diff --git a/src/presa_mano/api/work_plane_routes.py b/src/presa_mano/api/work_plane_routes.py
index 111a267..99f3382 100644
--- a/src/presa_mano/api/work_plane_routes.py
+++ b/src/presa_mano/api/work_plane_routes.py
@@ -31,7 +31,10 @@ class FitReq(BaseModel):
     v_min: int = Field(..., ge=0)
     u_max: int = Field(..., gt=0)
     v_max: int = Field(..., gt=0)
-    offset_mm: float = Field(5.0, ge=0.0, le=200.0)
+    # offset_mm può essere negativo: valore <0 esclude un buffer SOPRA il
+    # piano (toglie noise depth della superficie); valore >0 è permissivo
+    # (mantieni un buffer sotto il piano). Range esteso simmetricamente.
+    offset_mm: float = Field(5.0, ge=-200.0, le=200.0)
     max_preview_points: int = Field(5000, ge=100, le=20000)
 
 
diff --git a/web/index.html b/web/index.html
index 3f92714..dde0087 100644
--- a/web/index.html
+++ b/web/index.html
@@ -205,7 +205,7 @@
         </div>
         <div class="kv compact">
           <label>ROI <span id="wpRoiInfo" class="dim">— · clicca 2 angoli sull'RGB</span></label>
-          <label>offset (mm) <input type="number" id="wpOffset" value="5.0" step="0.5" min="0" max="200" /></label>
+          <label>offset (mm) <input type="number" id="wpOffset" value="5.0" step="0.5" min="-200" max="200" /></label>
         </div>
         <div class="actions">
           <button id="wpFitBtn" class="primary" disabled>Calcola piano (preview)</button>
diff --git a/web/work_plane.js b/web/work_plane.js
index a224ccc..3325b65 100644
--- a/web/work_plane.js
+++ b/web/work_plane.js
@@ -246,6 +246,21 @@ function renderDetails(plane) {
   $('wpDetails').textContent = JSON.stringify(plane, null, 2);
 }
 
+// FastAPI 422 returns `detail` as an array of validation errors:
+//   [{loc, msg, type, ...}, ...]
+// Plain HTTPException returns it as a string. Render either case usefully so
+// the status chip never says "[object Object]".
+function formatDetail(detail) {
+  if (typeof detail === 'string') return detail;
+  if (Array.isArray(detail)) {
+    return detail.map(e => {
+      const loc = Array.isArray(e.loc) ? e.loc.join('.') : e.loc;
+      return loc ? `${loc}: ${e.msg}` : (e.msg || JSON.stringify(e));
+    }).join('; ');
+  }
+  return JSON.stringify(detail);
+}
+
 async function doFit() {
   if (!roi) return;
   setChip('fit…');
@@ -259,7 +274,7 @@ async function doFit() {
       method: 'POST', headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify(body),
     }).then((r) => r.json());
-    if (r.detail) throw new Error(r.detail);
+    if (r.detail) throw new Error(formatDetail(r.detail));
     previewPlane = r.plane;
     previewPoints = r.points_camera;
     three.setPlane(previewPlane);
@@ -282,7 +297,7 @@ async function doSave() {
       method: 'POST', headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ plane: previewPlane }),
     }).then((r) => r.json());
-    if (r.detail) throw new Error(r.detail);
+    if (r.detail) throw new Error(formatDetail(r.detail));
     savedPlane = r.plane;
     previewPlane = null;
     $('wpStateBlock').innerHTML = `piano salvato · RMS ${savedPlane.fit_rms_mm.toFixed(1)} mm · n_inliers ${savedPlane.n_inliers} · offset ${savedPlane.offset_mm} mm`;