feat: no_std core runtime

Cargo.lock

@@ -5324,10 +5324,15 @@ dependencies = [
  "proptest",
  "proptest-state-machine",
  "serde",
+ "swactor-core",
  "tracing",
  "web-time 0.2.4",
 ]
 
+[[package]]
+name = "swactor-core"
+version = "0.1.0"
+
 [[package]]
 name = "swactor-datastore"
 version = "0.1.0"

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 members = [
     ".",
+    "crates/core",
     "crates/bindings/python",
     "crates/bindings/wasm-runtime",
     "crates/simulation",
@@ -41,6 +42,7 @@ transport = [
 wasm = ["no_random", "dep:web-time"] # browser/wasm32 target support
 
 [dependencies]
+swactor-core = { path = "crates/core" }
 getrandom = { version = "0.2", optional = true }
 serde = { version = "1", features = ["derive"], optional = true }
 tracing = { version = "0.1", optional = true }

crates/core/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "swactor-core"
+version = "0.1.0"
+edition = "2024"

crates/core/src/lib.rs

@@ -0,0 +1,273 @@
+//! Pure, synchronous, deterministic core for the swactor actor framework.
+//!
+//! This crate contains the decision logic that maps 1:1 to a TLA+ specification.
+//! Every public function is a pure function: `fn(inputs) -> output` with no side
+//! effects, no IO, no allocation, and no panics.
+//!
+//! The runtime crate (`swactor`) calls these functions to make decisions; this crate
+//! never calls back into the runtime. If you can't trivially transliterate a
+//! function's signature into TLA+, it doesn't belong here.
+
+#![no_std]
+#![deny(unsafe_code)]
+
+// ─── Actor Lifecycle Phase ───────────────────────────────────────────────────
+
+/// The lifecycle phase of an actor slot.
+///
+/// Replaces the four boolean fields (`started`, `poisoned`, `stopping`, `suspended`)
+/// with a single state machine. Every phase transition goes through
+/// [`lifecycle_transition`].
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ActorPhase {
+    /// Actor has been inserted into the pool but `on_start` has not been called yet.
+    Unstarted,
+    /// Actor is running normally — processing messages from its mailbox.
+    Running,
+    /// Actor is suspended — messages continue to queue but are not processed.
+    Suspended,
+    /// Actor has been requested to stop gracefully. `on_stop` will be called during cleanup.
+    Stopping,
+    /// Actor panicked during `on_start` or message handling. No further processing occurs.
+    Poisoned,
+}
+
+// ─── Stop / Exit Reasons ─────────────────────────────────────────────────────
+
+/// Reason an actor was removed from the runtime.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum StopReason {
+    /// Graceful stop (via `ctx.stop_self()` or `Runtime::stop_actor()`).
+    Normal,
+    /// Actor panicked and could not be restarted.
+    Panicked,
+    /// Actor stopped with a typed exit value (via `Ctx::stop_with`).
+    Completed,
+}
+
+/// Why an actor exited — delivered to watchers.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ExitReason {
+    /// Actor was explicitly stopped or removed from the pool.
+    Stopped,
+    /// Actor panicked during message handling.
+    Panicked,
+    /// The node hosting the actor left the cluster (SWIM Dead).
+    NodeDown,
+    /// Actor stopped with a typed exit value (via `Ctx::stop_with`).
+    Completed,
+}
+
+// ─── Mailbox ─────────────────────────────────────────────────────────────────
+
+/// What to do when a bounded mailbox is full.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum MailboxOverflow {
+    /// Drop the incoming message (newest). The message is silently discarded.
+    DropNewest,
+    /// Drop the oldest message in the queue to make room for the new one.
+    DropOldest,
+}
+
+/// Result of the mailbox capacity check.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum MailboxDecision {
+    /// The mailbox has room (or is unbounded). Accept the message.
+    Accept,
+    /// The mailbox is full and the policy is DropNewest — reject the incoming message.
+    RejectNewest,
+    /// The mailbox is full and the policy is DropOldest — evict the front of the queue.
+    EvictOldest,
+}
+
+// ─── Lifecycle Events ────────────────────────────────────────────────────────
+
+/// Events that drive the actor lifecycle state machine.
+///
+/// Each variant corresponds to a runtime action that may cause a phase transition.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum LifecycleEvent {
+    /// `on_start` completed successfully.
+    Started,
+    /// `on_start` panicked.
+    StartPanicked,
+    /// A message handler panicked.
+    MessagePanicked,
+    /// A graceful stop was requested (via `ctx.stop_self()` or `StopSignal`).
+    StopRequested,
+    /// A stop-with-value was requested (via `ctx.stop_with()`).
+    StopWithRequested,
+    /// The actor was suspended (via `ctx.suspend_self()`).
+    SuspendRequested,
+    /// A suspended actor was resumed (via `ResumeSignal`).
+    Resumed,
+}
+
+// ─── Deliver Decision ────────────────────────────────────────────────────────
+
+/// What should happen when a message arrives at an actor in a particular phase.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum DeliverDecision {
+    /// Enqueue the message in the actor's mailbox (subject to overflow policy).
+    Enqueue,
+    /// The actor is dead/stopping — discard the message silently.
+    Discard,
+    /// The actor is suspended — enqueue but do not process until resumed.
+    Queue,
+}
+
+// ─── Pure Functions ─────────────────────────────────────────────────────────
+
+// TLA+: LifecycleTransition
+/// The actor lifecycle state machine.
+///
+/// Given the current phase and an event, returns the new phase. Every phase
+/// transition in the runtime must go through this function.
+///
+/// Illegal transitions (e.g., `Resumed` on a non-suspended actor) return the
+/// phase unchanged — the runtime is responsible for not issuing nonsensical
+/// events, but the core never panics.
+#[must_use]
+pub fn lifecycle_transition(phase: ActorPhase, event: LifecycleEvent) -> ActorPhase {
+    match (phase, event) {
+        // Unstarted: waiting for on_start result
+        (ActorPhase::Unstarted, LifecycleEvent::Started) => ActorPhase::Running,
+        (ActorPhase::Unstarted, LifecycleEvent::StartPanicked) => ActorPhase::Poisoned,
+        (ActorPhase::Unstarted, LifecycleEvent::StopRequested) => ActorPhase::Stopping,
+        (ActorPhase::Unstarted, LifecycleEvent::StopWithRequested) => ActorPhase::Stopping,
+        (ActorPhase::Unstarted, LifecycleEvent::SuspendRequested) => ActorPhase::Suspended,
+        (ActorPhase::Unstarted, LifecycleEvent::MessagePanicked) => phase,
+        (ActorPhase::Unstarted, LifecycleEvent::Resumed) => phase,
+
+        // Running: normal operation
+        (ActorPhase::Running, LifecycleEvent::MessagePanicked) => ActorPhase::Poisoned,
+        (ActorPhase::Running, LifecycleEvent::StopRequested) => ActorPhase::Stopping,
+        (ActorPhase::Running, LifecycleEvent::StopWithRequested) => ActorPhase::Stopping,
+        (ActorPhase::Running, LifecycleEvent::SuspendRequested) => ActorPhase::Suspended,
+        (ActorPhase::Running, LifecycleEvent::Started) => phase,
+        (ActorPhase::Running, LifecycleEvent::StartPanicked) => phase,
+        (ActorPhase::Running, LifecycleEvent::Resumed) => phase,
+
+        // Suspended: waiting for resume or stop
+        (ActorPhase::Suspended, LifecycleEvent::Resumed) => ActorPhase::Running,
+        (ActorPhase::Suspended, LifecycleEvent::StopRequested) => ActorPhase::Stopping,
+        (ActorPhase::Suspended, LifecycleEvent::StopWithRequested) => ActorPhase::Stopping,
+        (ActorPhase::Suspended, LifecycleEvent::Started) => phase,
+        (ActorPhase::Suspended, LifecycleEvent::StartPanicked) => phase,
+        (ActorPhase::Suspended, LifecycleEvent::MessagePanicked) => phase,
+        (ActorPhase::Suspended, LifecycleEvent::SuspendRequested) => phase,
+
+        // Terminal states: Stopping and Poisoned absorb all events
+        (ActorPhase::Stopping, _) => ActorPhase::Stopping,
+        (ActorPhase::Poisoned, _) => ActorPhase::Poisoned,
+    }
+}
+
+// TLA+: MailboxAccept
+/// Decide what to do with an incoming message given mailbox state and policy.
+///
+/// `capacity` of 0 means unbounded (always accept). When the mailbox is full,
+/// the overflow policy determines whether the newest or oldest message is dropped.
+#[must_use]
+pub fn mailbox_accept(
+    mailbox_len: usize,
+    capacity: usize,
+    policy: MailboxOverflow,
+) -> MailboxDecision {
+    if capacity == 0 || mailbox_len < capacity {
+        MailboxDecision::Accept
+    } else {
+        match policy {
+            MailboxOverflow::DropNewest => MailboxDecision::RejectNewest,
+            MailboxOverflow::DropOldest => MailboxDecision::EvictOldest,
+        }
+    }
+}
+
+// TLA+: ShouldTickActor
+/// Whether an actor in the given phase should be ticked (i.e., have messages processed).
+///
+/// Returns `false` for `Poisoned`, `Stopping`, and `Suspended` — these actors
+/// either cannot process messages or must not.
+#[must_use]
+pub fn should_tick_actor(phase: ActorPhase) -> bool {
+    match phase {
+        ActorPhase::Unstarted | ActorPhase::Running => true,
+        ActorPhase::Suspended | ActorPhase::Stopping | ActorPhase::Poisoned => false,
+    }
+}
+
+// TLA+: BudgetExhausted
+/// Whether the per-actor message budget has been exhausted.
+///
+/// `budget` of 0 means unlimited — never exhausted.
+#[must_use]
+pub fn budget_exhausted(processed: usize, budget: usize) -> bool {
+    budget > 0 && processed >= budget
+}
+
+// TLA+: CleanupStopReason
+/// Determine the stop reason for a dead actor given its phase and whether
+/// it has a typed exit value.
+///
+/// - `Poisoned` → `StopReason::Panicked`
+/// - Has exit value → `StopReason::Completed`
+/// - Otherwise → `StopReason::Normal`
+#[must_use]
+pub fn cleanup_stop_reason(phase: ActorPhase, has_exit_value: bool) -> StopReason {
+    if phase == ActorPhase::Poisoned {
+        StopReason::Panicked
+    } else if has_exit_value {
+        StopReason::Completed
+    } else {
+        StopReason::Normal
+    }
+}
+
+// ─── Invariant Predicates ───────────────────────────────────────────────────
+
+// TLA+: PhaseIsTerminal
+/// Returns `true` if the phase is terminal — the actor will not process any
+/// further messages and is awaiting cleanup.
+#[must_use]
+pub fn phase_is_terminal(phase: ActorPhase) -> bool {
+    match phase {
+        ActorPhase::Stopping | ActorPhase::Poisoned => true,
+        ActorPhase::Unstarted | ActorPhase::Running | ActorPhase::Suspended => false,
+    }
+}
+
+// TLA+: ValidPhaseTransition
+/// Returns `true` only for transitions that exist in the lifecycle state machine.
+///
+/// This is an invariant predicate — useful for property-based testing and Kani
+/// harnesses to verify that no illegal transition is ever taken.
+#[must_use]
+pub fn valid_phase_transition(from: ActorPhase, to: ActorPhase) -> bool {
+    match (from, to) {
+        // Self-transitions are always valid (no-op events)
+        (a, b) if a == b => true,
+
+        // From Unstarted
+        (ActorPhase::Unstarted, ActorPhase::Running) => true,
+        (ActorPhase::Unstarted, ActorPhase::Poisoned) => true,
+        (ActorPhase::Unstarted, ActorPhase::Stopping) => true,
+        (ActorPhase::Unstarted, ActorPhase::Suspended) => true,
+
+        // From Running
+        (ActorPhase::Running, ActorPhase::Poisoned) => true,
+        (ActorPhase::Running, ActorPhase::Stopping) => true,
+        (ActorPhase::Running, ActorPhase::Suspended) => true,
+
+        // From Suspended
+        (ActorPhase::Suspended, ActorPhase::Running) => true,
+        (ActorPhase::Suspended, ActorPhase::Stopping) => true,
+
+        // Terminal states never transition out
+        (ActorPhase::Stopping, _) => false,
+        (ActorPhase::Poisoned, _) => false,
+
+        // Everything else is invalid
+        _ => false,
+    }
+}

