diff --git a/src/core/model/mod.rs b/src/core/model/mod.rs
index 2de6281..4fb4dc9 100644
--- a/src/core/model/mod.rs
+++ b/src/core/model/mod.rs
@@ -2,7 +2,7 @@
 pub const IDENTIFIER_SIZE_BYTES: usize = 32;
 
 pub mod address;
-pub(crate) mod direction;
+pub mod direction;
 pub mod identifier;
 pub mod identity;
 pub(crate) mod join;
diff --git a/src/lib.rs b/src/lib.rs
index af57158..30fbabf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod core;
 mod network;
 mod node;
+pub mod observability;
diff --git a/src/observability/labels.rs b/src/observability/labels.rs
new file mode 100644
index 0000000..977c500
--- /dev/null
+++ b/src/observability/labels.rs
@@ -0,0 +1,68 @@
+use crate::core::LookupTableLevel;
+
+/// Outcome of a completed identifier search, used as a metric label.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SearchOutcome {
+    /// A matching neighbor was returned by the search.
+    Found,
+    /// No matching neighbor existed at any level, so the search returned the
+    /// caller's own identifier (the Aspnes & Shah own-identifier fallback).
+    NotFound,
+}
+
+/// Type of a network message, used as a metric label. One variant per event kind.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum MessageType {
+    /// A test-only string payload (not used in production).
+    TestMessage,
+    /// An identifier-search request payload.
+    SearchByIdRequest,
+    /// An identifier-search response payload.
+    SearchByIdResponse,
+}
+
+/// The lower bound (inclusive) of the [`LevelBucket::Medium`] range.
+const MEDIUM_LEVEL_FLOOR: LookupTableLevel = 16;
+/// The lower bound (inclusive) of the [`LevelBucket::High`] range.
+const HIGH_LEVEL_FLOOR: LookupTableLevel = 64;
+/// The lower bound (inclusive) of the [`LevelBucket::Overflow`] range.
+const OVERFLOW_LEVEL_FLOOR: LookupTableLevel = 256;
+
+/// A coarse bucket over a [`LookupTableLevel`], used as a metric label.
+///
+/// Levels can number in the hundreds; labeling by raw level would explode
+/// cardinality, so it is bucketed. The mapping is total, with
+/// [`LevelBucket::Overflow`] catching levels at or beyond the expected maximum.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum LevelBucket {
+    /// Levels `0..16`.
+    Low,
+    /// Levels `16..64`.
+    Medium,
+    /// Levels `64..256`.
+    High,
+    /// Levels `256` and above (not expected in normal operation).
+    Overflow,
+}
+
+impl LevelBucket {
+    /// Maps a lookup-table level to its bucket. Total over all `usize` values.
+    #[must_use]
+    pub const fn from_level(level: LookupTableLevel) -> Self {
+        if level < MEDIUM_LEVEL_FLOOR {
+            LevelBucket::Low
+        } else if level < HIGH_LEVEL_FLOOR {
+            LevelBucket::Medium
+        } else if level < OVERFLOW_LEVEL_FLOOR {
+            LevelBucket::High
+        } else {
+            LevelBucket::Overflow
+        }
+    }
+}
+
+impl From<LookupTableLevel> for LevelBucket {
+    fn from(level: LookupTableLevel) -> Self {
+        LevelBucket::from_level(level)
+    }
+}
diff --git a/src/observability/labels_test.rs b/src/observability/labels_test.rs
new file mode 100644
index 0000000..b19574e
--- /dev/null
+++ b/src/observability/labels_test.rs
@@ -0,0 +1,32 @@
+use crate::core::LookupTableLevel;
+use crate::observability::labels::LevelBucket;
+
+/// Levels are bucketed into the expected fixed ranges, including the boundary
+/// values and the overflow tail.
