diff --git a/wfe-core/src/builder/inline_step.rs b/wfe-core/src/builder/inline_step.rs
new file mode 100644
index 0000000..9c1b5ea
--- /dev/null
+++ b/wfe-core/src/builder/inline_step.rs
@@ -0,0 +1,30 @@
+use async_trait::async_trait;
+
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+type InlineFn = Box<dyn Fn() -> ExecutionResult + Send + Sync>;
+
+/// A step that wraps an inline closure.
+pub struct InlineStep {
+    body: InlineFn,
+}
+
+impl InlineStep {
+    pub fn new(f: impl Fn() -> ExecutionResult + Send + Sync + 'static) -> Self {
+        Self { body: Box::new(f) }
+    }
+}
+
+impl Default for InlineStep {
+    fn default() -> Self {
+        Self::new(ExecutionResult::next)
+    }
+}
+
+#[async_trait]
+impl StepBody for InlineStep {
+    async fn run(&mut self, _context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        Ok((self.body)())
+    }
+}
diff --git a/wfe-core/src/builder/mod.rs b/wfe-core/src/builder/mod.rs
new file mode 100644
index 0000000..f748733
--- /dev/null
+++ b/wfe-core/src/builder/mod.rs
@@ -0,0 +1,7 @@
+mod inline_step;
+mod step_builder;
+mod workflow_builder;
+
+pub use inline_step::InlineStep;
+pub use step_builder::StepBuilder;
+pub use workflow_builder::WorkflowBuilder;
diff --git a/wfe-core/src/builder/step_builder.rs b/wfe-core/src/builder/step_builder.rs
new file mode 100644
index 0000000..7c90502
--- /dev/null
+++ b/wfe-core/src/builder/step_builder.rs
@@ -0,0 +1,221 @@
+use crate::models::{ErrorBehavior, ExecutionResult};
+use crate::primitives;
+use crate::traits::step::{StepBody, WorkflowData};
+
+use super::inline_step::InlineStep;
+use super::workflow_builder::WorkflowBuilder;
+
+/// Builder for configuring a single step in the workflow.
+///
+/// Owns the WorkflowBuilder, consuming self on each method call.
+/// This avoids all lifetime/borrow issues.
+pub struct StepBuilder<D: WorkflowData> {
+    builder: WorkflowBuilder<D>,
+    step_id: usize,
+}
+
+/// Builder for parallel branches.
+pub struct ParallelBuilder<D: WorkflowData> {
+    builder: WorkflowBuilder<D>,
+    container_id: usize,
+}
+
+impl<D: WorkflowData> StepBuilder<D> {
+    pub(crate) fn new(builder: WorkflowBuilder<D>, step_id: usize) -> Self {
+        Self { builder, step_id }
+    }
+
+    /// Set the display name of the current step.
+    pub fn name(mut self, name: &str) -> Self {
+        self.builder.steps[self.step_id].name = Some(name.to_string());
+        self
+    }
+
+    /// Set an external ID for forward references.
+    pub fn id(mut self, external_id: &str) -> Self {
+        self.builder.steps[self.step_id].external_id = Some(external_id.to_string());
+        self
+    }
+
+    /// Set the error handling behavior for this step.
+    pub fn on_error(mut self, behavior: ErrorBehavior) -> Self {
+        self.builder.steps[self.step_id].error_behavior = Some(behavior);
+        self
+    }
+
+    /// Add a compensation step for saga rollback.
+    pub fn compensate_with<C: StepBody + Default + 'static>(mut self) -> Self {
+        let comp_id = self.builder.add_step(std::any::type_name::<C>());
+        self.builder.steps[self.step_id].compensation_step_id = Some(comp_id);
+        self
+    }
+
+    /// Chain the next step. Wires an outcome from the current step to the new one.
+    pub fn then<S: StepBody + Default + 'static>(mut self) -> StepBuilder<D> {
+        let next_id = self.builder.add_step(std::any::type_name::<S>());
+        self.builder.wire_outcome(self.step_id, next_id, None);
+        self.builder.last_step = Some(next_id);
+        StepBuilder::new(self.builder, next_id)
+    }
+
+    /// Chain an inline function step.
+    pub fn then_fn(mut self, f: impl Fn() -> ExecutionResult + Send + Sync + 'static) -> StepBuilder<D> {
+        let next_id = self.builder.add_step(std::any::type_name::<InlineStep>());
+        self.builder.wire_outcome(self.step_id, next_id, None);
+        self.builder.last_step = Some(next_id);
+        self.builder.inline_closures.insert(next_id, Box::new(f));
+        StepBuilder::new(self.builder, next_id)
+    }
+
+    /// Insert a WaitFor step.