src/actor.rs

@@ -1,7 +1,9 @@
-use std::any::Any;
-use std::any::TypeId;
-use std::collections::{HashMap, HashSet};
-use std::sync::Arc;
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, String, Vec};
+
+use core::any::{Any, TypeId};
+use crate::compat::{Arc, TypeIdMap, TypeIdSet};
+use crate::delivery::AddrSet;
 
 use crate::Error;
 
@@ -36,8 +38,8 @@ impl PartialEq for ExitValue {
 
 impl Eq for ExitValue {}
 
-impl std::fmt::Debug for ExitValue {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for ExitValue {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         f.write_str("ExitValue(..)")
     }
 }
@@ -55,36 +57,60 @@ pub struct SystemInfo {
     pub uptime_ms: u64,
 }
 
-/// Why an actor exited.
-#[derive(Debug, Clone, PartialEq, Eq)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
-pub enum ExitReason {
-    /// Actor was explicitly stopped or removed from the pool.
-    Stopped,
-    /// Actor panicked during message handling.
-    Panicked,
-    /// The node hosting the actor left the cluster (SWIM Dead).
-    NodeDown,
-    /// Actor stopped with a typed exit value (via [`Ctx::stop_with`]).
-    Completed,
-}
+pub use swactor_core::ExitReason;
 
 /// Delivered to watchers when a watched actor exits.
 ///
 /// Implements `Message` (Clone + Send + Sync + 'static) so it can be
 /// delivered through normal mailbox channels.
 #[derive(Debug, Clone)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct ActorExited {
     /// The address of the actor that died.
     pub addr: ActorAddress,
     /// Why it exited.
     pub reason: ExitReason,
     /// Typed exit value if the actor called [`Ctx::stop_with`].
-    #[cfg_attr(feature = "serde", serde(skip))]
     pub exit_value: Option<ExitValue>,
 }
 
+#[cfg(feature = "serde")]
+impl serde::Serialize for ActorExited {
+    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        use serde::ser::SerializeStruct;
+        let mut s = serializer.serialize_struct("ActorExited", 2)?;
+        s.serialize_field("addr", &self.addr)?;
+        let reason_str = match self.reason {
+            ExitReason::Stopped => "Stopped",
+            ExitReason::Panicked => "Panicked",
+            ExitReason::NodeDown => "NodeDown",
+            ExitReason::Completed => "Completed",
+        };
+        s.serialize_field("reason", reason_str)?;
+        s.end()
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<'de> serde::Deserialize<'de> for ActorExited {
+    fn deserialize<D: serde::Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        #[derive(serde::Deserialize)]
+        struct Helper {
+            addr: ActorAddress,
+            reason: HelperExitReason,
+        }
+        #[derive(serde::Deserialize)]
+        enum HelperExitReason { Stopped, Panicked, NodeDown, Completed }
+        let h = Helper::deserialize(deserializer)?;
+        let reason = match h.reason {
+            HelperExitReason::Stopped => ExitReason::Stopped,
+            HelperExitReason::Panicked => ExitReason::Panicked,
+            HelperExitReason::NodeDown => ExitReason::NodeDown,
+            HelperExitReason::Completed => ExitReason::Completed,
+        };
+        Ok(ActorExited { addr: h.addr, reason, exit_value: None })
+    }
+}
+
 impl PartialEq for ActorExited {
     fn eq(&self, other: &Self) -> bool {
         self.addr == other.addr && self.reason == other.reason
@@ -140,9 +166,9 @@ pub struct ActorAddress(pub [u8; 32]);
 /// Custom Hash: only hash the first 8 bytes since all 32 are random.
 /// SipHash on 8 bytes is ~3x faster than on 32 bytes, with identical
 /// collision properties (2^64 possible values from cryptographic randomness).
-impl std::hash::Hash for ActorAddress {
+impl core::hash::Hash for ActorAddress {
     #[inline]
-    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+    fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
         // SAFETY: ActorAddress is always 32 bytes, so [..8] is valid.
         state.write_u64(u64::from_ne_bytes(
             self.0[..8].try_into().unwrap(),
@@ -150,8 +176,20 @@ impl std::hash::Hash for ActorAddress {
     }
 }
 
-impl std::fmt::Display for ActorAddress {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl PartialOrd for ActorAddress {
+    fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for ActorAddress {
+    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
+        self.0.cmp(&other.0)
+    }
+}
+
+impl core::fmt::Display for ActorAddress {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         for b in &self.0[..8] {
             write!(f, "{:02x}", b)?;
         }
@@ -177,14 +215,14 @@ impl ActorAddress {
 /// can clone individual entries cheaply (Arc bump) for copy-on-write overrides.
 #[derive(Clone, Default)]
 pub struct Environment {
-    inner: Arc<HashMap<TypeId, Arc<dyn Any + Send + Sync>>>,
+    inner: Arc<TypeIdMap<Arc<dyn Any + Send + Sync>>>,
 }
 
 impl Environment {
     /// Create an empty environment.
     pub fn new() -> Self {
         Self {
-            inner: Arc::new(HashMap::new()),
+            inner: Arc::new(TypeIdMap::new()),
         }
     }
 
@@ -223,14 +261,14 @@ impl Environment {
 ///
 /// Allows inserting/replacing typed values before freezing into an immutable `Environment`.
 pub struct EnvironmentBuilder {
-    map: HashMap<TypeId, Arc<dyn Any + Send + Sync>>,
+    map: TypeIdMap<Arc<dyn Any + Send + Sync>>,
 }
 
 impl EnvironmentBuilder {
     /// Create an empty builder.
     pub fn new() -> Self {
         Self {
-            map: HashMap::new(),
+            map: TypeIdMap::new(),
         }
     }
 
