What causes "Failed to serialize core event info to JSON" in dbt-labs/dbt-core?

Inspect the custom envs fed to JsonCompatLayer and ensure they are JSON-safe values (string keys, no NaN/Infinity)

Failed to serialize core event info to JSON

Error message

Failed to serialize core event info to JSON

What it means

The JSON compat layer builds a dbt-core-style CoreEventInfo struct for a progress/log event and converts it with serde_json::to_value, unwrapping via .expect. Panicking here means serde_json could not serialize the struct, an internal invariant violation since all CoreEventInfo fields are plain strings/ints plus an `extra` map of custom env values.

Source

Thrown at crates/dbt-common/src/tracing/layers/json_compat_layer.rs:1085

                // In debug build panic for unknown codes to catch missing mappings, in prod just skip
                #[cfg(debug_assertions)]
                panic!(
                    "Unhandled dbt_core_event_code '{}' in ProgressMessage. Add mapping in JsonCompatLayer `emit_progress_message` function.",
                    progress_msg.dbt_core_event_code.as_deref().unwrap()
                );
                #[cfg(not(debug_assertions))]
                return;
            }
            None => (None, None), // Fall back to generic handling
        };

        let info_json = serde_json::to_value(self.build_core_event_info(
            event_code,
            event_name,
            &log_record.severity_text,
            msg.clone(),
        ))
        .expect("Failed to serialize core event info to JSON");

        let mut data_obj = json!({
            "msg": msg,
        });

        // Include unique_id in node_info if available
        if let Some(unique_id) = progress_msg.unique_id.as_deref() {
            data_obj.as_object_mut().unwrap().insert(
                "node_info".to_string(),
                json!({
                    "unique_id": unique_id
                }),
            );
        }

        let value = json!({
            "info": info_json,
            "data": data_obj

View on GitHub (pinned to 0267ce9170)

Solutions

Inspect the custom envs fed to JsonCompatLayer and ensure they are JSON-safe values (string keys, no NaN/Infinity)
Replace the .expect with graceful handling that logs the event without the info block instead of aborting the run
Reproduce with RUST_BACKTRACE=1 and check which CoreEventInfo field fails; fix the type at the source

Example fix

// before
let info_json = serde_json::to_value(self.build_core_event_info(
    event_code,
    event_name,
    &log_record.severity_text,
    msg.clone(),
))
.expect("Failed to serialize core event info to JSON");
// after
let info_json = serde_json::to_value(self.build_core_event_info(
    event_code,
    event_name,
    &log_record.severity_text,
    msg.clone(),
))
.unwrap_or(serde_json::Value::Null);

Defensive patterns

Strategy: fallback

Validate before calling

fn is_json_safe(v: &serde_json::Value) -> bool {
    use serde_json::Value;
    match v {
        Value::Null | Value::Bool(_) | Value::String(_) | Value::Number(_) => true,
        Value::Array(a) => a.iter().all(is_json_safe),
        Value::Object(m) => m.iter().all(|(_k, v)| is_json_safe(v)),
    }
}
// assert every custom env value is_json_safe before constructing JsonCompatLayer

Try / catch

let info_json = serde_json::to_value(info).unwrap_or(serde_json::Value::Null);

Prevention

Only pass JSON-serializable custom env values (string keys, finite numbers) to the tracing layer
Add a round-trip unit test serializing CoreEventInfo in CI
Prefer unwrap_or(Value::Null) + warn over .expect in logging hot paths

When it happens

Trigger: on_log_record -> emit_progress_message runs while the JSON compat layer is active (json/machine output format) and the CoreEventInfo payload fails to serialize — practically only when a custom env entry injected into `extra` is not JSON-representable (non-string map key or non-finite float like NaN) or struct invariants are broken by a code change.

Common situations: Running dbt with --log-format json / machine output in CI; injecting custom env vars for log enrichment containing non-finite floats or exotic types; upgrading dbt-common after touching the tracing code.

Understand the failure class

Background: json.Marshal / "failed to marshal" errors in Go: why "unsupported type" happens and how to fix it — this error's family across 22 libraries.

Related errors

AI-assisted analysis of dbt-labs/dbt-core@0267ce9170 (2026-09-07). Data as JSON: /api/errors/fba2eba8a70b9769. Report an issue: GitHub.