feat: fillet fit option reduces a radius too large for its edges (#388)

* feat(kernel): typed TooLarge refusal and fillet_max search

Blend refusals caused only by the size (larger than the neighbouring faces,
taller than the walls, longer than the next edge, larger than a corner
radius) now return KernelError::TooLarge, displayed as before. fillet_max
finds the largest radius below a request at which the fillet of the edges
works, to three significant digits, by building candidates; it is an explicit
call (up to 16 fillets), not run on every refusal.

A corner edge that doesn't run square to the edge is now reported as such
instead of as a blend larger than the neighbouring faces.

Refs #293

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

* feat: fillet fit option reduces a radius too large for its edges

solid.fillet takes fit: exact (the default, as before: a radius too large
for the geometry around the edges is refused) or reduce: the radius is
reduced to the largest that works for all the picked edges (kernel
fillet_max, three significant digits, every candidate built). The feature
stores the requested radius, so it refits on every evaluation; when it
reduces, the feature's warning says so and the command returns
radius_applied. Pattern copies of the fillet use the same rule. The file
format writes fit only when it is reduce.

The Fillet dialog gets a Radius Fit row (Exact / Reduce to Fit, constant
radius) and, when the preview reduced the radius, an Applied Radius line;
editing the feature keeps the choice.

One radius for all edges for now; per-edge radii and taper follow.

Refs #293

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

---------

Co-authored-by: Claude Opus 5.5 <[email protected]>
Co-authored-by: bherbruck <[email protected]>

crates/doc/src/document.rs

@@ -328,6 +328,9 @@ pub enum FeatureKind {
         /// radius varying along each edge.
         #[serde(default, skip_serializing_if = "FilletStyle::is_constant")]
         style: FilletStyle,
+        /// What to do when the radius is too large for the geometry around the edges.
+        #[serde(default, skip_serializing_if = "FilletFit::is_exact")]
+        fit: FilletFit,
     },
     Chamfer {
         edges: Vec<Vec3>,
@@ -916,6 +919,22 @@ pub enum FilletStyle {
     },
 }
 
+/// A fillet radius too large for the geometry around its edges: refused (`Exact`, the default),
+/// or reduced to the largest that works for all of them (`Reduce`), with a warning.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum FilletFit {
+    #[default]
+    Exact,
+    Reduce,
+}
+
+impl FilletFit {
+    pub fn is_exact(&self) -> bool {
+        *self == FilletFit::Exact
+    }
+}
+
 impl FilletStyle {
     pub fn is_constant(&self) -> bool {
         *self == FilletStyle::Constant

crates/doc/src/eval.rs

@@ -256,6 +256,42 @@ fn thread_groove(t: &ThreadInfo, radius: f64, open: (bool, bool)) -> Result<solv
     Ok(kernel::sweep_helix_mesh(&plane, &section, base, k, p, (s1 - s0) / p, 48)?)
 }
 
+/// A constant fillet of `edges`; with [`FilletFit::Reduce`](crate::FilletFit::Reduce), a radius too
+/// large for the geometry around them is reduced to the largest that works for all of them, which
+/// is returned.
+fn fillet_fitted(
+    body: &kernel::Body,
+    edges: &[Vec3],
+    r: f64,
+    fit: crate::FilletFit,
+) -> std::result::Result<(kernel::Body, Option<f64>), kernel::KernelError> {
+    match kernel::fillet(body, edges, r) {
+        Err(kernel::KernelError::TooLarge(why)) if fit == crate::FilletFit::Reduce => match kernel::fillet_max(body, edges, r) {
+            Some((used, b)) => Ok((b, Some(used))),
+            None => Err(kernel::KernelError::TooLarge(why)),
+        },
+        other => other.map(|b| (b, None)),
+    }
+}
+
+/// A length in mm, without trailing zeros.
+fn show_mm(v: f64) -> String {
+    let s = format!("{v:.6}");
+    s.trim_end_matches('0').trim_end_matches('.').to_string()
+}
+
+impl ModelState {
+    /// The radius fillet `feature` used when it reduced a radius too large for its edges.
+    pub fn fitted_radius(&self, feature: u64) -> Option<f64> {
+        self.fitted.get(&feature).copied()
+    }
+
+    /// Every reduced fillet radius (by feature id).
+    pub fn fitted_radii(&self) -> impl Iterator<Item = (u64, f64)> + '_ {
+        self.fitted.iter().map(|(k, v)| (*k, *v))
+    }
+}
+
 /// Model after some prefix of the timeline.
 #[derive(Clone, Debug, Default)]
 pub struct ModelState {
@@ -275,6 +311,9 @@ pub struct ModelState {
     /// a pattern of the feature repeats these, not the points picked when it was made, which a
     /// parameter change may have moved off the edges.
     blend_edges: BTreeMap<u64, Vec<Vec3>>,
+    /// The radius a fillet used when it reduced a radius too large for its edges (by feature id,
+    /// see `FilletFit::Reduce`).
+    fitted: BTreeMap<u64, f64>,
 }
 
