as `<sandbox-share-mount>/<rootfs_rel>`.
        rootfs_rel: String,
        /// The host OCI runtime spec (config.json bytes). The agent extracts
        /// process/env/cwd/user/mounts/resources and grafts them onto its own
        /// guest bundle template; host-specific namespaces/paths are ignored.
        spec_json: String,
        /// Allocate a PTY for the init process.
        #[serde(default)]
        tty: bool,
    },

    /// Start a pod container created by `PodCreate` (or an exec process
    /// registered by `PodExec`), streaming its I/O on THIS connection:
    /// `Started` → `Stdout`/`Stderr`... → `Exited`. Stdin arrives via `Stdin`
    /// requests; PTY resize via `Resize`.
    PodStart {
        /// Container ID.
        id: String,
        /// Exec process to start instead of the init process.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        exec_id: Option<String>,
    },

    /// Register an exec process for a running pod container. Started later by
    /// `PodStart { exec_id }`.
    PodExec {
        /// Container ID.
        id: String,
        /// Exec process ID (unique within the container).
        exec_id: String,
        /// OCI Process JSON (containerd's ExecProcessRequest spec).
        process_json: String,
        /// Allocate a PTY for the exec process.
        #[serde(default)]
        tty: bool,
    },

    /// Signal a pod container's init process (or one exec process).
    PodSignal {
        /// Container ID.
        id: String,
        /// Exec process to signal instead of init.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        exec_id: Option<String>,
        /// Signal number (SIGKILL = 9, SIGTERM = 15, ...).
        signal: u32,
        /// Signal the whole container process group.
        #[serde(default)]
        all: bool,
    },

    /// List PIDs inside a pod container (guest view).
    PodPids {
        /// Container ID.
        id: String,
    },

    /// Sample a pod container's resource usage (guest view). The agent reads the
    /// container's process tree from /proc (there is no per-container cgroup); the
    /// shim maps the reply into containerd's cgroups metrics for CRI stats.
    PodStats {
        /// Container ID.
        id: String,
    },

    /// Remove a pod container's (or exec process's) guest resources after
    /// exit: bundle, cgroup, PTY. Exit status was already streamed by
    /// `PodStart`'s `Exited`.
    PodDelete {
        /// Container ID.
        id: String,
        /// Exec process to remove instead of the whole container.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        exec_id: Option<String>,
    },
}

