diff --git a/Cargo.lock b/Cargo.lock
index e2dcfe4..ca875c8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1807,10 +1807,13 @@ dependencies = [
 name = "op-work-packages"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "chrono",
  "op-core",
  "serde",
  "serde_json",
+ "thiserror 1.0.69",
+ "tokio",
 ]
 
 [[package]]
diff --git a/crates/op-work-packages/Cargo.toml b/crates/op-work-packages/Cargo.toml
index 684c4a9..9faf773 100644
--- a/crates/op-work-packages/Cargo.toml
+++ b/crates/op-work-packages/Cargo.toml
@@ -9,6 +9,9 @@ op-core = { path = "../op-core" }
 
 serde.workspace = true
 chrono.workspace = true
+async-trait.workspace = true
+thiserror.workspace = true
 
 [dev-dependencies]
 serde_json.workspace = true
+tokio.workspace = true
diff --git a/crates/op-work-packages/src/lib.rs b/crates/op-work-packages/src/lib.rs
index 66edeb4..9c40c8e 100644
--- a/crates/op-work-packages/src/lib.rs
+++ b/crates/op-work-packages/src/lib.rs
@@ -9,5 +9,10 @@
 //! concept `project_work_item` (codebook `0x0102`); see `op-canon`.
 
 pub mod work_package;
+pub mod work_package_service;
 
