Date objects in forwarded tail events arrive as strings in local dev

[winklemad]

What versions & operating system are you using?

main (fb6b51b), Node v22.17.1, macOS 15. Reproduced against packages/miniflare in the monorepo; the core of it reproduces in plain Node with no Workers involved.

Please provide a link to a minimal reproduction

Inlined below — it's six lines of plain JS against the replacer as it's currently written, so a linked repo would only add noise.

const SERIALIZED_DATE = "___serialized_date___";
const replacer = (_, value) =>
	value instanceof Date ? { [SERIALIZED_DATE]: value.toISOString() } : value;

console.log(JSON.stringify({ scheduledTime: new Date(0) }, replacer));
// {"scheduledTime":"1970-01-01T00:00:00.000Z"}   <- no tag, so nothing to revive

Describe the Bug

When tail events are forwarded between two local dev sessions through the dev registry, Date values arrive at the tail consumer as ISO strings rather than Date objects. event.scheduledTime.getTime() throws in local dev, while the same tail handler works in production. TraceItemAlarmEventInfo.scheduledTime is typed Date, so the types promise something local dev doesn't deliver.

The forwarding path serialises with JSON.stringify(events, tailEventsReplacer) and revives with tailEventsReviver (packages/miniflare/src/workers/core/dev-registry-proxy-shared.worker.ts). The replacer is meant to tag Dates so the reviver can rebuild them:

export function tailEventsReplacer(_: string, value: any) {
	if (value instanceof Date) {
		return { [SERIALIZED_DATE]: value.toISOString() };
	} else if (typeof value === "bigint") {
		return { [SERIALIZED_BIGINT]: value.toString() };
	}
	return value;
}

JSON.stringify() applies Date.prototype.toJSON() before it calls the replacer, so a real Date reaches the replacer already flattened to an ISO string. value instanceof Date is therefore never true for it, no tag is written, and the reviver has nothing to restore. The bigint arm is unaffected, which is why that half of the round-trip works.

Worth noting the instanceof check isn't dead: a value whose own toJSON() returns a Date reaches the replacer unconverted and is tagged correctly today, so that behaviour should be preserved rather than swapped out.

packages/vite-plugin-cloudflare/src/workers/vite-proxy-worker/index.ts carries its own copy of the pair with the same Date behaviour. The two aren't identical — the vite copy has no bigint handling — so they're the same Date issue in two places rather than one shared implementation.

This is a local-dev fidelity problem (dev diverging from production), not a production issue.

Please provide any relevant error logs

AssertionError: expected '2025-05-01T12:34:56.000Z' to be an instance of Date

I have a fix with tests and will open a PR shortly.

[workers-devprod]

• Reproduction: ✅ Successfully reproduced. JSON.stringify applies Date.prototype.toJSON() before tailEventsReplacer, so value instanceof Date is never true. Minimal repro shows Date round-trips as a string, not a Date object; bigint works correctly.

[workers-devprod]

Note

This is an automated, advisory triage report generated by workers-devprod. It is not an official maintainer response — a maintainer will follow up.

🤖 Automated triage report

Issue Triage: #15180

Summary

In local dev, tail events forwarded between dev sessions via the dev registry deliver Date values (e.g. scheduledTime) as ISO strings instead of Date objects, because JSON.stringify applies Date.prototype.toJSON() before tailEventsReplacer runs, so the value instanceof Date tag is never added and the reviver can't rebuild the Date.

Classification

• Type: Bug
• Component: packages/miniflare
• Severity: Medium
• Has Reproduction: Yes (inline minimal repro)
• Quality: Complete

Findings

• Created: not available in pre-fetched data (issue body truncated at 2000 chars by the read tool; only the body prefix was retrievable)
• Author: winklemad (Madan kumar)
• Version: main (fb6b51b), Node v22.17.1, macOS 15
• Labels: not available in pre-fetched data
• Comments: not available in pre-fetched data

Analysis

• Component: Clearly packages/miniflare. The reporter cites the exact file packages/miniflare/src/workers/core/dev-registry-proxy-shared.worker.ts and the tailEventsReplacer/tailEventsReviver pair used on the dev-registry tail-event forwarding path.
• Root cause is correctly diagnosed: JSON.stringify() invokes Date.prototype.toJSON() on a Date before the replacer sees the value, so the replacer receives an ISO string, not a Date. The value instanceof Date branch never fires, no ___serialized_date___ tag is emitted, and tailEventsReviver has nothing to revive. Result: event.scheduledTime is a string in local dev, so .getTime() throws — while the same handler works in production and TraceItemAlarmEventInfo.scheduledTime is typed Date.
• Reproducibility: High. The inline six-line plain-Node snippet demonstrates the flattening behavior without any Workers involvement; no external repo is needed.
• Severity: Medium — it's incorrect behavior (local dev diverges from production and from the declared type, causing a throw), but scoped to tail-event forwarding through the dev registry between local dev sessions.
• Difficulty: Easy — the fix is contained to the serialization logic (e.g. serialize Dates before JSON.stringify flattens them, or have the reviver detect ISO date strings for known Date-typed fields; note the parallel bigint branch already works because bigint has no toJSON).

Recommendation

Status: KEEP OPEN

Reasoning: This is a genuine, well-diagnosed bug in miniflare's dev-registry tail-event serialization with a minimal reproduction and a clear fix path. It is not spam, a duplicate, stale, or user error, and it has sufficient information to act on.

Action: Confirm the root cause in dev-registry-proxy-shared.worker.ts and fix the Date serialization so the replacer tag survives (or the reviver can rebuild it), then add a regression test covering a Date-valued tail event round-trip. A changeset is required.

Suggested Labels: (none — labels were not available in the pre-fetched data; omitting rather than guessing)

Suggested Comment

Thanks for the detailed write-up and the minimal repro — the diagnosis looks right. JSON.stringify calls Date.prototype.toJSON() before it invokes tailEventsReplacer, so the replacer receives an already-flattened ISO string and the value instanceof Date branch never runs, which is why nothing gets tagged for tailEventsReviver to rebuild. We'll fix the serialization on the dev-registry tail-event path so Date values survive the round-trip and match production behavior. If you'd like to open a PR, that'd be very welcome.

Structured summary (JSON)

{
  "issueNumber": 15180,
  "title": "Tail events forwarded via the dev registry deliver Date values as ISO strings instead of Date objects in local dev",
  "githubUrl": "https://github.com/cloudflare/workers-sdk/issues/15180",
  "recommendation": "KEEP OPEN",
  "difficulty": "easy",
  "reasoning": "Well-diagnosed miniflare bug: JSON.stringify applies Date.prototype.toJSON before tailEventsReplacer runs, so Date tags are never emitted and the reviver can't rebuild them. Includes a minimal reproduction and a clear fix path.",
  "suggestedAction": "Fix Date serialization in dev-registry-proxy-shared.worker.ts so Dates survive the replacer/reviver round-trip, add a regression test, and include a changeset.",
  "hasSuggestedComment": true,
  "suggestedLabels": []
}