impl AgentRequest {
    /// A log-safe one-line summary of the request.
    ///
    /// This string is written to the machine's console log, which is exposed
    /// over the logs API — so it must NEVER include credential- or data-bearing
    /// fields: registry `auth`, `env` (which can carry host-resolved secrets),
    /// `proxy` (may embed credentials), or `data` (file/stdin bytes). Only the
    /// variant name plus a non-secret identifier (image) is emitted.
    ///
    /// The match is exhaustive with no catch-all on purpose: adding a new
    /// variant forces a compile error here, so redaction is a deliberate
    /// decision rather than an accidental leak in some future request type.
    pub fn log_summary(&self) -> String {
        match self {
            AgentRequest::Ping => "Ping".into(),
            AgentRequest::FsNotify { events } => format!("FsNotify {{ count: {} }}", events.len()),
            AgentRequest::Pull { image, .. } => format!("Pull {{ image: {image} }}"),
            AgentRequest::Query { image, .. } => format!("Query {{ image: {image} }}"),
            AgentRequest::ListImages => "ListImages".into(),
            AgentRequest::GarbageCollect { .. } => "GarbageCollect".into(),
            AgentRequest::PrepareOverlay { .. } => "PrepareOverlay".into(),
            AgentRequest::CleanupOverlay { .. } => "CleanupOverlay".into(),
            AgentRequest::FormatStorage => "FormatStorage".into(),
            AgentRequest::StorageStatus => "StorageStatus".into(),
            AgentRequest::NetworkTest { .. } => "NetworkTest".into(),
            AgentRequest::Shutdown => "Shutdown".into(),
            AgentRequest::ExportLayer { .. } => "ExportLayer".into(),
            AgentRequest::VmExec { .. } => "VmExec".into(),
            AgentRequest::Run { image, .. } => format!("Run {{ image: {image} }}"),
            AgentRequest::Stdin { .. } => "Stdin".into(),
            AgentRequest::Resize { .. } => "Resize".into(),
            AgentRequest::FileWrite { .. } => "FileWrite".into(),
            AgentRequest::FileWriteBegin { .. } => "FileWriteBegin".into(),
            AgentRequest::FileWriteChunk { .. } => "FileWriteChunk".into(),
            AgentRequest::FileRead { .. } => "FileRead".into(),
            // Pod requests: spec/process JSON may carry env secrets — emit ids only.
            AgentRequest::PodCreate { id, .. } => format!("PodCreate {{ id: {id} }}"),
            AgentRequest::PodStart { id, exec_id } => match exec_id {
                Some(e) => format!("PodStart {{ id: {id}, exec: {e} }}"),
                None => format!("PodStart {{ id: {id} }}"),
            },
            AgentRequest::PodExec { id, exec_id, .. } => {
                format!("PodExec {{ id: {id}, exec: {exec_id} }}")
            }
            AgentRequest::PodSignal {
                id, signal, all, ..
            } => format!("PodSignal {{ id: {id}, signal: {signal}, all: {all} }}"),
            AgentRequest::PodPids { id } => format!("PodPids {{ id: {id} }}"),
            AgentRequest::PodStats { id } => format!("PodStats {{ id: {id} }}"),
            AgentRequest::PodDelete { id, exec_id } => match exec_id {
                Some(e) => format!("PodDelete {{ id: {id}, exec: {e} }}"),
                None => format!("PodDelete {{ id: {id} }}"),
            },
        }
    }
}

/// Agent response types.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "status", rename_all = "snake_case")]
pub enum AgentResponse {
    /// Operation completed successfully.
    Ok {
        /// Response data (varies by request type).
        #[serde(default, skip_serializing_if = "Option::is_none")]
        data: Option<serde_json::Value>,
    },

    /// Pong response to ping.
    Pong {
        /// Protocol version.
        version: u32,
    },

    /// Progress update (for long operations like pull).
    Progress {
        /// Human-readable message.
        message: String,
        /// Completion percentage (0-100).
        #[serde(default, skip_serializing_if = "Option::is_none")]
        percent: Option<u8>,
        /// Current layer being processed.
        #[serde(default, skip_serializing_if = "Option::is_none")]
        layer: Option<String>,
    },

    /// Operation failed.
    Error {
        /// Error message.
        message: String,
        /// Error code (for programmatic handling).
        #[serde(default, skip_serializing_if = "Option::is_none")]
        code: Option<String>,
    },

    /// Command execution completed (non-interactive mode).
    Completed {
        /// Exit code from the command.
        exit_code: i32,
        /// Standard output (may be truncated). `Vec<u8>` preserves binary
        /// output (image bytes, tarballs, etc.) that would be truncated by
        /// `String` at the first non-UTF-8 byte. Serialized as base64 JSON
        /// string — the same format as the streaming `Stdout` variant.
        #[serde(with = "base64_bytes")]
        stdout: Vec<u8>,
        /// Standard error (may be truncated).
        #[serde(with = "base64_bytes")]
        stderr: Vec<u8>,
    },

    /// Command started (interactive mode).
    /// Indicates the command is running and ready to receive stdin.
    Started,

    /// Stdout data from a running command (interactive mode).
    Stdout {
        /// Output data.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
    },

    /// Stderr data from a running command (interactive mode).
    Stderr {
        /// Error output data.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
    },

