feat(workers-ai-provider): add custom fetch option for credentials mode

[threepointone]

Summary

Adds an optional fetch parameter to workers-ai-provider when using credentials mode (accountId + apiKey). This matches the pattern used by @ai-sdk/openai and @ai-sdk/anthropic, enabling request interception for testing, middleware, and advanced use cases like durable response buffering.

Motivation

The Agents SDK is building a durable response buffer that survives DO eviction during long-running inference calls (RFC #1257). The buffer works by intercepting the provider's HTTP call and routing it through a proxy that persists the streaming response. For OpenAI and Anthropic, this is done via their fetch option. Workers AI lacked this option, requiring a workaround (fake AI binding whose run() routes to the buffer).

With this change, Workers AI in credentials mode supports the same clean pattern:

const model = createWorkersAI({
  accountId: "...",
  apiKey: "...",
  fetch: myCustomFetch  // intercepts all HTTP calls
})("@cf/moonshotai/kimi-k2.5");

Changes

src/index.ts

• Added fetch?: typeof globalThis.fetch to the credentials branch of WorkersAISettings (not the binding branch — bindings don't use HTTP)
• Passed fetch through to createRun()
• TypeScript enforces that fetch can only be used with credentials, not with bindings

src/utils.ts

• Added fetch? to CreateRunConfig
• createRun resolves fetchFn = config.fetch ?? globalThis.fetch and uses it for both the main request and the streaming retry fallback
• createRunBinary is unchanged (used for transcription, separate concern)

test/utils.test.ts

• Added test: custom fetch is called instead of global fetch
• Added test: custom fetch is used for the streaming retry fallback path (when a model returns JSON instead of SSE, createRun retries with stream: false — both calls go through the custom fetch)

Type safety

The fetch option is in the credentials branch of the discriminated union, so TypeScript rejects { binding: env.AI, fetch: myFetch } at the type level. This prevents confusion — bindings bypass HTTP entirely, so a custom fetch would have no effect.

Test plan

• All 38 existing + new tests pass (npx vitest run test/utils.test.ts)
• Type check passes (npx tsc --noEmit)
• Custom fetch test verifies globalThis.fetch is NOT called
• Streaming retry test verifies custom fetch is used for both attempts

Made with Cursor

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🦋 Changeset detected

Latest commit: 1c6bdad

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

This PR includes changesets to release 1 package

Name	Type
workers-ai-provider	Patch

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