 impl ModelState {
@@ -300,6 +339,10 @@ impl ModelState {
                 [p.x, p.y, p.z].map(f64::to_bits).hash(&mut h);
             }
         }
+        for (feature, r) in &self.fitted {
+            feature.hash(&mut h);
+            r.to_bits().hash(&mut h);
+        }
         serde_json::to_string(&self.threads).unwrap_or_default().hash(&mut h);
         serde_json::to_string(&self.sheets).unwrap_or_default().hash(&mut h);
         h.finish()
@@ -2780,6 +2823,7 @@ fn replay(
                 let Some(mb) = st.bodies.get(ti).cloned() else { continue };
                 let nb = match &src.kind {
                     FeatureKind::Chamfer { distance2, angle, flip, .. } => chamfer_with(vals, &mb.body, &moved, r, distance2, angle, *flip)?,
+                    FeatureKind::Fillet { fit, .. } => fillet_fitted(&mb.body, &moved, r, *fit)?.0,
                     _ => kernel::fillet(&mb.body, &moved, r)?,
                 };
                 let nb = Traced::step(nb, &[&mb.traced()]);
@@ -3518,7 +3562,18 @@ fn eval_feature(doc: &Document, vals: &BTreeMap<String, Value>, f: &Feature, st:
                         None => return Err(DocError::Invalid("no edges to fillet".into())),
                     }
                 }
-                FeatureKind::Fillet { .. } => (kernel::fillet(&mb.body, &edges, r)?, input),
+                FeatureKind::Fillet { fit, .. } => {
+                    let (b, used) = fillet_fitted(&mb.body, &edges, r, *fit)?;
+                    if let Some(used) = used {
+                        st.fitted.insert(f.id, used);
+                        let note = format!("the radius is reduced to {} mm, the largest that fits these edges", show_mm(used));
+                        *warning = Some(match warning.take() {
+                            Some(w) => format!("{w}; {note}"),
+                            None => note,
+                        });
+                    }
+                    (b, input)
+                }
                 FeatureKind::Chamfer { distance2, angle, flip, .. } => (chamfer_with(vals, &mb.body, &edges, r, distance2, angle, *flip)?, input),
                 _ => return Err(DocError::Invalid("blend".into())),
             };
@@ -3986,7 +4041,13 @@ mod chain_tests {
         )
         .unwrap();
         doc.add_feature(
-            FeatureKind::Fillet { edges: vec![Vec3::new(20.0, 0.0, 20.0)], radius: "4".into(), body: None, style: crate::FilletStyle::Constant },
+            FeatureKind::Fillet {
+                edges: vec![Vec3::new(20.0, 0.0, 20.0)],
+                radius: "4".into(),
+                body: None,
+                style: crate::FilletStyle::Constant,
+                fit: Default::default(),
+            },
             None,
         )
         .unwrap();

crates/doc/src/tests.rs

@@ -104,8 +104,17 @@ fn set_back_reports_restored_features() {
 #[test]
 fn box_fillet_cut_timeline() {
     let (mut doc, _) = plate_doc();
-    doc.add_feature(FeatureKind::Fillet { edges: vec![Vec3::new(0.0, 0.0, 10.0)], radius: "3".into(), body: None, style: Default::default() }, None)
-        .unwrap();
+    doc.add_feature(
+        FeatureKind::Fillet {
+            edges: vec![Vec3::new(0.0, 0.0, 10.0)],
+            radius: "3".into(),
+            body: None,
+            style: Default::default(),
+            fit: Default::default(),
+        },
+        None,
+    )
+    .unwrap();
     let mut sk = Sketch::new();
     sk.add_circle(Vec2::new(20.0, 15.0), 5.0, None, None).unwrap();
     let s2 = doc.add_feature(FeatureKind::Sketch { plane: PlaneRef::Origin { name: "XY".into() }, sketch: sk }, None).unwrap();
@@ -163,8 +172,17 @@ fn errors_are_reported_not_fatal() {
         None,
     )
     .unwrap();
-    doc.add_feature(FeatureKind::Fillet { edges: vec![Vec3::new(500.0, 0.0, 0.0)], radius: "1".into(), body: None, style: Default::default() }, None)
-        .unwrap();
+    doc.add_feature(
+        FeatureKind::Fillet {
+            edges: vec![Vec3::new(500.0, 0.0, 0.0)],
+            radius: "1".into(),
+            body: None,
+            style: Default::default(),
+            fit: Default::default(),
+        },
+        None,
+    )
+    .unwrap();
     doc.add_feature(
         FeatureKind::Box {
             corner: point_expr(Vec3::ZERO),

