diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1c77ace..df41d33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,9 +9,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
-* Progress reporting and cancellation: `Pipeline::run_with_progress` with a `CancelToken` and a per-phase progress callback (for driving desktop UIs from a worker thread).
-* Two-phase conversion for interactive tuning: `Pipeline::segment` caches the expensive clustering result as a reusable `Segmentation`, and `Pipeline::finish` re-runs only the cheap color-fitting / curve-fitting / optimization stages — so tuning those parameters no longer repays the clustering cost. (Speckle, color precision, and layer difference are clustering parameters and require a fresh `segment`.) Both have `*_with_progress` variants.
-* Binary thresholding methods: a tunable fixed threshold and Bradley–Roth adaptive thresholding (via visioncortex's summed-area table) for images with uneven lighting. Exposed on `Config` (`binary_threshold`, `binary_adaptive`, `binary_adaptive_window`, `binary_adaptive_t`), the CLI (`--threshold`, `--adaptive`, `--adaptive-window`, `--adaptive-t`), Python, and the Node package (`binaryThreshold`, `adaptive`, `adaptiveWindow`, `adaptiveT`).
+* Progress reporting and cancellation for driving a UI from a worker thread: `Pipeline::run_with_progress` with a `CancelToken` and a per-phase callback.
+* `Session`: interactive tuning that clusters an image once and re-renders per `Config` change, re-clustering only when a clustering parameter actually changes. Built on `Pipeline::segment`/`finish` (cache the segmentation, re-run just color/curve/optimize); `Config::segment_key` exposes what it compares.
+* Binary thresholding: a tunable fixed threshold and Bradley–Roth adaptive thresholding for uneven lighting — CLI `--threshold` / `--adaptive` (`--adaptive-window`, `--adaptive-t`), also on `Config`, Python, and Node.
 
 ## 1.0.0-alpha.1 - 2026-07-24
 
diff --git a/crates/vtracer/src/config.rs b/crates/vtracer/src/config.rs
index 882fda3..889921d 100644
--- a/crates/vtracer/src/config.rs
+++ b/crates/vtracer/src/config.rs
@@ -43,6 +43,22 @@ pub enum Preset {
     Photo,
 }
 
+/// The clustering-relevant projection of a [`Config`]. Two configs with equal
+/// keys produce the same [`Segmentation`](crate::Segmentation), so a cached one
+/// stays valid — this is what [`Session`](crate::Session) compares to decide
+/// whether to re-segment. Kept in sync with [`Config::frontend`] in one place.
+#[derive(Debug, Clone, PartialEq)]
+pub struct SegmentKey {
+    color_mode: ColorMode,
+    color_precision: i32,
+    layer_difference: i32,
+    filter_speckle: usize,
+    binary_threshold: u8,
+    binary_adaptive: bool,
+    binary_adaptive_window: u32,
+    binary_adaptive_t: f64,
+}
+
 /// High-level converter configuration. [`Config::build`] turns this into a
 /// concrete [`Pipeline`].
 #[derive(Debug, Clone)]
@@ -236,6 +252,24 @@ impl Config {
         }
     }
 
+    /// The clustering-relevant subset of this config. Changing any field it
+    /// captures (color mode, color precision, layer difference, speckle, or the
+    /// binary threshold settings) requires re-segmenting; changing anything else
+    /// — fit mode, curve params, compositing, palette, optimization — reuses a
+    /// cached segmentation. See [`Session`](crate::Session).
+    pub fn segment_key(&self) -> SegmentKey {
+        SegmentKey {
+            color_mode: self.color_mode,
+            color_precision: self.color_precision,
+            layer_difference: self.layer_difference,
+            filter_speckle: self.filter_speckle,
+            binary_threshold: self.binary_threshold,
+            binary_adaptive: self.binary_adaptive,
+            binary_adaptive_window: self.binary_adaptive_window,
+            binary_adaptive_t: self.binary_adaptive_t,
+        }
+    }
+
     /// Assemble a concrete pipeline from this configuration.
     pub fn build(&self) -> Result<Pipeline, Error> {
         let compositing = match self.hierarchical {
diff --git a/crates/vtracer/src/lib.rs b/crates/vtracer/src/lib.rs
index fd40d77..279ed7e 100644
--- a/crates/vtracer/src/lib.rs
+++ b/crates/vtracer/src/lib.rs
@@ -40,14 +40,16 @@ pub mod mosaic;
 pub mod optimize;
 pub mod pipeline;
 pub mod progress;
+pub mod session;
 pub mod svg;
 
-pub use config::{ColorMode, Config, FitMode, Hierarchical, Preset};
+pub use config::{ColorMode, Config, FitMode, Hierarchical, Preset, SegmentKey};
 pub use error::Error;
 pub use frontend::Threshold;
 pub use ir::{Segmentation, VectorDoc};
 pub use pipeline::Pipeline;
 pub use progress::{CancelToken, Phase, Progress};
+pub use session::Session;
 
