diff --git a/crates/doc/src/document.rs b/crates/doc/src/document.rs
index d5e14a5d..edd0719d 100644
--- a/crates/doc/src/document.rs
+++ b/crates/doc/src/document.rs
@@ -792,6 +792,9 @@ pub struct Document {
     /// Placements of components in their parents (each non-root component has at least one).
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub occurrences: Vec<crate::Occurrence>,
+    /// Joints between occurrences, joint origins and motion links.
+    #[serde(default, skip_serializing_if = "crate::joints::Assembly::is_empty")]
+    pub assembly: crate::joints::Assembly,
     /// Favourite parameters (by name).
     #[serde(default, skip_serializing_if = "std::collections::BTreeSet::is_empty")]
     pub favorites: std::collections::BTreeSet<String>,
@@ -831,6 +834,7 @@ impl Document {
             next_id: 1,
             components: Vec::new(),
             occurrences: Vec::new(),
+            assembly: Default::default(),
             body_components: Default::default(),
             materials: Default::default(),
             favorites: Default::default(),
diff --git a/crates/doc/src/joints.rs b/crates/doc/src/joints.rs
new file mode 100644
index 00000000..502e2e5d
--- /dev/null
+++ b/crates/doc/src/joints.rs
@@ -0,0 +1,495 @@
+//! Joints between occurrences: joint origins (snap frames on component geometry, re-found on
+//! every solve so they follow edits), joints (rigid, revolute, slider, cylindrical, pin-slot,
+//! planar, ball) with their motion values and limits, motion links, and the solver that places
+//! occurrences from the joint graph.
+//!
+//! A joint makes B's joint frame coincide with A's frame moved by the joint's motion:
+//! `T_B · F_b = T_A · F_a · M(q) · Flip`, so `T_B = T_A · F_a · M(q) · Flip · F_b⁻¹`, where `T`
+//! are occurrence world transforms, `F` the joint frames in each component's own frame, and `q`
+//! the joint's values (angles in radians, distances in mm). The solver walks the graph from
+//! grounded occurrences (and the root, which never moves); joints that close a loop are checked
+//! and reported when they disagree.
+
+use std::collections::{BTreeMap, BTreeSet, VecDeque};
+
+use serde::{Deserialize, Serialize};
+use solvecraft_geom::Vec3;
+
+use crate::{DocError, Document, IDENTITY, Mat, ModelState, Result, apply_point, apply_vector, mat_inverse, mat_mul};
+
+const MAX_JOINTS: usize = 10_000;
+
+/// Where a joint origin sits on a component's geometry (component coordinates).
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum Snap {
+    /// An explicit frame (origin, z axis, x axis).
+    Frame {
+        origin: Vec3,
+        z: Vec3,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        x: Option<Vec3>,
+    },
+    /// The centre of the planar face nearest `pick`, z along its outward normal.
+    FaceCenter { pick: Vec3 },
+    /// The centre of the circular end nearest `pick` of the cylindrical face there, z along its
+    /// axis (pointing out of the face's end).
+    CircleCenter { pick: Vec3 },
+    /// A point, z along `z` (default +Z).
+    Point {
+        point: Vec3,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        z: Option<Vec3>,
+    },
+}
+
+/// A joint origin: a snap on an occurrence's component (occurrence 0 = the root design).
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct JointOrigin {
+    pub occurrence: u64,
+    pub snap: Snap,
+}
+
+/// A named joint origin (Joint Origin command), referenced by joints by name.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct NamedOrigin {
+    pub name: String,
+    pub origin: JointOrigin,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum JointKind {
+    Rigid,
+    Revolute,
+    Slider,
+    Cylindrical,
+    PinSlot,
+    Planar,
+    Ball,
+}
+
+impl JointKind {
+    pub fn parse(s: &str) -> Option<JointKind> {
+        Some(match s.to_ascii_lowercase().replace([' ', '_', '-'], "").as_str() {
+            "rigid" => JointKind::Rigid,
+            "revolute" | "rotate" => JointKind::Revolute,
+            "slider" | "slide" => JointKind::Slider,
+            "cylindrical" => JointKind::Cylindrical,
+            "pinslot" => JointKind::PinSlot,
+            "planar" => JointKind::Planar,
+            "ball" => JointKind::Ball,
+            _ => return None,
+        })
+    }
+    /// Names of the joint's values, in order (angles in radians, distances in mm).
+    pub fn dofs(self) -> &'static [&'static str] {
+        match self {
+            JointKind::Rigid => &[],
+            JointKind::Revolute => &["angle"],
+            JointKind::Slider => &["distance"],
+            JointKind::Cylindrical => &["angle", "distance"],
+            JointKind::PinSlot => &["angle", "slide"],
+            JointKind::Planar => &["x", "y", "angle"],
+            JointKind::Ball => &["yaw", "pitch", "roll"],
+        }
+    }
+    pub fn is_angle(self, i: usize) -> bool {
+        matches!(self.dofs().get(i), Some(&("angle" | "yaw" | "pitch" | "roll")))
+    }
+}
+
+/// A joint between two occurrences.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Joint {
+    pub id: u64,
+    pub name: String,
+    pub kind: JointKind,
+    pub a: JointOrigin,
+    pub b: JointOrigin,
+    /// Current values of the joint's degrees of freedom.
+    #[serde(default)]
+    pub values: Vec<f64>,
+    /// Per value: (min, max) limits.
+    #[serde(default)]
+    pub limits: Vec<Option<(f64, f64)>>,
+    /// Turn B over (its z opposite to A's).
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub flip: bool,
+    /// Distance from A's frame to B's along z, and a turn about it (radians).
+    #[serde(default)]
+    pub offset: f64,
+    #[serde(default)]
+    pub angle: f64,
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub suppressed: bool,
+}
+
+/// Value of joint `b` (index `ib`) = ratio · value of joint `a` (index `ia`) + offset.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct MotionLink {
+    pub a: u64,
+    #[serde(default)]
+    pub ia: usize,
+    pub b: u64,
+    #[serde(default)]
+    pub ib: usize,
+    pub ratio: f64,
+    #[serde(default)]
+    pub offset: f64,
+}
+
+/// Joints, joint origins and motion links of a design.
+#[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize)]
+pub struct Assembly {
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub joints: Vec<Joint>,
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub origins: Vec<NamedOrigin>,
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub links: Vec<MotionLink>,
+}
+
+impl Assembly {
+    pub fn is_empty(&self) -> bool {
+        self.joints.is_empty() && self.origins.is_empty() && self.links.is_empty()
+    }
+}
+
+/// A frame: origin and orthonormal axes, as a transform (local → parent).
+fn frame(o: Vec3, z: Vec3, x: Option<Vec3>) -> Option<Mat> {
+    let z = z.normalized()?;
+    let x = x.and_then(|x| (x - z * x.dot(z)).normalized()).unwrap_or_else(|| z.any_perp());
+    let y = z.cross(x).normalized()?;
+    Some([[x.x, x.y, x.z, 0.0], [y.x, y.y, y.z, 0.0], [z.x, z.y, z.z, 0.0], [o.x, o.y, o.z, 1.0]])
+}
+
+fn rot_z(a: f64) -> Mat {
+    let (s, c) = a.sin_cos();
+    [[c, s, 0.0, 0.0], [-s, c, 0.0, 0.0], [0.0, 0.0, 1.0, 0.0], [0.0, 0.0, 0.0, 1.0]]
+}
+fn rot_y(a: f64) -> Mat {
+    let (s, c) = a.sin_cos();
+    [[c, 0.0, -s, 0.0], [0.0, 1.0, 0.0, 0.0], [s, 0.0, c, 0.0], [0.0, 0.0, 0.0, 1.0]]
+}
+fn shift(x: f64, y: f64, z: f64) -> Mat {
+    let mut m = IDENTITY;
+    m[3] = [x, y, z, 1.0];
+    m
+}
+
+/// The joint's motion in A's frame for its values.
