diff --git a/wfe-core/Cargo.toml b/wfe-core/Cargo.toml
new file mode 100644
index 0000000..c74b99b
--- /dev/null
+++ b/wfe-core/Cargo.toml
@@ -0,0 +1,26 @@
+[package]
+name = "wfe-core"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Core traits, models, builder, and executor for the WFE workflow engine"
+
+[features]
+default = []
+test-support = []
+
+[dependencies]
+tokio = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+async-trait = { workspace = true }
+uuid = { workspace = true }
+chrono = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }
+tokio-util = "0.7"
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+rstest = { workspace = true }
+tokio = { workspace = true, features = ["test-util"] }
diff --git a/wfe-core/src/error.rs b/wfe-core/src/error.rs
new file mode 100644
index 0000000..2700580
--- /dev/null
+++ b/wfe-core/src/error.rs
@@ -0,0 +1,39 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum WfeError {
+    #[error("Workflow not found: {0}")]
+    WorkflowNotFound(String),
+
+    #[error("Workflow definition not found: {id} v{version}")]
+    DefinitionNotFound { id: String, version: u32 },
+
+    #[error("Event not found: {0}")]
+    EventNotFound(String),
+
+    #[error("Subscription not found: {0}")]
+    SubscriptionNotFound(String),
+
+    #[error("Step not found: {0}")]
+    StepNotFound(usize),
+
+    #[error("Lock acquisition failed for: {0}")]
+    LockFailed(String),
+
+    #[error("Persistence error: {0}")]
+    Persistence(String),
+
+    #[error("Serialization error: {0}")]
+    Serialization(#[from] serde_json::Error),
+
+    #[error("Step execution error: {0}")]
+    StepExecution(String),
+
+    #[error("Workflow cancelled")]
+    Cancelled,
+
+    #[error(transparent)]
+    Other(#[from] Box<dyn std::error::Error + Send + Sync>),
+}
+
+pub type Result<T> = std::result::Result<T, WfeError>;
diff --git a/wfe-core/src/lib.rs b/wfe-core/src/lib.rs
new file mode 100644
index 0000000..73ca959
--- /dev/null
+++ b/wfe-core/src/lib.rs
@@ -0,0 +1,11 @@
+pub mod builder;
+pub mod error;
+pub mod executor;
+pub mod models;
+pub mod primitives;
+pub mod traits;
+
+#[cfg(any(test, feature = "test-support"))]
+pub mod test_support;
+
+pub use error::{Result, WfeError};
diff --git a/wfe-core/src/models/error_behavior.rs b/wfe-core/src/models/error_behavior.rs
new file mode 100644
index 0000000..68917b9
--- /dev/null
+++ b/wfe-core/src/models/error_behavior.rs
@@ -0,0 +1,80 @@
+use std::time::Duration;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum ErrorBehavior {
+    Retry {
+        #[serde(with = "duration_millis")]
+        interval: Duration,
+        #[serde(default = "default_max_retries")]
+        max_retries: u32,
+    },
+    Suspend,
+    Terminate,
+    Compensate,
+}
+
+fn default_max_retries() -> u32 {
+    3
+}
+
+impl Default for ErrorBehavior {
+    fn default() -> Self {
+        Self::Retry {
+            interval: Duration::from_secs(60),
+            max_retries: default_max_retries(),
+        }
+    }
+}
+
+mod duration_millis {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Deserializer, Serializer};
+
+    pub fn serialize<S: Serializer>(duration: &Duration, serializer: S) -> Result<S::Ok, S::Error> {
+        serializer.serialize_u64(duration.as_millis() as u64)
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Duration, D::Error> {
+        let millis = u64::deserialize(deserializer)?;
+        Ok(Duration::from_millis(millis))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn default_is_retry_60s() {
+        let behavior = ErrorBehavior::default();
+        assert_eq!(
+            behavior,
+            ErrorBehavior::Retry {
+                interval: Duration::from_secs(60),
+                max_retries: 3,
+            }
+        );
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let variants = vec![
+            ErrorBehavior::Retry {
+                interval: Duration::from_secs(30),
+                max_retries: 3,
+            },
+            ErrorBehavior::Suspend,
+            ErrorBehavior::Terminate,
+            ErrorBehavior::Compensate,
+        ];
+        for variant in variants {
+            let json = serde_json::to_string(&variant).unwrap();
+            let deserialized: ErrorBehavior = serde_json::from_str(&json).unwrap();
+            assert_eq!(variant, deserialized);
+        }
+    }
+}
diff --git a/wfe-core/src/models/event.rs b/wfe-core/src/models/event.rs
new file mode 100644
index 0000000..484ec9d
--- /dev/null
+++ b/wfe-core/src/models/event.rs
@@ -0,0 +1,119 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Event {
+    pub id: String,
+    pub event_name: String,
+    pub event_key: String,
+    pub event_data: serde_json::Value,
+    pub event_time: DateTime<Utc>,
+    pub is_processed: bool,
+}
+
+impl Event {
+    pub fn new(
+        event_name: impl Into<String>,
+        event_key: impl Into<String>,
+        event_data: serde_json::Value,
+    ) -> Self {
+        Self {
+            id: uuid::Uuid::new_v4().to_string(),
+            event_name: event_name.into(),
+            event_key: event_key.into(),
+            event_data,
+            event_time: Utc::now(),
+            is_processed: false,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct EventSubscription {
+    pub id: String,
+    pub workflow_id: String,
+    pub step_id: usize,
+    pub execution_pointer_id: String,
+    pub event_name: String,
+    pub event_key: String,
+    pub subscribe_as_of: DateTime<Utc>,
+    pub subscription_data: Option<serde_json::Value>,
+    pub external_token: Option<String>,
+    pub external_worker_id: Option<String>,
+    pub external_token_expiry: Option<DateTime<Utc>>,
+}
+
+impl EventSubscription {
+    pub fn new(
+        workflow_id: impl Into<String>,
+        step_id: usize,
+        execution_pointer_id: impl Into<String>,
+        event_name: impl Into<String>,
+        event_key: impl Into<String>,
+        subscribe_as_of: DateTime<Utc>,
+    ) -> Self {
+        Self {
+            id: uuid::Uuid::new_v4().to_string(),
+            workflow_id: workflow_id.into(),
+            step_id,
+            execution_pointer_id: execution_pointer_id.into(),
+            event_name: event_name.into(),
+            event_key: event_key.into(),
+            subscribe_as_of,
+            subscription_data: None,
+            external_token: None,
+            external_worker_id: None,
+            external_token_expiry: None,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn new_event_defaults() {
+        let event = Event::new("order.created", "order-456", serde_json::json!({"amount": 100}));
+        assert_eq!(event.event_name, "order.created");
+        assert_eq!(event.event_key, "order-456");
+        assert!(!event.is_processed);
+    }
+
+    #[test]
+    fn new_event_generates_unique_ids() {
+        let e1 = Event::new("test", "key", serde_json::json!(null));
+        let e2 = Event::new("test", "key", serde_json::json!(null));
+        assert_ne!(e1.id, e2.id);
+    }
+
+    #[test]
+    fn event_serde_round_trip() {
+        let event = Event::new("test", "key", serde_json::json!({"data": true}));
