onshape

name

onshape

description

Protocols for driving Onshape CAD via the onshape-mcp plugin. Render-first and entity-first workflows, unit + coordinate conventions, iteration discipline, when to reach for FeatureScript, and the gotchas (REMOVE-on-face auto-flip, Variable Studios as separate elements, deterministic ID remapping). Load before building anything in Onshape — the plugin's MCP tool surface makes more sense with this doc in context.

Jarvis Onshape MCP — Claude Skill Guide

Loaded as context for any Claude session driving the Jarvis Onshape MCP plugin. Encodes the protocols that keep CAD builds from silently failing. Short, imperative.

Think out loud

Before every non-trivial tool call (sketch, extrude, boolean, fillet, chamfer, describe, export), emit a short plan text (1-3 sentences, plain assistant output) saying WHY you're making this call and WHAT you expect to see / build. Example before an extrude:

"Extruding the base rectangle 30 mm up. Expect a flat plate with the four corner arcs intact, bbox (500, 780, 30)."

After each result, also say 1-2 sentences about what actually happened — especially if the result surprised you. If a describe_part_studio shows a feature at the wrong Z, name that out loud before deciding how to fix.

Don't let your only visible output be tool-call JSON. The observer watching the run relies on these thought lines to catch bugs in your reasoning.

Units

Onshape's API works in meters. Two safe ways to pass lengths:

• String with explicit suffix: "30 mm", "0.5 in", "15 deg". Always unambiguous; prefer this form.
• Bare number: treated as millimeters by default (CAD industry norm). 60 means 60 mm.

Never assume inches when reading a tool's schema even if the legacy description says "in inches." The builders were ported from an inch-default codebase; the bare-number default is now mm. When a tool response shows unexpected geometry sizes, suspect units first.

This applies on the assembly side too: transform_instance translations, set_instance_position x/y/z, and every create_*_mate / create_mate_connector offset accept bare numbers as mm or explicit unit strings. Slider / cylindrical mate minLimit/maxLimit are lengths (mm-default). Revolute minLimit/maxLimit are ANGLES in degrees (float). get_assembly_positions returns mm in its report text.

Coordinate frames

• Top plane = XY, normal +Z. corner1: [0, 0] in a Top-plane sketch is on the world origin.
• Front plane = XZ, normal -Y. Note the sign flip (classic trap).
• Right plane = YZ, normal +X.

When sketching on a picked face (faceId from list_entities), the sketch-local axes are defined by that face's own coordinate system. Geometry you sketch is interpreted in the face's plane, not world space.

The render-first protocol

After every feature that creates or modifies visible geometry:

1. Call describe_part_studio (not individual render_ calls — describe gives structured text + images in one shot).
2. Verify against expectations in the text first: is the new feature present in FEATURE TREE with status OK/INFO? Are the expected new faces/edges in BODIES? Does MASS PROPERTIES: volume line up with what you predicted?
3. If text checks out, glance at the iso view for anything visually off (asymmetric when should be symmetric, missing material, flipped direction).
4. If suspicious, crop_image on the specific area. Normalized [x1,y1,x2,y2] in [0,1].

Text checks catch arithmetic and counting errors (my weak spot). Image checks catch orientation and topology errors. Neither alone is sufficient.

Sketches: coordinate-first vs constraint-first

create_sketch has two surfaces. The right one depends on what you're modeling.

Coordinate-first (simple sketches, 1-3 primitives)

Pass entity dicts without id and no constraints. Positions are what you type.

{
  "entities": [
    {"type": "circle", "center": [0, 0], "radius": "25 mm"},
    {"type": "circle", "center": [100, 0], "radius": "12 mm"}
  ]
}

Fast. Use for mounting plates, single holes, obvious rectangles. Don't use this when the drawing specifies tangencies, concentricities, or dimensional chains — the solver makes those easy, hand-computed coordinates don't round-trip cleanly.