+pub fn motion(kind: JointKind, q: &[f64]) -> Mat {
+    let v = |i: usize| q.get(i).copied().unwrap_or(0.0);
+    match kind {
+        JointKind::Rigid => IDENTITY,
+        JointKind::Revolute => rot_z(v(0)),
+        JointKind::Slider => shift(0.0, 0.0, v(0)),
+        JointKind::Cylindrical => mat_mul(&rot_z(v(0)), &shift(0.0, 0.0, v(1))),
+        JointKind::PinSlot => mat_mul(&shift(v(1), 0.0, 0.0), &rot_z(v(0))),
+        JointKind::Planar => mat_mul(&shift(v(0), v(1), 0.0), &rot_z(v(2))),
+        JointKind::Ball => mat_mul(&mat_mul(&rot_z(v(0)), &rot_y(v(1))), &rot_z(v(2))),
+    }
+}
+
+/// Bodies of a component in the model (its own frame).
+fn component_bodies<'a>(doc: &Document, st: &'a ModelState, component: u64) -> Vec<&'a crate::ModelBody> {
+    st.bodies.iter().filter(|b| doc.body_component(&b.name, b.feature) == component).collect()
+}
+
+/// The component an occurrence places (0 for the root).
+fn component_of(doc: &Document, occ: u64) -> Result<u64> {
+    if occ == 0 {
+        return Ok(0);
+    }
+    doc.occurrences.iter().find(|o| o.id == occ).map(|o| o.component).ok_or_else(|| DocError::Unknown(format!("occurrence {occ}")))
+}
+
+/// Resolve a joint origin to a frame in its component's coordinates.
+pub fn resolve_origin(doc: &Document, st: &ModelState, o: &JointOrigin) -> Result<Mat> {
+    let bad = || DocError::Invalid("joint origin: degenerate frame".into());
+    match &o.snap {
+        Snap::Frame { origin, z, x } => frame(*origin, *z, *x).ok_or_else(bad),
+        Snap::Point { point, z } => frame(*point, z.unwrap_or(Vec3::Z), None).ok_or_else(bad),
+        Snap::FaceCenter { pick } => {
+            let comp = component_of(doc, o.occurrence)?;
+            let mut best: Option<(f64, Vec3, Vec3)> = None;
+            for b in component_bodies(doc, st, comp) {
+                let tol = (b.body.size() * 2e-3).max(1e-3);
+                for f in b.body.faces(tol)? {
+                    let Some(n) = f.plane_normal else { continue };
+                    let d = (*pick - f.centroid).dot(n).abs() + 1e-3 * (*pick - f.centroid).len();
+                    if best.as_ref().is_none_or(|(bd, _, _)| d < *bd) {
+                        best = Some((d, f.centroid, n));
+                    }
+                }
+            }
+            let (_, c, n) = best.ok_or_else(|| DocError::Invalid("joint origin: no planar face there".into()))?;
+            frame(c, n, None).ok_or_else(bad)
+        }
+        Snap::CircleCenter { pick } => {
+            let comp = component_of(doc, o.occurrence)?;
+            let cyl = component_bodies(doc, st, comp)
+                .into_iter()
+                .filter_map(|b| solvecraft_kernel::cylinder_face_at(&b.body, *pick))
+                .min_by(|a, b| {
+                    let off = |c: &solvecraft_kernel::CylinderFace| {
+                        let v = *pick - c.axis_point;
+                        ((v - c.axis * v.dot(c.axis)).len() - c.radius).abs()
+                    };
+                    off(a).total_cmp(&off(b))
+                })
+                .ok_or_else(|| DocError::Invalid("joint origin: no cylindrical face there".into()))?;
+            let s = (*pick - cyl.axis_point).dot(cyl.axis);
+            let (at, dir) = if (s - cyl.start).abs() <= (cyl.end - s).abs() { (cyl.start, -1.0) } else { (cyl.end, 1.0) };
+            frame(cyl.axis_point + cyl.axis * at, cyl.axis * dir, None).ok_or_else(bad)
+        }
+    }
+}
+
+/// World transform of an occurrence (through its parents).
+pub fn occurrence_world(doc: &Document, occ: u64) -> Mat {
+    let mut m = IDENTITY;
+    let mut cur = doc.occurrences.iter().find(|o| o.id == occ);
+    for _ in 0..1000 {
+        let Some(o) = cur else { break };
+        m = mat_mul(&o.transform, &m);
+        if o.parent == 0 {
+            break;
+        }
+        cur = doc.occurrence_of(o.parent);
+    }
+    m
+}
+
+/// Clamp values to the joint's limits.
+pub fn clamp(j: &Joint, q: &mut [f64]) {
+    for (i, v) in q.iter_mut().enumerate() {
+        if let Some(Some((lo, hi))) = j.limits.get(i) {
+            *v = v.clamp(lo.min(*hi), hi.max(*lo));
+        }
+    }
+}
+
+/// The result of placing occurrences by their joints.
+#[derive(Clone, Debug, Default, Serialize)]
+pub struct Solution {
+    /// New world transforms of occurrences the joints place.
+    #[serde(skip)]
+    pub world: BTreeMap<u64, Mat>,
+    /// Joints that disagree with the placement found through others: (joint id, name, why).
+    pub conflicts: Vec<(u64, String, String)>,
+    /// Joints that could not be applied (bad origin…): (joint id, name, why).
+    pub errors: Vec<(u64, String, String)>,
+    /// Remaining degrees of freedom per occurrence (6 = free, 0 = fixed).
+    pub dof: BTreeMap<u64, usize>,
+    /// Occurrences fixed in place (grounded, or the root's).
+    pub fixed: Vec<u64>,
+}
+
+/// How far two placements differ: (translation mm, rotation rad).
+fn mismatch(a: &Mat, b: &Mat) -> (f64, f64) {
+    let t = Vec3::new(a[3][0] - b[3][0], a[3][1] - b[3][1], a[3][2] - b[3][2]).len();
+    let mut tr = 0.0;
+    for i in 0..3 {
+        for k in 0..3 {
+            tr += a[i][k] * b[i][k];
+        }
+    }
+    (t, ((tr - 1.0) / 2.0).clamp(-1.0, 1.0).acos())
+}
+
+/// Effective values of every joint after motion links and limits.
+pub fn joint_values(asm: &Assembly) -> BTreeMap<u64, Vec<f64>> {
+    let mut vals: BTreeMap<u64, Vec<f64>> = asm
+        .joints
+        .iter()
+        .map(|j| {
+            let mut q = j.values.clone();
+            q.resize(j.kind.dofs().len(), 0.0);
+            (j.id, q)
+        })
+        .collect();
+    // Links in order, a few passes for chains.
+    for _ in 0..4 {
+        for l in &asm.links {
+            let Some(src) = vals.get(&l.a).and_then(|q| q.get(l.ia)).copied() else { continue };
+            if let Some(v) = vals.get_mut(&l.b).and_then(|q| q.get_mut(l.ib)) {
+                *v = l.ratio * src + l.offset;
+            }
+        }
+    }
+    for j in &asm.joints {
+        if let Some(q) = vals.get_mut(&j.id) {
+            clamp(j, q);
+        }
+    }
+    vals
+}
+
+/// Place occurrences from the joint graph.
+pub fn solve(doc: &Document, st: &ModelState) -> Solution {
+    let asm = &doc.assembly;
+    let mut sol = Solution::default();
+    let joints: Vec<&Joint> = asm.joints.iter().filter(|j| !j.suppressed).take(MAX_JOINTS).collect();
+    let vals = joint_values(asm);
+    // Joint frames in component coordinates (errors leave the joint out).
+    let mut frames: BTreeMap<u64, (Mat, Mat)> = BTreeMap::new();
+    for j in &joints {
+        match (resolve_origin(doc, st, &j.a), resolve_origin(doc, st, &j.b)) {
+            (Ok(fa), Ok(fb)) => {
+                frames.insert(j.id, (fa, fb));
+            }
+            (Err(e), _) | (_, Err(e)) => sol.errors.push((j.id, j.name.clone(), e.to_string())),
+        }
+    }
+    // Fixed: the root, grounded occurrences; with neither involved, the first joint's A side.