crates/engine/src/cmd/features.rs

@@ -31,7 +31,7 @@ pub static COMMANDS: &[CommandSpec] = &[
         .at("SOLID", "MODIFY")
         .icon("fillet")
         .key("F")
-        .params("edges: [[x,y,z] point on edge | {body, index}]; radius: expr; body?; type?: constant|chord (radius is the width across)|variable (radius at the end nearest start, radius2 at the other); radius2?; start?: [x,y,z]; mid?: [[position 0…1 from start, radius expr], …] (variable radii between the ends); smooth?: bool (smooth instead of straight between the radii)"),
+        .params("edges: [[x,y,z] point on edge | {body, index}]; radius: expr; body?; type?: constant|chord (radius is the width across)|variable (radius at the end nearest start, radius2 at the other); radius2?; start?: [x,y,z]; mid?: [[position 0…1 from start, radius expr], …] (variable radii between the ends); smooth?: bool (smooth instead of straight between the radii); fit?: exact|reduce (exact, the default: a radius too large for the geometry around the edges is refused; reduce: it is reduced to the largest that fits all of them, returned as `radius_applied` and noted in the feature's warning)"),
     CommandSpec::new("solid.chamfer", "Chamfer", chamfer).at("SOLID", "MODIFY").icon("chamfer").params("edges: [[x,y,z] | {body, index}]; distance: expr; distance2?: expr (along the second face) | angle?: expr (from the first face); flip?: bool (which face is first: by default the one facing up most); body?"),
     CommandSpec::new("solid.combine", "Combine", combine).at("SOLID", "MODIFY").icon("combine").params("target: body; tools: [body]; operation?: join|cut|intersect; keep_tools?: bool; new_component?: bool (the result goes into a new component; component_name?)"),
     CommandSpec::new("solid.pattern.rectangular", "Rectangular Pattern", pattern_rect)
@@ -785,7 +785,19 @@ fn fillet(s: &mut Session, p: &Value) -> Result<Value> {
         }
         Some(o) => return Err(bad(cmd, format!("unknown fillet type `{o}` (constant, chord or variable)"))),
     };
-    add_feature(s, p, FeatureKind::Fillet { edges, radius, body: str_(p, "body").map(str::to_string), style })
+    let fit = match str_(p, "fit").map(str::to_ascii_lowercase).as_deref() {
+        None | Some("exact") => solvecraft_doc::FilletFit::Exact,
+        Some("reduce") => solvecraft_doc::FilletFit::Reduce,
+        Some(o) => return Err(bad(cmd, format!("unknown fit `{o}` (exact or reduce)"))),
+    };
+    let mut out = add_feature(s, p, FeatureKind::Fillet { edges, radius, body: str_(p, "body").map(str::to_string), style, fit })?;
+    // The radius the fillet used, when `fit: reduce` reduced it.
+    if let Some(id) = out.get("feature").and_then(Value::as_u64)
+        && let Some(r) = s.model.result(id).and_then(|res| res.state.fitted_radius(id))
+    {
+        out["radius_applied"] = serde_json::json!(r);
+    }
+    Ok(out)
 }
 
