feat: support split edge functions

[teemingc]

This PR removes the arbitrary limitation where Netlify edge functions cannot be split.

Note: Netlify split edge CI job won't run until it's in the main branch

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 77b8cd9:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/77b8cd901e3dc73adfdbcc67517452b728afbd3d

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/17022

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🦋 Changeset detected

Latest commit: 77b8cd9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-netlify	Minor

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📝 Walkthrough

Walkthrough

The Netlify adapter now supports split and edge together. It discovers split routes, builds named Edge Functions, and writes aggregate Edge configuration. The Edge entry point now exports init(server). Deployment automation and platform tests cover the combined mode. Test routes and configuration validation verify generated function names. Documentation, declarations, and release metadata reflect the new support.

Sequence Diagram(s)

sequenceDiagram
  participant GitHubActions
  participant NetlifyAdapter
  participant Netlify
  GitHubActions->>NetlifyAdapter: build split deployment with EDGE=true
  NetlifyAdapter->>NetlifyAdapter: discover routes and generate Edge Functions
  NetlifyAdapter->>Netlify: emit functions and Edge path configuration
  GitHubActions->>Netlify: deploy split Edge app
  Netlify-->>GitHubActions: return deployment URL

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Merge Risk: 🟡 Moderate · up to 5e905

Netlify split Edge support is implemented, but the updated split test can fail for serverless builds before validating generated functions. The assertion should be conditioned on deployment mode before merging.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly states that the PR adds support for splitting Netlify Edge Functions and removes the previous limitation. It also identifies the CI timing constraint and includes relevant chec…
Title check	✅ Passed	The title clearly and concisely summarises the main change: support for split Netlify Edge Functions.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description clearly states that the PR adds support for splitting Netlify Edge Functions and removes the previous limitation. It also identifies the CI timing constraint and includes relevant checklist and test information.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch teemingc-netlify-edge-split

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Preview: https://svelte-dev-git-preview-kit-17022-svelte.vercel.app/

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Actionable comments posted: 2

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Inline comments:
In @.github/actions/netlify-deploy/action.yml:
- Line 38: Add a build step before the netlify deploy command in the action,
invoking the split fixture’s existing package build with EDGE=true so
vite.config.ts generates the build output before deployment. Preserve the
current deployment directory and configuration.

In `@packages/adapter-netlify/index.js`:
- Around line 191-192: Update the function-name generation near the name
declaration to track generated names separately from the seen route-pattern set.
When the normalized name derived from parts collides with an existing generated
name, append a deterministic unique suffix before writing output files or
emitting add_edge_function_config entries; preserve the existing name for
non-colliding routes.

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• .changeset/green-pens-split.md
• .github/actions/netlify-deploy/action.yml
• .github/workflows/platform-tests-netlify.yml
• documentation/docs/25-build-and-deploy/80-adapter-netlify.md
• packages/adapter-netlify/files/edge.js
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• packages/adapter-netlify/test/apps/split/vite.config.ts

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Inline comments:
In `@packages/adapter-netlify/test/apps/split/test/test.js`:
- Around line 30-31: Update the configuration assertion around functions and
names so it branches on EDGE: for Edge builds, retain the existing
config.functions/edge_functions validation; for serverless builds, avoid mapping
an absent functions property and instead verify the generated
.netlify/v1/functions/*.mjs files.

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[Nic-Polumeyv]

I don't think we should key this header on NETLIFY_FUNCTIONS_TOKEN. It's injected by the Lambda runtime and isn't documented for either runtime, so we don't know whether Deno gets it. If it's undefined the header becomes x-sveltekit-original-url-undefined, reroute keeps working, and any client can set event.url (origin included) on every split edge function. The split edge job only runs post-merge, so nothing here would catch it.

I'd mint the secret at build time and inline it into both templates

const original_url_header = `x-sveltekit-original-url-${crypto.randomUUID()}`;

and only take pathname and search from the header, same as __pathname in adapter-vercel's serverless.js:

const original = new URL(request.headers.get(original_url_header));
request = new Request(new URL(original.pathname + original.search, request.url), request);                                      

Also, Netlify's edge functions guide says to avoid same-site fetch() from edge functions. A native rewrite with the original URL (in a reserved query param) would avoid this hop.