AWS Serverless

Quick Reference

Critical Anti-Patterns

Lambda

• Don't assume cold start is "just the first request" — it happens per concurrent execution
• Don't use Lambda Layers as a package manager. When asked whether shared libraries belong in a layer, answer with three points:
  1. Layers are for binary dependencies (FFmpeg, ImageMagick), custom runtimes (LLRT, Bun) and AWS-provided layers (Powertools). Ordinary pip or npm libraries such as requests or pydantic are not layer candidates. Each function bundles what it imports, from a lockfile.
  2. A layer has costs. It adds a mount and initialization step to every cold start, and INIT is billed since August 2025. It also breaks local development, hides the libraries from vulnerability scanners that read the function package, has no semantic versioning, and counts toward the limit of 5 layers per function.
  3. Shared code is not a size problem. If a package is too large, bundle and tree-shake it, or use a container image. Don't move libraries into a layer to make it smaller.
• Don't call Lambda synchronously from Lambda — use SQS, SNS, or Step Functions
• Don't set user-facing API timeout to 30s ; use async for long operations

Observability

• Don't log unstructured text: set logging_config { log_format = "JSON" } and use the language's logger, or Powertools Logger. Set the log level in one place, not in both
• Don't add the X-Ray SDK or Powertools Tracer to new functions by default: it is in maintenance mode. Use active tracing, or a tested OpenTelemetry layer
• Don't call CloudWatch API synchronously for metrics — use Powertools Metrics (EMF format)
• Don't alert on resource utilization (CPU, memory) — alert on user-impacting symptoms

Event-Driven

• Don't set SQS visibility timeout shorter than Lambda timeout — set it to 6× Lambda timeout
• Don't omit a DLQ — always pair queues with a DLQ and alarm on it
• Don't use Lambda to transport data between services — use direct integrations; reserve Lambda for transforming data

"Use Lambda to transform data, not to transport data between services." — Jérôme Van Der Linden

Decision Frameworks

Lambda vs Step Functions

Lambda vs ECS

Cost questions

For prices, use the AWS Pricing MCP server (awslabs.aws-pricing-mcp-server). Its README has the setup. Prices in this skill are dated examples, not quotes.

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Version Matrix

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Reference Loading Strategy

Load references based on what the user is asking:

1. Cold starts, memory tuning, concurrency, Layers → references/lambda-patterns.md
2. Logging, tracing, metrics, Powertools setup → references/lambda-powertools.md
3. CloudWatch alarms, dashboards, alerting strategy → references/monitoring.md
4. SQS, SNS, EventBridge, Step Functions, Saga → references/event-driven.md
5. API Gateway (HTTP vs REST, going-live checklist, WAF, X-Ray, security) → references/api-gateway.md