feat(miniflare): add CF-Worker header to outgoing fetch requests

[penalosa]

Fixes #4367.

Adds the CF-Worker header to outgoing fetch requests in Miniflare's local development environment to match production behavior. In production, Cloudflare adds this header to subrequests made by Workers, with the value set to the zone/domain.

The implementation reuses the existing strip-cf-connecting-ip wrapper worker that already intercepts outbound requests. A new optional zone option is exposed to allow specifying the header value. Wrangler now passes the zone value (extracted from the configured routes/host) to Miniflare automatically.

Header value behavior:

• If zone option is provided: uses that value (e.g., example.com)
• If zone is not provided but name is: uses ${name}.example.com (e.g., my-worker.example.com)
• If neither is provided: uses worker.example.com

Updates since last revision:

• Wrangler now passes the zone value to Miniflare via ConfigBundle.zone, extracted from dev.origin.hostname (which comes from routes configuration)

Human review checklist:

• Verify that event.config.dev?.origin?.hostname is the correct source for the zone value in Wrangler
• Consider if there are edge cases where the origin hostname might differ from the actual zone

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• 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 aligns local behavior with existing production behavior that is already documented.

Link to Devin run: https://app.devin.ai/sessions/3c5448d105c44a5fbbc723f94bbf35b8
Devin PR requested by @penalosa

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[changeset-bot]

🦋 Changeset detected

Latest commit: fd5fb0f

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[devin-ai-integration]

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create-cloudflare

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wrangler

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commit: fd5fb0f

[penalosa]

The Cf-worker header should be set to the zone. Miniflare should expose an (optional) option to pass in a zone, which Wrangler will use. Additionally, Miniflare should default cf-worker to ${worker-name}.example.com if no zone is passed in

[penalosa]

Devin, please make this change

[claude]

Claude finished @penalosa's task —— View job

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Changeset Review

Todo List:

• Read changeset guidelines from .changeset/README.md
• Read and validate .changeset/cf-worker-header.md
• Check version type appropriateness
• Verify changelog quality
• Check for markdown headers (h1/h2/h3)
• Verify no analytics concerns

---

✅ All changesets look good

The changeset .changeset/cf-worker-header.md is properly formatted:

• Version type: minor is appropriate for this new configuration option
• Changelog quality: Clear description with explanation of the new zone option and default behavior
• No markdown headers: No h1/h2/h3 headers present
• Package coverage: Correctly targets miniflare package
• Format: Follows the title + body format with good detail

[penalosa]

Devin, it doesn't look like Wrangler passes the zone value in this PR

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional flags.

[NuroDev]

Nit pick: Not needed if we're using satisfies ExportedHandler<Env>

Suggested change

	fetch(request, env: Env) {
	fetch(request, env) {

[devin-ai-integration]

Devin Review found 1 new potential issue.

View issue and 6 additional flags in Devin Review.

[devin-ai-integration]

🟡 CF-Worker header not added when stripCfConnectingIp is disabled

The CF-Worker header is only added to outbound requests when stripCfConnectingIp is true (the default). When a user explicitly sets stripCfConnectingIp: false, the CF-Worker header is not added, which doesn't match production behavior where CF-Worker is always added to subrequests.

Click to expand

Root Cause

The CF-Worker header logic is coupled with the strip-cf-connecting-ip worker at packages/miniflare/src/plugins/core/index.ts:851-853:

globalOutbound: options.stripCfConnectingIp
    ? { name: getStripCfConnectingIpName(workerIndex) }
    : getGlobalOutbound(workerIndex, options),

When stripCfConnectingIp is false, the strip-cf-connecting-ip worker is bypassed entirely, and the CF-Worker header (set at line 9 in strip-cf-connecting-ip.worker.ts) is never added.

Impact

Users who disable CF-Connecting-IP stripping for testing purposes will also lose the CF-Worker header on outbound requests, which may cause their local development environment to behave differently from production in unexpected ways.

Actual vs Expected

• Actual: When stripCfConnectingIp: false, no CF-Worker header is added
• Expected: CF-Worker header should be added regardless of the stripCfConnectingIp setting to match production behavior

(Refers to lines 851-853)

Recommendation: Consider decoupling the CF-Worker header logic from the CF-Connecting-IP stripping, or rename the worker to reflect its combined purpose and add a separate option for controlling the CF-Worker header.

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[penalosa]

This is fine. stripCfConnectingIp really means "this miniflare instance is a proxy and shouldn't do weird things to requests" which applies to both stripCfConnectingIp and adding a CF Worker header