diff --git a/crates/protocols/src/responses.rs b/crates/protocols/src/responses.rs
index 55463a3b7..b27fe367b 100644
--- a/crates/protocols/src/responses.rs
+++ b/crates/protocols/src/responses.rs
@@ -1531,12 +1531,25 @@ fn default_reasoning_effort() -> Option<ReasoningEffort> {
 #[derive(Debug, Clone, Deserialize, Serialize, schemars::JsonSchema)]
 #[serde(rename_all = "snake_case")]
 pub enum ReasoningEffort {
+    None,
     Minimal,
     Low,
     Medium,
     High,
 }
 
+impl ReasoningEffort {
+    pub const fn as_str(&self) -> &'static str {
+        match self {
+            Self::None => "none",
+            Self::Minimal => "minimal",
+            Self::Low => "low",
+            Self::Medium => "medium",
+            Self::High => "high",
+        }
+    }
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, schemars::JsonSchema)]
 #[serde(rename_all = "snake_case")]
 pub enum ReasoningSummary {
diff --git a/crates/protocols/tests/responses.rs b/crates/protocols/tests/responses.rs
index c8cc6075c..5d4bf40a0 100644
--- a/crates/protocols/tests/responses.rs
+++ b/crates/protocols/tests/responses.rs
@@ -10,6 +10,16 @@ use openai_protocol::{
 use serde_json::json;
 use validator::Validate;
 
+#[test]
+fn reasoning_effort_none_round_trips() {
+    let payload = json!({"effort": "none"});
+    let reasoning: ResponseReasoningParam =
+        serde_json::from_value(payload.clone()).expect("reasoning effort none should deserialize");
+
+    assert!(matches!(reasoning.effort, Some(ReasoningEffort::None)));
+    assert_eq!(serde_json::to_value(reasoning).expect("serialize"), payload);
+}
+
 #[test]
 fn summary_text_content_round_trips_spec_shape() {
     // Spec: `summary: array of SummaryTextContent { text, type: "summary_text" }`.
diff --git a/model_gateway/src/routers/grpc/harmony/builder.rs b/model_gateway/src/routers/grpc/harmony/builder.rs
index f10efeba3..27c1f929e 100644
--- a/model_gateway/src/routers/grpc/harmony/builder.rs
+++ b/model_gateway/src/routers/grpc/harmony/builder.rs
@@ -415,28 +415,29 @@ impl HarmonyBuilder {
         self.build_system_message(reasoning_effort, has_tools)
     }
 
-    /// Build system message from ResponsesRequest
+    /// Convert Responses reasoning effort to Harmony's supported values.
     ///
     /// # Arguments
     /// * `request` - The ResponsesRequest
-    /// * `with_custom_tools` - Whether custom tools (beyond built-ins) are present
-    fn build_system_message_from_responses(
-        &self,
+    ///
+    /// Returns an error for `none`, which Harmony cannot represent.
+    fn reasoning_effort_from_responses(
         request: &ResponsesRequest,
-        with_custom_tools: bool,
-    ) -> HarmonyMessage {
-        let reasoning_effort = request
+    ) -> Result<Option<ReasoningEffort>, String> {
+        request
             .reasoning
             .as_ref()
             .and_then(|r| r.effort.as_ref())
             .map(|effort| match effort {
-                ResponsesReasoningEffort::High => ReasoningEffort::High,
-                ResponsesReasoningEffort::Medium => ReasoningEffort::Medium,
-                ResponsesReasoningEffort::Low => ReasoningEffort::Low,
-                ResponsesReasoningEffort::Minimal => ReasoningEffort::Low,
-            });
-
-        self.build_system_message(reasoning_effort, with_custom_tools)
+                ResponsesReasoningEffort::None => {
+                    Err("reasoning.effort 'none' is not supported by Harmony models".to_string())
+                }
+                ResponsesReasoningEffort::High => Ok(ReasoningEffort::High),
+                ResponsesReasoningEffort::Medium => Ok(ReasoningEffort::Medium),
+                ResponsesReasoningEffort::Low => Ok(ReasoningEffort::Low),
+                ResponsesReasoningEffort::Minimal => Ok(ReasoningEffort::Low),
+            })
+            .transpose()
     }
 
     /// Build developer message with common logic
@@ -535,6 +536,7 @@ impl HarmonyBuilder {
         request: &ResponsesRequest,
     ) -> Result<Vec<HarmonyMessage>, String> {
         let mut all_messages = Vec::new();
+        let reasoning_effort = Self::reasoning_effort_from_responses(request)?;
 
         // Handle new vs continuing conversation
         if request.previous_response_id.is_none() {
@@ -549,7 +551,7 @@ impl HarmonyBuilder {
             let with_custom_tools = has_custom_tools(&tool_types);
 
             // Add system message
-            let sys_msg = self.build_system_message_from_responses(request, with_custom_tools);
+            let sys_msg = self.build_system_message(reasoning_effort, with_custom_tools);
             all_messages.push(sys_msg);
 
             // Add developer message if we have custom tools or instructions
@@ -1169,6 +1171,27 @@ mod tests {
 
     use super::*;
 