 fn chamfer(s: &mut Session, p: &Value) -> Result<Value> {

crates/engine/src/tests.rs

@@ -2790,3 +2790,31 @@ fn a_design_saved_before_format_14_keeps_generic_names_past_body_name() {
     assert_eq!(names, ["Ribs", "Body1", "Body2"]);
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn fillet_fit_reduce_uses_the_largest_radius_that_fits() {
+    // #293: 3.5 is too large for the 6 mm end faces of a 14 x 6 x 10 box; reduce gives 3 (a full
+    // round), says so, and refits when the box changes.
+    let mut s = Session::default();
+    run(&mut s, "parameters.add", json!({"name": "w", "expression": "6 mm"}));
+    run(&mut s, "solid.box", json!({"length": 14, "width": "w", "height": 10, "corner": [14, 0, 0]}));
+    let mids = json!([[14, 0, 5], [28, 0, 5], [14, 6, 5], [28, 6, 5]]);
+    assert!(s.execute("solid.fillet", &json!({"edges": mids, "radius": 3.5})).is_err());
+    let out = run(&mut s, "solid.fillet", json!({"edges": mids, "radius": 3.5, "fit": "reduce", "name": "Round"}));
+    assert_eq!(out["radius_applied"].as_f64(), Some(3.0), "{out}");
+    let full = 840.0 - (4.0 - PI) * 9.0 * 10.0;
+    assert!(rel(volume(&mut s), full) < 2e-4, "{} vs {full}", volume(&mut s));
+    let id = out["feature"].as_u64().unwrap();
+    let warning = s.model.result(id).and_then(|r| r.warning.clone()).unwrap_or_default();
+    assert!(warning.contains("reduced to 3 mm"), "{warning}");
+    // Wider: the requested 3.5 fits, nothing is reduced.
+    let r = run(&mut s, "parameters.change", json!({"name": "w", "expression": "8 mm"}));
+    assert_eq!(r["errors"].as_array().map(Vec::len), Some(0), "{r}");
+    assert_eq!(s.model.result(id).and_then(|r| r.state.fitted_radius(id)), None);
+    assert!(s.model.result(id).and_then(|r| r.warning.clone()).is_none());
+    // Saved only when set.
+    let json = serde_json::to_string(&s.doc.features).unwrap();
+    assert_eq!(json.matches("\"fit\":\"reduce\"").count(), 1, "{json}");
+    // An unknown mode is refused.
+    assert!(s.execute("solid.fillet", &json!({"edges": mids, "radius": 1, "fit": "taper"})).is_err());
+}

crates/io/src/lib.rs

@@ -575,7 +575,13 @@ mod tests {
         )
         .unwrap();
         doc.add_feature(
-            FeatureKind::Fillet { edges: vec![Vec3::new(0.0, 0.0, 5.0)], radius: "4".into(), body: None, style: Default::default() },
+            FeatureKind::Fillet {
+                edges: vec![Vec3::new(0.0, 0.0, 5.0)],
+                radius: "4".into(),
+                body: None,
+                style: Default::default(),
+                fit: Default::default(),
+            },
             None,
         )
         .unwrap();

crates/kernel/src/blend.rs

@@ -27,6 +27,11 @@ pub(crate) fn unsupported(msg: &str) -> KernelError {
     KernelError::Failed(format!("not supported yet: {msg}"))
 }
 
+/// A blend too large for the faces around it.
+pub(crate) fn too_large(msg: &str) -> KernelError {
+    KernelError::TooLarge(msg.into())
+}
+
 pub(crate) fn vtx(v: &mt::Vertex) -> Vec3 {
     from_p3(v.point())
 }
@@ -286,7 +291,7 @@ fn blend_geometry(
             // A rail ending before the old corner extends the neighbouring rim edge away from its
             // other vertex: valid only within the trimmed side face.
             if along >= len - tol || (along < 0.0 && !faces.get(fi).is_some_and(|f| extension_fits(f, p, q, tol))) {
-                return Err(unsupported("the blend is larger than the neighbouring faces"));
+                return Err(too_large("the blend is larger than the neighbouring faces"));
             }
         }
         // Across two end faces: where the blend's cylinder crosses the edge between them, on the
@@ -332,7 +337,7 @@ fn blend_geometry(
         return Err(unsupported("both ends touch the same face"));
     }
     if (end1.a - end0.a).dot(d) <= tol || (end1.b - end0.b).dot(d) <= tol {
-        return Err(unsupported("the blend is larger than the neighbouring faces"));
+        return Err(too_large("the blend is larger than the neighbouring faces"));
     }
     // A side face whose corner edges are consumed at both ends is used up: its remaining edge,
     // parallel to the old edge at the setback, becomes the rail and the face is removed.
@@ -616,7 +621,10 @@ fn blend(body: &Body, edges: &[Vec3], r: f64, shape: Shape, what: &str) -> Resul
                             .collect();
                         guard(what, || crate::curveblend::curve_blend(&solid, &ids, size, r, shape == Shape::Round))
                             .map(|o| (o, if on_chain.is_empty() { vec![j] } else { on_chain }))
-                            .map_err(|e2| KernelError::Failed(format!("{e2} ({e})")))
+                            .map_err(|e2| match e2 {
+                                KernelError::TooLarge(m) => KernelError::TooLarge(format!("{m} ({e})")),
+                                e2 => KernelError::Failed(format!("{e2} ({e})")),
+                            })
                     }
                     None => Err(e),
                 },
@@ -893,7 +901,9 @@ fn loop_split(cur: &Body, pts: &[Vec3]) -> Result<Option<(Vec<Vec<Vec3>>, Vec<Ve
     Ok((!groups.is_empty()).then_some((groups, rest)))
 }
 