    /// Command exited (interactive mode).
    Exited {
        /// Exit code from the command.
        exit_code: i32,
        /// The container was terminated by the cgroup OOM killer. The shim
        /// turns this into a TaskOOM event so the CRI reports
        /// `reason=OOMKilled`. Only ever set on a pod container's init exit.
        #[serde(default)]
        oom: bool,
    },

    /// PIDs inside a pod container (`PodPids` reply).
    Pids {
        /// Guest PIDs, container-init first when known.
        pids: Vec<u32>,
    },

    /// Resource usage sample for a pod container (`PodStats` reply). Summed over
    /// the container's process tree read from /proc (no per-container cgroup).
    Stats {
        /// Cumulative CPU time of the process tree, in nanoseconds.
        cpu_usage_ns: u64,
        /// Resident memory of the process tree, in bytes.
        memory_bytes: u64,
    },

    /// Streaming binary-data chunk.
    ///
    /// Used by every streaming download direction: the agent sends
    /// one or more `DataChunk` responses in sequence, with `done: true`
    /// on the final chunk. Current producers: `ExportLayer` and
    /// `FileRead`.
    ///
    /// Payload size per chunk should stay under
    /// [`LAYER_CHUNK_SIZE`] so the encoded frame (~1.33× after
    /// base64) fits inside [`MAX_FRAME_SIZE`] with JSON overhead to
    /// spare.
    DataChunk {
        /// Chunk bytes. Empty allowed on the final frame (common for
        /// EOF-on-clean-boundary cases).
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
        /// True on the final chunk of the stream.
        done: bool,
    },
}

// ============================================================================
// Error Code Constants
// ============================================================================
//
// Standard error codes for AgentResponse::Error. Using constants ensures
// consistency across the codebase and makes error handling more reliable.

/// Error codes for agent responses.
pub mod error_codes {
    /// Request payload was invalid or malformed.
    pub const INVALID_REQUEST: &str = "INVALID_REQUEST";
    /// Requested resource was not found.
    pub const NOT_FOUND: &str = "NOT_FOUND";
    /// Internal error during operation.
    pub const INTERNAL_ERROR: &str = "INTERNAL_ERROR";
    /// Image pull operation failed.
    pub const PULL_FAILED: &str = "PULL_FAILED";
    /// Image query operation failed.
    pub const QUERY_FAILED: &str = "QUERY_FAILED";
    /// Command execution failed.
    pub const RUN_FAILED: &str = "RUN_FAILED";
    /// Command execution failed in container.
    pub const EXEC_FAILED: &str = "EXEC_FAILED";
    /// Process spawn failed.
    pub const SPAWN_FAILED: &str = "SPAWN_FAILED";
    /// Mount operation failed.
    pub const MOUNT_FAILED: &str = "MOUNT_FAILED";
    /// File I/O operation failed.
    pub const FILE_IO_FAILED: &str = "FILE_IO_FAILED";
    /// Overlay filesystem operation failed.
    pub const OVERLAY_FAILED: &str = "OVERLAY_FAILED";
    /// Cleanup operation failed.
    pub const CLEANUP_FAILED: &str = "CLEANUP_FAILED";
    /// Storage format operation failed.
    pub const FORMAT_FAILED: &str = "FORMAT_FAILED";
    /// Storage status query failed.
    pub const STATUS_FAILED: &str = "STATUS_FAILED";
    /// List operation failed.
    pub const LIST_FAILED: &str = "LIST_FAILED";
    /// Garbage collection failed.
    pub const GC_FAILED: &str = "GC_FAILED";
    /// Container creation failed.
    pub const CREATE_FAILED: &str = "CREATE_FAILED";
    /// Container start failed.
    pub const START_FAILED: &str = "START_FAILED";
    /// Container stop failed.
    pub const STOP_FAILED: &str = "STOP_FAILED";
    /// Container delete failed.
    pub const DELETE_FAILED: &str = "DELETE_FAILED";
    /// Export operation failed.
    pub const EXPORT_FAILED: &str = "EXPORT_FAILED";
    /// Serialization error.
    pub const SERIALIZATION_ERROR: &str = "SERIALIZATION_ERROR";
    /// Message size exceeds maximum.
    pub const MESSAGE_TOO_LARGE: &str = "MESSAGE_TOO_LARGE";
    /// Process wait operation failed.
    pub const WAIT_FAILED: &str = "WAIT_FAILED";
}

