diff --git a/crates/feder-core/src/lib.rs b/crates/feder-core/src/lib.rs
index b82752b..7d4bc37 100644
--- a/crates/feder-core/src/lib.rs
+++ b/crates/feder-core/src/lib.rs
@@ -1,3 +1,166 @@
 //! Portable ActivityPub core logic for Feder.
+#![no_std]
+
+extern crate alloc;
+
+use alloc::{string::String, vec::Vec};
 
 pub use feder_vocab as vocab;
+
+/// Portable core state and decision logic.
+#[derive(Debug, Default)]
+pub struct FederCore;
+
+impl FederCore {
+    #[must_use]
+    pub fn new() -> Self {
+        Self
+    }
+
+    /// Handle one core input and return runtime actions to perform later.
+    ///
+    /// This method intentionally performs no I/O. Follow acceptance, object
+    /// storage, and delivery behavior are added by later Phase 1 issues.
+    #[must_use]
+    pub fn handle(&mut self, input: Input) -> HandleResult {
+        match input {
+            Input::ReceivedFollow(_) | Input::UserCreateNote(_) => HandleResult::default(),
+        }
+    }
+}
+
+/// Something entering the portable core from a runtime.
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Input {
+    ReceivedFollow(vocab::Follow),
+    UserCreateNote(UserCreateNote),
+}
+
+/// Runtime-provided data for creating a local note.
+///
+/// IDs and timestamps are inputs so the core does not depend on clocks,
+/// randomness, or platform-specific ID generation.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct UserCreateNote {
+    pub note_id: vocab::Iri,
+    pub create_id: vocab::Iri,
+    pub actor: vocab::Reference<vocab::Actor>,
+    pub content: String,
+    pub published: Option<String>,
+}
+
+/// Something the runtime should perform after core handling.
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Action {
+    StoreFollower(StoreFollower),
+    StoreObject(StoreObject),
+    SendActivity(SendActivity),
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct StoreFollower {
+    pub follower: vocab::Reference<vocab::Actor>,
+    pub following: vocab::Reference<vocab::Actor>,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct StoreObject {
+    pub object: Object,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct SendActivity {
+    pub activity: Activity,
+    pub inbox: vocab::Iri,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Activity {
+    Accept(vocab::Accept),
+    CreateNote(vocab::Create<vocab::Note>),
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Object {
+    Note(vocab::Note),
+}
+
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct HandleResult {
+    pub actions: Vec<Action>,
+}
+
+impl HandleResult {
+    #[must_use]
+    pub fn new(actions: Vec<Action>) -> Self {
+        Self { actions }
+    }
+
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
+        self.actions.is_empty()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use alloc::format;
+    use alloc::string::ToString;
+
+    fn iri(value: &str) -> vocab::Iri {
+        value.parse().expect("valid test IRI")
+    }
+
+    fn actor(id: &str) -> vocab::Actor {
+        vocab::Actor::person(
+            iri(id),
+            iri(&format!("{id}/inbox")),
+            iri(&format!("{id}/outbox")),
+        )
+    }
+
+    #[test]
+    fn received_follow_enters_core_without_runtime_io() {
+        let mut core = FederCore::new();
+        let follow = vocab::Follow::new(
+            iri("https://remote.example/activities/follow/1"),
+            vocab::Reference::id(iri("https://remote.example/users/bob")),
+            vocab::Reference::object(actor("https://example.com/users/alice")),
+        );
+
+        let result = core.handle(Input::ReceivedFollow(follow));
+
+        assert!(result.is_empty());
+    }
+
+    #[test]
+    fn user_create_note_input_carries_nondeterministic_values() {
+        let input = UserCreateNote {
+            note_id: iri("https://example.com/notes/1"),
+            create_id: iri("https://example.com/activities/create/1"),
+            actor: vocab::Reference::id(iri("https://example.com/users/alice")),
+            content: "Hello from Feder.".to_string(),
+            published: Some("2026-06-10T00:00:00Z".to_string()),
+        };
+
+        let mut core = FederCore::new();
+        let result = core.handle(Input::UserCreateNote(input));
+
+        assert!(result.is_empty());
+    }
+
+    #[test]
+    fn handle_result_wraps_action_lists() {
+        let result = HandleResult::new(Vec::from([Action::StoreFollower(StoreFollower {
+            follower: vocab::Reference::id(iri("https://remote.example/users/bob")),
+            following: vocab::Reference::id(iri("https://example.com/users/alice")),
+        })]));
+
+        assert_eq!(result.actions.len(), 1);
+    }
+}