 pub use work_package::{DoneRatio, WorkPackage};
+pub use work_package_service::{
+    MemoryWorkPackageStore, NewWorkPackage, WorkPackageError, WorkPackageResult,
+    WorkPackageService, WorkPackageStore,
+};
diff --git a/crates/op-work-packages/src/work_package_service.rs b/crates/op-work-packages/src/work_package_service.rs
new file mode 100644
index 0000000..060d671
--- /dev/null
+++ b/crates/op-work-packages/src/work_package_service.rs
@@ -0,0 +1,324 @@
+//! Work package service
+//!
+//! Mirrors: `app/services/work_packages/create_service.rb` (+ the set-attributes
+//! write path). A thin `CreateService` over a pluggable [`WorkPackageStore`]:
+//! deserialize a [`NewWorkPackage`] write-model, validate, build the domain
+//! [`WorkPackage`], persist, return it with its assigned id.
+//!
+//! Structure follows the `op-journals` house pattern: an error enum +
+//! `Result` alias, an async `Store` trait, the service, and an in-memory
+//! store for tests.
+
+use std::sync::atomic::{AtomicI64, Ordering};
+use std::sync::{Arc, RwLock};
+
+use async_trait::async_trait;
+use op_core::traits::Id;
+use op_core::types::Formattable;
+use serde::Deserialize;
+use thiserror::Error;
+
+use crate::work_package::WorkPackage;
+
+/// Work package service errors.
+#[derive(Debug, Error)]
+pub enum WorkPackageError {
+    /// No work package with the given id.
+    #[error("work package not found: {0}")]
+    NotFound(Id),
+    /// The write-model failed validation (e.g. blank subject).
+    #[error("validation failed: {0}")]
+    Validation(String),
+    /// The underlying store failed.
+    #[error("database error: {0}")]
+    Database(String),
+}
+
+/// Result alias for the service surface.
+pub type WorkPackageResult<T> = Result<T, WorkPackageError>;
+
+/// Create write-model — the input DTO for `POST /work_packages`.
+///
+/// Only the NOT NULL columns are required (`subject`, `project_id`,
+/// `type_id`, `status_id`, `author_id`); everything else is optional,
+/// matching the persisted shape (`op-db` carries the rest as `Option`).
+/// Deserializable so an API layer can parse a request body straight into it.
+#[derive(Debug, Clone, Deserialize)]
+pub struct NewWorkPackage {
+    /// One-line headline. Required (non-blank — see [`WorkPackageService::create`]).
+    pub subject: String,
+    /// Owning project FK.
+    pub project_id: Id,
+    /// Work-package type FK.
+    pub type_id: Id,
+    /// Status FK.
+    pub status_id: Id,
+    /// Author FK.
+    pub author_id: Id,
+    /// Priority FK (optional — nullable column).
+    #[serde(default)]
+    pub priority_id: Option<Id>,
+    /// Assignee FK (optional).
+    #[serde(default)]
+    pub assigned_to_id: Option<Id>,
+    /// Parent WP FK for the hierarchy (optional).
+    #[serde(default)]
+    pub parent_id: Option<Id>,
+    /// Description body as markdown source (optional).
+    #[serde(default)]
+    pub description: Option<String>,
+}
+
+impl NewWorkPackage {
+    /// The minimal required create input; optionals default to `None`.
+    pub fn new(
+        subject: impl Into<String>,
+        project_id: Id,
+        type_id: Id,
+        status_id: Id,
+        author_id: Id,
+    ) -> Self {
+        Self {
+            subject: subject.into(),
+            project_id,
+            type_id,
+            status_id,
+            author_id,
+            priority_id: None,
+            assigned_to_id: None,
+            parent_id: None,
+            description: None,
+        }
+    }
+
+    /// Map the write-model onto a fresh (unpersisted) domain entity.
+    pub fn to_work_package(&self) -> WorkPackage {
+        let mut wp = WorkPackage::new(
+            self.subject.clone(),
+            self.project_id,
+            self.type_id,
+            self.status_id,
+            self.author_id,
+        );
+        wp.priority_id = self.priority_id;
+        wp.assigned_to_id = self.assigned_to_id;
+        wp.parent_id = self.parent_id;
+        if let Some(body) = &self.description {
+            wp.description = Formattable::markdown(body.clone());
+        }
+        wp
+    }
+}
+
+/// Persistence port for work packages (mirrors OpenProject's repository).
+#[async_trait]
+pub trait WorkPackageStore: Send + Sync {
+    /// Insert a new work package; returns the assigned id.
+    async fn insert(&self, wp: &WorkPackage) -> WorkPackageResult<Id>;
+    /// Fetch by id.
+    async fn get(&self, id: Id) -> WorkPackageResult<Option<WorkPackage>>;
+    /// All work packages in a project.
+    async fn list_for_project(&self, project_id: Id) -> WorkPackageResult<Vec<WorkPackage>>;
+}
+
+/// Create/read service over a [`WorkPackageStore`].
+pub struct WorkPackageService {
+    store: Arc<dyn WorkPackageStore>,
+}
+
+impl WorkPackageService {
+    /// Build the service over a store.
+    pub fn new(store: Arc<dyn WorkPackageStore>) -> Self {
+        Self { store }
+    }
+
+    /// Create a work package from the write-model: validate, build, persist.
+    /// Returns the persisted entity with its assigned id.
+    ///
+    /// # Errors
+    ///
+    /// [`WorkPackageError::Validation`] if `subject` is blank;
+    /// [`WorkPackageError::Database`] if the store fails.
+    pub async fn create(&self, input: NewWorkPackage) -> WorkPackageResult<WorkPackage> {
+        if input.subject.trim().is_empty() {
+            return Err(WorkPackageError::Validation(
+                "subject can't be blank".to_string(),
+            ));
+        }
+        let wp = input.to_work_package();
+        let id = self.store.insert(&wp).await?;
+        let mut created = wp;
+        created.id = Some(id);