impl AgentResponse {
    /// Create an error response with the given message and code.
    ///
    /// # Example
    ///
    /// ```
    /// use smolvm_protocol::{AgentResponse, error_codes};
    ///
    /// let response = AgentResponse::error("image not found", error_codes::NOT_FOUND);
    /// ```
    pub fn error(message: impl Into<String>, code: &str) -> Self {
        AgentResponse::Error {
            message: message.into(),
            code: Some(code.to_string()),
        }
    }

    /// Create an error response from a Result's error, with the given code.
    ///
    /// # Example
    ///
    /// ```ignore
    /// let response = some_operation()
    ///     .map(|data| AgentResponse::ok_with_data(data))
    ///     .unwrap_or_else(|e| AgentResponse::from_err(e, error_codes::PULL_FAILED));
    /// ```
    pub fn from_err<E: std::fmt::Display>(err: E, code: &str) -> Self {
        AgentResponse::Error {
            message: err.to_string(),
            code: Some(code.to_string()),
        }
    }

    /// Create an Ok response with optional JSON data.
    pub fn ok(data: Option<serde_json::Value>) -> Self {
        AgentResponse::Ok { data }
    }

    /// Create an Ok response with JSON-serializable data.
    ///
    /// Returns an error response if serialization fails.
    pub fn ok_with_data<T: serde::Serialize>(data: T) -> Self {
        match serde_json::to_value(data) {
            Ok(value) => AgentResponse::Ok { data: Some(value) },
            Err(e) => AgentResponse::error(
                format!("failed to serialize response: {}", e),
                error_codes::SERIALIZATION_ERROR,
            ),
        }
    }

    /// Convert a Result into an AgentResponse.
    ///
    /// On success, serializes the value to JSON. On error, creates an error response.
    ///
    /// # Example
    ///
    /// ```ignore
    /// let response = AgentResponse::from_result(
    ///     storage::pull_image(image),
    ///     error_codes::PULL_FAILED,
    /// );
    /// ```
    pub fn from_result<T, E>(result: Result<T, E>, error_code: &str) -> Self
    where
        T: serde::Serialize,
        E: std::fmt::Display,
    {
        match result {
            Ok(data) => Self::ok_with_data(data),
            Err(e) => Self::from_err(e, error_code),
        }
    }
}

/// Image information returned by Query/ListImages.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ImageInfo {
    /// Image reference.
    pub reference: String,
    /// Image digest (sha256:...).
    pub digest: String,
    /// Image size in bytes.
    pub size: u64,
    /// Creation timestamp (ISO 8601).
    pub created: Option<String>,
    /// Platform architecture.
    pub architecture: String,
    /// Platform OS.
    pub os: String,
    /// Number of layers.
    pub layer_count: usize,
    /// Layer digests in order.
    pub layers: Vec<String>,
    /// Image entrypoint (from OCI config).
    #[serde(default)]
    pub entrypoint: Vec<String>,
    /// Image default command (from OCI config).
    #[serde(default)]
    pub cmd: Vec<String>,
    /// Image environment variables (from OCI config).
    #[serde(default)]
    pub env: Vec<String>,
    /// Image working directory (from OCI config).
    #[serde(default)]
    pub workdir: Option<String>,
    /// Image default user (from OCI config).
    #[serde(default)]
    pub user: Option<String>,
}