+    let mut placed: BTreeMap<u64, Mat> = BTreeMap::new();
+    let mut dof: BTreeMap<u64, usize> = BTreeMap::new();
+    placed.insert(0, IDENTITY);
+    dof.insert(0, 0);
+    for o in doc.occurrences.iter().filter(|o| o.grounded) {
+        placed.insert(o.id, occurrence_world(doc, o.id));
+        dof.insert(o.id, 0);
+    }
+    let involved: BTreeSet<u64> = joints.iter().flat_map(|j| [j.a.occurrence, j.b.occurrence]).collect();
+    if !involved.iter().any(|o| placed.contains_key(o))
+        && let Some(j) = joints.first()
+    {
+        placed.insert(j.a.occurrence, occurrence_world(doc, j.a.occurrence));
+        dof.insert(j.a.occurrence, 0);
+    }
+    sol.fixed = placed.keys().copied().collect();
+    let flip_m = |j: &Joint| {
+        let mut m = rot_z(j.angle);
+        m[3][2] = j.offset;
+        if j.flip { mat_mul(&m, &[[1.0, 0.0, 0.0, 0.0], [0.0, -1.0, 0.0, 0.0], [0.0, 0.0, -1.0, 0.0], [0.0, 0.0, 0.0, 1.0]]) } else { m }
+    };
+    // B's world transform from A's (or A's from B's, going the other way).
+    let through = |j: &Joint, from_a: bool, t: &Mat| -> Option<Mat> {
+        let (fa, fb) = frames.get(&j.id)?;
+        let q = vals.get(&j.id).cloned().unwrap_or_default();
+        let link = mat_mul(&mat_mul(&motion(j.kind, &q), &flip_m(j)), &mat_inverse(fb)?);
+        if from_a {
+            Some(mat_mul(&mat_mul(t, fa), &link))
+        } else {
+            // T_A = T_B · link⁻¹ · F_a⁻¹
+            Some(mat_mul(&mat_mul(t, &mat_inverse(&link)?), &mat_inverse(fa)?))
+        }
+    };
+    let mut used: BTreeSet<u64> = BTreeSet::new();
+    let mut queue: VecDeque<u64> = placed.keys().copied().collect();
+    while let Some(occ) = queue.pop_front() {
+        let Some(t) = placed.get(&occ).copied() else { continue };
+        for j in &joints {
+            if used.contains(&j.id) || !frames.contains_key(&j.id) {
+                continue;
+            }
+            let (other, from_a) = if j.a.occurrence == occ {
+                (j.b.occurrence, true)
+            } else if j.b.occurrence == occ {
+                (j.a.occurrence, false)
+            } else {
+                continue;
+            };
+            if placed.contains_key(&other) {
+                continue;
+            }
+            let Some(m) = through(j, from_a, &t) else { continue };
+            used.insert(j.id);
+            placed.insert(other, m);
+            dof.insert(other, dof.get(&occ).copied().unwrap_or(0) + j.kind.dofs().len());
+            queue.push_back(other);
+        }
+    }
+    // Joints that close loops: do they agree?
+    for j in &joints {
+        if used.contains(&j.id) || !frames.contains_key(&j.id) {
+            continue;
+        }
+        let (Some(ta), Some(tb)) = (placed.get(&j.a.occurrence), placed.get(&j.b.occurrence)) else { continue };
+        let Some(want) = through(j, true, ta) else { continue };
+        let (dt, da) = mismatch(&want, tb);
+        if dt > 1e-6 || da > 1e-8 {
+            sol.conflicts.push((
+                j.id,
+                j.name.clone(),
+                format!("off by {dt:.4} mm and {:.3}° from the placement the other joints give", da.to_degrees()),
+            ));
+        }
+    }
+    for o in &doc.occurrences {
+        dof.entry(o.id).or_insert(6);
+    }
+    placed.remove(&0);
+    sol.world = placed;
+    sol.dof = dof;
+    sol
+}
+
+impl Document {
+    /// Apply a solution: occurrence transforms (in their parents) for the occurrences it places.
+    /// Returns true when something moved.
+    pub fn apply_joint_solution(&mut self, sol: &Solution) -> bool {
+        let mut changed = false;
+        let parents: BTreeMap<u64, u64> = self.occurrences.iter().map(|o| (o.id, o.parent)).collect();
+        for (occ, world) in &sol.world {
+            let parent = parents.get(occ).copied().unwrap_or(0);
+            let pw = if parent == 0 { IDENTITY } else { self.component_transform(parent) };
+            let Some(local) = mat_inverse(&pw).map(|inv| mat_mul(&inv, world)) else { continue };
+            if let Some(o) = self.occurrences.iter_mut().find(|o| o.id == *occ) {
+                let (dt, da) = mismatch(&o.transform, &local);
+                if dt > 1e-9 || da > 1e-10 {
+                    o.transform = local;
+                    changed = true;
+                }
+            }
+        }
+        changed
+    }
+
+    /// A frame for an as-built joint: the world frame `w` seen from an occurrence.
+    pub fn frame_in_occurrence(&self, occ: u64, w: &Mat) -> Option<Snap> {
+        let t = if occ == 0 { IDENTITY } else { occurrence_world(self, occ) };
+        let local = mat_mul(&mat_inverse(&t)?, w);
+        let o = apply_point(&local, Vec3::ZERO);
+        let z = apply_vector(&local, Vec3::Z);
+        let x = apply_vector(&local, Vec3::X);
+        Some(Snap::Frame { origin: o, z, x: Some(x) })
+    }
+
+    /// A joint origin's frame in the world (for as-built joints and display).
+    pub fn origin_world(&self, st: &ModelState, o: &JointOrigin) -> Result<Mat> {
+        let f = resolve_origin(self, st, o)?;
+        let t = if o.occurrence == 0 { IDENTITY } else { occurrence_world(self, o.occurrence) };
+        Ok(mat_mul(&t, &f))
+    }
+}
+
+/// The frame matrix for an explicit origin / z / x (public for commands).
+pub fn frame_of(o: Vec3, z: Vec3, x: Option<Vec3>) -> Option<Mat> {
+    frame(o, z, x)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn close(a: &Mat, b: &Mat) -> bool {
+        let (t, r) = mismatch(a, b);
+        t < 1e-9 && r < 1e-9
+    }
+
+    #[test]
+    fn motions_compose() {
+        assert!(close(&motion(JointKind::Revolute, &[std::f64::consts::FRAC_PI_2]), &rot_z(std::f64::consts::FRAC_PI_2)));
+        let m = motion(JointKind::Cylindrical, &[0.0, 5.0]);
+        assert!((m[3][2] - 5.0).abs() < 1e-12);
+        let m = motion(JointKind::PinSlot, &[0.0, 3.0]);
+        assert!((m[3][0] - 3.0).abs() < 1e-12);
+        assert_eq!(JointKind::Ball.dofs().len(), 3);
+        assert!(JointKind::parse("Pin Slot").is_some() && JointKind::parse("nope").is_none());
+        let f = frame(Vec3::new(1.0, 2.0, 3.0), Vec3::Z, None).unwrap();
+        assert!(close(&mat_mul(&f, &mat_inverse(&f).unwrap()), &IDENTITY));
+    }
+}
diff --git a/crates/doc/src/lib.rs b/crates/doc/src/lib.rs
index 0ad015f0..d281188e 100644
--- a/crates/doc/src/lib.rs
+++ b/crates/doc/src/lib.rs
@@ -10,6 +10,7 @@ mod clipboard;
 mod document;
 mod eval;
 pub mod expr;
+pub mod joints;
 mod params;
 pub mod project;
 mod project3d;
diff --git a/crates/engine/src/cmd/joints.rs b/crates/engine/src/cmd/joints.rs
new file mode 100644
index 00000000..b6db903e
--- /dev/null
+++ b/crates/engine/src/cmd/joints.rs
@@ -0,0 +1,548 @@
+//! Joints between occurrences (`solvecraft_doc::joints`): Joint Origin, Joint, As-built Joint,
+//! Rigid Group, Drive Joints, Motion Link, limits, and Interference. Every change re-solves the
+//! joint graph and moves the occurrences it places; the session re-solves after any edit too,
+//! so joints follow the geometry their origins snap to.