+        let json = serde_json::to_string(&event).unwrap();
+        let deserialized: Event = serde_json::from_str(&json).unwrap();
+        assert_eq!(event.id, deserialized.id);
+        assert_eq!(event.event_name, deserialized.event_name);
+        assert_eq!(event.event_data, deserialized.event_data);
+    }
+
+    #[test]
+    fn new_subscription_defaults() {
+        let sub = EventSubscription::new("wf-1", 0, "ptr-1", "evt", "key", Utc::now());
+        assert_eq!(sub.workflow_id, "wf-1");
+        assert_eq!(sub.step_id, 0);
+        assert!(sub.external_token.is_none());
+        assert!(sub.subscription_data.is_none());
+    }
+
+    #[test]
+    fn subscription_serde_round_trip() {
+        let sub = EventSubscription::new("wf-1", 2, "ptr-1", "evt", "key", Utc::now());
+        let json = serde_json::to_string(&sub).unwrap();
+        let deserialized: EventSubscription = serde_json::from_str(&json).unwrap();
+        assert_eq!(sub.id, deserialized.id);
+        assert_eq!(sub.workflow_id, deserialized.workflow_id);
+        assert_eq!(sub.event_name, deserialized.event_name);
+    }
+}
diff --git a/wfe-core/src/models/execution_error.rs b/wfe-core/src/models/execution_error.rs
new file mode 100644
index 0000000..73ac619
--- /dev/null
+++ b/wfe-core/src/models/execution_error.rs
@@ -0,0 +1,48 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ExecutionError {
+    pub error_time: DateTime<Utc>,
+    pub workflow_id: String,
+    pub execution_pointer_id: String,
+    pub message: String,
+}
+
+impl ExecutionError {
+    pub fn new(
+        workflow_id: impl Into<String>,
+        execution_pointer_id: impl Into<String>,
+        message: impl Into<String>,
+    ) -> Self {
+        Self {
+            error_time: Utc::now(),
+            workflow_id: workflow_id.into(),
+            execution_pointer_id: execution_pointer_id.into(),
+            message: message.into(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn new_error_captures_fields() {
+        let err = ExecutionError::new("wf-1", "ptr-1", "something went wrong");
+        assert_eq!(err.workflow_id, "wf-1");
+        assert_eq!(err.execution_pointer_id, "ptr-1");
+        assert_eq!(err.message, "something went wrong");
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let err = ExecutionError::new("wf-1", "ptr-1", "fail");
+        let json = serde_json::to_string(&err).unwrap();
+        let deserialized: ExecutionError = serde_json::from_str(&json).unwrap();
+        assert_eq!(err.workflow_id, deserialized.workflow_id);
+        assert_eq!(err.message, deserialized.message);
+    }
+}
diff --git a/wfe-core/src/models/execution_pointer.rs b/wfe-core/src/models/execution_pointer.rs
new file mode 100644
index 0000000..2ed4714
--- /dev/null
+++ b/wfe-core/src/models/execution_pointer.rs
@@ -0,0 +1,101 @@
+use std::collections::HashMap;
+
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+use super::status::PointerStatus;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ExecutionPointer {
+    pub id: String,
+    pub step_id: usize,
+    pub active: bool,
+    pub status: PointerStatus,
+    pub sleep_until: Option<DateTime<Utc>>,
+    pub persistence_data: Option<serde_json::Value>,
+    pub start_time: Option<DateTime<Utc>>,
+    pub end_time: Option<DateTime<Utc>>,
+    pub event_name: Option<String>,
+    pub event_key: Option<String>,
+    pub event_published: bool,
+    pub event_data: Option<serde_json::Value>,
+    pub step_name: Option<String>,
+    pub retry_count: u32,
+    pub children: Vec<String>,
+    pub context_item: Option<serde_json::Value>,
+    pub predecessor_id: Option<String>,
+    pub outcome: Option<serde_json::Value>,
+    pub scope: Vec<String>,
+    pub extension_attributes: HashMap<String, serde_json::Value>,
+}
+
+impl ExecutionPointer {
+    pub fn new(step_id: usize) -> Self {
+        Self {
+            id: uuid::Uuid::new_v4().to_string(),
+            step_id,
+            active: true,
+            status: PointerStatus::Pending,
+            sleep_until: None,
+            persistence_data: None,
+            start_time: None,
+            end_time: None,
+            event_name: None,
+            event_key: None,
+            event_published: false,
+            event_data: None,
+            step_name: None,
+            retry_count: 0,
+            children: Vec::new(),
+            context_item: None,
+            predecessor_id: None,
+            outcome: None,
+            scope: Vec::new(),
+            extension_attributes: HashMap::new(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn new_pointer_has_correct_defaults() {
+        let pointer = ExecutionPointer::new(0);
+        assert_eq!(pointer.step_id, 0);
+        assert!(pointer.active);
+        assert_eq!(pointer.status, PointerStatus::Pending);
+        assert_eq!(pointer.retry_count, 0);
+        assert!(pointer.children.is_empty());
+        assert!(pointer.scope.is_empty());
+        assert!(!pointer.event_published);
+    }
+
+    #[test]
+    fn new_pointer_generates_unique_ids() {
+        let p1 = ExecutionPointer::new(0);
+        let p2 = ExecutionPointer::new(0);
+        assert_ne!(p1.id, p2.id);
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let mut pointer = ExecutionPointer::new(3);
+        pointer.status = PointerStatus::Running;
+        pointer.retry_count = 2;
+        pointer.persistence_data = Some(serde_json::json!({"step_state": true}));
+        pointer.children = vec!["child-1".into(), "child-2".into()];
+
+        let json = serde_json::to_string(&pointer).unwrap();
+        let deserialized: ExecutionPointer = serde_json::from_str(&json).unwrap();
+
+        assert_eq!(pointer.id, deserialized.id);
+        assert_eq!(pointer.step_id, deserialized.step_id);
+        assert_eq!(pointer.status, deserialized.status);
+        assert_eq!(pointer.retry_count, deserialized.retry_count);
+        assert_eq!(pointer.persistence_data, deserialized.persistence_data);
+        assert_eq!(pointer.children, deserialized.children);
+    }
+}
diff --git a/wfe-core/src/models/execution_result.rs b/wfe-core/src/models/execution_result.rs
new file mode 100644
index 0000000..9551e53
--- /dev/null
+++ b/wfe-core/src/models/execution_result.rs
@@ -0,0 +1,188 @@
+use std::time::Duration;
+
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+use super::poll_config::PollEndpointConfig;
+
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct ExecutionResult {
+    /// Whether the workflow should proceed to the next step.
+    pub proceed: bool,
+    /// Outcome value for decision-based routing.
+    pub outcome_value: Option<serde_json::Value>,
+    /// Duration to sleep before re-executing.
+    #[serde(default, with = "super::option_duration_millis")]
+    pub sleep_for: Option<Duration>,
+    /// Step-specific state to persist between executions.
+    pub persistence_data: Option<serde_json::Value>,
+    /// Event name to wait for.
+    pub event_name: Option<String>,
+    /// Event key to match.
+    pub event_key: Option<String>,
+    /// Only consider events published after this time.
