apache/maven · error · MavenException

The consumer POM for {} cannot be created because the POM co

Error message

The consumer POM for {} cannot be created because the POM contains mixins. Mixins are not supported in the default consumer POM format. You have the following options to resolve this:
  1. Preserve the model version by setting 'preserve.model.version=true' to generate a consumer POM with <modelVersion>4.2.0</modelVersion>, which supports mixins
  2. Enable flattening by setting the property 'maven.consumer.pom.flatten=true' to remove mixins during transformation
  3. Remove the mixins from your POM

What it means

DefaultConsumerPomBuilder refuses to generate the default 4.0.0 consumer POM when the model uses <mixins>: mixins are a 4.1+ model feature that the 4.0.0 consumer format cannot represent. The build() method only proceeds when flattening (maven.consumer.pom.flatten=true, which inlines/removes mixins) is enabled or the model version is preserved (preserve.model.version=true, keeping a 4.2.0 consumer POM).

Source

Thrown at impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java:129

    @SuppressWarnings("checkstyle:ParameterNumber")
    DefaultConsumerPomBuilder(LifecycleBindingsInjector lifecycleBindingsInjector) {
        this.lifecycleBindingsInjector = lifecycleBindingsInjector;
    }

    @Override
    public Model build(RepositorySystemSession session, MavenProject project, ModelSource src)
            throws ModelBuilderException {
        Model model = project.getModel().getDelegate();
        boolean flattenEnabled = Features.consumerPomFlatten(session.getConfigProperties());
        String packaging = model.getPackaging();
        String originalPackaging = project.getOriginalModel().getPackaging();

        // Check if this is a BOM (original packaging is "bom")
        boolean isBom = BOM_PACKAGING.equals(originalPackaging);

        // Check if mixins are present without flattening enabled
        if (!model.getMixins().isEmpty() && !flattenEnabled && !model.isPreserveModelVersion()) {
            throw new MavenException("The consumer POM for "
                    + project.getId()
                    + " cannot be created because the POM contains mixins. "
                    + "Mixins are not supported in the default consumer POM format. "
                    + "You have the following options to resolve this:" + System.lineSeparator()
                    + "  1. Preserve the model version by setting 'preserve.model.version=true' to generate a consumer POM with <modelVersion>4.2.0</modelVersion>, which supports mixins"
                    + System.lineSeparator()
                    + "  2. Enable flattening by setting the property 'maven.consumer.pom.flatten=true' to remove mixins during transformation"
                    + System.lineSeparator()
                    + "  3. Remove the mixins from your POM");
        }

        // Check if consumer POM flattening is disabled
        if (!flattenEnabled) {
            // When flattening is disabled, treat non-POM projects like parent POMs
            // Apply only basic transformations without flattening dependency management
            // BOMs always need the effective (interpolated) model because transformBom()
            // strips parent and properties — any ${...} references would become dangling.
            // The flatten flag has no semantic effect on BOMs (transformBom always produces

View on GitHub (pinned to e4093d4e12)

Solutions

1. Set 'maven.consumer.pom.flatten=true' (e.g. in .mvn/maven.config or the POM properties) so mixins are flattened away during transformation
2. Or set 'preserve.model.version=true' to emit a 4.2.0 consumer POM that still supports mixins (Maven 4 consumers only)
3. Or remove the <mixins> element and inline its content manually
4. Verify the property actually reaches the session — it must be resolvable at deploy time, not only locally

Example fix

<!-- before -->
<project>
  <mixins><mixin>org.acme:shared-config:1.0</mixin></mixins>
  <!-- no properties: build fails with this error on install/deploy -->

<!-- after -->
<properties>
  <maven.consumer.pom.flatten>true</maven.consumer.pom.flatten>
</properties>

// before install/deploy
boolean hasMixins = !model.getMixins().isEmpty();
boolean flatten = "true".equals(System.getProperty("maven.consumer.pom.flatten", props.get("maven.consumer.pom.flatten")));
if (hasMixins && !flatten && !model.isPreserveModelVersion()) {
    throw new IllegalStateException("Configure maven.consumer.pom.flatten=true before deploying");
}

catch (MavenException e) {
    if (e.getMessage().contains("contains mixins")) {
        // apply option 1/2/3 from the message, then retry the deploy
    }
}

When it happens

Trigger: Running install/deploy on a project whose POM declares <mixins>, with both 'maven.consumer.pom.flatten' unset/false and 'preserve.model.version' unset/false. The consumer POM builder then cannot downgrade to 4.0.0 and throws MavenException before writing the attached artifact.

Common situations: Adopting Maven 4 mixins for POM composition and then running a stock 'mvn deploy' without the companion properties; CI pipelines that flatten was forgotten to be configured; building a 4.x project with older tooling assumptions where mixins were silently ignored.

Related errors

• The consumer POM for %s cannot be downgraded to model versio
• Error installing metadata: {}
• Error while deploying metadata: {}
• The use of the root attribute on the model requires the buil
• Duplicate dependency: {}

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/1997a1cb74bcb9af. Report an issue: GitHub.