Top level date parsing fails on client

[SebbeBroman]

What version of Elysia is running?

1.4.28

What platform is your computer?

Darwin 25.3.0 arm64 arm

What environment are you using

Bun 1.3.11

Are you using dynamic mode?

Same behavior in both

What steps can reproduce the bug?

import { Elysia } from 'elysia'
import { treaty } from '@elysiajs/eden'

export const app = new Elysia()
	.get('myroute', () => new Date(), {})
	.listen(3000)

const apiRPC = treaty(app)
const api = treaty<typeof app>('localhost:3000')

const { data: rpcData } = await apiRPC.myroute.get()
console.log(rpcData)
console.log(rpcData?.getFullYear())

const { data: apiData } = await api.myroute.get()
console.log(apiData)
console.log(apiData?.getFullYear())

2026-03-20T07:56:11.000Z
2026
ArrayBuffer(66) [ 70, 114, 105, 32, 77, 97, 114, 32, 50, 48, 32, 50, 48, 50, 54, 32, 48, 56, 58, 53, 54, 58, 49, 49, 32, 71, 77, 84, 43, 48, 49, 48, 48, 32, 40, 67, 101, 110, 116, 114, 97, 108, 32, 69, 117, 114, 111, 112, 101, 97, 110, 32, 83, 116, 97, 110, 100, 97, 114, 100, 32, 84, 105, 109, 101, 41 ]
12 | console.log(rpcData)
13 | console.log(rpcData?.getFullYear())
14 |
15 | const { data: apiData } = await api.myroute.get()
16 | console.log(apiData)
17 | console.log(apiData?.getFullYear())
                          ^
TypeError: apiData?.getFullYear is not a function. (In 'apiData?.getFullYear()', 'apiData?.getFullYear' is undefined)

What is the expected behavior?

That the results are the same if you pass the app or the connection url.

What do you see instead?

No response

Additional information

When passing the app instance to treaty it parses the date correctly, however, passing the url the date is parsed correctly resulting in an arraybuffer.
It seems like when its a bare date elysia/bun returns it as a application/octet-stream with the date string as bytes.

Have you try removing the node_modules and bun.lockb and try again yet?

No response

[SebbeBroman]

Sorry, maybe this issue fits better under the eden repo?

[SaltyAom]

yeah, this is more likely an eden problem

[SebbeBroman]

Should I close this here and reopen? I tried looking into it a bit, the only solution I could find was to decode octet-stream and try to parse it as a date, that seems like it could have some performance overhead and maybe it's not worth supporting parsing raw dates.

[computer4000]

Root Cause

When using treaty(app) (local), Elysia returns the actual Date object. When using treaty(url) (remote), the response is serialized as JSON, then Eden needs to deserialize it.

The issue: Bun serializes Date as ISO string, but sends it with Content-Type: application/octet-stream instead of application/json, so Eden's JSON parser doesn't run.

Fix in Eden

Eden Treaty needs to handle Date parsing in the response transformer:

// In Eden's response handling
if (typeof data === 'string' && ISO_DATE_REGEX.test(data)) {
  return new Date(data)
}

Workaround for Now

Return dates explicitly as ISO strings and parse on client:

.get('/myroute', () => ({ date: new Date().toISOString() }))

Or use a custom response serializer that handles Dates.