diff --git a/envs/opencode_env/sandbox/interception.py b/envs/opencode_env/sandbox/interception.py
index 131d410243..674d8d9a7a 100644
--- a/envs/opencode_env/sandbox/interception.py
+++ b/envs/opencode_env/sandbox/interception.py
@@ -43,6 +43,7 @@
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Any
+from urllib.parse import urlsplit
 
 import httpx
 import uvicorn
@@ -216,9 +217,14 @@ def _model_uses_max_completion_tokens(model: str) -> bool:
 
 
 def _resolve_upstream_url(upstream: str) -> str:
-    """Build the fully qualified chat-completions URL from a base URL."""
+    """Build the fully qualified chat-completions URL from a base URL.
+
+    ``/v1`` is only added when the base has no path. A base that already has
+    one (``/v1``, ``/v1beta/openai``, ``/api/paas/v4``) is used as-is, which is
+    the URL opencode itself would call in ``black_box`` mode.
+    """
     base = upstream.rstrip("/")
-    if base.endswith("/v1"):
+    if urlsplit(base).path:
         return f"{base}/chat/completions"
     return f"{base}{CHAT_COMPLETIONS_PATH}"
 
diff --git a/tests/envs/test_opencode_env.py b/tests/envs/test_opencode_env.py
index b286118742..4dc77c114d 100644
--- a/tests/envs/test_opencode_env.py
+++ b/tests/envs/test_opencode_env.py
@@ -346,6 +346,27 @@ def fake_serve(cfg) -> None:
     assert captured["cfg"].upstream_api_key == "sk-from-env"
 
 
+def test_interception_keeps_upstream_base_path() -> None:
+    from opencode_env.sandbox.interception import _resolve_upstream_url
+
+    assert (
+        _resolve_upstream_url("https://api.openai.com/v1/")
+        == "https://api.openai.com/v1/chat/completions"
+    )
+    assert (
+        _resolve_upstream_url("http://localhost:8000")
+        == "http://localhost:8000/v1/chat/completions"
+    )
+    assert (
+        _resolve_upstream_url("https://generativelanguage.googleapis.com/v1beta/openai")
+        == "https://generativelanguage.googleapis.com/v1beta/openai/chat/completions"
+    )
+    assert (
+        _resolve_upstream_url("https://api.z.ai/api/paas/v4")
+        == "https://api.z.ai/api/paas/v4/chat/completions"
+    )
+
+
 # ---------------------------------------------------------------------------
 # Integration — only runs when E2B + endpoint creds are present and the
 # user explicitly opts in via ``pytest -m integration``.