+    pub fn wait_for(mut self, event_name: &str, event_key: &str) -> StepBuilder<D> {
+        let next_id = self.builder.add_step(std::any::type_name::<primitives::wait_for::WaitForStep>());
+        self.builder.wire_outcome(self.step_id, next_id, None);
+        self.builder.last_step = Some(next_id);
+        self.builder.steps[next_id].step_config = Some(serde_json::json!({
+            "event_name": event_name,
+            "event_key": event_key,
+        }));
+        StepBuilder::new(self.builder, next_id)
+    }
+
+    /// Insert a Delay step.
+    pub fn delay(mut self, duration: std::time::Duration) -> StepBuilder<D> {
+        let next_id = self.builder.add_step(std::any::type_name::<primitives::delay::DelayStep>());
+        self.builder.wire_outcome(self.step_id, next_id, None);
+        self.builder.last_step = Some(next_id);
+        self.builder.steps[next_id].step_config = Some(serde_json::json!({
+            "duration_millis": duration.as_millis() as u64,
+        }));
+        StepBuilder::new(self.builder, next_id)
+    }
+
+    /// Insert an If container step with child steps built by the closure.
+    /// The type parameter S is the step type used for the If condition evaluation.
+    pub fn if_do<S: StepBody + Default + 'static>(
+        mut self,
+        build_children: impl FnOnce(&mut WorkflowBuilder<D>),
+    ) -> StepBuilder<D> {
+        let if_id = self.builder.add_step(std::any::type_name::<primitives::if_step::IfStep>());
+        self.builder.wire_outcome(self.step_id, if_id, None);
+
+        // Build children
+        let before_count = self.builder.steps.len();
+        build_children(&mut self.builder);
+        let after_count = self.builder.steps.len();
+
+        // Register children with the If step
+        for child_id in before_count..after_count {
+            self.builder.add_child(if_id, child_id);
+        }
+
+        self.builder.last_step = Some(if_id);
+        StepBuilder::new(self.builder, if_id)
+    }
+
+    /// Insert a While container step.
+    pub fn while_do<S: StepBody + Default + 'static>(
+        mut self,
+        build_children: impl FnOnce(&mut WorkflowBuilder<D>),
+    ) -> StepBuilder<D> {
+        let while_id = self.builder.add_step(std::any::type_name::<primitives::while_step::WhileStep>());
+        self.builder.wire_outcome(self.step_id, while_id, None);
+
+        let before_count = self.builder.steps.len();
+        build_children(&mut self.builder);
+        let after_count = self.builder.steps.len();
+
+        for child_id in before_count..after_count {
+            self.builder.add_child(while_id, child_id);
+        }
+
+        self.builder.last_step = Some(while_id);
+        StepBuilder::new(self.builder, while_id)
+    }
+
+    /// Insert a ForEach container step.
+    pub fn for_each<S: StepBody + Default + 'static>(
+        mut self,
+        build_children: impl FnOnce(&mut WorkflowBuilder<D>),
+    ) -> StepBuilder<D> {
+        let fe_id = self.builder.add_step(std::any::type_name::<primitives::foreach_step::ForEachStep>());
+        self.builder.wire_outcome(self.step_id, fe_id, None);
+
+        let before_count = self.builder.steps.len();
+        build_children(&mut self.builder);
+        let after_count = self.builder.steps.len();
+
+        for child_id in before_count..after_count {
+            self.builder.add_child(fe_id, child_id);
+        }
+
+        self.builder.last_step = Some(fe_id);
+        StepBuilder::new(self.builder, fe_id)
+    }
+
+    /// Insert a Saga container step with child steps.
+    pub fn saga(
+        mut self,
+        build_children: impl FnOnce(&mut WorkflowBuilder<D>),
+    ) -> StepBuilder<D> {
+        let saga_id = self.builder.add_step(std::any::type_name::<primitives::saga_container::SagaContainerStep>());
+        self.builder.steps[saga_id].saga = true;
+        self.builder.wire_outcome(self.step_id, saga_id, None);
+
+        let before_count = self.builder.steps.len();
+        build_children(&mut self.builder);
+        let after_count = self.builder.steps.len();
+
+        for child_id in before_count..after_count {
+            self.builder.add_child(saga_id, child_id);
+        }
+
+        self.builder.last_step = Some(saga_id);
+        StepBuilder::new(self.builder, saga_id)
+    }
+
+    /// Start a parallel block.
