diff --git a/DESIGN.md b/DESIGN.md
index 51363d3..4043566 100644
--- a/DESIGN.md
+++ b/DESIGN.md
@@ -7,8 +7,8 @@ Work is tracked in Gitea issues; the tracking epic links every piece in dependen
 ## Decisions
 
 - **Placement model:** freeform plops (not a square-foot grid), scaled by real plant spacing. Grid snapping may come later as a toggle.
-- **A fill is one of two operations (#77).** A plop is a *clump*, not a plant, which is the right primitive for SKETCHING ("a few plops of garlic in a corner") but can't draw a real planting — a filled bed comes out as ~15 blobs, not 8 rows of garlic. So `FillRegion`/`FillNamedRegion` take a `FillLayout`: `clump` (default; plop radius 1.5×spacing, ~7 plants each — quick coverage) or `grid` (radius spacing/2, pitch = spacing, ONE plant per plop — a layout you could plant from). Same `hexCenters` lattice and #75 edge rule for both; only the radius→spacing relationship differs. Surfaced on `POST /objects/:id/fill` (`layout`) and the agent's `fill_region` (`mode`). A grid-filled bed approaches the low-hundreds-of-plops the SVG budget was sized for, which the semantic-zoom tiers already anticipate.
-- **Spacing is a plant-to-plant rule, so bed edges get half of it.** A bed edge is not a competitor for soil, light or water, so the outer row owes it half the spacing rather than a full one. `FillRegion` centres its lattice accordingly, and lets a plop — a *clump* three spacings across — cross the edge by up to half a spacing so its outermost plants land at that half-spacing. The rule, the square-foot-chart arithmetic behind it, and the failure mode it prevents are written out once in `hexCenters`; #75 is what getting it wrong looked like.
+- **A fill is one of two operations (#77).** A plop is a *clump*, not a plant, which is the right primitive for SKETCHING ("a few plops of garlic in a corner") but can't draw a real planting — a filled bed comes out as ~15 blobs, not 8 rows of garlic. So `FillRegion`/`FillNamedRegion` take a `FillLayout`: `clump` (default; plop radius 1.5×spacing, ~7 plants each — quick coverage) or `grid` (radius spacing/2, pitch = spacing, ONE plant per plop — a layout you could plant from). Same `hexCenters` lattice and #75 edge rule for both; only the radius→spacing relationship differs. Surfaced on `POST /objects/:id/fill` (`layout`) and the agent's `fill_region` (`mode`). Same centered `hexCenters` lattice for both, but BOTH the plop radius (`plopRadiusFor`) and the edge inset (`edgeInset`) differ by layout: a grid plant sits at the plop's centre, so it insets a half-spacing; a clump's plants reach its rim, so it insets radius-less-a-half and overhangs the edge by that half — reusing the clump formula for grid would inset by zero and plant flush on the edge. A grid-filled bed approaches the low-hundreds-of-plops the SVG budget was sized for, which the semantic-zoom tiers already anticipate.
+- **Spacing is a plant-to-plant rule, so bed edges get half of it.** A bed edge is not a competitor for soil, light or water, so the outer row owes it half the spacing rather than a full one. `FillRegion` centres its lattice accordingly, and lets a plop — a *clump* three spacings across — cross the edge by up to half a spacing so its outermost plants land at that half-spacing. The rule, the square-foot-chart arithmetic behind it, and how it differs by layout are written out once in `edgeInset` (which `hexCenters` then honours); #75 is what getting it wrong looked like.
 - **Stack:** Go 1.26.x backend, module `gitea.stevedudenhoeffer.com/steve/pansy`; React + TypeScript + Vite + Tailwind frontend, production build embedded via `embed.FS` → one static binary (`CGO_ENABLED=0`).
