diff --git a/litellm/proxy/litellm_pre_call_utils.py b/litellm/proxy/litellm_pre_call_utils.py
index f752d7cfa89..d026c5510e6 100644
--- a/litellm/proxy/litellm_pre_call_utils.py
+++ b/litellm/proxy/litellm_pre_call_utils.py
@@ -669,6 +669,21 @@ def _extract_codex_session_id_from_headers(
     )
 
 
+def _extract_bare_session_id_from_headers(
+    normalized: Mapping[str, str],
+) -> str | None:
+    """
+    Read a vendor-less ``x-session-id`` header (opencode sends ``X-Session-Id``
+    alongside ``x-session-affinity`` on every turn of a session). Checked after
+    the ``x-<vendor>-session-id`` scan so a more specific header such as
+    opencode's ``x-parent-session-id`` on subagent calls keeps winning.
+    """
+    value: Final = normalized.get("x-session-id")
+    if isinstance(value, str) and _SESSION_ID_VALUE_RE.match(value):
+        return value
+    return None
+
+
 def get_chain_id_from_headers(headers: dict[str, str] | None) -> str | None:
     """
     Extract chain id for call chaining from request headers.
@@ -679,6 +694,7 @@ def get_chain_id_from_headers(headers: dict[str, str] | None) -> str | None:
     3. Any ``x-<vendor>-session-id`` header whose value looks like a session id
        (alphanumeric / UUID, at least 8 chars).  E.g. ``x-claude-code-session-id``.
     4. Codex's unprefixed ``session-id`` / ``thread-id``, for Codex callers only.
+    5. A vendor-less ``x-session-id`` header (e.g. opencode), same value rules.
 
     Header keys are matched case-insensitively so this works with raw header
     dicts from any transport.
@@ -694,6 +710,7 @@ def get_chain_id_from_headers(headers: dict[str, str] | None) -> str | None:
         or normalized.get("x-litellm-session-id")
         or _extract_generic_session_id_from_headers(normalized)
         or _extract_codex_session_id_from_headers(normalized)
+        or _extract_bare_session_id_from_headers(normalized)
     )
 
 
diff --git a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
index 72d37650963..7070617ce3e 100644
--- a/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
+++ b/tests/test_litellm/proxy/test_litellm_pre_call_utils.py
@@ -3343,6 +3343,62 @@ def test_add_litellm_metadata_groups_codex_turns_into_one_session():
         assert turn["litellm_metadata"]["session_id"] == CODEX_SESSION_UUID
 
 
+OPENCODE_SESSION_ID = "ses_f91e6e825ffeuhlu5EbglxjAN2"
+OPENCODE_HEADERS = {
+    "x-session-affinity": OPENCODE_SESSION_ID,
+    "X-Session-Id": OPENCODE_SESSION_ID,
+    "User-Agent": "opencode/1.18.28",
+}
+
+
+def test_add_litellm_metadata_groups_opencode_turns_into_one_session():
+    """Every turn of an opencode session must land on metadata.session_id, which is what
+    DeploymentAffinityCheck reads for session pinning, instead of a fresh per-call id."""
+    turns = [{"metadata": {}}, {"metadata": {}}]
+    for turn in turns:
+        LiteLLMProxyRequestSetup.add_litellm_metadata_from_request_headers(
+            headers=OPENCODE_HEADERS, data=turn, _metadata_variable_name="metadata"
+        )
+
+    for turn in turns:
+        assert turn["metadata"]["session_id"] == OPENCODE_SESSION_ID
+        assert turn["metadata"]["trace_id"] == OPENCODE_SESSION_ID
+        assert turn["litellm_session_id"] == OPENCODE_SESSION_ID
+        assert turn["litellm_trace_id"] == OPENCODE_SESSION_ID
+
+
+@pytest.mark.parametrize("value", ["short", "has spaces!!", ""])
+def test_get_chain_id_from_headers_bare_session_id_ignores_implausible_value(value: str):
+    from litellm.proxy.litellm_pre_call_utils import get_chain_id_from_headers
+
+    assert get_chain_id_from_headers({"x-session-id": value}) is None
+
+
+@pytest.mark.parametrize(
+    "other_header",
+    [
+        "x-litellm-trace-id",
+        "x-litellm-session-id",
+        "x-claude-code-session-id",
+        "x-parent-session-id",
+    ],
+)
+def test_get_chain_id_from_headers_bare_session_id_loses_to_more_specific_header(other_header: str):
+    """opencode subagent calls carry x-parent-session-id next to X-Session-Id; explicit and
+    vendor-scoped headers must keep winning over the bare header."""
+    from litellm.proxy.litellm_pre_call_utils import get_chain_id_from_headers
+
+    assert (
+        get_chain_id_from_headers(
+            {
+                "x-session-id": OPENCODE_SESSION_ID,
+                other_header: "e96634a3-fa28-4083-b354-55542e2dca01",
+            }
+        )
+        == "e96634a3-fa28-4083-b354-55542e2dca01"
+    )
+
+
 def test_trace_id_from_traceparent_valid():
     from litellm.proxy.litellm_pre_call_utils import _trace_id_from_traceparent