diff --git a/src/aituner/http_client.py b/src/aituner/http_client.py
index e840731..746bba4 100644
--- a/src/aituner/http_client.py
+++ b/src/aituner/http_client.py
@@ -179,6 +179,9 @@ def chat_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"llm_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        # TimeoutError (socket.timeout), URLError, ConnectionError all subclass OSError.
+        raise HttpClientError(f"llm_completion failed: {exc}") from exc
 
 
 def stream_text_completion(
@@ -232,6 +235,8 @@ def stream_text_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"stream_text_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        raise HttpClientError(f"stream_text_completion failed: {exc}") from exc
     return "".join(parts)
 
 
@@ -293,6 +298,10 @@ def stream_chat_completion(
     except urllib.error.HTTPError as exc:
         detail = exc.read().decode("utf-8", errors="replace")
         raise HttpClientError(f"stream_chat_completion failed: {exc.code} {detail}") from exc
+    except OSError as exc:
+        # A request that exceeds request_timeout_s raises TimeoutError mid-stream;
+        # treat it as a failed request (SLO miss), not a crashed trial.
+        raise HttpClientError(f"stream_chat_completion failed: {exc}") from exc
     ttft_ms = None if first_token_at is None else (first_token_at - start) * 1000.0
     if completion_tokens is None and chunk_token_count > 0:
         completion_tokens = chunk_token_count
diff --git a/tests/test_core_flow.py b/tests/test_core_flow.py
index b1cbfe7..2772159 100644
--- a/tests/test_core_flow.py
+++ b/tests/test_core_flow.py
@@ -16,6 +16,7 @@ from aituner.cli import main as cli_main
 from aituner.compare import _aggregate_summary, load_compare_spec, run_compare
 from aituner.engine import build_launch_recipe
 from aituner.http_client import (
+    HttpClientError,
     StreamMetrics,
     _auth_headers,
     _openai_url,
@@ -560,6 +561,34 @@ class CoreFlowTests(unittest.TestCase):
         self.assertTrue(ev(0, 3900).passed)
         self.assertFalse(ev(0, 4100).passed)
 
+    def test_streaming_socket_timeout_is_a_failed_request_not_a_crash(self) -> None:
+        # A request that exceeds request_timeout_s raises TimeoutError mid-stream;
+        # it must surface as HttpClientError (a failed request), never escape to
+        # crash the trial.
+        with mock.patch(
+            "aituner.http_client._urlopen", side_effect=TimeoutError("timed out")
+        ):
+            with self.assertRaises(HttpClientError):
+                stream_chat_completion(
+                    base_url="http://127.0.0.1:1/v1",
+                    body={"messages": [{"role": "user", "content": "hi"}], "stream": True},
+                    timeout_s=0.5,
+                )
+            outcome = _run_one_request(
+                TraceRequest(
+                    row_id="r",
+                    arrival_s=0.0,
+                    sampling_u=1.0,
+                    body={"messages": [{"role": "user", "content": "hi"}], "stream": True},
+                    prompt_tokens_hint=10,
+                    completion_tokens_hint=None,
+                ),
+                base_url="http://127.0.0.1:1/v1",
+                timeout_s=0.5,
+            )
+        self.assertFalse(outcome.success)
+        self.assertIn("timed out", outcome.error)
+
     def test_lca_similarity_matrix_separates_different_profiles(self) -> None:
         window = WindowRecord(
             window_id="base",