Constraint-first (drawing transcription)

Give each entity a user-level id, list constraints. Onshape's solver resolves positions from the constraints.

{
  "entities": [
    {"id": "hub",   "type": "circle", "center": [0, 0],   "radius": "25 mm"},
    {"id": "tip",   "type": "circle", "center": [100, 0], "radius": "12 mm"},
    {"id": "upper", "type": "line",   "start": [0, 25], "end": [100, 12]}
  ],
  "constraints": [
    {"type": "DIAMETER", "entity": "hub", "value": "50 mm"},
    {"type": "DIAMETER", "entity": "tip", "value": "24 mm"},
    {"type": "DISTANCE", "entities": ["hub.center", "tip.center"], "value": "100 mm", "direction": "HORIZONTAL"},
    {"type": "COINCIDENT", "entities": ["upper.start", "hub"]},
    {"type": "COINCIDENT", "entities": ["upper.end", "tip"]},
    {"type": "TANGENT", "entities": ["upper", "hub"]},
    {"type": "TANGENT", "entities": ["upper", "tip"]}
  ]
}

Entity refs are ids with optional sub-point suffixes: line.start, line.end, circle.center, arc.center. Seed positions (center, radius on circles/arcs; start/end on lines are optional) are just solver starting guesses — the constraints drive final geometry.

14 constraint types

Entity-ref only (no value): HORIZONTAL, VERTICAL (lines only), COINCIDENT, TANGENT, CONCENTRIC, PARALLEL, PERPENDICULAR, EQUAL, MIDPOINT.

Dimensioned (require value): DIAMETER, RADIUS, DISTANCE (with direction: MINIMUM | HORIZONTAL | VERTICAL), ANGLE (value in degrees default; "90 deg" / "1.57 rad" for units).

Binary pair: OFFSET (offset entity ↔ master; pair with a DISTANCE constraint on the same two entities for the offset length).

Aliases: HORIZONTAL_DISTANCE → DISTANCE(direction=HORIZONTAL), VERTICAL_DISTANCE → DISTANCE(direction=VERTICAL), LENGTH → DISTANCE(direction=MINIMUM) (for line length or slot end-to-end). POINT_ON is rejected — use COINCIDENT with a point sub-ref.

Pinning to the sketch origin

There's no magic origin keyword. To anchor a sketch to the plane origin (prevents drift on parametric resize), add an explicit point entity at [0, 0] and COINCIDENT the geometry you want anchored to it:

{
  "entities": [
    {"id": "origin", "type": "point", "at": [0, 0]},
    {"id": "hub",    "type": "circle", "center": [0, 0], "radius": "25 mm"}
  ],
  "constraints": [
    {"type": "COINCIDENT", "entities": ["hub.center", "origin"]},
    {"type": "DIAMETER",   "entity": "hub", "value": "50 mm"}
  ]
}

The sketch-local origin point acts as a fixed anchor. Without it, dimensions parametrize fine but positions drift when variables change — hub.center can slide ~1 mm when you retarget dimensions.

Gotchas

• HORIZONTAL/VERTICAL work on LINE entities only. On a POINT (like hub.center), use DISTANCE(direction=VERTICAL, value="0 mm") to pin to the horizontal axis.
• Circle + arc seeds are required. Onshape's solver needs a starting guess even when DIAMETER/RADIUS will drive final values.
• Arcs default to the short way. arc specs take start_angle and end_angle (degrees default; strings "38 deg" / "1.5 rad" for explicit units). If the CCW sweep from start to end exceeds 180°, the builder silently swaps endpoints so the arc goes the shorter way — matches Onshape UI's three-point-arc default. Need the long way? Pass "short_arc": false on the arc entity.
• Entity IDs must be unique within a sketch. Duplicate id → raise.
• Sub-point refs aren't validated. circle.start makes no sense but the builder won't catch it; Onshape rejects at solve time.
• Over-constraint surfaces as SKETCH_SOLVE_FAILED / SKETCH_UNSOLVABLE_CONSTRAINT WARNING. Onshape's REST API does NOT return per-constraint diagnostics — silence is a platform limitation. Recovery is client-side bisection: give every addConstraint an explicit id, and when a solve fails use edit_sketch with removeIds to drop half the last-added constraints, re-POST, and binary-search. Typical culprits: mutually exclusive pairs (PARALLEL + PERPENDICULAR on the same two lines), redundant positional constraints (COINCIDENT chain over-specifying endpoints), or tangent-line geometry that forces an arc into an impossible shape.