+
+use serde_json::{Value, json};
+use solvecraft_doc::expr::Kind;
+use solvecraft_doc::joints::{self, Joint, JointKind, JointOrigin, MotionLink, NamedOrigin, Snap};
+use solvecraft_geom::Vec3;
+
+use super::CommandSpec;
+use crate::params::{bad, bool_, num, str_, string_list, vec3};
+use crate::{EngineError, Result, Session};
+
+pub static COMMANDS: &[CommandSpec] = &[
+    CommandSpec::new("JointOrigin", "Joint Origin", joint_origin).at("SOLID", "CREATE").icon("joint").params(
+        "name?, occurrence?: id|name (default: from the pick), face: [x,y,z] (centre of a planar face) | circle: [x,y,z] (centre of a round end) | point: [x,y,z], z?: [x,y,z]",
+    ),
+    CommandSpec::new("JointAssembleCmdNew", "Joint", joint).at("SOLID", "ASSEMBLE").icon("joint").key("J").params(
+        "type: rigid|revolute|slider|cylindrical|pin_slot|planar|ball; a, b: joint origins ({occurrence, face|circle|point: [x,y,z] in world} | origin name); \
+         values?: [angles in deg or expressions, distances]; limits?: [[min, max] | null…]; flip?: bool; offset?; angle?; name?",
+    ),
+    CommandSpec::new("JointAsBuiltCmd", "As-Built Joint", as_built).at("SOLID", "ASSEMBLE").icon("joint").params(
+        "type, a: occurrence, b: occurrence (stay where they are); at?: joint origin (default: b's origin) — the joint's frame",
+    ),
+    CommandSpec::new("RigidGroupCmd", "Rigid Group", rigid_group).at("SOLID", "ASSEMBLE").icon("joint").params("occurrences: [ids or names] (move together, as they are)"),
+    CommandSpec::new("FusionMoveJointsCommand", "Drive Joints", drive).at("SOLID", "ASSEMBLE").icon("joint").params(
+        "joint: id|name; value | values: [angles in deg or expressions, distances] (clamped to the limits)",
+    ),
+    CommandSpec::new("FusionMotionRelationshipCommand", "Motion Link", motion_link)
+        .at("SOLID", "ASSEMBLE")
+        .icon("joint")
+        .params("a, b: joints; ratio (b = ratio · a + offset); offset?; ia?, ib?: value indices (default 0)"),
+    CommandSpec::new("joint.limits", "Joint Limits", limits).params("joint; index?: value index (default 0); min?, max? (omit both to clear)"),
+    CommandSpec::new("joint.edit", "Edit Joint", edit).params("joint; type?, flip?, offset?, angle?, suppressed?, name?"),
+    CommandSpec::new("joint.delete", "Delete Joint", delete).params("joint: id|name (or origin: name, link: index)"),
+    CommandSpec::new("joint.list", "List Joints", list).noundo().params("→ joints (type, occurrences, values, limits), origins, links, degrees of freedom, conflicts"),
+    CommandSpec::new("joint.solve", "Solve Joints", solve_cmd).params("re-place the occurrences from the joints → conflicts, errors, dof"),
+    CommandSpec::new("InterferenceCheckCommand", "Interference", interference).at("SOLID", "INSPECT").icon("measure").noundo().params(
+        "bodies?: [names] (default: all, in world placement) → pairs that overlap with the overlap volume, area, bounding box and centre",
+    ),
+];
+
+/// Re-solve the joints and move the occurrences they place (after any edit).
+pub fn resolve(s: &mut Session) -> joints::Solution {
+    let st = s.model.state();
+    let sol = joints::solve(&s.doc, &st);
+    if !sol.world.is_empty() {
+        let mut next = (*s.doc).clone();
+        if next.apply_joint_solution(&sol) {
+            *s.doc_mut() = next;
+            s.revision += 1;
+        }
+    }
+    sol
+}
+
+fn sol_json(sol: &joints::Solution) -> Value {
+    json!({
+        "conflicts": sol.conflicts.iter().map(|(id, n, w)| json!({"joint": id, "name": n, "why": w})).collect::<Vec<_>>(),
+        "errors": sol.errors.iter().map(|(id, n, w)| json!({"joint": id, "name": n, "why": w})).collect::<Vec<_>>(),
+        "dof": sol.dof.iter().map(|(o, d)| json!({"occurrence": o, "dof": d})).collect::<Vec<_>>(),
+        "fixed": sol.fixed,
+    })
+}
+
+fn occurrence_id(s: &Session, v: Option<&Value>, cmd: &str) -> Result<u64> {
+    let key = match v {
+        Some(Value::Number(n)) => n.to_string(),
+        Some(Value::String(x)) => x.clone(),
+        _ => return Err(bad(cmd, "an occurrence must be an id or a name")),
+    };
+    if key == "0" || key.eq_ignore_ascii_case("root") {
+        return Ok(0);
+    }
+    if let Some(o) = s.doc.occurrences.iter().find(|o| o.id.to_string() == key || o.name == key) {
+        return Ok(o.id);
+    }
+    // A component name: its first occurrence.
+    s.doc.find_component(&key).and_then(|c| s.doc.occurrence_of(c)).map(|o| o.id).ok_or_else(|| bad(cmd, format!("no occurrence `{key}`")))
+}
+
+/// The occurrence whose bodies are nearest a world point (0 = root bodies).
+fn occurrence_at(s: &Session, p: Vec3) -> Option<u64> {
+    let st = s.model.state();
+    let mut best: Option<(f64, u64)> = None;
+    for b in &st.bodies {
+        let comp = s.doc.body_component(&b.name, b.feature);
+        let occs: Vec<u64> = if comp == 0 { vec![0] } else { s.doc.occurrences.iter().filter(|o| o.component == comp).map(|o| o.id).collect() };
+        for occ in occs {
+            let t = if occ == 0 { solvecraft_doc::IDENTITY } else { joints::occurrence_world(&s.doc, occ) };
+            let inv = solvecraft_doc::mat_inverse(&t)?;
+            let q = solvecraft_doc::apply_point(&inv, p);
+            let bb = b.mesh().bounds();
+            let c = Vec3::new(q.x.clamp(bb.min.x, bb.max.x), q.y.clamp(bb.min.y, bb.max.y), q.z.clamp(bb.min.z, bb.max.z));
+            let d = c.dist(q);
+            if best.is_none_or(|(bd, _)| d < bd) {
+                best = Some((d, occ));
+            }
+        }
+    }
+    best.map(|(_, o)| o)
+}
+
+/// A joint origin from parameters: a named origin, or a snap given in world coordinates.
+fn origin_param(s: &Session, v: Option<&Value>, cmd: &str) -> Result<JointOrigin> {
+    let v = v.ok_or_else(|| bad(cmd, "joint origins `a` and `b` are required"))?;
+    if let Value::String(name) = v {
+        return s
+            .doc
+            .assembly
+            .origins
+            .iter()
+            .find(|o| &o.name == name)
+            .map(|o| o.origin.clone())
+            .ok_or_else(|| bad(cmd, format!("no joint origin `{name}`")));
+    }
+    let pick = ["face", "circle", "point"].iter().find_map(|k| v.get(*k).and_then(vec3).map(|p| (*k, p)));
+    let (what, world) = match (pick, v.get("frame")) {
+        (Some(x), _) => x,
+        (None, Some(f)) => ("frame", f.get("origin").and_then(vec3).ok_or_else(|| bad(cmd, "`frame` needs origin and z"))?),
+        _ => return Err(bad(cmd, "a joint origin needs face, circle, point or frame [x, y, z]")),
+    };
+    let occurrence = match v.get("occurrence") {
+        Some(o) => occurrence_id(s, Some(o), cmd)?,
+        None => occurrence_at(s, world).ok_or_else(|| bad(cmd, "no body there"))?,
+    };
+    // World → the component's own coordinates.