+        Ok(created)
+    }
+
+    /// Fetch a work package by id, erroring [`WorkPackageError::NotFound`] if
+    /// absent.
+    ///
+    /// # Errors
+    ///
+    /// [`WorkPackageError::NotFound`] / [`WorkPackageError::Database`].
+    pub async fn find(&self, id: Id) -> WorkPackageResult<WorkPackage> {
+        self.store
+            .get(id)
+            .await?
+            .ok_or(WorkPackageError::NotFound(id))
+    }
+
+    /// All work packages in a project.
+    ///
+    /// # Errors
+    ///
+    /// [`WorkPackageError::Database`] on store failure.
+    pub async fn list_for_project(&self, project_id: Id) -> WorkPackageResult<Vec<WorkPackage>> {
+        self.store.list_for_project(project_id).await
+    }
+}
+
+/// In-memory [`WorkPackageStore`] for tests and prototyping.
+pub struct MemoryWorkPackageStore {
+    rows: RwLock<Vec<WorkPackage>>,
+    next_id: AtomicI64,
+}
+
+impl Default for MemoryWorkPackageStore {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl MemoryWorkPackageStore {
+    /// Empty store, ids starting at 1.
+    pub fn new() -> Self {
+        Self {
+            rows: RwLock::new(Vec::new()),
+            next_id: AtomicI64::new(1),
+        }
+    }
+}
+
+#[async_trait]
+impl WorkPackageStore for MemoryWorkPackageStore {
+    async fn insert(&self, wp: &WorkPackage) -> WorkPackageResult<Id> {
+        let id = self.next_id.fetch_add(1, Ordering::SeqCst);
+        let mut stored = wp.clone();
+        stored.id = Some(id);
+        self.rows
+            .write()
+            .map_err(|e| WorkPackageError::Database(e.to_string()))?
+            .push(stored);
+        Ok(id)
+    }
+
+    async fn get(&self, id: Id) -> WorkPackageResult<Option<WorkPackage>> {
+        let rows = self
+            .rows
+            .read()
+            .map_err(|e| WorkPackageError::Database(e.to_string()))?;
+        Ok(rows.iter().find(|wp| wp.id == Some(id)).cloned())
+    }
+
+    async fn list_for_project(&self, project_id: Id) -> WorkPackageResult<Vec<WorkPackage>> {
+        let rows = self
+            .rows
+            .read()
+            .map_err(|e| WorkPackageError::Database(e.to_string()))?;
+        Ok(rows
+            .iter()
+            .filter(|wp| wp.project_id == project_id)
+            .cloned()
+            .collect())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn service() -> WorkPackageService {
+        WorkPackageService::new(Arc::new(MemoryWorkPackageStore::new()))
+    }
+
+    // subject, project, type, status, author
+    fn input() -> NewWorkPackage {
+        NewWorkPackage::new("Fix the thing", 1, 2, 3, 10)
+    }
+
+    #[tokio::test]
+    async fn create_persists_and_assigns_id() {
+        let svc = service();
+        let created = svc.create(input()).await.unwrap();
+        assert!(created.id.is_some(), "persisted row gets an id");
+        assert_eq!(created.subject, "Fix the thing");
+        // Round-trips through the store.
+        let found = svc.find(created.id.unwrap()).await.unwrap();
+        assert_eq!(found.subject, "Fix the thing");
+        assert_eq!(found.project_id, 1);
+    }
+
+    #[tokio::test]
+    async fn create_rejects_blank_subject() {
+        let svc = service();
+        let mut bad = input();
+        bad.subject = "   ".to_string();
+        let err = svc.create(bad).await.unwrap_err();
+        assert!(
+            matches!(err, WorkPackageError::Validation(_)),
+            "blank subject → Validation, got {err:?}"
+        );
+    }
+
+    #[tokio::test]
+    async fn create_maps_optional_write_model_fields() {
+        let svc = service();
+        let mut inp = input();
+        inp.priority_id = Some(5);
+        inp.assigned_to_id = Some(20);
+        inp.parent_id = Some(7);
+        inp.description = Some("**body**".to_string());
+        let wp = svc.create(inp).await.unwrap();
+        assert_eq!(wp.priority_id, Some(5));
+        assert_eq!(wp.assigned_to_id, Some(20));
+        assert_eq!(wp.parent_id, Some(7));
+        assert_eq!(wp.description.raw, "**body**");
+    }
+
+    #[tokio::test]
+    async fn find_unknown_id_is_not_found() {
+        let svc = service();
+        let err = svc.find(999).await.unwrap_err();
+        assert!(matches!(err, WorkPackageError::NotFound(999)), "{err:?}");
+    }
+
+    #[tokio::test]
+    async fn list_for_project_filters_by_project() {
+        let svc = service();
+        // Two in project 1, one in project 2.
+        svc.create(NewWorkPackage::new("a", 1, 2, 3, 10))
+            .await
+            .unwrap();
+        svc.create(NewWorkPackage::new("b", 1, 2, 3, 10))
+            .await
+            .unwrap();
+        svc.create(NewWorkPackage::new("c", 2, 2, 3, 10))
+            .await
+            .unwrap();
+        assert_eq!(svc.list_for_project(1).await.unwrap().len(), 2);
+        assert_eq!(svc.list_for_project(2).await.unwrap().len(), 1);
+        assert_eq!(svc.list_for_project(99).await.unwrap().len(), 0);
+    }
+
+    #[test]
+    fn write_model_deserializes_with_optional_fields_defaulting() {
+        // An API body with only the required fields parses; optionals default.
+        let json = r#"{"subject":"S","project_id":1,"type_id":2,"status_id":3,"author_id":10}"#;
+        let nwp: NewWorkPackage = serde_json::from_str(json).unwrap();
+        assert_eq!(nwp.subject, "S");
+        assert!(nwp.priority_id.is_none());
+        assert!(nwp.description.is_none());
+        // And it maps to a fresh, unpersisted entity.
+        let wp = nwp.to_work_package();
+        assert!(wp.id.is_none());
+        assert_eq!(wp.status_id, 3);
+    }
+}