+    pub event_as_of: Option<DateTime<Utc>>,
+    /// Values to branch execution on (for ForEach/Parallel).
+    pub branch_values: Option<Vec<serde_json::Value>>,
+    /// Poll endpoint configuration for external service polling.
+    pub poll_endpoint: Option<PollEndpointConfig>,
+}
+
+impl ExecutionResult {
+    /// Continue to the next step.
+    pub fn next() -> Self {
+        Self {
+            proceed: true,
+            ..Default::default()
+        }
+    }
+
+    /// Continue with an outcome value for decision routing.
+    pub fn outcome(value: impl Into<serde_json::Value>) -> Self {
+        Self {
+            proceed: true,
+            outcome_value: Some(value.into()),
+            ..Default::default()
+        }
+    }
+
+    /// Pause execution and persist step-specific data.
+    pub fn persist(data: serde_json::Value) -> Self {
+        Self {
+            proceed: false,
+            persistence_data: Some(data),
+            ..Default::default()
+        }
+    }
+
+    /// Create child branches for parallel/foreach execution.
+    pub fn branch(values: Vec<serde_json::Value>, persistence_data: Option<serde_json::Value>) -> Self {
+        Self {
+            proceed: false,
+            branch_values: Some(values),
+            persistence_data,
+            ..Default::default()
+        }
+    }
+
+    /// Sleep for a duration before re-executing.
+    pub fn sleep(duration: Duration, persistence_data: Option<serde_json::Value>) -> Self {
+        Self {
+            proceed: false,
+            sleep_for: Some(duration),
+            persistence_data,
+            ..Default::default()
+        }
+    }
+
+    /// Wait for an external event.
+    pub fn wait_for_event(
+        event_name: impl Into<String>,
+        event_key: impl Into<String>,
+        as_of: DateTime<Utc>,
+    ) -> Self {
+        Self {
+            proceed: false,
+            event_name: Some(event_name.into()),
+            event_key: Some(event_key.into()),
+            event_as_of: Some(as_of),
+            ..Default::default()
+        }
+    }
+
+    /// Poll an external endpoint until a condition is met.
+    pub fn poll_endpoint(config: PollEndpointConfig) -> Self {
+        Self {
+            proceed: false,
+            poll_endpoint: Some(config),
+            ..Default::default()
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn next_proceeds_with_no_data() {
+        let result = ExecutionResult::next();
+        assert!(result.proceed);
+        assert!(result.outcome_value.is_none());
+        assert!(result.sleep_for.is_none());
+        assert!(result.persistence_data.is_none());
+        assert!(result.event_name.is_none());
+        assert!(result.branch_values.is_none());
+        assert!(result.poll_endpoint.is_none());
+    }
+
+    #[test]
+    fn outcome_proceeds_with_value() {
+        let result = ExecutionResult::outcome(serde_json::json!(42));
+        assert!(result.proceed);
+        assert_eq!(result.outcome_value, Some(serde_json::json!(42)));
+    }
+
+    #[test]
+    fn persist_does_not_proceed() {
+        let data = serde_json::json!({"counter": 5});
+        let result = ExecutionResult::persist(data.clone());
+        assert!(!result.proceed);
+        assert_eq!(result.persistence_data, Some(data));
+    }
+
+    #[test]
+    fn branch_creates_child_values() {
+        let values = vec![serde_json::json!(1), serde_json::json!(2), serde_json::json!(3)];
+        let result = ExecutionResult::branch(values.clone(), None);
+        assert!(!result.proceed);
+        assert_eq!(result.branch_values, Some(values));
+    }
+
+    #[test]
+    fn sleep_sets_duration() {
+        let result = ExecutionResult::sleep(Duration::from_secs(30), None);
+        assert!(!result.proceed);
+        assert_eq!(result.sleep_for, Some(Duration::from_secs(30)));
+    }
+
+    #[test]
+    fn wait_for_event_sets_event_fields() {
+        let now = Utc::now();
+        let result = ExecutionResult::wait_for_event("order.completed", "order-123", now);
+        assert!(!result.proceed);
+        assert_eq!(result.event_name.as_deref(), Some("order.completed"));
+        assert_eq!(result.event_key.as_deref(), Some("order-123"));
+        assert_eq!(result.event_as_of, Some(now));
+    }
+
+    #[test]
+    fn poll_endpoint_sets_config() {
+        use super::super::poll_config::*;
+        use std::collections::HashMap;
+
+        let config = PollEndpointConfig {
+            url: "https://api.example.com/status".into(),
+            method: HttpMethod::Get,
+            headers: HashMap::new(),
+            body: None,
+            interval: Duration::from_secs(10),
+            timeout: Duration::from_secs(300),
+            condition: PollCondition::StatusCode(200),
+        };
+        let result = ExecutionResult::poll_endpoint(config.clone());
+        assert!(!result.proceed);
+        assert_eq!(result.poll_endpoint, Some(config));
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let result = ExecutionResult::sleep(Duration::from_secs(30), Some(serde_json::json!({"x": 1})));
+        let json = serde_json::to_string(&result).unwrap();
+        let deserialized: ExecutionResult = serde_json::from_str(&json).unwrap();
+        assert_eq!(result.proceed, deserialized.proceed);
+        assert_eq!(result.sleep_for, deserialized.sleep_for);
+        assert_eq!(result.persistence_data, deserialized.persistence_data);
+    }
+}
diff --git a/wfe-core/src/models/lifecycle.rs b/wfe-core/src/models/lifecycle.rs
new file mode 100644
index 0000000..880cb55
--- /dev/null
+++ b/wfe-core/src/models/lifecycle.rs
@@ -0,0 +1,104 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LifecycleEvent {
+    pub event_time_utc: DateTime<Utc>,
+    pub workflow_instance_id: String,
+    pub workflow_definition_id: String,
+    pub version: u32,
+    pub reference: Option<String>,
+    pub event_type: LifecycleEventType,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum LifecycleEventType {
+    Started,
+    Resumed,
+    Suspended,
+    Completed,
+    Terminated,
+    Error { message: String },
+    StepStarted { step_id: usize, step_name: Option<String> },
+    StepCompleted { step_id: usize, step_name: Option<String> },
+}
+
+impl LifecycleEvent {
+    pub fn new(
+        workflow_instance_id: impl Into<String>,
+        workflow_definition_id: impl Into<String>,
+        version: u32,
+        event_type: LifecycleEventType,
+    ) -> Self {
+        Self {
+            event_time_utc: Utc::now(),
+            workflow_instance_id: workflow_instance_id.into(),
+            workflow_definition_id: workflow_definition_id.into(),
+            version,
+            reference: None,
+            event_type,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn lifecycle_event_types_are_distinct() {
+        assert_ne!(LifecycleEventType::Started, LifecycleEventType::Completed);
+    }
+
+    #[test]
+    fn serde_round_trip_simple_variant() {
+        let event = LifecycleEvent::new("wf-1", "def-1", 1, LifecycleEventType::Started);
+        let json = serde_json::to_string(&event).unwrap();
+        let deserialized: LifecycleEvent = serde_json::from_str(&json).unwrap();
+        assert_eq!(event.workflow_instance_id, deserialized.workflow_instance_id);
+        assert_eq!(event.event_type, deserialized.event_type);
+    }
+
+    #[test]
+    fn serde_round_trip_error_variant() {
+        let event = LifecycleEvent::new(
+            "wf-1",
+            "def-1",
+            1,
+            LifecycleEventType::Error {
