diff --git a/Justfile b/Justfile
index ce4a73b..2e67f20 100644
--- a/Justfile
+++ b/Justfile
@@ -32,9 +32,15 @@ dev:
     uv run --directory codetracer-python-recorder maturin develop --uv
 
 # Run unit tests of dev build
-test:
-    uv run --group dev --group test pytest
+test: cargo-test py-test
+
+# Run Rust unit tests without default features to link Python C library
+cargo-test:
+    uv run cargo test --manifest-path codetracer-python-recorder/Cargo.toml --no-default-features
 
+py-test:
+    uv run --group dev --group test pytest
+    
 # Run tests only on the pure recorder
 test-pure:
     uv run --group dev --group test pytest codetracer-pure-python-recorder
diff --git a/codetracer-python-recorder/Cargo.lock b/codetracer-python-recorder/Cargo.lock
index da12a63..e0c1fa0 100644
--- a/codetracer-python-recorder/Cargo.lock
+++ b/codetracer-python-recorder/Cargo.lock
@@ -68,6 +68,7 @@ checksum = "9555578bc9e57714c812a1f84e4fc5b4d21fcb063490c624de019f7464c91268"
 name = "codetracer-python-recorder"
 version = "0.1.0"
 dependencies = [
+ "bitflags",
  "pyo3",
  "runtime_tracing",
 ]
diff --git a/codetracer-python-recorder/Cargo.toml b/codetracer-python-recorder/Cargo.toml
index e3c31c3..b25b86c 100644
--- a/codetracer-python-recorder/Cargo.toml
+++ b/codetracer-python-recorder/Cargo.toml
@@ -8,8 +8,16 @@ repository = "https://github.com/metacraft-labs/codetracer-python-recorder"
 
 [lib]
 name = "codetracer_python_recorder"
-crate-type = ["cdylib"]
+crate-type = ["cdylib", "rlib"]
+
+[features]
+extension-module = ["pyo3/extension-module"]
+default = ["extension-module"]
 
 [dependencies]
-pyo3 = { version = "0.25.1", features = ["extension-module"] }
-runtime_tracing = "0.14.0"
\ No newline at end of file
+pyo3 = { version = "0.25.1" }
+runtime_tracing = "0.14.0"
+bitflags = "2.4"
+
+[dev-dependencies]
+pyo3 = { version = "0.25.1", features = ["auto-initialize"] }
diff --git a/codetracer-python-recorder/src/lib.rs b/codetracer-python-recorder/src/lib.rs
index 92782b5..31d6b52 100644
--- a/codetracer-python-recorder/src/lib.rs
+++ b/codetracer-python-recorder/src/lib.rs
@@ -3,6 +3,9 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use pyo3::exceptions::PyRuntimeError;
 use pyo3::prelude::*;
 
+pub mod tracer;
+pub use crate::tracer::{install_tracer, uninstall_tracer, EventSet, Tracer};
+
 /// Global flag tracking whether tracing is active.
 static ACTIVE: AtomicBool = AtomicBool::new(false);
 
