diff --git a/sdk/agentserver/azure-ai-agentserver-ghcopilot/CHANGELOG.md b/sdk/agentserver/azure-ai-agentserver-ghcopilot/CHANGELOG.md
index c5242a31a831..2e932973eb42 100644
--- a/sdk/agentserver/azure-ai-agentserver-ghcopilot/CHANGELOG.md
+++ b/sdk/agentserver/azure-ai-agentserver-ghcopilot/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Release History
 
+## 1.0.0b3 (Unreleased)
+
+### Bugs Fixed
+
+- `_derive_resource_url_from_project_endpoint` now matches the project endpoint host by suffix instead of substring, so a look-alike host (e.g. `<name>.services.ai.azure.com.example`) no longer derives an attacker-controlled resource URL.
+
 ## 1.0.0b2 (2026-04-24)
 
 ### Breaking Changes
diff --git a/sdk/agentserver/azure-ai-agentserver-ghcopilot/azure/ai/agentserver/githubcopilot/_copilot_adapter.py b/sdk/agentserver/azure-ai-agentserver-ghcopilot/azure/ai/agentserver/githubcopilot/_copilot_adapter.py
index f82e5e9ef8c6..af536b47a366 100644
--- a/sdk/agentserver/azure-ai-agentserver-ghcopilot/azure/ai/agentserver/githubcopilot/_copilot_adapter.py
+++ b/sdk/agentserver/azure-ai-agentserver-ghcopilot/azure/ai/agentserver/githubcopilot/_copilot_adapter.py
@@ -128,8 +128,9 @@ def _derive_resource_url_from_project_endpoint(project_endpoint: str) -> str:
         (".services.ai.azure.cn", ".cognitiveservices.azure.cn"),
         (".services.ai.azure.us", ".cognitiveservices.azure.us"),
     ]:
-        if project_pat in hostname:
-            return f"https://{hostname.replace(project_pat, resource_pat)}"
+        if len(hostname) > len(project_pat) and hostname.endswith(project_pat):
+            resource_host = hostname[: -len(project_pat)] + resource_pat
+            return f"https://{resource_host}"
 
     raise ValueError(f"Cannot derive RESOURCE_URL from: {project_endpoint}")
 
diff --git a/sdk/agentserver/azure-ai-agentserver-ghcopilot/tests/unit_tests/test_replat_features.py b/sdk/agentserver/azure-ai-agentserver-ghcopilot/tests/unit_tests/test_replat_features.py
index 069773579952..ebc53364e330 100644
--- a/sdk/agentserver/azure-ai-agentserver-ghcopilot/tests/unit_tests/test_replat_features.py
+++ b/sdk/agentserver/azure-ai-agentserver-ghcopilot/tests/unit_tests/test_replat_features.py
@@ -268,6 +268,20 @@ def test_derive_resource_url_invalid(self):
         with pytest.raises(ValueError, match="Cannot derive"):
             _derive_resource_url_from_project_endpoint("https://unknown.example.com/foo")
 
+    def test_derive_resource_url_rejects_lookalike_host(self):
+        """A look-alike host that only contains the suffix is rejected."""
+        with pytest.raises(ValueError, match="Cannot derive"):
+            _derive_resource_url_from_project_endpoint(
+                "https://victim.services.ai.azure.com.attacker.example/api/projects/myproject"
+            )
+
+    def test_derive_resource_url_rejects_bare_suffix_host(self):
+        """A host that is exactly the suffix (no resource label) is rejected."""
+        with pytest.raises(ValueError, match="Cannot derive"):
+            _derive_resource_url_from_project_endpoint(
+                "https://.services.ai.azure.com/api/projects/myproject"
+            )
+
     def test_get_project_endpoint_new_var(self):
         """Prefers FOUNDRY_PROJECT_ENDPOINT over legacy name."""
         with patch.dict(os.environ, {