If an Android app crashes only in an optimized release build, start by retracing its stack trace with the mapping.txt generated for that exact build. Then inspect the merged R8 rules and the code path named by the retraced trace—especially code accessed indirectly through reflection—and add the narrowest rule that preserves the required runtime behavior. Confirm the fix in the optimized release variant; treat global R8 disable flags as temporary diagnostic aids, not the solution.
First establish whether R8 is involved
A debug-versus-release difference is a clue, not proof that R8 caused the crash. Release builds may differ in configuration and dependencies as well as optimization. Capture the exact failing release artifact, the full exception and stack trace, the crash time, device and Android version, and the project’s Android Gradle Plugin (AGP) and R8 versions. Compare only traces that belong to the same build and mapping file.
Android Developers notes in its R8 optimization troubleshooting guide that a crash after optimization is typically due to broken reflection. Reflection is an important lead because R8’s static analysis may not see references made dynamically, but it does not establish the cause of any particular app’s crash.
Retrace the crash with the matching mapping file
R8 can rename classes and members, so a release stack trace may not identify the original code. Retrace it using the mapping.txt from the exact release build that produced the crash. Android documents this command-line form:
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$ANDROID_HOME/cmdline-tools/latest/bin/retrace app/build/outputs/mapping/<releaseVariant>/mapping.txt trace.txtThe mapping file is under build/outputs/mapping/ and later builds can overwrite it. Archive the mapping file for every shipped release variant so a future crash can be decoded against the right build. Android’s instructions for decoding an obfuscated stack trace also describe the Retrace workflow. Android Studio Logcat may retrace automatically in some versions; if it is unavailable or the result is unclear, use the command-line tool and the archived mapping.
After retracing, identify the exception type and the first meaningful application frame. A missing class or member, or a failure in serialization, dependency injection, JNI, or reflective lookup, can point toward a dynamic runtime contract that R8 cannot infer. Treat these as investigation leads: verify the code path and configuration before changing rules.
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Inspect the rules R8 actually received
The app’s proguard-rules.pro is not necessarily the whole configuration. App modules and consumed libraries can contribute rules, so inspect the merged configuration (the troubleshooting guide refers to configuration.txt) for rules that affect the suspect class or its members. The exact output location and tooling can vary with AGP and Android Studio versions; consult the documentation for the versions used by the project.
When you need to know why R8 retained a particular class or member, temporarily add -whyareyoukeeping locally. It reports a shortest path to a kept item and may not show every reason. Android warns that the diagnostic rule can slow builds and should not be checked into the codebase. It is for investigating retained code, not a substitute for examining why a dynamically used item may have been removed or transformed. See Android’s optimization troubleshooting guidance.
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Choose a targeted keep rule
A keep rule should preserve the runtime contract that the failing path depends on—not a broad package simply because one class is implicated. Depending on how the code is used, it may need to remain present, retain a class or member name, or keep particular attributes. Those requirements are distinct; preserving a class does not automatically mean every reflective name or metadata item is preserved.
- Find the specific class, method, field, or metadata required by the dynamic access.
- Check the relevant library’s R8 or consumer-rule guidance if a dependency owns that behavior.
- Use the narrowest rule that preserves the required behavior, leaving unrelated code eligible for optimization.
- Check syntax and rule location against the project’s AGP version before copying an example. Android’s keep-rules overview documents an AGP 9.3-and-later DSL location at
src/<variant>/keepRulesas well as legacy DSL support; do not assume that path applies to an older project.
R8 full mode has been the default starting with AGP 8.0. It makes stronger assumptions about dynamically accessed code, so a project should not be debugged on the assumption that older compatibility-mode behavior applies. Record the AGP/R8 versions when comparing results; see Android’s full-mode documentation.
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Verify the fix with optimization on
- Build the affected release variant with the targeted rule and normal optimization settings.
- Install that optimized artifact on a device or configuration where the failure can be reproduced.
- Run the same scenario that triggered the crash and inspect the resulting trace if it still fails.
- Check that the rule is no broader than needed and that unrelated code remains eligible for optimization.
Android recommends testing app behavior after optimization; its optimization guide describes enabling optimization for release builds. A successful debug build does not demonstrate that the optimized release path is fixed.
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Options such as -dontoptimize, -dontshrink, and -dontobfuscate can help determine whether a category of R8 processing is implicated when used as temporary local diagnostics. They are not a targeted repair: disabling a category changes the build broadly and can conceal the underlying runtime contract. Android says these options should be used only temporarily during debugging or development. If a disable flag helps isolate the issue, remove it, add the focused rule, and verify the optimized release scenario again. See the R8 global options guidance.
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