diff --git a/codetracer-python-recorder/src/tracer.rs b/codetracer-python-recorder/src/tracer.rs
new file mode 100644
index 0000000..2842906
--- /dev/null
+++ b/codetracer-python-recorder/src/tracer.rs
@@ -0,0 +1,244 @@
+use std::sync::{Mutex, OnceLock};
+use pyo3::{
+    exceptions::PyRuntimeError,
+    prelude::*,
+    types::{PyAny, PyCFunction, PyModule},
+};
+
+const MONITORING_TOOL_NAME: &str = "codetracer";
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct EventId(pub i32);
+
+#[allow(non_snake_case)]
+#[derive(Clone, Copy, Debug)]
+pub struct MonitoringEvents {
+    pub BRANCH: EventId,
+    pub CALL: EventId,
+    pub C_RAISE: EventId,
+    pub C_RETURN: EventId,
+    pub EXCEPTION_HANDLED: EventId,
+    pub INSTRUCTION: EventId,
+    pub JUMP: EventId,
+    pub LINE: EventId,
+    pub PY_RESUME: EventId,
+    pub PY_RETURN: EventId,
+    pub PY_START: EventId,
+    pub PY_THROW: EventId,
+    pub PY_UNWIND: EventId,
+    pub PY_YIELD: EventId,
+    pub RAISE: EventId,
+    pub RERAISE: EventId,
+    pub STOP_ITERATION: EventId,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct ToolId {
+    pub id: u8,
+}
+
+pub type CallbackFn<'py> = Bound<'py, PyCFunction>;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct EventSet(pub i32);
+
+pub const NO_EVENTS: EventSet = EventSet(0);
+
+impl EventSet {
+    pub const fn empty() -> Self {
+        NO_EVENTS
+    }
+    pub fn contains(&self, ev: &EventId) -> bool {
+        (self.0 & ev.0) != 0
+    }
+}
+
+pub fn acquire_tool_id(py: Python<'_>) -> PyResult<ToolId> {
+    let monitoring = py.import("sys")?.getattr("monitoring")?;
+    const FALLBACK_ID: u8 = 5;
+    monitoring.call_method1("use_tool_id", (FALLBACK_ID, MONITORING_TOOL_NAME))?;
+    Ok(ToolId { id: FALLBACK_ID })
+}
+
+pub fn load_monitoring_events(py: Python<'_>) -> PyResult<MonitoringEvents> {
+    let monitoring = py.import("sys")?.getattr("monitoring")?;
+    let events = monitoring.getattr("events")?;
+    Ok(MonitoringEvents {
+        BRANCH: EventId(events.getattr("BRANCH")?.extract()?),
+        CALL: EventId(events.getattr("CALL")?.extract()?),
+        C_RAISE: EventId(events.getattr("C_RAISE")?.extract()?),
+        C_RETURN: EventId(events.getattr("C_RETURN")?.extract()?),
+        EXCEPTION_HANDLED: EventId(events.getattr("EXCEPTION_HANDLED")?.extract()?),
+        INSTRUCTION: EventId(events.getattr("INSTRUCTION")?.extract()?),
+        JUMP: EventId(events.getattr("JUMP")?.extract()?),
+        LINE: EventId(events.getattr("LINE")?.extract()?),
+        PY_RESUME: EventId(events.getattr("PY_RESUME")?.extract()?),
+        PY_RETURN: EventId(events.getattr("PY_RETURN")?.extract()?),
+        PY_START: EventId(events.getattr("PY_START")?.extract()?),
+        PY_THROW: EventId(events.getattr("PY_THROW")?.extract()?),
+        PY_UNWIND: EventId(events.getattr("PY_UNWIND")?.extract()?),
+        PY_YIELD: EventId(events.getattr("PY_YIELD")?.extract()?),
+        RAISE: EventId(events.getattr("RAISE")?.extract()?),
+        RERAISE: EventId(events.getattr("RERAISE")?.extract()?),
+        STOP_ITERATION: EventId(events.getattr("STOP_ITERATION")?.extract()?),
+    })
+}
+
+static MONITORING_EVENTS: OnceLock<MonitoringEvents> = OnceLock::new();
+
+pub fn monitoring_events(py: Python<'_>) -> PyResult<&'static MonitoringEvents> {
+    if let Some(ev) = MONITORING_EVENTS.get() {
+        return Ok(ev);
+    }
+    let ev = load_monitoring_events(py)?;