 - **Users:** multi-user with ownership. Users own gardens; a garden can be shared with other users as viewer (read) or editor (edit content). Owner additionally shares/deletes. The first registered user is `is_admin` (set race-free inside the INSERT); admin gates instance-wide Settings (`requireAdmin`), the only thing that reads that flag.
 - **Auth:** OIDC-first (Authentik is the primary IdP), local argon2id passwords as an optional fallback.
diff --git a/internal/api/ops_test.go b/internal/api/ops_test.go
index 013b41a..7a091ed 100644
--- a/internal/api/ops_test.go
+++ b/internal/api/ops_test.go
@@ -95,22 +95,39 @@ func TestFillAndClearAPI(t *testing.T) {
 	if n := int(decodeMap(t, w.Body.Bytes())["cleared"].(float64)); n != 0 {
 		t.Errorf("second clear removed %d, want 0", n)
 	}
+}
 
-	// Grid layout (#77) packs individual plants at true spacing — many more, small
-	// plops than the clump default on the same bed.
+// TestFillLayoutAPI covers the layout selector (#77): grid packs denser than the
+// clump default, and an unknown layout is a 400 rather than a silent clump fill.
+func TestFillLayoutAPI(t *testing.T) {
+	r := authEngine(t, localCfg())
+	cookie := registerAndCookie(t, r, "layout@example.com")
+	_, objID, plantID := seedFillableBed(t, r, cookie, 200, 200, 20)
+
+	// Clump (default) for the baseline count.
+	w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
+		"plantId": plantID, "region": "all",
+	}, cookie)
+	if w.Code != http.StatusOK {
+		t.Fatalf("clump fill: status %d, body %s", w.Code, w.Body.String())
+	}
+	clumpN := int(decodeMap(t, w.Body.Bytes())["created"].(float64))
+	if w := doJSON(t, r, http.MethodPost, clearPath(objID), nil, cookie); w.Code != http.StatusOK {
+		t.Fatalf("clear between fills: %d", w.Code)
+	}
+
+	// Grid packs individual plants at true spacing — many more, small plops.
 	w = doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
 		"plantId": plantID, "region": "all", "layout": "grid",
 	}, cookie)
 	if w.Code != http.StatusOK {
 		t.Fatalf("grid fill: status %d, body %s", w.Code, w.Body.String())
 	}
-	if gridN := int(decodeMap(t, w.Body.Bytes())["created"].(float64)); gridN <= created {
-		t.Errorf("grid fill created %d, want more than the clump fill's %d", gridN, created)
+	if gridN := int(decodeMap(t, w.Body.Bytes())["created"].(float64)); gridN <= clumpN {
+		t.Errorf("grid fill created %d, want more than the clump fill's %d", gridN, clumpN)
 	}
+
 	// An unknown layout is a 400, not a silent clump fill.
-	if w := doJSON(t, r, http.MethodPost, clearPath(objID), nil, cookie); w.Code != http.StatusOK {
-		t.Fatalf("clear before bad-layout: %d", w.Code)
-	}
 	if w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
 		"plantId": plantID, "region": "all", "layout": "spiral",
 	}, cookie); w.Code != http.StatusBadRequest {
diff --git a/internal/service/ops.go b/internal/service/ops.go
index 085371d..ec6e764 100644
--- a/internal/service/ops.go
+++ b/internal/service/ops.go
@@ -127,6 +127,33 @@ func plopRadiusFor(spacingCM float64, layout FillLayout) float64 {
 	return defaultPlopRadius(spacingCM)
 }
 
+// edgeInset is how far a plop's CENTRE must stay inside the region edge. It
+// differs by layout because the half-spacing rule is about where the PLANT lands,
+// and the plant sits in a different place within the plop.
+//
+// Spacing is a constraint between neighbouring plants competing for the same soil,
+// light and water; a bed edge is nobody's neighbour, so the outer plant owes it
+// only HALF the spacing — the half it would otherwise share. That is the
+// square-foot-chart arithmetic: garlic at 9-per-square sits 2" from the frame, not
+// 6".