@@ -304,14 +342,14 @@ impl LogicalName {
 #[derive(Clone, Debug)]
 pub struct ServiceBinding<S: 'static + Send + Sync> {
     pub addr: ActorAddress,
-    _marker: std::marker::PhantomData<S>,
+    _marker: core::marker::PhantomData<S>,
 }
 
 impl<S: 'static + Send + Sync> ServiceBinding<S> {
     pub fn new(addr: ActorAddress) -> Self {
         Self {
             addr,
-            _marker: std::marker::PhantomData,
+            _marker: core::marker::PhantomData,
         }
     }
 }
@@ -328,11 +366,11 @@ impl<S: 'static + Send + Sync> ServiceBinding<S> {
 /// Built via fluent API: `CapabilitySet::new().with_send(addr).with_spawn()`.
 #[derive(Clone, Default)]
 pub struct CapabilitySet {
-    send_any: HashSet<ActorAddress, crate::delivery::AddrBuildHasher>,
-    send_typed: HashSet<(TypeId, ActorAddress)>,
+    send_any: AddrSet,
+    send_typed: crate::compat::TypedAddrSet,
     can_spawn: bool,
-    service_types: HashSet<TypeId>,
-    monitor_targets: HashSet<ActorAddress, crate::delivery::AddrBuildHasher>,
+    service_types: TypeIdSet,
+    monitor_targets: AddrSet,
 }
 
 impl CapabilitySet {
@@ -386,8 +424,8 @@ impl CapabilitySet {
     }
 }
 
-impl std::fmt::Debug for CapabilitySet {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for CapabilitySet {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         f.debug_struct("CapabilitySet")
             .field("send_any", &self.send_any.len())
             .field("send_typed", &self.send_typed.len())
@@ -442,7 +480,7 @@ where
         let msg = match msg.downcast::<A::Incoming>() {
             Ok(typed) => {
                 self.inner.handle(ctx, *typed);
-                return Some(std::any::type_name::<A::Incoming>());
+                return Some(core::any::type_name::<A::Incoming>());
             }
             Err(msg) => msg,
         };
@@ -484,16 +522,7 @@ impl MonitorRef {
     }
 }
 
-/// Reason an actor was removed from the runtime.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub enum StopReason {
-    /// Graceful stop (via `ctx.stop_self()` or `Runtime::stop_actor()`).
-    Normal,
-    /// Actor panicked and could not be restarted.
-    Panicked,
-    /// Actor stopped with a typed exit value (via [`Ctx::stop_with`]).
-    Completed,
-}
+pub use swactor_core::StopReason;
 
 /// Death notification delivered as a normal message when a monitored actor dies.
 ///

src/channel.rs

@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use crate::compat::Arc;
 
 use crossbeam_queue::{ArrayQueue, SegQueue};
 

src/compat.rs

@@ -0,0 +1,190 @@
+// Compatibility module: re-exports types from std or core/alloc depending on feature flags.
+//
+// Core types (Any, fmt, atomics, etc.) are imported directly from `core::` in each file.
+// This module provides conditional re-exports for types that differ between std and no_std.
+
+// --- alloc types (conditional on std feature) ---
+#[cfg(feature = "std")]
+pub use std::sync::Arc;
+#[cfg(not(feature = "std"))]
+pub use alloc::sync::Arc;
+
+#[cfg(feature = "std")]
+pub use std::collections::VecDeque;
+#[cfg(not(feature = "std"))]
+pub use alloc::collections::VecDeque;
+
+// Vec, String, Box — in std prelude normally, but need explicit import in no_std.
+// vec! and format! macros are available via `extern crate alloc` in lib.rs.
+#[cfg(not(feature = "std"))]
+pub use alloc::{boxed::Box, string::String, string::ToString, vec::Vec};
+
+// --- sync primitives (std vs spin-based) ---
+#[cfg(feature = "std")]
+pub use std::sync::{RwLock, OnceLock};
+#[cfg(not(feature = "std"))]
+pub use crate::sync_impl::SpinRwLock as RwLock;
+
+// --- collections (std vs alloc-based alternatives) ---
+#[cfg(feature = "std")]
+pub use std::collections::HashMap;
+#[cfg(not(feature = "std"))]
+pub use alloc::collections::BTreeMap as HashMap;
+
+// --- TypeId-keyed map/set ---
+//
+// TypeId does not implement Ord, so BTreeMap cannot be used in no_std.
+// In std mode: thin wrappers around HashMap/HashSet.
+// In no_std mode: linear-scan Vec wrappers (<10 entries typical).
+
+use core::any::TypeId as StdTypeId;
+
+/// A map keyed by `TypeId`.
+/// std: HashMap<TypeId, V>. no_std: linear-scan Vec.
+#[derive(Clone)]
+pub struct TypeIdMap<V> {
+    #[cfg(feature = "std")]
+    inner: std::collections::HashMap<StdTypeId, V>,
+    #[cfg(not(feature = "std"))]
+    inner: Vec<(StdTypeId, V)>,
+}
+
+impl<V> TypeIdMap<V> {
+    pub fn new() -> Self {
+        Self {
+            #[cfg(feature = "std")]
+            inner: std::collections::HashMap::new(),
+            #[cfg(not(feature = "std"))]
+            inner: Vec::new(),
+        }
+    }
+
+    #[cfg(feature = "std")]
+    pub fn get(&self, key: &StdTypeId) -> Option<&V> {
+        self.inner.get(key)
+    }
+
+    #[cfg(not(feature = "std"))]
+    pub fn get(&self, key: &StdTypeId) -> Option<&V> {
+        self.inner.iter().find(|(k, _)| k == key).map(|(_, v)| v)
+    }
+
+    #[cfg(feature = "std")]
+    pub fn contains_key(&self, key: &StdTypeId) -> bool {
+        self.inner.contains_key(key)
+    }
+
+    #[cfg(not(feature = "std"))]
+    pub fn contains_key(&self, key: &StdTypeId) -> bool {
+        self.inner.iter().any(|(k, _)| k == key)
+    }
+
+    #[cfg(feature = "std")]
+    pub fn insert(&mut self, key: StdTypeId, value: V) -> Option<V> {
+        self.inner.insert(key, value)
+    }
+
+    #[cfg(not(feature = "std"))]
+    pub fn insert(&mut self, key: StdTypeId, value: V) -> Option<V> {
+        for entry in &mut self.inner {
+            if entry.0 == key {
+                let old = core::mem::replace(&mut entry.1, value);
+                return Some(old);
+            }
+        }
+        self.inner.push((key, value));
+        None
+    }
+
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.inner.is_empty()
+    }
+}
+
+impl<V> Default for TypeIdMap<V> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// A set of `TypeId` values.
+/// std: HashSet<TypeId>. no_std: linear-scan Vec.
+#[derive(Clone, Default)]
+pub struct TypeIdSet {
+    #[cfg(feature = "std")]
+    inner: std::collections::HashSet<StdTypeId>,
+    #[cfg(not(feature = "std"))]
+    inner: Vec<StdTypeId>,
+}
+
+impl TypeIdSet {
+    #[allow(dead_code)]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn insert(&mut self, id: StdTypeId) -> bool {
+        #[cfg(feature = "std")]
+        { self.inner.insert(id) }
+        #[cfg(not(feature = "std"))]
+        {
+            if self.inner.contains(&id) {
+                false
+            } else {
+                self.inner.push(id);
+                true
+            }
+        }
+    }
+
+    pub fn contains(&self, id: &StdTypeId) -> bool {
+        self.inner.contains(id)
+    }
+
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+/// A set of `(TypeId, ActorAddress)` pairs.
+/// Used for typed send capabilities. std: HashSet. no_std: linear-scan Vec.
+#[derive(Clone, Default)]
+pub struct TypedAddrSet {
+    #[cfg(feature = "std")]
+    inner: std::collections::HashSet<(StdTypeId, crate::actor::ActorAddress)>,
+    #[cfg(not(feature = "std"))]
+    inner: Vec<(StdTypeId, crate::actor::ActorAddress)>,
+}
+
+impl TypedAddrSet {
+    #[allow(dead_code)]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn insert(&mut self, pair: (StdTypeId, crate::actor::ActorAddress)) -> bool {
+        #[cfg(feature = "std")]
+        { self.inner.insert(pair) }
+        #[cfg(not(feature = "std"))]
+        {
+            if self.inner.contains(&pair) {
+                false
+            } else {
+                self.inner.push(pair);
+                true
+            }
+        }
+    }
+
+    pub fn contains(&self, pair: &(StdTypeId, crate::actor::ActorAddress)) -> bool {
+        self.inner.contains(pair)
+    }
+
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+}

src/config.rs

@@ -1,11 +1,4 @@
-/// What to do when a bounded mailbox is full.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum MailboxOverflow {
-    /// Drop the incoming message (newest). The message is silently discarded.
-    DropNewest,
-    /// Drop the oldest message in the queue to make room for the new one.
-    DropOldest,
-}
+pub use swactor_core::MailboxOverflow;
 
