diff --git a/okf/decisions/concave-edge-classifier-can-select-wrong-edges.md b/okf/decisions/concave-edge-classifier-can-select-wrong-edges.md
new file mode 100644
index 0000000..004ef7b
--- /dev/null
+++ b/okf/decisions/concave-edge-classifier-can-select-wrong-edges.md
@@ -0,0 +1,121 @@
+---
+type: decision
+title: Shape.concaveEdges() can return the wrong edges entirely, not just a threshold quirk
+description: On OCCTSwift 1.x, concaveEdges() on an extruded L-profile returns two top-cap boundary edges rather than the one true reentrant edge, and classifies that edge convex. Fixed in the 2.0.0 line. Select fillet/chamfer edges geometrically while on 1.x.
+resource: https://github.com/SecondMouseAU/OCCTSwiftScripts/issues/105
+tags: [decision, occtswift, fillet, topology, recipes, edge-selection]
+timestamp: 2026-08-05
+---
+
+# Decision
+
+On the OCCTSwift 1.x line, do not trust `Shape.concaveEdges()` / `Shape.convexEdges()` to
+find the edge you mean to fillet or chamfer, even when you can name the one edge you expect
+geometrically. Verify by
+inspecting the returned edges' actual positions, and prefer `Shape.edges(where:)` with an
+explicit geometric predicate once you know what you are looking for.
+
+# Version scope: 1.x only, fixed in 2.0.0
+
+This is an **OCCTSwift 1.x defect**. It is already fixed in the 2.0.0 line, verified against the
+published `v2.0.0-kernel.1` prerelease with the same repro:
+
+```
+1.17.0          L-prism concave=2 (expected 1)   insideCorner inConcave=false   MISMATCH
+2.0.0-kernel.1  L-prism concave=1 (expected 1)   insideCorner inConcave=true    OK
+```
+
+A T-prism with two reentrant edges reports 3 on 1.17.0 and 2 on 2.0.0-kernel.1. A box, having no
+reentrant edges, is correct on both.
+
+The fix was too involved to backport to the 1.x line, so it is carried by the 2.0.0 refactor.
+Raised upstream as [OCCTSwift#695](https://github.com/SecondMouseAU/OCCTSwift/issues/695).
+
+**So the geometric selection below is a 1.x workaround with a known end date.** When this repo
+moves to the 2.0.0 line, `concaveEdges()` becomes usable for this case again, and recipe 01 could
+return to it. That would be a legitimate simplification rather than a regression. Re-run the
+repro above before relying on it, rather than assuming the migration carried the fix.
+
+# Why
+
+Recipe 01's L-bracket profile has exactly one reentrant vertex, at `(thickness, thickness)`.
+Extruded along Z, that vertex produces exactly one concave edge: a line parallel to the
+extrusion axis, running its full length.
+
+`prism.concaveEdges()` does not return that edge. It returns two different edges instead,
+each length `legLength - thickness` (45 mm on the recipe's own parameters), lying in the
+*end cap* plane (`z = width`) rather than running the extrusion axis:
+
+```
+edge[7]  (the true reentrant edge)   isLine, len=40 (=width), bounds x:[5,5] y:[5,5] z:[0,40]  -> classified CONVEX
+edge[9]  (concaveEdges() result #1)  isLine, len=45,           bounds x:[5,50] y:[5,5] z:[40,40] -> classified CONCAVE
+edge[12] (concaveEdges() result #2)  isLine, len=45,           bounds x:[5,5] y:[5,50] z:[40,40] -> classified CONCAVE
+```
+
+`edge[7]` is exactly where the reentrant vertex's profile predicts a concave edge should be.
+It is classified **convex**. The two edges classified concave are boundary segments of the
+top cap, at the far end of the part from where the reentrant corner's own edge runs.
+
+This is not a rounding or threshold problem (`concaveEdges(angle:)`'s default tolerance is
+irrelevant here): the classifier is naming structurally different edges than the geometric
+feature it is supposed to describe.
+
+## Consequence: the wrong edges have the wrong feasible radius
+
+The two edges `concaveEdges()` returns are bounded by the 5 mm leg thickness (they sit on
+the boundary between the end cap and a wall that is only 5 mm across), so filleting them
+fails above roughly that radius. The true edge (`edge[7]`) is bounded only by
+`legLength - thickness` (45 mm here), since it runs along the full unconstrained length of
