Every empty-response retry re-sends the full conversation input at full
price. On large contexts a single turn that produces no visible output
could bill the user several dollars across the 3-retry + fallback-chain
walk (reported: ~$2.33 for one empty answer on a ~26K-token session).
Signaled refusals (finish_reason=content_filter, Anthropic refusal
stop_reason, guardrail interventions) are already terminal today and
never reach this loop. The uncovered class is *unsignaled* refusals:
the provider returns 200 with zero output tokens and a generic finish
reason. Those are deterministic — resending the identical prompt
reproduces the same empty — so burning the remaining retry budget only
multiplies the charge.
New agent/empty_response_guard.py, two independent guards, both failing
OPEN to today's behaviour:
- Deterministic-empty detection: two consecutive empty attempts with
usage present, output_tokens == 0 (reasoning tokens count as output),
and identical (model, provider, finish_reason) skip the remaining
retries and go straight to the fallback chain — a different model may
well answer. Missing usage, nonzero output, or any signature change
keeps the full budget.
- Cost-aware retry budget: when one attempt's estimated input cost
exceeds HERMES_EMPTY_RETRY_COST_THRESHOLD_USD (default $0.25), the
empty-retry budget drops 3 -> 1 for that streak. Unknown pricing or
included/subscription routes are untouched.
At exhaustion the status trace now includes the estimated cost of the
empty attempts so the charge is at least explained in-session.
Streak state lives on the agent and self-clears whenever
_empty_content_retries resets to 0, transparently honouring every
existing reset site (turn start, tool success, compaction, fallback
activation) without touching them.
Set HERMES_DETERMINISTIC_EMPTY_GUARD=0 to disable both guards.
Tests: tests/agent/test_empty_response_guard.py (26 unit tests) plus
two loop-level integration tests in tests/run_agent/test_run_agent.py
proving the api_call reduction and the fail-open path.
Refs NS-503.
Shannon Sands
ac06c2ff8b