/// Overlay preparation result.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct OverlayInfo {
    /// Path to the merged overlay rootfs.
    pub rootfs_path: String,
    /// Path to the upper (writable) directory.
    pub upper_path: String,
    /// Path to the work directory.
    pub work_path: String,
}

/// Storage status information.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct StorageStatus {
    /// Whether the storage is formatted and ready.
    pub ready: bool,
    /// Total size in bytes.
    pub total_bytes: u64,
    /// Used size in bytes.
    pub used_bytes: u64,
    /// Number of cached layers.
    pub layer_count: usize,
    /// Number of cached images.
    pub image_count: usize,
}

/// Registry authentication credentials for pulling images.
///
/// `Debug` is hand-written to redact the password: this value is carried inside
/// `AgentRequest::Pull`, and any `{:?}` of that request (e.g. a tracing span)
/// would otherwise serialize the token verbatim into the machine's console log,
/// which is exposed over the logs API.
#[derive(Clone, Serialize, Deserialize)]
pub struct RegistryAuth {
    /// Username for authentication.
    pub username: String,
    /// Password or token for authentication.
    pub password: String,
}

impl std::fmt::Debug for RegistryAuth {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("RegistryAuth")
            .field("username", &self.username)
            .field("password", &"***")
            .finish()
    }
}

// ============================================================================
// Workload VM Protocol (Command Execution)
// ============================================================================

/// Messages from host to workload VM.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "snake_case")]
pub enum HostMessage {
    /// Authentication request.
    Auth {
        /// Authentication token (base64).
        token: String,
        /// Protocol version.
        protocol_version: u32,
    },

    /// Run a command.
    Run {
        /// Request ID for correlating responses.
        request_id: u64,
        /// Command and arguments.
        command: Vec<String>,
        /// Environment variables.
        env: Vec<(String, String)>,
        /// Working directory.
        workdir: Option<String>,
    },

    /// Execute a command in running VM.
    Exec {
        /// Request ID.
        request_id: u64,
        /// Command and arguments.
        command: Vec<String>,
        /// Allocate a TTY.
        tty: bool,
    },

    /// Send a signal to a running command.
    Signal {
        /// Request ID of the command.
        request_id: u64,
        /// Signal number.
        signal: i32,
    },

    /// Request graceful shutdown.
    Stop {
        /// Timeout in milliseconds.
        timeout_ms: u64,
    },
}

/// Messages from workload VM to host.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "snake_case")]
pub enum GuestMessage {
    /// Authentication successful.
    AuthOk,

    /// Authentication failed.
    AuthFailed,

    /// VM is ready to receive commands.
    Ready,

    /// Command started.
    Started {
        /// Request ID.
        request_id: u64,
    },

    /// Stdout data from command.
    Stdout {
        /// Request ID.
        request_id: u64,
        /// Output data.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
        /// Whether output was truncated.
        truncated: bool,
    },

    /// Stderr data from command.
    Stderr {
        /// Request ID.
        request_id: u64,
        /// Output data.
        #[serde(with = "base64_bytes")]
        data: Vec<u8>,
        /// Whether output was truncated.
        truncated: bool,
    },

    /// Command exited.
    Exit {
        /// Request ID.
        request_id: u64,
        /// Exit code.
        code: i32,
        /// Exit reason.
        reason: String,
    },

    /// Error occurred.
    Error {
        /// Request ID (if applicable).
        request_id: Option<u64>,
        /// Error message.
        message: String,
    },
}

// ============================================================================
// Wire Format Helpers
// ============================================================================

/// Envelope that wraps any message with an optional trace ID for correlation.
///
/// On the wire, the trace_id is flattened into the JSON alongside the message
/// fields: `{"trace_id":"abc123","method":"ping"}`.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Envelope<T> {
    /// Trace ID for correlating host API requests to agent operations.
    #[serde(skip_serializing_if = "Option::is_none", default)]
    pub trace_id: Option<String>,
    /// The wrapped message.
    #[serde(flatten)]
    pub body: T,
}

impl<T> Envelope<T> {
    /// Create an envelope with no trace ID.
    pub fn new(body: T) -> Self {
        Self {
            trace_id: None,
            body,
        }
    }

