> ## Documentation Index
> Fetch the complete documentation index at: https://wundergraphinc-brendan-add-sof-link.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Relaxed Field Selection Merging Nullability

> Allow fields with differing nullability across union member types in federated graphs.

In a federated graph, different subgraphs may define the same field on different concrete union member types with differing nullability. For example, one subgraph might define `upc: ID!` on `Cosmo` while another defines `upc: ID` (nullable) on `Consultancy`, both members of a `Products` union.

By default, the router rejects queries that select such fields across union members with a validation error like:

```
fields 'upc' conflict because they return conflicting types 'ID' and 'ID!'
```

The `relax_subgraph_operation_field_selection_merging_nullability` option relaxes this validation when the enclosing types are **non-overlapping concrete object types** (i.e., different union members). This is useful when subgraphs legitimately disagree on nullability for the same field.

## Examples

### Scalar fields with differing nullability

Given two union member types where `upc` has different nullability:

```graphql theme={null}
# Subgraph A
type Cosmo {
  upc: ID!
}

# Subgraph B
type Consultancy {
  upc: ID  # nullable
}

union Products = Cosmo | Consultancy
```

The following query selects `upc` on both types:

```graphql theme={null}
{
  products {
    ... on Consultancy { upc }
    ... on Cosmo { upc }
  }
}
```

**Default behavior:** The router rejects this with a field conflict error.

**With relaxed nullability enabled:** The query is accepted because `Consultancy` and `Cosmo` are non-overlapping concrete types — a result can only ever be one or the other.

### Object fields with differing nullability

The same applies to object-typed fields:

```graphql theme={null}
# Subgraph A
type Cosmo {
  lead: Employee!
}

# Subgraph B
type Consultancy {
  lead: Employee  # nullable
}
```

```graphql theme={null}
{
  products {
    ... on Consultancy { lead { id } }
    ... on Cosmo { lead { id } }
  }
}
```

**Default behavior:** The router rejects this with a type conflict error.

**With relaxed nullability enabled:** The query is accepted.

## Configuration

<Tabs>
  <Tab title="Environment Variable">
    ```bash theme={null}
    ENGINE_RELAX_SUBGRAPH_OPERATION_FIELD_SELECTION_MERGING_NULLABILITY=true
    ```
  </Tab>

  <Tab title="YAML">
    ```yaml config.yaml theme={null}
    version: "1"

    engine:
      relax_subgraph_operation_field_selection_merging_nullability: true
    ```
  </Tab>
</Tabs>

This option is disabled by default. It only affects validation of field selections where the enclosing inline fragment types are non-overlapping concrete union members. See the [Router Configuration reference](/router/configuration#router-engine-configuration) for all engine flags.