# Selecting Reachable Grasp

> Walk a ranked list of candidate grasp poses and take the first the arm can actually reach — IK solves it, the solution lands where it was asked, and (where `sim.clearance` is available) the arm is clear of the scene there. Use after any propose rung. This is the fix for taking candidate 0 on faith and closing the jaws on nothing.

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

---


# selecting-reachable-grasp

`poses` is a list of `Se3Pose`, best first -- a propose skill's `poses`
output, or the `poses` field of a `GraspCandidates`. It is declared here in
prose rather than under `required_inputs` because the registry has no name for
a bare list of poses, and a `GraspCandidates` is a dict the script does not take.

One strategy for the **select** stage: reachability, then clearance, first
match wins.

## Why this is its own skill

A proposer ranks by the object's geometry, which is the only thing it knows.
Whether the *arm* can hold a pose is a different question, and entry 0 is
routinely the one it cannot. Nothing downstream notices — IK returns something,
the servo drives to it, and the jaws close on air. A study packet put it
exactly: *"gripper closed on nothing while grasping ring_spanner_19"*.

Taking the **first** that passes rather than the best: the list arrives ranked
by a proposer that knows things about the object this skill does not, and
re-ranking here would silently overrule it.

## When to use

- After every propose rung. There is no case where taking candidate 0 unchecked
  is better than walking the list.

## When NOT to use

- As a substitute for a collision-aware planner on the *carry*. This gates the
  grasp configuration, not the path to it. The two are different halves: a
  corridor test refuses a grasp whose descent goes through a shelf; a planner
  routes the carry around a beam. Neither substitutes for the other.

## Recommended subgraph state flow

```text
select → (selected | none_reachable)
```

A single script node. Inputs: `poses` (built from the proposer's stations and
`robot.grasp_frame`'s rotation), `arm_id`, `object_name`, `check_clearance`.
Router on `route`.

Downstream, execute the selection with
`motion.plan_linear(orientation="lock")` → `robot.execute_trajectory` so the
closing heading is actually held — `robot.go_to_pose` leaves the wrist roll
free and discards it.

## Required end states

| End state | Meaning |
|---|---|
| `selected` | `pose` and `joint_config`. Read `index` to see how far down the list it had to go, and `corridor_checked` to see whether clearance ran. |
| `none_reachable` | Nothing in the list solved. Route to another propose rung, not back to this one. |