How to Diagnose ':app:compileDebugKotlin' Compilation Errors in Android Gradle Builds

By the end of this page, you will understand what :app:compileDebugKotlin means, why the message Compilation error. See log for more details is usually only a wrapper error, and how to find the real Kotlin compiler message that actually explains the problem. You will also learn how Gradle, the Android plugin, and Kotlin versions work together, what to check in build.gradle, and a practical workflow for fixing build failures in Android projects.

The main concept behind this error is build troubleshooting in Gradle-based Android projects, especially when Kotlin compilation fails.

The message:

Execution failed for task ':app:compileDebugKotlin'.
> Compilation error. See log for more details

is usually not the root cause. It means Gradle ran the Kotlin compiler for the debug build of the app module, and that compiler reported an error. Gradle then wraps that compiler failure in a more generic task failure message.

What :app:compileDebugKotlin means

• app = the module being built
• compileDebugKotlin = the task that compiles Kotlin code for the debug variant
• Execution failed = that task did not complete successfully

Why this matters

In real Android development, many errors appear as Gradle task failures, but the true problem may be one of these:

• a Kotlin syntax or type error in source code
• a mismatched Kotlin plugin and standard library version
• incompatible Android Gradle plugin, Gradle, or Kotlin versions
• missing dependency or wrong repository
• generated code not being created correctly
• outdated dependency declarations or plugins

The key idea

A often shows , but not . The important part is the , where you usually see file names, line numbers, and actual Kotlin error messages such as:

Think of Gradle as a project manager and the Kotlin compiler as a specialist worker.

• Gradle says: "Compile the Kotlin code for the debug build."
• The Kotlin compiler starts working.
• If it finds a problem, it reports: "I cannot continue."
• Gradle then reports back with a generic message: "The task failed. See the log."

So the Gradle error is like hearing:

"The construction job failed. Ask the electrician for details."

The important information is not that the whole job failed. The important information is the electrician's exact complaint, such as:

• wrong wire connected
• missing component
• incompatible voltage

In Android terms, those become:

• syntax error
• unresolved reference
• incompatible dependency or plugin version
• missing repository or artifact

Core commands for diagnosing the error

Start with commands that show the actual compiler output in the terminal:

./gradlew :app:compileDebugKotlin --info

If that is not enough:

./gradlew :app:compileDebugKotlin --debug

To capture all output, including standard error:

./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

Notice the difference from this command:

./gradlew build --stacktrace > logs.txt 2>logErrors.txt

That splits normal output and error output into separate files. Kotlin compiler messages may appear in standard output, so you can miss the real message if you only inspect logErrors.txt.

What a useful compiler error looks like

A real Kotlin compile error usually looks more like this:

e: /app/src/main/java/com/example/MainActivity.kt:15:12 Unresolved reference: TextView

That is actionable because it tells you:

• file path
• line number
• column number
• exact reason

Example: generic Gradle failure vs real source-code error

Suppose you have this Kotlin code:

Example debugging workflow

Assume your build fails with:

Execution failed for task ':app:compileDebugKotlin'.
> Compilation error. See log for more details

Step 1: Run only the failing task

./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

This narrows the problem to one task and stores all output in one file.

Step 2: Search for the first Kotlin compiler error

Open build-output.txt and look for lines starting with:

• e:
• Unresolved reference
• Type mismatch
• file paths ending in .kt

Example:

e: /project/app/src/main/java/com/example/LoginActivity.kt:42:23 Unresolved reference: snackbar

Step 3: Ignore the long Gradle stack trace at first

The stack trace tells you the failure moved through Gradle internals:

• TaskExecutionException
• GradleException
• KotlinCompile

1. Fixing broken Android app builds

This is the most direct use case. When a CI pipeline or local Android Studio build fails, developers need to identify whether the issue is:

• broken Kotlin code
• a missing dependency
• a plugin mismatch
• a repository problem

2. Working with legacy Android projects

Your example uses older Android and Kotlin versions. Many real teams still maintain legacy apps. In those projects, build debugging often involves:

• older support libraries
• deprecated compile dependencies
• Fabric or other old plugins
• older Gradle plugin behavior

Knowing how to read the true compiler error is essential.

3. Continuous Integration failures

On CI servers, you often only see a summary like:

Task :app:compileDebugKotlin FAILED

A developer must then inspect the full logs to find the first real compiler error.

4. Dependency upgrades

After upgrading Kotlin, Android Gradle plugin, or Google libraries, compilation errors can appear because APIs changed or versions are incompatible. The same diagnostic workflow helps isolate whether the break is in code or in configuration.

