[Bug]: DeepSeek PR review fails after three reasoning-only responses

Summary

The Codex PR Review workflow is routed to the DeepSeek review script for the current provider configuration. On PR #428, the configured deepseek-v4-flash model returned an empty final message.content on all three attempts, while returning substantial reasoning_content. The script failed before posting any GitHub review.

This is a review-model/output integration failure, not a code-review rejection or a failed product test. The deeper cause (provider behavior, thinking configuration, or output-budget exhaustion) is not yet proven by the retained logs.

Evidence

• Affected PR: #428
• Reviewed head: abf810e9e7f7bcfd3e1e1d915c817e81e144101d
• Failed workflow: https://github.com/OpenCoworkAI/open-codesign/actions/runs/35456727130
• Failed job / step: pr-review / Run DeepSeek for PR Review
• Time: 2026-09-19, approximately 17:00:52–17:02:36 UTC
• Configured model: deepseek-v4-flash; effort: high
• Response model field: deepseek-flash

Sanitized failure shape:

DeepSeek output invalid on attempt 1/3: DeepSeek API returned no message content
DeepSeek output invalid on attempt 2/3: DeepSeek API returned no message content
Error: DeepSeek API returned no message content
Process completed with exit code 1

The response previews show message.content: "" with non-empty reasoning_content. The workflow did not run the Codex action fallback and did not submit any review. Request credentials/endpoints are intentionally omitted here.

Current implementation and suspected contributors

• Review entrypoint does not override the output budget.
• Shared request helper defaults to max_tokens: 8192, requests JSON output, and only explicitly enables thinking for deepseek-reasoner / deepseek-v4-pro*. For deepseek-v4-flash it omits the thinking setting; the observed response nevertheless contains reasoning.
• Retry handling adds a JSON-format reminder but keeps the same budget/model/thinking policy.
• Empty-content diagnostics truncate the raw response before the useful trailing finish_reason / usage fields, so the logs do not establish whether the output budget was exhausted.

A large review input (27 changed files, 3938 additions in this case) plus reasoning consuming the output budget is a plausible hypothesis, not a confirmed root cause.

Expected behavior

A valid, non-empty JSON review body is posted, or the workflow clearly reports that automated review was unavailable. Model/provider failures must not be presented as code findings, and reasoning text must not be substituted for a completed review or approval.

Suggested follow-up / acceptance criteria

• Log safe structured metadata on empty output: requested/returned model, finish reason, usage, and content/reasoning lengths; do not dump credentials, prompts, or raw reasoning.
• Verify the provider's current thinking/output-budget semantics for deepseek-v4-flash and configure them explicitly where supported.
• Add a bounded, cost-aware recovery policy for reasoning-only or length-exhausted output instead of repeating the same ineffective request three times (appropriate output budget, smaller review chunks, or an explicitly configured fallback).
• Add mocked API regression tests for reasoning-only output, finish_reason=length, valid final JSON, malformed JSON, and exhausted retries; assert no blank/fabricated review is posted.
• Reproduce with a similarly sized PR and verify successful review submission or an actionable infrastructure-failure summary.

Merge disposition

Per the maintainer's explicit request, #428 was squash-merged after verifying that CI (lint/typecheck/tests/build), CodeQL, and Dependency Review all passed, with no submitted blocking review. The only failing check was this automated review execution. The merge used an exact head-SHA guard and did not use an admin override or change repository protections.

Merge commit: c25a5765c6ce7eec6c7e184643613bfcd3bfa403.