+each wall. A radius that is completely reasonable for the real feature (8 mm, comfortably
+under 45 mm) reads as infeasible when applied to the wrong edges (over the 5 mm limit), and
+`prism.filleted(edges:radius:)` returns `nil`. That nil, hidden behind a `?? prism`
+fallback, is what recipe 01 shipped for as long as it trusted `concaveEdges()`
+([OCCTSwiftScripts#105](https://github.com/SecondMouseAU/OCCTSwiftScripts/issues/105)).
+
+## A concave fillet adds material; verify the sign, not just a nonzero delta
+
+Filleting the true reentrant edge **increases** the bracket's volume: it fills part of the
+sharp inside corner with a rounded blend. `prism.filleted(edges: [edge7], radius: r).volume`
+is `prism.volume + r² · (1 - π/4) · length`, not minus. Filleting the two edges
+`concaveEdges()` actually returns *removes* material instead, because those two are
+ordinary 90-degree corners from the fillet's point of view (the same
+`r² · (1 - π/4) · length` formula, sign flipped): measured 176.42 mm3 removed at r=3 mm
+across both wrong edges (2 × 45 mm), against 77.26 mm3 that a single correct fillet at r=3
+mm over the 40 mm extrusion length would add. A volume check that only asserts "some
+material moved" would have passed on the wrong edges; the sign and the magnitude both have
+to match the specific edge you intended.
+
+# How to select instead
+
+Once you know the geometric feature you want (here: a line parallel to the extrusion axis,
+positioned at the profile's reentrant vertex), select it directly rather than filtering a
+classifier's output:
+
+```swift
+let insideCorner = prism.edges { edge in
+    guard edge.isLine else { return false }
+    let b = edge.bounds
+    let runsFullWidth = abs((b.max.z - b.min.z) - width) < 1e-6
+        && abs(b.max.x - b.min.x) < 1e-6 && abs(b.max.y - b.min.y) < 1e-6
+    guard runsFullWidth else { return false }
+    return abs(b.min.x - thickness) < 1e-6 && abs(b.min.y - thickness) < 1e-6
+}
+```
+
+# Related
+
+[Revolve seams cannot be chamfered](revolve-seams-cannot-be-chamfered.md) already noted, on
+the pipe flange (#103/#104), that `convexEdges()` / `concaveEdges()` disagreed with what
+chamfering the edges actually did volumetrically. This is the same finding on a second,
+unrelated shape, which is why it is worth its own entry rather than a footnote: two
+independent recipes have now hit a classifier/reality mismatch on `concaveEdges()` and
+`convexEdges()`. Treat both as a hint, to be checked against the shape's actual geometry and
+the operation's actual volumetric effect, not as ground truth.
+
+This is also the third instance of a broader pattern tracked across #100, #103, and #105: an
+optional-returning geometry operation degrades silently via a `?? fallback`, the emitted
+output stays volumetrically plausible, and the docs keep describing the intended behaviour.
+A fourth, dormant instance (`recipes/06-fan-blade`'s `blade.union(hub) ?? blade`, which has
+never actually failed) was fixed alongside #105 once the audit turned it up. Any new
+`Shape`-returning call in a recipe that can return `nil` should fail loudly (force-unwrap or
+an explicit `guard ... else { fatalError(...) }`), never degrade through `??`.
diff --git a/okf/decisions/index.md b/okf/decisions/index.md
index 0e4b881..dbba3b4 100644
--- a/okf/decisions/index.md
+++ b/okf/decisions/index.md
@@ -17,3 +17,6 @@ that need standalone rationale.
   called as `f || status=1` must `return 1` explicitly or its checks are decorative.
 * [Revolve seams cannot be chamfered](revolve-seams-cannot-be-chamfered.md): the all-edge
   `chamfered(distance:)` always fails on a full revolve; select edges explicitly.
+* [Concave edge classifier can select wrong edges](concave-edge-classifier-can-select-wrong-edges.md):
+  `concaveEdges()` returned two unrelated edges instead of an L-bracket's one true reentrant
+  edge; verify a classifier's output geometrically before trusting it.
diff --git a/okf/log.md b/okf/log.md
index 4ef82e8..0624f5c 100644
--- a/okf/log.md
+++ b/okf/log.md
@@ -1,5 +1,19 @@
 # Knowledge Log
 
