diff --git a/index.html b/index.html
index 28adabf..bcf6499 100644
--- a/index.html
+++ b/index.html
@@ -53,6 +53,13 @@
             width: 6rem;
         }
 
+        /* Flag an amount that's too precise for the resource (e.g. a fractional
+           non-Electrum amount, or finer than a tenth of Electrum): the step
+           attribute makes it :invalid and the server rejects it too. */
+        input:invalid {
+            outline: 2px solid #dc2626;
+        }
+
         button {
             font: inherit;
             padding: .3rem .7rem;
@@ -462,6 +469,11 @@
             return e;
         }
         function num(value, attrs = {}) {return el("input", {type: "number", value, ...attrs});}
+        // Smallest amount a resource can be specified in: Electrum is tracked in
+        // tenths (0.1 steps), every other resource is whole-unit (integers). Used
+        // to set the <input step> so over-precise amounts read as :invalid; the
+        // server enforces the same rule. "renown" (constraints) is whole-unit too.
+        function resStep(resource) {return resource === "electrum" ? "0.1" : "1";}
         function selectEl(opts, value) {
             const s = el("select");
             for (const o of opts) {
@@ -595,7 +607,7 @@
         const startInputs = {};
         const tradeInputs = {};
         for (const r of RESOURCES) {
-            const inp = num(r === "express" || r === "trade_goods" ? 0 : 3, {min: 0});
+            const inp = num(r === "express" || r === "trade_goods" ? 0 : 3, {min: 0, step: resStep(r)});
             startInputs[r] = inp;
             document.getElementById("start").append(
                 el("label", {class: "field"}, [el("span", {}, r), inp]));
@@ -830,7 +842,8 @@
             const card = el("div", {class: "card"});
             const res = selectEl(SCORE_KEYS, c.resource || "capital");
             const op = selectEl([">=", "<=", "=="], c.op || ">=");
-            const value = num(c.value ?? 0);
+            const value = num(c.value ?? 0, {step: resStep(res.value)});
+            res.onchange = () => {value.step = resStep(res.value);};
             const turn = turnSelect(c.turn);
             card._get = () => {
                 const o = {resource: res.value, op: op.value, value: +value.value};
@@ -852,9 +865,11 @@
         function addConversion(c = {}) {
             const card = el("div", {class: "card"});
             const from = selectEl(RESOURCES, c.from || "trade_goods");
-            const fromAmt = num(c.from_amount ?? c.amount ?? 1, {min: 0});
+            const fromAmt = num(c.from_amount ?? c.amount ?? 1, {min: 0, step: resStep(from.value)});
             const to = selectEl(RESOURCES, c.to || "capital");
-            const toAmt = num(c.to_amount ?? c.amount ?? 1, {min: 0});
+            const toAmt = num(c.to_amount ?? c.amount ?? 1, {min: 0, step: resStep(to.value)});
+            from.onchange = () => {fromAmt.step = resStep(from.value);};
+            to.onchange = () => {toAmt.step = resStep(to.value);};
             const turn = turnSelect(c.turn);
             card._get = () => {
                 const o = {
@@ -880,10 +895,12 @@
         function addOptionalConversion(c = {}) {
             const card = el("div", {class: "card"});
             const from = selectEl(RESOURCES, c.from || "trade_goods");
-            const fromAmt = num(c.from_amount ?? c.amount ?? 1, {min: 0});
+            const fromAmt = num(c.from_amount ?? c.amount ?? 1, {min: 0, step: resStep(from.value)});
             const to = selectEl(RESOURCES, c.to || "capital");
-            const toAmt = num(c.to_amount ?? c.amount ?? 1, {min: 0});
-            const maxCount = num(c.max_count ?? 1, {min: 1});
+            const toAmt = num(c.to_amount ?? c.amount ?? 1, {min: 0, step: resStep(to.value)});
+            from.onchange = () => {fromAmt.step = resStep(from.value);};
+            to.onchange = () => {toAmt.step = resStep(to.value);};
+            const maxCount = num(c.max_count ?? 1, {min: 1, step: "1"});
             // Multiple turns may be selected; each chosen turn yields its own copy of
             // this conversion in the problem JSON (one entry per turn).