 // Re-export the visioncortex value types callers need at the boundary.
 pub use visioncortex::{Color, ColorImage, PointF64, PointI32};
diff --git a/crates/vtracer/src/session.rs b/crates/vtracer/src/session.rs
new file mode 100644
index 0000000..98be553
--- /dev/null
+++ b/crates/vtracer/src/session.rs
@@ -0,0 +1,118 @@
+//! Interactive tuning session: cache the expensive clustering, re-render on the
+//! cheap stages, and re-segment automatically only when it's actually needed.
+//!
+//! A desktop app loads an image once, then calls [`Session::render`] on every
+//! slider change with a fresh [`Config`]. The session compares the config's
+//! [`SegmentKey`](crate::config::SegmentKey) to what it last clustered and
+//! re-segments only if a clustering parameter changed — the caller never has to
+//! know which parameters those are.
+//!
+//! ```no_run
+//! use vtracer::{Config, Session, ColorImage};
+//! # fn load() -> ColorImage { todo!() }
+//! let mut session = Session::new(load());
+//!
+//! // First render clusters the image.
+//! let mut cfg = Config::default();
+//! let _svg = session.render_svg(&cfg).unwrap();
+//!
+//! // Tuning a curve parameter reuses the cached segmentation (no re-cluster).
+//! cfg.corner_threshold = 90;
+//! let _svg = session.render_svg(&cfg).unwrap();
+//!
+//! // Changing a clustering parameter re-segments automatically.
+//! cfg.filter_speckle = 8;
+//! let _svg = session.render_svg(&cfg).unwrap();
+//! ```
+
+use visioncortex::ColorImage;
+
+use crate::config::{Config, SegmentKey};
+use crate::error::Error;
+use crate::ir::{Segmentation, VectorDoc};
+use crate::progress::{CancelToken, Progress};
+
+/// A reusable converter for one image: clusters once, re-renders many times.
+///
+/// Build it with the source [`ColorImage`] and drive it with a [`Config`] per
+/// render. The cached [`Segmentation`] is refreshed transparently whenever the
+/// config's clustering parameters change.
+pub struct Session {
+    img: ColorImage,
+    /// The segmentation and the key it was produced with (`None` until the
+    /// first render).
+    cache: Option<(SegmentKey, Segmentation)>,
+}
+
+impl Session {
+    /// Start a session over `img`. Nothing is clustered until the first render.
+    pub fn new(img: ColorImage) -> Self {
+        Self { img, cache: None }
+    }
+
+    /// Whether the cached segmentation is missing or was clustered with
+    /// different parameters than `key`.
+    fn stale(&self, key: &SegmentKey) -> bool {
+        self.cache.as_ref().map_or(true, |(k, _)| k != key)
+    }
+
+    /// The cached segmentation. Only call after ensuring the cache is fresh.
+    fn segmentation(&self) -> &Segmentation {
+        &self.cache.as_ref().expect("cache populated by caller").1
+    }
+
+    /// Render to the document IR, re-segmenting only if `cfg`'s clustering
+    /// parameters differ from the cached segmentation's.
+    pub fn render(&mut self, cfg: &Config) -> Result<VectorDoc, Error> {
+        let pipeline = cfg.build()?;
+        let key = cfg.segment_key();
+        if self.stale(&key) {
+            self.cache = Some((key, pipeline.segment(&self.img)?));
+        }
+        pipeline.finish(self.segmentation())
+    }
+
+    /// [`render`](Session::render), serialized to an SVG string.
+    pub fn render_svg(&mut self, cfg: &Config) -> Result<String, Error> {
+        let pipeline = cfg.build()?;
+        let key = cfg.segment_key();
+        if self.stale(&key) {
+            self.cache = Some((key, pipeline.segment(&self.img)?));
+        }
+        Ok(pipeline.writer.write(&pipeline.finish(self.segmentation())?))
+    }
+
+    /// [`render`](Session::render) with progress reporting and cancellation.
+    ///
+    /// When a re-segmentation is needed, progress covers the [`Phase::Segment`]
+    /// stage first, then the finish stages; on a cache hit only the finish
+    /// stages report. Hand a clone of `cancel` to the UI to abort a long
+    /// clustering pass.
+    ///
+    /// [`Phase::Segment`]: crate::Phase::Segment
+    pub fn render_with_progress(
+        &mut self,
+        cfg: &Config,
+        cancel: &CancelToken,
+        on_progress: &mut dyn FnMut(Progress),
+    ) -> Result<VectorDoc, Error> {
+        let pipeline = cfg.build()?;
+        let key = cfg.segment_key();
+        if self.stale(&key) {
+            let seg = pipeline.segment_with_progress(&self.img, cancel, on_progress)?;
+            self.cache = Some((key, seg));
+        }