-/// Constant-radius fillet of the edges nearest to the given points.
+/// Constant-radius fillet of the edges nearest to the given points. A radius too large for the
+/// faces around the edges is refused with [`KernelError::TooLarge`] ([`fillet_max`] finds the
+/// largest that works).
 pub fn fillet(body: &Body, edges: &[Vec3], radius: f64) -> Result<Body> {
     body.require_brep("fillet")?;
     // Corners the local blends refuse (an earlier round at an edge's end): radius per edge.
@@ -903,6 +913,41 @@ pub fn fillet(body: &Body, edges: &[Vec3], radius: f64) -> Result<Body> {
     })
 }
 
+/// The step of the radius search: three significant digits of the requested radius.
+fn radius_step(radius: f64) -> f64 {
+    10f64.powf(radius.log10().floor() - 3.0)
+}
+
+/// The largest radius below `radius` (to three significant digits) at which the fillet of
+/// these edges works, with its result; `None` when none does. Each candidate is a real fillet
+/// of the edges, so the radius returned is one that was built; the search runs up to 16 of
+/// them, so it is for an explicit request, not every refusal.
+pub fn fillet_max(body: &Body, edges: &[Vec3], radius: f64) -> Option<(f64, Body)> {
+    if !(radius.is_finite() && radius > 1e-6) {
+        return None;
+    }
+    let q = radius_step(radius);
+    // Radii n·q for n in [0, hi): hi fails (it is the request, or above it), lo works.
+    let mut hi = (radius / q).ceil() as u64;
+    let mut lo = 0u64;
+    let mut best: Option<Body> = None;
+    // log2(10⁴) steps reach the step size; the cap keeps a slow body from stalling.
+    for _ in 0..16 {
+        if hi - lo <= 1 {
+            break;
+        }
+        let mid = lo + (hi - lo) / 2;
+        match fillet(body, edges, mid as f64 * q) {
+            Ok(b) => {
+                lo = mid;
+                best = Some(b);
+            }
+            Err(_) => hi = mid,
+        }
+    }
+    best.map(|b| (lo as f64 * q, b))
+}
+
 /// Equal-distance chamfer of the edges nearest to the given points. Each edge is cut with a
 /// prism whose only face inside the body is the chamfer plane (so neighbouring chamfers meet in
 /// a mitre); the local operation is the fallback.

crates/kernel/src/curveblend.rs

@@ -691,7 +691,7 @@ fn curve_blend_with_limit(solid: &Solid, ids: &[mt::EdgeID], size: f64, r: f64,
             let (ta, oa) = a.as_ref().map(at).unwrap_or((0.0, 0.0));
             let (tb, ob) = b.as_ref().map(at).unwrap_or((1.0, 0.0));
             if oa > size * 1e-5 || ob > size * 1e-5 || ta < -1e-9 || tb > 1.0 + 1e-9 || tb - ta < 1e-6 {
-                return Err(unsupported("the blend is larger than the neighbouring faces"));
+                return Err(crate::blend::too_large("the blend is larger than the neighbouring faces"));
             }
             let a = a.unwrap_or_else(|| seam.front().clone());
             let b = b.unwrap_or_else(|| seam.back().clone());

crates/kernel/src/lib.rs

@@ -66,7 +66,7 @@ mod thread_tests;
 mod topo;
 mod tspline;
 
-pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet};
+pub use blend::{ChamferSide, chamfer, chamfer_sides, fillet, fillet_max};
 pub use blend_corner::fillet_radii;
 pub use blend_variable::{fillet_chord, fillet_variable, fillet_variable_points};
 pub use body::{Body, CapSignature, EdgeInfo, FaceInfo, FacePaint, FaceSource, Paint};
@@ -105,6 +105,10 @@ pub enum KernelError {
     Invalid(String),
     #[error("the operation failed: {0}")]
     Failed(String),
+    /// A blend or other size too large for the geometry around it (a smaller size may work);
+    /// shown like any feature that isn't supported yet.
+    #[error("the operation failed: not supported yet: {0}")]
+    TooLarge(String),
     /// The third-party kernel panicked; the model is unchanged.
     #[error("kernel internal error: {0}")]
     Internal(String),

crates/kernel/src/loopblend.rs

@@ -356,7 +356,7 @@ fn build(
         }
         let p = joint_point(i).ok_or_else(|| unsupported("degenerate loop joint"))?;
         if !crate::blend::extension_fits(f, p, *q, size * 1e-7) {
-            return Err(unsupported("the blend is larger than the neighbouring faces"));
+            return Err(crate::blend::too_large("the blend is larger than the neighbouring faces"));
         }
     }
     // The offset arcs must not turn inside out; an arc of exactly the blend's radius shrinks
@@ -371,12 +371,12 @@ fn build(
                     *x = true;
                 }
             } else if toward && a.dist(c) <= r + tol {
-                return Err(unsupported("the blend is larger than a corner radius"));
+                return Err(crate::blend::too_large("the blend is larger than a corner radius"));
             }
         }
     }
     if collapsed.iter().all(|x| *x) {
-        return Err(unsupported("the blend is larger than a corner radius"));
+        return Err(crate::blend::too_large("the blend is larger than a corner radius"));
     }
     let mut va: Vec<mt::Vertex> = pa.iter().map(|p| builder::vertex(p3(*p))).collect();
     // A collapsed piece starts and ends at one vertex.
@@ -546,7 +546,11 @@ fn build(
                 let along = (from_p3(other.point()) - from_p3(v.point())).dot(target - from_p3(v.point()));
                 let reach = (target - from_p3(v.point())).len2();
                 if along <= reach + tol {
-                    return Err(unsupported(if face_side { "the blend is longer than the next edge" } else { "the blend is taller than the walls" }));
+                    return Err(crate::blend::too_large(if face_side {
+                        "the blend is longer than the next edge"
+                    } else {
+                        "the blend is taller than the walls"
+                    }));
                 }
                 let nb = if face_side { va.get(i) } else { vb.get(i) }.ok_or_else(|| KernelError::Failed("joint".into()))?;
                 let slot = seam_ends.entry(e.id()).or_insert_with(|| (e.absolute_clone(), None, None));
@@ -562,7 +566,7 @@ fn build(
         let a = a.unwrap_or_else(|| e.absolute_front().clone());
         let b = b.unwrap_or_else(|| e.absolute_back().clone());
         if from_p3(a.point()).dist(from_p3(b.point())) < tol * 10.0 {
-            return Err(unsupported("the blend is longer than the next edge"));
+            return Err(crate::blend::too_large("the blend is longer than the next edge"));
         }
         seam_subst.insert(id, builder::line(&a, &b));
     }

crates/kernel/src/tests.rs

@@ -323,6 +323,39 @@ fn fillet_consuming_a_side_face_makes_a_full_round() {
     assert_eq!(measure(&b).unwrap().volume, 840.0);
 }
 
