Release Flutter Web S3

Scope

Use this skill from the root of a Flutter application repository that publishes its Web build to AWS S3 or an S3-compatible object storage provider such as Cloudflare R2, MinIO, Tigris, Backblaze B2, or DigitalOcean Spaces.

Default assumptions:

• Version source: pubspec.yaml
• Build output: build/web
• Main entry point: this skill's scripts/releasewebs3.py
• Version helper: this skill's scripts/preparewebrelease.py
• Local deploy config: deploy/s3.env, ignored by git
• Default tag pattern: web-v<pubspec version>
• Package manager/runtime preference: uv for Python helpers, fvm when the project uses FVM
• Fingerprinted live deploys are enabled by default via FINGERPRINTWEBBUILD=1

Adapt these defaults to the target project before publishing. Do not assume the project has Slang, a specific GitHub Actions workflow, or a specific object storage provider.

Do not print secret values from deploy/s3.env or GitHub secrets. Any compatibility switch such as BUILDWASM, FINGERPRINTWEBBUILD, S3ADDRESSINGSTYLE, AWSREQUESTCHECKSUMCALCULATION, AWSRESPONSECHECKSUMVALIDATION, or ALLOWDIRTY must be explicitly called out before use because these can hide provider or runtime differences.

Main Usage

Use scripts/releasewebs3.py as the only release entry point.

Parameters

• --project-root: manually specify the Flutter project root; by default the script auto-detects it from git or pubspec.yaml
• --pubspec: override the pubspec.yaml path relative to the project root
• --tag-match: specify the git tag glob used to locate the previous Web release; default web-v*
• --tag-prefix: specify the git tag prefix for the new release; default web-v
• --version: explicitly set the release version and bypass the suggested version
• --bump {major|minor|patch}: override the detected semantic version bump
• --dry-run: print planned file, git, build, and S3 actions without making changes
• --skip-tests: skip the Flutter test step during validation
• --no-tag: do not create the git release tag after publishing
• --no-push: do not push the git release tag to remote
• --yes / -y: non-interactive mode; fail instead of prompting for confirmation
• --promote <release-id>: skip version/build flow and promote an existing immutable release to the live prefix

Workflow Covered By The Script

The script handles the whole fixed flow automatically:

1. Preflight

- check git status and branch state - verify Flutter Web project structure - detect fvm / flutter / dart - ensure deploy/s3.env exists - verify S3 connectivity

1. Version

- inspect commits since the latest matching tag - suggest a SemVer bump - optionally accept an explicit or overridden version - write the chosen version back to pubspec.yaml

1. Validate

- run flutter pub get - run build_runner if needed - run slang if needed - run tests unless --skip-tests - perform an S3 upload dry-run

1. Publish

- build Flutter Web - write build/web/version.json - fingerprint cacheable build artifacts when enabled - upload immutable release assets - refresh live assets - create and optionally push a git tag

1. Promote

- copy an existing immutable release to the live prefix without rebuilding

Configuration

Create deploy/s3.env.example with non-secret placeholders, then create a local deploy/s3.env from it and ensure deploy/s3.env is git-ignored.

Example:

S3_ENDPOINT_URL=https://s3.example.com
S3_BUCKET=my-bucket
S3_REGION=auto
S3_LIVE_PREFIX=web
S3_RELEASE_PREFIX=web/releases
BASE_HREF=/
PWA_STRATEGY=none
# AWS_PROFILE=my-profile

Required runtime configuration:

• S3ENDPOINTURL: provider endpoint URL
• S3_BUCKET: target bucket
• credentials via AWS_PROFILE, CI secrets, or the provider's supported AWS CLI mechanism
• for local uploads, prefer configuring a named AWS CLI profile with aws configure --profile <new-profile-name> and setting AWS_PROFILE=<profile>

Optional project configuration:

• S3_REGION: region passed to AWS CLI; default auto
• S3LIVEPREFIX: live object prefix; default web
• S3RELEASEPREFIX: immutable release prefix; default web/releases
• BASE_HREF: passed to flutter build web --base-href
• PWA_STRATEGY: passed to flutter build web --pwa-strategy
• BUILDWEBDIR: override when the web output is not build/web
• BUILD_WASM=1: add --wasm, only after browser/runtime compatibility is verified
• FINGERPRINTWEBBUILD=1: rewrite cacheable web artifacts to content-hashed paths and emit build/webfingerprintmanifest.json
• FLUTTERCMD, DARTCMD: override command discovery for non-standard projects
• PREBUILDCMD, BUILDRUNNERCMD, SLANGCMD, TESTCMD: override build/test/codegen commands
• RELEASECACHECONTROL, LIVECACHECONTROL, LIVESTATICCACHECONTROL, LIVEHASHEDCACHECONTROL: customize cache headers for uploaded assets

