diff --git a/apps/solvecraft-cli/src/main.rs b/apps/solvecraft-cli/src/main.rs
index 2be465ba..cf7cc1c2 100644
--- a/apps/solvecraft-cli/src/main.rs
+++ b/apps/solvecraft-cli/src/main.rs
@@ -57,6 +57,8 @@ fn main() -> ExitCode {
         Some("mcp") => cmd_mcp(&args[1..]),
         // Hidden, for the kill-mid-save test: save a large design over and over.
         Some("save-stress") => cmd_save_stress(&args[1..]),
+        // Hidden: time recomputes after parameter edits (docs/perf.md).
+        Some("bench-edit") => cmd_bench_edit(&args[1..]),
         Some("--version" | "-V") => {
             println!("solvecraft-cli {}", env!("CARGO_PKG_VERSION"));
             Ok(())
@@ -241,3 +243,33 @@ fn cmd_save_stress(args: &[String]) -> Result<(), String> {
     }
     Ok(())
 }
+
+/// `bench-edit <script.json|recipe.json> <param> <expr>`: build the design, then time a
+/// parameter edit, setting it back, undo and redo (ms and features recomputed each).
+fn cmd_bench_edit(args: &[String]) -> Result<(), String> {
+    let (Some(path), Some(param), Some(expr)) = (args.first(), args.get(1), args.get(2)) else {
+        return Err("usage: solvecraft-cli bench-edit <script.json|recipe.json> <param> <expr>".into());
+    };
+    let v = read_json(path)?;
+    let script = if v.get("features").is_some() { recipe::to_script(&v)? } else { v };
+    let mut s = Session::default();
+    let t = std::time::Instant::now();
+    s.run_script(&script).map_err(|e| e.to_string())?;
+    let build = t.elapsed().as_secs_f64() * 1000.0;
+    let old = s.doc.all_param_exprs().into_iter().find(|(n, _, _)| n == param).map(|(_, e, _)| e).ok_or(format!("no parameter `{param}`"))?;
+    let mut out = vec![json!({"step": "build", "ms": build, "features": s.doc.features.len()})];
+    let mut time = |s: &mut Session, step: &str, id: &str, p: Value| -> Result<(), String> {
+        let t = std::time::Instant::now();
+        let r = s.execute(id, &p).map_err(|e| format!("{step}: {e}"))?;
+        let n = r.get("recomputed").cloned().unwrap_or_else(|| json!(s.model.last_recomputed));
+        out.push(json!({"step": step, "ms": t.elapsed().as_secs_f64() * 1000.0, "recomputed": n}));
+        Ok(())
+    };
+    let expr = expr.replace("{old}", &old);
+    time(&mut s, "edit", "ChangeParameterCommand", json!({"name": param, "expression": expr}))?;
+    time(&mut s, "set back", "ChangeParameterCommand", json!({"name": param, "expression": old}))?;
+    time(&mut s, "undo", "UndoCommand", json!({}))?;
+    time(&mut s, "redo", "RedoCommand", json!({}))?;
+    println!("{}", pretty(&json!(out)));
+    Ok(())
+}
diff --git a/crates/doc/src/eval.rs b/crates/doc/src/eval.rs
index ff2a1af8..49474a0b 100644
--- a/crates/doc/src/eval.rs
+++ b/crates/doc/src/eval.rs
@@ -20,11 +20,14 @@ pub struct ModelBody {
     /// Feature that created it.
     pub feature: u64,
     mesh: OnceLock<Arc<Mesh>>,
+    /// Identity of this body's content for the evaluation cache: the key of the evaluation
+    /// that produced it (kept while later features pass it through unchanged).
+    key: u64,
 }
 
 impl ModelBody {
     pub fn new(name: String, body: Body, feature: u64) -> Self {
-        ModelBody { name, body, feature, mesh: OnceLock::new() }
+        ModelBody { name, body, feature, mesh: OnceLock::new(), key: 0 }
     }
     /// Display mesh (chord tolerance 1/1000 of the body size), computed once.
     pub fn mesh(&self) -> Arc<Mesh> {
@@ -187,6 +190,85 @@ pub struct ModelState {
     pub body_counter: usize,
     /// Sheet metal bodies (by body name) and their flat patterns.
     pub sheets: Vec<crate::sheet::SheetBody>,
+    /// Evaluation-cache keys of the sketches (by sketch feature id).
