# Lambda runtimes

Which runtime identifiers [Lambda](/docs/services/lambda/index.md) accepts, and what a rejection
means.

Identifiers are AWS's modelled `Runtime` enum; the deprecation, block-create and
block-update dates are AWS's published runtime lifecycle dates. Request
validation, the runtime-to-image mapping and the console's create wizard all
read the same table.

## What each rejection means

| Situation | Response |
| --- | --- |
| Not a value of the modelled `Runtime` enum, and not a runtime-version ARN | `400 InvalidParameterValueException` listing every modelled value |
| Modelled, but past AWS's deprecation phase for the operation | `400 InvalidParameterValueException` naming the recommended successor |
| Modelled and accepted by AWS, but Overcast has no execution image for it | `501 NotImplemented` — an emulator gap; nothing is persisted |

A missing execution image answers `501` rather than an invented `400`: the
request was valid and AWS would have accepted it. The check applies to `Zip`
packages; an `Image` function brings its own.

## Deprecation is observed in three steps

AWS retires a runtime in three steps and Overcast observes each against the
current date:

| Date | Effect |
| --- | --- |
| Deprecation date | Support ends; the runtime stays deployable and invokable |
| Block function create | `CreateFunction` starts refusing it |
| Block function update | `UpdateFunctionConfiguration` starts refusing it |

So a deprecated runtime is not automatically refused — `python3.9` lost support in
December 2025 but stays deployable until its block-create date.

## Which runtimes have an image

Overcast maps an official base image to every runtime AWS still accepts for
`CreateFunction`, across Node.js, Python, Java (including the Amazon Linux 2023
variants), .NET, Ruby and the `provided` custom runtimes. Runtimes AWS has already
blocked from `CreateFunction` — `go1.x`, `java8`, `python3.7` and older,
`nodejs14.x` and older, the `dotnetcore` family, `ruby2.x` and `provided` — carry
no image, so the `400` above is the only response they can produce.

`LAMBDA_SEED_RUNTIME_IMAGES=true` pre-pulls only the images for runtimes AWS still
supports; a deprecated-but-deployable runtime is pulled on demand at its first
cold start. `GET /_overcast/lambda/runtimes` returns the catalogue with each
runtime's `supported`, `deprecated`, `createBlocked` and `updateBlocked` flags.

## Related

- [Lambda](/docs/services/lambda/index.md) — quick start and what works
- [Lambda limitations](/docs/services/lambda/limitations/index.md) — the divergence table
- [Lambda examples](/docs/services/lambda/examples/index.md) — container images, layers, extensions
- [Lambda troubleshooting](/docs/services/lambda/troubleshooting/index.md) — throttles, layer errors, extension endpoints
- [Environment variable reference](/docs/configuration/reference/index.md) — every `LAMBDA_*` variable