+#[test]
+fn test_level_bucket_ranges() {
+    let cases: &[(LookupTableLevel, LevelBucket)] = &[
+        (0, LevelBucket::Low),
+        (15, LevelBucket::Low),
+        (16, LevelBucket::Medium),
+        (63, LevelBucket::Medium),
+        (64, LevelBucket::High),
+        (255, LevelBucket::High),
+        (256, LevelBucket::Overflow),
+        (usize::MAX, LevelBucket::Overflow),
+    ];
+
+    for (level, expected) in cases {
+        assert_eq!(
+            LevelBucket::from_level(*level),
+            *expected,
+            "level {level} bucketed incorrectly"
+        );
+        // `From` must agree with `from_level`.
+        assert_eq!(
+            LevelBucket::from(*level),
+            *expected,
+            "from disagreed at {level}"
+        );
+    }
+}
diff --git a/src/observability/metrics.rs b/src/observability/metrics.rs
new file mode 100644
index 0000000..aa15547
--- /dev/null
+++ b/src/observability/metrics.rs
@@ -0,0 +1,33 @@
+use crate::core::model::direction::Direction;
+use crate::observability::labels::{LevelBucket, MessageType, SearchOutcome};
+use std::time::Duration;
+
+/// Records metrics for identifier-search operations.
+#[unimock::unimock(api = SearchMetricsMock)]
+pub trait SearchMetrics: Send + Sync {
+    /// Records a completed search by its outcome, hop count, and duration.
+    fn record_search(&self, outcome: SearchOutcome, hops: usize, elapsed: Duration);
+}
+
+/// Records metrics for lookup-table mutations.
+#[unimock::unimock(api = LookupTableMetricsMock)]
+pub trait LookupTableMetrics: Send + Sync {
+    /// Records installation of a neighbor at the given level bucket and direction.
+    fn record_neighbor_install(&self, level: LevelBucket, direction: Direction);
+}
+
+/// Records metrics for network message flow.
+#[unimock::unimock(api = NetworkMetricsMock)]
+pub trait NetworkMetrics: Send + Sync {
+    /// Records that a message of the given type was sent.
+    fn record_message_sent(&self, message_type: MessageType);
+
+    /// Records that a message of the given type was received.
+    fn record_message_received(&self, message_type: MessageType);
+}
+
+/// The full metric surface a node records against, composing the per-subsystem traits.
+pub trait NodeMetrics: SearchMetrics + LookupTableMetrics + NetworkMetrics {}
+
+// Backends implement the three sub-traits; `NodeMetrics` follows automatically and is never implemented directly.
+impl<T> NodeMetrics for T where T: SearchMetrics + LookupTableMetrics + NetworkMetrics {}
diff --git a/src/observability/mod.rs b/src/observability/mod.rs
new file mode 100644
index 0000000..bb1715f
--- /dev/null
+++ b/src/observability/mod.rs
@@ -0,0 +1,30 @@
+//! Observability interfaces for Skip Graph nodes.
+//!
+//! Defines the interface the node code calls to record metric events. It is
+//! backend-free (no OpenTelemetry, Prometheus, or other metrics dependency);
+//! concrete backends are implemented elsewhere.
+//!
+//! # Infallibility contract
+//!
+//! Every metric method returns `()` and must never panic: emitting a metric
+//! cannot fail or influence the operation being measured. Implementations
+//! swallow their own backend errors.
+//!
+//! # Cardinality contract
+//!
+//! Labels are closed enums only ([`SearchOutcome`], [`MessageType`],
+//! [`LevelBucket`], [`Direction`]), never identifiers or other unbounded
+//! values, so the type system makes a high-cardinality label unrepresentable.
+
+mod labels;
+mod metrics;
+
+#[cfg(test)]
+mod labels_test;
+
+pub use crate::core::model::direction::Direction;
+pub use labels::{LevelBucket, MessageType, SearchOutcome};
+pub use metrics::{
+    LookupTableMetrics, LookupTableMetricsMock, NetworkMetrics, NetworkMetricsMock, NodeMetrics,
+    SearchMetrics, SearchMetricsMock,
+};