# aws-lambda

This Cursor rule guides AWS Lambda code structure, security, performance, and testing. Use it while building or reviewing a serverless AWS project.

- rule · `type=rule` · Cursor · ★ 3580
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/aws-lambda.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/aws-lambda

## What it does

The rule covers Lambda handler design, configuration, logging, permissions, and event processing. It recommends creating SDK clients outside the handler, using environment variables for operational settings, and keeping IAM permissions limited to the actions a function needs. Repeated events call for idempotency; workflows with several steps can be coordinated with AWS Step Functions.

Its examples use `aws-lambda-powertools`, AWS CDK v2, and DynamoDB. For local checks, it names AWS SAM CLI and LocalStack; for input validation, it points to Pydantic or the Parser utility in `aws-lambda-powertools`. Secrets can be stored with AWS Secrets Manager or AWS Systems Manager Parameter Store. Broad permissions such as `s3:*` run against the rule’s least-privilege guidance, so review the execution role before applying it.

## Requirements
- Works with: AWS

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/aws-lambda.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/aws-lambda.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/aws-lambda.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/aws-lambda.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/aws-lambda.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/aws-lambda.mdc

## When to use it
- move SDK client creation out of the handler
- add structured JSON logging to a Lambda function
- guard event processing against duplicate invocations
- restrict an IAM role to specific actions
- test a Lambda locally or against AWS services

## Example prompts
- Apply the `aws-lambda` rule and move `boto3` client creation outside the handler.
- Review this Lambda function for duplicate event handling and excessive IAM permissions.
- Show me how to test this handler locally with AWS SAM CLI using the `aws-lambda` rule.

## Source text · aws-lambda.mdc · CC0-1.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 3a4a5ef150ff2c65.

BEGIN THIRD-PARTY FILE 3a4a5ef150ff2c65
````text
---
description: Definitive guidelines for writing secure, performant, and maintainable AWS Lambda functions using modern best practices.
globs: **/*
---
# aws-lambda Best Practices

This guide outlines the definitive best practices for developing AWS Lambda functions. Adhere to these principles to build reliable, cost-effective, and secure serverless applications.

## Code Organization and Structure

**1. Single Responsibility Principle:** Each Lambda function must perform one distinct task. Decompose complex workflows into smaller, focused functions orchestrated by services like AWS Step Functions.

**2. Initialize Outside the Handler:** Leverage execution environment reuse by initializing SDK clients, database connections, and heavy dependencies in the global scope.

❌ BAD:
```python
import boto3

def handler(event, context):
    s3 = boto3.client('s3') # Initialized on every invocation
    # ...
```

✅ GOOD:
```python
import boto3

s3 = boto3.client('s3') # Initialized once per execution environment

def handler(event, context):
    # s3 client is reused across invocations
    # ...
```

**3. Use Lambda Layers for Shared Code:** Keep deployment packages small and promote code reuse for common libraries and dependencies.

**4. Configuration via Environment Variables:** Never hardcode operational parameters. Use environment variables for dynamic configuration. For sensitive data, use AWS Secrets Manager or AWS Systems Manager Parameter Store.

**5. Structured JSON Logging:** Output logs in JSON format to CloudWatch. This makes logs easily queryable and analyzable. Use `aws-lambda-powertools` Logger utility.

❌ BAD:
```python
print(f"Processing event: {event}")
```

✅ GOOD:
```python
from aws_lambda_powertools import Logger
logger = Logger()

@logger.inject_lambda_context
def handler(event, context):
    logger.info("Processing event", event=event)
    # ...
```

**6. Infrastructure as Code (AWS CDK v2):** Define your Lambda functions and their surrounding infrastructure (API Gateway, DynamoDB, IAM roles) using AWS CDK v2. This ensures version control, repeatability, and safe deployments.

✅ GOOD:
```python
# Example using AWS CDK v2 (Python)
from aws_cdk import Stack, aws_lambda as lambda_
from constructs import Construct

class MyLambdaStack(Stack):
    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        lambda_.Function(self, "MyHandler",
            runtime=lambda_.Runtime.PYTHON_3_12,
            handler="index.handler",
            code=lambda_.Code.from_asset("lambda"), # 'lambda' directory contains index.py
            environment={
                "TABLE_NAME": "MyTable",
            },
            # Add more configurations like memory, timeout, IAM roles
        )
```

## Common Patterns and Anti-patterns

**1. Implement Idempotency:** Design functions to produce the same result even if invoked multiple times with the same input. Use `aws-lambda-powertools` for robust idempotency.

❌ BAD:
```python
# No idempotency check, might process duplicate events
def handler(event, context):
    order_id = event['detail']['orderId']
    # Process order, could be duplicated
    # ...
```

✅ GOOD:
```python
from aws_lambda_powertools.utilities.idempotency import idempotent
from aws_lambda_powertools.utilities.idempotency.persistence import DynamoDBPersistenceLayer
import os

persistence_layer = DynamoDBPersistenceLayer(table_name=os.environ["IDEMPOTENCY_TABLE"])

@idempotent(persistence_layer=persistence_layer)
def handler(event, context):
    order_id = event['detail']['orderId']
    # Process order, guaranteed to run once
    # ...
```
````
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