Iteration with edit_sketch

Don't delete-and-rebuild a sketch. edit_sketch takes a sketchFeatureId + addEntities / addConstraints / removeIds and splices.

edit_sketch({
  "sketchFeatureId": "Fabc_0",
  "addEntities": [{"id": "bore", "type": "circle", "center": [0,0], "radius": "18 mm"}],
  "addConstraints": [
    {"id": "d_bore", "type": "DIAMETER", "entity": "bore", "value": "36 mm"},
    {"id": "c_bore", "type": "CONCENTRIC", "entities": ["bore", "hub"]}
  ]
})

Removing an entity cascades: any constraint referencing it (directly or via sub-point) gets auto-dropped and reported in cascaded_removals: [{constraint_id, referenced}]. Read that field — otherwise a silent 48→12 constraint scrub bites three turns later.

The entity-first protocol

Never sketch on a face, fillet an edge, chamfer an edge, or mate to a face without calling list_entities or describe_part_studio first and picking the entity by reading its description.

Good picks:

• "the top face of the plate" → filter type == "PLANE" and normal_axis == "+Z", then max(by origin[2]).
• "the cylindrical hole" → filter type == "CYLINDER", then by radius.
• "the outer edges of the top face" → filter edges by direction_axis in ("+X","+Y","-X","-Y") at z_max.

Face/edge IDs are strings like JHK, JNC, JHl. Drop them verbatim into tool args as faceId or edgeIds: ["JHK", ...].

The regen-check protocol

apply_feature_and_check (used by every mutating tool) returns {ok, status, feature_id, feature_name, error_message}. Interpret:

• status == "OK" → feature built cleanly.
• status == "INFO" → Onshape auto-adjusted something. ok=True but READ error_message: common notes include "extrude was through-all auto-clamped" (fine) and "nothing was cut" (bad — you probably got the extrude direction wrong).
• status == "WARNING" → feature built but Onshape is concerned. Read and decide.
• status == "ERROR" → feature did not build. Do NOT add more features on top of it. Either fix the parameters and re-POST via update_feature, or delete_feature_by_name and retry.

Mate handlers (create_fastened_mate / create_revolute_mate / create_slider_mate / create_cylindrical_mate / create_mate_connector) share the same {ok, status, ...} contract via apply_assembly_feature_and_check. A mate that silently flips an instance still shows up as status="ERROR" or "WARNING" on the mate-level response — no need to visually check every mate just to catch a solver rejection. The 4-mate-for-2-part bracket dogfood burned ~50 turns to the now-fixed prose-return of this path.

Extrude-on-face direction trap

Cutting a hole from a +Z face (e.g. sketch on the top of a plate, then REMOVE-extrude to make a hole): the default direction is the sketch normal, which points away from the material (+Z into air). The cut removes nothing; Onshape returns INFO: nothing was cut.

Auto-default (current): when create_extrude sees operationType=REMOVE on a sketch placed on a picked face (any non-standard plane), it now defaults oppositeDirection=true so the cut goes INTO the material. The structured response includes a notes entry like "auto-set oppositeDirection=true because this REMOVE extrude is sketched on a picked face -- cutting INTO the material, not out into air" so you know what got auto-decided.

Override: pass oppositeDirection: false explicitly on create_extrude if you actually want to cut away from the face (e.g. cutting through from underneath into a body that hangs below). The auto-flip only fires when oppositeDirection is omitted from the tool args.

