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+# Units inventory (#579)
+
+Every place a dimensional number enters or leaves FieldCraft today, the unit it is assumed to be
+in, and where the SI conversion goes. Today nothing converts: every stored `f64` means "whatever
+consistent system the user chose", labelled by `model.units` (`UnitSystem`,
+`crates/model/src/units/mod.rs`). The SI table that later steps convert through is
+`crates/model/src/units/` (`Quantity`, `Unit`, `DisplaySystem`). **Risky** marks an implicit
+assumption, a hard-coded factor or a missing dimension.
+
+## Model storage (`crates/model/src`): all bare `f64`, no unit tag
+Conversion goes at the command/file boundary; storage becomes SI.
+
+| Where | Fields | Quantity |
+|---|---|---|
+| `lib.rs` | `units: UnitSystem` (serde default `unspecified`); `MAX_COORD = 1e9` "model units" | label; length |
+| `mesh.rs`, `datums.rs`, `constraints.rs`, `relaxations.rs`, `mesh_plans.rs` | node/datum/reference point xyz, plan centre/radius/size/tolerance | length |
+| `sizes.rs`, `seeds.rs`, `virtual_faces.rs`, `tie.rs` | size regions, seed sizes, tolerances, position_tolerance | length (bias, growth: dimensionless) |
+| `materials.rs` | young, shear, hyperelastic c01/c20/c30, yield_stress, hardening_modulus, hardening table stress, JC a/b | pressure |
+| | poisson, strains, JC n/c/m, taylor_quinney, damage d1–d5 | dimensionless |
+| | JC reference_strain_rate | frequency (1/s) |
+| | JC room/melt temperature, thermal reference_temperature | temperature (absolute) |
+| | conductivity, specific_heat, expansion | conductivity, specific heat, expansion (1/ΔT) |
+| | density; fluid viscosity (doc: "1e-9 MPa·s for water") | density; dynamic viscosity |
+| `sections.rs` | profile dims, shell thickness/offset, 2D thickness, area | length, area |
+| `loads.rs` | force, moment, traction, pressure, gravity, body force, centrifugal ω/point, surface flux, film, sink, heat source power, temperature | force, moment, pressure, acceleration, force/volume, angular velocity, heat flux, HTC, temperature, heat generation |
+| `bcs.rs` | displacement `[Option<f64>; 6]` (3 lengths + 3 rotations rad), temperature | length, angle, temperature |
+| `steps.rs` | frequency min/max_hz, buckle shift, end_time/increments/intervals, initial velocity, rigid wall point, hourglass/bulk viscosity coefs, heat theta/initial_temperature, imperfection scale, displacement control | frequency, time, velocity, length, temperature (coefs dimensionless) |
+| `contact.rs` | μ static/kinetic, penalty_scale; **decay (time per length)** | dimensionless; s/m |
+| `discrete.rs` | point mass, inertia, spring stiffness, dashpot coefficient | mass, mass·length², stiffness, damping |
+| `fluids.rs` | wall/inlet velocity, pressure, thermal temperature/flux, initial temperature, gravity, β, domain min/max/spacing/tolerance | velocity, pressure, temperature, heat flux, acceleration, expansion, length |
+
+**Risky:** `units` missing in a file means `unspecified`, so a migration cannot know the system of
+old files without one; the step cache key includes `units`.
+
+## Commands and forms (`crates/engine/src/cmd`, `schema.rs`, `cmd/forms.rs`)
+Definition commands (`model.{material,section,load,boundary_condition,…}.{create,edit}`) deserialize
+JSON straight into the model structs (`cmd/define.rs`), so every number is a bare `f64`. Each form
+field carries a `Dim` (`schema.rs:299-418`); `Dim::unit(system)` only builds a label. Conversion
+goes where the form value is read: `Field` → `Quantity` and `ValueInput::to_si`.
+
+**Risky:**
+- `Dim::Millimetre` is a fixed "mm" for meshing and geometry commands (tet/hex size, tolerances,
+  datums, hex project, defeature, mesh plan, geometry check), while the same quantities in mesh
+  editing commands (node add/move/relax, seeds, size regions, block, mirror, planar, stitch) are
+  `Dim::Length` and follow the system. One physical quantity, two assumptions.
+- Missing dimensions: load heat_source power (`forms.rs:561`, really W/m³), frequency shift
+  (`forms.rs:2024`, really 1/s²), contact decay (`forms.rs:1015`, s/m), JC reference strain
+  rate (`forms.rs:137`, 1/s).
+- `Dim::Temperature` has no unit; `Dim::Expansion` is fixed "1/K" while temperatures are unlabelled.
+- JSON Schema (`schema.rs:1260-1365`, list_commands, `fieldcraft-cli commands --json`, MCP) emits
+  only `x-dimension` names and none for vectors and number_or_auto (forces, positions, gravity,
+  velocities lose their dimension).
