OpenAPITools/openapi-generator · error · MojoExecutionException

Invalid mergeConflictStrategy value '" + mergeConflictStrate

Error message

Invalid mergeConflictStrategy value '" + mergeConflictStrategy + "'. Valid values are: WARN, FAIL

What it means

Thrown in ValidateMojo.mergeInDirectory's file-list branch when mergeMode resolved to DEEP and MergeConflictStrategy.valueOf(mergeConflictStrategy.toUpperCase(Locale.ROOT)) throws IllegalArgumentException — the <mergeConflictStrategy> is neither WARN nor FAIL (case-insensitive, untrimmed). Consistent with the generate goal, the strategy is only validated when deep merging is active.

Source

Thrown at modules/openapi-generator-maven-plugin/src/main/java/org/openapitools/codegen/plugin/ValidateMojo.java:303

      }
      MergedSpecBuilder.MergeMode resolvedMergeMode;
      try {
        resolvedMergeMode = MergedSpecBuilder.MergeMode.valueOf(mergeMode.toUpperCase(Locale.ROOT));
      } catch (IllegalArgumentException e) {
        throw new MojoExecutionException("Invalid mergeMode value '" + mergeMode
            + "'. Valid values are: REF, DEEP");
      }

      MergedSpecBuilder builder = new MergedSpecBuilder(inputSpecFiles, mergedFileOutputDir.getAbsolutePath(),
          mergedFileName, mergedFileInfoName, mergedFileInfoDescription, mergedFileInfoVersion, auth)
          .withMergeMode(resolvedMergeMode);

      if (resolvedMergeMode == MergedSpecBuilder.MergeMode.DEEP) {
        try {
          builder.withConflictStrategy(
              MergedSpecBuilder.MergeConflictStrategy.valueOf(mergeConflictStrategy.toUpperCase(Locale.ROOT)));
        } catch (IllegalArgumentException e) {
          throw new MojoExecutionException("Invalid mergeConflictStrategy value '" + mergeConflictStrategy
              + "'. Valid values are: WARN, FAIL");
        }
      }

      String mergedSpec = builder.buildMergedSpec();
      LOGGER.info("Merged input spec from explicit file list: {}", mergedSpec);
      return Optional.of(mergedSpec);
    }

    // Directory scan mode
    Optional<String> mergedSpec = Optional.empty();
    if (StringUtils.isNotBlank(inputSpecRootDirectory)) {
      inputSpecRootDirectory = replaceBackslashesToSlashes(inputSpecRootDirectory);

      MergedSpecBuilder.MergeMode resolvedMergeMode;
      try {
        resolvedMergeMode = MergedSpecBuilder.MergeMode.valueOf(mergeMode.toUpperCase(Locale.ROOT));
      } catch (IllegalArgumentException e) {

View on GitHub (pinned to fcec517be3)

Solutions

Set <mergeConflictStrategy>WARN</mergeConflictStrategy> or <mergeConflictStrategy>FAIL</mergeConflictStrategy>.
Trim whitespace from the value; the lookup uppercases but does not trim.
Share the merge settings with the generate goal via plugin-level <configuration> to keep both consistent.

Example fix

<!-- before -->
<mergeMode>DEEP</mergeMode>
<mergeConflictStrategy>THROW</mergeConflictStrategy>

<!-- after -->
<mergeMode>DEEP</mergeMode>
<mergeConflictStrategy>FAIL</mergeConflictStrategy>

Defensive patterns

Strategy: validation

Validate before calling

CS=$(mvn -q help:evaluate -Dexpression=mergeConflictStrategy -DforceStdout 2>/dev/null)
echo "$CS" | tr -d '[:space:]' | tr '[:lower:]' '[:upper:]' | grep -qx '\(WARN\|FAIL\)' || {
  echo "mergeConflictStrategy must be WARN or FAIL (got '$CS')"; exit 1;
}

Prevention

Validate mergeConflictStrategy even while in REF mode — the plugin only checks it under DEEP, so a typo stays hidden until you switch.
Use WARN during exploration and FAIL once merged specs are considered authoritative.
Document the two valid values next to the property in your POM comments.

When it happens

Trigger: Validate goal with <inputSpecFiles>, <mergeMode>DEEP</mergeMode>, and an invalid conflict strategy such as "THROW", "ignore", or " WARN". The same bad value under REF mode never triggers this error.

Common situations: Enabling deep merge for validation of merged multi-file specs and guessing strategy names; values supplied via -DmergeConflictStrategy=... in pipelines differing from the POM default; whitespace introduced during XML editing.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/798ee73abbdb78dc. Report an issue: GitHub.