+                message: "boom".into(),
+            },
+        );
+        let json = serde_json::to_string(&event).unwrap();
+        let deserialized: LifecycleEvent = serde_json::from_str(&json).unwrap();
+        assert_eq!(
+            deserialized.event_type,
+            LifecycleEventType::Error {
+                message: "boom".into()
+            }
+        );
+    }
+
+    #[test]
+    fn serde_round_trip_step_variant() {
+        let event = LifecycleEvent::new(
+            "wf-1",
+            "def-1",
+            1,
+            LifecycleEventType::StepStarted {
+                step_id: 3,
+                step_name: Some("ProcessOrder".into()),
+            },
+        );
+        let json = serde_json::to_string(&event).unwrap();
+        let deserialized: LifecycleEvent = serde_json::from_str(&json).unwrap();
+        assert_eq!(
+            deserialized.event_type,
+            LifecycleEventType::StepStarted {
+                step_id: 3,
+                step_name: Some("ProcessOrder".into()),
+            }
+        );
+    }
+}
diff --git a/wfe-core/src/models/mod.rs b/wfe-core/src/models/mod.rs
new file mode 100644
index 0000000..169099f
--- /dev/null
+++ b/wfe-core/src/models/mod.rs
@@ -0,0 +1,65 @@
+pub mod error_behavior;
+pub mod event;
+pub mod execution_error;
+pub mod execution_pointer;
+pub mod execution_result;
+pub mod lifecycle;
+pub mod poll_config;
+pub mod queue_type;
+pub mod scheduled_command;
+pub mod status;
+pub mod workflow_definition;
+pub mod workflow_instance;
+
+pub use error_behavior::ErrorBehavior;
+pub use event::{Event, EventSubscription};
+pub use execution_error::ExecutionError;
+pub use execution_pointer::ExecutionPointer;
+pub use execution_result::ExecutionResult;
+pub use lifecycle::{LifecycleEvent, LifecycleEventType};
+pub use poll_config::{HttpMethod, PollCondition, PollEndpointConfig};
+pub use queue_type::QueueType;
+pub use scheduled_command::{CommandName, ScheduledCommand};
+pub use status::{PointerStatus, WorkflowStatus};
+pub use workflow_definition::{StepOutcome, WorkflowDefinition, WorkflowStep};
+pub use workflow_instance::WorkflowInstance;
+
+/// Serde helper for `Option<Duration>` as milliseconds.
+pub(crate) mod option_duration_millis {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Deserializer, Serializer};
+
+    pub fn serialize<S: Serializer>(
+        duration: &Option<Duration>,
+        serializer: S,
+    ) -> Result<S::Ok, S::Error> {
+        match duration {
+            Some(d) => serializer.serialize_some(&(d.as_millis() as u64)),
+            None => serializer.serialize_none(),
+        }
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(
+        deserializer: D,
+    ) -> Result<Option<Duration>, D::Error> {
+        let millis: Option<u64> = Option::deserialize(deserializer)?;
+        Ok(millis.map(Duration::from_millis))
+    }
+}
+
+/// Serde helper for `Duration` as milliseconds (non-optional).
+pub(crate) mod duration_millis {
+    use std::time::Duration;
+
+    use serde::{Deserialize, Deserializer, Serializer};
+
+    pub fn serialize<S: Serializer>(duration: &Duration, serializer: S) -> Result<S::Ok, S::Error> {
+        serializer.serialize_u64(duration.as_millis() as u64)
+    }
+
+    pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<Duration, D::Error> {
+        let millis = u64::deserialize(deserializer)?;
+        Ok(Duration::from_millis(millis))
+    }
+}
diff --git a/wfe-core/src/models/poll_config.rs b/wfe-core/src/models/poll_config.rs
new file mode 100644
index 0000000..b8e7aa7
--- /dev/null
+++ b/wfe-core/src/models/poll_config.rs
@@ -0,0 +1,83 @@
+use std::collections::HashMap;
+use std::time::Duration;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum HttpMethod {
+    Get,
+    Post,
+    Put,
+    Patch,
+    Delete,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum PollCondition {
+    /// Check a JSON path equals a value: e.g. JsonPathEquals("$.status", "complete")
+    JsonPathEquals {
+        path: String,
+        value: serde_json::Value,
+    },
+    /// Check HTTP status code
+    StatusCode(u16),
+    /// Check response body contains string
+    BodyContains(String),
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PollEndpointConfig {
+    /// URL template. Supports `{placeholder}` interpolation from workflow data.
+    pub url: String,
+    pub method: HttpMethod,
+    #[serde(default)]
+    pub headers: HashMap<String, String>,
+    #[serde(default)]
+    pub body: Option<serde_json::Value>,
+    #[serde(with = "super::duration_millis")]
+    pub interval: Duration,
+    #[serde(with = "super::duration_millis")]
+    pub timeout: Duration,
+    pub condition: PollCondition,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn poll_config_serde_round_trip() {
+        let config = PollEndpointConfig {
+            url: "https://api.example.com/status/{id}".into(),
+            method: HttpMethod::Get,
+            headers: HashMap::from([("Authorization".into(), "Bearer token123".into())]),
+            body: None,
+            interval: Duration::from_secs(30),
+            timeout: Duration::from_secs(3600),
+            condition: PollCondition::JsonPathEquals {
+                path: "$.status".into(),
+                value: serde_json::Value::String("complete".into()),
+            },
+        };
+        let json = serde_json::to_string(&config).unwrap();
+        let deserialized: PollEndpointConfig = serde_json::from_str(&json).unwrap();
+        assert_eq!(config, deserialized);
+    }
+
+    #[test]
+    fn poll_condition_status_code() {
+        let cond = PollCondition::StatusCode(200);
+        let json = serde_json::to_string(&cond).unwrap();
+        let deserialized: PollCondition = serde_json::from_str(&json).unwrap();
+        assert_eq!(cond, deserialized);
+    }
+
+    #[test]
+    fn poll_condition_body_contains() {
+        let cond = PollCondition::BodyContains("success".into());
+        let json = serde_json::to_string(&cond).unwrap();
+        let deserialized: PollCondition = serde_json::from_str(&json).unwrap();
+        assert_eq!(cond, deserialized);
+    }
+}
diff --git a/wfe-core/src/models/queue_type.rs b/wfe-core/src/models/queue_type.rs
new file mode 100644
index 0000000..c2804e1
--- /dev/null
+++ b/wfe-core/src/models/queue_type.rs
@@ -0,0 +1,30 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum QueueType {
+    Workflow,
+    Event,
+    Index,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn queue_types_are_distinct() {
+        assert_ne!(QueueType::Workflow, QueueType::Event);
+        assert_ne!(QueueType::Event, QueueType::Index);
+        assert_ne!(QueueType::Workflow, QueueType::Index);
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        for qt in [QueueType::Workflow, QueueType::Event, QueueType::Index] {
+            let json = serde_json::to_string(&qt).unwrap();
+            let deserialized: QueueType = serde_json::from_str(&json).unwrap();
+            assert_eq!(qt, deserialized);
+        }
+    }
+}
diff --git a/wfe-core/src/models/scheduled_command.rs b/wfe-core/src/models/scheduled_command.rs
new file mode 100644
index 0000000..dfc18ab
--- /dev/null
+++ b/wfe-core/src/models/scheduled_command.rs
@@ -0,0 +1,65 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum CommandName {
+    ProcessWorkflow,
+    ProcessEvent,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ScheduledCommand {
+    pub command_name: CommandName,
+    pub data: String,
+    /// Epoch milliseconds when the command should execute.