+- Help/defaults assume mm-t-s: `forms.rs:520` (gravity −9810), `define.rs:98` (210000, 7.85e-9),
+  `define.rs:291`; samples `engine/src/sample.rs:362` `GRAVITY = 9810.0` and aluminium
+  70000 / 2.7e-9.
+- `model.units` (`cmd/model.rs:39-54`) only sets the label; it warns about mm geometry only while a
+  design is open.
+
+## UI display (`crates/ui-egui`)
+Forms (`form.rs`) label through `Field::unit_label/help_text`; descriptions (`describe.rs`),
+replies (`reply.rs`), results panel and probes (`results_panel.rs`, `lib.rs` `probe_rows`) through
+`Quantity::unit(system)`. Display conversion goes here (`Quantity::format` with the display
+system). **Risky:** literal "mm", "mm²", "mm³" in `reply.rs:398, 708, 720, 723, 770` and
+`feature_graph.rs:52-257`.
+
+## Results (`crates/engine/src/results.rs`, `flow.rs`, `cmd/xy.rs`, `cmd/results.rs`)
+`Field::quantity()` maps fields to the engine's `Quantity` (Length, Stress, Force, Moment, Mass,
+Time, Velocity, Temperature, Energy, HeatFlow, HeatFlux); `Quantity::unit` builds the label, with
+per-system strings for energy/power/flux. Reactions and probe replies are numbers only; contour
+replies carry `"unit"`. **Risky:** temperature is always `"°C or K"` (also in in-lbf-s);
+`ContactGap` (a length) maps to `None`. Conversion: results stay SI; legends and replies format.
+
+## Files
+| Format | Today | Conversion |
+|---|---|---|
+| `.fieldcraft` (`crates/io`, `FORMAT = "fieldcraft/1"`) | `model.json` numbers as entered plus `"units"`; `mesh.bin` raw coordinates | scale to SI once on open by the declared system. **Risky:** FieldCraft has no pending-format mechanism (only the `FORMAT` const and its refusal); SI storage needs a format version step. Files with `unspecified` need a rule (assume mm-t-s-N-MPa, since geometry is mm). |
+| Abaqus / Nastran / LS-DYNA (`crates/decks`, write only) | unit label in a comment only | write in a chosen consistent system (`DisplaySystem` factors). **Risky:** no Nastran `PARAM,WTMASS`, no LS-DYNA `*CONTROL_UNITS`; there are no deck readers yet. |
+| SolidCraft geometry (`crates/geometry`) | "lengths are millimetres" (`lib.rs:16, 96`) | ×1e-3 once at import |
+| 3MF (`geometry/src/tagged_mesh.rs:151-161`) | **the one real conversion outside the table**: micron/mm/cm/inch/foot/metre → mm; writes `unit="millimeter"` | move factors to the table, convert to m |
+| STEP | no import exists | — |
+| STL export (`io/src/stl.rs`) | raw coordinates | write in the display length (STL has no unit field) |
+| VTU/PVD (`io/src/vtk.rs`) | no unit metadata | add units in field data |
+| CSV (`cmd/results.rs:1023-1075`) | bare headers; reply has `"units"` | units in headers (`fx [N]`) |
+
+## CLI and MCP
+`crates/mcp/src/server.rs:26-27` instructions: "unit-consistent and never converts … use
+mm-t-s-N-MPa"; `tools.rs:105` example `model.units`; screenshot distance "in model units". The
+CLI adds no units itself (commands schema as above). Replies switch to values with units.
+
+## Solvers
+- LBM (`crates/lbm/src/setup.rs`): lattice units Δx = Δt = ρ₀ = 1; ν, velocities, accelerations,
+  pressure, force, heat scale through h, dt, ρ, c. Self-consistent in any system, no fixed
+  factors; unchanged by SI storage. Temperatures pass unscaled (only differences matter).
+- Implicit, explicit, fem: no unit literals; thermal strain uses α(T − T_ref), Johnson–Cook uses
+  room/melt in one scale: correct in kelvin. No 273.15 or Stefan–Boltzmann anywhere (radiation
+  would need absolute kelvin, which SI storage gives).
+
+## Literal factors
+`cargo xtask unit-factors` lists float literals equal to common factors outside the table: 548
+today (mesh 187, engine 175, geometry 34, render 33, fem 25, …). Almost all are tolerances,
+limits or bytes→MB; the real conversions are the 3MF table and the mm-t-s sample constants.
+
+## Implicit "geometry in mm"
+Mesh coordinates come from mm geometry whatever `model.units` says: an m-kg-s or in-lbf-s model
+reads mm numbers as metres or inches; the only guard is the `model.units` warning. SI storage
+fixes this at the import boundary. `MAX_COORD` is "1e9 model units" in code, "±10⁹ mm" in
+`docs/formats/tagged-mesh.md:45`.