When to measure vs when to eyeball

• Measure: parallelism, perpendicularity, distances between known faces, hole depth, thickness, concentricity. Always more precise than reading a render.
• Eyeball (with the critic if available): overall symmetry, topology sanity ("does it look like a motor mount"), unexpected extra features, missing holes the brief asked for.

measure(entityAId, entityBId) returns point_distance_m always, angle_deg, parallel/perpendicular flags, and — for the special cases (face-face with parallel normals, face-point) — a projected_distance_m which is the actual geometric distance.

Gemini second opinion

If GEMINI_API_KEY is set, every inspect step in the CAD driver harness auto-routes the render through Gemini for a strict visual review. Output goes to the snapshot .txt file. If Gemini disagrees (missing or wrong features) and you're confident your text checks pass, go look at the render yourself — Gemini can be wrong but is often right when its signal says "I don't see the ø30 housing".

Variables

Parametric variables live in a separate Variable Studio element, not in the Part Studio itself. Workflow:

1. Call create_variable_studio(...) once per document to create a VS element. Reuse its element id across all variable writes in the same workspace.
2. set_variable(vs_element_id, name, expression) — writes to the VS element, NOT the Part Studio.
3. Sketch tools accept variableWidth/variableHeight/variableRadius/variableCenter args that reference these names as #name expressions. The variable resolver walks workspace VS elements, so any Part Studio in the same workspace can use them.

Trap: GET variables on a Part Studio element id returns [] even when variables are set — you must read from the VS element. If a diagnostic says "no variables" after a set, you're reading the wrong element.

Historical gotcha (fixed 2026-04-17, commit [sketch-vars-face] e6dd198): earlier set_variable used a POST that REPLACED the VS contents, so each call silently wiped prior variables. Now it upserts by name. If you see a sketch WARNING referencing #varname and think "but I just set that variable," verify the VS still contains ALL the variables you expected — not just the most recent one.

Known-broken: variableCenter. The per-axis DISTANCE-from-origin constraint references localFirst: "origin", but Onshape sketches don't expose the Part Studio origin as a local sketch entity, so the payload produces SKETCH_MISSING_LOCAL_REFERENCE (WARNING) and the center is never actually driven by the variable. Investigation: scratchpad/signed-variable-center-investigation.md. Until the helper is fixed to use externalFirst with a query of the Origin feature (or a mirror-pattern helper is built):

• Do NOT pass variableCenter. It fails silently (sketch still places the seed geometry, so it often looks correct until you change the variable).
• For parametric hole positions, use two explicit coordinates per hole. Write #hole_offset and #minus_hole_offset variables, set the second to -hole_offset in your VS, and pass them as separate values in each hole's center field: center: ["#hole_offset", "#hole_offset"] for the +,+ corner, ["#minus_hole_offset", "#hole_offset"] for -,+, etc. Bare numbers still work for non-parametric cases.
• The variableRadius path is safe — it uses a RADIUS constraint that resolves correctly.

When to use FeatureScript

The plugin wraps sketches, extrudes, revolves, thickens, fillets, chamfers, patterns, booleans, mates, shells (create_shell), and offset planes (create_offset_plane). Anything else — threads (ISO, UTS), drafts, lofts, sweeps, helical cuts, patterns along a path, variable-radius fillets — needs FeatureScript via write_featurescript_feature.

When to write a FeatureScript custom feature

Before writing FS from scratch, check docs/fs-cookbook/ in this repo for a proven snippet to adapt. Each cookbook file is a complete defineFeature(...) you can paste into featureScript, change a few numbers, and ship. Direct authorship of less-common ops burns 5-15 turns on opaque REGEN_ERRORs that the cookbook recipes have already worked through. Today's index: helix.fs (threads, springs, augers, helical ribs — avoids the broken opHelix API). Add a recipe when you find a pattern that works after >2 failed attempts.

