git:20260914.0515e4e to git:20260929.fd2c146

---

name: kernelcad-params

description: Editable symbolic parameters — param(), params(), ParamRef arithmetic, MCP set_param. 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);

```

### Typed params: boolean / choice / string

`param()` also infers **choice** and **string** kinds from the default value and `meta`:

```typescript

// Boolean — still a ParamRef<boolean> (keeps its symbolic Editable<boolean>

// use in `enabled: hasLid` opts), but ALSO carries an eager `.value` snapshot

// for script control flow — see below.

const hasLid = param('HasLid', true, { description: 'chamfer the top edge as a lid seat' });

// Choice — string default + meta.choices -> TypedParamRef<string>, not a ParamRef.

const screw = param('Screw', 'M4', { choices: ['M3', 'M4', 'M5'] });

// Plain string — string default, no choices -> TypedParamRef<string>.

const label = param('Label', 'KCAD', { maxLength: 24 });

```

`choice`/`string` params return a **`TypedParamRef`**, a different type from

the numeric/boolean `ParamRef`. It does NOT support `.add()`/`.multiply()`/etc

(meaningless for strings) and cannot be coerced to a number — read `.value`

at evaluate time instead. Boolean `ParamRef`s work the same way for `.value`

(booleans have no arithmetic to preserve, so there's no symbolic-vs-eager

tension — every boolean `param()` call is a leaf):

```typescript

if (hasLid.value) { body = body.chamfer(1, { face: 'top' }); }

const diameters: Record<string, number> = { M3: 3.4, M4: 4.5, M5: 5.5 };

body = body.hole('top', { u: 0, v: 0, diameter: diameters[screw.value], depth: 'through' });

const text = sketch.text(label.value, { size: 6, align: 'center', position: [0, 0] });

```

A numeric `ParamRef.value` stays `undefined` — numeric params remain purely

symbolic (`_expr` walked against a live `ParamTable`) so a Studio slider can

re-lower a dimension without re-running the whole script. Only read

`.value` on boolean/choice/string params.

Unlike numeric `ParamRef`s, `TypedParamRef`s resolve **eagerly** (the value

at `param()` call time, not a re-evaluated symbolic AST) — because control

flow (`if`), object-key lookups, and text content have no meaningful

partial re-lower; `set_param` re-runs the whole script body with the new

value, which is exactly what eager resolution gives you.

`set_param({ code, param_name: 'Screw', new_value: 'M5' })` works the same

way for typed params as for numeric ones — a value outside the declared

`choices` set (or longer than `maxLength`) surfaces `feature.invalid-args`

(hint `invalid-args.param.choice-invalid` / `invalid-args.param.value-out-of-range`)

on re-evaluation. Booleans set via `set_param` take a literal `true`/`false`,

not a string.

The batched `params({...})` shorthand still only accepts `number | boolean`

defaults — declare choice/string params individually via `param()`.

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

Use MCP `inspect({ of: 'params' })` to list a model's params, then `set_param({ code, param_name: 'boltDia', new_value: 6 })` to rewrite the `param()` default in the source. The edit is source-only — it returns the modified code plus diagnostics from re-evaluating; the caller persists the returned code.

`inspect({ of: 'params' })` accepts the same `{ file? }` / `{ code? }` input every other `inspect({ of: ... })` reader does — pass one to evaluate that file/code fresh. With neither, it falls back to reading whatever session `evaluate_script` last left active (so a bare `inspect({ of: 'params' })` right after `evaluate_script` still works with no arguments) — it does NOT search for or reuse an unrelated session.

```typescript

// Via MCP, evaluating a file/code fresh (no prior evaluate_script needed):

inspect({ of: 'params', file: 'part.kcad.ts' })

// → { params: [

// { name: 'boltDia', type: 'number', value: 5, defaultValue: 5, min: 3, max: 10, description: '...' },

// { name: 'HasLid', type: 'boolean', value: true, defaultValue: true },

// { name: 'Screw', type: 'choice', value: 'M4', defaultValue: 'M4', choices: ['M3', 'M4', 'M5'] },

// { name: 'Label', type: 'string', value: 'KCAD', defaultValue: 'KCAD' },

// ] }

set_param({ code, param_name: 'boltDia', new_value: 6 })

// → { ok: true, code: '...rewritten source...', diagnostics: [] }

set_param({ code, param_name: 'Screw', new_value: 'M5' })

// → { ok: true, code: '...rewritten source...', diagnostics: [] }

```

`set_param` VALIDATES `new_value` against the param's declared kind BEFORE

rewriting anything: a value outside `[min, max]`, a non-boolean for a

boolean param, a non-string for a string param, or a value outside the

declared `choices` set is REFUSED — `{ ok: false, error }`, no `code` in the

result, source untouched:

```typescript

set_param({ code, param_name: 'HasLid', new_value: 'yes' })

// → { ok: false, error: "param 'HasLid' is boolean, got string (... invalid-args.param.type-mismatch ...)" }

set_param({ code, param_name: 'Screw', new_value: 'M9' })

// → { ok: false, error: "param 'Screw' value 'M9' is not one of the declared choices: M3, M4, M5 (... invalid-args.param.choice-invalid ...)" }

```

### `sweep_tolerance` is numeric-only

`kinematic.sweepTolerance({ params: {...} })` only accepts `number` params

(`{ values: [...] }` or `{ min, max, steps }`) — sweeping a boolean/choice/

string param has no meaningful range and fails fast with

`feature.invalid-args` (hint `invalid-args.param.type-mismatch`) before any

combo is evaluated.

## 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 |

| `feature.invalid-args` (hint `invalid-args.param.choice-invalid`) | A `choice` param's value is not in its declared `meta.choices`, or a `choice` param is declared without `meta.choices` |

| `feature.invalid-args` (hint `invalid-args.param.value-out-of-range`) | A `string` param's value exceeds `meta.maxLength` |

| `feature.invalid-args` (hint `invalid-args.param.type-mismatch`) | A `TypedParamRef` (choice/string) is coerced to a number, used where a numeric `Editable` is expected, swept via `sweep_tolerance`, or `set_param`'s `new_value` doesn't match the param's declared JS type (e.g. a string for a boolean param) |

Boundary errors from `set_param` setting a value outside `[min, max]` also surface as `feature.invalid-args` when the rewritten source is re-evaluated.

## 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 `set_param` sets a new value, it must satisfy `min` / `max`; otherwise re-evaluating the rewritten source 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` — `set_param({ code, param_name, new_value })` rewrites a `param()` default in the source.