# Fix 'not supported on device' DeviceErrors in PennyLane

Fix PennyLane DeviceErrors by classifying the rejection (operator vs observable vs measurement) from the exact error text, then checking analytic vs finite-shot mode: CV vs qubit gate sets, simulator-only qml.state(), and shots requirements for sampling.

Exact reference: {"kind":"skill_version","skill_id":"skl_UHy4DyLd_dVAOQJjFDaTnA","version_id":"skv_IY3II0cw70Bn4xlAqOjwjw"}

Applicability: [{"constraint":">=0.33","technology":"PennyLane","version_scheme":"semver"}]

# Fix "not supported on device" DeviceErrors in PennyLane

Use this when executing a QNode raises `pennylane.exceptions.DeviceError`
saying an operator, observable, or measurement is not supported or not accepted
on the device. The error names which of the three was rejected — classify that
first, then check the execution mode, before editing the circuit.

## 1. Read the error — it tells you which of three things is rejected

| Error pattern | What's rejected | Typical cause |
|---|---|---|
| `Operator X(wires=[...]) not supported on device and does not provide a decomposition.` (older versions: `Gate X not supported on device ...`) | a gate | The device's native gate set lacks the op and PennyLane found no decomposition path — e.g. `DeviceError: Gate RX not supported on device default.gaussian` |
| `Observable ... not supported on device` | an observable inside `qml.expval`/`qml.var` | The observable is outside the device's accepted set |
| `Measurement ... not accepted for analytic simulation on device.` | the measurement type | The measurement is invalid in the current mode: `qml.state()`/`qml.density_matrix()` on hardware (simulator-only measurements), or sample-based measurements with `shots=None` |

## 2. Classify, then check the mode

- **Op rejected**: CV devices (`default.gaussian`) support only Gaussian
  operations — use `qml.Rotation` instead of `qml.RX`, or move the circuit to
  `default.qubit`. On hardware plugins, check the plugin's native gate set;
  PennyLane decomposes automatically when a decomposition exists, so this error
  means no decomposition path was found — swap in a natively supported
  equivalent rather than forcing the op.
- **Observable rejected**: rewrite the observable from supported pieces
  (e.g. a `qml.Hamiltonian`/`qml.LinearCombination` of Paulis on qubit devices)
  instead of a single unsupported observable.
- **Measurement rejected**: `qml.state()` and `qml.density_matrix()` are
  simulator-only — on hardware or remote devices (e.g. Strawberry Fields
  remote) they are rejected outright; use `qml.expval`, `qml.probs`, or
  `qml.sample` instead. Sample-based measurements (`qml.sample`,
  `qml.counts`) need finite shots: `qml.device("default.qubit", wires=2,
  shots=1000)`. Devices validate measurements differently for analytic
  (`shots=None`) vs finite-shot runs, so the same circuit can pass on one mode
  and fail on the other.

## 3. Diagnose before editing

```python
import pennylane as qml

specs = qml.specs(circuit)(params)   # device_name, gate counts, depth
print(specs["device_name"])
print(qml.draw(circuit, wire_order=dev.wires)(params))
```

`qml.specs` confirms which device actually executed the tape and breaks down
the gate set in use — the fastest way to spot the offending op before guessing.

## 4. Checklist for the failing execution

1. Copy the exact DeviceError; identify op / observable / measurement from the
   table in step 1.
2. Confirm the mode: `shots=None` (analytic) vs finite shots — acceptance
   rules differ between the two.
3. Op rejected: swap for a natively supported equivalent, or move to a more
   general device (`default.qubit`).
4. Measurement rejected: `qml.state()` means simulator only; sampling means
   set `shots`.
5. Re-run `qml.specs` after the change to confirm the fix, not just the
   absence of the error.


## Supporting basis and limitations

Built from the PennyLane device preprocessing docs (validate_observables / validate_measurements / decompose error texts: 'Observable ... not supported on device', 'Measurement ... not accepted for analytic simulation on device', 'Operator ... not supported on device and does not provide a decomposition'), forum threads (RX rejected on default.gaussian; qml.state/density_matrix unsupported on Strawberry Fields remote), and qml.specs / qml.draw for diagnosis.

## Change and rationale

New skill: fix 'not supported on device' DeviceErrors in PennyLane.

'Not supported on device' is a recurring PennyLane failure with three distinct causes (unsupported op, unsupported observable, measurement rejected in this mode) and three distinct fixes, but users typically guess. The error text already names the rejected category. This skill adds a classify-first decision procedure keyed on the exact error patterns from the device preprocessing docs.