    /// Create an envelope with an optional trace ID.
    pub fn with_trace_id(body: T, trace_id: Option<String>) -> Self {
        Self { trace_id, body }
    }
}

/// Encode a message to wire format (length-prefixed JSON).
pub fn encode_message<T: Serialize>(msg: &T) -> Result<Vec<u8>, serde_json::Error> {
    let json = serde_json::to_vec(msg)?;
    let len = json.len() as u32;

    let mut buf = Vec::with_capacity(4 + json.len());
    buf.extend_from_slice(&len.to_be_bytes());
    buf.extend_from_slice(&json);

    Ok(buf)
}

/// Decode a message from wire format.
pub fn decode_message<T: for<'de> Deserialize<'de>>(data: &[u8]) -> Result<T, DecodeError> {
    if data.len() < 4 {
        return Err(DecodeError::TooShort);
    }

    let len = u32::from_be_bytes([data[0], data[1], data[2], data[3]]) as usize;

    if len > MAX_FRAME_SIZE as usize {
        return Err(DecodeError::TooLarge(len));
    }

    if data.len() < 4 + len {
        return Err(DecodeError::Incomplete {
            expected: len,
            got: data.len() - 4,
        });
    }

    serde_json::from_slice(&data[4..4 + len]).map_err(DecodeError::Json)
}

/// Error decoding a wire message.
#[derive(Debug)]
pub enum DecodeError {
    /// Data too short to contain length header.
    TooShort,
    /// Frame size exceeds maximum.
    TooLarge(usize),
    /// Incomplete frame.
    Incomplete {
        /// Expected length.
        expected: usize,
        /// Actual length.
        got: usize,
    },
    /// JSON parse error.
    Json(serde_json::Error),
}

impl std::fmt::Display for DecodeError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            DecodeError::TooShort => write!(f, "data too short for length header"),
            DecodeError::TooLarge(size) => write!(f, "frame too large: {} bytes", size),
            DecodeError::Incomplete { expected, got } => {
                write!(
                    f,
                    "incomplete frame: expected {} bytes, got {}",
                    expected, got
                )
            }
            DecodeError::Json(e) => write!(f, "JSON decode error: {}", e),
        }
    }
}

impl std::error::Error for DecodeError {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_encode_decode_roundtrip() {
        let req = AgentRequest::Pull {
            image: "alpine:latest".to_string(),
            oci_platform: Some("linux/arm64".to_string()),
            auth: None,
            proxy: None,
            no_proxy: None,
        };

        let encoded = encode_message(&req).unwrap();
        let decoded: AgentRequest = decode_message(&encoded).unwrap();

        let AgentRequest::Pull {
            image,
            oci_platform,
            auth,
            proxy,
            no_proxy,
        } = decoded
        else {
            panic!("expected Pull variant, got {:?}", decoded);
        };
        assert_eq!(image, "alpine:latest");
        assert_eq!(oci_platform, Some("linux/arm64".to_string()));
        assert!(auth.is_none());
        assert!(proxy.is_none());
        assert!(no_proxy.is_none());
    }

    #[test]
    fn test_encode_decode_with_auth() {
        let req = AgentRequest::Pull {
            image: "ghcr.io/owner/repo:latest".to_string(),
            oci_platform: None,
            auth: Some(RegistryAuth {
                username: "testuser".to_string(),
                password: "testpass".to_string(),
            }),
            proxy: None,
            no_proxy: None,
        };

        let encoded = encode_message(&req).unwrap();
        let decoded: AgentRequest = decode_message(&encoded).unwrap();

        let AgentRequest::Pull {
            image,
            oci_platform,
            auth,
            proxy: _,
            no_proxy: _,
        } = decoded
        else {
            panic!("expected Pull variant, got {:?}", decoded);
        };
        assert_eq!(image, "ghcr.io/owner/repo:latest");
        assert!(oci_platform.is_none());
        let auth = auth.expect("auth should be Some");
        assert_eq!(auth.username, "testuser");
        assert_eq!(auth.password, "testpass");
    }