+## 2026-08-05 (fix/105-bracket-fillet)
+
+* **Update**: Fixed recipe 01's inside-corner fillet, which had never applied (#105).
+  `prism.concaveEdges()` returns the wrong two edges on this shape (top-cap boundary
+  segments bounded by the 5 mm leg thickness), not the one true reentrant edge (bounded
+  only by `legLength - thickness`, 45 mm). `filletRadius = 8` was infeasible for the wrong
+  edges and a `?? prism` fallback hid the resulting `nil`. Now selects the true edge
+  geometrically with `Shape.edges(where:)`; the same `filletRadius = 8` now applies,
+  adding 549.38 mm3 (matches the analytic `r² · (1 - pi/4) · width` prediction exactly).
+  Also fixed `recipes/06-fan-blade`'s `blade.union(hub) ?? blade`, the same pattern found
+  dormant during the `??`-fallback audit #105 requested (the union has never actually
+  failed; behaviour is unchanged).
+* **Creation**: Recorded the concave-edge-classifier-can-select-wrong-edges decision.
+
 ## 2026-08-05
 
 * **Update**: Fixed two cookbook recipes that emitted shells while documenting themselves as
diff --git a/recipes/01-mounting-bracket/README.md b/recipes/01-mounting-bracket/README.md
index a32747d..abe2dec 100644
--- a/recipes/01-mounting-bracket/README.md
+++ b/recipes/01-mounting-bracket/README.md
@@ -18,28 +18,45 @@ An L-shaped mounting bracket with a rounded inside corner and four through-holes
 ## Algorithm
 
 The L cross-section is built as a closed polygon in the XY plane and extruded along Z into
-a prism. The inside corner is then rounded by filleting the solid's **concave edge**,
-located geometrically with `Shape.concaveEdges()`: no fragile edge-index bookkeeping, and
-it tracks the corner as parameters change. The fillet is applied *before* drilling so
-`concaveEdges()` returns only the reentrant corner. Finally four holes are cut: two through
-the base leg (drilled along Y) and two through the upright leg (drilled along X). Each drill
-starts 1 mm outside the entry face and over-runs the exit by 1 mm so the resulting cut faces
-are clean and coincident-face artifacts are avoided.
+a prism. The reentrant vertex at `(thickness, thickness)` extrudes to exactly one concave
+edge: a straight line parallel to the extrusion axis. That is the edge this recipe fillets,
+adding a rounded blend that fills part of the sharp inside corner. It is selected
+**geometrically**, with `Shape.edges(where:)`, not with `Shape.concaveEdges()`: on this
+shape that call returns a different pair of edges (see Gotchas), so the selector checks
+directly for a line edge parallel to Z sitting at `(thickness, thickness)`. This tracks the
+corner as parameters change without a fragile edge index. The fillet is applied *before*
+drilling so the selector only ever sees the corner edge, not a drilled hole's rim. Finally
+four holes are cut: two through the base leg (drilled along Y) and two through the upright
+leg (drilled along X). Each drill starts 1 mm outside the entry face and over-runs the exit
+by 1 mm so the resulting cut faces are clean and coincident-face artifacts are avoided.
 
 ## OCCTSwift APIs used
 
 - `Wire.polygon(_:closed:)`: L-shaped cross-section
 - `Shape.extrude(profile:direction:length:)`: profile → prism
-- `Shape.concaveEdges()`: find the reentrant inside-corner edge (OCCTSwift v1.3.1)
+- `Shape.edges(where:)`: select the inside-corner edge geometrically (OCCTSwift v1.2.1)
 - `Shape.filleted(edges:radius:)`: round that edge
 - `Shape.drilled(at:direction:radius:depth:)`: the four through-holes
-- `Shape.volume`: sanity print
+- `Shape.volume`: sanity print, and the check that the fillet actually ran
 
