[BUG] Queue: Queue::send default content_type changed v8 → json (0.7 → 0.8) — silent cross-version message drops #1012

Is there an existing issue for this?

• I have searched the existing issues

What version of workers-rs are you using?

0.8.4

What version of wrangler are you using?

4.100.0

Describe the bug

Summary

In worker 0.8.x, Queue::send defaults outgoing messages to content_type = Json. A consumer written against the 0.7.x default (which delivered a structured-clone object body) silently drops these messages: send() returns Ok, the consumer batch returns Ok, no exception, no log, zero messages processed. During a gradual upgrade — or when one queue is shared by services on different worker versions — this breaks invisibly.

Environment

• worker 0.8.4 producer interoperating with a 0.7.x consumer over a Cloudflare Queue.
• wasm32-unknown-unknown, Rust 1.96, worker-build 0.8.4.

Where it comes from

impl<T: Serialize> From<T> for SendMessage<T> hard-codes the default:

// worker-0.8.4/src/queue.rs
impl<T: Serialize> From<T> for SendMessage<T> {
    fn from(message: T) -> Self {
        Self { message, options: Some(QueueSendOptions {
            content_type: Some(QueueContentType::Json), // <-- default (was v8 in 0.7.x)
            delay_seconds: None,
        })}
    }
}

A consumer expecting the old structured-clone object typically does:

let body = raw_message.body();      // JsValue
if !body.is_object() { continue; }  // a `json`-typed body fails this → SILENTLY skipped

Why this was painful

The wire format changed across a minor-version bump with no error and no obvious note. It took a long debugging session to find, because every layer reports success.

Asks

• Call out the default-content_type change prominently in the CHANGELOG / a 0.7→0.8 migration note — it's a wire-format behavior change, not just an API change.
• Consider keeping the prior default, or at least making the default a clearly-documented, pinnable constant, with a note that producer and consumer must agree on content_type.
• Optionally: give consumers a way to detect a content_type their decoder didn't expect, instead of silently yielding a non-object body.

Steps To Reproduce

1. Producer (0.8.x): queue.send(MyMsg { .. }).await? — default content_type = Json.
2. Consumer (0.7.x, or anything expecting a structured-clone object): if !body.is_object() { continue }.
3. Observe: producer Ok, consumer Ok, 0 messages processed, no error anywhere.

Workaround (producer side):

queue.send(MessageBuilder::new(msg).content_type(QueueContentType::V8).build()).await?;

Related to #1013