diff --git a/crates/domain/src/message.rs b/crates/domain/src/message.rs
deleted file mode 100644
index 4901382..0000000
--- a/crates/domain/src/message.rs
+++ /dev/null
@@ -1,9 +0,0 @@
-use types::bounded::CapSlot;
-
-pub const MAX_MESSAGE_WORDS: usize = 4;
-pub const MAX_CAP_TRANSFERS: usize = 4;
-
-pub struct Message {
-    words: [usize; MAX_MESSAGE_WORDS],
-    caps: [Option<CapSlot>; MAX_CAP_TRANSFERS],
-}
diff --git a/crates/domain/src/message/mod.rs b/crates/domain/src/message/mod.rs
new file mode 100644
index 0000000..3bf5fa7
--- /dev/null
+++ b/crates/domain/src/message/mod.rs
@@ -0,0 +1,76 @@
+use types::bounded::CapSlot;
+
+#[cfg(test)]
+#[path = "tests.rs"]
+mod tests;
+
+pub const MAX_MESSAGE_WORDS: usize = 4;
+pub const MAX_CAP_TRANSFERS: usize = 4;
+
+pub struct Message {
+    words: [usize; MAX_MESSAGE_WORDS],
+    caps: [Option<CapSlot>; MAX_CAP_TRANSFERS],
+}
+
+impl Message {
+    pub fn empty() -> Self {
+        Self {
+            words: [0; MAX_MESSAGE_WORDS],
+            caps: [None; MAX_CAP_TRANSFERS],
+        }
+    }
+
+    pub fn from_words(words: [usize; MAX_MESSAGE_WORDS]) -> Self {
+        Self {
+            words,
+            caps: [None; MAX_CAP_TRANSFERS],
+        }
+    }
+
+    pub fn words(&self) -> &[usize; MAX_MESSAGE_WORDS] {
+        &self.words
+    }
+
+    pub fn word(&self, index: usize) -> usize {
+        self.words[index]
+    }
+
+    pub fn caps(&self) -> &[Option<CapSlot>; MAX_CAP_TRANSFERS] {
+        &self.caps
+    }
+
+    pub fn cap(&self, index: usize) -> Option<CapSlot> {
+        self.caps[index]
+    }
+
+    pub fn builder() -> MessageBuilder {
+        MessageBuilder {
+            message: Message::empty(),
+        }
+    }
+}
+
+pub struct MessageBuilder {
+    message: Message,
+}
+
+impl MessageBuilder {
+    pub fn word(mut self, index: usize, value: usize) -> Self {
+        self.message.words[index] = value;
+        self
+    }
+
+    pub fn words(mut self, words: [usize; MAX_MESSAGE_WORDS]) -> Self {
+        self.message.words = words;
+        self
+    }
+
+    pub fn cap(mut self, index: usize, slot: CapSlot) -> Self {
+        self.message.caps[index] = Some(slot);
+        self
+    }
+
+    pub fn build(self) -> Message {
+        self.message
+    }
+}
diff --git a/crates/domain/src/message/tests.rs b/crates/domain/src/message/tests.rs
new file mode 100644
index 0000000..1627a18
--- /dev/null
+++ b/crates/domain/src/message/tests.rs
@@ -0,0 +1,86 @@
+use super::*;
+use types::bounded::CapSlot;
+
+fn slot(n: u8) -> CapSlot {
+    CapSlot::new(n).unwrap()
+}
+
+// --- Construction ---
+
+#[test]
+fn empty_message_has_zeroed_words_and_no_caps() {
+    let msg = Message::empty();
+    assert_eq!(msg.words(), &[0; MAX_MESSAGE_WORDS]);
+    for i in 0..MAX_CAP_TRANSFERS {
+        assert!(msg.cap(i).is_none());
+    }
+}
+
+#[test]
+fn from_words_sets_words_and_no_caps() {
+    let msg = Message::from_words([10, 20, 30, 40]);
+    assert_eq!(msg.word(0), 10);
+    assert_eq!(msg.word(1), 20);
+    assert_eq!(msg.word(2), 30);
+    assert_eq!(msg.word(3), 40);
+    assert!(msg.cap(0).is_none());
+}
+
+// --- Accessors ---
+
+#[test]
+fn words_returns_full_array() {
+    let msg = Message::from_words([1, 2, 3, 4]);
+    assert_eq!(msg.words(), &[1, 2, 3, 4]);
+}
+
+#[test]
+fn caps_returns_full_array() {
+    let msg = Message::empty();
+    assert_eq!(msg.caps(), &[None; MAX_CAP_TRANSFERS]);
+}
+
+// --- Builder ---
+
+#[test]
+fn builder_sets_individual_words() {
+    let msg = Message::builder()
+        .word(0, 42)
+        .word(2, 99)
+        .build();
+    assert_eq!(msg.word(0), 42);
+    assert_eq!(msg.word(1), 0);
+    assert_eq!(msg.word(2), 99);
+    assert_eq!(msg.word(3), 0);
+}
+
+#[test]
+fn builder_sets_caps() {
+    let msg = Message::builder()
+        .cap(0, slot(5))
+        .cap(1, slot(10))
+        .build();
+    assert_eq!(msg.cap(0), Some(slot(5)));
+    assert_eq!(msg.cap(1), Some(slot(10)));
+    assert!(msg.cap(2).is_none());
+}
+
+#[test]
+fn builder_sets_words_and_caps_together() {
+    let msg = Message::builder()
+        .word(0, 100)
+        .word(1, 200)
+        .cap(0, slot(3))
+        .build();
+    assert_eq!(msg.word(0), 100);
+    assert_eq!(msg.word(1), 200);
+    assert_eq!(msg.cap(0), Some(slot(3)));
+}
+
+#[test]
+fn builder_with_all_words() {
+    let msg = Message::builder()
+        .words([11, 22, 33, 44])
+        .build();
+    assert_eq!(msg.words(), &[11, 22, 33, 44]);
+}