5. Multi-module Android apps

In bigger codebases, failures may happen in a specific module such as:

:feature-login:compileDebugKotlin

The same rule applies: identify the exact failing task, then inspect the compiler output for that module.

How developers usually handle this in real projects

Run the smallest failing task first

Instead of rebuilding everything, developers run:

./gradlew :app:compileDebugKotlin

This shortens logs and speeds up debugging.

Use guard-style diagnosis

A common mental checklist is:

1. Is there an actual .kt compiler error with file and line number?
2. If not, is a dependency missing?
3. If not, are plugin versions compatible?
4. If not, is a repository or generated source step broken?

This is similar to a guard clause approach in code: eliminate simple causes first.

Keep versions centralized

Real projects often store versions in one place:

ext.kotlin_version = '1.1.2-2'
ext.supportLibraryVersion = '25.3.1'

This reduces accidental version drift.

Validate dependency setup

Developers commonly check whether the dependency needed by the Kotlin file is actually present. For example, an unresolved class might come from:

• missing library dependency
• wrong artifact version
• repository not declared

Be cautious with broad updates

In legacy projects, upgrading Kotlin alone can break compatibility with:

• Android Gradle plugin
• Gradle wrapper
• old support libraries

Mistake 1: Treating the stack trace as the real error

Beginners often focus on lines like:

org.gradle.api.tasks.TaskExecutionException
org.gradle.api.GradleException: Compilation error. See log for more details

These are wrapper errors.

Better approach

Look earlier in the output for:

• file names
• line numbers
• e: lines
• Kotlin compiler messages

Mistake 2: Redirecting logs in a way that hides the real compiler output

Broken approach:

./gradlew build --stacktrace > logs.txt 2>logErrors.txt

If you only inspect logErrors.txt, you may miss the actual compiler messages.

Better approach

Capture both streams together:

./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

Mistake 3: Updating one build tool without checking compatibility

For example, changing only Kotlin while leaving an old Android Gradle plugin can create new build issues.

Better approach

Check compatibility among:

• Kotlin Gradle plugin
• Kotlin stdlib
• Android Gradle plugin

Quick diagnosis checklist

./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

Then inspect build-output.txt for:

• e: lines
• .kt: file references
• Unresolved reference
• Type mismatch
• Expecting
• Could not resolve

What the error means

Execution failed for task ':app:compileDebugKotlin'

Means:

• module: app
• build variant: debug
• task: compile Kotlin
• status: failed

It does not automatically tell you the root cause.

Best commands

./gradlew :app:compileDebugKotlin
./gradlew :app:compileDebugKotlin --info
./gradlew :app:compileDebugKotlin --debug
./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

Things to verify in Gradle

Why does compileDebugKotlin fail even when Gradle sync succeeds?

Gradle sync checks project configuration and dependencies, but actual compilation happens during build tasks. Kotlin code errors may only appear during compilation.

Is the stack trace enough to fix a Kotlin compilation error?

Usually no. The stack trace shows how Gradle failed. The real cause is often earlier in the log as a Kotlin compiler message.

What is the best command to debug this error?

A good starting point is:

./gradlew :app:compileDebugKotlin --stacktrace > build-output.txt 2>&1

Then inspect the first actual compiler error in the file.

Can dependency version mismatch cause compileDebugKotlin errors?

Yes. If Kotlin, the Android Gradle plugin, libraries, or repositories are incompatible, the compile task can fail.

Should I run build or :app:compileDebugKotlin first?

Run :app:compileDebugKotlin first. It is smaller, faster, and produces more focused logs.

Why is Compilation error. See log for more details so vague?

Because it is a generic Gradle wrapper message. The detailed reason comes from the Kotlin compiler output earlier in the log.

Can a bad build.gradle repository block cause compilation errors?

• Gradle build lifecycle — helps you understand tasks, phases, and why failures happen inside specific tasks.
• Kotlin compiler errors — the real source of many compileDebugKotlin failures.
• Android Gradle plugin compatibility — important when Kotlin, Gradle, and Android plugin versions must work together.
• Dependency management in Gradle — useful for fixing unresolved references and missing libraries.
• Repositories in Gradle — relevant when artifacts cannot be downloaded.
• Debug vs release build variants — explains why a problem may appear only in one build type.
• Android module structure — helps you understand paths like :app:compileDebugKotlin.
• Reading build logs — essential for finding the first meaningful error instead of only the final stack trace.