+    sketch_keys: BTreeMap<u64, u64>,
+}
+
+impl ModelState {
+    /// A digest of everything later features can read: bodies (by content identity), sketches,
+    /// the body counter, threads and sheet bodies.
+    fn digest(&self) -> u64 {
+        let mut h = std::collections::hash_map::DefaultHasher::new();
+        for b in &self.bodies {
+            b.name.hash(&mut h);
+            b.key.hash(&mut h);
+            b.feature.hash(&mut h);
+        }
+        for s in &self.sketches {
+            s.feature.hash(&mut h);
+            self.sketch_keys.get(&s.feature).hash(&mut h);
+        }
+        self.body_counter.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()
+    }
+
+    /// After a feature evaluated to this state from `before` with cache key `key`: bodies it
+    /// made or changed, and sketches it made, take that key; the rest keep theirs.
+    fn stamp(&mut self, before: &ModelState, feature: u64, key: u64) {
+        for b in &mut self.bodies {
+            let kept = before.bodies.iter().find(|x| x.name == b.name && x.body.same(&b.body)).map(|x| x.key);
+            b.key = kept.unwrap_or_else(|| {
+                let mut h = std::collections::hash_map::DefaultHasher::new();
+                (key, &b.name).hash(&mut h);
+                h.finish()
+            });
+        }
+        if self.sketches.iter().any(|s| s.feature == feature) {
+            self.sketch_keys.insert(feature, key);
+        }
+    }
+}
+
+/// Evaluation results by cache key, shared by a model and its copies (previews, scratch
+/// sessions), so a result computed once is reused wherever the same inputs come back.
+#[derive(Debug, Default)]
+pub struct EvalCache {
+    map: std::collections::HashMap<u64, CachedEval>,
+    order: std::collections::VecDeque<u64>,
+    pub hits: usize,
+}
+
+#[derive(Clone, Debug)]
+struct CachedEval {
+    error: Option<String>,
+    warning: Option<String>,
+    ms: f64,
+    state: Arc<ModelState>,
+}
+
+/// Results kept (oldest dropped first).
+const CACHE_ENTRIES: usize = 4096;
+
+impl EvalCache {
+    fn get(&mut self, key: u64) -> Option<CachedEval> {
+        let r = self.map.get(&key).cloned();
+        if r.is_some() {
+            self.hits += 1;
+        }
+        r
+    }
+    fn put(&mut self, key: u64, e: CachedEval) {
+        if self.map.insert(key, e).is_none() {
+            self.order.push_back(key);
+        }
+        while self.order.len() > CACHE_ENTRIES {
+            if let Some(k) = self.order.pop_front() {
+                self.map.remove(&k);
+            }
+        }
+    }
 }
 
 impl ModelState {
@@ -221,6 +303,10 @@ pub struct Model {
     empty: Arc<ModelState>,
     /// Features recomputed by the last `evaluate`.
     pub last_recomputed: usize,
+    /// Results by inputs, shared with this model's copies.
+    pub cache: Arc<std::sync::Mutex<EvalCache>>,
+    /// Evaluate everything from scratch (no reuse; tests compare the two).
+    pub no_cache: bool,
 }
 
 /// Parameters a feature uses directly (inputs and sketch dimensions), sorted.
@@ -253,6 +339,56 @@ fn fingerprint(prev: u64, f: &Feature, vals: &BTreeMap<String, Value>, rolled_ba
     h.finish()
 }
 
+/// Document-level inputs any feature may read: construction planes (resolved by name), sheet
+/// metal and plastic rules (and the parameters they use), components and body placement.