+    pub execute_time: i64,
+}
+
+impl ScheduledCommand {
+    pub fn process_workflow(workflow_id: impl Into<String>, execute_time: i64) -> Self {
+        Self {
+            command_name: CommandName::ProcessWorkflow,
+            data: workflow_id.into(),
+            execute_time,
+        }
+    }
+
+    pub fn process_event(event_id: impl Into<String>, execute_time: i64) -> Self {
+        Self {
+            command_name: CommandName::ProcessEvent,
+            data: event_id.into(),
+            execute_time,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn process_workflow_factory() {
+        let cmd = ScheduledCommand::process_workflow("wf-123", 1000);
+        assert_eq!(cmd.command_name, CommandName::ProcessWorkflow);
+        assert_eq!(cmd.data, "wf-123");
+        assert_eq!(cmd.execute_time, 1000);
+    }
+
+    #[test]
+    fn process_event_factory() {
+        let cmd = ScheduledCommand::process_event("evt-456", 2000);
+        assert_eq!(cmd.command_name, CommandName::ProcessEvent);
+        assert_eq!(cmd.data, "evt-456");
+        assert_eq!(cmd.execute_time, 2000);
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let cmd = ScheduledCommand::process_workflow("wf-1", 500);
+        let json = serde_json::to_string(&cmd).unwrap();
+        let deserialized: ScheduledCommand = serde_json::from_str(&json).unwrap();
+        assert_eq!(cmd.command_name, deserialized.command_name);
+        assert_eq!(cmd.data, deserialized.data);
+        assert_eq!(cmd.execute_time, deserialized.execute_time);
+    }
+}
diff --git a/wfe-core/src/models/status.rs b/wfe-core/src/models/status.rs
new file mode 100644
index 0000000..c76aa3e
--- /dev/null
+++ b/wfe-core/src/models/status.rs
@@ -0,0 +1,73 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum WorkflowStatus {
+    #[default]
+    Runnable,
+    Suspended,
+    Complete,
+    Terminated,
+}
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum PointerStatus {
+    #[default]
+    Pending,
+    Running,
+    Complete,
+    Sleeping,
+    WaitingForEvent,
+    Failed,
+    Compensated,
+    Cancelled,
+    PendingPredecessor,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn workflow_status_default_is_runnable() {
+        assert_eq!(WorkflowStatus::default(), WorkflowStatus::Runnable);
+    }
+
+    #[test]
+    fn pointer_status_default_is_pending() {
+        assert_eq!(PointerStatus::default(), PointerStatus::Pending);
+    }
+
+    #[test]
+    fn workflow_status_serde_round_trip() {
+        for status in [
+            WorkflowStatus::Runnable,
+            WorkflowStatus::Suspended,
+            WorkflowStatus::Complete,
+            WorkflowStatus::Terminated,
+        ] {
+            let json = serde_json::to_string(&status).unwrap();
+            let deserialized: WorkflowStatus = serde_json::from_str(&json).unwrap();
+            assert_eq!(status, deserialized);
+        }
+    }
+
+    #[test]
+    fn pointer_status_serde_round_trip() {
+        for status in [
+            PointerStatus::Pending,
+            PointerStatus::Running,
+            PointerStatus::Complete,
+            PointerStatus::Sleeping,
+            PointerStatus::WaitingForEvent,
+            PointerStatus::Failed,
+            PointerStatus::Compensated,
+            PointerStatus::Cancelled,
+            PointerStatus::PendingPredecessor,
+        ] {
+            let json = serde_json::to_string(&status).unwrap();
+            let deserialized: PointerStatus = serde_json::from_str(&json).unwrap();
+            assert_eq!(status, deserialized);
+        }
+    }
+}
diff --git a/wfe-core/src/models/workflow_definition.rs b/wfe-core/src/models/workflow_definition.rs
new file mode 100644
index 0000000..1ff89ca
--- /dev/null
+++ b/wfe-core/src/models/workflow_definition.rs
@@ -0,0 +1,121 @@
+use std::time::Duration;
+
+use serde::{Deserialize, Serialize};
+
+use super::error_behavior::ErrorBehavior;
+
+/// A compiled workflow definition ready for execution.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct WorkflowDefinition {
+    pub id: String,
+    pub version: u32,
+    pub description: Option<String>,
+    pub steps: Vec<WorkflowStep>,
+    pub default_error_behavior: ErrorBehavior,
+    #[serde(default, with = "super::option_duration_millis")]
+    pub default_error_retry_interval: Option<Duration>,
+}
+
+impl WorkflowDefinition {
+    pub fn new(id: impl Into<String>, version: u32) -> Self {
+        Self {
+            id: id.into(),
+            version,
+            description: None,
+            steps: Vec::new(),
+            default_error_behavior: ErrorBehavior::default(),
+            default_error_retry_interval: None,
+        }
+    }
+}
+
+/// A single step in a workflow definition.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct WorkflowStep {
+    pub id: usize,
+    pub name: Option<String>,
+    pub external_id: Option<String>,
+    pub step_type: String,
+    pub children: Vec<usize>,
+    pub outcomes: Vec<StepOutcome>,
+    pub error_behavior: Option<ErrorBehavior>,
+    pub compensation_step_id: Option<usize>,
+    pub do_compensate: bool,
+    #[serde(default)]
+    pub saga: bool,
+    /// Serializable configuration for primitive steps (e.g. event_name, duration).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub step_config: Option<serde_json::Value>,
+}
+
+impl WorkflowStep {
+    pub fn new(id: usize, step_type: impl Into<String>) -> Self {
+        Self {
+            id,
+            name: None,
+            external_id: None,
+            step_type: step_type.into(),
+            children: Vec::new(),
+            outcomes: Vec::new(),
+            error_behavior: None,
+            compensation_step_id: None,
+            do_compensate: false,
+            saga: false,
+            step_config: None,
+        }
+    }
+}
+
+/// Routing outcome from a step.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct StepOutcome {
+    pub next_step: usize,
+    pub label: Option<String>,
+    pub value: Option<serde_json::Value>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn definition_defaults() {
+        let def = WorkflowDefinition::new("test-workflow", 1);
+        assert_eq!(def.id, "test-workflow");
+        assert_eq!(def.version, 1);
+        assert!(def.steps.is_empty());
+        assert_eq!(def.default_error_behavior, ErrorBehavior::default());
+        assert!(def.default_error_retry_interval.is_none());
+    }
+
+    #[test]
+    fn step_defaults() {
+        let step = WorkflowStep::new(0, "MyStep");
+        assert_eq!(step.id, 0);
+        assert_eq!(step.step_type, "MyStep");
+        assert!(step.children.is_empty());
+        assert!(step.outcomes.is_empty());
