vectordotdev/vector · error

should not fail to encode output event

Error message

should not fail to encode output event

What it means

The output driver optionally encodes each received output event using the provided encoder (e.g. a codec configured for the test) to compute received-bytes metrics. Encoding is expected to succeed for any event that the topology produced, so an Err aborts with this expect. It indicates an encoder/event incompatibility rather than a test logic failure.

Solutions

1. Log the encoding error and the offending event (replace expect with unwrap_or_else) to see the incompatible value.
2. Choose a codec in the test's encoding config that supports all value types the component emits (e.g. JSON with fallible handling).
3. If a codec upgrade caused it, adjust the event normalization or pin/patch the codec version.

// ensure codec supports all value types the component can emit before the test
assert!(encoder_supports(&output_event), "codec cannot encode event");

encoder.clone().encode(output_event, &mut buffer)
    .unwrap_or_else(|e| panic!("encode failed for event {output_event:?}: {e}"));

When it happens

Trigger: spawn_output_driver receiving an event the configured encoder cannot serialize — e.g. a JSON/text codec asked to encode an event containing non-UTF-8 bytes, unsupported value types after a codec change, or a codec with strict serialization rules (e.g. Avro schema mismatch).

Common situations: Validation tests using an encoding config whose codec can't represent all event value types produced by the transform under test; codec library upgrades adding strictness; events containing binary/regex-derived values passed through a text encoder.

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

• should not fail to encode input event
• Timestamp out of range
• a sink must always have an external resource
• a source must always have an external resource
• breaking fragment ' ' has an invalid anchor ' '. Add ` }`…

AI-assisted analysis of vectordotdev/vector@bdb87aeaa4 (2026-09-16). Data as JSON: /api/errors/e02996d17fae0d60. Report an issue: GitHub.

Appendix: source

Thrown at src/components/validation/runner/mod.rs:693

                        output_events.extend(events.clone());

                        // Update the runner metrics for the received event. This will later
                        // be used in the Validators, as the "expected" case.
                        let mut output_runner_metrics = output_runner_metrics.lock().await;

                        if component_type != ComponentType::Sink {
                            for output_event in events {
                                // The event is wrapped in a Vec to match the actual event storage in
                                // the real topology
                                output_runner_metrics.received_event_bytes_total +=
                                    vec![&output_event].estimated_json_encoded_size_of().get() as u64;

                                if let Some(encoder) = maybe_encoder.as_ref() {
                                    let mut buffer = BytesMut::new();
                                    encoder
                                        .clone()
                                        .encode(output_event, &mut buffer)
                                        .expect("should not fail to encode output event");

                                    output_runner_metrics.received_events_total += 1;
                                    output_runner_metrics.received_bytes_total += buffer.len() as u64;
                                }
                            }
                        }
                        if output_events.len() >= expected_events {
                            info!("Output driver has received all expected events.");
                            break
                        }
                    } else {
                        // The channel closed on us.
                        // This shouldn't happen because in the runner we should not shutdown the external
                        // resource until this output driver task is complete.
                        error!("Output driver channel with external resource closed.");
                        break
                    }
                }

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