+//
+//   - Grid: one plant, at the plop's centre. Put that centre a half-spacing in and
+//     the outer row lands exactly where the rule wants it — inset = spacing/2.
+//   - Clump: a fat plop (radius 1.5×spacing) whose plants fill out to its RIM.
+//     Insetting the whole circle would push the outer row a full 1.5 spacings in,
+//     three times the rule. Instead the clump may hang over by a half-spacing (rim
+//     at spacing/2 past the edge), landing its outermost plants that same
+//     half-spacing in — inset = radius − spacing/2. A grid plop reusing THAT
+//     formula would inset by radius − spacing/2 = 0 and plant flush on the edge,
+//     which is the bug this split fixes.
+func edgeInset(radius, spacing float64, layout FillLayout) float64 {
+	half := math.Max(0, spacing) / 2
+	if layout == FillGrid {
+		return half
+	}
+	return math.Max(0, radius-half)
+}
+
 // validFillLayout normalizes a layout: empty defaults to clump (so existing
 // callers are unchanged), a known value passes, anything else is rejected.
 func validFillLayout(l FillLayout) (FillLayout, bool) {
@@ -140,14 +167,15 @@ func validFillLayout(l FillLayout) (FillLayout, bool) {
 	}
 }
 
-// FillRegion lays a hex-packed field of plops of one plant across a region of a
-// plantable object the actor can edit. Plop radius comes from the plant's spacing
-// (or spacingOverride) via defaultPlopRadius; centers sit on a hex lattice at 2×
-// radius pitch, centered in the region, and set in from each edge by the plop's
-// radius less half a spacing — see hexCenters for why that half-spacing is what
-// the edge is owed. A candidate is skipped when its plop would sit entirely
-// inside an existing active plop (so re-filling doesn't stack duplicates).
-// Returns the plops it created.
+// FillRegion lays a field of plops of one plant across a region of a plantable
+// object the actor can edit. The layout picks the primitive: FillClump drops fat
+// clumps for quick sketching, FillGrid lays out individual plants at true spacing
+// (see FillLayout). Plop radius comes from the plant's spacing (or spacingOverride)
+// via plopRadiusFor; centers sit on a centered hex lattice at 2×radius pitch, set
+// in from each edge by edgeInset — a half-spacing for grid, radius-less-a-half-
+// spacing for a clump (see edgeInset for the why). A candidate is skipped when its
+// plop would sit entirely inside an existing active plop (so re-filling doesn't
+// stack duplicates). Returns the plops it created.
 func (s *Service) FillRegion(ctx context.Context, actorID, objectID int64, region Region, plantID int64, spacingOverride *float64, layout FillLayout) ([]domain.Planting, error) {
 	o, _, err := s.objectForRole(ctx, actorID, objectID, roleEditor)
 	if err != nil {
@@ -157,9 +185,10 @@ func (s *Service) FillRegion(ctx context.Context, actorID, objectID int64, regio
 }
 
 // fillLoaded is the shared body of FillRegion/FillNamedRegion given an object
-// already loaded and authorized (roleEditor). It rejects a non-finite region,
-// clamps the region to the object's bounds, refuses fills over maxFillPlops, and
-// inserts the whole batch in one transaction rather than one round-trip per plop.
+// already loaded and authorized (roleEditor). It validates the layout, rejects a
+// non-finite region, clamps the region to the object's bounds, refuses fills over
+// maxFillPlops, and inserts the whole batch in one transaction rather than one
+// round-trip per plop.