+        assert!(step.error_behavior.is_none());
+        assert!(step.compensation_step_id.is_none());
+    }
+
+    #[test]
+    fn definition_serde_round_trip() {
+        let mut def = WorkflowDefinition::new("wf", 3);
+        let mut step = WorkflowStep::new(0, "StepA");
+        step.outcomes.push(StepOutcome {
+            next_step: 1,
+            label: Some("next".into()),
+            value: None,
+        });
+        def.steps.push(step);
+        def.steps.push(WorkflowStep::new(1, "StepB"));
+
+        let json = serde_json::to_string(&def).unwrap();
+        let deserialized: WorkflowDefinition = serde_json::from_str(&json).unwrap();
+        assert_eq!(def.id, deserialized.id);
+        assert_eq!(def.steps.len(), deserialized.steps.len());
+        assert_eq!(def.steps[0].outcomes[0].next_step, 1);
+    }
+}
diff --git a/wfe-core/src/models/workflow_instance.rs b/wfe-core/src/models/workflow_instance.rs
new file mode 100644
index 0000000..9fac9f4
--- /dev/null
+++ b/wfe-core/src/models/workflow_instance.rs
@@ -0,0 +1,141 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+use super::execution_pointer::ExecutionPointer;
+use super::status::{PointerStatus, WorkflowStatus};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct WorkflowInstance {
+    pub id: String,
+    pub workflow_definition_id: String,
+    pub version: u32,
+    pub description: Option<String>,
+    pub reference: Option<String>,
+    pub execution_pointers: Vec<ExecutionPointer>,
+    pub next_execution: Option<i64>,
+    pub status: WorkflowStatus,
+    pub data: serde_json::Value,
+    pub create_time: DateTime<Utc>,
+    pub complete_time: Option<DateTime<Utc>>,
+}
+
+impl WorkflowInstance {
+    pub fn new(workflow_definition_id: impl Into<String>, version: u32, data: serde_json::Value) -> Self {
+        Self {
+            id: uuid::Uuid::new_v4().to_string(),
+            workflow_definition_id: workflow_definition_id.into(),
+            version,
+            description: None,
+            reference: None,
+            execution_pointers: Vec::new(),
+            next_execution: Some(0),
+            status: WorkflowStatus::Runnable,
+            data,
+            create_time: Utc::now(),
+            complete_time: None,
+        }
+    }
+
+    /// Check if all execution pointers in a given scope have completed.
+    pub fn is_branch_complete(&self, scope: &[String]) -> bool {
+        self.execution_pointers
+            .iter()
+            .filter(|p| p.scope == scope)
+            .all(|p| {
+                matches!(
+                    p.status,
+                    PointerStatus::Complete
+                        | PointerStatus::Compensated
+                        | PointerStatus::Cancelled
+                        | PointerStatus::Failed
+                )
+            })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn new_instance_defaults() {
+        let instance = WorkflowInstance::new("test-workflow", 1, serde_json::json!({}));
+        assert_eq!(instance.workflow_definition_id, "test-workflow");
+        assert_eq!(instance.version, 1);
+        assert_eq!(instance.status, WorkflowStatus::Runnable);
+        assert_eq!(instance.next_execution, Some(0));
+        assert!(instance.execution_pointers.is_empty());
+        assert!(instance.complete_time.is_none());
+    }
+
+    #[test]
+    fn is_branch_complete_empty_scope_returns_true() {
+        let instance = WorkflowInstance::new("test", 1, serde_json::json!({}));
+        assert!(instance.is_branch_complete(&[]));
+    }
+
+    #[test]
+    fn is_branch_complete_all_complete() {
+        let scope = vec!["parent-1".to_string()];
+        let mut instance = WorkflowInstance::new("test", 1, serde_json::json!({}));
+
+        let mut p1 = ExecutionPointer::new(0);
+        p1.scope = scope.clone();
+        p1.status = PointerStatus::Complete;
+
+        let mut p2 = ExecutionPointer::new(1);
+        p2.scope = scope.clone();
+        p2.status = PointerStatus::Compensated;
+
+        instance.execution_pointers = vec![p1, p2];
+        assert!(instance.is_branch_complete(&scope));
+    }
+
+    #[test]
+    fn is_branch_complete_with_active_pointer() {
+        let scope = vec!["parent-1".to_string()];
+        let mut instance = WorkflowInstance::new("test", 1, serde_json::json!({}));
+
+        let mut p1 = ExecutionPointer::new(0);
+        p1.scope = scope.clone();
+        p1.status = PointerStatus::Complete;
+
+        let mut p2 = ExecutionPointer::new(1);
+        p2.scope = scope.clone();
+        p2.status = PointerStatus::Running;
+
+        instance.execution_pointers = vec![p1, p2];
+        assert!(!instance.is_branch_complete(&scope));
+    }
+
+    #[test]
+    fn is_branch_complete_ignores_different_scope() {
+        let scope_a = vec!["parent-a".to_string()];
+        let scope_b = vec!["parent-b".to_string()];
+        let mut instance = WorkflowInstance::new("test", 1, serde_json::json!({}));
+
+        let mut p1 = ExecutionPointer::new(0);
+        p1.scope = scope_a.clone();
+        p1.status = PointerStatus::Complete;
+
+        let mut p2 = ExecutionPointer::new(1);
+        p2.scope = scope_b.clone();
+        p2.status = PointerStatus::Running;
+
+        instance.execution_pointers = vec![p1, p2];
+        assert!(instance.is_branch_complete(&scope_a));
+    }
+
+    #[test]
+    fn serde_round_trip() {
+        let instance = WorkflowInstance::new("my-workflow", 2, serde_json::json!({"key": "value"}));
+        let json = serde_json::to_string(&instance).unwrap();
+        let deserialized: WorkflowInstance = serde_json::from_str(&json).unwrap();
+        assert_eq!(instance.id, deserialized.id);
+        assert_eq!(instance.workflow_definition_id, deserialized.workflow_definition_id);
+        assert_eq!(instance.version, deserialized.version);
+        assert_eq!(instance.status, deserialized.status);
+        assert_eq!(instance.data, deserialized.data);
+    }
+}
diff --git a/wfe-core/src/traits/lifecycle.rs b/wfe-core/src/traits/lifecycle.rs
new file mode 100644
index 0000000..847582d
--- /dev/null
+++ b/wfe-core/src/traits/lifecycle.rs
@@ -0,0 +1,9 @@
+use async_trait::async_trait;
+
+use crate::models::LifecycleEvent;
+
+/// Publishes lifecycle events for workflow state transitions.
+#[async_trait]
+pub trait LifecyclePublisher: Send + Sync {
+    async fn publish(&self, event: LifecycleEvent) -> crate::Result<()>;
+}
diff --git a/wfe-core/src/traits/lock.rs b/wfe-core/src/traits/lock.rs
new file mode 100644
index 0000000..cd24d09
--- /dev/null
+++ b/wfe-core/src/traits/lock.rs
@@ -0,0 +1,10 @@
+use async_trait::async_trait;
+
+/// Distributed lock provider for preventing concurrent execution of the same workflow.