+    let t = if occurrence == 0 { solvecraft_doc::IDENTITY } else { joints::occurrence_world(&s.doc, occurrence) };
+    let inv = solvecraft_doc::mat_inverse(&t).ok_or_else(|| bad(cmd, "occurrence transform"))?;
+    let local = |p: Vec3| solvecraft_doc::apply_point(&inv, p);
+    let local_dir = |d: Vec3| solvecraft_doc::apply_vector(&inv, d);
+    let snap = match what {
+        "face" => Snap::FaceCenter { pick: local(world) },
+        "circle" => Snap::CircleCenter { pick: local(world) },
+        "point" => Snap::Point { point: local(world), z: v.get("z").and_then(vec3).map(local_dir) },
+        _ => {
+            let f = v.get("frame").cloned().unwrap_or_default();
+            let z = f.get("z").and_then(vec3).ok_or_else(|| bad(cmd, "`frame` needs a z axis"))?;
+            Snap::Frame { origin: local(world), z: local_dir(z), x: f.get("x").and_then(vec3).map(local_dir) }
+        }
+    };
+    let o = JointOrigin { occurrence, snap };
+    joints::resolve_origin(&s.doc, &s.model.state(), &o).map_err(|e| bad(cmd, e.to_string()))?;
+    Ok(o)
+}
+
+fn joint_id(s: &Session, v: Option<&Value>, cmd: &str) -> Result<u64> {
+    let key = match v {
+        Some(Value::Number(n)) => n.to_string(),
+        Some(Value::String(x)) => x.clone(),
+        _ => return Err(bad(cmd, "`joint` must be an id or a name")),
+    };
+    s.doc.assembly.joints.iter().find(|j| j.id.to_string() == key || j.name == key).map(|j| j.id).ok_or_else(|| bad(cmd, format!("no joint `{key}`")))
+}
+
+/// A joint value: numbers are degrees for angles and mm for distances; text is an expression.
+fn value(s: &Session, v: &Value, angle: bool, cmd: &str) -> Result<f64> {
+    let kind = if angle { Kind::Angle } else { Kind::Length };
+    let e = match v {
+        Value::Number(n) => n.as_f64().filter(|x| x.is_finite() && x.abs() < 1e9).map(|x| x.to_string()),
+        Value::String(t) => Some(t.clone()),
+        _ => None,
+    }
+    .ok_or_else(|| bad(cmd, "values must be numbers or expressions"))?;
+    s.doc.eval(&e, kind).map_err(|e| bad(cmd, e.to_string()))
+}
+
+fn values_param(s: &Session, p: &Value, kind: JointKind, cmd: &str) -> Result<Vec<f64>> {
+    let n = kind.dofs().len();
+    let list: Vec<Value> = match (p.get("values"), p.get("value")) {
+        (Some(Value::Array(a)), _) => a.clone(),
+        (_, Some(v)) => vec![v.clone()],
+        _ => Vec::new(),
+    };
+    if list.len() > n {
+        return Err(bad(cmd, format!("a {kind:?} joint has {n} values ({})", kind.dofs().join(", "))));
+    }
+    let mut out = vec![0.0; n];
+    for (i, v) in list.iter().enumerate() {
+        if let Some(slot) = out.get_mut(i) {
+            *slot = value(s, v, kind.is_angle(i), cmd)?;
+        }
+    }
+    Ok(out)
+}
+
+fn limits_param(s: &Session, p: &Value, kind: JointKind, cmd: &str) -> Result<Vec<Option<(f64, f64)>>> {
+    let n = kind.dofs().len();
+    let mut out = vec![None; n];
+    if let Some(Value::Array(a)) = p.get("limits") {
+        if a.len() > n {
+            return Err(bad(cmd, "more limits than values"));
+        }
+        for (i, l) in a.iter().enumerate() {
+            if let (Some(lo), Some(hi)) = (l.get(0), l.get(1))
+                && let Some(slot) = out.get_mut(i)
+            {
+                *slot = Some((value(s, lo, kind.is_angle(i), cmd)?, value(s, hi, kind.is_angle(i), cmd)?));
+            }
+        }
+    }
+    Ok(out)
+}
+
+fn next_joint_id(s: &Session) -> u64 {
+    s.doc.assembly.joints.iter().map(|j| j.id).max().unwrap_or(0) + 1
+}
+
+fn add_joint(s: &mut Session, j: Joint) -> Result<Value> {
+    if s.doc.assembly.joints.len() >= 10_000 {
+        return Err(EngineError::Other("too many joints".into()));
+    }
+    if j.a.occurrence == j.b.occurrence {
+        return Err(bad("JointAssembleCmdNew", "a joint needs two different occurrences"));
+    }
+    let id = j.id;
+    let name = j.name.clone();
+    s.doc_mut().assembly.joints.push(j);
+    let sol = resolve(s);
+    let mut out = sol_json(&sol);
+    out["joint"] = json!(id);
+    out["name"] = json!(name);
+    Ok(out)
+}
+
+fn joint_origin(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "JointOrigin";
+    let origin = origin_param(s, Some(p), cmd)?;
+    let name = match str_(p, "name").map(str::trim).filter(|n| !n.is_empty()) {
+        Some(n) => n.to_string(),
+        None => (1..).map(|k| format!("Joint Origin{k}")).find(|n| !s.doc.assembly.origins.iter().any(|o| &o.name == n)).unwrap_or_default(),
+    };
+    if s.doc.assembly.origins.iter().any(|o| o.name == name) {
+        return Err(bad(cmd, format!("there is already a joint origin `{name}`")));
+    }
+    s.doc_mut().assembly.origins.push(NamedOrigin { name: name.clone(), origin });
+    Ok(json!({"origin": name}))
+}
+
+fn kind_param(p: &Value, cmd: &str) -> Result<JointKind> {
+    str_(p, "type")
+        .and_then(JointKind::parse)
+        .ok_or_else(|| bad(cmd, "`type` must be rigid, revolute, slider, cylindrical, pin_slot, planar or ball"))
+}
+
+fn joint(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "JointAssembleCmdNew";
+    let kind = kind_param(p, cmd)?;
+    let a = origin_param(s, p.get("a"), cmd)?;
+    let b = origin_param(s, p.get("b"), cmd)?;
+    let id = next_joint_id(s);
+    let name = str_(p, "name").map(str::to_string).unwrap_or_else(|| format!("Joint{id}"));
+    let offset = match p.get("offset") {
+        Some(v) => value(s, v, false, cmd)?,
+        None => 0.0,
+    };
+    let angle = match p.get("angle") {
+        Some(v) => value(s, v, true, cmd)?,
+        None => 0.0,
+    };
+    let j = Joint {
+        id,
+        name,
+        kind,
+        a,
+        b,
+        values: values_param(s, p, kind, cmd)?,
+        limits: limits_param(s, p, kind, cmd)?,
+        flip: bool_(p, "flip").unwrap_or(false),
+        offset,
+        angle,
+        suppressed: false,
+    };
+    add_joint(s, j)
+}
+
+fn as_built(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "JointAsBuiltCmd";
+    let kind = kind_param(p, cmd)?;
+    let (oa, ob) = (occurrence_id(s, p.get("a"), cmd)?, occurrence_id(s, p.get("b"), cmd)?);
+    // The joint's frame in the world: the given origin, or B's component origin.
+    let w = match p.get("at") {
+        Some(v) => {
+            let o = origin_param(s, Some(v), cmd)?;
+            s.doc.origin_world(&s.model.state(), &o).map_err(|e| bad(cmd, e.to_string()))?