 func (s *Service) fillLoaded(ctx context.Context, actorID int64, o *domain.GardenObject, region Region, plantID int64, spacingOverride *float64, layout FillLayout) ([]domain.Planting, error) {
 	if !o.Plantable {
 		return nil, domain.ErrInvalidInput
@@ -197,7 +226,7 @@ func (s *Service) fillLoaded(ctx context.Context, actorID int64, o *domain.Garde
 	}
 
 	region = region.clampTo(o.WidthCM/2, o.HeightCM/2)
-	centers, total := hexCenters(region, radius, spacing, maxFillPlops)
+	centers, total := hexCenters(region, radius, edgeInset(radius, spacing, layout), maxFillPlops)
 	if total > maxFillPlops {
 		return nil, domain.ErrInvalidInput // region too large for this spacing; ask for less
 	}
@@ -208,13 +237,17 @@ func (s *Service) fillLoaded(ctx context.Context, actorID int64, o *domain.Garde
 	}
 	today := s.now().UTC().Format(dateLayout)
 	batch := make([]*domain.Planting, 0, len(centers))
+	// Only the plops that were ALREADY here can cover a candidate: every plop this
+	// fill makes shares one radius and sits on a distinct lattice point, and a plop
+	// is "covered" only when it lies entirely inside another — impossible between
+	// two equal-radius circles at different centres. So skip against `existing` as
+	// loaded and don't grow it per plop, which made an empty-bed grid fill's check
+	// needlessly quadratic.
 	for _, c := range centers {
 		if coveredByExisting(c.x, c.y, radius, existing) {
 			continue
 		}
-		p := &domain.Planting{ObjectID: o.ID, PlantID: plantID, XCM: c.x, YCM: c.y, RadiusCM: radius, PlantedAt: &today}
-		batch = append(batch, p)
-		existing = append(existing, *p) // so later candidates in THIS fill don't stack on it
+		batch = append(batch, &domain.Planting{ObjectID: o.ID, PlantID: plantID, XCM: c.x, YCM: c.y, RadiusCM: radius, PlantedAt: &today})
 	}
 	created, err := s.store.CreatePlantings(ctx, batch)
 	if err != nil {
@@ -243,24 +276,14 @@ type localPoint struct{ x, y float64 }
 //
 // # How close to the edge the outer row goes
 //
-// Spacing is a constraint BETWEEN NEIGHBOURING PLANTS competing for the same
-// soil, light and water. A bed edge is not a competitor, so the outer row only
-// owes it HALF the spacing — the half it would otherwise share with a neighbour.
-// That is the arithmetic inside every square-foot-gardening chart: 4 per square
-// is 6" apart and 3" from the square's edge; 9 per square is 4" apart and 2"
-// from the edge. Garlic at 9 per square goes in 2" from the frame, not 6".
+// The caller passes `inset`: the margin the outer row keeps from every edge. It
+// encodes the half-spacing rule (spacing is owed between neighbouring plants, and
+// a bed edge is nobody's neighbour) and differs by layout — see edgeInset, which
+// derives it. hexCenters just honours it on all four sides.
 //
-// A plop is a CLUMP, not a plant — defaultPlopRadius makes it 1.5×spacing, so
-// three spacings across — and its plants sit out to its rim. So keeping the whole
-// circle inside the bed would inset the outer row by a full 1.5 spacings, three
-// times what the rule allows. Instead the clump may hang over the edge by up to
-// half a spacing, which puts its outermost plants exactly the half-spacing from
-// the edge that the rule asks for. Overhang is capped there and nowhere near the
-// full radius: a clump mostly outside the bed is a drawing of plants in the path.
-//
-// Do not "simplify" this back to anchoring at the region's min corner. That is
-// what #75 was: staggered rows start a full pitch in, and the leftover all lands
-// on the far edge, where clumps hang outside a bed that nothing clips them to.
+// Do not "simplify" the centering back to anchoring at the region's min corner.
+// That is what #75 was: staggered rows start a full pitch in, and the leftover all
+// lands on the far edge, where clumps hang outside a bed that nothing clips them to.
 //
 // # Counting before building
 //
@@ -268,7 +291,7 @@ type localPoint struct{ x, y float64 }
 // BEFORE building anything: a fill large enough to be refused shouldn't allocate
 // its whole lattice first just to be counted and thrown away. Over `limit` it
 // returns (nil, total), so the caller can still refuse with the real number.
