Logging Agent Skill
Use when adding logs, debugging, or working with the Logger across the SDK and container runtime. Covers the constructor-injection pattern, child loggers, env-var configuration, and test mocking. (project)
Install
one command, takes just this skill from the repositorynpx skills add https://github.com/cloudflare/sandbox-sdk --skill loggingThe instruction itself
6 sections, as written by the authorLogging
Pattern: Explicit Constructor Injection
Loggers are passed explicitly via constructor injection throughout the codebase. There is no global/ambient logger.
import type { Logger } from '@repo/shared';
class MyService {
constructor(private logger: Logger) {}
async doWork(context: WorkContext) {
const childLogger = this.logger.child({ operation: 'work' });
childLogger.info('Working', { context });
}
}
Child loggers
Use logger.child({ ... }) to attach structured context that will appear on every log line from that child. Prefer child loggers at the boundary of a unit of work (request, operation, session) rather than re-passing context on every call.
Configuration
Two environment variables, both read once at startup:
| Var | Values | Purpose |
| -------------------- | -------------------------------------- | --------------------- |
| SANDBOX_LOG_LEVEL | debug \| info \| warn \| error | Minimum level emitted |
| SANDBOX_LOG_FORMAT | json \| pretty | Output format |
Use json in production (machine-parseable) and pretty for local dev.
In Tests
Use createNoOpLogger() from @repo/shared to silence logging in tests:
import { createNoOpLogger } from '@repo/shared';
const service = new MyService(createNoOpLogger());
Don't construct real loggers in unit tests — they add noise and can mask real failures with log output.
When Adding Logs
- Log at info for significant lifecycle events (operation started/completed)
- Log at debug for fine-grained tracing (request bodies, intermediate state)
- Log at warn for recoverable anomalies
- Log at error for failures that surface to the caller; include the error object as structured context:
logger.error('Failed', { err }) - Pass structured context as the second argument, not via string interpolation
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How to use it
Take cloudflare/logging from the repository into ~/.claude/skills for personal
use, or into .claude/skills inside a project.
The agent identifies a skill by the name field in its header. Two skills with the
same name cannot sit side by side — one of them will be ignored.