+        }
+        None => joints::occurrence_world(&s.doc, ob),
+    };
+    let fa = s.doc.frame_in_occurrence(oa, &w).ok_or_else(|| bad(cmd, "occurrence transform"))?;
+    let fb = s.doc.frame_in_occurrence(ob, &w).ok_or_else(|| bad(cmd, "occurrence transform"))?;
+    let id = next_joint_id(s);
+    let name = str_(p, "name").map(str::to_string).unwrap_or_else(|| format!("Joint{id}"));
+    let n = kind.dofs().len();
+    let j = Joint {
+        id,
+        name,
+        kind,
+        a: JointOrigin { occurrence: oa, snap: fa },
+        b: JointOrigin { occurrence: ob, snap: fb },
+        values: vec![0.0; n],
+        limits: limits_param(s, p, kind, cmd)?,
+        flip: false,
+        offset: 0.0,
+        angle: 0.0,
+        suppressed: false,
+    };
+    add_joint(s, j)
+}
+
+fn rigid_group(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "RigidGroupCmd";
+    let list = p.get("occurrences").and_then(Value::as_array).cloned().unwrap_or_default();
+    if list.len() < 2 || list.len() > 1000 {
+        return Err(bad(cmd, "`occurrences` must list 2…1000 occurrences"));
+    }
+    let ids: Vec<u64> = list.iter().map(|v| occurrence_id(s, Some(v), cmd)).collect::<Result<_>>()?;
+    let first = ids.first().copied().unwrap_or(0);
+    let mut made = Vec::new();
+    for other in ids.iter().skip(1) {
+        let r = as_built(s, &json!({"type": "rigid", "a": first, "b": other, "name": format!("Rigid Group {first}:{other}")}))?;
+        made.push(r["joint"].clone());
+    }
+    Ok(json!({"joints": made}))
+}
+
+fn drive(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionMoveJointsCommand";
+    let id = joint_id(s, p.get("joint"), cmd)?;
+    let kind = s.doc.assembly.joints.iter().find(|j| j.id == id).map(|j| j.kind).ok_or_else(|| bad(cmd, "joint"))?;
+    if kind == JointKind::Rigid {
+        return Err(bad(cmd, "a rigid joint does not move"));
+    }
+    let mut q = values_param(s, p, kind, cmd)?;
+    if p.get("values").is_none() && p.get("value").is_none() {
+        return Err(bad(cmd, "give `value` or `values`"));
+    }
+    let doc = s.doc_mut();
+    let Some(j) = doc.assembly.joints.iter_mut().find(|j| j.id == id) else { return Err(bad(cmd, "joint")) };
+    // Values not given keep theirs.
+    if let Some(Value::Array(a)) = p.get("values") {
+        for (i, v) in q.iter_mut().enumerate().skip(a.len()) {
+            *v = j.values.get(i).copied().unwrap_or(*v);
+        }
+    }
+    joints::clamp(j, &mut q);
+    j.values = q.clone();
+    let sol = resolve(s);
+    let mut out = sol_json(&sol);
+    out["joint"] = json!(id);
+    out["values"] = json!(q);
+    Ok(out)
+}
+
+fn motion_link(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "FusionMotionRelationshipCommand";
+    let (a, b) = (joint_id(s, p.get("a"), cmd)?, joint_id(s, p.get("b"), cmd)?);
+    if a == b {
+        return Err(bad(cmd, "link two different joints"));
+    }
+    let ratio = num(p, "ratio").ok_or_else(|| bad(cmd, "`ratio` must be a number"))?;
+    let (ia, ib) = (num(p, "ia").unwrap_or(0.0).max(0.0) as usize, num(p, "ib").unwrap_or(0.0).max(0.0) as usize);
+    let dofs = |id: u64| s.doc.assembly.joints.iter().find(|j| j.id == id).map(|j| j.kind.dofs().len()).unwrap_or(0);
+    if ia >= dofs(a) || ib >= dofs(b) {
+        return Err(bad(cmd, "the linked joints need moving values at those indices"));
+    }
+    let offset = num(p, "offset").unwrap_or(0.0);
+    s.doc_mut().assembly.links.push(MotionLink { a, ia, b, ib, ratio, offset });
+    let sol = resolve(s);
+    Ok(sol_json(&sol))
+}
+
+fn limits(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "joint.limits";
+    let id = joint_id(s, p.get("joint"), cmd)?;
+    let i = num(p, "index").unwrap_or(0.0).max(0.0) as usize;
+    let kind = s.doc.assembly.joints.iter().find(|j| j.id == id).map(|j| j.kind).ok_or_else(|| bad(cmd, "joint"))?;
+    if i >= kind.dofs().len() {
+        return Err(bad(cmd, "no such value"));
+    }
+    let lim = match (p.get("min"), p.get("max")) {
+        (Some(lo), Some(hi)) => Some((value(s, lo, kind.is_angle(i), cmd)?, value(s, hi, kind.is_angle(i), cmd)?)),
+        (None, None) => None,
+        _ => return Err(bad(cmd, "give both `min` and `max`, or neither to clear")),
+    };
+    let doc = s.doc_mut();
+    let Some(j) = doc.assembly.joints.iter_mut().find(|j| j.id == id) else { return Err(bad(cmd, "joint")) };
+    j.limits.resize(kind.dofs().len(), None);
+    if let Some(slot) = j.limits.get_mut(i) {
+        *slot = lim;
+    }
+    let mut q = j.values.clone();
+    q.resize(kind.dofs().len(), 0.0);
+    joints::clamp(j, &mut q);
+    j.values = q;
+    let sol = resolve(s);
+    Ok(sol_json(&sol))
+}
+
+fn edit(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "joint.edit";
+    let id = joint_id(s, p.get("joint"), cmd)?;
+    let kind = match str_(p, "type") {
+        Some(t) => Some(JointKind::parse(t).ok_or_else(|| bad(cmd, "unknown joint type"))?),
+        None => None,
+    };
+    let offset = match p.get("offset") {
+        Some(v) => Some(value(s, v, false, cmd)?),
+        None => None,
+    };
+    let angle = match p.get("angle") {
+        Some(v) => Some(value(s, v, true, cmd)?),
+        None => None,
+    };
+    let doc = s.doc_mut();
+    let Some(j) = doc.assembly.joints.iter_mut().find(|j| j.id == id) else { return Err(bad(cmd, "joint")) };
+    if let Some(k) = kind {
+        j.kind = k;
+        j.values.resize(k.dofs().len(), 0.0);
+        j.limits.resize(k.dofs().len(), None);
+    }
+    if let Some(o) = offset {
+        j.offset = o;
+    }
+    if let Some(a) = angle {
+        j.angle = a;
+    }
+    if let Some(f) = bool_(p, "flip") {
+        j.flip = f;
+    }
+    if let Some(x) = bool_(p, "suppressed") {
+        j.suppressed = x;
+    }
+    if let Some(n) = str_(p, "name").map(str::trim).filter(|n| !n.is_empty() && n.len() <= 128) {
+        j.name = n.to_string();
+    }
+    let sol = resolve(s);
+    Ok(sol_json(&sol))
+}
+
+fn delete(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "joint.delete";
+    if let Some(name) = str_(p, "origin") {
+        let used = s.doc.assembly.joints.len();
+        let doc = s.doc_mut();
+        let before = doc.assembly.origins.len();
+        doc.assembly.origins.retain(|o| o.name != name);
+        if doc.assembly.origins.len() == before {
+            return Err(bad(cmd, format!("no joint origin `{name}`")));
+        }
+        return Ok(json!({"deleted_origin": name, "joints": used}));
+    }
+    if let Some(i) = num(p, "link") {
+        let i = i.max(0.0) as usize;
+        let doc = s.doc_mut();
+        if i >= doc.assembly.links.len() {
+            return Err(bad(cmd, "no such motion link"));
+        }
+        doc.assembly.links.remove(i);
+        return Ok(json!({"deleted_link": i}));
+    }
+    let id = joint_id(s, p.get("joint"), cmd)?;
+    let doc = s.doc_mut();
+    doc.assembly.joints.retain(|j| j.id != id);
+    doc.assembly.links.retain(|l| l.a != id && l.b != id);
+    let sol = resolve(s);
+    let mut out = sol_json(&sol);