+fn context_digest(doc: &Document, vals: &BTreeMap<String, Value>) -> u64 {
+    let mut h = std::collections::hash_map::DefaultHasher::new();
+    for f in doc.features.iter().filter(|f| matches!(f.kind, FeatureKind::ConstructionPlane { .. })) {
+        fingerprint(0, f, vals, false).hash(&mut h);
+    }
+    serde_json::to_string(&doc.sheet).unwrap_or_default().hash(&mut h);
+    serde_json::to_string(&doc.plastic).unwrap_or_default().hash(&mut h);
+    serde_json::to_string(&doc.components).unwrap_or_default().hash(&mut h);
+    serde_json::to_string(&doc.body_components).unwrap_or_default().hash(&mut h);
+    for e in doc
+        .sheet
+        .rules
+        .iter()
+        .flat_map(|r| [&r.thickness, &r.k_factor, &r.bend_radius, &r.relief_width, &r.relief_depth, &r.corner_relief, &r.hem_gap, &r.gap])
+    {
+        for n in expr::references(e) {
+            vals.get(&n).map(|v| v.v.to_bits()).hash(&mut h);
+        }
+    }
+    for r in &doc.plastic.rules {
+        for (_, e, _) in r.fields() {
+            for n in expr::references(e) {
+                vals.get(&n).map(|v| v.v.to_bits()).hash(&mut h);
+            }
+        }
+    }
+    h.finish()
+}
+
+/// The cache key of a feature evaluation: its own inputs, the document context and the state it
+/// starts from. Features that replay other features' definitions (patterns and mirrors of
+/// features) also take the chain fingerprint, which covers every earlier definition.
+fn cache_key(f: &Feature, chain_fp: u64, local: u64, ctx: u64, state: &ModelState) -> u64 {
+    let mut h = std::collections::hash_map::DefaultHasher::new();
+    local.hash(&mut h);
+    ctx.hash(&mut h);
+    state.digest().hash(&mut h);
+    f.id.hash(&mut h);
+    let replays = match &f.kind {
+        FeatureKind::Pattern { features, .. } | FeatureKind::Mirror { features, .. } => !features.is_empty(),
+        _ => false,
+    };
+    if replays {
+        chain_fp.hash(&mut h);
+    }
+    h.finish()
+}
+
 fn now() -> web_time::Instant {
     web_time::Instant::now()
 }
@@ -284,9 +420,10 @@ impl Model {
         let mut prev_fp = 0u64;
         let mut state = self.empty.clone();
         let mut out: Vec<FeatureResult> = Vec::with_capacity(doc.features.len());
-        let mut reuse = true;
+        let mut reuse = !self.no_cache;
         let mut recomputed = 0;
         let marker = doc.marker.unwrap_or(usize::MAX);
+        let ctx = context_digest(doc, &vals);
         for (i, f) in doc.features.iter().enumerate() {
             let rolled_back = i >= marker;
             let mut fp = fingerprint(prev_fp, f, &vals, rolled_back);
@@ -344,6 +481,26 @@ impl Model {
                 });
                 continue;
             }
+            // The same feature on the same inputs: its earlier result.
+            let key = cache_key(f, fp, fingerprint(0, f, &vals, false), ctx, &state);
+            if !self.no_cache
+                && let Some(c) = self.cache.lock().ok().and_then(|mut m| m.get(key))
+            {
+                if c.error.is_none() {
+                    state = c.state.clone();
+                }
+                out.push(FeatureResult {
+                    id: f.id,
+                    name: f.name.clone(),
+                    error: c.error,
+                    warning: c.warning,
+                    ms: c.ms,
+                    skipped: false,
+                    fingerprint: fp,
+                    state: state.clone(),
+                });
+                continue;
+            }
             recomputed += 1;
             let t0 = now();
             let mut next = (*state).clone();
@@ -359,11 +516,17 @@ impl Model {
             let ms = t0.elapsed().as_secs_f64() * 1000.0;
             let error = match r {
                 Ok(()) => {
+                    next.stamp(&state, f.id, key);
                     state = Arc::new(next);
                     None
                 }
                 Err(e) => Some(e.to_string().trim_start_matches("the operation failed: ").to_string()),
             };
+            if !self.no_cache
+                && let Ok(mut m) = self.cache.lock()
+            {
+                m.put(key, CachedEval { error: error.clone(), warning: warning.clone(), ms, state: state.clone() });
+            }
             out.push(FeatureResult { id: f.id, name: f.name.clone(), error, warning, ms, skipped: false, fingerprint: fp, state: state.clone() });
         }
         self.results = out;
diff --git a/crates/doc/src/format.rs b/crates/doc/src/format.rs
index be1c0078..2ee161ad 100644
--- a/crates/doc/src/format.rs
+++ b/crates/doc/src/format.rs
@@ -83,9 +83,8 @@ pub fn read(s: &str) -> Result<Document> {
             return Err(DocError::Invalid(format!("damaged design: occurrence `{}` has an invalid placement", o.name)));
         }
     }
-    if version < 2 {
-        v1_to_v2(&mut d)?;
-    }
+    // Fill in what older designs (and hand-edited files) may lack; a no-op otherwise.