             const turns = multiTurnSelect(c.turn != null ? [c.turn] : []);
diff --git a/solve.py b/solve.py
index c7d1a32..220fe73 100644
--- a/solve.py
+++ b/solve.py
@@ -451,6 +451,22 @@ class Solution:
 # Model construction
 # --------------------------------------------------------------------------- #
 
+def _scaled_int(value: float, scale: int, what: str) -> int:
+    """Represent ``value`` as the integer the model stores for it, where ``scale``
+    is the resource's internal sub-unit count (1 for whole-unit resources,
+    ELECTRUM_SCALE for Electrum's tenths).
+
+    Rather than silently rounding, reject any amount too precise to represent
+    exactly: a non-integer for a whole-unit resource, or finer than a tenth for
+    Electrum. ``what`` names the field for the error message."""
+    scaled = value * scale
+    nearest = round(scaled)
+    if abs(scaled - nearest) > 1e-6:
+        unit = "a whole number" if scale == 1 else f"a multiple of {1 / scale:g}"
+        raise ValueError(f"{what} must be {unit}; got {value}")
+    return int(nearest)
+
+
 class _Builder:
     def __init__(self, problem: Problem):
         self.p = problem
@@ -1183,8 +1199,8 @@ class _Builder:
             # precision; every other resource stays an integer.
             from_scale = ELECTRUM_SCALE if src == "electrum" else 1
             to_scale = ELECTRUM_SCALE if dst == "electrum" else 1
-            from_amt = int(round(raw_from * from_scale))
-            to_amt = int(round(raw_to * to_scale))
+            from_amt = _scaled_int(raw_from, from_scale, f"conversion from_amount ({src})")
+            to_amt = _scaled_int(raw_to, to_scale, f"conversion to_amount ({dst})")
             if from_amt < 0 or to_amt < 0:
                 raise ValueError("conversion amounts must be non-negative")
             self._add_delta(src, t, -from_amt)
@@ -1226,8 +1242,10 @@ class _Builder:
             # precision; every other resource stays an integer.
             from_scale = ELECTRUM_SCALE if src == "electrum" else 1
             to_scale = ELECTRUM_SCALE if dst == "electrum" else 1
-            from_amt = int(round(raw_from * from_scale))
-            to_amt = int(round(raw_to * to_scale))
+            from_amt = _scaled_int(
+                raw_from, from_scale, f"optional conversion from_amount ({src})")
+            to_amt = _scaled_int(
+                raw_to, to_scale, f"optional conversion to_amount ({dst})")
             if from_amt < 0 or to_amt < 0:
                 raise ValueError("optional conversion amounts must be non-negative")
             max_count = int(c.get("max_count", 1))
@@ -1284,7 +1302,7 @@ class _Builder:
         T = self.T
         for r in RESOURCES:
             scale = ELECTRUM_SCALE if r == "electrum" else 1
-            start = int(round(self.p.start.get(r, 0) * scale))
+            start = _scaled_int(self.p.start.get(r, 0), scale, f"starting {r}")
             self.res[r] = []
             for t in range(T):
                 v = m.NewIntVar(0, self.MAXR * scale, f"res_{r}_t{t}")
@@ -1376,9 +1394,10 @@ class _Builder:
             if op not in self._OPS:
                 raise ValueError(f"resource_constraints op must be one of {self._OPS}")
             var = self._resource_at(c["resource"], c.get("turn"))
-            # Electrum is compared in tenths; its bound may carry a decimal.
+            # Electrum is compared in tenths; its bound may carry one decimal.
             scale = ELECTRUM_SCALE if c["resource"] == "electrum" else 1
-            value = int(round(float(c["value"]) * scale))
+            value = _scaled_int(
+                float(c["value"]), scale, f"constraint value ({c['resource']})")
             if op == ">=":
                 m.Add(var >= value)
             elif op == "<=":