diff --git a/crates/shared/src/protocol.rs b/crates/shared/src/protocol.rs
index 4b68e06..13a2945 100644
--- a/crates/shared/src/protocol.rs
+++ b/crates/shared/src/protocol.rs
@@ -2,8 +2,9 @@
 
 //! Network protocol types and constants.
 //!
-//! The module is split by stream purpose, starting with the [`control`]-stream handshake and disconnect messages. Control-stream types are re-exported here so callers continue to refer to `shared::protocol::<Type>` regardless of the internal layout.
+//! The module is split by stream purpose: [`control`]-stream handshake and disconnect messages, and the [`chunk`]-sync request/delivery messages. Control-stream types are re-exported here so callers continue to refer to `shared::protocol::<Type>` regardless of the internal layout, while the chunk types stay namespaced under `shared::protocol::chunk` to keep the two protocols visually distinct.
 
+pub mod chunk;
 mod control;
 
 pub use control::{
diff --git a/crates/shared/src/protocol/chunk.rs b/crates/shared/src/protocol/chunk.rs
new file mode 100644
index 0000000..236443f
--- /dev/null
+++ b/crates/shared/src/protocol/chunk.rs
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Chunk-sync message types carried between server and client.
+
+use crate::world::{ChunkData, ChunkPos};
+use serde::{Deserialize, Serialize};
+
+/// A client's request for the chunks it wants resident, expressed as a center and radius.
+// TODO: per-chunk request/ack + flow control for the fuller protocol.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct ChunkSubscribe {
+    /// Chunk-space center the client wants chunks around (derived from its camera/player).
+    pub center: ChunkPos,
+    /// Load radius in chunks. The server clamps this to a server-side maximum.
+    pub radius: u16,
+}
+
+/// One chunk delivered to the client.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum ChunkMessage {
+    /// Full chunk payload at a position.
+    // TODO: LOD tier selection so a chunk can be delivered at a coarser stream tier.
+    Chunk {
+        /// Position of the delivered chunk.
+        pos: ChunkPos,
+        /// Serializable chunk contents (reuses the save/diff representation).
+        data: ChunkData,
+    },
+    /// The server has dropped this chunk from the client's set; the client should unload it. This is the server-authoritative counterpart to the client's own radius-based unload: the server can force a discard even when the chunk is still within the client's radius.
+    Drop {
+        /// Position the client should discard.
+        pos: ChunkPos,
+    },
+}
+
+#[cfg(test)]
+#[path = "../tests/protocol_chunk.rs"]
+mod tests;
diff --git a/crates/shared/src/tests/protocol_chunk.rs b/crates/shared/src/tests/protocol_chunk.rs
new file mode 100644
index 0000000..07b6de7
--- /dev/null
+++ b/crates/shared/src/tests/protocol_chunk.rs
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+use super::*;
+use crate::world::BlockId;
+
+/// Round-trips a `ChunkSubscribe` through postcard and asserts the decoded value is identical.
+fn roundtrip_subscribe(msg: &ChunkSubscribe) -> Result<(), postcard::Error> {
+    let bytes = postcard::to_stdvec(msg)?;
+    let decoded: ChunkSubscribe = postcard::from_bytes(&bytes)?;
+    assert_eq!(msg, &decoded);
+    Ok(())
+}
+
+/// Round-trips a `ChunkMessage` through postcard and asserts the decoded value is identical.
+fn roundtrip_message(msg: &ChunkMessage) -> Result<(), postcard::Error> {
+    let bytes = postcard::to_stdvec(msg)?;
+    let decoded: ChunkMessage = postcard::from_bytes(&bytes)?;
+    assert_eq!(msg, &decoded);
+    Ok(())
+}
+
+#[test]
+fn roundtrip_chunk_subscribe() -> Result<(), postcard::Error> {
+    // Negative coordinates exercise the signed ChunkPos fields under serialization.
+    let msg = ChunkSubscribe {
+        center: ChunkPos::new(-4, 0, 7),
+        radius: 12,
+    };
+    roundtrip_subscribe(&msg)
+}
+
+#[test]
+fn roundtrip_chunk_message_chunk() -> Result<(), postcard::Error> {
+    // A non-empty ChunkData: private fields force construction via new + set.
+    let mut data = ChunkData::new(ChunkPos::new(1, 2, 3), 0);
+    data.set(0, BlockId(1));
+    data.set(42, BlockId(1));
+    let msg = ChunkMessage::Chunk {
+        pos: ChunkPos::new(1, 2, 3),
+        data,
+    };
+    roundtrip_message(&msg)
+}
+
+#[test]
+fn roundtrip_chunk_message_drop() -> Result<(), postcard::Error> {
+    let msg = ChunkMessage::Drop {
+        pos: ChunkPos::new(0, -1, 0),
+    };
+    roundtrip_message(&msg)
+}