Java Agent that captures runtime exceptions, classifies them as INTERNAL (your code) or EXTERNAL (dependency/API/IO/network), and opens GitHub Draft PRs with AI-generated fixes — all without touching a single line of your project.

your-app.jar  ──►  -javaagent:callisto.jar
                         │
                         ▼
                   captures exceptions
                         │
                         ▼
               .callisto/bugs.jsonl
                         │
              ┌──────────┼──────────┐
              ▼          ▼          ▼
          summary       show     draft-pr
          (stats)    (details)  (GitHub PR)

Option A — pre-built JAR (recommended)

Download callisto.jar from the Releases page.

Option B — build from source

git clone https://github.com/yourorg/callisto
cd callisto
mvn clean package -DskipTests
# Output: callisto-agent/target/callisto.jar

1. Attach the agent to your JVM

java -javaagent:/path/to/callisto.jar -jar your-app.jar

Callisto captures every exception silently in the background. No code changes needed.

2. View captured bugs

java -jar callisto.jar summary

Callisto Summary
================
Total bugs:  12

By classification:
  INTERNAL      4  (████     33%)
  EXTERNAL      7  (██████   58%)
  UNCERTAIN     1  (█         8%)

Top exception types:
  NullPointerException          3
  HttpConnectTimeoutException   2

3. Inspect a bug

java -jar callisto.jar show BUG-a1b2c3

Bug BUG-a1b2c3
==============
Exception    : java.lang.NullPointerException
Message      : Cannot invoke "String.toUpperCase()" because "username" is null
Thread       : main
Captured     : 2026-04-21T10:30:00Z
Occurrences  : 3
Classification: INTERNAL
Reasoning    : Stack originates in com.example.myapp — matches project prefix

4. Create a Draft PR with an AI fix

export GITHUB_TOKEN=ghp_xxxxxxxxxxxx
java -jar callisto.jar draft-pr BUG-a1b2c3

Callisto will:

1. Ask Claude Code CLI to generate a patch
2. Create branch callisto/fix/BUG-a1b2c3 on GitHub
3. Commit the patch and open a Draft PR
4. Run your test suite to validate the fix (retries up to maxAttempts)

Use --dir PATH on any command to point at a project other than the current directory.

Create callisto.json in your project root. All fields are optional.

{
  "projectPackagePrefix": ["com.example.myapp"],
  "outputDir": ".callisto",
  "github": {
    "token": "",
    "repo": "owner/repo"
  },
  "test": {
    "command": "mvn test",
    "maxAttempts": 3
  }
}

Unknown fields in callisto.json cause a parse error — this prevents silent misconfiguration from typos.

Check .callisto/ exists in the working directory where the JVM launched. If missing, look for agent init errors on stderr:

[Callisto] WARN: Could not read callisto.json: ...

Also verify the agent flag is actually reaching the JVM — some launchers (Spring Boot Maven plugin, Gradle application plugin) need JAVA_OPTS or jvmArgs set explicitly:

# Maven / Spring Boot
export JAVA_OPTS="-javaagent:/path/to/callisto.jar"

# Gradle (build.gradle)
run { jvmArgs '-javaagent:/path/to/callisto.jar' }

This fallback triggers when:

• claude CLI is not installed or not in PATH
• claude auth has expired — run claude auth login to refresh
• Claude could not determine a patch (no file change generated)

Verify with:

claude --version
claude auth status