 ## Gotchas
 
-- Fillet **before** drilling: `concaveEdges()` classifies *every* concave edge, and a
-  drilled hole's rim can read as concave. Filleting first keeps the selection to just the
-  inside corner. (Tighten `concaveEdges(angle:)` if a near-flat junction sneaks in.)
+- **`Shape.concaveEdges()` picks the wrong edges on this shape.** It returns two edges
+  instead of one: the top-cap boundary segments at `z = width` where each wall meets the
+  end face (each running the leg length, not the extrusion width), rather than the true
+  reentrant edge. Those two are bounded by the 5 mm leg thickness, so a fillet on them fails
+  above roughly that radius; that mismatch is what let `filletRadius = 8` silently no-op
+  behind a `?? prism` fallback for as long as this recipe used `concaveEdges()`
+  (OCCTSwiftScripts #105). The true inside-corner edge has no such limit (its bound is
+  `legLength − thickness`, 45 mm here), which is why the same `filletRadius = 8` works fine
+  once the correct edge is selected.
+- **A concave fillet adds material, it does not remove it.** Rounding the inside corner
+  fills part of the sharp reentrant point with a blend, so `bracket.volume` after the
+  fillet is *larger* than the prism's, by `filletRadius² · (1 − π/4) · width`. Do not expect
+  a volume decrease as evidence the fillet ran; check the increase against that formula
+  instead.
+- Fillet **before** drilling: a drilled hole's rim sitting near the corner could otherwise
+  confuse a looser selector. Filleting first keeps the selection to just the inside corner.
 - Drill start points sit *outside* the part and `depth` over-runs the thickness so the
   hole punches fully through; drilling exactly on a face can leave a sliver.
 - The bracket is a single solid emitted as `body-0` (the reference `output.brep`).
diff --git a/recipes/01-mounting-bracket/main.swift b/recipes/01-mounting-bracket/main.swift
index 08d5027..ebf3cbc 100644
--- a/recipes/01-mounting-bracket/main.swift
+++ b/recipes/01-mounting-bracket/main.swift
@@ -3,10 +3,21 @@
 // Inputs:  none (edit the parameter block below)
 // Outputs: one solid body: an L-bracket with a filleted inside corner and four
 //          through-holes (two per leg).
-// Notes:   The inside corner is rounded by filleting the solid's concave edge, found
-//          geometrically with Shape.concaveEdges() (OCCTSwift v1.3.1, #171) rather than
-//          by a fragile edge index. Fillet before drilling so concaveEdges() returns only
-//          the reentrant corner. Holes are drilled through the leg thickness with a small
+// Notes:   The reentrant corner at (thickness, thickness) extrudes to exactly one
+//          concave edge, a straight line parallel to the extrusion axis. That is the
+//          edge this recipe fillets. It is NOT the edge `Shape.concaveEdges()` finds:
+//          on this shape that call returns two different edges instead, the top-cap
+//          boundary segments at z = width where each wall meets the end face, each
+//          running the leg length rather than the extrusion width (OCCTSwiftScripts
+//          #105). Those two are bounded by the 5 mm leg thickness and a fillet there
+//          fails above roughly that radius, which is why `filletRadius = 8` used to
+//          silently no-op behind a `?? prism` fallback. The true inside-corner edge has
+//          no such limit (its bound is legLength − thickness, 45 mm here), so this
+//          recipe selects it explicitly with `Shape.edges(where:)`: a line parallel to
+//          the extrusion axis positioned at (thickness, thickness), the same
+//          geometric-selection approach recipe 03 uses for the pipe flange (#103).
+//          Fillet before drilling so the selector only ever sees the corner edge, not a
+//          drilled hole's rim. Holes are drilled through the leg thickness with a small
 //          overshoot so the cut faces stay clean.
 //
 // Run:  swift run occtkit run recipes/01-mounting-bracket/main.swift --format brep
@@ -18,7 +29,8 @@ import ScriptHarness
 let legLength: Double  = 50    // length of each leg, measured from the heel (mm)
 let thickness: Double  = 5     // material thickness of each leg (mm)
 let width: Double      = 40    // bracket width (extrusion depth, mm)
-let filletRadius: Double = 8   // inside-corner radius (mm)
+let filletRadius: Double = 8   // inside-corner radius (mm); fits comfortably under the
+                                // legLength − thickness = 45 mm geometric limit (see below)
 let holeRadius: Double  = 3.5  // mounting-hole radius (mm)
 
 let ctx = ScriptContext(metadata: ManifestMetadata(
@@ -37,7 +49,29 @@ let lProfile = Wire.polygon([
 