+#[async_trait]
+pub trait DistributedLockProvider: Send + Sync {
+    async fn acquire_lock(&self, resource: &str) -> crate::Result<bool>;
+    async fn release_lock(&self, resource: &str) -> crate::Result<()>;
+    async fn start(&self) -> crate::Result<()>;
+    async fn stop(&self) -> crate::Result<()>;
+}
diff --git a/wfe-core/src/traits/middleware.rs b/wfe-core/src/traits/middleware.rs
new file mode 100644
index 0000000..7bb4152
--- /dev/null
+++ b/wfe-core/src/traits/middleware.rs
@@ -0,0 +1,93 @@
+use async_trait::async_trait;
+
+use crate::models::{ExecutionResult, WorkflowInstance};
+use crate::traits::step::StepExecutionContext;
+
+/// Workflow-level middleware with default no-op implementations.
+#[async_trait]
+pub trait WorkflowMiddleware: Send + Sync {
+    async fn pre_workflow(&self, _instance: &WorkflowInstance) -> crate::Result<()> {
+        Ok(())
+    }
+    async fn post_workflow(&self, _instance: &WorkflowInstance) -> crate::Result<()> {
+        Ok(())
+    }
+}
+
+/// Step-level middleware with default no-op implementations.
+#[async_trait]
+pub trait StepMiddleware: Send + Sync {
+    async fn pre_step(&self, _context: &StepExecutionContext<'_>) -> crate::Result<()> {
+        Ok(())
+    }
+    async fn post_step(
+        &self,
+        _context: &StepExecutionContext<'_>,
+        _result: &ExecutionResult,
+    ) -> crate::Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::{ExecutionPointer, ExecutionResult, WorkflowInstance};
+
+    struct NoOpWorkflowMiddleware;
+    impl WorkflowMiddleware for NoOpWorkflowMiddleware {}
+
+    struct NoOpStepMiddleware;
+    impl StepMiddleware for NoOpStepMiddleware {}
+
+    #[tokio::test]
+    async fn workflow_middleware_default_pre_workflow() {
+        let mw = NoOpWorkflowMiddleware;
+        let instance = WorkflowInstance::new("wf", 1, serde_json::json!({}));
+        mw.pre_workflow(&instance).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn workflow_middleware_default_post_workflow() {
+        let mw = NoOpWorkflowMiddleware;
+        let instance = WorkflowInstance::new("wf", 1, serde_json::json!({}));
+        mw.post_workflow(&instance).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn step_middleware_default_pre_step() {
+        use crate::models::WorkflowStep;
+        let mw = NoOpStepMiddleware;
+        let instance = WorkflowInstance::new("wf", 1, serde_json::json!({}));
+        let pointer = ExecutionPointer::new(0);
+        let step = WorkflowStep::new(0, "test_step");
+        let ctx = StepExecutionContext {
+            item: None,
+            execution_pointer: &pointer,
+            persistence_data: None,
+            step: &step,
+            workflow: &instance,
+            cancellation_token: tokio_util::sync::CancellationToken::new(),
+        };
+        mw.pre_step(&ctx).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn step_middleware_default_post_step() {
+        use crate::models::WorkflowStep;
+        let mw = NoOpStepMiddleware;
+        let instance = WorkflowInstance::new("wf", 1, serde_json::json!({}));
+        let pointer = ExecutionPointer::new(0);
+        let step = WorkflowStep::new(0, "test_step");
+        let ctx = StepExecutionContext {
+            item: None,
+            execution_pointer: &pointer,
+            persistence_data: None,
+            step: &step,
+            workflow: &instance,
+            cancellation_token: tokio_util::sync::CancellationToken::new(),
+        };
+        let result = ExecutionResult::next();
+        mw.post_step(&ctx, &result).await.unwrap();
+    }
+}
diff --git a/wfe-core/src/traits/mod.rs b/wfe-core/src/traits/mod.rs
new file mode 100644
index 0000000..85151f2
--- /dev/null
+++ b/wfe-core/src/traits/mod.rs
@@ -0,0 +1,20 @@
+pub mod lifecycle;
+pub mod lock;
+pub mod middleware;
+pub mod persistence;
+pub mod queue;
+pub mod registry;
+pub mod search;
+pub mod step;
+
+pub use lifecycle::LifecyclePublisher;
+pub use lock::DistributedLockProvider;
+pub use middleware::{StepMiddleware, WorkflowMiddleware};
+pub use persistence::{
+    EventRepository, PersistenceProvider, ScheduledCommandRepository, SubscriptionRepository,
+    WorkflowRepository,
+};
+pub use queue::QueueProvider;
+pub use registry::WorkflowRegistry;
+pub use search::{Page, SearchFilter, SearchIndex, WorkflowSearchResult};
+pub use step::{StepBody, StepExecutionContext, WorkflowData};
diff --git a/wfe-core/src/traits/persistence.rs b/wfe-core/src/traits/persistence.rs
new file mode 100644
index 0000000..9b00fe2
--- /dev/null
+++ b/wfe-core/src/traits/persistence.rs
@@ -0,0 +1,95 @@
+use async_trait::async_trait;
+use chrono::{DateTime, Utc};
+
+use crate::models::{
+    Event, EventSubscription, ExecutionError, ScheduledCommand, WorkflowInstance,
+};
+
+/// Persistence for workflow instances.
+#[async_trait]
+pub trait WorkflowRepository: Send + Sync {
+    async fn create_new_workflow(&self, instance: &WorkflowInstance) -> crate::Result<String>;
+    async fn persist_workflow(&self, instance: &WorkflowInstance) -> crate::Result<()>;
+    async fn persist_workflow_with_subscriptions(
+        &self,
+        instance: &WorkflowInstance,
+        subscriptions: &[EventSubscription],
+    ) -> crate::Result<()>;
+    async fn get_runnable_instances(&self, as_at: DateTime<Utc>) -> crate::Result<Vec<String>>;
+    async fn get_workflow_instance(&self, id: &str) -> crate::Result<WorkflowInstance>;
+    async fn get_workflow_instances(&self, ids: &[String]) -> crate::Result<Vec<WorkflowInstance>>;
+}
+
+/// Persistence for event subscriptions.
+#[async_trait]
+pub trait SubscriptionRepository: Send + Sync {
+    async fn create_event_subscription(
+        &self,
+        subscription: &EventSubscription,
+    ) -> crate::Result<String>;
+    async fn get_subscriptions(
+        &self,
+        event_name: &str,
+        event_key: &str,
+        as_of: DateTime<Utc>,
+    ) -> crate::Result<Vec<EventSubscription>>;
+    async fn terminate_subscription(&self, subscription_id: &str) -> crate::Result<()>;
+    async fn get_subscription(&self, subscription_id: &str) -> crate::Result<EventSubscription>;
+    async fn get_first_open_subscription(
+        &self,
+        event_name: &str,
+        event_key: &str,
+        as_of: DateTime<Utc>,
+    ) -> crate::Result<Option<EventSubscription>>;
+    async fn set_subscription_token(
+        &self,
+        subscription_id: &str,
+        token: &str,
+        worker_id: &str,
+        expiry: DateTime<Utc>,
+    ) -> crate::Result<bool>;
+    async fn clear_subscription_token(
+        &self,
+        subscription_id: &str,
+        token: &str,
+    ) -> crate::Result<()>;
+}
+
+/// Persistence for events.
