diff --git a/src/research_handler.py b/src/research_handler.py
index 3484c46..381530b 100644
--- a/src/research_handler.py
+++ b/src/research_handler.py
@@ -30,6 +30,24 @@ def _bounded_int(value, *, default: int, minimum: int, maximum: int) -> int:
     return max(minimum, min(maximum, n))
 
 
+def _format_probe_failure(model: str, exc: Exception) -> str:
+    """Turn a failed research model probe into a user-facing message."""
+    detail = getattr(exc, "detail", None)
+    status = getattr(exc, "status_code", None)
+    err = str(detail if detail is not None else exc).strip()
+
+    if status in {401, 403} or "401" in err or "API key" in err or "Unauthorized" in err:
+        return f"Model '{model}' requires an API key. Check your endpoint configuration."
+
+    if status and err:
+        return f"Model '{model}' probe failed: {err}"
+
+    if err:
+        return f"Cannot reach model '{model}' — {err}"
+
+    return f"Cannot reach model '{model}' — check that the endpoint is running and accessible."
+
+
 class ResearchHandler:
     """Handles research service operations with iterative deep research."""
 
@@ -634,14 +652,7 @@ class ResearchHandler:
             logger.info(f"Endpoint probe OK: {model}")
         except Exception as e:
             logger.error(f"Probe failed for {model}: {e}")
-            err = str(e)
-            if "401" in err or "API key" in err or "Unauthorized" in err:
-                raise RuntimeError(
-                    f"Model '{model}' requires an API key. Check your endpoint configuration."
-                ) from e
-            raise RuntimeError(
-                f"Cannot reach model '{model}' — check that the endpoint is running and accessible."
-            ) from e
+            raise RuntimeError(_format_probe_failure(model, e)) from e
 
     async def call_research_service(
         self,
diff --git a/tests/test_research_probe_errors.py b/tests/test_research_probe_errors.py
new file mode 100644
index 0000000..8418090
--- /dev/null
+++ b/tests/test_research_probe_errors.py
@@ -0,0 +1,61 @@
+"""Regression tests for Deep Research model probe error messages.
+
+Deep Research probes the selected model before starting a long run. When the
+upstream returned a concrete model/API error, the probe used to collapse it into
+"Cannot reach model", hiding the real issue from the UI.
+"""
+import pytest
+from fastapi import HTTPException
+
+from src.research_handler import ResearchHandler, _format_probe_failure
+
+
+def test_probe_failure_preserves_upstream_model_errors():
+    exc = HTTPException(
+        status_code=400,
+        detail="OpenAI returned HTTP 400: Unsupported parameter: temperature",
+    )
+
+    msg = _format_probe_failure("o3-mini", exc)
+
+    assert msg == (
+        "Model 'o3-mini' probe failed: "
+        "OpenAI returned HTTP 400: Unsupported parameter: temperature"
+    )
+
+
+def test_probe_failure_keeps_api_key_guidance():
+    exc = HTTPException(status_code=401, detail="OpenAI authentication failed")
+
+    assert _format_probe_failure("gpt-4o", exc) == (
+        "Model 'gpt-4o' requires an API key. Check your endpoint configuration."
+    )
+
+
+def test_probe_failure_keeps_reachability_guidance_for_plain_errors():
+    msg = _format_probe_failure("local-model", RuntimeError("connection refused"))
+
+    assert msg == "Cannot reach model 'local-model' — connection refused"
+
+
+@pytest.mark.asyncio
+async def test_probe_endpoint_surfaces_http_exception_detail(monkeypatch):
+    async def _raise(*args, **kwargs):
+        raise HTTPException(
+            status_code=400,
+            detail="OpenAI returned HTTP 400: max_tokens is not supported",
+        )
+
+    monkeypatch.setattr("src.llm_core.llm_call_async", _raise)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        await ResearchHandler._probe_endpoint(
+            "https://api.openai.com/v1/chat/completions",
+            "o3-mini",
+            {"Authorization": "Bearer test"},
+        )
+
+    msg = str(excinfo.value)
+    assert "Model 'o3-mini' probe failed" in msg
+    assert "max_tokens is not supported" in msg
+    assert "Cannot reach model" not in msg