[BUG] Queue: typed `MessageBatch<T>` deserialize via `serde_wasm_bindgen::from_value` mangles payloads — offer a JSON-based path

[Ujang360]

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

The typed queue-consumer path (MessageBatch<T> / Message::try_from(RawMessage)) deserializes each body with serde_wasm_bindgen::from_value. For real-world payloads we found its V8↔serde type coercion unreliable — it produces wrong data or an opaque deserialize error. Bypassing it with JSON.stringify(body) + serde_json::from_str works reliably for the exact same payloads.

Environment

• worker 0.8.4, wasm32-unknown-unknown, Rust 1.96.
• Payload: a struct with serde_json::Values and tuples, e.g. substitutes: Vec<(String, serde_json::Value)>, plus Vec<String>, strings.

Reliable workaround (consumer side)

use js_sys::JSON;
fn parse<T: serde::de::DeserializeOwned>(body: &JsValue) -> Result<T, String> {
    let s = JSON::stringify(body).ok().and_then(|s| s.as_string())
        .ok_or("JSON.stringify produced no string")?;
    serde_json::from_str(&s).map_err(|e| e.to_string())
}

Round-trips through plain JSON and Just Works where serde_wasm_bindgen::from_value mangled the data. (This is also what the previous generation of our consumer hand-rolled — the footgun is long-standing.)

Asks

• Document the limitation + this JSON.stringify + serde_json escape hatch in the Queues guide — it was the single biggest footgun we hit.
• Consider a content_type-aware / JSON-based deserialize helper on MessageBatch / Message, e.g. msg.body_json::<T>(), that uses JSON.stringify + serde_json for json-typed bodies instead of serde_wasm_bindgen::from_value.
• More generally: surface the message's content_type to the consumer so it can choose the right decoder.

Steps To Reproduce

// worker-0.8.4/src/queue.rs
impl<T> TryFrom<RawMessage> for Message<T> where T: DeserializeOwned {
    fn try_from(value: RawMessage) -> Result<Self, Error> {
        let body = serde_wasm_bindgen::from_value(value.body())?; // <-- V8 coercion
        ...
    }
}

serde_wasm_bindgen's coercion misbehaves on serde_json::Value, tuples (Vec<(String, Value)>), maps, and number / undefined edge cases.

Related to #1012

[SameerVers3]

Also hitting this.

serde_wasm_bindgen::from_value breaks on serde_json::Value fields and tuples in queue payloads.

The json.stringify + serde_json::from_str workaround works, but a built-in body_json::() helper plus exposing the message's content_type would be much cleaner.