+    let _ = MONITORING_EVENTS.set(ev);
+    Ok(MONITORING_EVENTS.get().unwrap())
+}
+
+pub fn register_callback(
+    py: Python<'_>,
+    tool: &ToolId,
+    event: &EventId,
+    cb: Option<&CallbackFn<'_>>,
+) -> PyResult<()> {
+    let monitoring = py.import("sys")?.getattr("monitoring")?;
+    match cb {
+        Some(cb) => {
+            monitoring.call_method("register_callback", (tool.id, event.0, cb), None)?;
+        }
+        None => {
+            monitoring.call_method("register_callback", (tool.id, event.0, py.None()), None)?;
+        }
+    }
+    Ok(())
+}
+
+pub fn events_union(ids: &[EventId]) -> EventSet {
+    let mut bits = 0i32;
+    for id in ids {
+        bits |= id.0;
+    }
+    EventSet(bits)
+}
+
+pub fn set_events(py: Python<'_>, tool: &ToolId, set: EventSet) -> PyResult<()> {
+    let monitoring = py.import("sys")?.getattr("monitoring")?;
+    monitoring.call_method1("set_events", (tool.id, set.0))?;
+    Ok(())
+}
+
+pub fn free_tool_id(py: Python<'_>, tool: &ToolId) -> PyResult<()> {
+    let monitoring = py.import("sys")?.getattr("monitoring")?;
+    monitoring.call_method1("free_tool_id", (tool.id,))?;
+    Ok(())
+}
+
+
+/// Trait implemented by tracing backends.
+///
+/// Each method corresponds to an event from `sys.monitoring`. Default
+/// implementations allow implementers to only handle the events they care
+/// about.
+pub trait Tracer: Send {
+    /// Return the set of events the tracer wants to receive.
+    fn interest(&self, _events: &MonitoringEvents) -> EventSet {
+        NO_EVENTS
+    }
+
+    /// Called on Python function calls.
+    fn on_call(
+        &mut self,
+        _py: Python<'_>,
+        _code: &Bound<'_, PyAny>,
+        _offset: i32,
+        _callable: &Bound<'_, PyAny>,
+        _arg0: Option<&Bound<'_, PyAny>>,
+    ) {
+    }
+
+    /// Called on line execution.
+    fn on_line(&mut self, _py: Python<'_>, _code: &Bound<'_, PyAny>, _lineno: u32) {}
+}
+
+struct Global {
+    tracer: Box<dyn Tracer>,
+    mask: EventSet,
+    tool: ToolId,
+}
+
+static GLOBAL: Mutex<Option<Global>> = Mutex::new(None);
+
+/// Install a tracer and hook it into Python's `sys.monitoring`.
+pub fn install_tracer(py: Python<'_>, tracer: Box<dyn Tracer>) -> PyResult<()> {
+    let mut guard = GLOBAL.lock().unwrap();
+    if guard.is_some() {
+        return Err(PyRuntimeError::new_err("tracer already installed"));
+    }
+
+    let tool = acquire_tool_id(py)?;
+    let events = monitoring_events(py)?;
+
+    let module = PyModule::new(py, "_codetracer_callbacks")?;
+
+    let mask = tracer.interest(events);
+
+    if mask.contains(&events.CALL) {
+        let cb = wrap_pyfunction!(callback_call, &module)?;
+
+        register_callback(py, &tool, &events.CALL, Some(&cb))?;
+
+    }
+    if mask.contains(&events.LINE) {
+        let cb = wrap_pyfunction!(callback_line, &module)?;
+        register_callback(py, &tool, &events.LINE, Some(&cb))?;
+    }
+    set_events(py, &tool, mask)?;
+    
+
+    *guard = Some(Global {
+        tracer,
+	mask,
+        tool,
+    });
+    Ok(())
+}
+
+/// Remove the installed tracer if any.
+pub fn uninstall_tracer(py: Python<'_>) -> PyResult<()> {
+    let mut guard = GLOBAL.lock().unwrap();
+    if let Some(global) = guard.take() {
+        let events = monitoring_events(py)?;
+        if global.mask.contains(&events.CALL) {
+            register_callback(py, &global.tool, &events.CALL, None)?;
+        }