+    out["deleted"] = json!(id);
+    Ok(out)
+}
+
+fn list(s: &mut Session, _p: &Value) -> Result<Value> {
+    let st = s.model.state();
+    let sol = joints::solve(&s.doc, &st);
+    let vals = joints::joint_values(&s.doc.assembly);
+    let js: Vec<Value> = s
+        .doc
+        .assembly
+        .joints
+        .iter()
+        .map(|j| {
+            let q = vals.get(&j.id).cloned().unwrap_or_default();
+            let named: Vec<Value> = j
+                .kind
+                .dofs()
+                .iter()
+                .enumerate()
+                .map(|(i, n)| {
+                    let v = q.get(i).copied().unwrap_or(0.0);
+                    json!({"name": n, "value": if j.kind.is_angle(i) { v.to_degrees() } else { v }, "unit": if j.kind.is_angle(i) { "deg" } else { "mm" },
+                        "limits": j.limits.get(i).copied().flatten().map(|(a, b)| if j.kind.is_angle(i) { [a.to_degrees(), b.to_degrees()] } else { [a, b] })})
+                })
+                .collect();
+            let world = |o: &JointOrigin| s.doc.origin_world(&st, o).ok().map(|m| json!({"origin": [m[3][0], m[3][1], m[3][2]], "z": [m[2][0], m[2][1], m[2][2]]}));
+            json!({"id": j.id, "name": j.name, "type": j.kind, "a": j.a.occurrence, "b": j.b.occurrence, "values": named,
+                "flip": j.flip, "offset": j.offset, "suppressed": j.suppressed, "frame_a": world(&j.a), "frame_b": world(&j.b)})
+        })
+        .collect();
+    let links: Vec<Value> =
+        s.doc.assembly.links.iter().map(|l| json!({"a": l.a, "ia": l.ia, "b": l.b, "ib": l.ib, "ratio": l.ratio, "offset": l.offset})).collect();
+    let origins: Vec<Value> = s.doc.assembly.origins.iter().map(|o| json!({"name": o.name, "occurrence": o.origin.occurrence})).collect();
+    let mut out = sol_json(&sol);
+    out["joints"] = json!(js);
+    out["links"] = json!(links);
+    out["origins"] = json!(origins);
+    Ok(out)
+}
+
+fn solve_cmd(s: &mut Session, _p: &Value) -> Result<Value> {
+    let sol = resolve(s);
+    Ok(sol_json(&sol))
+}
+
+fn interference(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "InterferenceCheckCommand";
+    let want = string_list(p, "bodies");
+    let st = s.world_state();
+    let bodies: Vec<&solvecraft_doc::ModelBody> =
+        st.bodies.iter().filter(|b| want.is_empty() || want.contains(&b.name)).filter(|b| !b.body.is_mesh()).collect();
+    if bodies.len() > 200 {
+        return Err(bad(cmd, "too many bodies (200 at most)"));
+    }
+    let mut hits = Vec::new();
+    for (i, a) in bodies.iter().enumerate() {
+        for b in bodies.iter().skip(i + 1) {
+            let (ba, bb) = (a.mesh().bounds(), b.mesh().bounds());
+            let apart = ba.max.x < bb.min.x
+                || bb.max.x < ba.min.x
+                || ba.max.y < bb.min.y
+                || bb.max.y < ba.min.y
+                || ba.max.z < bb.min.z
+                || bb.max.z < ba.min.z;
+            if apart {
+                continue;
+            }
+            let Ok(Some(o)) = solvecraft_kernel::boolean(&a.body, &b.body, solvecraft_kernel::BoolOp::Intersect) else { continue };
+            let Ok(m) = solvecraft_kernel::measure(&o) else { continue };
+            if m.volume > 1e-9 * a.body.size().max(1.0).powi(3) {
+                hits.push(json!({"a": a.name, "b": b.name, "volume_mm3": m.volume, "area_mm2": m.area, "center": m.centroid, "bbox": {"min": m.bbox.min, "max": m.bbox.max}}));
+            }
+        }
+    }
+    Ok(json!({"interferences": hits, "count": hits.len(), "checked": bodies.len()}))
+}
+
+#[cfg(test)]
+#[path = "joints_tests.rs"]
+mod tests;
diff --git a/crates/engine/src/cmd/joints_tests.rs b/crates/engine/src/cmd/joints_tests.rs
new file mode 100644
index 00000000..2c6f3e10
--- /dev/null
+++ b/crates/engine/src/cmd/joints_tests.rs
@@ -0,0 +1,188 @@
+use std::f64::consts::FRAC_PI_2;
+
+use serde_json::{Value, json};
+use solvecraft_geom::Vec3;
+
+use crate::Session;
+
+fn run(s: &mut Session, id: &str, p: Value) -> Value {
+    match s.execute(id, &p) {
+        Ok(v) => v,
+        Err(e) => panic!("{id} {p}: {e}"),
+    }
+}
+
+fn occ(s: &Session, component: &str) -> u64 {
+    let c = s.doc.find_component(component).unwrap_or(0);
+    s.doc.occurrence_of(c).map(|o| o.id).unwrap_or(0)
+}
+
+fn place(s: &Session, component: &str, p: Vec3) -> Vec3 {
+    solvecraft_doc::apply_point(&solvecraft_doc::joints::occurrence_world(&s.doc, occ(s, component)), p)
+}
+
+fn near(a: Vec3, b: Vec3) -> bool {
+    a.dist(b) < 1e-6
+}
+
+/// A hinge leaf (oracle 63's knuckle geometry): a 40 x 30 x 3 plate and an r4 knuckle along X
+/// at y = 33, z = 4.
+fn leaf(s: &mut Session, name: &str) {
+    run(s, "component.activate", json!({"component": "root"}));
+    run(s, "FusionCreateNewComponentCommand", json!({"name": name}));
+    run(s, "PrimitiveBox", json!({"length": 40, "width": 30, "height": 3, "corner": [0, 0, 0]}));
+    run(s, "PrimitiveCylinder", json!({"base": [0, 33, 4], "axis": [1, 0, 0], "radius": 4, "height": 40, "operation": "new"}));
+}
+
+#[test]
+fn revolute_hinge_turns_about_the_knuckle() {
+    let mut s = Session::default();
+    leaf(&mut s, "LeafA");
+    leaf(&mut s, "LeafB");
+    let o = occ(&s, "LeafA");
+    run(&mut s, "occurrence.ground", json!({"occurrence": o, "grounded": true}));
+    let r = run(
+        &mut s,
+        "JointAssembleCmdNew",
+        json!({
+        "type": "revolute", "a": {"occurrence": "LeafA", "circle": [1, 37, 4]}, "b": {"occurrence": "LeafB", "circle": [1, 37, 4]}, "name": "Hinge"}),
+    );
+    assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
+    // Same frames: B stays put at angle 0.
+    assert!(near(place(&s, "LeafB", Vec3::new(20.0, 0.0, 0.0)), Vec3::new(20.0, 0.0, 0.0)));
+    let l = run(&mut s, "joint.list", json!({}));
+    let dof = l["dof"].as_array().unwrap().iter().find(|d| d["occurrence"] == occ(&s, "LeafB")).unwrap()["dof"].clone();
+    assert_eq!(dof, 1, "{l}");
+    // Drive 90°: B turns about the knuckle axis (through (0, 33, 4), along the frame's z = −X).
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Hinge", "value": 90}));
+    let want = solvecraft_doc::rigid(Vec3::ZERO, Vec3::new(0.0, 33.0, 4.0), Vec3::new(-1.0, 0.0, 0.0), FRAC_PI_2);
+    let p = Vec3::new(20.0, 0.0, 0.0);
+    assert!(near(place(&s, "LeafB", p), solvecraft_doc::apply_point(&want, p)), "{:?}", place(&s, "LeafB", p));
+    assert!(near(place(&s, "LeafB", p), Vec3::new(20.0, 29.0, 37.0)));
+    // LeafA did not move; the knuckle axis is fixed.
+    assert!(near(place(&s, "LeafA", p), p));
+    assert!(near(place(&s, "LeafB", Vec3::new(10.0, 33.0, 4.0)), Vec3::new(10.0, 33.0, 4.0)));
+    // Expressions drive too, and undo puts it back.
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Hinge", "value": "pi rad"}));
+    assert!(near(place(&s, "LeafB", p), Vec3::new(20.0, 66.0, 8.0)));
+    run(&mut s, "UndoCommand", json!({}));
+    assert!(near(place(&s, "LeafB", p), Vec3::new(20.0, 29.0, 37.0)));
+}
+
+/// Two 10 mm cubes in components A (grounded) and B.
