diff --git a/headroom/copilot_auth.py b/headroom/copilot_auth.py
index d8c779e44..a1aed76a8 100644
--- a/headroom/copilot_auth.py
+++ b/headroom/copilot_auth.py
@@ -1007,7 +1007,12 @@ def build_copilot_upstream_url(base_url: str, path: str) -> str:
         # chat/responses and Anthropic messages all build their upstream URL
         # here), so mark the request for provider relabeling downstream.
         mark_request_routed_to_copilot()
-        if normalized_path.startswith("/v1/"):
+        # Copilot serves its OpenAI-compatible surface WITHOUT a ``/v1`` prefix
+        # (``/chat/completions``, ``/responses``, ...), so strip it there. But its
+        # Anthropic surface for Claude models IS ``/v1/messages`` (with the
+        # ``/v1``); stripping it forwarded ``/messages`` and Copilot returned 404
+        # for claude-* models (#2409). Keep ``/v1`` for the messages endpoint.
+        if normalized_path.startswith("/v1/") and not normalized_path.startswith("/v1/messages"):
             normalized_path = normalized_path[3:]
     else:
         reset_request_routed_to_copilot()
diff --git a/tests/test_copilot_auth.py b/tests/test_copilot_auth.py
index 68f7fea97..e1af9a11b 100644
--- a/tests/test_copilot_auth.py
+++ b/tests/test_copilot_auth.py
@@ -612,6 +612,34 @@ def test_build_copilot_upstream_url_strips_v1_only_for_copilot_hosts() -> None:
     )
 
 
+def test_build_copilot_upstream_url_preserves_v1_messages_for_copilot() -> None:
+    # Copilot's Anthropic surface for Claude models is /v1/messages (with the
+    # /v1); stripping it forwarded /messages and Copilot 404'd (#2409).
+    assert (
+        copilot_auth.build_copilot_upstream_url(
+            "https://api.githubcopilot.com",
+            "/v1/messages",
+        )
+        == "https://api.githubcopilot.com/v1/messages"
+    )
+    # Batches under the messages endpoint keep /v1 too.
+    assert (
+        copilot_auth.build_copilot_upstream_url(
+            "https://api.githubcopilot.com",
+            "/v1/messages/batches",
+        )
+        == "https://api.githubcopilot.com/v1/messages/batches"
+    )
+    # A GHE Copilot host keeps /v1/messages as well.
+    assert (
+        copilot_auth.build_copilot_upstream_url(
+            "https://copilot-api.acme.ghe.com",
+            "/v1/messages",
+        )
+        == "https://copilot-api.acme.ghe.com/v1/messages"
+    )
+
+
 def test_build_copilot_upstream_url_strips_v1_for_ghe_copilot_hosts() -> None:
     assert (
         copilot_auth.build_copilot_upstream_url(