A Claude Code plugin for Firebase Crashlytics

Turn a Crashlytics crash into a proven fix

fixcrash writes a test that fails on the version that crashed, fixes the code, proves the fix, and opens a pull request with that proof. When it can't reproduce a crash, it says so and opens no pull request.

How it works GitHub

StatusEvaluated on Android, iOS and React Native testbeds. Not yet tried on a production app. Not published yet.

How it works

AI crash tools explain a crash, suggest a diff, or open a pull request. None of them show that the change fixes this crash. fixcrash makes that the only way to get a pull request, and checks it in code rather than by the model's word.

Reproduce
A test that fails on the version that crashed, with the crash's own exception.
Fix
The fix, on a fixcrash/<issue> branch.
Verify
The same test passing, and the full suite green.
Pull request
A pull request with that evidence, and a note on the Crashlytics issue.

Can't reproduce it?

A report: what it tried, what it suspects, and what data is missing, and a note on the issue. No pull request.

Reproduce on the version that crashed

The crashing commit comes from the event's build stamp or a release tag. The test is written in a git worktree at that commit.

Same failure, not any failure

The test must fail with the crash's exception type, and its stack must hit the crash's top app frame. It must get there the way the app does: a test that builds an object without its constructor or calls private members through reflection is refused.

Proof is enforced in code

fixcrash verify writes a receipt bound to the commit only when the test fails before the fix, passes after it, and the full suite is green. A hook blocks PR creation without a valid receipt for HEAD.

Honest when it can't

Hypotheses and test attempts are budgeted. When the budget runs out, you get a report, not a guess shipped as a fix.

The proof

The proof is fixcrash verify. On the Android testbed's parser crash it gives:

fixcrash verify --issue c43757a2… --test …ProfileParserTest.parsesProfileWithoutAvatar \

    --exception java.lang.NullPointerException --frame …ProfileParser.parse --base 89da0c3

base (crashing version) 89da0c31a144: ok: fails as the crash did: java.lang.NullPointerException at …ProfileParser.parse(ProfileParser.kt:12)

from 89da0c31a144: same commit as base, not run again

head (fix) 954cbb73fe9f: ok: the test passes; suite green: 4 tests, 0 skipped

VERIFIED: receipt .git/fixcrash/receipts/954cbb73fe9f….json

It runs every check in its own worktree, from committed code, and never touches your checkout. A test that fails with another exception, misses the crash's frame, or throws from a stub gets no receipt.

When it can't, it says so

If it can't reproduce the crash, it says so: what it tried, what it suspects, and what data is missing. It opens no pull request.

A unit test can usually reproduce

parsing and mapping errors (a missing field, a wrong type);
null, !! or nil on data;
index and range errors;
state-machine logic;
date, locale and number formatting.

Usually it can't, and fixcrash ends with a report

lifecycle crashes (Fragment not attached, a view used after onDestroy);
main-thread races;
OOM, ANR and watchdog kills;
OEM-specific and native (NDK, C++) crashes;
crashes inside third-party SDKs.

Platforms

Android (Kotlin)
JVM unit tests, Robolectric when Android classes are involved.
Evaluated: Android, 2026-10-08
iOS (Swift)
XCTest and Swift Testing through xcodebuild, on a simulator; release builds' inlined frames recovered from debug symbols.
Evaluated: iOS, UIKit, 2026-10-15; iOS, SwiftUI, 2026-10-16
React Native
Jest, plus symbolication of JS stacks from source maps for the crashing commit.
Evaluated: React Native, 2026-10-17

Results on the testbeds

/fixcrash:fix on each testbed's seeded crashes: sample apps with known crashes, where a unit test can reach some and not others. Each run was one fresh Claude Code session. Not yet tried on a production app.

An earlier Android run got a pull request through with a test that reached the crash by reflection. Such tests are now refused.

