Pumpkin-MC/Pumpkin · error · WritingError

Failed to serialize packet

Error message

Failed to serialize packet: {0}

What it means

WritingError::Message(String) is the generic fallback variant of the writing error enum, used for packet-level serialization failures that are not IO, not serde-custom, and not version gating — for example a packet implementation explicitly reporting that it could not serialize its own fields. It carries a free-form description of what failed.

Solutions

1. Read the message text — it is produced by the packet implementation and names the failure.
2. Ensure all required fields are populated before writing the packet.
3. Check for newer library versions if the failing packet is a built-in one; the implementation may have a fix.
4. If you wrote the packet type, return a more specific WritingError variant instead of a bare string.

Example fix

// before: packet built with a required field left default
let p = Packet::default();
p.write(stream).await?; // "failed to serialize packet: missing field"

// after: construct explicitly with all fields
let p = Packet { id, state, data: payload };
p.write(stream).await?;

// ensure every required packet field is set before writing
fn packet_ready(p: &Packet) -> bool { p.id != 0 && !p.data.is_empty() }

match packet.write(&mut stream).await {
    Err(WritingError::Message(msg)) => {
        error!("packet serialization failed: {msg}");
        // fix construction site; connection can usually stay open
    }
    other => /* ... */,
}

When it happens

Trigger: A packet's write/serialize implementation constructs Message directly when its internal encoding steps fail (e.g. malformed state machine, missing required sub-field, or a wrapper that flattens heterogeneous encode errors into the writing error).

Common situations: Custom or third-party packet implementations reporting internal failures; misuse of packet-builder APIs leaving required fields unset; wrappers converting lower-level errors into a string and re-raising as Message.

Understand the failure class

Background: "JSON serialization failed", "not JSON serializable", "Failed to serialize": why JSON marshaling errors happen and how to fix them — this error's family across 46 libraries.

Related errors

• Serde failure
• {0}
• JSON parse error
• Invalid slot
• Player ' ' tried to interact with a closed container

AI-assisted analysis of Pumpkin-MC/Pumpkin@8d4639e25a (2026-09-09). Data as JSON: /api/errors/35354a75443ce7a8. Report an issue: GitHub.

Appendix: source

Thrown at crates/pumpkin-protocol/src/ser/mod.rs:45

    #[error("{0}")]
    Message(String),
}

impl serde::de::Error for ReadingError {
    fn custom<T: std::fmt::Display>(msg: T) -> Self {
        Self::Message(msg.to_string())
    }
}

#[derive(Debug, Error)]
pub enum WritingError {
    #[error("IO error: {0}")]
    IoError(#[from] std::io::Error),
    #[error("Serde failure: {0}")]
    Serde(String),
    #[error("Packet is not supported in Minecraft version {0:?}")]
    UnsupportedVersion(JavaMinecraftVersion),
    #[error("Failed to serialize packet: {0}")]
    Message(String),
}

impl serde::ser::Error for WritingError {
    fn custom<T: std::fmt::Display>(msg: T) -> Self {
        Self::Serde(msg.to_string())
    }
}

struct NetworkReadDataSource<'a, R: NetworkReadExt + ?Sized>(&'a mut R);

impl<'a, R: NetworkReadExt + ?Sized> pumpkin_nbt::deserializer::NbtDataSource<'a>
    for NetworkReadDataSource<'a, R>
{
    fn read_u8(&mut self) -> Result<u8, pumpkin_nbt::Error> {
        self.0.get_u8().map_err(|e| {
            pumpkin_nbt::Error::Incomplete(std::io::Error::new(
                std::io::ErrorKind::UnexpectedEof,

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