+fn two_cubes() -> Session {
+    let mut s = Session::default();
+    for (name, x) in [("A", 0), ("B", 20)] {
+        run(&mut s, "component.activate", json!({"component": "root"}));
+        run(&mut s, "FusionCreateNewComponentCommand", json!({"name": name}));
+        run(&mut s, "PrimitiveBox", json!({"length": 10, "width": 10, "height": 10, "corner": [x, 0, 0]}));
+    }
+    let o = occ(&s, "A");
+    run(&mut s, "occurrence.ground", json!({"occurrence": o, "grounded": true}));
+    s
+}
+
+#[test]
+fn slider_limits_clamp() {
+    let mut s = two_cubes();
+    // B's bottom face onto A's top face (faces meet), sliding along their common normal.
+    let r = run(
+        &mut s,
+        "JointAssembleCmdNew",
+        json!({"type": "slider", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "flip": true, "limits": [[0, 10]], "name": "Rail"}),
+    );
+    assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
+    // B now sits on A.
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 10.0)), "{:?}", place(&s, "B", Vec3::new(25.0, 5.0, 0.0)));
+    let r = run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Rail", "value": 25}));
+    assert_eq!(r["values"], json!([10.0]), "{r}");
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 20.0)));
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Rail", "value": -5}));
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 10.0)));
+    // Changing the limits clamps the current value.
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Rail", "value": 8}));
+    run(&mut s, "joint.limits", json!({"joint": "Rail", "min": 0, "max": 4}));
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 14.0)));
+    // Joints follow edits: A gets taller, B rides on its new top.
+    let h = s
+        .doc
+        .features
+        .iter()
+        .find(|f| matches!(f.kind, solvecraft_doc::FeatureKind::Box { .. }))
+        .and_then(|f| f.param_names.get(2).cloned())
+        .unwrap_or_default();
+    run(&mut s, "ChangeParameterCommand", json!({"name": h, "expression": "15"}));
+    assert!(near(place(&s, "B", Vec3::new(25.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 19.0)), "{:?}", place(&s, "B", Vec3::new(25.0, 5.0, 0.0)));
+}
+
+#[test]
+fn rigid_cycles_solve_and_conflicts_report() {
+    let mut s = two_cubes();
+    run(&mut s, "component.activate", json!({"component": "root"}));
+    run(&mut s, "FusionCreateNewComponentCommand", json!({"name": "C"}));
+    run(&mut s, "PrimitiveBox", json!({"length": 10, "width": 10, "height": 10, "corner": [40, 0, 0]}));
+    run(&mut s, "component.activate", json!({"component": "root"}));
+    // A rigid group and a closing as-built joint: a consistent loop.
+    run(&mut s, "RigidGroupCmd", json!({"occurrences": ["A", "B", "C"]}));
+    let r = run(&mut s, "JointAsBuiltCmd", json!({"type": "rigid", "a": "B", "b": "C"}));
+    assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0), "{r}");
+    assert!(near(place(&s, "C", Vec3::new(40.0, 0.0, 0.0)), Vec3::new(40.0, 0.0, 0.0)));
+    let l = run(&mut s, "joint.list", json!({}));
+    assert!(l["dof"].as_array().unwrap().iter().all(|d| d["dof"] == 0), "{l}");
+    // A joint that wants C elsewhere conflicts and says so.
+    let r = run(
+        &mut s,
+        "JointAssembleCmdNew",
+        json!({"type": "rigid", "a": {"occurrence": "A", "point": [0, 0, 0]}, "b": {"occurrence": "C", "point": [40, 0, 0]}, "name": "Wrong"}),
+    );
+    let c = r["conflicts"].as_array().cloned().unwrap_or_default();
+    assert_eq!(c.len(), 1, "{r}");
+    assert!(c[0]["why"].as_str().unwrap_or("").contains("off by"), "{r}");
+    // Suppressing it clears the conflict.
+    let r = run(&mut s, "joint.edit", json!({"joint": "Wrong", "suppressed": true}));
+    assert_eq!(r["conflicts"].as_array().map(Vec::len), Some(0));
+    assert!(
+        s.execute(
+            "JointAssembleCmdNew",
+            &json!({"type": "rigid", "a": {"occurrence": "A", "point": [0, 0, 0]}, "b": {"occurrence": "A", "point": [1, 0, 0]}})
+        )
+        .is_err()
+    );
+    assert!(s.execute("JointAssembleCmdNew", &json!({"type": "hinge"})).is_err());
+}
+
+#[test]
+fn motion_links_and_interference() {
+    let mut s = two_cubes();
+    run(
+        &mut s,
+        "JointAssembleCmdNew",
+        json!({"type": "cylindrical", "a": {"face": [5, 5, 10]}, "b": {"face": [25, 5, 0]}, "flip": true, "name": "Screw"}),
+    );
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Screw", "values": [0, 0]}));
+    // No overlap while B sits on A; then push B 4 mm into A.
+    let i = run(&mut s, "InterferenceCheckCommand", json!({}));
+    assert_eq!(i["count"], 0, "{i}");
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Screw", "values": [0, -4]}));
+    let i = run(&mut s, "InterferenceCheckCommand", json!({}));
+    assert_eq!(i["count"], 1, "{i}");
+    assert!((i["interferences"][0]["volume_mm3"].as_f64().unwrap_or(0.0) - 400.0).abs() < 1e-3, "{i}");
+    // A motion link: a second joint's value follows.
+    run(&mut s, "component.activate", json!({"component": "root"}));
+    run(&mut s, "FusionCreateNewComponentCommand", json!({"name": "D"}));
+    run(&mut s, "PrimitiveBox", json!({"length": 10, "width": 10, "height": 10, "corner": [60, 0, 0]}));
+    run(
+        &mut s,
+        "JointAssembleCmdNew",
+        json!({"type": "slider", "a": {"occurrence": "A", "face": [5, 5, 10]}, "b": {"occurrence": "D", "face": [65, 5, 0]}, "flip": true, "name": "Follower"}),
+    );
+    run(&mut s, "FusionMotionRelationshipCommand", json!({"a": "Screw", "ia": 1, "b": "Follower", "ratio": 2}));
+    run(&mut s, "FusionMoveJointsCommand", json!({"joint": "Screw", "values": [0, 3]}));
+    assert!(near(place(&s, "D", Vec3::new(65.0, 5.0, 0.0)), Vec3::new(5.0, 5.0, 16.0)), "{:?}", place(&s, "D", Vec3::new(65.0, 5.0, 0.0)));
+    // Saved and reopened, joints come back.
+    let back = solvecraft_doc::Document::from_json(&s.doc.to_json()).unwrap();
+    assert_eq!(back.assembly, s.doc.assembly);
+    let mut t = Session::new(back);
+    let l = run(&mut t, "joint.list", json!({}));
+    assert_eq!(l["joints"].as_array().map(Vec::len), Some(2));
+}
diff --git a/crates/engine/src/cmd/mod.rs b/crates/engine/src/cmd/mod.rs
index 6c0b44fd..96a06f68 100644
--- a/crates/engine/src/cmd/mod.rs
+++ b/crates/engine/src/cmd/mod.rs
@@ -12,6 +12,7 @@ mod features_more;
 pub use features::auto_operation;
 mod file;
 mod inspect;
+pub(crate) mod joints;
 mod measure_sel;
 mod section;
 pub(crate) use measure_sel::measure_items;
@@ -126,6 +127,7 @@ pub fn command_specs() -> Vec<&'static CommandSpec> {
     v.extend(features::COMMANDS.iter());
     v.extend(features_more::COMMANDS.iter());
     v.extend(component::COMMANDS.iter());
+    v.extend(joints::COMMANDS.iter());
     v.extend(browser::COMMANDS.iter());
     v.extend(edit::COMMANDS.iter());
     v.extend(clipboard::COMMANDS.iter());
diff --git a/crates/engine/src/lib.rs b/crates/engine/src/lib.rs
index 006556cd..130ba8c4 100644
--- a/crates/engine/src/lib.rs
+++ b/crates/engine/src/lib.rs
@@ -249,6 +249,10 @@ impl Session {
     pub fn refresh(&mut self) {
         self.model.evaluate(&self.doc);
         self.revision += 1;
+        // Joints follow the geometry their origins snap to.
+        if !self.doc.assembly.joints.is_empty() {
+            cmd::joints::resolve(self);
+        }
         if let Some(id) = self.active_sketch
             && self.doc.feature(id).is_none()
         {