Tool responses now include a hints list that points at this section; don't ignore it. Three triggers:

1. You've done the same 3+ feature pattern twice. Threads (sketch helix + sweep + cosmetic), bolt-circle patterns (variable + sketch + N circles + cut + annotation), standard grooves (offset + sweep-cut + fillet). The second time you write it by hand you're paying a turns-tax that never amortizes.
2. A primitive tool can't express it. Helical cut, loft between profiles, draft on a set of faces, sweep along a 3D path, variable-radius fillet, per-face-thickness shell. These have no MCP tool today and won't grow one; FS is the answer. (Uniform-thickness shells DO have a primitive — create_shell. Offset construction planes DO too — create_offset_plane. Reach for FS only when you need the non-uniform / multi-face version.)
3. You need per-instance parameters beyond what update_feature handles. If you want set_variable(hole_d_m3) to retarget 8 through-holes and 8 counterbores in one shot, package the pair as a custom feature whose definition takes hole_d + cbore_d parameters. update_feature only tweaks one feature's params; a custom feature lets one variable bump drive the whole chain.

Minimal template (copy, adapt, pass as featureScript to write_featurescript_feature):

FeatureScript 2909;
import(path : "onshape/std/geometry.fs", version : "2909.0");

annotation { "Feature Type Name" : "My Custom Feature" }
export const myCustomFeature = defineFeature(function(context is Context, id is Id, definition is map)
    precondition
    {
        annotation { "Name" : "Length" }
        isLength(definition.length, LENGTH_BOUNDS);
        // Add more parameters here: isAngle, isReal, isInteger, isBoolean, etc.
    }
    {
        // Body of the feature -- call op* primitives from onshape/std.
        // opPlane(context, id + "plane1", { "plane" : plane(vector(0,0,1)*definition.length, vector(0,0,1)) });
        // opExtrude(context, id + "ext1", { "entities": ..., "direction": ..., "endBound": ..., "endBoundEntity": ... });
    });

Call from the MCP layer:

write_featurescript_feature(
  documentId, workspaceId, elementId,
  feature_type="myCustomFeature",
  feature_name="Instance name",
  feature_script="<the FS source above>",
  parameters=[{"id": "length", "type": "quantity", "value": "15 mm"}],
)

The orchestrator creates a Feature Studio element, uploads the source, pulls the microversion, and instantiates via BTMFeature-134 with the correct e<eid>::m<mv> namespace. You get back a FeatureApplyResult with the usual {status, feature_id, error_message, hints} contract — regen errors in your FS body propagate through exactly the same way as starter-feature errors.

ToolSearch efficiency

This MCP server exposes many deferred tools. Every time you call a tool that hasn't been loaded, the runtime spends a round-trip loading the schema. Batch-load the tool surface in one ToolSearch call upfront:

ToolSearch(query="select:mcp__onshape__create_sketch,mcp__onshape__create_sketch_rectangle,mcp__onshape__create_sketch_circle,mcp__onshape__create_extrude,mcp__onshape__create_fillet,mcp__onshape__create_chamfer,mcp__onshape__create_shell,mcp__onshape__create_offset_plane,mcp__onshape__list_entities,mcp__onshape__describe_part_studio,mcp__onshape__measure,mcp__onshape__get_mass_properties,mcp__onshape__export_part_studio,mcp__onshape__create_document,mcp__onshape__create_part_studio", max_results=15)

Saves 3-5 individual search calls per session. The multi-entity create_sketch collapses a lot of small cases; prefer it over per-primitive tools.

Iteration discipline

This is a CAD session, not a script. When a feature builds wrong:

1. Don't stack a "fix" feature on top. Diagnose the wrong one.
2. update_feature to tweak parameters if the shape is close. delete_feature_by_name + re-add if it's the wrong shape entirely.
3. Re-run the same describe + assert you used to catch it.
4. If you're fighting the same feature three times, pause and ask whether the approach is wrong. Maybe you need a FeatureScript custom feature, or the whole build should be re-ordered.