tail_sampling: "Unexpected sampling decision"

[jgournet]

I've got this fairly sample tail_sampling configuration:

        processors:
          tail_sampling:
            #decision_wait: 30s
            #num_traces: 50000
            #expected_new_traces_per_sec: 0
            #decision_cache:
            #  sampled_cache_size: 100_000
            #  non_sampled_cache_size: 100_000
            policies:
              - name: status-code-sample
                type: status_code
                status_code:
                  status_codes:
                    - ERROR
              - name: latency-sample
                type: latency
                latency:
                  threshold_ms: 2000
              - name: percent-sample
                type: probabilistic
                probabilistic:
                  sampling_percentage: 1
              - name: drop-large-traces
                type: drop
                drop:
                  drop_sub_policy:
                    - name: drop-large-traces
                      type: span_count
                      span_count:
                        min_spans: 10001

idea is hopefully fairly obvious, but I want to:

• sample all ERROR traces
• sample all long traces (more than 2sec)
• sample 1% of other traces
• AND from all those sampled traces, drop any that have more than 10.000 spans

Without the "drop-large-traces", things are going fine, except for some traces that have several million spans, which break jaeger/elasticsearch

So I'm trying to drop any traces with more than 10.000 spans.

When I add that drop policy, I get some "Unexpected sampling decision" in logs.
Any idea why ?

2026-01-15T23:07:22.074Z warn    [email protected]/processor.go:620 Unexpected sampling decision    {"resource": {"service.instance.id": "3beabbb9-0c27-4270-b4f1-e7d5929ed526", "service.name": "otelcol-k8s", "service.version": "0.138.0"}, "otelcol.component.id": "tail_sampling", "otelcol.component.kind": "processor", "otelcol.pipeline.id": "traces/jaeger", "otelcol.signal": "traces", "decision": 4}