+        pipeline.finish_with_progress(self.segmentation(), cancel, on_progress)
+    }
+
+    /// Drop the cached segmentation, forcing the next render to re-cluster.
+    /// Use after replacing the source image out of band; normally unnecessary.
+    pub fn invalidate(&mut self) {
+        self.cache = None;
+    }
+
+    /// The source image this session renders.
+    pub fn image(&self) -> &ColorImage {
+        &self.img
+    }
+}
diff --git a/crates/vtracer/tests/session.rs b/crates/vtracer/tests/session.rs
new file mode 100644
index 0000000..ae74326
--- /dev/null
+++ b/crates/vtracer/tests/session.rs
@@ -0,0 +1,124 @@
+//! `Session` caches the segmentation and re-segments only when a clustering
+//! parameter changes — verified both at the key level and end-to-end.
+
+use vtracer::{ColorImage, Config, Session};
+
+/// A few colored blocks — several clusters.
+fn blocks() -> ColorImage {
+    let (w, h) = (48usize, 48usize);
+    let mut pixels = Vec::with_capacity(w * h * 4);
+    for y in 0..h {
+        for x in 0..w {
+            let c = match (x / 16, y / 16) {
+                (0, _) => (220u8, 40, 40),
+                (1, 0) => (40, 200, 60),
+                (1, _) => (50, 60, 220),
+                _ => (230, 210, 40),
+            };
+            pixels.extend_from_slice(&[c.0, c.1, c.2, 255]);
+        }
+    }
+    ColorImage {
+        pixels,
+        width: w,
+        height: h,
+    }
+}
+
+/// The key partition: finish-phase params share a segment key; clustering
+/// params change it. This is the contract `Session` relies on.
+#[test]
+fn segment_key_tracks_only_clustering_params() {
+    let base = Config::default();
+
+    // Finish-phase changes → same key (segmentation is reusable).
+    for tweaked in [
+        Config {
+            corner_threshold: 90,
+            ..base.clone()
+        },
+        Config {
+            optimize: 0,
+            ..base.clone()
+        },
+        Config {
+            hierarchical: vtracer::Hierarchical::Cutout,
+            ..base.clone()
+        },
+        Config {
+            max_colors: Some(4),
+            ..base.clone()
+        },
+    ] {
+        assert_eq!(
+            base.segment_key(),
+            tweaked.segment_key(),
+            "finish-phase param must not change the segment key"
+        );
+    }
+
+    // Clustering changes → different key (must re-segment).
+    for tweaked in [
+        Config {
+            filter_speckle: base.filter_speckle + 4,
+            ..base.clone()
+        },
+        Config {
+            color_precision: 4,
+            ..base.clone()
+        },
+        Config {
+            layer_difference: 32,
+            ..base.clone()
+        },
+        Config {
+            color_mode: vtracer::ColorMode::Binary,
+            ..base.clone()
+        },
+    ] {
+        assert_ne!(
+            base.segment_key(),
+            tweaked.segment_key(),
+            "clustering param must change the segment key"
+        );
+    }
+}
+
+/// A `Session` render equals the one-shot pipeline — for a finish-only change
+/// (reuses the cache) and for a clustering change (re-segments). Correctness is
+/// identical either way; the cache is a transparent optimization.
+#[test]
+fn session_matches_one_shot() {
+    let img = blocks();
+    let mut session = Session::new(img.clone());
+
+    let base = Config::default();
+    let svg0 = session.render_svg(&base).unwrap();
+    assert_eq!(
+        svg0,
+        base.build().unwrap().to_svg(&img).unwrap(),
+        "first render must match the one-shot pipeline"
+    );
+
+    // Finish-only change: reuses the cached segmentation.
+    let tuned = Config {
+        corner_threshold: 90,
+        ..base.clone()
+    };
+    assert_eq!(
+        session.render_svg(&tuned).unwrap(),
+        tuned.build().unwrap().to_svg(&img).unwrap(),
+        "reused-segmentation render must match the one-shot pipeline"
+    );
+
+    // Clustering change: re-segments, still matches the one-shot.
+    let respeckled = Config {
+        filter_speckle: base.filter_speckle + 4,
+        ..base.clone()
+    };
+    assert_eq!(
+        session.render_svg(&respeckled).unwrap(),
+        respeckled.build().unwrap().to_svg(&img).unwrap(),
+        "re-segmented render must match the one-shot pipeline"
+    );
+}
diff --git a/docs/images/desktop-app.png b/docs/images/desktop-app.png
new file mode 100644
index 0000000..bb4d608
Binary files /dev/null and b/docs/images/desktop-app.png differ