bug: diagnose whitespace-only LLM responses from third-party relays

[ZhulongNT]

Summary

GenericAgent can receive a syntactically valid but semantically empty completion: for example, the response payload contains one text block whose value is a single whitespace character (" "). The TUI then shows:

· LLM returned an empty response. Retrying...

The retry succeeds in the observed case, so this is recoverable, but it is confusing to diagnose.

Observation

• Session trace: model_responses_858881.txt, 2026-09-21 16:27:46.
• The response was represented as a Python object equivalent to:

  [{"type": "text", "text": " "}]

• There was no meaningful content or thinking; GA correctly treated it as blank and regenerated.
• The relevant guard is ga.py / do_no_tool() (currently around lines 459–467). It emits [Warn] LLM returned an empty response. Retrying... and routes through _retry_or_exit().
• The TUI replaces [Warn] with ·, which makes the displayed line look less like an actionable warning.

Important context: third-party relays / gateways

This has been observed when using a non-official API relay/gateway rather than the model provider's official endpoint. The relay may return an HTTP-successful response while dropping, truncating, or normalizing streamed content to whitespace. This is an observation, not a claim that every relay is faulty; the same symptom could also originate upstream. However, intermediaries make the failure mode substantially harder to attribute because the request technically succeeds.

There is a related lower-level path in llmcore.py where a stream produces no output and is retried as ConnectionError("empty response"), but the whitespace-only case reaches the agent-level blank-response guard instead.

Why this deserves a separate issue

#201 added the useful retry limit (retry up to three times, then stop). It does not expose enough context to distinguish:

1. a provider-side empty completion,
2. a relay/gateway that returned a malformed or whitespace-only successful response, and
3. a stream that ended before producing any content.

Suggested improvements

1. Keep the current safe retry behavior.
2. Log a compact, redacted diagnostic for blank responses: session/model name, configured base URL host (no credentials), response block types, text lengths / whitespace-only flag, whether any thinking was present, and retry count.
3. Make the TUI warning retain a visible severity label, e.g. · [Warn] Empty LLM response; retrying (1/3).
4. Optionally classify whitespace_only, no_stream_output, and empty_payload separately in the trace/logs.
5. Documentation: recommend reproducing against the official endpoint before filing provider-specific issues when a third-party relay is in the path.

This should make relay-related incidents diagnosable without weakening compatibility with OpenAI-compatible gateways.