tracel-ai/burn · error

state of fusion optimization `{}` failed to decode: {err}

Error message

state of fusion optimization `{}` failed to decode: {err}

What it means

Decode guard on stored fusion-optimization state: `OptimizationState::decode::<T>` expects the CBOR bytes to deserialize into the requested type T. The panic fires when the stored state was written by a different optimization that happens to share the same `name`, or by an incompatible version of the same optimization, so the bytes do not match T's schema.

Source

Thrown at crates/burn-cubecl-fusion/src/optim/base.rs:181

            name: name.to_string(),
            state: {
                let mut bytes = Vec::new();
                ciborium::into_writer(state, &mut bytes)
                    .expect("fusion optimization state must be serializable");
                bytes
            },
        }
    }

    /// Decode the optimization's state.
    ///
    /// # Panics
    ///
    /// When the bytes don't decode as `T` — the state was produced by a
    /// different optimization sharing the name, or by an incompatible version.
    pub fn decode<T: DeserializeOwned>(&self) -> T {
        ciborium::from_reader(self.state.as_slice()).unwrap_or_else(|err| {
            panic!(
                "state of fusion optimization `{}` failed to decode: {err}",
                self.name
            )
        })
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn state_round_trips_through_bytes() {
        let state = CubeOptimizationState::new("custom", &vec![3u32, 7]);

        assert_eq!(state.name, "custom");
        assert_eq!(state.decode::<Vec<u32>>(), vec![3, 7]);
    }

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Decode with the exact state type the optimization originally serialized (matching name AND type).
2. Bump or namespace the optimization name when its state layout changes, so old states are not mistaken for new ones.
3. Delete the stale checkpoint/cache containing the incompatible state and let the optimization re-initialize.

When it happens

Trigger: Thrown at crates/burn-cubecl-fusion/src/optim/base.rs:181 when the library encounters an invalid state.

Common situations: See trigger scenarios.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/ce2fa9710ee1c36e. Report an issue: GitHub.