+    normalise(&mut d)?;
     d.format = format_string(FORMAT);
     Ok(d)
 }
@@ -134,8 +133,8 @@ fn rigid(m: &crate::Mat) -> bool {
         && t[..3].iter().all(|v| v.abs() <= MAX_REAL)
 }
 
-/// Format 1 files may predate feature input names and occurrences.
-fn v1_to_v2(d: &mut Document) -> Result<()> {
+/// Feature input names and component occurrences, which format 1 files may predate.
+fn normalise(d: &mut Document) -> Result<()> {
     let ids: Vec<u64> = d.features.iter().map(|f| f.id).collect();
     for id in ids {
         d.name_feature_inputs(id);
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index f459dd30..d127ca49 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -1074,3 +1074,73 @@ fn appearances_by_face_body_and_component() {
     assert!(s.execute("AppearanceCommand", &json!({"color": "#ffffff"})).is_err(), "no target");
     assert!(s.execute("AppearanceCommand", &json!({"faces": [[500, 500, 500]], "color": "#ffffff"})).is_err(), "no face there");
 }
+
+/// The evaluation cache gives exactly what evaluating from scratch gives, through edits,
+/// suppression, roll-back, undo and redo.
+#[test]
+fn cached_evaluation_matches_from_scratch() {
+    let mut s = Session::default();
+    s.run_script(&crate::sample::script()).unwrap();
+    run(
+        &mut s,
+        "PatternRectangular",
+        json!({"bodies": ["Plate"], "dir1": [1, 0, 0], "count1": 3, "spacing1": 100, "dir2": [0, 1, 0], "count2": 2, "spacing2": 70}),
+    );
+    run(&mut s, "PrimitiveBox", json!({"corner": [-50, 0, 0], "length": 10, "width": 10, "height": 10, "body_name": "Side"}));
+    let same = |s: &Session, what: &str| {
+        let mut fresh = solvecraft_doc::Model::new();
+        fresh.no_cache = true;
+        fresh.evaluate(&s.doc);
+        let errs = |m: &solvecraft_doc::Model| m.results.iter().map(|r| (r.id, r.error.clone(), r.skipped)).collect::<Vec<_>>();
+        assert_eq!(errs(&s.model), errs(&fresh), "{what}: feature results");
+        let shape = |st: &solvecraft_doc::ModelState| {
+            st.bodies
+                .iter()
+                .map(|b| {
+                    let m = solvecraft_kernel::measure(&b.body).unwrap();
+                    // Volume and area to 1e-9: two evaluations from scratch differ in the last
+                    // bits too (the kernel's tessellation is not bit-for-bit repeatable).
+                    (b.name.clone(), b.feature, (m.volume * 1e6).round(), (m.area * 1e6).round(), m.faces, m.edges, m.vertices, m.merged.faces)
+                })
+                .collect::<Vec<_>>()
+        };
+        assert_eq!(shape(&s.model.state()), shape(&fresh.state()), "{what}: bodies");
+        assert_eq!(s.model.state().sketches.len(), fresh.state().sketches.len(), "{what}: sketches");
+    };
+    // Two evaluations from scratch agree to the same precision.