+    pub fn parallel(
+        mut self,
+        build_branches: impl FnOnce(ParallelBuilder<D>) -> ParallelBuilder<D>,
+    ) -> StepBuilder<D> {
+        let seq_id = self.builder.add_step(std::any::type_name::<primitives::sequence::SequenceStep>());
+        self.builder.wire_outcome(self.step_id, seq_id, None);
+
+        let pb = ParallelBuilder {
+            builder: self.builder,
+            container_id: seq_id,
+        };
+        let pb = build_branches(pb);
+        let mut builder = pb.builder;
+        builder.last_step = Some(seq_id);
+        StepBuilder::new(builder, seq_id)
+    }
+
+    /// Mark this step as the terminal step (no further outcomes).
+    pub fn end_workflow(self) -> WorkflowBuilder<D> {
+        self.builder
+    }
+
+    /// Access the compiled definition directly (shortcut for end_workflow().build()).
+    pub fn build(self, id: impl Into<String>, version: u32) -> crate::models::WorkflowDefinition {
+        self.builder.build(id, version)
+    }
+}
+
+impl<D: WorkflowData> ParallelBuilder<D> {
+    /// Add a parallel branch.
+    pub fn branch(
+        mut self,
+        build_branch: impl FnOnce(&mut WorkflowBuilder<D>),
+    ) -> Self {
+        let before_count = self.builder.steps.len();
+        build_branch(&mut self.builder);
+        let after_count = self.builder.steps.len();
+
+        for child_id in before_count..after_count {
+            self.builder.add_child(self.container_id, child_id);
+        }
+        self
+    }
+}
diff --git a/wfe-core/src/builder/workflow_builder.rs b/wfe-core/src/builder/workflow_builder.rs
new file mode 100644
index 0000000..d67128b
--- /dev/null
+++ b/wfe-core/src/builder/workflow_builder.rs
@@ -0,0 +1,356 @@
+use std::collections::HashMap;
+use std::marker::PhantomData;
+
+use crate::models::{
+    ExecutionResult, StepOutcome, WorkflowDefinition, WorkflowStep,
+};
+use crate::traits::step::{StepBody, WorkflowData};
+
+use super::inline_step::InlineStep;
+use super::step_builder::StepBuilder;
+
+/// Type alias for boxed inline step closures.
+pub type InlineClosureBox = Box<dyn Fn() -> ExecutionResult + Send + Sync>;
+
+/// Fluent builder for constructing workflow definitions.
+///
+/// Uses an owned-self pattern: each method consumes and returns the builder,
+/// avoiding lifetime issues with mutable borrows.
+///
+/// # Example
+/// ```ignore
+/// let def = WorkflowBuilder::<MyData>::new()
+///     .start_with::<StepA>()
+///     .name("Step A")
+///     .then::<StepB>()
+///     .name("Step B")
+///     .end_workflow()
+///     .build("my-workflow", 1);
+/// ```
+pub struct WorkflowBuilder<D: WorkflowData> {
+    pub(crate) steps: Vec<WorkflowStep>,
+    pub(crate) last_step: Option<usize>,
+    /// Inline closures keyed by step id, stored for later registration.
+    pub(crate) inline_closures: HashMap<usize, InlineClosureBox>,
+    _phantom: PhantomData<D>,
+}
+
+impl<D: WorkflowData> WorkflowBuilder<D> {
+    pub fn new() -> Self {
+        Self {
+            steps: Vec::new(),
+            last_step: None,
+            inline_closures: HashMap::new(),
+            _phantom: PhantomData,
+        }
+    }
+
+    /// Add the first step of the workflow.
+    pub fn start_with<S: StepBody + Default + 'static>(mut self) -> StepBuilder<D> {
+        let id = self.steps.len();
+        let step = WorkflowStep::new(id, std::any::type_name::<S>());
+        self.steps.push(step);
+        self.last_step = Some(id);
+        StepBuilder::new(self, id)
+    }
+
+    /// Add a step by type name. Used by container builder closures.
+    pub fn add_step(&mut self, step_type: &str) -> usize {
+        let id = self.steps.len();
+        self.steps.push(WorkflowStep::new(id, step_type));
+        id
+    }
+
+    /// Wire an outcome from `from_step` to `to_step`.