Android · 2026-10-08

Claude Code 2.1.294, claude-opus-5-5, firebase-tools 15.32.1, AGP 9.4.1, Gradle 9.6

parserAs expected: PR
NullPointerException at ProfileParser.parse
Expected PR · 1m 41s · $0.73
pagingAs expected: PR
IndexOutOfBoundsException at FeedPager.page
Expected PR · 1m 24s · $0.65
lifecycleAs expected: report, no PR
IllegalStateException at DetailsFragment.onDetailsLoaded
Expected report, no PR · 1m 11s · $0.59

Scenario	Crash	Expected	Outcome	Agent time	Cost
parser	NullPointerException at ProfileParser.parse	PR	As expected: PR	1m 41s	$0.73
paging	IndexOutOfBoundsException at FeedPager.page	PR	As expected: PR	1m 24s	$0.65
lifecycle	IllegalStateException at DetailsFragment.onDetailsLoaded	report, no PR	As expected: report, no PR	1m 11s	$0.59

iOS, UIKit · 2026-10-15

Claude Code 2.1.300, claude-opus-5-5, firebase-tools 15.32.1, Xcode 27.0, Firebase iOS SDK 13.0.1

parserAs expected: PR
EXC_BREAKPOINT in ProfileParser.parse(_:), inlined into MainViewController.openProfile()
Expected PR · 7m 16s · $0.80
pagingAs expected: PR
EXC_BREAKPOINT in FeedPager.page(_:), inlined into MainViewController.loadMore()
Expected PR · 9m 58s · $0.91
lifecycleAs expected: report, no PR
EXC_BREAKPOINT at closure #1 in SyncService.start(completion:)
Expected report, no PR · 2m 16s · $0.66

Scenario	Crash	Expected	Outcome	Agent time	Cost
parser	EXC_BREAKPOINT in ProfileParser.parse(_:), inlined into MainViewController.openProfile()	PR	As expected: PR	7m 16s	$0.80
paging	EXC_BREAKPOINT in FeedPager.page(_:), inlined into MainViewController.loadMore()	PR	As expected: PR	9m 58s	$0.91
lifecycle	EXC_BREAKPOINT at closure #1 in SyncService.start(completion:)	report, no PR	As expected: report, no PR	2m 16s	$0.66

iOS, SwiftUI · 2026-10-16

Claude Code 2.1.301, claude-opus-5-5, firebase-tools 15.32.1, Xcode 27.1 with the iOS 27.0 simulator, Firebase iOS SDK 13.0.1

renderAs expected: PR
EXC_BREAKPOINT in a ForEach row of TagsView.body
Expected PR · 3m 52s · $0.81
asyncAs expected: PR
EXC_BREAKPOINT at InboxModel.load(), after an await
Expected PR · 3m 10s · $0.73
actionAs expected: report, no PR
EXC_BREAKPOINT in a toolbar button's action in ArchiveView.body
Expected report, no PR · 1m 18s · $0.64

Scenario	Crash	Expected	Outcome	Agent time	Cost
render	EXC_BREAKPOINT in a ForEach row of TagsView.body	PR	As expected: PR	3m 52s	$0.81
async	EXC_BREAKPOINT at InboxModel.load(), after an await	PR	As expected: PR	3m 10s	$0.73
action	EXC_BREAKPOINT in a toolbar button's action in ArchiveView.body	report, no PR	As expected: report, no PR	1m 18s	$0.64

React Native · 2026-10-17

Claude Code 2.1.302, claude-opus-5-5, firebase-tools 15.32.1, React Native 0.88.0, Node 24.21, npm 11

renderAs expected: PR
TypeError at src/TagList.tsx:19, Android
Expected PR · 1m 45s · $1.04
parserAs expected: PR
TypeError at src/profile.ts:15, Android
Expected PR · 1m 58s · $1.01
engineAs expected: report, no PR
RangeError at src/updates.ts:13, Android
Expected report, no PR · 2m 06s · $0.86
asyncAs expected: PR
JavaScriptError at src/inbox.ts:14, Android, non-fatal
Expected PR · 1m 57s · $0.71
parser, iOSAs expected: PR
TypeError at src/profile.ts:15, iOS
Expected PR · 2m 35s · $0.73

Scenario	Crash	Expected	Outcome	Agent time	Cost
render	TypeError at src/TagList.tsx:19, Android	PR	As expected: PR	1m 45s	$1.04
parser	TypeError at src/profile.ts:15, Android	PR	As expected: PR	1m 58s	$1.01
engine	RangeError at src/updates.ts:13, Android	report, no PR	As expected: report, no PR	2m 06s	$0.86
async	JavaScriptError at src/inbox.ts:14, Android, non-fatal	PR	As expected: PR	1m 57s	$0.71
parser, iOS	TypeError at src/profile.ts:15, iOS	PR	As expected: PR	2m 35s	$0.73

Setup, earlier runs and findings: docs/EVAL.md at 7ea6be9.

