fix(providers): refuse an empty Responses input before sending instead of retrying a guaranteed 400

[waleedlatif1]

Summary

• An Agent block whose system and user messages are absent or resolve to empty (e.g. <start.input> is "") sent the OpenAI Responses API input: [], because empty-content messages are dropped when the input is built. OpenAI rejects that with a 400 on every attempt, yet it surfaced as a plain internal error, logged at ERROR, and burned block retries and model fallbacks
• executeResponsesProviderRequest now refuses an empty input before any request is sent, throwing EmptyProviderInputError — a UserFailure (classified as the author's to fix) with retryable = false (skips block retries and the fallback chain)
• The message tells the author the Agent has nothing to send and to add a system/user message or check the referenced value
• Applies to every Responses caller (OpenAI, Azure OpenAI, Kie); none use previous_response_id, prompt, or conversation, so an empty input could never succeed on any of them
• Unchanged: system-prompt-only requests, whitespace-only content (only zero-length content is dropped, as before), file-only user messages, tool-loop continuation turns

Type of Change

• Bug fix

Testing

• Extended providers/openai/core.response-status.test.ts: empty system prompt + empty messages rejects with the user failure (isNonRetryableExecutionError true, classifyFailure → user) and never calls fetch; a system-prompt-only request still sends [{ role: 'system', … }]
• Red on the pre-fix code (removing the guard yields ProviderError: OpenAI API error (400)), green with it
• OpenAI, Azure OpenAI and Kie provider tests pass (154 tests)

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Oct 18, 2026 9:05pm UTC

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Turn on auto-fix | Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

Summary

Rejects empty Responses input before sending a request.

• Responses requests with no input stop before they reach the provider.

_{Reviews (1) · Last reviewed commit: "fix(providers): refuse an empty Response..." · Reviewed by Greptile}