    #[test]
    fn test_encode_decode_with_proxy() {
        let req = AgentRequest::Pull {
            image: "alpine:latest".to_string(),
            oci_platform: None,
            auth: None,
            proxy: Some("http://192.168.127.254:3128".to_string()),
            no_proxy: Some("127.0.0.1,localhost,.internal".to_string()),
        };

        let encoded = encode_message(&req).unwrap();
        let decoded: AgentRequest = decode_message(&encoded).unwrap();

        let AgentRequest::Pull {
            proxy, no_proxy, ..
        } = decoded
        else {
            panic!("expected Pull variant, got {:?}", decoded);
        };
        assert_eq!(proxy.as_deref(), Some("http://192.168.127.254:3128"));
        assert_eq!(no_proxy.as_deref(), Some("127.0.0.1,localhost,.internal"));
    }

    #[test]
    fn test_decode_too_short() {
        let data = [0u8; 2];
        let result: Result<AgentRequest, _> = decode_message(&data);
        assert!(matches!(result, Err(DecodeError::TooShort)));
    }

    #[test]
    fn test_decode_incomplete() {
        let mut data = vec![0, 0, 0, 100]; // claims 100 bytes
        data.extend_from_slice(b"{}"); // only 2 bytes of payload
        let result: Result<AgentRequest, _> = decode_message(&data);
        assert!(matches!(result, Err(DecodeError::Incomplete { .. })));
    }

    #[test]
    fn test_agent_request_serialization() {
        let req = AgentRequest::Ping;
        let json = serde_json::to_string(&req).unwrap();
        assert!(json.contains("ping"));

        let req = AgentRequest::PrepareOverlay {
            image: "ubuntu:22.04".to_string(),
            workload_id: "wl-123".to_string(),
        };
        let json = serde_json::to_string(&req).unwrap();
        assert!(json.contains("prepare_overlay"));
    }

    #[test]
    fn test_agent_response_serialization() {
        let resp = AgentResponse::Pong {
            version: PROTOCOL_VERSION,
        };
        let json = serde_json::to_string(&resp).unwrap();
        assert!(json.contains("pong"));

        let resp = AgentResponse::Progress {
            message: "Pulling layer 1/3".to_string(),
            percent: Some(33),
            layer: Some("sha256:abc123".to_string()),
        };
        let json = serde_json::to_string(&resp).unwrap();
        assert!(json.contains("progress"));
    }

    #[test]
    fn file_write_begin_roundtrips() {
        let req = AgentRequest::FileWriteBegin {
            path: "/tmp/target".into(),
            mode: Some(0o600),
            total_size: 123_456_789,
        };
        let bytes = encode_message(&req).unwrap();
        let back: AgentRequest = decode_message(&bytes).unwrap();
        match back {
            AgentRequest::FileWriteBegin {
                path,
                mode,
                total_size,
            } => {
                assert_eq!(path, "/tmp/target");
                assert_eq!(mode, Some(0o600));
                assert_eq!(total_size, 123_456_789);
            }
            _ => panic!("wrong variant"),
        }
    }

    #[test]
    fn file_write_chunk_roundtrips_binary_data() {
        // Binary data (bytes outside UTF-8) must survive the base64
        // trip intact. If the encoding ever silently lossifies, this
        // fires.
        let payload: Vec<u8> = (0u8..=255).collect();
        let req = AgentRequest::FileWriteChunk {
            data: payload.clone(),
            done: true,
        };
        let bytes = encode_message(&req).unwrap();
        let back: AgentRequest = decode_message(&bytes).unwrap();
        match back {
            AgentRequest::FileWriteChunk { data, done } => {
                assert_eq!(data, payload);
                assert!(done);
            }
            _ => panic!("wrong variant"),
        }
    }

