diff --git a/Cargo.lock b/Cargo.lock
index afc300e..e6e6733 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -33,6 +33,7 @@ dependencies = [
  "ahash",
  "hashbrown",
  "mino",
+ "serde",
 ]
 
 [[package]]
diff --git a/fish/Cargo.toml b/fish/Cargo.toml
index 75bcbb1..9d6adb7 100644
--- a/fish/Cargo.toml
+++ b/fish/Cargo.toml
@@ -5,10 +5,12 @@ version = "0.1.0"
 edition = "2021"
 
 [features]
-default = []
+default = ["io"]
+io = ["serde"]
 
 [dependencies]
 mino = { path = "../mino" }
 
 ahash = "0.8"
 hashbrown = "0.13"
+serde = { version = "1.0", features = ["derive"], optional = true }
diff --git a/fish/src/io.rs b/fish/src/io.rs
new file mode 100644
index 0000000..d253832
--- /dev/null
+++ b/fish/src/io.rs
@@ -0,0 +1,194 @@
+use alloc::boxed::Box;
+use alloc::string::String;
+use alloc::vec::Vec;
+use mino::srs::PieceType;
+use mino::MatBuf;
+use mino::{srs::Piece, Loc};
+use serde::{Deserialize, Serialize};
+
+/// Deserializable description of an initial game state. This is intentionally similar to
+/// the TBP "start" message.
+#[derive(Clone, Debug, Deserialize)]
+pub struct InputState {
+    #[serde(flatten)]
+    pub queue: InputQueue,
+    #[allow(dead_code)]
+    #[serde(flatten)]
+    _attack: AttackState,
+    // XXX(iitalics): TBP uses the terminology "board" but we use "matrix".
+    #[serde(rename = "board")]
+    pub matrix: InputMatrix,
+}
+
+/// Deserializable description of the queue.
+#[derive(Clone, Debug, Deserialize)]
+pub struct InputQueue {
+    #[serde(deserialize_with = "deserialize_hold")]
+    pub hold: Option<PieceType>,
+    #[serde(deserialize_with = "deserialize_previews")]
+    #[serde(rename = "queue")]
+    pub previews: Vec<PieceType>,
+}
+
+#[derive(Clone, Debug, Deserialize)]
+#[allow(dead_code)]
+struct AttackState {
+    combo: u32,
+    back_to_back: u32,
+    // TODO: use this?
+}
+
+// use FromStr to deserialize a PieceType
+
+fn deserialize_hold<'de, D>(de: D) -> Result<Option<PieceType>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    Option::<String>::deserialize(de)?
+        .map(|s| s.parse())
+        .transpose()
+        .map_err(serde::de::Error::custom)
+}
+
+fn deserialize_previews<'de, D>(de: D) -> Result<Vec<PieceType>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    Vec::<String>::deserialize(de)?
+        .iter()
+        .map(|s| s.parse())
+        .collect::<Result<Vec<_>, _>>()
+        .map_err(serde::de::Error::custom)
+}
+
+/// Deserializable description of the game matrix.
+#[derive(Clone, Debug)]
+pub struct InputMatrix {
+    pub cells: Box<[[Option<Color>; 10]; 40]>,
+}
+
+impl InputMatrix {
+    /// Converts this matrix to a [`MatBuf`], which discards color information but is
+    /// stored more efficiently.
+    pub fn to_mat(&self) -> MatBuf {
+        let mut mat = MatBuf::new();
+        for (y, row) in self.cells.iter().enumerate() {
+            for (x, cell) in row.iter().enumerate() {
+                if cell.is_some() {
+                    mat.set(x as i16, y as i16);
+                }
+            }
+        }
+        mat
+    }
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Debug, Deserialize)]
+#[repr(u8)]
+pub enum Color {
+    I = 1,
+    J = 2,
+    L = 3,
+    O = 4,
+    S = 5,
+    T = 6,
+    Z = 7,
+    G = 8,
+}
+
+impl<'de> serde::de::Deserialize<'de> for InputMatrix {
+    fn deserialize<D>(de: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        // XXX(iitalics): serde doesn't let you deserialize a [T; 40] so we have to go
+        // through Vec first and then check the length :|
+        let cells: Vec<_> = serde::de::Deserialize::deserialize(de)?;
+        let cells: [_; 40] = cells
+            .try_into()
+            .map_err(|_| serde::de::Error::custom("board must contain 40 rows"))?;
+        Ok(Self {
+            cells: Box::new(cells),
+        })
+    }
+}
+
+#[derive(Clone, Debug, Serialize, Default)]
+pub struct OutputMoves {
+    pub moves: Vec<OutputMove>,
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub struct OutputMove {
+    pub location: OutputLocation,
+    pub spin: Spin,
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub struct OutputLocation {
+    #[serde(serialize_with = "serialize_piece_type")]
+    #[serde(rename = "type")]
+    pub ty: PieceType,
+    #[serde(serialize_with = "serialize_loc")]
+    #[serde(flatten)]
+    pub location: Loc,
+}
+
+impl From<Piece> for OutputLocation {
+    fn from(pc: Piece) -> Self {
+        Self {
+            ty: pc.ty,
+            location: pc.loc,
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Default, Serialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Spin {
+    #[default]
+    None,
+    Mini,
+    Full,
+}
+
+fn serialize_piece_type<S>(ty: &PieceType, ser: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    ty.as_char().serialize(ser)
+}
+
+fn serialize_loc<S>(loc: &Loc, ser: S) -> Result<S::Ok, S::Error>
+where
+    S: serde::Serializer,
+{
+    use serde::ser::SerializeStruct;
+    let mut ser = ser.serialize_struct("Location", 3)?;
+    ser.serialize_field("x", &loc.x)?;
+    ser.serialize_field("y", &loc.y)?;
+    let r = ["north", "east", "south", "west"][loc.r as usize];
+    ser.serialize_field("orientation", r)?;
+    ser.end()
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_color_repr() {
+        assert_eq!(
+            core::mem::size_of::<Option<Color>>(),
+            core::mem::size_of::<u8>()
+        );
+        assert_eq!(
+            core::mem::size_of::<[Option<Color>; 10]>(),
+            core::mem::size_of::<[u8; 10]>()
+        );
+        assert_eq!(
+            unsafe { core::mem::transmute::<Option<Color>, u8>(None) },
+            0u8
+        );
+    }
+}
diff --git a/fish/src/lib.rs b/fish/src/lib.rs
index f6deba4..f53dd73 100644
--- a/fish/src/lib.rs
+++ b/fish/src/lib.rs
@@ -4,4 +4,7 @@ extern crate alloc;
 
 pub mod find;
 
+#[cfg(feature = "io")]
+pub mod io;
+
 pub use find::find_locations;