diff --git a/frontends/tests/test_mixin_partial_stream.py b/frontends/tests/test_mixin_partial_stream.py
new file mode 100644
index 000000000..1e746555f
--- /dev/null
+++ b/frontends/tests/test_mixin_partial_stream.py
@@ -0,0 +1,104 @@
+from __future__ import annotations
+
+import importlib.util
+import threading
+import sys
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(ROOT))
+
+spec = importlib.util.spec_from_file_location("llmcore_mixin_under_test", ROOT / "llmcore.py")
+llmcore = importlib.util.module_from_spec(spec)
+assert spec.loader is not None
+spec.loader.exec_module(llmcore)
+MixinSession = llmcore.MixinSession
+
+
+class FakeSession:
+    def __init__(self, name, chunks):
+        self.name = name
+        self.chunks = list(chunks)
+        self.calls = 0
+        self.history = []
+        self.system = ""
+        self.tools = None
+        self.lock = threading.Lock()
+        self.max_retries = 0
+        self.closed = 0
+
+    def make_messages(self, messages):
+        return messages
+
+    def raw_ask(self, messages):
+        self.calls += 1
+        try:
+            for chunk in self.chunks:
+                yield chunk
+        finally:
+            self.closed += 1
+        return [{"type": "text", "text": "".join(c for c in self.chunks if not c.startswith("!!!Error:"))}]
+
+
+def _mixin(*sessions, retries=1):
+    mixin = MixinSession.__new__(MixinSession)
+    mixin._sessions = list(sessions)
+    mixin._retries = retries
+    mixin._base_delay = 0
+    mixin._spring_sec = 300
+    mixin._cur_idx = 0
+    mixin._switched_at = 0
+    mixin._native = False
+    mixin.name = "|".join(s.name for s in sessions)
+    mixin.history = []
+    mixin.system = ""
+    mixin.tools = None
+    mixin.lock = threading.Lock()
+    return mixin
+
+
+def _collect(generator):
+    chunks = []
+    try:
+        while True: chunks.append(next(generator))
+    except StopIteration as exc:
+        return chunks, exc.value
+
+
+def test_failure_before_output_can_fail_over_transparently():
+    primary = FakeSession("A", ["!!!Error: overloaded"])
+    backup = FakeSession("B", ["B-complete"])
+    chunks, _ = _collect(_mixin(primary, backup).raw_ask([{"role": "user", "content": "x"}]))
+    assert chunks == ["B-complete"]
+    assert primary.calls == backup.calls == 1
+
+
+def test_failure_after_output_does_not_concatenate_attempts():
+    primary = FakeSession("A", ["A-part", "!!!Error: upstream reset"])
+    backup = FakeSession("B", ["B-complete"])
+    mixin = _mixin(primary, backup)
+    chunks, blocks = _collect(mixin.raw_ask([{"role": "user", "content": "x"}]))
+    assert chunks == ["A-part", "!!!Error: upstream reset"]
+    assert blocks == [{"type": "text", "text": "A-part"}]
+    assert backup.calls == 0
+    assert mixin.current_name == "B"
+    assert primary.closed == 1
+
+
+def test_partial_failure_keeps_remaining_chunks_from_the_same_attempt():
+    primary = FakeSession("A", ["A-part", "!!!Error: upstream reset", "A-tail"])
+    backup = FakeSession("B", ["B-complete"])
+    chunks, _ = _collect(_mixin(primary, backup).raw_ask([{"role": "user", "content": "x"}]))
+    assert chunks == ["A-part", "!!!Error: upstream reset", "A-tail"]
+    assert backup.calls == 0
+
+
+def test_backup_is_used_on_the_next_request_after_partial_failure():
+    primary = FakeSession("A", ["A-part", "!!!Error: upstream reset"])
+    backup = FakeSession("B", ["B-complete"])
+    mixin = _mixin(primary, backup)
+    _collect(mixin.raw_ask([{"role": "user", "content": "first"}]))
+    chunks, _ = _collect(mixin.raw_ask([{"role": "user", "content": "second"}]))
+    assert chunks == ["B-complete"]
+    assert backup.calls == 1
diff --git a/llmcore.py b/llmcore.py
index 423503248..8e2460a8d 100644
--- a/llmcore.py
+++ b/llmcore.py
@@ -1125,13 +1125,26 @@ def raw_ask(self, messages):
             session = self._sessions[idx]
             gen = session.raw_ask(self._prepare(idx, messages))
             print(f'[MixinSession] Using session ({session.name})')
-            last_chunk, return_val, yielded = None, [], False
+            last_chunk, return_val, yielded, failed_after_output = None, [], False, False
             try:
                 while True:
                     chunk = next(gen); last_chunk = chunk
-                    if not yielded and test_error(chunk): continue
-                    yield chunk; yielded = True
+                    if failed_after_output:
+                        yield chunk
+                        continue
+                    if test_error(chunk):
+                        if yielded:
+                            yield chunk
+                            failed_after_output = True
+                            continue
+                        continue
+                    yield chunk; yielded = yielded or bool(chunk)
             except StopIteration as e: return_val = e.value or []
+            if failed_after_output:
+                self._cur_idx = (idx + 1) % n if n > 1 else idx
+                self._switched_at = time.time()
+                print(f'[MixinSession] Partial failure, next call → s{self._cur_idx} ({self.current.name})')
+                return return_val
             is_err = test_error(last_chunk)
             if not is_err:
                 if attempt > 0: self._cur_idx = idx; self._switched_at = time.time()