diff --git a/agent/opencode_affinity.py b/agent/opencode_affinity.py
index 6df0bf1cc9..321312c455 100644
--- a/agent/opencode_affinity.py
+++ b/agent/opencode_affinity.py
@@ -18,6 +18,7 @@ matching per-model wire-format decision for the auxiliary client.
 
 from __future__ import annotations
 
+import uuid
 from typing import Any, Optional
 
 OPENCODE_SESSION_HEADER = "x-opencode-session"
@@ -100,7 +101,13 @@ def opencode_session_headers(
         )
     except Exception:
         key = str(session_id or "")
-    return {OPENCODE_SESSION_HEADER: key} if key else {}
+    if not key:
+        # Stateless one-shot requests (commit messages, summaries, standalone prompts outside
+        # a session) lack an ambient conversation or session id. OpenCode Go strictly requires
+        # x-opencode-session on every request (HTTP 400 MissingSessionID if absent, #105841)
+        # so generate an ephemeral session id fallback.
+        key = f"oneshot-{uuid.uuid4().hex[:16]}"
+    return {OPENCODE_SESSION_HEADER: key}
 
 
 def merge_opencode_session_headers(
diff --git a/tests/agent/test_opencode_session_affinity.py b/tests/agent/test_opencode_session_affinity.py
index ee7877816f..35ea5cd725 100644
--- a/tests/agent/test_opencode_session_affinity.py
+++ b/tests/agent/test_opencode_session_affinity.py
@@ -147,3 +147,17 @@ def test_tui_gateway_oneshot_runtime_snapshot_carries_the_session(monkeypatch, o
     aux.call_llm(task="title_generation", main_runtime=_main_runtime_from_agent(agent), messages=_MSGS)
 
     assert captured["extra_headers"]["x-opencode-session"] == "sess-desktop-1"
+
+
+def test_stateless_oneshot_still_sends_an_opencode_session_header(out_of_turn):
+    """A one-shot with no live session (Desktop commit-message generation from the review panel with
+    no active chat, standalone aux calls) has no conversation identity at all, yet the relay rejects
+    header-less requests with 400 MissingSessionID (#105841). It must carry an ephemeral key instead
+    of nothing; non-OpenCode targets stay untouched."""
+    from agent.opencode_affinity import opencode_session_headers
+
+    kwargs = aux._build_call_kwargs("opencode-go", "glm-5", _MSGS, base_url="https://opencode.ai/zen/go/v1")
+    assert kwargs["extra_headers"]["x-opencode-session"]
+
+    assert opencode_session_headers("opencode-go", None, session_id=None).get("x-opencode-session")
+    assert opencode_session_headers("openrouter", "https://openrouter.ai/api/v1", session_id=None) == {}