> ## Documentation Index
> Fetch the complete documentation index at: https://docs.thrindex.com/llms.txt
> Use this file to discover all available pages before exploring further.

# E0203 — OutputShapeMismatch (backend)

> The hardware backend returned an output raster with different dimensions than the reference.

<Warning>E0203: backend output shape mismatch</Warning>

## What happened

A hardware backend ran inference and returned an output raster, but the raster has a different number of timesteps (`got_t ≠ t`) or output neurons (`got_n ≠ n`) than the model's declared output shape.

## Why

This is a backend implementation bug. The backend must run exactly `T` timesteps and return exactly `N_out` neurons per the artifact's declared output layer.

## How to fix

1. If you are integrating a hardware backend: verify that your backend's `run_batch` implementation returns a tensor of shape `[batch, T, N_out]` where `T` and `N_out` come from the artifact.
2. Check for off-by-one errors in timestep unrolling.
3. If this occurs with a production THRINDEX release, file a bug with the artifact, target name, and the exact error message.

## Example

```
E0203: backend output shape mismatch — expected [100, 20], got [99, 20]
Why: the backend returned fewer/more timesteps or neurons than the reference simulation
Fix: verify the backend correctly unrolls T timesteps and outputs all N_out neurons
Docs: https://docs.thrindex.com/errors/E0203
```

## Related

* [E0206](/errors/E0206) — the same check performed per-sample in the conformance harness