+        if global.mask.contains(&events.LINE) {
+            register_callback(py, &global.tool, &events.LINE, None)?;
+        }
+        set_events(py, &global.tool, NO_EVENTS)?;
+        free_tool_id(py, &global.tool)?;
+    }
+    Ok(())
+}
+
+#[pyfunction]
+fn callback_call(
+    py: Python<'_>,
+    code: Bound<'_, PyAny>,
+    offset: i32,
+    callable: Bound<'_, PyAny>,
+    arg0: Option<Bound<'_, PyAny>>,
+) -> PyResult<()> {
+    if let Some(global) = GLOBAL.lock().unwrap().as_mut() {
+        global.tracer.on_call(py, &code, offset, &callable, arg0.as_ref());
+    }
+    Ok(())
+}
+
+#[pyfunction]
+fn callback_line(py: Python<'_>, code: Bound<'_, PyAny>, lineno: u32) -> PyResult<()> {
+    if let Some(global) = GLOBAL.lock().unwrap().as_mut() {
+        global.tracer.on_line(py, &code, lineno);
+    }
+    Ok(())
+}
diff --git a/codetracer-python-recorder/tests/print_tracer.rs b/codetracer-python-recorder/tests/print_tracer.rs
new file mode 100644
index 0000000..5972af7
--- /dev/null
+++ b/codetracer-python-recorder/tests/print_tracer.rs
@@ -0,0 +1,47 @@
+use codetracer_python_recorder::{install_tracer, uninstall_tracer, EventSet, Tracer};
+use codetracer_python_recorder::tracer::{MonitoringEvents, events_union};
+use pyo3::prelude::*;
+use std::ffi::CString;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+static CALL_COUNT: AtomicUsize = AtomicUsize::new(0);
+
+struct PrintTracer;
+
+impl Tracer for PrintTracer {
+    fn interest(&self, events:&MonitoringEvents) -> EventSet {
+	events_union(&[events.CALL])
+    }
+
+    fn on_call(
+        &mut self,
+        _py: Python<'_>,
+        _code: &pyo3::Bound<'_, pyo3::types::PyAny>,
+        _offset: i32,
+        _callable: &pyo3::Bound<'_, pyo3::types::PyAny>,
+        _arg0: Option<&pyo3::Bound<'_, pyo3::types::PyAny>>,
+    ) {
+        CALL_COUNT.fetch_add(1, Ordering::SeqCst);
+    }
+}
+
+#[test]
+fn tracer_prints_on_call() {
+    Python::with_gil(|py| {
+        CALL_COUNT.store(0, Ordering::SeqCst);
+        if let Err(e) = install_tracer(py, Box::new(PrintTracer)) {
+            e.print(py);
+            panic!("Install Tracer failed");
+        }
+        let code = CString::new("def foo():\n    return 1\nfoo()").expect("CString::new failed");
+        if let Err(e) = py.run(code.as_c_str(), None, None) {
+            e.print(py);
+            uninstall_tracer(py).ok();
+            panic!("Python raised an exception");
+        }
+        uninstall_tracer(py).unwrap();
+        let count = CALL_COUNT.load(Ordering::SeqCst);
+        assert!(count >= 1, "expected at least one CALL event, got {}", count);
+    });
+}
+
diff --git a/design-docs/design-001.md b/design-docs/design-001.md
index 1945608..063e668 100644
--- a/design-docs/design-001.md
+++ b/design-docs/design-001.md
@@ -8,6 +8,13 @@ The tracer collects `sys.monitoring` events, converts them to `runtime_tracing`
 
 ## Architecture
 
+### Tracer Abstraction
+Rust code exposes a `Tracer` trait representing callbacks for Python
+`sys.monitoring` events. Implementations advertise their desired events via an
+`EventMask` bit flag returned from `interest`. A `Dispatcher` wraps a trait
+object and forwards events only when the mask contains the corresponding flag,
+allowing tracers to implement just the methods they care about.
+
 ### Tool Initialization
 - Acquire a tool identifier via `sys.monitoring.use_tool_id`; store it for the lifetime of the tracer.
   ```rs