-func hexCenters(r Region, radius, spacing float64, limit int) ([]localPoint, int) {
+func hexCenters(r Region, radius, inset float64, limit int) ([]localPoint, int) {
 	if radius <= 0 {
 		return nil, 0
 	}
@@ -281,11 +304,6 @@ func hexCenters(r Region, radius, spacing float64, limit int) ([]localPoint, int
 	pitch := 2 * radius
 	rowH := pitch * math.Sqrt(3) / 2
 
-	// How far a clump's centre must stay inside the edge: its own radius, less the
-	// half-spacing of overhang the rule allows. Never negative, and never past the
-	// centre of the clump.
-	inset := math.Max(0, radius-math.Max(0, spacing)/2)
-
 	rows, y0 := fitAxis(r.MaxY-r.MinY, rowH, inset)
 	cols, x0 := fitAxis(r.MaxX-r.MinX, pitch, inset)
 
diff --git a/internal/service/ops_test.go b/internal/service/ops_test.go
index f18cc36..ba2810a 100644
--- a/internal/service/ops_test.go
+++ b/internal/service/ops_test.go
@@ -96,7 +96,7 @@ func TestHexCentersEdgeInset(t *testing.T) {
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			r := rect(-tc.w/2, -tc.h/2, tc.w/2, tc.h/2)
-			pts, total := hexCenters(r, tc.radius, tc.spacing, maxFillPlops)
+			pts, total := hexCenters(r, tc.radius, edgeInset(tc.radius, tc.spacing, FillClump), maxFillPlops)
 			if len(pts) == 0 {
 				t.Fatal("no centers")
 			}
@@ -173,7 +173,7 @@ func TestHexCentersTinyRegion(t *testing.T) {
 		{"off in a corner", rect(20, -40, 30, -30), 25, -35},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
-			pts, _ := hexCenters(tc.r, 15, 10, maxFillPlops)
+			pts, _ := hexCenters(tc.r, 15, edgeInset(15, 10, FillClump), maxFillPlops)
 			if len(pts) != 1 || pts[0].x != tc.wantX || pts[0].y != tc.wantY {
 				t.Errorf("got %+v, want one plop at (%v,%v)", pts, tc.wantX, tc.wantY)
 			}
@@ -323,6 +323,7 @@ func TestFillGridLaysOutIndividualPlants(t *testing.T) {
 		t.Errorf("grid produced %d plops, clump %d — grid should be denser", len(grid), len(clump))
 	}
 	// Each grid plop is one plant at radius spacing/2 = 5.
+	maxAbs := 0.0
 	for _, p := range grid {
 		if p.RadiusCM != 5 {
 			t.Errorf("grid plop radius = %v, want 5 (spacing/2)", p.RadiusCM)
@@ -330,6 +331,14 @@ func TestFillGridLaysOutIndividualPlants(t *testing.T) {
 		if p.DerivedCount != 1 {
 			t.Errorf("grid plop derived count = %d, want 1 (one plant per plop)", p.DerivedCount)
 		}
+		maxAbs = math.Max(maxAbs, math.Max(math.Abs(p.XCM), math.Abs(p.YCM)))
+	}
+	// The half-spacing edge rule: a grid plant sits AT the plop centre, so the
+	// outer row is inset a half-spacing (spacing/2 = 5) — at ±25 on this 60cm bed,
+	// not flush on ±30. Regression guard: the clump inset formula (radius − half)
+	// collapses to 0 for grid and would plant one on the very edge.
+	if edge := 30.0; maxAbs > edge-5+1e-6 {
+		t.Errorf("outermost grid plop at |coord|=%.2f — only %.2fcm from the edge; want a half-spacing (5cm) in", maxAbs, edge-maxAbs)
 	}
 }