 /// The tunable settings for the runtime.
 pub struct RuntimeConfig {

src/delivery.rs

@@ -1,8 +1,16 @@
-use std::any::Any;
-use std::collections::{HashMap, HashSet};
-use std::hash::{BuildHasher, Hasher};
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::sync::{Arc, OnceLock, RwLock};
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, Vec};
+
+use core::any::Any;
+#[cfg(feature = "std")]
+use core::hash::{BuildHasher, Hasher};
+#[cfg(feature = "std")]
+use std::collections::HashMap;
+use core::sync::atomic::{AtomicUsize, Ordering};
+use crate::compat::{Arc, RwLock};
+#[cfg(feature = "std")]
+use crate::compat::OnceLock;
+#[cfg(feature = "std")]
 use std::thread::Thread;
 
 use crate::actor::{ActorAddress, Message, SpawnRequest};
@@ -21,8 +29,10 @@ use crate::Error;
 ///
 /// This is safe because the input is already random (uniform distribution),
 /// so additional mixing would be redundant.
+#[cfg(feature = "std")]
 pub struct AddrHasher(u64);
 
+#[cfg(feature = "std")]
 impl Hasher for AddrHasher {
     #[inline]
     fn finish(&self) -> u64 {
@@ -41,9 +51,11 @@ impl Hasher for AddrHasher {
 }
 
 /// BuildHasher for creating AddrHasher instances.
+#[cfg(feature = "std")]
 #[derive(Default, Clone)]
 pub struct AddrBuildHasher;
 
+#[cfg(feature = "std")]
 impl BuildHasher for AddrBuildHasher {
     type Hasher = AddrHasher;
 
@@ -55,10 +67,18 @@ impl BuildHasher for AddrBuildHasher {
 
 /// HashMap optimized for ActorAddress keys.
 /// Uses identity hashing since ActorAddress bytes are already random.
+/// In no_std: BTreeMap using ActorAddress's Ord impl.
+#[cfg(feature = "std")]
 pub type AddrMap<V> = HashMap<ActorAddress, V, AddrBuildHasher>;
+#[cfg(not(feature = "std"))]
+pub type AddrMap<V> = alloc::collections::BTreeMap<ActorAddress, V>;
 
 /// HashSet optimized for ActorAddress keys.
-pub type AddrSet = HashSet<ActorAddress, AddrBuildHasher>;
+/// In no_std: BTreeSet using ActorAddress's Ord impl.
+#[cfg(feature = "std")]
+pub type AddrSet = std::collections::HashSet<ActorAddress, AddrBuildHasher>;
+#[cfg(not(feature = "std"))]
+pub type AddrSet = alloc::collections::BTreeSet<ActorAddress>;
 
 // ─── Address Map Types ───────────────────────────────────────────────────────
 
@@ -80,12 +100,20 @@ pub(crate) struct AddressMap {
 }
 
 impl AddressMap {
+    #[cfg(feature = "std")]
     pub fn with_capacity(cap: usize) -> Self {
         Self {
             inner: RwLock::new(HashMap::with_capacity_and_hasher(cap, AddrBuildHasher)),
         }
     }
 
+    #[cfg(not(feature = "std"))]
+    pub fn with_capacity(_cap: usize) -> Self {
+        Self {
+            inner: RwLock::new(AddrMap::new()),
+        }
+    }
+
     pub fn insert(&self, addr: ActorAddress, worker: WorkerId) {
         self.inner.write().unwrap().insert(addr, worker);
     }
@@ -200,18 +228,26 @@ pub(crate) struct InboxRegistry {
 }
 
 impl InboxRegistry {
+    #[cfg(feature = "std")]
     pub fn new() -> Self {
         Self {
             senders: RwLock::new(HashMap::with_hasher(AddrBuildHasher)),
         }
     }
 
+    #[cfg(not(feature = "std"))]
+    pub fn new() -> Self {
+        Self {
+            senders: RwLock::new(AddrMap::new()),
+        }
+    }
+
     pub fn register(&self, addr: ActorAddress, sender: Arc<dyn SenderT>) {
         self.senders.write().unwrap().insert(addr, sender);
     }
 
     /// Check if an address is registered without consuming a message.
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub fn contains(&self, addr: &ActorAddress) -> bool {
         self.senders.read().unwrap().contains_key(addr)
     }
@@ -242,14 +278,16 @@ pub(crate) struct TickContext<'a> {
     pub(crate) extension: Option<&'a dyn crate::extension::RuntimeExtension>,
     pub(crate) stats_hook: Option<&'a dyn crate::stats::StatsHook>,
     /// Thread handles for waking parked workers on cross-worker sends.
+    #[cfg(feature = "std")]
     pub(crate) worker_threads: &'a [OnceLock<Thread>],
     /// Per-worker stats for summing total_actors across workers.
     pub(crate) worker_stats: &'a [Arc<WorkerStats>],
     /// Runtime creation time for computing uptime_ms.
+    #[cfg(feature = "std")]
     pub(crate) created_at: crate::Instant,
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub(crate) codec_registry: Option<&'a crate::transport::CodecRegistry>,
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub(crate) transport_router: Option<&'a crate::transport::TransportRouter>,
 }
 
@@ -261,7 +299,7 @@ impl<'a> TickContext<'a> {
         addr: ActorAddress,
         msg: Box<dyn Any + Send>,
     ) -> Result<(), Error> {
-        #[cfg(feature = "transport")]
+        #[cfg(all(feature = "transport", feature = "std"))]
         {
             if self.inbox_registry.contains(&addr) {
                 return self.inbox_registry.try_deliver(addr, msg);

src/error.rs

@@ -1,3 +1,6 @@
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, String, ToString};
+
 /// Simple, ergonomic, local `Error` type.
 /// # Usage
 /// ```
@@ -13,16 +16,29 @@
 /// }
 /// ```
 #[derive(Debug)]
-pub struct Error(Box<dyn std::error::Error + Send + Sync + 'static>);
+pub struct Error(Box<dyn core::error::Error + Send + Sync + 'static>);
+
+/// Small wrapper so `String` can become `Box<dyn core::error::Error>` in both
+/// std and no_std (where String doesn't auto-impl core::error::Error via blanket).
+#[derive(Debug)]
+struct StringError(String);
+
+impl core::fmt::Display for StringError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
+impl core::error::Error for StringError {}
 
 impl<T: AsRef<str>> From<T> for Error {
     fn from(value: T) -> Self {
-        Error(value.as_ref().to_string().into())
+        Error(Box::new(StringError(value.as_ref().to_string())))
     }
 }
 
-impl std::fmt::Display for Error {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Display for Error {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         write!(f, "{}", self.0)
     }
 }

src/extension.rs

@@ -1,4 +1,7 @@
-use std::any::Any;
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, Vec};
+
+use core::any::Any;
 
 use crate::actor::{ActorAddress, Environment, ExitValue, StopReason};
 

src/lib.rs

@@ -1,3 +1,12 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(feature = "std"))]
+extern crate alloc;
+
+pub(crate) mod compat;
+#[cfg(not(feature = "std"))]
+pub(crate) mod sync_impl;
+
 pub mod actor;
 pub mod extension;
 pub mod worker;
@@ -10,7 +19,9 @@ pub(crate) mod error;
 pub use error::Error;
 
 // Re-export identity hashing types for ActorAddress-keyed collections.
-pub use delivery::{AddrBuildHasher, AddrMap, AddrSet};
+#[cfg(feature = "std")]
+pub use delivery::AddrBuildHasher;
+pub use delivery::{AddrMap, AddrSet};
 
 pub mod config;
 pub(crate) mod delivery;
@@ -18,7 +29,7 @@ pub mod stats;
 
 pub mod runtime;
 
-#[cfg(feature = "transport")]
+#[cfg(all(feature = "transport", feature = "std"))]
 pub mod transport;
 
 #[cfg(feature = "std")]
@@ -26,9 +37,10 @@ pub mod std;
 
 // Platform-aware Instant: web_time on wasm, std::time on native.
 // web_time is a no-op re-export of std::time::Instant on non-wasm targets.
+// In no_std (without wasm), Instant is not available — timing returns 0.
 #[cfg(feature = "wasm")]
 pub(crate) use web_time::Instant;
-#[cfg(not(feature = "wasm"))]
+#[cfg(all(not(feature = "wasm"), feature = "std"))]
 pub(crate) use ::std::time::Instant;
 
 #[cfg(feature = "getrandom")]
@@ -36,7 +48,7 @@ pub(crate) fn get_random(buf: &mut [u8]) {
     getrandom::getrandom(buf).unwrap()
 }
 
-#[cfg(all(feature = "no_random", not(feature = "getrandom")))]
+#[cfg(not(feature = "getrandom"))]
 pub(crate) fn get_random(buf: &mut [u8]) {
     use core::sync::atomic::{AtomicUsize, Ordering};
 

src/runtime.rs

@@ -1,11 +1,16 @@
-use std::any::Any;
-use std::cell::RefCell;
-use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::{Arc, OnceLock};
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, Vec};
+
+use core::any::Any;
+use core::cell::RefCell;
+use core::sync::atomic::{AtomicBool, Ordering};
+use crate::compat::Arc;
+#[cfg(feature = "std")]
+use crate::compat::OnceLock;
+#[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
 use std::thread::{self, JoinHandle};
+#[cfg(feature = "std")]
 use std::thread::Thread;
-use crate::Instant;
 
 use crate::actor::{Actor, ActorAddress, ActorInterface, AnyActor, Environment, ExitValue, Message, ResumeSignal, SpawnRequest, StopSignal, StopWithSignal, SystemInfo};
 use crate::channel::{Receiver, Sender};
@@ -69,13 +74,13 @@ impl<R: Message> Ask<R> {
 }
 