+#[async_trait]
+pub trait EventRepository: Send + Sync {
+    async fn create_event(&self, event: &Event) -> crate::Result<String>;
+    async fn get_event(&self, id: &str) -> crate::Result<Event>;
+    async fn get_runnable_events(&self, as_at: DateTime<Utc>) -> crate::Result<Vec<String>>;
+    async fn get_events(
+        &self,
+        event_name: &str,
+        event_key: &str,
+        as_of: DateTime<Utc>,
+    ) -> crate::Result<Vec<String>>;
+    async fn mark_event_processed(&self, id: &str) -> crate::Result<()>;
+    async fn mark_event_unprocessed(&self, id: &str) -> crate::Result<()>;
+}
+
+/// Persistence for scheduled commands.
+#[async_trait]
+pub trait ScheduledCommandRepository: Send + Sync {
+    fn supports_scheduled_commands(&self) -> bool;
+    async fn schedule_command(&self, command: &ScheduledCommand) -> crate::Result<()>;
+    async fn process_commands(
+        &self,
+        as_of: DateTime<Utc>,
+        handler: &(dyn Fn(ScheduledCommand) -> std::pin::Pin<Box<dyn std::future::Future<Output = crate::Result<()>> + Send>>
+              + Send
+              + Sync),
+    ) -> crate::Result<()>;
+}
+
+/// Composite persistence provider combining all repository traits.
+#[async_trait]
+pub trait PersistenceProvider:
+    WorkflowRepository + EventRepository + SubscriptionRepository + ScheduledCommandRepository
+{
+    async fn persist_errors(&self, errors: &[ExecutionError]) -> crate::Result<()>;
+    async fn ensure_store_exists(&self) -> crate::Result<()>;
+}
diff --git a/wfe-core/src/traits/queue.rs b/wfe-core/src/traits/queue.rs
new file mode 100644
index 0000000..c221f18
--- /dev/null
+++ b/wfe-core/src/traits/queue.rs
@@ -0,0 +1,13 @@
+use async_trait::async_trait;
+
+use crate::models::QueueType;
+
+/// Queue provider for distributing workflow execution across workers.
+#[async_trait]
+pub trait QueueProvider: Send + Sync {
+    async fn queue_work(&self, id: &str, queue: QueueType) -> crate::Result<()>;
+    async fn dequeue_work(&self, queue: QueueType) -> crate::Result<Option<String>>;
+    fn is_dequeue_blocking(&self) -> bool;
+    async fn start(&self) -> crate::Result<()>;
+    async fn stop(&self) -> crate::Result<()>;
+}
diff --git a/wfe-core/src/traits/registry.rs b/wfe-core/src/traits/registry.rs
new file mode 100644
index 0000000..f9a6e7a
--- /dev/null
+++ b/wfe-core/src/traits/registry.rs
@@ -0,0 +1,10 @@
+use crate::models::WorkflowDefinition;
+
+/// Registry for workflow definitions with version support.
+pub trait WorkflowRegistry: Send + Sync {
+    fn register(&mut self, definition: WorkflowDefinition);
+    fn get_definition(&self, id: &str, version: Option<u32>) -> Option<&WorkflowDefinition>;
+    fn is_registered(&self, id: &str, version: u32) -> bool;
+    fn deregister(&mut self, id: &str, version: u32) -> bool;
+    fn get_all_definitions(&self) -> Vec<&WorkflowDefinition>;
+}
diff --git a/wfe-core/src/traits/search.rs b/wfe-core/src/traits/search.rs
new file mode 100644
index 0000000..829410d
--- /dev/null
+++ b/wfe-core/src/traits/search.rs
@@ -0,0 +1,48 @@
+use async_trait::async_trait;
+
+use crate::models::WorkflowInstance;
+
+/// Result from a search query.
+#[derive(Debug, Clone)]
+pub struct WorkflowSearchResult {
+    pub id: String,
+    pub workflow_definition_id: String,
+    pub version: u32,
+    pub status: crate::models::WorkflowStatus,
+    pub reference: Option<String>,
+    pub description: Option<String>,
+}
+
+/// Filter for search queries.
+#[derive(Debug, Clone)]
+pub enum SearchFilter {
+    Status(crate::models::WorkflowStatus),
+    DateRange {
+        field: String,
+        before: Option<chrono::DateTime<chrono::Utc>>,
+        after: Option<chrono::DateTime<chrono::Utc>>,
+    },
+    Reference(String),
+}
+
+/// Paginated search results.
+#[derive(Debug, Clone)]
+pub struct Page<T> {
+    pub data: Vec<T>,
+    pub total: u64,
+}
+
+/// Search index for querying workflows.
+#[async_trait]
+pub trait SearchIndex: Send + Sync {
+    async fn index_workflow(&self, instance: &WorkflowInstance) -> crate::Result<()>;
+    async fn search(
+        &self,
+        terms: &str,
+        skip: u64,
+        take: u64,
+        filters: &[SearchFilter],
+    ) -> crate::Result<Page<WorkflowSearchResult>>;
+    async fn start(&self) -> crate::Result<()>;
+    async fn stop(&self) -> crate::Result<()>;
+}
diff --git a/wfe-core/src/traits/step.rs b/wfe-core/src/traits/step.rs
new file mode 100644
index 0000000..47b4b14
--- /dev/null
+++ b/wfe-core/src/traits/step.rs
@@ -0,0 +1,35 @@
+use async_trait::async_trait;
+use serde::de::DeserializeOwned;
+use serde::Serialize;
+
+use crate::models::{ExecutionPointer, ExecutionResult, WorkflowInstance, WorkflowStep};
+
+/// Marker trait for all data types that flow between workflow steps.
+/// Anything that is serializable and deserializable qualifies.
+pub trait WorkflowData: Serialize + DeserializeOwned + Send + Sync + Clone + 'static {}
+
+/// Blanket implementation: any type satisfying the bounds is WorkflowData.
+impl<T> WorkflowData for T where T: Serialize + DeserializeOwned + Send + Sync + Clone + 'static {}
+
+/// Context available to a step during execution.
+#[derive(Debug)]
+pub struct StepExecutionContext<'a> {
+    /// The current item when iterating (ForEach).
+    pub item: Option<&'a serde_json::Value>,
+    /// The current execution pointer.
+    pub execution_pointer: &'a ExecutionPointer,
+    /// Persistence data from a previous execution of this step.
+    pub persistence_data: Option<&'a serde_json::Value>,
+    /// The step definition.
+    pub step: &'a WorkflowStep,
+    /// The running workflow instance.
+    pub workflow: &'a WorkflowInstance,
+    /// Cancellation token.
+    pub cancellation_token: tokio_util::sync::CancellationToken,
+}
+
+/// The core unit of work in a workflow. Each step implements this trait.
+#[async_trait]
+pub trait StepBody: Send + Sync {
+    async fn run(&mut self, context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult>;
+}