diff --git a/Cargo.lock b/Cargo.lock
index 02c4cdf..4cb5376 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -288,6 +288,15 @@ dependencies = [
  "tokio-util",
 ]
 
+[[package]]
+name = "maelstrom-protocol"
+version = "0.0.1"
+dependencies = [
+ "serde",
+ "serde_json",
+ "serde_repr",
+]
+
 [[package]]
 name = "memchr"
 version = "2.7.2"
@@ -473,6 +482,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_repr"
+version = "0.1.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c64451ba24fc7a6a2d60fc75dd9c83c90903b19028d4eff35e88fc1e86564e9"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "signal-hook-registry"
 version = "1.4.2"
diff --git a/Cargo.toml b/Cargo.toml
index 728b983..68a8a92 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,5 +1,5 @@
 [workspace]
-members = ["gg-echo", "gg-uid", "gg-broadcast"]
+members = ["maelstrom-protocol", "gg-echo", "gg-uid", "gg-broadcast"]
 resolver = "2"
 
 [workspace.package]
diff --git a/maelstrom-protocol/Cargo.toml b/maelstrom-protocol/Cargo.toml
new file mode 100644
index 0000000..42ca124
--- /dev/null
+++ b/maelstrom-protocol/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "maelstrom-protocol"
+edition = "2021"
+version.workspace = true
+authors.workspace = true
+
+[dependencies]
+serde = { version = "1", default-features = false, features = ["derive"] }
+serde_json = { version = "1", default-features = false, features = ["std"] }
+serde_repr = "0.1"
diff --git a/maelstrom-protocol/src/lib.rs b/maelstrom-protocol/src/lib.rs
new file mode 100644
index 0000000..950e422
--- /dev/null
+++ b/maelstrom-protocol/src/lib.rs
@@ -0,0 +1,92 @@
+use serde::{Deserialize, Serialize};
+use serde_json::{Map, Value};
+use serde_repr::{Deserialize_repr, Serialize_repr};
+
+pub type Payload = Map<String, Value>;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Message {
+    src: String,
+    dest: String,
+    body: Body,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Body {
+    #[serde(rename = "type")]
+    pub typ: String,
+    #[serde(default, skip_serializing_if = "u64_zero_by_ref")]
+    pub msg_id: u64,
+    #[serde(default, skip_serializing_if = "u64_zero_by_ref")]
+    pub in_reply_to: u64,
+    #[serde(flatten)]
+    pub payload: Payload,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum ErrorCode {
+    Definite(DefiniteError),
+    Indefinite(IndefiniteError),
+}
+
+#[derive(Debug, Clone, Serialize_repr, Deserialize_repr)]
+#[repr(u64)]
+pub enum DefiniteError {
+    NodeNotFound = 2,
+    NotSupported = 10,
+    TemporarilyUnavailable = 11,
+    MalformedRequest = 12,
+    Abort = 14,
+    KeyNotFound = 20,
+    KeyAlreadyExists = 21,
+    PreconditionFailed = 22,
+    TxnConflict = 30,
+}
+
+#[derive(Debug, Clone, Serialize_repr, Deserialize_repr)]
+#[repr(u64)]
+pub enum IndefiniteError {
+    Timeout = 0,
+    Crash = 13,
+}
+
+pub fn init_ok(msg_id: u64, in_reply_to: u64) -> Body {
+    Body {
+        typ: "init_ok".to_string(),
+        msg_id,
+        in_reply_to,
+        payload: Payload::new(),
+    }
+}
+
+pub fn error(msg_id: u64, in_reply_to: u64, code: ErrorCode, text: Option<&str>) -> Body {
+    Body {
+        typ: "error".to_string(),
+        msg_id,
+        in_reply_to,
+        payload: [
+            ("code".to_string(), serde_json::to_value(code).unwrap()),
+            ("text".to_string(), serde_json::to_value(text).unwrap()),
+        ]
+        .into_iter()
+        .collect(),
+    }
+}
+
+#[allow(clippy::trivially_copy_pass_by_ref)]
+fn u64_zero_by_ref(num: &u64) -> bool {
+    *num == 0
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn error_codes() {
+        let ec = ErrorCode::Definite(DefiniteError::Abort);
+        let e = serde_json::to_string(&ec).unwrap();
+        assert_eq!(&e, "14");
+    }
+}