diff --git a/src/gitea_codex_bot/services/reviewer.py b/src/gitea_codex_bot/services/reviewer.py
index c3a8b03..482dc98 100644
--- a/src/gitea_codex_bot/services/reviewer.py
+++ b/src/gitea_codex_bot/services/reviewer.py
@@ -166,8 +166,42 @@ def _call_openai_review(settings: Settings, prompt: str) -> dict[str, Any]:
     raise ReviewError("OpenAI response did not contain JSON output text.")
 
 
-def _fallback_review(diff_context: dict[str, Any]) -> dict[str, Any]:
-    findings = []
+def _summarize_openai_failure(exc: Exception) -> str:
+    if isinstance(exc, httpx.HTTPStatusError):
+        status = exc.response.status_code
+        response_text = exc.response.text.strip()
+        if response_text:
+            compact = " ".join(response_text.split())
+            if len(compact) > 400:
+                compact = f"{compact[:400]}..."
+            return f"OpenAI API HTTP {status}: {compact}"
+        return f"OpenAI API HTTP {status}."
+    if isinstance(exc, httpx.TimeoutException):
+        return "OpenAI API request timed out."
+    message = str(exc).strip()
+    if message:
+        return message
+    return f"{exc.__class__.__name__} (no details)"
+
+
+def _fallback_review(diff_context: dict[str, Any], *, failure_reason: str | None = None) -> dict[str, Any]:
+    findings: list[dict[str, Any]] = []
+    summary = "Fallback analysis was used because OpenAI review was unavailable."
+
+    if failure_reason:
+        summary = f"OpenAI review failed. Error: {failure_reason}"
+        findings.append(
+            {
+                "severity": "high",
+                "file": "unknown",
+                "line_start": 1,
+                "line_end": 1,
+                "title": "OpenAI review request failed",
+                "body": failure_reason,
+                "suggestion": "Fix API/auth/network issues and rerun @codex review.",
+            }
+        )
+
     if "TODO" in diff_context["diff"]:
         findings.append(
             {
@@ -183,7 +217,7 @@ def _fallback_review(diff_context: dict[str, Any]) -> dict[str, Any]:
     return {
         "verdict": "correct" if not findings else "has_issues",
         "confidence": 0.4 if not findings else 0.6,
-        "summary": "Fallback analysis was used because OpenAI review was unavailable.",
+        "summary": summary,
         "findings": findings,
     }
 
@@ -198,8 +232,8 @@ def run_review_for_pr(
     prompt, diff_context, repo_cfg = prepare_review_prompt(settings, gitea, repo, pr_number, command)
     try:
         result = _call_openai_review(settings, prompt)
-    except Exception:
-        result = _fallback_review(diff_context)
+    except Exception as exc:
+        result = _fallback_review(diff_context, failure_reason=_summarize_openai_failure(exc))
     return normalize_review_result(result), repo_cfg
 
 
diff --git a/tests/test_reviewer_fallback.py b/tests/test_reviewer_fallback.py
new file mode 100644
index 0000000..9a818e6
--- /dev/null
+++ b/tests/test_reviewer_fallback.py
@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+import httpx
+
+from gitea_codex_bot.config import get_settings
+from gitea_codex_bot.services.repo_config import RepoReviewConfig
+from gitea_codex_bot.services.reviewer import _fallback_review, run_review_for_pr
+from gitea_codex_bot.types import ParsedCommand
+
+
+def test_fallback_review_surfaces_failure_reason() -> None:
+    result = _fallback_review({"diff": ""}, failure_reason="OpenAI API HTTP 401: invalid_api_key")
+
+    assert result["verdict"] == "has_issues"
+    assert result["summary"] == "OpenAI review failed. Error: OpenAI API HTTP 401: invalid_api_key"
+    assert result["findings"][0]["title"] == "OpenAI review request failed"
+    assert result["findings"][0]["body"] == "OpenAI API HTTP 401: invalid_api_key"
+
+
+def test_run_review_for_pr_uses_openai_http_error_in_fallback(monkeypatch) -> None:
+    def _fake_prepare(*_args, **_kwargs):
+        return "prompt", {"diff": "TODO: tighten validation"}, RepoReviewConfig()
+
+    def _raise_http_error(*_args, **_kwargs):
+        request = httpx.Request("POST", "https://api.openai.com/v1/responses")
+        response = httpx.Response(429, request=request, text='{"error":{"message":"rate_limited"}}')
+        raise httpx.HTTPStatusError("rate limited", request=request, response=response)
+
+    monkeypatch.setattr("gitea_codex_bot.services.reviewer.prepare_review_prompt", _fake_prepare)
+    monkeypatch.setattr("gitea_codex_bot.services.reviewer._call_openai_review", _raise_http_error)
+
+    settings = get_settings()
+    command = ParsedCommand(name="review", raw="@codex review")
+    result, _repo_cfg = run_review_for_pr(settings, object(), "acme/repo", 9, command)
+
+    assert result["summary"].startswith("OpenAI review failed. Error: OpenAI API HTTP 429:")
+    assert result["findings"][0]["title"] == "OpenAI review request failed"
+    assert "rate_limited" in result["findings"][0]["body"]
+    assert any(finding["title"] == "TODO marker in diff" for finding in result["findings"])