    #[test]
    fn file_write_size_constants_are_frame_safe() {
        // Sanity: a single streaming chunk at FILE_WRITE_CHUNK_SIZE
        // must fit inside MAX_FRAME_SIZE after base64 (+ ~33%) and
        // JSON overhead. If anyone bumps CHUNK_SIZE past the limit,
        // this test fires before production does.
        let chunk_bytes = FILE_WRITE_CHUNK_SIZE as u64;
        let base64_bytes = chunk_bytes.div_ceil(3) * 4; // ceil(n/3)*4
        let json_overhead = 256u64; // method tag, done bool, quotes
        let total = base64_bytes + json_overhead;
        assert!(
            total < MAX_FRAME_SIZE as u64,
            "FILE_WRITE_CHUNK_SIZE of {} bytes would produce a frame \
             of ~{} bytes which exceeds MAX_FRAME_SIZE of {}",
            chunk_bytes,
            total,
            MAX_FRAME_SIZE
        );
    }

    #[test]
    fn test_ports_constants() {
        assert_eq!(ports::WORKLOAD_CONTROL, 5000);
        assert_eq!(ports::WORKLOAD_LOGS, 5001);
        assert_eq!(ports::AGENT_CONTROL, 6000);
        assert_eq!(ports::SSH_AGENT, 6001);
    }

    #[test]
    fn test_cid_constants() {
        assert_eq!(cid::HOST, 2);
        assert_eq!(cid::GUEST, 3);
    }

    #[test]
    fn test_envelope_serialization_with_trace_id() {
        let req = AgentRequest::Ping;
        let envelope = Envelope::with_trace_id(&req, Some("abc123".to_string()));
        let json = serde_json::to_string(&envelope).unwrap();

        // trace_id should be flattened alongside the method tag
        assert!(json.contains("\"trace_id\":\"abc123\""));
        assert!(json.contains("\"method\":\"ping\""));

        // Deserialize back — Envelope<AgentRequest> with flatten
        let parsed: Envelope<AgentRequest> = serde_json::from_str(&json).unwrap();
        assert_eq!(parsed.trace_id.as_deref(), Some("abc123"));
        assert!(matches!(parsed.body, AgentRequest::Ping));
    }

    #[test]
    fn test_envelope_without_trace_id() {
        let req = AgentRequest::Ping;
        let envelope = Envelope::new(&req);
        let json = serde_json::to_string(&envelope).unwrap();

        // No trace_id field (skip_serializing_if = None)
        assert!(!json.contains("trace_id"));
        assert!(json.contains("\"method\":\"ping\""));
    }

    #[test]
    fn test_envelope_backward_compat_bare_request() {
        // A bare AgentRequest (no Envelope) should fail to parse as Envelope
        // but succeed as bare AgentRequest — this is the agent's fallback path
        let bare_json = r#"{"method":"ping"}"#;

        // Envelope parse should fail (no body field to flatten into)
        // Actually with flatten, this may work — let's verify
        let envelope_result = serde_json::from_str::<Envelope<AgentRequest>>(bare_json);
        let bare_result = serde_json::from_str::<AgentRequest>(bare_json);

        // At least one must succeed for backward compat
        assert!(
            envelope_result.is_ok() || bare_result.is_ok(),
            "Neither Envelope nor bare parse succeeded"
        );

        // Bare parse must always work
        assert!(bare_result.is_ok());
        assert!(matches!(bare_result.unwrap(), AgentRequest::Ping));

        // If Envelope works, trace_id should be None
        if let Ok(env) = envelope_result {
            assert!(env.trace_id.is_none());
        }
    }
}