 // ── Extrude to a solid prism, then round the concave (inside-corner) edge ─────
 let prism = Shape.extrude(profile: lProfile, direction: SIMD3(0, 0, 1), length: width)!
-var bracket = prism.filleted(edges: prism.concaveEdges(), radius: filletRadius) ?? prism
+
+// The inside corner is the one straight edge parallel to the extrusion axis (Z) that
+// sits at (thickness, thickness): select it geometrically rather than trusting
+// concaveEdges(), which picks the wrong edges on this shape (see the header note).
+// That classifier defect is OCCTSwift 1.x only: it is fixed in the 2.0.0 line
+// (verified on v2.0.0-kernel.1, upstream OCCTSwift#695). This geometric selection is
+// therefore a 1.x workaround, and this recipe could return to concaveEdges() once the
+// package moves to 2.0.0. Re-run the check in the OKF entry before doing so.
+let insideCornerEdges = prism.edges { edge in
+    guard edge.isLine else { return false }
+    let b = edge.bounds
+    let runsFullWidth = abs((b.max.z - b.min.z) - width) < 1e-6
+        && abs(b.max.x - b.min.x) < 1e-6 && abs(b.max.y - b.min.y) < 1e-6
+    guard runsFullWidth else { return false }
+    return abs(b.min.x - thickness) < 1e-6 && abs(b.min.y - thickness) < 1e-6
+}
+// Guard the selector separately from the fillet. `filleted(edges: [], radius:)` does
+// return nil today (checked on both 1.17.0 and 2.0.0-kernel.1), so the force-unwrap
+// below would catch an empty match, but only as an anonymous nil-unwrap crash. This
+// names the actual fault, and avoids depending on undocumented nil-on-empty behaviour
+// if a parameter change or an upstream tweak ever silently breaks the predicate.
+guard !insideCornerEdges.isEmpty else { fatalError("inside-corner edge selector matched nothing") }
+var bracket = prism.filleted(edges: insideCornerEdges, radius: filletRadius)!
 
 // ── Four through-holes: two in the base leg (drill along Y), two in the upright
 //    leg (drill along X). Start just outside the entry face and over-run the exit.
diff --git a/recipes/01-mounting-bracket/output.brep b/recipes/01-mounting-bracket/output.brep
index 44253a0..2a15ead 100644
--- a/recipes/01-mounting-bracket/output.brep
+++ b/recipes/01-mounting-bracket/output.brep
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diff --git a/recipes/01-mounting-bracket/output.png b/recipes/01-mounting-bracket/output.png
index 7151596..c4ab90b 100644
Binary files a/recipes/01-mounting-bracket/output.png and b/recipes/01-mounting-bracket/output.png differ
diff --git a/recipes/06-fan-blade/main.swift b/recipes/06-fan-blade/main.swift
index 7b7a8a0..8f14930 100644
--- a/recipes/06-fan-blade/main.swift
+++ b/recipes/06-fan-blade/main.swift
@@ -69,7 +69,7 @@ guard var blade = Shape.loft(profiles: profiles, solid: true) else { fatalError(
 let hubR = rootChord * 0.55
 let hub = Shape.cylinder(at: SIMD3(0, 0, -12), direction: SIMD3(0, 0, 1),
                          radius: hubR, height: 14)!
-blade = blade.union(hub) ?? blade
+blade = blade.union(hub)!
 
 try ctx.add(blade, color: C.steel, name: "Fan blade")
 
diff --git a/recipes/README.md b/recipes/README.md
index bc587d7..f0931f4 100644
--- a/recipes/README.md
+++ b/recipes/README.md
@@ -9,7 +9,7 @@ tweak the parameters.
 
 | # | Recipe | Preview | What it shows |
 |---|--------|---------|---------------|
-| 01 | [Mounting bracket](01-mounting-bracket/) | <img src="01-mounting-bracket/output.png" width="160"> | sketch → extrude → `concaveEdges` fillet → drill |
+| 01 | [Mounting bracket](01-mounting-bracket/) | <img src="01-mounting-bracket/output.png" width="160"> | sketch → extrude → geometrically-selected inside-corner fillet → drill |
 | 02 | [Helical compression spring](02-helical-spring/) | <img src="02-helical-spring/output.png" width="160"> | helix path + circular section → pipe sweep |
 | 03 | [Pipe flange](03-pipe-flange/) | <img src="03-pipe-flange/output.png" width="160"> | revolve + `circularPatternCut` bolt circle + chamfer |
 | 04 | [Involute spur gear](04-spur-gear/) | <img src="04-spur-gear/output.png" width="160"> | involute tooth math → polygon → extrude → bore |