sentry-lwa

Drop-in AwsLambdaIntegration for ASGI apps on AWS Lambda behind the Lambda Web Adapter.

by Weird Sheep Labs

Why this exists

The Lambda Web Adapter ships its own Rust-based runtime interface client (the lambda_http crate) and uses it to poll the Lambda Runtime API directly. Your ASGI app runs as a plain HTTP server behind it — awslambdaric is never imported.

Sentry's native AwsLambdaIntegration monkeypatches awslambdaric's bootstrap module. Under the Web Adapter, setup_once() finds nothing to patch and returns early. The flush hook, timeout warning, and Lambda context enrichment are all inert. Lambda freezes the execution environment as soon as the response is sent, and any Sentry spans still queued at that point are silently lost.

This is a known issue. Others have reported identical data loss with DataDog and OpenTelemetry:

• #466 — DataDog traces lost because Lambda freezes before post-response middleware completes
• #404 — maintainer confirms background tasks are killed on freeze, suggests the internal extension pattern as a workaround
• #590 — the official fastapi-background-tasks extension example does not work (still open)

What this does

sentry-lwa provides a single AwsLambdaIntegration that replaces the native one. It:

1. Registers an internal Lambda extension that delays the freeze by holding off its Next call until the Sentry transport queue is flushed.
2. Wraps Starlette.__call__ to bridge the ASGI request lifecycle to the extension thread.
3. Enriches every Sentry event with Lambda metadata (function name, ARN, CloudWatch log link, execution duration) from the x-amzn-lambda-context header.
4. Optionally fires a ServerlessTimeoutWarning ~1.5 s before the invocation deadline.

Works with any ASGI framework that runs on Starlette — FastAPI, MCP servers via fastmcp, plain Starlette apps.

Installation

pip install sentry-lwa

Usage

Swap one import. Use whatever native Sentry integration your framework needs — AwsLambdaIntegration handles everything that breaks under the Web Adapter.

FastAPI

import sentry_sdk
from sentry_sdk.integrations.fastapi import FastApiIntegration
from sentry_lwa import AwsLambdaIntegration

sentry_sdk.init(
    integrations=[
        FastApiIntegration(),
        AwsLambdaIntegration(timeout_warning=True),
    ],
)

MCP server (fastmcp)

import sentry_sdk
from sentry_sdk.integrations.mcp import MCPIntegration
from sentry_lwa import AwsLambdaIntegration

sentry_sdk.init(
    integrations=[
        MCPIntegration(),
        AwsLambdaIntegration(timeout_warning=True),
    ],
)

Parameters

Behaviour

• Inside Lambda — the extension registers with the Extensions API, waits for each request to finish, flushes the Sentry queue (up to 2 s, capped by the invocation deadline), then calls Next to release the freeze. Every Sentry event is enriched with Lambda metadata.
• Outside Lambda (AWS_LAMBDA_RUNTIME_API not set) — the extension does not register and every request passes through unchanged. No overhead.

How it works

1. AwsLambdaIntegration subclasses the native AwsLambdaIntegration but does not call super().setup_once() (it would no-op under the Web Adapter).
2. setup_once() wraps Starlette.__call__ with a layer that parses the x-amzn-lambda-context header, adds a per-request event processor to the Sentry scope, optionally starts a timeout warning thread, signals the extension thread when the request starts and finishes.
3. setup_once_with_options() registers a SentryFlushExtension — an internal Lambda extension running in a background thread.
4. The extension thread (blocked on the INVOKE event from the Extensions API) wakes when the ASGI thread signals done, calls sentry_sdk.flush(), then calls Next — allowing Lambda to freeze.