[miniflare] Lazily initialize remote binding RPC sessions

[razethion]

Fixes #15351.

Remote bindings use two paths through Miniflare's shared proxy client:

• fetch() requests use HTTP.
• RPC method calls use a WebSocket RPC session through Cap'n Web.

The proxy client previously created the RPC session eagerly in its constructor. This happened even for fetch-only bindings such as D1 and R2. Their successful requests continued through HTTP, while the unused RPC session could fail later and emit delayed internal error; reference = ... messages.

This change creates and caches the RPC session only when an RPC method is called. Fetch-only bindings therefore no longer open an unrelated WebSocket connection. RPC bindings continue to use one shared session when needed.

Investigation

RPC support for remote bindings was introduced in #10249. The intended design was to keep methods implemented by the proxy worker, particularly fetch(), on their direct path and forward other methods through an RPC stub. The original review discussion notes that direct fetch requests were intentionally kept outside JSRPC because that path is more efficient.

However, the fallback RPC stub was created before the proxy knew whether it would be used. #10724 later replaced the original JSRPC implementation with Cap'n Web while preserving this eager session lifecycle.

Simply delaying initialization until an unknown property is accessed is not sufficient because workerd probes optional WorkerEntrypoint handlers during startup. The proxy now returns a deferred method for unknown properties and initializes the RPC session only if that method is invoked.

This explains why affected D1 requests still returned 200: the request used the HTTP path successfully, while the separate unused RPC session produced the delayed errors.

Validation

The new Miniflare tests verify that:

• A fetch-only remote binding makes its HTTP request without opening an RPC WebSocket.
• Calling a real RPC method opens one WebSocket session and returns the expected result.

A packaged alpha build was also installed in the affected application and tested with the original remote D1 reproduction. The delayed internal-error flood no longer occurred.

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: This fixes internal remote-binding proxy behavior without changing the public API or configuration.

---

[changeset-bot]

🦋 Changeset detected

Latest commit: 7c17adf

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

This PR includes changesets to release 8 packages

Name	Type
@cloudflare/vite-plugin	Patch
@cloudflare/vitest-plugin	Patch
miniflare	Patch
wrangler	Patch
@cloudflare/deploy-helpers	Patch
@cloudflare/pages-shared	Patch
@cloudflare/remote-bindings	Patch
@cloudflare/runtime-types	Patch

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[workers-devprod]

Codeowners approval required for this PR:

• @cloudflare/wrangler

Show detailed file reviewers

• .changeset/clean-dodos-travel.md: [@cloudflare/wrangler]
• packages/miniflare/src/workers/shared/remote-proxy-client.worker.ts: [@cloudflare/wrangler]
• packages/miniflare/test/plugins/shared/remote-bindings-proxy-client.spec.ts: [@cloudflare/wrangler]

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commit: 7c17adf