Fingerprint Mode

When FINGERPRINTWEBBUILD=1, the skill fingerprints cacheable build artifacts and rewrites references across the Flutter Web output so the live prefix can safely serve:

• stable entrypoints with short cache
• hashed assets with long immutable cache

Current stable entrypoints:

• index.html
• flutter.js
• flutterserviceworker.js
• assets/AssetManifest.bin
• assets/NOTICES

Current hashed targets include:

• flutter_bootstrap.js
• manifest.json
• favicon.png
• version.json
• main.dart.js
• main.dart.mjs
• main.dart.wasm
• icons/**
• assets/** except the stable files listed above
• canvaskit/** as one hashed directory prefix

The fingerprint pass also emits build/webfingerprintmanifest.json. The release script uses this manifest to split live uploads into:

• LIVECACHECONTROL for stable files
• LIVEHASHEDCACHE_CONTROL for hashed files

Recommended live cache policy when fingerprinting is enabled:

LIVE_CACHE_CONTROL=no-cache, no-store, must-revalidate
LIVE_HASHED_CACHE_CONTROL=public,max-age=31536000,immutable

Supported Flutter Output Assumptions

The fingerprint helper is intentionally coupled to current Flutter Web build structure. Before sharing this skill across projects, assume it needs review whenever Flutter changes its web output conventions.

This skill currently expects all of the following to exist in build/web after flutter build web:

• index.html
• flutter_bootstrap.js
• main.dart.js
• main.dart.mjs
• main.dart.wasm
• manifest.json
• assets/AssetManifest.bin.json
• assets/FontManifest.json
• canvaskit/

If a target project uses a different Flutter version or custom post-processing that changes these filenames or removes these files, re-verify the skill before reuse.

Project-Specific App Code

This skill does not inject application-level migration logic into shared projects. If a project needs one-time cleanup for old service workers or legacy browser caches, keep that logic in the project repo, not in the shared skill.

Version Helper

If you only need to inspect or write the version without running the full release flow, use preparewebrelease.py directly.

It can:

• inspect the current version from pubspec.yaml
• find the latest matching git tag
• inspect commits since the last release
• suggest a SemVer bump
• generate a tag name from the selected version
• optionally write the version back to pubspec.yaml

Known Issues

S3RELEASEPREFIX as a sub-path of S3LIVEPREFIX

When S3RELEASEPREFIX is a child path of S3LIVEPREFIX (for example LIVEPREFIX=web, RELEASEPREFIX=web/releases), a naive live sync can accidentally delete immutable releases.

Symptom: after publishing, older immutable releases under web/releases/<version>/ are missing their files.

Fix: releasewebs3.py explicitly excludes releases/** during live sync and live refresh operations. The script also warns when it detects nested prefix configuration.

Flutter Web Output Drift

The fingerprint helper rewrites generated Flutter artifacts, not source templates. A Flutter SDK upgrade can change:

• bootstrap structure
• runtime asset lookup paths
• manifest formats
• canvaskit/skwasm layout

After upgrading Flutter in any project that uses this skill, rebuild once and run the fingerprint verifier before publishing.

Verification

For a shared-skill smoke test after flutter build web, run:

uv run python .agents/skills/release-flutter-web-s3/scripts/fingerprint_web_build.py build/web --manifest-out build/web_fingerprint_manifest.json
uv run python .agents/skills/release-flutter-web-s3/scripts/verify_fingerprint_web_build.py build/web --manifest build/web_fingerprint_manifest.json

Resources

scripts/

• releasewebs3.py: main release entry point; handles versioning, validation, upload, publish, and promote
• fingerprintwebbuild.py: fingerprint cacheable Flutter Web output files and rewrite internal references
• verifyfingerprintweb_build.py: verify that a fingerprinted Flutter Web build is internally consistent
• preparewebrelease.py: inspect commits since the latest web-v* tag, suggest a SemVer version, generate web-v<version>, draft release notes, and optionally update pubspec.yaml
• references/awscli-setup.md: local setup guide for installing AWS CLI, obtaining ak/sk, creating a named profile, and using that profile with releasewebs3.py