skills: agentic hex meshing (fieldcraft-meshing)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

skills/fieldcraft-meshing/SKILL.md

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+---
+name: fieldcraft-meshing
+description: Agentic hex meshing (AHM) in FieldCraft — read the part, plan partitions, and build a structured all-hex (or hex-dominant) mesh an analyst would accept. Use before any mesh.* command on a part that matters, and whenever a mesh comes back with fans, slivers or a low scaled Jacobian.
+---
+
+# Agentic hex meshing (AHM)
+
+You mesh like an experienced analyst: you look first, plan the blocks, and only then call a mesher.
+A mesh is judged by its worst elements, not its count. A fast mesh with fans, slivers or collapsed
+corners is a failed mesh, even when every element is "valid".
+
+## 1. Look before you mesh (always)
+
+Before the first mesh command:
+1. `inspect_model` / `geometry.bodies`: bodies, volumes, bounding boxes, faces, holes, fillets.
+2. Take screenshots from the principal views and an iso view. Name, in words, what the part is.
+3. **Symmetry.** Test it geometrically (mirror the bounding box and the key faces). If the part (and
+   the loads you will apply) are symmetric, mesh one half or quarter only and mirror the mesh
+   (`mesh.mirror`). Never mesh a symmetric part whole.
+4. **Primary vs secondary features.**
+   - Primary: what carries the physics and sets the topology — the main body, walls, bosses,
+     the load path, thin directions, the direction a section is swept along.
+   - Secondary: small fillets, chamfers, logos, small holes far from the region of interest.
+   Start the block structure from the primary features. Secondary features are fitted into it
+   (or defeatured), never allowed to drive it.
+5. **Where to split.** Before meshing, list the cuts: planes or curved surfaces that turn every
+   region into a six-sided block or a sweepable prism. Say why each cut is there.
+
+## 2. Plan the blocks (the rules)
+
+- **Never make sharp corners.** No element angle near 0° or 180°. Where a block would have a
+  corner angle far from 90°, cut differently or add a block (an O-grid) so every corner in the mesh
+  is close to 90°.
+- **Partitions must never create skewed, tapered or high-aspect-ratio hex.** Check every cell you
+  create: if it is a wedge, a sliver or a long thin strip relative to the element size, the cut is
+  wrong. Move or remove it.
+- **Butterfly (O-grid) around curved parts.** Around a hole, a boss, a cylinder or a fillet, cut a
+  square block around it and an O-grid ring between the square and the curve, so element lines run
+  radially and the corners of the square sit at 45° on the curve. Never let a mesh fan into a hole
+  or pinch at a tangency.
+- **Split and merge to make it square.** When a region is far from a square (an L, a long strip
+  with a bump), split it into pieces and merge pieces again so each final cell is closer to a cube.
+  Aim for cells with aspect ratio near 1 at the target size.
+- **Curved splits are allowed.** A cut does not have to be a plane: use a cylindrical or offset
+  surface (a ring around a boss, an offset of a fillet) when a plane would create slivers.
+- **Never cut along an existing face** (a plane coincident with a face within tolerance leaves
+  micrometre slivers, #384) and never through the axis of a small hole (degenerate tessellation at the
+  hole edge). Offset the cut to just outside the feature instead.
+- **Keep cuts consistent across touching bodies** so their meshes can match, or tie them
+  deliberately (`interaction.tie`; linear faces only today).
+
+## 3. Mesh
+
+- Mesh each cell with the structured method that fits (`mesh.hex.generate` extrude/sweep/revolve/
+  mapped; `mesh.cells` for partitioned parts; `mesh.hex.blocks` when every cell is a block).
+- Use edge seeds (`mesh.seed.edge`) so counts match across shared edges and so there are enough
+  elements through thin walls (at least 2–3 through a wall for bending, 1 for pure conduction only).
+- Tet (`tet10` preferred for stress) only for cells that are not worth blocking, and only where
+  they are far from the region of interest.
+
+## 4. Verify (every time, before solving)
+
+1. `mesh.quality`: all valid; scaled Jacobian min ≥ 0.3 (≥ 0.5 in the region of interest);
+   edge ratio max ≤ 10; no element below these is acceptable without a stated reason.
+2. Look at it: screenshots of each body and a section. Fans, pinches, wedges, slivers → back to §2.
+3. Volume: `mesh.quality` volume equals the geometry volume (a mismatch means orphan or missing
+   cells, #385).
+4. `mesh.check.duplicates`: no cracks or free edges inside a body.
+5. After solving, refine once and compare the quantity of interest (convergence).
+
+## 5. Report
+
+State: symmetry used, primary features, every cut and why, method per cell, element counts,
+quality numbers, and anything you defeatured. If the plan failed, say which rule failed and the
+next cut you will try — don't silently fall back to a worse mesh.