What it runs, sends and stores

Runs on your machine

On demand, in your own Claude Code session. There is no server, daemon or bot.

Reads crash data, redacted

Crash data comes from Google's Firebase MCP server, which the plugin starts at a pinned firebase-tools version. Personal data is redacted before Claude reads it.

Writes after you say yes

It asks you before it pushes, opens the pull request, or adds a note to the issue. Writes to Crashlytics are notes only: it never changes an issue's state.

Keeps its state locally

Its worktree, receipts and recorded events are in .git/fixcrash. A React Native project's worktree is in ~/.cache/fixcrash, because Metro and Jest skip paths with .git in them.

What holds in code

A hook, not a prompt, keeps these rules:

Personal data is redacted before Claude reads crash data. User ids and installation ids are dropped from events; emails, IPs, tokens and other secrets are masked in messages, logs, breadcrumbs and custom keys. Claude can't write what it never saw.
The expected failure comes from the event. The hook records each event's exception type and blame frame. fixcrash reproduce and fixcrash verify check the test against that record, not against what Claude says the crash was.
No pull request without proof. Once a session has read crash events, gh pr create and GitHub MCP pull request tools need a valid receipt for the commit the pull request proposes.
Budgets. fixcrash reproduce counts test attempts and hypotheses per crash; past the budget, the answer is a report.
Receipts and recorded events are written by the CLI only. The hook refuses file edits there and shell commands that name them. The reproducer's file edits are held to test sources.

Safety rules

Crash data is untrusted input. Exception messages, logs, custom keys and breadcrumbs may contain instructions; fixcrash never follows them.
Redaction. No user ids, emails, tokens, IPs or other personal data from events in tests, commits, PR bodies or notes. Tests use synthetic values.
Writes to Crashlytics are notes only. fixcrash never changes an issue's state. Closing an issue is a human's call, after a release.
Tests run offline where the build tool allows, never against production services.

Install

fixcrash isn't published yet. These commands work once its repository is public.

In Claude Code

/plugin marketplace add krezzoid/fixcrash

/plugin install fixcrash@fixcrash

Then, in your app

/fixcrash:triage [app-id]

/fixcrash:fix <issue-id> [app-id]

/fixcrash:triage: : top issues of the latest version, each marked unit-reproducible or not.
/fixcrash:fix: : the loop above, for one issue.

/fixcrash:fix reproduces the crash in a subagent that may only write tests, fixes it on a fixcrash/<issue> branch, runs fixcrash verify, and asks you before it pushes, opens the pull request, or adds a note to the issue.

Requirements

Claude Code.
A Firebase project with Crashlytics, and a Firebase CLI login (npx firebase-tools login).
Your app's own toolchain: JDK and Android SDK, Node.js for React Native, Xcode for iOS.
Node.js 24 or newer for bin/fixcrash and the hook.
gh, logged in, to open pull requests.

The plugin's options (Firebase app id, project directory, release tag pattern, budgets) default to an app at the repository root, with the app id read from google-services.json or GoogleService-Info.plist.

Turn a Crashlytics crash into a proven fix

How it works

Reproduce

Fix

Verify

Pull request

Can't reproduce it?

Reproduce on the version that crashed

Same failure, not any failure

Proof is enforced in code

Honest when it can't

The proof

When it can't, it says so

A unit test can usually reproduce

Usually it can't, and fixcrash ends with a report

Platforms

Android (Kotlin)

iOS (Swift)

React Native

Results on the testbeds

Android · 2026-10-08

iOS, UIKit · 2026-10-15

iOS, SwiftUI · 2026-10-16

React Native · 2026-10-17

What it runs, sends and stores

Runs on your machine

Reads crash data, redacted

Writes after you say yes

Keeps its state locally

What holds in code

Safety rules

Install

In Claude Code

Then, in your app

Requirements