diff --git a/crates/protocols/src/responses.rs b/crates/protocols/src/responses.rs
index ca6440b07..21d3a90be 100644
--- a/crates/protocols/src/responses.rs
+++ b/crates/protocols/src/responses.rs
@@ -452,6 +452,30 @@ pub enum ResponseTool {
     LocalShell,
 }
 
+impl ResponseTool {
+    /// Wire `type` tag for this variant — matches the variant's
+    /// `#[serde(rename = ...)]` attribute. Stable across serde
+    /// roundtrips and safe to use as a discriminator string.
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            ResponseTool::Function(_) => "function",
+            ResponseTool::WebSearchPreview(_) => "web_search_preview",
+            ResponseTool::WebSearch(_) => "web_search",
+            ResponseTool::CodeInterpreter(_) => "code_interpreter",
+            ResponseTool::Mcp(_) => "mcp",
+            ResponseTool::FileSearch(_) => "file_search",
+            ResponseTool::ImageGeneration(_) => "image_generation",
+            ResponseTool::Computer => "computer",
+            ResponseTool::ComputerUsePreview(_) => "computer_use_preview",
+            ResponseTool::Custom(_) => "custom",
+            ResponseTool::Namespace(_) => "namespace",
+            ResponseTool::Shell(_) => "shell",
+            ResponseTool::ApplyPatch => "apply_patch",
+            ResponseTool::LocalShell => "local_shell",
+        }
+    }
+}
+
 /// Payload carried by [`ResponseTool::Namespace`].
 ///
 /// Using a dedicated struct (rather than inline struct-variant fields) lets
@@ -3883,3 +3907,24 @@ impl ResponseReasoningContent {
         Self::ReasoningText { text }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Lock `as_str()` to the canonical serde tag for the unit variants
+    /// — drift between the two would produce inconsistent wire labels
+    /// across dispatch paths and serializers.
+    #[test]
+    fn response_tool_as_str_matches_serde_tag_for_unit_variants() {
+        for tool in [
+            ResponseTool::Computer,
+            ResponseTool::ApplyPatch,
+            ResponseTool::LocalShell,
+        ] {
+            let serialized = serde_json::to_value(&tool).unwrap();
+            let serde_tag = serialized.get("type").and_then(|v| v.as_str()).unwrap();
+            assert_eq!(tool.as_str(), serde_tag);
+        }
+    }
+}
diff --git a/model_gateway/src/routers/grpc/harmony/builder.rs b/model_gateway/src/routers/grpc/harmony/builder.rs
index 3efaacd30..f10efeba3 100644
--- a/model_gateway/src/routers/grpc/harmony/builder.rs
+++ b/model_gateway/src/routers/grpc/harmony/builder.rs
@@ -543,28 +543,7 @@ impl HarmonyBuilder {
             let tool_types: Vec<&str> = request
                 .tools
                 .as_ref()
-                .map(|tools| {
-                    tools
-                        .iter()
-                        .map(|tool| match tool {
-                            ResponseTool::Function(_) => "function",
-                            ResponseTool::WebSearchPreview(_) => "web_search_preview",
-                            ResponseTool::WebSearch(_) => "web_search",
-                            ResponseTool::CodeInterpreter(_) => "code_interpreter",
-                            ResponseTool::Mcp(_) => "mcp",
-                            ResponseTool::FileSearch(_) => "file_search",
-                            ResponseTool::ImageGeneration(_) => "image_generation",
-                            ResponseTool::Computer => "computer",
-                            ResponseTool::ComputerUsePreview(_) => "computer_use_preview",
-                            ResponseTool::Custom(_) => "custom",
-                            ResponseTool::Namespace(_) => "namespace",
-                            ResponseTool::Shell(_) => "shell",
-                            ResponseTool::ApplyPatch => "apply_patch",
-                            // T5 schema-only: forced-cascade arm, no behavior.
-                            ResponseTool::LocalShell => "local_shell",
-                        })
-                        .collect()
-                })
+                .map(|tools| tools.iter().map(ResponseTool::as_str).collect())
                 .unwrap_or_default();
 
             let with_custom_tools = has_custom_tools(&tool_types);