diff --git a/Cargo.lock b/Cargo.lock
index 19dc1d4..c8b320e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -147,11 +147,16 @@ version = "0.1.0"
 name = "kaptan-drivers"
 version = "0.1.0"
 dependencies = [
+ "kaptan-i2c",
  "kaptan-sensor",
  "kaptan-spi",
  "kaptan-time",
 ]
 
+[[package]]
+name = "kaptan-i2c"
+version = "0.1.0"
+
 [[package]]
 name = "kaptan-kakute-f7-mini-v3"
 version = "0.1.0"
@@ -188,6 +193,7 @@ name = "kaptan-runtime"
 version = "0.1.0"
 dependencies = [
  "kaptan-drivers",
+ "kaptan-i2c",
  "kaptan-sensor",
  "kaptan-spi",
  "kaptan-time",
@@ -210,6 +216,7 @@ version = "0.1.0"
 dependencies = [
  "cortex-m",
  "kaptan-dma",
+ "kaptan-i2c",
  "kaptan-spi",
  "kaptan-time",
  "stm32f7",
diff --git a/Cargo.toml b/Cargo.toml
index 4bb4a42..9d22a2b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,6 +4,7 @@ members = [
     "libs/kaptan-actuator",
     "libs/kaptan-dma",
     "libs/kaptan-drivers",
+    "libs/kaptan-i2c",
     "libs/kaptan-mavlink",
     "libs/kaptan-runtime",
     "libs/kaptan-sensor",
diff --git a/libs/kaptan-drivers/Cargo.toml b/libs/kaptan-drivers/Cargo.toml
index 6361f79..871ba91 100644
--- a/libs/kaptan-drivers/Cargo.toml
+++ b/libs/kaptan-drivers/Cargo.toml
@@ -11,5 +11,6 @@ workspace = true
 
 [dependencies]
 kaptan-sensor = { path = "../kaptan-sensor" }
+kaptan-i2c = { path = "../kaptan-i2c" }
 kaptan-spi = { path = "../kaptan-spi" }
 kaptan-time = { path = "../kaptan-time" }
diff --git a/libs/kaptan-i2c/Cargo.toml b/libs/kaptan-i2c/Cargo.toml
new file mode 100644
index 0000000..edf98fc
--- /dev/null
+++ b/libs/kaptan-i2c/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "kaptan-i2c"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+[lints]
+workspace = true
diff --git a/libs/kaptan-i2c/src/lib.rs b/libs/kaptan-i2c/src/lib.rs
new file mode 100644
index 0000000..ba3a20f
--- /dev/null
+++ b/libs/kaptan-i2c/src/lib.rs
@@ -0,0 +1,50 @@
+//! Non-blocking I²C transaction contract.
+//!
+//! Controllers own in-flight transfer storage. Implementations may use
+//! interrupts or DMA, but must never make a driver wait for a peripheral
+//! status flag.
+
+#![no_std]
+
+/// Terminal outcome reported by an I²C controller.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum I2cCompletion {
+    /// The bound interrupt had no terminal transaction event.
+    NotPending,
+    /// The submitted transaction completed and read bytes are valid.
+    Complete,
+    /// The submitted transaction failed and read bytes are unusable.
+    TransferFault,
+}
+
+/// Owns a non-blocking I²C controller route and its in-flight storage.
+///
+/// Each operation sends a STOP on successful completion. Controllers copy a
+/// submitted write into owned storage before returning. A completed read is
+/// copied into the caller-provided destination only from
+/// [`Self::complete_transaction`], after the hardware no longer accesses the
+/// controller-owned storage. This avoids retaining pointers to caller slices
+/// across interrupt boundaries.
+pub trait I2cBus {
+    /// Failure to submit a transaction.
+    type StartError;
+
+    /// Starts one address-scoped write transaction ending with a STOP.
+    fn start_write(&mut self, address: u8, write: &[u8]) -> Result<(), Self::StartError>;
+
+    /// Starts one address-scoped read transaction ending with a STOP.
+    fn start_read(&mut self, address: u8, length: usize) -> Result<(), Self::StartError>;
+
+    /// Releases controller ownership after a hardware ISR acknowledged the
+    /// terminal event and, for a successful read, copies into `read`.
+    ///
+    /// This must not poll or wait. `read` must match the length submitted to
+    /// [`Self::start_read`] for the completed read transaction.
+    fn complete_transaction(&mut self, completion: I2cCompletion, read: Option<&mut [u8]>);
+
+    /// Cancels an in-flight transaction after a board-selected timeout.
+    ///
+    /// Implementations must release controller-owned transaction state and
+    /// arrange for any required STOP or peripheral recovery without waiting.
+    fn cancel_transaction(&mut self);
+}
diff --git a/libs/kaptan-runtime/Cargo.toml b/libs/kaptan-runtime/Cargo.toml
index 7e39dbe..072715b 100644
--- a/libs/kaptan-runtime/Cargo.toml
+++ b/libs/kaptan-runtime/Cargo.toml
@@ -11,6 +11,7 @@ workspace = true
 
 [dependencies]
 kaptan-drivers = { path = "../kaptan-drivers" }
+kaptan-i2c = { path = "../kaptan-i2c" }
 kaptan-sensor = { path = "../kaptan-sensor" }
 kaptan-spi = { path = "../kaptan-spi" }
 kaptan-time = { path = "../kaptan-time" }
diff --git a/libs/kaptan-stm32f7/Cargo.toml b/libs/kaptan-stm32f7/Cargo.toml
index d753bf6..dccbec2 100644
--- a/libs/kaptan-stm32f7/Cargo.toml
+++ b/libs/kaptan-stm32f7/Cargo.toml
@@ -16,6 +16,7 @@ stm32f722 = ["stm32f7/stm32f722"]
 [dependencies]
 cortex-m = "0.7"
 kaptan-dma = { path = "../kaptan-dma" }
+kaptan-i2c = { path = "../kaptan-i2c" }
 kaptan-spi = { path = "../kaptan-spi" }
 kaptan-time = { path = "../kaptan-time" }
 stm32f7 = { version = "0.16", default-features = false }