+    let mut a = solvecraft_doc::Model::new();
+    a.no_cache = true;
+    a.evaluate(&s.doc);
+    let mut b = solvecraft_doc::Model::new();
+    b.no_cache = true;
+    b.evaluate(&s.doc);
+    let vols = |m: &solvecraft_doc::Model| {
+        m.state().bodies.iter().map(|x| (solvecraft_kernel::measure(&x.body).unwrap().volume * 1e6).round()).collect::<Vec<_>>()
+    };
+    assert_eq!(vols(&a), vols(&b));
+    same(&s, "built");
+    let hits = |s: &Session| s.model.cache.lock().map(|c| c.hits).unwrap_or(0);
+    for (step, (id, p)) in [
+        ("ChangeParameterCommand", json!({"name": "width", "expression": "90 mm"})),
+        ("ChangeParameterCommand", json!({"name": "width", "expression": "80 mm"})),
+        ("ChangeParameterCommand", json!({"name": "thickness", "expression": "10 mm"})),
+        ("UndoCommand", json!({})),
+        ("RedoCommand", json!({})),
+        ("timeline.suppress", json!({"feature": "Holes", "suppressed": true})),
+        ("timeline.suppress", json!({"feature": "Holes", "suppressed": false})),
+        ("timeline.rollTo", json!({"position": 6})),
+        ("timeline.rollTo", json!({})),
+        ("UndoCommand", json!({})),
+        ("UndoCommand", json!({})),
+    ]
+    .into_iter()
+    .enumerate()
+    {
+        let before = hits(&s);
+        run(&mut s, id, p.clone());
+        same(&s, &format!("step {step}: {id} {p}"));
+        if step == 1 {
+            assert!(hits(&s) > before, "setting the width back reuses the earlier results");
+        }
+    }
+}
diff --git a/crates/kernel/src/body.rs b/crates/kernel/src/body.rs
index b089e58e..72bbeae4 100644
--- a/crates/kernel/src/body.rs
+++ b/crates/kernel/src/body.rs
@@ -250,6 +250,17 @@ fn polyline_mid(pts: &[Vec3]) -> (Vec3, f64) {
 }
 
 impl Body {
+    /// The very same body (shared data), not just an equal-looking one.
+    pub fn same(&self, other: &Body) -> bool {
+        std::sync::Arc::ptr_eq(&self.solid, &other.solid)
+            && match (&self.mesh, &other.mesh) {
+                (Some(a), Some(b)) => std::sync::Arc::ptr_eq(a, b),
+                (None, None) => true,
+                _ => false,
+            }
+            && self.color == other.color
+    }
+
     pub(crate) fn new(solid: Solid) -> Result<Body> {
         if solid.boundaries().is_empty() {
             return Err(KernelError::Failed("empty result".into()));
diff --git a/docs/perf.md b/docs/perf.md
index 228ad33c..ee864ceb 100644
--- a/docs/perf.md
+++ b/docs/perf.md
@@ -38,3 +38,46 @@ What changed: rows scrolled out of view only reserve their height. They register
 and paint nothing (no text layout, icons or eye/fold buttons), so the cost follows the rows on
 screen rather than the rows in the tree. Scrolling, Find in Browser and drag and drop still see
 every row's place.
+
+# Recompute performance
+
+`solvecraft-cli bench-edit <script> <param> "(<old>) * 1.02"` builds a design from its script
+(timing the build), then times a parameter edit, setting the parameter back to its old
+expression, undo and redo. Each cell is before → after the evaluation cache, with the number of
+features recomputed in parentheses. Release build, on a machine shared with other build jobs (load average about 45; the
+times vary by ±50 % run to run, so compare the recompute counts and the zeros). "Early" edits a parameter of the
+first sketch or feature, "late" one of the last feature.
