# Seating A Cable Crossing

> Streams one planned crossing leg of a cable onto a routing station, lets the rod settle, then LOOKS — segments the rod, fits its centreline and counts how much of the curve lies inside the station's seat box on the side the route requires — and reports the fraction honestly rather than assuming the crossing took. It measures and does not correct. Use when a deformable-linear-object route is laid one crossing at a time and each crossing must be verified by vision before the next is planned.

- Skill: `graph-robots/seating-a-cable-crossing` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add graph-robots/seating-a-cable-crossing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/graph-robots/seating-a-cable-crossing/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: Apache-2.0
- Author: graph-robots (https://skillmd.com/u/graph-robots)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/graph-robots/seating-a-cable-crossing

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# seating-a-cable-crossing

Seat one crossing, then look, and hand on an honest number.

## Why this is its own skill

A weave that streams its legs and hopes never finds out which crossing failed.
The whole difference between a full solve and a third of one on the weave
bench was one crossing that did not take and was never looked at again. This
skill closes that loop for a single crossing: stream the planner's `place`
leg, wait for the rod to come to rest, then segment the rod, fit its
centreline and count the densified curve inside the station's seat box.

"Did not take" is a **fraction, not a touch**. The task scores containment as
the share of the rod inside a box a few centimetres wide beside the station,
on the side the route requires (`>= 0.047` on the weave bench). Measured on the
failures, a losing station is almost never on the wrong side: it is on the
right side with too little of it there.

## What it deliberately does not do

It **measures and does not correct**. The obvious correction — push the hand
further onto the required side when the box is short — was built and measured
and made the seat worse (0.016 to 0.000 after a 14 mm nudge): the cable is not
short of the box, it is crossing it at an angle, and driving the contact point
further in rotates it further. A correction wants a different move, arriving
along the seat rather than across it, and that belongs in the carry planner.

## Routing

The script always returns `route: done`; the verdict is in the outputs.
`seated` is `fraction >= seat_fraction`; `fraction` is `0.0` when the seat
could not be measured at all (no camera frame, no depth, no rod mask, a fit too
short to be a curve) and `detail` says so. A graph that wants to branch on the
verdict reads `seated`, not the exit. `blocked` is the exit a subgraph declares
as `on_error` for a tool that raises during the place or the look.

## Which station

The plan's `station` field names the station the planner actually worked, and
that — not the loop's pass index — selects the seat box. The planner does not
work stations in pass order (far-side crossings first, near-side seats last,
each from the anchored end outwards), so indexing the perceived list by the
pass number measured a different station's box from the one just worked on
every pass. `index` is the fallback only when the plan carries no `station`.

## Inputs and parameters

| Input | Shape |
|---|---|
| `plan` | JSON: `{"stages": [{"stage": "place", "trajectory": Trajectory, ...}], "station": int, "side": ±1}` from the crossing planner |
| `scene` | JSON: `{"stations": [[x, y], ...], "sides": [±1, ...]}` from the fixture survey |
| `index` | the pass number; used only when the plan names no station |
| `arm_id` | the work arm that streams the place leg |

Parameters with the weave bench's numbers as defaults: `seat_fraction` (0.047),
`seat_inner` / `seat_outer` (0.003 / 0.045 m out from the station on the
required side), `seat_half_y` (0.035 m along the station line), `camera`
(`"cable"`), `rod_query` (`"thin white cable"`), `curve_nodes` (42). Read a
different bench's off its own task document rather than reusing them.
`grip_open_m` / `grip_close_m` are accepted for uniform binding and unused.

## What a CPU test can and cannot exercise

The fraction arithmetic (densification, the side sign, the box bounds) and the
plan-station indexing run against canned observations and a canned centreline
fit. What a CPU test cannot exercise is the perception behind
`sam3.segment_text` and `curve.fit_centerline` on a real frame, and whether a
streamed `place` leg actually lays cable where the plan said.

## Required end states

| End state | Meaning |
|---|---|
| `done` | Streamed and measured; read `seated` / `fraction` / `detail`. |
| `blocked` | A tool raised; the subgraph's `on_error`. |