+    pub(crate) fn wire_outcome(&mut self, from_step: usize, to_step: usize, value: Option<serde_json::Value>) {
+        if let Some(step) = self.steps.get_mut(from_step) {
+            step.outcomes.push(StepOutcome {
+                next_step: to_step,
+                label: None,
+                value,
+            });
+        }
+    }
+
+    /// Add a child step ID to a parent container step.
+    pub(crate) fn add_child(&mut self, parent: usize, child: usize) {
+        if let Some(step) = self.steps.get_mut(parent) {
+            step.children.push(child);
+        }
+    }
+
+    /// Compile the builder into a WorkflowDefinition.
+    pub fn build(self, id: impl Into<String>, version: u32) -> WorkflowDefinition {
+        let mut def = WorkflowDefinition::new(id, version);
+        def.steps = self.steps;
+        // Note: inline closures are dropped here. Use `build_with_closures` to retain them.
+        def
+    }
+
+    /// Compile the builder into a WorkflowDefinition and return any inline closures
+    /// keyed by step id.
+    pub fn build_with_closures(
+        self,
+        id: impl Into<String>,
+        version: u32,
+    ) -> (WorkflowDefinition, HashMap<usize, InlineClosureBox>) {
+        let mut def = WorkflowDefinition::new(id, version);
+        def.steps = self.steps;
+        (def, self.inline_closures)
+    }
+
+    /// Register all inline closures from this builder into the given step registry.
+    ///
+    /// Each inline closure is registered under a unique key derived from the
+    /// `InlineStep` type name and step id.
+    pub fn register_inline_steps(
+        self,
+        registry: &mut crate::executor::StepRegistry,
+        id: impl Into<String>,
+        version: u32,
+    ) -> WorkflowDefinition {
+        let mut def = WorkflowDefinition::new(id, version);
+        def.steps = self.steps;
+        for (step_id, closure) in self.inline_closures {
+            let closure = std::sync::Arc::new(closure);
+            let key = format!("{}::{step_id}", std::any::type_name::<InlineStep>());
+            // Update the step_type so the executor resolves correctly.
+            if let Some(step) = def.steps.get_mut(step_id) {
+                step.step_type = key.clone();
+            }
+            let closure = closure.clone();
+            registry.register_factory(&key, move || {
+                let c = closure.clone();
+                Box::new(InlineStep::new(move || (c)()))
+            });
+        }
+        def
+    }
+}
+
+impl<D: WorkflowData> Default for WorkflowBuilder<D> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::{ErrorBehavior, ExecutionResult};
+    use crate::traits::step::StepExecutionContext;
+    use pretty_assertions::assert_eq;
+    use serde::{Deserialize, Serialize};
+
+    #[derive(Debug, Clone, Default, Serialize, Deserialize)]
+    struct TestData {
+        counter: i32,
+    }
+
+    #[derive(Default)]
+    struct StepA;
+
+    #[async_trait::async_trait]
+    impl StepBody for StepA {
+        async fn run(&mut self, _ctx: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+            Ok(ExecutionResult::next())
+        }
+    }
+
+    #[derive(Default)]
+    struct StepB;
+
+    #[async_trait::async_trait]
+    impl StepBody for StepB {
+        async fn run(&mut self, _ctx: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+            Ok(ExecutionResult::next())
+        }
+    }
+
+    #[derive(Default)]
+    struct StepC;
+
+    #[async_trait::async_trait]
+    impl StepBody for StepC {
+        async fn run(&mut self, _ctx: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+            Ok(ExecutionResult::next())
+        }
+    }
+
+    #[test]
+    fn build_empty_workflow() {
+        let def = WorkflowBuilder::<TestData>::new().build("empty", 1);
+        assert_eq!(def.id, "empty");
+        assert_eq!(def.version, 1);
+        assert!(def.steps.is_empty());
+    }
+
+    #[test]
+    fn start_with_adds_first_step() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .end_workflow()
+            .build("test", 1);
+        assert_eq!(def.steps.len(), 1);
+        assert!(def.steps[0].step_type.contains("StepA"));
+    }
+
+    #[test]
+    fn then_chains_two_steps_with_outcome() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .then::<StepB>()
+            .end_workflow()
+            .build("test", 1);
+        assert_eq!(def.steps.len(), 2);
+        // Step 0 should have outcome pointing to step 1
+        assert_eq!(def.steps[0].outcomes.len(), 1);