 /// Handle for dealing with a runtime that has started via the `Runtime::run()` method.
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
 pub struct RuntimeHandle {
     pub runtime: Arc<Runtime>,
     threads: Vec<JoinHandle<()>>,
 }
 
-#[cfg(not(target_arch = "wasm32"))]
+#[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
 impl RuntimeHandle {
     pub fn join(self) {
         for handle in self.threads {
@@ -110,11 +115,13 @@ pub struct Runtime {
     /// Workers available for tick(). run() drains this and moves workers to threads.
     tick_workers: RefCell<Vec<Worker>>,
     /// Thread handles for waking parked workers. Set by workers on startup via OnceLock.
+    #[cfg(feature = "std")]
     worker_threads: Arc<Vec<OnceLock<Thread>>>,
-    created_at: Instant,
-    #[cfg(feature = "transport")]
+    #[cfg(feature = "std")]
+    created_at: crate::Instant,
+    #[cfg(all(feature = "transport", feature = "std"))]
     codec_registry: Option<Arc<crate::transport::CodecRegistry>>,
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     transport_router: Option<Arc<crate::transport::TransportRouter>>,
 }
 
@@ -129,8 +136,8 @@ unsafe impl Sync for Runtime {}
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct RuntimeAddress(pub [u8; 32]);
 
-impl std::fmt::Display for RuntimeAddress {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Display for RuntimeAddress {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         for b in &self.0[..8] {
             write!(f, "{:02x}", b)?;
         }
@@ -155,6 +162,7 @@ impl RuntimeAddress {
 pub struct ExternalSender {
     address_map: Arc<AddressMap>,
     transfer_txs: Vec<Sender<Envelope>>,
+    #[cfg(feature = "std")]
     worker_threads: Arc<Vec<OnceLock<Thread>>>,
 }
 
@@ -163,6 +171,7 @@ impl Clone for ExternalSender {
         Self {
             address_map: self.address_map.clone(),
             transfer_txs: self.transfer_txs.clone(),
+            #[cfg(feature = "std")]
             worker_threads: self.worker_threads.clone(),
         }
     }
@@ -182,6 +191,7 @@ impl ExternalSender {
             Some(wid) => {
                 self.transfer_txs[wid.as_usize()]
                     .send(Envelope::new(addr, Box::new(msg)));
+                #[cfg(feature = "std")]
                 notify_worker(&self.worker_threads, wid.as_usize());
                 Ok(())
             }
@@ -232,6 +242,7 @@ impl Runtime {
 
         let placement = Placement::new(num_workers, worker_stats.clone());
 
+        #[cfg(feature = "std")]
         let worker_threads: Arc<Vec<OnceLock<Thread>>> =
             Arc::new((0..num_workers).map(|_| OnceLock::new()).collect());
 
@@ -247,11 +258,13 @@ impl Runtime {
             worker_stats,
             stats_hook: None,
             tick_workers: RefCell::new(workers),
+            #[cfg(feature = "std")]
             worker_threads,
-            created_at: Instant::now(),
-            #[cfg(feature = "transport")]
+            #[cfg(feature = "std")]
+            created_at: crate::Instant::now(),
+            #[cfg(all(feature = "transport", feature = "std"))]
             codec_registry: None,
-            #[cfg(feature = "transport")]
+            #[cfg(all(feature = "transport", feature = "std"))]
             transport_router: None,
         };
 
@@ -369,6 +382,7 @@ impl Runtime {
         ExternalSender {
             address_map: self.address_map.clone(),
             transfer_txs: self.transfer_txs.iter().cloned().collect(),
+            #[cfg(feature = "std")]
             worker_threads: self.worker_threads.clone(),
         }
     }
@@ -383,12 +397,14 @@ impl Runtime {
             config: &self.config,
             extension: self.extension.as_deref(),
             stats_hook: self.stats_hook.as_deref(),
+            #[cfg(feature = "std")]
             worker_threads: &self.worker_threads,
             worker_stats: &self.worker_stats,
+            #[cfg(feature = "std")]
             created_at: self.created_at,
-            #[cfg(feature = "transport")]
+            #[cfg(all(feature = "transport", feature = "std"))]
             codec_registry: self.codec_registry.as_deref(),
-            #[cfg(feature = "transport")]
+            #[cfg(all(feature = "transport", feature = "std"))]
             transport_router: self.transport_router.as_deref(),
         }
     }
@@ -414,7 +430,7 @@ impl Runtime {
     /// In single-threaded mode, one background thread is spawned.
     ///
     /// Not available on wasm32 — use the browser crate's Web Worker-based run instead.
-    #[cfg(not(target_arch = "wasm32"))]
+    #[cfg(all(not(target_arch = "wasm32"), feature = "std"))]
     pub fn run(self) -> Result<RuntimeHandle, Error> {
         self.is_running.store(true, Ordering::Release);
 
@@ -464,7 +480,10 @@ impl Runtime {
             .map(|ws| ws.drain_tick_timings())
             .collect();
 
+        #[cfg(feature = "std")]
         let uptime_ms = self.created_at.elapsed().as_millis() as u64;
+        #[cfg(not(feature = "std"))]
+        let uptime_ms = 0u64;
 
         RuntimeStats { num_workers, uptime_ms, actors, workers, actor_details: Vec::new(), tick_timings }
     }
@@ -480,6 +499,7 @@ impl Runtime {
             Some(wid) => {
                 self.transfer_txs[wid.as_usize()]
                     .send(Envelope::new(addr, Box::new(StopSignal)));
+                #[cfg(feature = "std")]
                 notify_worker(&self.worker_threads, wid.as_usize());
                 Ok(())
             }
@@ -494,9 +514,12 @@ impl Runtime {
 
         self.is_running.store(false, Ordering::Release);
         // Wake all parked workers so they see the shutdown flag immediately
-        for thread in self.worker_threads.iter() {
-            if let Some(t) = thread.get() {
-                t.unpark();
+        #[cfg(feature = "std")]
+        {
+            for thread in self.worker_threads.iter() {
+                if let Some(t) = thread.get() {
+                    t.unpark();
+                }
             }
         }
     }
@@ -509,13 +532,13 @@ impl Runtime {
     }
 
     /// Set the codec registry for remote transport.
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub fn set_codec_registry(&mut self, registry: Arc<crate::transport::CodecRegistry>) {
         self.codec_registry = Some(registry);
     }
 
     /// Set the transport router for remote message delivery.
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub fn set_transport_router(&mut self, router: Arc<crate::transport::TransportRouter>) {
         self.transport_router = Some(router);
     }
@@ -524,7 +547,7 @@ impl Runtime {
     ///
     /// Used by [`CodecRegistry::receive`](crate::transport::CodecRegistry::receive)
     /// to inject incoming messages from remote runtimes.
-    #[cfg(feature = "transport")]
+    #[cfg(all(feature = "transport", feature = "std"))]
     pub fn deliver_raw(
         &self,
         addr: ActorAddress,
@@ -543,6 +566,7 @@ impl Runtime {
 
 /// Wake a parked worker thread so it can process new work.
 /// No-op if the thread handle hasn't been registered yet (single-threaded tick mode).
+#[cfg(feature = "std")]
 #[inline]
 pub(crate) fn notify_worker(threads: &[OnceLock<Thread>], wid: usize) {
     if let Some(t) = threads.get(wid).and_then(|o| o.get()) {
@@ -557,6 +581,7 @@ impl ContextInner for Runtime {
             Some(wid) => {
                 self.transfer_txs[wid.as_usize()]
                     .send(Envelope::new(addr, msg));
+                #[cfg(feature = "std")]
                 notify_worker(&self.worker_threads, wid.as_usize());
                 Ok(())
             }
@@ -569,6 +594,7 @@ impl ContextInner for Runtime {
         self.address_map.insert(request.addr, worker_id);
         self.spawn_txs[worker_id.as_usize()]
             .send(request);
+        #[cfg(feature = "std")]
         notify_worker(&self.worker_threads, worker_id.as_usize());
     }
 
@@ -577,6 +603,7 @@ impl ContextInner for Runtime {
         if let Some(wid) = self.address_map.lookup(&addr) {
             self.transfer_txs[wid.as_usize()]
                 .send(Envelope::new(addr, Box::new(StopSignal)));
+            #[cfg(feature = "std")]
             notify_worker(&self.worker_threads, wid.as_usize());
         }
     }
@@ -585,19 +612,22 @@ impl ContextInner for Runtime {
         if let Some(wid) = self.address_map.lookup(&addr) {
             self.transfer_txs[wid.as_usize()]
                 .send(Envelope::new(addr, Box::new(StopWithSignal(value))));
+            #[cfg(feature = "std")]
             notify_worker(&self.worker_threads, wid.as_usize());
         }
     }
 
