diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9d0368fb0..dba81821d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 
 ### Fixed
+- The Anthropic Messages route (`POST /v1/messages`) now honors the
+  `x-headroom-base-url` per-request upstream override. It previously ignored the
+  header and always forwarded to `api.anthropic.com`, so clients that speak the
+  Anthropic Messages wire format while authenticating against a non-Anthropic
+  gateway (e.g. OpenCode Zen) were rejected upstream with `401 invalid
+  x-api-key`. The route now forwards to `<x-headroom-base-url>/v1/messages`,
+  consistent with the OpenAI-compatible and passthrough routes
+  ([#1760](https://github.com/headroomlabs-ai/headroom/issues/1760)).
 - **proxy:** the savings store now fsyncs its parent directory after the
   atomic rename, so the most recent `proxy_savings.json` write survives a
   power-loss or crash. `_save_locked` fsynced the temp file's contents but
diff --git a/docs/content/docs/configuration.mdx b/docs/content/docs/configuration.mdx
index 73b5f5854..9201e8f20 100644
--- a/docs/content/docs/configuration.mdx
+++ b/docs/content/docs/configuration.mdx
@@ -127,6 +127,33 @@ response = client.chat.completions.create(
 </Tab>
 </Tabs>
 
+### Proxy upstream override (`x-headroom-base-url`)
+
+When using the proxy, send the `x-headroom-base-url` request header to route a
+single request to a different upstream instead of the configured provider URL.
+This lets a client that speaks a provider's wire format authenticate against a
+compatible gateway (for example an OpenAI-compatible endpoint, or an
+Anthropic-Messages gateway such as OpenCode Zen) without changing the proxy
+configuration.
+
+The header is honored by the OpenAI-compatible routes, the Anthropic Messages
+route (`POST /v1/messages`), and the generic passthrough route. The proxy
+forwards the request to `<x-headroom-base-url>` + the original request path
+(e.g. `/v1/messages`). An empty or whitespace-only value is ignored and the
+configured upstream is used.
+
+```bash
+curl http://127.0.0.1:8787/v1/messages \
+  -H "content-type: application/json" \
+  -H "anthropic-version: 2023-06-01" \
+  -H "x-headroom-base-url: https://opencode.ai/zen/go" \
+  -H "x-api-key: <gateway-api-key>" \
+  -d '{"model":"glm-5.2","max_tokens":16,"messages":[{"role":"user","content":"hi"}]}'
+```
+
+When `HEADROOM_STRIP_INTERNAL_HEADERS` is `enabled` (the default), the proxy
+reads this header for routing and then strips it before forwarding upstream.
+
 ## SmartCrusher Configuration
 
 Fine-tune JSON compression behavior:
diff --git a/headroom/providers/proxy_routes.py b/headroom/providers/proxy_routes.py
index ce6b5fe08..80abd2544 100644
--- a/headroom/providers/proxy_routes.py
+++ b/headroom/providers/proxy_routes.py
@@ -491,6 +491,15 @@ def register_provider_routes(app: FastAPI, proxy: Any) -> None:
 
     @app.post("/v1/messages")
     async def anthropic_messages(request: Request):
+        # Honor the per-request upstream override so clients that speak the
+        # Anthropic Messages wire format but authenticate against a
+        # non-Anthropic gateway route correctly, consistent with the
+        # OpenAI-compatible and generic passthrough routes.
+        custom_base = request.headers.get("x-headroom-base-url", "").strip()
+        if custom_base:
+            return await proxy.handle_anthropic_messages(
+                request, upstream_base_url=custom_base.rstrip("/")
+            )
         return await proxy.handle_anthropic_messages(request)
 
     @app.post("/anthropic/v1/messages")
diff --git a/tests/test_proxy/test_anthropic_upstream_header.py b/tests/test_proxy/test_anthropic_upstream_header.py
new file mode 100644
index 000000000..cea62070f
--- /dev/null
+++ b/tests/test_proxy/test_anthropic_upstream_header.py
@@ -0,0 +1,102 @@
+"""Tests that the ``/v1/messages`` route honors ``x-headroom-base-url``.
+
+The Anthropic Messages route must forward the per-request upstream
+override header to ``handle_anthropic_messages`` so clients that speak
+the Anthropic wire format but authenticate against a non-Anthropic
+gateway (e.g. OpenCode Zen's "Go" tier) route correctly — consistent
+with the OpenAI-compatible routes and the generic passthrough route,
+which already honor it (see ``providers/proxy_routes.py``).
+
+Contract pinned here:
+- header present            → its value is passed as ``upstream_base_url``
+- header absent             → no override (``upstream_base_url`` unset/None)
+- header empty/whitespace   → no override (must not blank the upstream)
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock
+
+import pytest
+
+fastapi = pytest.importorskip("fastapi")
+
+from fastapi.responses import JSONResponse  # noqa: E402
+from fastapi.testclient import TestClient  # noqa: E402
+
+from headroom.proxy.server import ProxyConfig, create_app  # noqa: E402
+
+MESSAGES = "/v1/messages"
+BODY = {"model": "glm-5.2", "max_tokens": 16, "messages": [{"role": "user", "content": "hi"}]}
+
+
+def _make_config(**overrides) -> ProxyConfig:
+    base = {
+        "optimize": False,
+        "cache_enabled": False,
+        "rate_limit_enabled": False,
+        "mode": "token",
+    }
+    base.update(overrides)
+    return ProxyConfig(**base)
+
+
+def _install_handler_spy(proxy) -> AsyncMock:
+    """Replace handle_anthropic_messages with a spy returning a 200."""
+    spy = AsyncMock(return_value=JSONResponse({"ok": True}))
+    proxy.handle_anthropic_messages = spy
+    return spy
+
+
+def _base_url_kwarg(spy: AsyncMock):
+    call = spy.call_args
+    return call.kwargs.get("upstream_base_url")
+
+
+def test_header_passed_as_upstream_base_url():
+    app = create_app(_make_config())
+    with TestClient(app) as client:
+        spy = _install_handler_spy(client.app.state.proxy)
+        resp = client.post(
+            MESSAGES,
+            json=BODY,
+            headers={"x-headroom-base-url": "https://opencode.ai/zen/go"},
+        )
+
+    assert resp.status_code == 200
+    assert _base_url_kwarg(spy) == "https://opencode.ai/zen/go"
+
+
+def test_missing_header_leaves_upstream_unset():
+    app = create_app(_make_config())
+    with TestClient(app) as client:
+        spy = _install_handler_spy(client.app.state.proxy)
+        resp = client.post(MESSAGES, json=BODY)
+
+    assert resp.status_code == 200
+    assert _base_url_kwarg(spy) is None
+
+
+def test_empty_or_whitespace_header_leaves_upstream_unset():
+    for value in ("", "   "):
+        app = create_app(_make_config())
+        with TestClient(app) as client:
+            spy = _install_handler_spy(client.app.state.proxy)
+            resp = client.post(MESSAGES, json=BODY, headers={"x-headroom-base-url": value})
+
+        assert resp.status_code == 200
+        assert _base_url_kwarg(spy) is None
+
+
+def test_header_value_is_trimmed_and_trailing_slash_stripped():
+    app = create_app(_make_config())
+    with TestClient(app) as client:
+        spy = _install_handler_spy(client.app.state.proxy)
+        resp = client.post(
+            MESSAGES,
+            json=BODY,
+            headers={"x-headroom-base-url": "  https://opencode.ai/zen/go/  "},
+        )
+
+    assert resp.status_code == 200
+    assert _base_url_kwarg(spy) == "https://opencode.ai/zen/go"