# Structural Diff

A [weight diff](/docs/compare-and-morph/weight-diff) assumes two tensors are the same shape and asks how their values differ. A structural diff asks a prior question: do the two checkpoints even have the same *shape of model* - same layers, same modules, same tensor shapes and dtypes - before any value-level comparison is meaningful.

## What a structural diff surfaces

Comparing two checkpoints' [Semantic Model Graphs](/docs/core-concepts/model-architecture-layer-module) directly, rather than their weight values, surfaces:

-   **Added or removed layers/modules** - present in one checkpoint's tree but not the other's.
-   **Shape changes** - the same-named tensor with a different shape between the two (a resized embedding table after vocabulary expansion, for instance).
-   **Dtype changes** - the same-named, same-shaped tensor with a different dtype (commonly from a [quantization](/docs/working-with-models/quantized-models) pass applied to one side only).

Each of these appears in the Outliner as a per-node structural marker (added/removed/changed-type), independent of and prior to whatever the [Diff view](/docs/editors/comparison-editor) shows for value-level changes on the tensors that *do* still match.

## Why this matters before diffing values

A weight diff computed across two structurally different checkpoints without first checking structure produces numbers that are technically defined but not meaningful - comparing a tensor at the "same" name across two checkpoints whose architectures actually diverged there is comparing the wrong things, not measuring a real change.

This is also the distinction [Merge Conflicts](/docs/compare-and-morph/merge-conflicts#what-counts-as-a-conflict) draws between numerically resolvable and structural conflicts - a structural diff is what determines which category a given difference falls into before any resolution approach is chosen.

## Related resources

-   [Weight Diff](/docs/compare-and-morph/weight-diff) - the value-level comparison that structural diffing is a precondition for.
-   [Merge Conflicts](/docs/compare-and-morph/merge-conflicts) - where the resolvable-vs-structural distinction determines available resolution options.
-   [Model, Architecture, Layer and Module](/docs/core-concepts/model-architecture-layer-module) - the Semantic Model Graph a structural diff compares.