-    fn request_suspend(&self, addr: ActorAddress) {
+    fn request_suspend(&self, _addr: ActorAddress) {
         // Outside worker context — not supported (suspend is per-actor, from handler)
-        eprintln!("swactor: request_suspend called outside worker context for {addr} — ignored");
+        #[cfg(feature = "std")]
+        eprintln!("swactor: request_suspend called outside worker context for {_addr} — ignored");
     }
 
     fn request_resume(&self, addr: ActorAddress) {
         if let Some(wid) = self.address_map.lookup(&addr) {
             self.transfer_txs[wid.as_usize()]
                 .send(Envelope::new(addr, Box::new(ResumeSignal)));
+            #[cfg(feature = "std")]
             notify_worker(&self.worker_threads, wid.as_usize());
         }
     }
@@ -605,6 +635,7 @@ impl ContextInner for Runtime {
     fn post_worker_request(&self, _request: Box<dyn Any + Send>) {
         // Worker requests (e.g., timers) are per-worker; posting from outside
         // a worker context (e.g., rt.spawn() callback) is not supported.
+        #[cfg(feature = "std")]
         eprintln!("swactor: post_worker_request called outside worker context — ignored");
     }
 
@@ -617,11 +648,15 @@ impl ContextInner for Runtime {
         let total_actors: usize = self.worker_stats.iter()
             .map(|ws| ws.num_actors.load(Ordering::Relaxed))
             .sum();
+        #[cfg(feature = "std")]
+        let uptime_ms = self.created_at.elapsed().as_millis() as u64;
+        #[cfg(not(feature = "std"))]
+        let uptime_ms = 0u64;
         SystemInfo {
             worker_id: 0,
             num_workers,
             total_actors,
-            uptime_ms: self.created_at.elapsed().as_millis() as u64,
+            uptime_ms,
         }
     }
 }

src/stats.rs

@@ -1,4 +1,7 @@
-use std::sync::atomic::{AtomicU64, AtomicUsize};
+#[cfg(not(feature = "std"))]
+use crate::compat::{String, Vec};
+
+use core::sync::atomic::{AtomicU64, AtomicUsize};
 
 use crossbeam_queue::ArrayQueue;
 
@@ -83,7 +86,7 @@ impl WorkerStats {
 
     /// Create a point-in-time snapshot as a [`WorkerInfo`].
     pub fn snapshot(&self, id: usize) -> WorkerInfo {
-        use std::sync::atomic::Ordering::Relaxed;
+        use core::sync::atomic::Ordering::Relaxed;
         WorkerInfo {
             id,
             num_actors: self.num_actors.load(Relaxed),

src/sync_impl.rs

@@ -0,0 +1,230 @@
+//! Spin-based sync primitives for `#[cfg(not(feature = "std"))]`.
+//!
+//! These are minimal implementations sufficient for the swactor runtime
+//! in single-threaded cooperative (no_std) environments. They use atomic
+//! spin loops and are NOT suitable for highly contended multi-threaded use.
+#![allow(dead_code)]
+
+use core::cell::UnsafeCell;
+use core::ops::{Deref, DerefMut};
+use core::sync::atomic::{AtomicBool, AtomicU8, AtomicUsize, Ordering};
+
+// ─── SpinMutex ──────────────────────────────────────────────────────────────
+
+/// A simple spin-lock mutex.
+///
+/// Returns `Result<Guard, E>` from `lock()` to match `std::sync::Mutex` API
+/// (the codebase calls `.lock().unwrap()`). The `Err` variant is never produced.
+pub struct SpinMutex<T> {
+    locked: AtomicBool,
+    data: UnsafeCell<T>,
+}
+
+unsafe impl<T: Send> Send for SpinMutex<T> {}
+unsafe impl<T: Send> Sync for SpinMutex<T> {}
+
+impl<T> SpinMutex<T> {
+    pub const fn new(value: T) -> Self {
+        Self {
+            locked: AtomicBool::new(false),
+            data: UnsafeCell::new(value),
+        }
+    }
+
+    pub fn lock(&self) -> Result<SpinMutexGuard<'_, T>, core::convert::Infallible> {
+        while self
+            .locked
+            .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+            .is_err()
+        {
+            core::hint::spin_loop();
+        }
+        Ok(SpinMutexGuard { lock: self })
+    }
+}
+
+pub struct SpinMutexGuard<'a, T> {
+    lock: &'a SpinMutex<T>,
+}
+
+impl<T> Deref for SpinMutexGuard<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+impl<T> DerefMut for SpinMutexGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+impl<T> Drop for SpinMutexGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.locked.store(false, Ordering::Release);
+    }
+}
+
+// ─── SpinRwLock ─────────────────────────────────────────────────────────────
+
+/// A simple spin-lock reader-writer lock.
+///
+/// State encoding in `state: AtomicUsize`:
+/// - 0 = unlocked
+/// - 1..usize::MAX-1 = N active readers
+/// - usize::MAX = writer holds the lock
+const WRITER: usize = usize::MAX;
+
+pub struct SpinRwLock<T> {
+    state: AtomicUsize,
+    data: UnsafeCell<T>,
+}
+
+unsafe impl<T: Send> Send for SpinRwLock<T> {}
+unsafe impl<T: Send + Sync> Sync for SpinRwLock<T> {}
+
+impl<T> SpinRwLock<T> {
+    pub const fn new(value: T) -> Self {
+        Self {
+            state: AtomicUsize::new(0),
+            data: UnsafeCell::new(value),
+        }
+    }
+
+    pub fn read(&self) -> Result<SpinRwLockReadGuard<'_, T>, core::convert::Infallible> {
+        loop {
+            let s = self.state.load(Ordering::Relaxed);
+            if s == WRITER {
+                core::hint::spin_loop();
+                continue;
+            }
+            if self
+                .state
+                .compare_exchange_weak(s, s + 1, Ordering::Acquire, Ordering::Relaxed)
+                .is_ok()
+            {
+                return Ok(SpinRwLockReadGuard { lock: self });
+            }
+            core::hint::spin_loop();
+        }
+    }
+
+    pub fn write(&self) -> Result<SpinRwLockWriteGuard<'_, T>, core::convert::Infallible> {
+        loop {
+            if self
+                .state
+                .compare_exchange_weak(0, WRITER, Ordering::Acquire, Ordering::Relaxed)
+                .is_ok()
+            {
+                return Ok(SpinRwLockWriteGuard { lock: self });
+            }
+            core::hint::spin_loop();
+        }
+    }
+}
+
+pub struct SpinRwLockReadGuard<'a, T> {
+    lock: &'a SpinRwLock<T>,
+}
+
+impl<T> Deref for SpinRwLockReadGuard<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+impl<T> Drop for SpinRwLockReadGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.state.fetch_sub(1, Ordering::Release);
+    }
+}
+
+pub struct SpinRwLockWriteGuard<'a, T> {
+    lock: &'a SpinRwLock<T>,
+}
+
+impl<T> Deref for SpinRwLockWriteGuard<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        unsafe { &*self.lock.data.get() }
+    }
+}
+
+impl<T> DerefMut for SpinRwLockWriteGuard<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.lock.data.get() }
+    }
+}
+
+impl<T> Drop for SpinRwLockWriteGuard<'_, T> {
+    fn drop(&mut self) {
+        self.lock.state.store(0, Ordering::Release);
+    }
+}
+
+// ─── SpinOnceLock ───────────────────────────────────────────────────────────
+
+const EMPTY: u8 = 0;
+const INITIALIZING: u8 = 1;
+const READY: u8 = 2;
+
+/// A spin-based `OnceLock` matching the `std::sync::OnceLock` API surface.
+pub struct SpinOnceLock<T> {
+    state: AtomicU8,
+    data: UnsafeCell<Option<T>>,
+}
+
+unsafe impl<T: Send + Sync> Send for SpinOnceLock<T> {}
+unsafe impl<T: Send + Sync> Sync for SpinOnceLock<T> {}
+
+impl<T> SpinOnceLock<T> {
+    pub const fn new() -> Self {
+        Self {
+            state: AtomicU8::new(EMPTY),
+            data: UnsafeCell::new(None),
+        }
+    }
+
+    pub fn get(&self) -> Option<&T> {
+        if self.state.load(Ordering::Acquire) == READY {
+            unsafe { (*self.data.get()).as_ref() }
+        } else {
+            None
+        }
+    }
+
+    pub fn set(&self, value: T) -> Result<(), T> {
+        if self
+            .state
+            .compare_exchange(EMPTY, INITIALIZING, Ordering::Acquire, Ordering::Relaxed)
+            .is_ok()
+        {
+            unsafe { *self.data.get() = Some(value) };
+            self.state.store(READY, Ordering::Release);
+            Ok(())
+        } else {
+            Err(value)
+        }
+    }
+
+    pub fn get_or_init<F: FnOnce() -> T>(&self, f: F) -> &T {
+        if self.state.load(Ordering::Acquire) == READY {
+            return unsafe { (*self.data.get()).as_ref().unwrap() };
+        }
+        if self
+            .state
+            .compare_exchange(EMPTY, INITIALIZING, Ordering::Acquire, Ordering::Relaxed)
+            .is_ok()
+        {
+            unsafe { *self.data.get() = Some(f()) };
+            self.state.store(READY, Ordering::Release);
+        } else {
+            while self.state.load(Ordering::Acquire) != READY {
+                core::hint::spin_loop();
+            }
+        }
+        unsafe { (*self.data.get()).as_ref().unwrap() }
+    }
+}

src/transport.rs

@@ -8,7 +8,7 @@
 //! - **[`Codec<M>`]**: HOW bytes are encoded — gRPC/protobuf, bincode, custom, etc.
 //! - **[`Transport`]**: WHERE bytes are sent — in-memory, gRPC channel, etc.
 
