kernelcad-params git:20260613.15c8fd5A

Immutable. This exact content never changes and is served at /api/v1/blob/d1b6cb10553059ad.

---
name: kernelcad-params
description: Editable symbolic parameters — param(), params(), ParamRef arithmetic, MCP params_update. Use when the model needs values the agent or studio can change live.
---

# kernelCAD — editable parameters

Use `param()` for values the user may want to tweak after the first build; use literals for incidental dimensions. A param returns a symbolic `ParamRef`, not a number, so do not do JS arithmetic with it (`paramRef / 2` throws `feature.invalid-args` at the coercion site — the result would be a frozen number that stops re-evaluating on param updates). For derived dimensions, chain the arithmetic methods on the ParamRef itself: `.add`, `.subtract`, `.multiply`, `.divide`, `.negate` each return a new ParamRef built on a structured expression that re-evaluates whenever the underlying param changes — e.g. `param('r', 5).divide(2)` for a half-radius, or `param('w', 80).subtract(param('margin', 5).multiply(2))` for a derived inset width.

## `param()` / `params()` API

```typescript
// Single named param — returns a symbolic ParamRef<T>.
param<T extends number | boolean>(name: string, defaultValue: T, opts?: {
  min?: number;
  max?: number;
  description?: string;
}): ParamRef<T>;

// Batched shorthand — every key becomes a ParamRef of the same type as
// its default value. Accepts only number and boolean defaults.
params({ width: 60, addCablePort: true }): {
  width: ParamRef<number>;
  addCablePort: ParamRef<boolean>;
};
```

### ParamRef arithmetic

`ParamRef` is a branded symbolic node. Do **not** use JS operators; use the chain methods:

| Method | Returns | Notes |
|--------|---------|-------|
| `.add(n)` | `ParamRef<number>` | `paramRef.add(5)` |
| `.subtract(n)` | `ParamRef<number>` | `paramRef.subtract(margin)` |
| `.multiply(n)` | `ParamRef<number>` | `paramRef.multiply(2)` |
| `.divide(n)` | `ParamRef<number>` | `paramRef.divide(2)` |
| `.negate()` | `ParamRef<number>` | `-paramRef` symbolic |

Each method accepts either a plain number or another `ParamRef`. Chains stay fully symbolic: `radius.multiply(2).add(1)` is a `ParamRef`, not a number, and re-evaluates on each param change.

### Usage example

```typescript
const boltDia = param('boltDia', 5, { min: 3, max: 10, description: 'bolt hole diameter' });
const addCablePort = param('addCablePort', true, { description: 'include cable pass-through' });

return box(80, 50, 6)
  .holes('top', {
    positions: [{ u: -30, v: -20 }, { u: 30, v: -20 }],
    diameter: boltDia,
    depth: 'through',
    name: 'mountBolts',
  })
  .cutout(
    path().moveTo(-8, -5).lineTo(8, -5).lineTo(8, 5).lineTo(-8, 5).close(),
    { face: 'top', depth: 'through', name: 'cablePort', enabled: addCablePort },
  );
```

The batched declaration form is useful for compact top-of-file param blocks:

```typescript
const p = params({ plateW: 80, plateD: 50, plateT: 6 });
return box(p.plateW, p.plateD, p.plateT);
```

### MCP: `inspect({ of: 'params' })` / `params_update`

For post-build edits, use MCP `inspect({ of: 'params' })` to inspect the active evaluated session, then `params_update({ edits: [{ name: 'boltDia', value: 6 }] })`. Updates validate atomically, re-lower only affected records plus their downstream dependents, and return soft warnings when a boolean-gated feature makes a named downstream reference become a passthrough.

```typescript
// Via MCP (introspection session):
inspect({ of: 'params' })
// → [{ name: 'boltDia', value: 5, min: 3, max: 10, description: '...' }, ...]

params_update({ edits: [{ name: 'boltDia', value: 6 }] })
// → { applied: true, warnings: [] }
```

## Parametric assembly frames

Every Vec3 surface in the assembly API accepts `Editable<number>` per coord, so connector frames and joint frames can be built from `param()` values. Beyond plain tuples, the `worldOrigin` of a connector is itself a symbolic Vec3 that can be passed back into another assembly input — when downstream consumers read it (e.g. a joint origin = `parent.connector('tip').worldOrigin`), edits to the underlying params propagate live through the chain.

```typescript
const baseX = param('baseX', 70);
const plate = box(baseX, 46, 4);
const arm = assembly('arm');
const base = arm.part('base', plate, {
  connectors: { pivot: { origin: [baseX.divide(2), 23, 4], axis: [0, 0, 1] } },
});
const shoulder = arm.part('shoulder', shoulderLink, {
  connectors: { root: { origin: [0, 9, 2], axis: [0, 1, 0] } },
  connect: { connector: 'root', to: base.connector('pivot'), name: 'base-to-shoulder' },
});
arm.mate('yaw', 'base.pivot', 'shoulder.root', 'revolute', {});
```

`setParamValue('baseX', 100)` reactively rebuilds the plate AND the connector frame AND the joint origin AND the dependent shoulder placement — all in one re-lower. Axis vectors normalize at lower time; an axis whose components resolve to `[0, 0, 0]` raises `feature.invalid-args` with hint `invalid-args.axis.zero`.

## Diagnostic codes

The following diagnostic codes surface in the `diagnostics[]` array returned by `kernelcad evaluate`:

| Code | Trigger |
|------|---------|
| `feature.invalid-args` | `param()` default is outside declared `min`/`max`, or a `ParamRef` axis resolves to `[0,0,0]` |
| `feature.invalid-args` (hint `invalid-args.axis.zero`) | A Vec3 built from params resolves to the zero vector at lower time |

Boundary errors from `params_update` set a value outside `[min, max]` also surface as `feature.invalid-args` before the update is applied (atomic validation).

## Verification gates

After authoring with editable parameters, run before reporting done:

| Gate | Pass criterion |
|------|----------------|
| G-eval | `kernelcad evaluate` exits 0 — no `param.*` diagnostics |
| G-default-in-range | Every `param()` default value satisfies its declared `min` / `max` |
| G-unique-names | No two `param()` calls share a `name` in the same script — duplicates fail at capture time |
| G-paramref-survives-arithmetic | `ParamRef.add/.subtract/.multiply/.divide/.negate` return new ParamRefs, not numbers; chains like `radius.multiply(2).add(1)` stay symbolic |
| G-params-update-keeps-bounds | When the studio or MCP `params_update` sets a new value, it must satisfy `min` / `max`; otherwise the kernel refuses the edit and surfaces a diagnostic |

## Related skills

- `kernelcad-authoring` — primitives + sketches accept Editable<number> wherever a number is accepted.
- `kernelcad-features` — fillet radius / hole diameter / shell thickness all accept ParamRefs.
- `kernelcad-assemblies` — pose ParamRefs drive mate angles / translations via `.solvedModel()`.
- `kernelcad-mcp` — `params_update({ name, value })` edits a live model.