+#[test]
+fn fillet_max_finds_the_largest_radius_that_works() {
+    let too_large = |b: &Body, edges: &[Vec3], r: f64| matches!(fillet(b, edges, r), Err(KernelError::TooLarge(_)));
+    // Two rounds on a 6 mm face: up to 3 (a full round), although after the first round at
+    // 3.5 the next edge alone would take only 2.5.
+    let b = box_solid(Vec3::new(14.0, 0.0, 0.0), Vec3::new(28.0, 6.0, 10.0)).unwrap();
+    let mids = [Vec3::new(14.0, 0.0, 5.0), Vec3::new(28.0, 0.0, 5.0), Vec3::new(14.0, 6.0, 5.0), Vec3::new(28.0, 6.0, 5.0)];
+    assert!(too_large(&b, &mids, 3.5));
+    let (r, body) = fillet_max(&b, &mids, 3.5).unwrap();
+    assert_eq!(r, 3.0);
+    assert!(rel(measure(&body).unwrap().volume, 840.0 - (4.0 - PI) * 90.0) < 2e-4);
+    // A 2 mm disc's top edge: up to 2 (the wall used up).
+    let disc = cylinder(Vec3::ZERO, Vec3::Z, 13.0, 2.0).unwrap();
+    let top = [Vec3::new(0.0, 13.0, 2.0), Vec3::new(0.0, -13.0, 2.0)];
+    assert!(too_large(&disc, &top, 2.5));
+    assert_eq!(fillet_max(&disc, &top, 2.5).map(|x| x.0), Some(2.0));
+    // Walls of different heights: at the shortest one's height only that wall would be used up,
+    // which is refused, so the largest radius is just below it (to the search's 3 digits).
+    let notched = boolean(
+        &box_solid(Vec3::ZERO, Vec3::new(40.0, 30.0, 10.0)).unwrap(),
+        &box_solid(Vec3::new(35.0, -1.0, -1.0), Vec3::new(41.0, 5.0, 3.0)).unwrap(),
+        BoolOp::Cut,
+    )
+    .unwrap()
+    .unwrap();
+    let outline = [Vec3::new(20.0, 0.0, 10.0), Vec3::new(40.0, 15.0, 10.0), Vec3::new(20.0, 30.0, 10.0), Vec3::new(0.0, 15.0, 10.0)];
+    assert!(too_large(&notched, &outline, 8.0));
+    let (r, _) = fillet_max(&notched, &outline, 8.0).unwrap();
+    assert!((r - 6.999).abs() < 1e-9, "{r}");
+    // No edge there: no radius.
+    assert!(fillet_max(&b, &[Vec3::new(500.0, 0.0, 5.0)], 1.0).is_none());
+}
+
 #[test]
 fn tapered_extrudes() {
     // 40 x 40 square, 30 up, 10° outward: a frustum.

crates/ui-egui/src/dialogs.rs

@@ -1577,6 +1577,22 @@ pub fn show(app: &mut SolveApp, ctx: &egui::Context) {
                         );
                         enter |= field(ui, radius);
                         ui.end_row();
+                        // A constant radius too large for the edges: refused, or reduced to the
+                        // largest that fits (the preview shows what was used).
+                        if !*chamfer && rtype == 0 {
+                            row_label(ui, "Radius Fit");
+                            let fit = extra_choice(ui, &mut d.extra, "fi_fit", "fit", &[("Exact", "exact"), ("Reduce to Fit", "reduce")]);
+                            ui.end_row();
+                            if fit == 1
+                                && let Some(r) = app.preview.applied
+                            {
+                                row_label(ui, "Applied Radius");
+                                ui.label(RichText::new(crate::prefs::show_mm(app, r, 1)).color(t.warning));
+                                ui.end_row();
+                            }
+                        } else if !*chamfer {
+                            d.extra.remove("fit");
+                        }
                         if *chamfer && *ctype == 1 {
                             row_label(ui, "Distance 2");
                             enter |= field(ui, distance2);
@@ -3303,6 +3319,10 @@ pub fn for_feature(app: &SolveApp, id: u64, marker: Option<usize>) -> Option<Dia
                     *flip = *f;
                 }
             }
+            // A fillet that reduces a radius too large for its edges.
+            if let FeatureKind::Fillet { fit: solvecraft_engine::doc::FilletFit::Reduce, .. } = &f.kind {
+                d.extra.insert("fit".into(), json!("reduce"));
+            }
             // A chord-length or variable fillet.
             if let FeatureKind::Fillet { style, .. } = &f.kind {
                 match style {

crates/ui-egui/src/preselect_tests.rs

@@ -512,6 +512,39 @@ fn chamfer_types_send_their_values() {
     assert!(matches!(&e.kind, Kind::Fillet { chamfer: true, ctype: 2, angle, flip: true, .. } if angle == "30 deg"), "{:?}", e.kind);
 }
 