-use std::any::{Any, TypeId};
+use core::any::{Any, TypeId};
 use std::collections::HashMap;
 use std::sync::{Arc, RwLock};
 

src/worker.rs

@@ -1,15 +1,22 @@
-use std::any::Any;
-use std::cell::RefCell;
-use std::collections::{HashMap, VecDeque};
-use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
+#[cfg(not(feature = "std"))]
+use crate::compat::{Box, Vec};
+
+use core::any::Any;
+use core::cell::RefCell;
+use core::sync::atomic::Ordering;
+#[cfg(feature = "std")]
+use core::sync::atomic::AtomicBool;
+use crate::compat::{Arc, HashMap, VecDeque};
+#[cfg(feature = "std")]
 use std::thread;
+#[cfg(feature = "std")]
 use crate::Instant;
 
 use crate::actor::{ActorAddress, AnyActor, ContextInner, Ctx, Environment, ExitValue, ResumeSignal, SpawnRequest, StopReason, StopSignal, StopWithSignal, SystemInfo};
 use crate::channel::Receiver;
 use crate::config::MailboxOverflow;
-use crate::delivery::{AddrBuildHasher, AddrMap, Envelope, TickContext, WorkerId};
+use crate::delivery::{AddrMap, Envelope, TickContext, WorkerId};
+use swactor_core::{ActorPhase, LifecycleEvent, lifecycle_transition};
 use crate::stats::{ActorSnapshot, TickTiming, WorkerStats};
 use crate::Error;
 
@@ -29,6 +36,7 @@ fn route_to_pool_or_remote(
         match tc.address_map.lookup(&dest) {
             Some(wid) => {
                 tc.transfer_txs[wid.as_usize()].send(Envelope::new(dest, msg));
+                #[cfg(feature = "std")]
                 crate::runtime::notify_worker(tc.worker_threads, wid.as_usize());
             }
             None => {
@@ -86,7 +94,11 @@ impl Worker {
         let mut spawn_count: usize = 0;
         while let Some(mut req) = self.spawn_rx.try_recv() {
             if let Some(ext) = tc.extension {
-                req.env = ext.on_spawn(req.addr, req.parent, req.env, tc.created_at.elapsed().as_millis() as u64);
+                #[cfg(feature = "std")]
+                let uptime_ms = tc.created_at.elapsed().as_millis() as u64;
+                #[cfg(not(feature = "std"))]
+                let uptime_ms = 0u64;
+                req.env = ext.on_spawn(req.addr, req.parent, req.env, uptime_ms);
             }
             self.pool.insert(req);
             #[cfg(feature = "tracing")]
@@ -169,10 +181,12 @@ impl Worker {
         }
 
         let mut did_work = false;
+        #[cfg(feature = "std")]
         let t0 = Instant::now();
 
         // 1. Drain spawn queue → add actors to pool
         did_work |= self.drain_spawns(tc);
+        #[cfg(feature = "std")]
         let t1 = Instant::now();
 
         // 2. Drain transfer queue → deliver envelopes to actors
@@ -182,6 +196,7 @@ impl Worker {
             self.pool.deliver(&dest, payload);
             did_work = true;
         }
+        #[cfg(feature = "std")]
         let t2 = Instant::now();
 
         // 2.5. Fire per-worker extension (e.g., timers) → deliver before tick_all
@@ -218,6 +233,7 @@ impl Worker {
                 did_work = true;
             }
         }
+        #[cfg(feature = "std")]
         let t3 = Instant::now();
 
         #[cfg(feature = "tracing")]
@@ -232,6 +248,7 @@ impl Worker {
         // 4. Drain spawn queue again — actors spawned during step 3
         //    must be in the pool before pending_local delivery.
         did_work |= self.drain_spawns(tc);
+        #[cfg(feature = "std")]
         let t4 = Instant::now();
 
         // 5. Drain pending_local buffer → deliver to local actors
@@ -250,6 +267,7 @@ impl Worker {
             }
         }
 
+        #[cfg(feature = "std")]
         let t5 = Instant::now();
 
         // 6. Publish stats (skip entirely when idle to avoid allocation + mutex)
@@ -268,10 +286,12 @@ impl Worker {
             }
         }
 
+        #[cfg(feature = "std")]
         let t6 = Instant::now();
 
         // Record tick timing
         let timing = TickTiming {
+            #[cfg(feature = "std")]
             phase_us: [
                 t1.duration_since(t0).as_micros() as u64,
                 t2.duration_since(t1).as_micros() as u64,
@@ -280,6 +300,8 @@ impl Worker {
                 t5.duration_since(t4).as_micros() as u64,
                 t6.duration_since(t5).as_micros() as u64,
             ],
+            #[cfg(not(feature = "std"))]
+            phase_us: [0; 6],
             messages_processed: processed,
             did_work,
         };
@@ -303,6 +325,7 @@ impl Worker {
         did_work
     }
 
+    #[cfg(feature = "std")]
     pub(crate) fn run(&mut self, tc: &TickContext, is_running: &AtomicBool) {
         #[cfg(feature = "tracing")]
         let _span = tracing::info_span!("worker.run", worker_id = self.id.0).entered();
@@ -343,6 +366,7 @@ impl ContextInner for WorkerContext<'_> {
             Some(wid) => {
                 self.stats.cross_sends.fetch_add(1, Ordering::Relaxed);
                 self.tc.transfer_txs[wid.as_usize()].send(Envelope::new(addr, msg));
+                #[cfg(feature = "std")]
                 crate::runtime::notify_worker(self.tc.worker_threads, wid.as_usize());
                 Ok(())
             }
@@ -358,6 +382,7 @@ impl ContextInner for WorkerContext<'_> {
         self.tc.address_map.insert(request.addr, worker_id);
         self.tc.spawn_txs[worker_id.as_usize()]
             .send(request);
+        #[cfg(feature = "std")]
         crate::runtime::notify_worker(self.tc.worker_threads, worker_id.as_usize());
     }
 
@@ -392,11 +417,15 @@ impl ContextInner for WorkerContext<'_> {
         let total_actors: usize = self.tc.worker_stats.iter()
             .map(|ws| ws.num_actors.load(Ordering::Relaxed))
             .sum();
+        #[cfg(feature = "std")]
+        let uptime_ms = self.tc.created_at.elapsed().as_millis() as u64;
+        #[cfg(not(feature = "std"))]
+        let uptime_ms = 0u64;
         SystemInfo {
             worker_id: self.worker_id.0,
             num_workers,
             total_actors,
-            uptime_ms: self.tc.created_at.elapsed().as_millis() as u64,
+            uptime_ms,
         }
     }
 }
@@ -404,13 +433,8 @@ impl ContextInner for WorkerContext<'_> {
 struct ActorSlot {
     mailbox: VecDeque<Box<dyn Any + Send>>,
     actor: Box<dyn AnyActor>,
-    poisoned: bool,
-    /// Graceful stop requested (via StopSignal).
-    stopping: bool,
-    /// Whether on_start has been called for this actor.
-    started: bool,
-    /// Actor is suspended — messages queue but are not processed.
-    suspended: bool,
+    /// Lifecycle phase — replaces the old `started`, `poisoned`, `stopping`, `suspended` booleans.
+    phase: ActorPhase,
     last_msg_type: Option<&'static str>,
     messages_processed: u64,
     /// Per-message-type counters (bounded to 32 entries).
@@ -438,7 +462,10 @@ pub(crate) struct ActorPool {
 impl ActorPool {
     pub fn new(default_mailbox_capacity: usize, default_overflow_policy: MailboxOverflow) -> Self {
         Self {
-            actors: HashMap::with_hasher(AddrBuildHasher),
+            #[cfg(feature = "std")]
+            actors: std::collections::HashMap::with_hasher(crate::delivery::AddrBuildHasher),
+            #[cfg(not(feature = "std"))]
+            actors: AddrMap::new(),
             default_mailbox_capacity,
             default_overflow_policy,
             drops_this_tick: 0,
@@ -451,10 +478,7 @@ impl ActorPool {
         self.actors.insert(req.addr, ActorSlot {
             mailbox: VecDeque::with_capacity(prealloc),
             actor: req.actor,
-            poisoned: false,
-            stopping: false,
-            started: false,
-            suspended: false,
+            phase: ActorPhase::Unstarted,
             last_msg_type: None,
             messages_processed: 0,
             msg_type_counts: HashMap::new(),
@@ -472,13 +496,13 @@ impl ActorPool {
         if let Some(slot) = self.actors.get_mut(addr) {
             // Intercept control signals for suspended actors: they skip tick_all
             // so we must handle resume/stop at delivery time.
-            if slot.suspended {
+            if slot.phase == ActorPhase::Suspended {
                 if msg.is::<ResumeSignal>() {
-                    slot.suspended = false;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::Resumed);
                     return true;
                 }
                 if msg.is::<StopSignal>() {
-                    slot.stopping = true;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopRequested);
                     slot.mailbox.clear();
                     return true;
                 }
@@ -486,21 +510,20 @@ impl ActorPool {
                     if let Ok(sig) = msg.downcast::<StopWithSignal>() {
                         slot.exit_value = Some(sig.0);
                     }
-                    slot.stopping = true;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopWithRequested);
                     slot.mailbox.clear();
                     return true;
                 }
             }
-            if slot.mailbox_capacity > 0 && slot.mailbox.len() >= slot.mailbox_capacity {
-                match slot.overflow_policy {
-                    MailboxOverflow::DropNewest => {
-                        self.drops_this_tick += 1;
-                        return true;
-                    }
-                    MailboxOverflow::DropOldest => {
-                        slot.mailbox.pop_front();
-                        self.drops_this_tick += 1;
-                    }
+            match swactor_core::mailbox_accept(slot.mailbox.len(), slot.mailbox_capacity, slot.overflow_policy) {
+                swactor_core::MailboxDecision::Accept => {}
+                swactor_core::MailboxDecision::RejectNewest => {
+                    self.drops_this_tick += 1;
+                    return true;
+                }
+                swactor_core::MailboxDecision::EvictOldest => {
+                    slot.mailbox.pop_front();
+                    self.drops_this_tick += 1;
                 }
             }
             slot.mailbox.push_back(msg);
@@ -512,7 +535,7 @@ impl ActorPool {
 