+        assert_eq!(def.steps[0].outcomes[0].next_step, 1);
+    }
+
+    #[test]
+    fn then_chains_three_steps() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .then::<StepB>()
+            .then::<StepC>()
+            .end_workflow()
+            .build("test", 1);
+        assert_eq!(def.steps.len(), 3);
+        assert_eq!(def.steps[0].outcomes[0].next_step, 1);
+        assert_eq!(def.steps[1].outcomes[0].next_step, 2);
+        assert!(def.steps[2].outcomes.is_empty());
+    }
+
+    #[test]
+    fn name_sets_step_name() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .name("First Step")
+            .end_workflow()
+            .build("test", 1);
+        assert_eq!(def.steps[0].name, Some("First Step".into()));
+    }
+
+    #[test]
+    fn on_error_sets_behavior() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .on_error(ErrorBehavior::Suspend)
+            .end_workflow()
+            .build("test", 1);
+        assert_eq!(def.steps[0].error_behavior, Some(ErrorBehavior::Suspend));
+    }
+
+    #[test]
+    fn if_do_inserts_container_with_children() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .if_do::<StepB>(|b| {
+                let id = b.add_step(std::any::type_name::<StepC>());
+                b.last_step = Some(id);
+            })
+            .end_workflow()
+            .build("test", 1);
+
+        // Steps: 0=StepA, 1=IfStep, 2=StepC (child)
+        // StepA -> IfStep -> (after if)
+        assert!(def.steps.len() >= 3);
+        // The If step should have StepC as a child
+        assert!(def.steps[1].step_type.contains("IfStep"));
+        assert!(def.steps[1].children.contains(&2));
+    }
+
+    #[test]
+    fn while_do_inserts_container() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .while_do::<StepB>(|b| {
+                b.add_step(std::any::type_name::<StepC>());
+            })
+            .end_workflow()
+            .build("test", 1);
+
+        assert!(def.steps.len() >= 3);
+        assert!(def.steps[1].step_type.contains("WhileStep"));
+    }
+
+    #[test]
+    fn for_each_inserts_container() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .for_each::<StepB>(|b| {
+                b.add_step(std::any::type_name::<StepC>());
+            })
+            .end_workflow()
+            .build("test", 1);
+
+        assert!(def.steps.len() >= 3);
+        assert!(def.steps[1].step_type.contains("ForEachStep"));
+    }
+
+    #[test]
+    fn parallel_creates_branches() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .parallel(|branches| {
+                branches
+                    .branch(|b| {
+                        b.add_step(std::any::type_name::<StepB>());
+                    })
+                    .branch(|b| {
+                        b.add_step(std::any::type_name::<StepC>());
+                    })
+            })
+            .end_workflow()
+            .build("test", 1);
+
+        // Steps: 0=StepA, 1=Sequence(parallel container), 2=StepB, 3=StepC
+        assert!(def.steps.len() >= 4);
+        assert!(def.steps[1].step_type.contains("SequenceStep"));
+        assert!(def.steps[1].children.len() >= 2);
+    }
+
+    #[test]
+    fn saga_with_compensation() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .saga(|b| {
+                b.add_step(std::any::type_name::<StepB>());
+                b.add_step(std::any::type_name::<StepC>());
+            })
+            .end_workflow()
+            .build("test", 1);
+
+        // Saga container should exist and have children
+        assert!(def.steps[1].step_type.contains("SagaContainerStep"));
+        assert!(def.steps[1].saga);
+        assert!(!def.steps[1].children.is_empty());
+    }
+
+    #[test]
+    fn compensate_with_sets_compensation_step() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .compensate_with::<StepB>()
+            .end_workflow()
+            .build("test", 1);
+
+        // Step 0 (StepA) should have compensation pointing to step 1 (StepB)
+        assert_eq!(def.steps[0].compensation_step_id, Some(1));
+        assert!(def.steps[1].step_type.contains("StepB"));
+    }
+
+    #[test]
+    fn inline_step_via_then_fn() {
+        let def = WorkflowBuilder::<TestData>::new()
+            .start_with::<StepA>()
+            .then_fn(ExecutionResult::next)
+            .end_workflow()
+            .build("test", 1);
+
+        assert_eq!(def.steps.len(), 2);
+        assert!(def.steps[1].step_type.contains("InlineStep"));
+        assert_eq!(def.steps[0].outcomes[0].next_step, 1);
+    }
+}