sbt: Conflicting cross-version suffixes in: org.scala-lang.modules

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Find which dependency dragged in the other suffix
sbt "whatDependsOn org.scala-lang.modules scala-xml_2.12"
sbt dependencyTree | grep -i scala-xml

// Double %% appends the suffix for the current Scala version, single % does not
libraryDependencies += "org.scala-lang.modules" %% "scala-xml" % "2.3.0"
// not: "org.scala-lang.modules" % "scala-xml_2.12" % "1.3.0"

// A Java library must use a single %, it has no Scala suffix at all
libraryDependencies += "com.zaxxer" % "HikariCP" % "6.2.1"

// When a transitive dependency is the culprit, pin one version for everyone
ThisBuild / libraryDependencySchemes +=
  "org.scala-lang.modules" %% "scala-xml" % VersionScheme.Always

// Then confirm only one suffix survives
sbt "show compile:dependencyClasspath" | grep -c scala-xml

How to diagnose Scala errors

Scala's most distinctive error class is binary incompatibility: libraries are published per Scala major version, and mixing artifacts built for different versions produces confusing "not a member of" or NoSuchMethodError failures. The %% operator in sbt exists precisely to append the right suffix, and most unresolved-dependency errors are a % where %% was needed.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Check the Scala version suffix on every dependency. Use %% for Scala libraries and plain % only for Java ones.
2. Run sbt evicted to list dependencies that were evicted by version conflict. A frequent source of runtime NoSuchMethodError.
3. Use sbt dependencyTree to see the resolved graph and locate the transitive dependency pulling in an old version.
4. For inference failures, add explicit type annotations at the boundary rather than at the error site; inference failures usually propagate from further up.
5. Clear stale state with sbt clean and remove ~/.ivy2/cache or ~/.cache/coursier entries only for the specific failing artifact.

Tools worth reaching for

• sbt evicted
• sbt dependencyTree
• coursier resolve
• scalac -explain
• MiMa (binary compat checks)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 scala-sbt.org 📖 scala-sbt.org

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