+
+| Design | Edit | Build | Edit | Set back | Undo | Redo |
+|---|---|---|---|---|---|---|
+| sample plate, 20 × 20 pattern | early (`width`) | 7587 ms (10) → 1764 ms (10) | 7969 ms (10) → 1691 ms (10) | 7557 ms (10) → 0 ms (0) | 6534 ms (0) → 0 ms (0) | 4570 ms (0) → 0 ms (0) |
+| sample plate, 20 × 20 pattern | late (`d9`) | 7870 ms (10) → 1699 ms (10) | 3944 ms (1) → 812 ms (1) | 4039 ms (1) → 0 ms (0) | 1464 ms (0) → 0 ms (0) | 1398 ms (0) → 0 ms (0) |
+| oracle 58 phone stand | early (`d1`) | 163 ms (6) → 140 ms (6) | 228 ms (5) → 140 ms (5) | 171 ms (5) → 0 ms (0) | 158 ms (0) → 0 ms (0) | 130 ms (0) → 0 ms (0) |
+| oracle 58 phone stand | late (`d4`) | 144 ms (6) → 195 ms (6) | 118 ms (1) → 118 ms (1) | 145 ms (1) → 0 ms (0) | 149 ms (0) → 0 ms (0) | 187 ms (0) → 0 ms (0) |
+| oracle 59 enclosure base | late (`d8`) | 11582 ms (11) → 11179 ms (11) | 5772 ms (1) → 5742 ms (1) | 5606 ms (1) → 0 ms (0) | 6023 ms (0) → 0 ms (0) | 6119 ms (0) → 0 ms (0) |
+| oracle 60 pipe flange | early (`d1`) | 8137 ms (7) → 8348 ms (7) | 7569 ms (6) → 10143 ms (6) | 8354 ms (6) → 0 ms (0) | 8607 ms (0) → 0 ms (0) | 10034 ms (0) → 0 ms (0) |
+| oracle 60 pipe flange | late (`d6`) | 7661 ms (7) → 11990 ms (7) | 67 ms (1) → 361 ms (1) | 69 ms (1) → 0 ms (0) | 84 ms (0) → 0 ms (0) | 66 ms (0) → 0 ms (0) |
+| oracle 61 shaft keyway | early (`d1`) | 1723 ms (6) → 3016 ms (6) | 1785 ms (5) → 2745 ms (5) | 1760 ms (5) → 2323 ms (5) | 1667 ms (0) → 0 ms (0) | 1719 ms (0) → 0 ms (0) |
+| oracle 61 shaft keyway | late (`d4`) | 1905 ms (6) → 2247 ms (6) | 1734 ms (1) → 2101 ms (1) | 1682 ms (1) → 0 ms (0) | 1719 ms (0) → 0 ms (0) | 1876 ms (0) → 0 ms (0) |
+| oracle 62 heat sink | early (`d1`) | 382 ms (6) → 643 ms (6) | 362 ms (5) → 697 ms (5) | 233 ms (5) → 568 ms (5) | 245 ms (0) → 0 ms (0) | 360 ms (0) → 0 ms (0) |
+| oracle 62 heat sink | late (`d5`) | 479 ms (6) → 688 ms (6) | 270 ms (1) → 657 ms (1) | 321 ms (1) → 0 ms (0) | 413 ms (0) → 0 ms (0) | 283 ms (0) → 0 ms (0) |
+| oracle 63 hinge knuckle | early (`d1`) | 1305 ms (8) → 1671 ms (8) | 1329 ms (7) → 2397 ms (7) | 1971 ms (7) → 0 ms (0) | 1705 ms (0) → 0 ms (0) | 1321 ms (0) → 0 ms (0) |
+| oracle 63 hinge knuckle | late (`d4`) | 1430 ms (8) → 2072 ms (8) | 1387 ms (1) → 2073 ms (1) | 1480 ms (1) → 0 ms (0) | 1192 ms (0) → 0 ms (0) | 1035 ms (0) → 0 ms (0) |
+
+What changed: every feature evaluation is cached by a key made of the feature's own inputs
+(definition and parameter values), the document context it may read (construction planes,
+sheet metal and plastic rules, components) and a digest of the model it starts from, in which
+each body is identified by the evaluation that produced it (bodies a feature passes through
+unchanged keep their identity). The same feature on the same inputs is never evaluated twice:
+setting a value back, undo, redo, a preview followed by the same command, and the repeated
+evaluations while a script builds the design all reuse earlier results. The cache is shared by a
+model and its copies (previews, scratch sessions) and keeps the last 4096 results.
+
+An edit still recomputes the edited feature and every later feature whose starting model
+changed; in these parts every later feature works on the edited body, so early edits recompute
+almost everything, as they must. `cached_evaluation_matches_from_scratch` checks that cached and
+from-scratch evaluation give the same models (bodies, volumes and areas to 1e-6, topology,
+errors) through edits, suppression, roll-back, undo and redo. Setting a sketch dimension back
+can still recompute (oracle 61, 62): solving the sketch moves its points by rounding noise, so
+its inputs are not exactly the old ones. Oracle 59's first parameter edit fails (a recipe
+expression the bench's `* 1.02` can't scale) and is left out.