diff --git a/libs/kaptan-spi/Cargo.toml b/libs/kaptan-spi/Cargo.toml
new file mode 100644
index 0000000..4261e19
--- /dev/null
+++ b/libs/kaptan-spi/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "kaptan-spi"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+[lints]
+workspace = true
diff --git a/libs/kaptan-spi/src/lib.rs b/libs/kaptan-spi/src/lib.rs
new file mode 100644
index 0000000..e30f986
--- /dev/null
+++ b/libs/kaptan-spi/src/lib.rs
@@ -0,0 +1,44 @@
+//! Non-blocking, chip-select-scoped SPI transfer contract.
+//!
+//! A selected device owns electrical chip-select behavior. A driver retains its
+//! transfer storage until a terminal completion event; implementations submit
+//! DMA or interrupt-driven work and never wait for a peripheral flag.
+
+#![no_std]
+
+/// Terminal outcome reported by a selected SPI device.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum SpiCompletion {
+    /// The bound interrupt does not report a terminal event.
+    NotPending,
+    /// The submitted full-duplex transfer completed.
+    Complete,
+    /// The submitted transfer failed and its receive bytes are unusable.
+    TransferFault,
+}
+
+/// Owns one chip-select-scoped, non-blocking SPI route.
+///
+/// `start_transfer` begins a complete transfer and returns without waiting for
+/// a peripheral status flag. The caller owns both buffers until the board
+/// reports a terminal `SpiCompletion`; `complete_transfer` releases the route
+/// after that terminal outcome.
+pub trait SpiDevice {
+    /// Failure to submit a transfer.
+    type StartError;
+
+    /// Starts one complete full-duplex transfer.
+    ///
+    /// Implementations assert chip select before submitting and deassert it on
+    /// every terminal completion or submission failure. `receive` receives one
+    /// octet for each `transmit` octet; the two ranges must have equal length.
+    fn start_transfer(
+        &mut self,
+        transmit: &[u8],
+        receive: &mut [u8],
+    ) -> Result<(), Self::StartError>;
+
+    /// Releases transport ownership after a hardware ISR acknowledged a
+    /// terminal completion. This must not poll or wait.
+    fn complete_transfer(&mut self, completion: SpiCompletion);
+}