+/// Radius Fit: the fillet reduces a radius too large for its edges, the preview shows the radius
+/// it used, and editing the feature keeps the choice (#293).
+#[test]
+fn fillet_radius_fit_reduces_and_shows_the_applied_radius() {
+    let mut app = app_with(serde_json::json!([
+        {"command": "solid.box", "params": {"length": 14, "width": 6, "height": 10, "corner": [14, 0, 0]}}
+    ]));
+    let edges: Vec<Sel> = [Vec3::new(14.0, 0.0, 5.0), Vec3::new(28.0, 0.0, 5.0), Vec3::new(14.0, 6.0, 5.0), Vec3::new(28.0, 6.0, 5.0)]
+        .into_iter()
+        .map(|p| edge(&app, p))
+        .collect();
+    let mut d = start(&mut app, edges, "solid.fillet");
+    if let Kind::Fillet { radius, .. } = &mut d.kind {
+        *radius = "3.5 mm".into();
+    }
+    // Exact (the default) sends no fit and the preview refuses the radius.
+    let (id, p) = apply_commands(&app, &d).unwrap().remove(0);
+    assert!(p.get("fit").is_none(), "{p}");
+    assert!(app.session.preview(&[(id.clone(), p)]).is_err());
+    // Reduce to Fit: the preview builds it, at the radius that fits.
+    d.extra.insert("fit".into(), json!("reduce"));
+    let (id, p) = apply_commands(&app, &d).unwrap().remove(0);
+    assert_eq!(p["fit"], json!("reduce"));
+    let job = crate::preview::compute(&app.session, &[(id.clone(), p.clone())], crate::preview::Colors::from_tokens(&crate::theme::Tokens::dark()))
+        .unwrap();
+    assert_eq!(job.applied, Some(3.0));
+    // Applied and edited again, the fillet keeps its fit.
+    app.session.execute(&id, &p).unwrap();
+    let fillet = app.session.doc.features.last().unwrap().id;
+    let e = crate::dialogs::for_feature(&app, fillet, None).unwrap();
+    assert_eq!(e.extra.get("fit"), Some(&json!("reduce")));
+}
+
 /// Apply a dialog's commands to the session; the new feature's id.
 fn apply(app: &mut SolveApp, d: &Dialog) -> u64 {
     for (id, p) in apply_commands(app, d).unwrap() {

crates/ui-egui/src/preview.rs

@@ -41,6 +41,8 @@ pub struct Built {
     pub error: Option<String>,
     /// Extent of what the preview shows (the view's depth range must cover it).
     pub bounds: Aabb3,
+    /// The radius a fillet used when it reduced one too large for its edges (`fit: reduce`).
+    pub applied: Option<f64>,
 }
 
 type Job = Result<Built, String>;
@@ -66,6 +68,8 @@ pub struct PreviewState {
     /// Time the last preview took (ms), for `ui.inspect`.
     pub ms: f64,
     started: f64,
+    /// The radius the previewed fillet used when it reduced the one asked for.
+    pub applied: Option<f64>,
 }
 
 impl PreviewState {
@@ -73,6 +77,7 @@ impl PreviewState {
         self.running = None;
         self.shown_key = None;
         self.error = None;
+        self.applied = None;
         self.busy = false;
         if self.active || !self.replaced.is_empty() {
             self.active = false;
@@ -98,9 +103,13 @@ impl PreviewState {
                 self.bounds = b.bounds;
                 self.active = true;
                 self.error = b.error;
+                self.applied = b.applied;
                 self.set_scene(b.scene);
             }
-            Err(e) => self.error = Some(e),
+            Err(e) => {
+                self.error = Some(e);
+                self.applied = None;
+            }
         }
     }
 
@@ -220,6 +229,7 @@ pub fn compute(s: &Session, cmds: &[(String, Value)], colors: Colors) -> Job {
             let p = real.map_err(|e| e.to_string())?;
             let mut b = guard(&|| build(&p.before, &p.after, colors))?;
             b.bounds = state_bounds(&p.after);
+            b.applied = p.after.fitted_radii().find(|(id, _)| p.before.fitted_radius(*id).is_none()).map(|(_, r)| r);
             Ok(b)
         }
     }
@@ -256,7 +266,7 @@ fn build_tool(before: &ModelState, after: &ModelState, c: Colors, op: &str) -> B
             }
         }
     }
-    Built { scene: sc, replaced: Vec::new(), error: None, bounds: Aabb3::EMPTY }
+    Built { scene: sc, replaced: Vec::new(), error: None, bounds: Aabb3::EMPTY, applied: None }
 }
 
 /// The same commands making a new body instead of joining, cutting or intersecting, and the
@@ -357,7 +367,7 @@ pub fn build(before: &ModelState, after: &ModelState, c: Colors) -> Built {
             replaced.push(b.name.clone());
         }
     }
-    Built { scene: sc, replaced, error: None, bounds: Aabb3::EMPTY }
+    Built { scene: sc, replaced, error: None, bounds: Aabb3::EMPTY, applied: None }
 }
 
 /// Is face `f` of `m` new surface, i.e. not on the old mesh? Decided by a vote of points on it