+    #[test]
+    fn responses_reasoning_effort_none_is_rejected_for_all_harmony_requests() {
+        for previous_response_id in [None, Some("resp_previous".to_string())] {
+            let request = ResponsesRequest {
+                input: ResponseInput::Text("Answer briefly".to_string()),
+                previous_response_id,
+                reasoning: Some(openai_protocol::responses::ResponseReasoningParam {
+                    effort: Some(openai_protocol::responses::ReasoningEffort::None),
+                    summary: None,
+                }),
+                ..Default::default()
+            };
+
+            let result = HarmonyBuilder::new().construct_input_messages_with_harmony(&request);
+            assert_eq!(
+                result.err().as_deref(),
+                Some("reasoning.effort 'none' is not supported by Harmony models")
+            );
+        }
+    }
+
     /// Invariant: `image_generation` must never be advertised as a
     /// gpt-oss native builtin tool. If a future change re-adds it,
     /// gpt-oss's behavior becomes undefined (hallucinated tool call
diff --git a/model_gateway/src/routers/grpc/regular/responses/conversions.rs b/model_gateway/src/routers/grpc/regular/responses/conversions.rs
index 08c92def4..4b8dd0ba5 100644
--- a/model_gateway/src/routers/grpc/regular/responses/conversions.rs
+++ b/model_gateway/src/routers/grpc/regular/responses/conversions.rs
@@ -240,6 +240,11 @@ pub(crate) fn responses_to_chat(req: &ResponsesRequest) -> Result<ChatCompletion
             None
         },
         parallel_tool_calls: req.parallel_tool_calls,
+        reasoning_effort: req
+            .reasoning
+            .as_ref()
+            .and_then(|reasoning| reasoning.effort.as_ref())
+            .map(|effort| effort.as_str().to_string()),
         top_logprobs: req.top_logprobs,
         top_p: req.top_p,
         skip_special_tokens: true,
@@ -446,6 +451,21 @@ mod tests {
         assert_eq!(chat_req.temperature, Some(0.7));
     }
 
+    #[test]
+    fn test_reasoning_effort_none_conversion() {
+        let req = ResponsesRequest {
+            input: ResponseInput::Text("Answer briefly".to_string()),
+            reasoning: Some(openai_protocol::responses::ResponseReasoningParam {
+                effort: Some(openai_protocol::responses::ReasoningEffort::None),
+                summary: None,
+            }),
+            ..Default::default()
+        };
+
+        let chat_req = responses_to_chat(&req).unwrap();
+        assert_eq!(chat_req.reasoning_effort.as_deref(), Some("none"));
+    }
+
     #[test]
     fn test_items_input_conversion() {
         let req = ResponsesRequest {
diff --git a/model_gateway/src/routers/openai/responses/route.rs b/model_gateway/src/routers/openai/responses/route.rs
index 47bed0cdb..3c9a139e3 100644
--- a/model_gateway/src/routers/openai/responses/route.rs
+++ b/model_gateway/src/routers/openai/responses/route.rs
@@ -215,8 +215,8 @@ mod tests {
     use openai_protocol::{
         common::Detail,
         responses::{
-            Annotation, FileDetail, ResponseContentPart, ResponseInput, ResponseInputOutputItem,
-            ResponsesRequest,
+            Annotation, FileDetail, ReasoningEffort, ResponseContentPart, ResponseInput,
+            ResponseInputOutputItem, ResponseReasoningParam, ResponsesRequest,
         },
     };
     use serde_json::{json, to_value};
@@ -305,6 +305,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn router_serialization_preserves_reasoning_effort_none() {
+        let req = ResponsesRequest {
+            model: "gpt-5.4".to_string(),
+            input: ResponseInput::Text("Answer briefly".to_string()),
+            reasoning: Some(ResponseReasoningParam {
+                effort: Some(ReasoningEffort::None),
+                summary: None,
+            }),
+            ..Default::default()
+        };
+
+        let payload = serialize_like_router(&req);
+        assert_eq!(payload["reasoning"]["effort"], json!("none"));
+    }
+
     #[test]
     fn router_serialization_omits_empty_input_image_fields() {
         // `file_id` / `image_url` / `detail` are all optional; the wire
diff --git a/model_gateway/tests/api/api_endpoints_test.rs b/model_gateway/tests/api/api_endpoints_test.rs
index c37425397..84c16d885 100644
--- a/model_gateway/tests/api/api_endpoints_test.rs
+++ b/model_gateway/tests/api/api_endpoints_test.rs
@@ -686,6 +686,37 @@ mod responses_endpoint_tests {
         ctx.shutdown().await;
     }
 
+    #[tokio::test]
+    async fn test_v1_responses_accepts_reasoning_effort_none() {
+        let ctx = AppTestContext::new(vec![MockWorkerConfig {
+            port: 18959,
+            worker_type: WorkerType::Regular,
+            health_status: HealthStatus::Healthy,
+            response_delay_ms: 0,
+            fail_rate: 0.0,
+        }])
+        .await;
+
+        let app = ctx.create_app();
+        let payload = json!({
+            "input": "Answer briefly",
+            "model": "mock-model",
+            "reasoning": {"effort": "none"},
+            "stream": false
+        });
+        let req = Request::builder()
+            .method("POST")
+            .uri("/v1/responses")
+            .header(CONTENT_TYPE, "application/json")
+            .body(Body::from(serde_json::to_string(&payload).unwrap()))
+            .unwrap();
+
+        let resp = app.clone().oneshot(req).await.unwrap();
+        assert_eq!(resp.status(), StatusCode::OK);
+
+        ctx.shutdown().await;
+    }
+
     #[tokio::test]
     async fn test_v1_responses_streaming() {
         let ctx = AppTestContext::new(vec![MockWorkerConfig {