     /// Take and reset the drop counter for this tick.
     pub fn take_drops(&mut self) -> usize {
-        std::mem::replace(&mut self.drops_this_tick, 0)
+        core::mem::replace(&mut self.drops_this_tick, 0)
     }
 
     /// Tick all actors in the pool. Returns the number of messages processed.
@@ -530,14 +553,11 @@ impl ActorPool {
     ) -> usize {
         let mut count = 0;
         for (&addr, slot) in self.actors.iter_mut() {
-            if slot.poisoned || slot.stopping {
-                // Discard all messages for poisoned/stopping actors
-                slot.mailbox.clear();
-                continue;
-            }
-
-            // Skip suspended actors — messages keep queueing
-            if slot.suspended {
+            if !swactor_core::should_tick_actor(slot.phase) {
+                // Discard mailbox for poisoned/stopping; suspended keeps queueing
+                if slot.phase == ActorPhase::Poisoned || slot.phase == ActorPhase::Stopping {
+                    slot.mailbox.clear();
+                }
                 continue;
             }
 
@@ -554,26 +574,30 @@ impl ActorPool {
             let ctx = Ctx::new(inner, addr, slot.parent_addr, slot.env.clone(), snap_processed, snap_depth, snap_type_counts);
 
             // Call on_start once, before first message
-            if !slot.started {
+            if slot.phase == ActorPhase::Unstarted {
+                #[cfg(feature = "std")]
                 let start_result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                     slot.actor.on_start(&ctx);
                 }));
-                slot.started = true;
+                #[cfg(not(feature = "std"))]
+                let start_result: Result<(), ()> = { slot.actor.on_start(&ctx); Ok(()) };
                 if start_result.is_err() {
                     stats.panics.fetch_add(1, Ordering::Relaxed);
+                    #[cfg(feature = "std")]
                     eprintln!("swactor: actor {addr} panicked in on_start — poisoned");
                     #[cfg(feature = "tracing")]
                     tracing::error!(actor_addr = %addr, "actor.on_start_panicked");
-                    slot.poisoned = true;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StartPanicked);
                     slot.mailbox.clear();
                     continue;
                 }
+                slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::Started);
                 // Check if on_start requested stop or stop_with
                 {
                     let stops = stop_requests.borrow();
                     if !stops.is_empty() && stops.contains(&addr) {
                         drop(stops);
-                        slot.stopping = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopRequested);
                         stats.stops.fetch_add(1, Ordering::Relaxed);
                         slot.mailbox.clear();
                         // Check for stop_with value
@@ -591,7 +615,7 @@ impl ActorPool {
                     if let Some(pos) = sws.iter().position(|(a, _)| *a == addr) {
                         let (_, val) = sws.swap_remove(pos);
                         slot.exit_value = Some(val);
-                        slot.stopping = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopWithRequested);
                         stats.stops.fetch_add(1, Ordering::Relaxed);
                         slot.mailbox.clear();
                         continue;
@@ -602,7 +626,7 @@ impl ActorPool {
                     let suspends = suspend_requests.borrow();
                     if !suspends.is_empty() && suspends.contains(&addr) {
                         drop(suspends);
-                        slot.suspended = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::SuspendRequested);
                         continue;
                     }
                 }
@@ -612,7 +636,7 @@ impl ActorPool {
             while let Some(msg) = slot.mailbox.pop_front() {
                 // Intercept StopSignal (from external runtime.stop_actor)
                 if msg.is::<StopSignal>() {
-                    slot.stopping = true;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopRequested);
                     stats.stops.fetch_add(1, Ordering::Relaxed);
                     slot.mailbox.clear();
                     #[cfg(feature = "tracing")]
@@ -625,15 +649,18 @@ impl ActorPool {
                     if let Ok(sig) = msg.downcast::<StopWithSignal>() {
                         slot.exit_value = Some(sig.0);
                     }
-                    slot.stopping = true;
+                    slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopWithRequested);
                     stats.stops.fetch_add(1, Ordering::Relaxed);
                     slot.mailbox.clear();
                     break;
                 }
 
+                #[cfg(feature = "std")]
                 let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                     slot.actor.handle_any(&ctx, msg)
                 }));
+                #[cfg(not(feature = "std"))]
+                let result: Result<Option<&'static str>, ()> = Ok(slot.actor.handle_any(&ctx, msg));
                 match result {
                     Ok(None) => {
                         stats.type_mismatches.fetch_add(1, Ordering::Relaxed);
@@ -641,10 +668,11 @@ impl ActorPool {
                     Err(_) => {
                         stats.panics.fetch_add(1, Ordering::Relaxed);
                         slot.mailbox.clear();
+                        #[cfg(feature = "std")]
                         eprintln!("swactor: actor {addr} panicked — poisoned, future messages will be discarded");
                         #[cfg(feature = "tracing")]
                         tracing::error!(actor_addr = %addr, "actor.panicked");
-                        slot.poisoned = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::MessagePanicked);
                         slot.mailbox.clear();
                         break;
                     }
@@ -675,7 +703,7 @@ impl ActorPool {
                             slot.exit_value = Some(val);
                         }
                         drop(sws);
-                        slot.stopping = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::StopRequested);
                         stats.stops.fetch_add(1, Ordering::Relaxed);
                         slot.mailbox.clear();
                         break;
@@ -688,12 +716,12 @@ impl ActorPool {
                     let suspends = suspend_requests.borrow();
                     if !suspends.is_empty() && suspends.contains(&addr) {
                         drop(suspends);
-                        slot.suspended = true;
+                        slot.phase = lifecycle_transition(slot.phase, LifecycleEvent::SuspendRequested);
                         break; // stop processing this actor's messages this tick
                     }
                 }
 
-                if budget > 0 && actor_count >= budget {
+                if swactor_core::budget_exhausted(actor_count, budget) {
                     break;
                 }
             }
@@ -723,21 +751,15 @@ impl ActorPool {
         let dead_addrs: Vec<ActorAddress> = self
             .actors
             .iter()
-            .filter(|(_, slot)| slot.poisoned || slot.stopping)
+            .filter(|(_, slot)| slot.phase == ActorPhase::Poisoned || slot.phase == ActorPhase::Stopping)
             .map(|(&addr, _)| addr)
             .collect();
         let mut dead = Vec::with_capacity(dead_addrs.len());
         for addr in dead_addrs {
             if let Some(mut slot) = self.actors.remove(&addr) {
-                let reason = if slot.poisoned {
-                    StopReason::Panicked
-                } else if slot.exit_value.is_some() {
-                    StopReason::Completed
-                } else {
-                    StopReason::Normal
-                };
+                let reason = swactor_core::cleanup_stop_reason(slot.phase, slot.exit_value.is_some());
                 // Call on_stop for gracefully stopping actors only
-                if slot.stopping && !slot.poisoned {
+                if slot.phase == ActorPhase::Stopping {
                     let mut type_counts: Vec<(&'static str, u64)> =
                         slot.msg_type_counts.iter().map(|(&k, &v)| (k, v)).collect();
                     type_counts.sort_by(|a, b| b.1.cmp(&a.1));
@@ -747,9 +769,12 @@ impl ActorPool {
                         slot.mailbox.len(),
                         type_counts,
                     );
-                    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+                    #[cfg(feature = "std")]
+                    { let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
                         slot.actor.on_stop(&ctx);
-                    }));
+                    })); }
+                    #[cfg(not(feature = "std"))]
+                    { slot.actor.on_stop(&ctx); }
                 }
                 dead.push((addr, reason, slot.exit_value.take()));
                 // slot is dropped here — actor resources freed
@@ -770,7 +795,7 @@ impl ActorPool {
                 mailbox_depth: slot.mailbox.len(),
                 last_msg_type: slot.last_msg_type,
                 messages_processed: slot.messages_processed,
-                poisoned: slot.poisoned,
+                poisoned: slot.phase == ActorPhase::Poisoned,
                 message_type_counts: type_counts,
             }
         }));