diff --git a/open-sse/config/kiroConstants.js b/open-sse/config/kiroConstants.js
index 93b8f4569e..120b5e38df 100644
--- a/open-sse/config/kiroConstants.js
+++ b/open-sse/config/kiroConstants.js
@@ -144,9 +144,13 @@ export function extractKiroEffortLevel(body) {
   return null;
 }
 
-export function buildKiroAdditionalModelRequestFields(body) {
+export function buildKiroAdditionalModelRequestFields(body, effortPath = "output_config") {
   const effort = extractKiroEffortLevel(body);
   if (!effort) return undefined;
+  if (effortPath === "reasoning") {
+    // Mirrors Kiro CLI/KAS buildEffortRequestFields("reasoning") for GPT.
+    return { reasoning: { effort } };
+  }
   // Mirrors Kiro CLI/KAS buildEffortRequestFields("output_config").
   return {
     thinking: { type: "adaptive", display: "summarized" },
@@ -154,12 +158,15 @@ export function buildKiroAdditionalModelRequestFields(body) {
   };
 }
 
-export function supportsKiroAdditionalModelRequestFields(model) {
-  if (typeof model !== "string") return false;
+export function resolveKiroEffortPath(model) {
+  if (typeof model !== "string") return null;
   const normalized = model.toLowerCase().replace(/-/g, ".");
-  if (!normalized.includes("claude")) return false;
+  if (/(?:^|[/.])gpt[/.]5[/.]6(?:[/.]|$)/.test(normalized)) {
+    return "reasoning";
+  }
+  if (!normalized.includes("claude")) return null;
   const match = normalized.match(/(?:^|[/.])claude(?:[/.][a-z]+)*[/.](\d+)(?:[/.](\d+))?(?:[/.]|$)/);
-  if (!match) return false;
+  if (!match) return null;
   const [, majorText, minorText] = match;
   const major = Number(majorText);
   const minor = minorText === undefined ? null : Number(minorText);
@@ -167,12 +174,19 @@ export function supportsKiroAdditionalModelRequestFields(model) {
   // Kiro rejected additionalModelRequestFields on legacy 4.5 models in live smoke.
   // Default future Claude/Kiro models to supported so new model releases do not
   // need a code allowlist update.
-  return !(major < 4 || (major === 4 && (minor === null || minor <= 5 || dateSuffixMinor)));
+  return major < 4 || (major === 4 && (minor === null || minor <= 5 || dateSuffixMinor))
+    ? null
+    : "output_config";
+}
+
+export function supportsKiroAdditionalModelRequestFields(model) {
+  return resolveKiroEffortPath(model) !== null;
 }
 
 export function buildKiroAdditionalModelRequestFieldsForModel(body, model) {
-  if (!supportsKiroAdditionalModelRequestFields(model)) return undefined;
-  return buildKiroAdditionalModelRequestFields(body);
+  const effortPath = resolveKiroEffortPath(model);
+  if (!effortPath) return undefined;
+  return buildKiroAdditionalModelRequestFields(body, effortPath);
 }
 
 /**
diff --git a/open-sse/translator/request/openai-to-kiro.js b/open-sse/translator/request/openai-to-kiro.js
index a2681b5a7d..b49185374b 100644
--- a/open-sse/translator/request/openai-to-kiro.js
+++ b/open-sse/translator/request/openai-to-kiro.js
@@ -511,12 +511,10 @@ function convertMessages(messages, tools, model) {
  *    Kiro's 2-3 minute server timeout. The suffix is stripped before being
  *    sent upstream.
  *
- * 2. Thinking / reasoning. Kiro does not accept `thinking.type` or
- *    `reasoning_effort` natively. The only way to enable reasoning is to
- *    inject `<thinking_mode>enabled</thinking_mode>` into the user content
- *    sent upstream. Detection covers Anthropic-Beta header, Claude API
+ * 2. Thinking / reasoning. Detection covers Anthropic-Beta header, Claude API
  *    `thinking`, OpenAI `reasoning_effort`, AMP/Cursor magic tags, and model
- *    name hints.
+ *    name hints. Kiro's prompt tags remain for compatibility, while supported
+ *    models also receive the same schema-specific effort fields as Kiro CLI.
  */
 export function openaiToKiroRequest(model, body, stream, credentials) {
   const messages = body.messages || [];
diff --git a/tests/translator/claude-kiro-direct.test.js b/tests/translator/claude-kiro-direct.test.js
index 01e217092a..00d7745621 100644
--- a/tests/translator/claude-kiro-direct.test.js
+++ b/tests/translator/claude-kiro-direct.test.js
@@ -112,6 +112,17 @@ describe("Claude → Kiro (direct route)", () => {
     expect(out.systemPrompt).toContain("<max_thinking_length>24576</max_thinking_length>");
   });
 
+  it("maps Claude-format effort to GPT-5.6 Kiro CLI reasoning fields", () => {
+    const out = C2K({
+      output_config: { effort: "low" },
+      messages: [{ role: "user", content: "think lightly" }],
+    }, null, "gpt-5.6-sol");
+
+    expect(out.additionalModelRequestFields).toEqual({
+      reasoning: { effort: "low" },
+    });
+  });
+
   it("sends Claude system as top-level systemPrompt and keeps a user-content fallback", () => {
     const out = C2K({
       system: "system-only instruction",
diff --git a/tests/unit/openai-to-kiro.test.js b/tests/unit/openai-to-kiro.test.js
index 8b1c97e868..13f67bef72 100644
--- a/tests/unit/openai-to-kiro.test.js
+++ b/tests/unit/openai-to-kiro.test.js
@@ -316,6 +316,44 @@ describe("openaiToKiroRequest", () => {
       });
     });
 
+    it("maps GPT-5.6 reasoning.effort high to Kiro CLI reasoning fields", () => {
+      const body = {
+        reasoning: { effort: "high" },
+        messages: [{ role: "user", content: "Think deeply" }]
+      };
+
+      const result = openaiToKiroRequest("gpt-5.6-sol", body, true, {});
+
+      expect(systemPromptOf(result)).toContain("<max_thinking_length>24576</max_thinking_length>");
+      expect(result.additionalModelRequestFields).toEqual({
+        reasoning: { effort: "high" },
+      });
+    });
+
+    it("maps prefixed GPT-5.6 reasoning_effort medium to Kiro CLI reasoning fields", () => {
+      const body = {
+        reasoning_effort: "medium",
+        messages: [{ role: "user", content: "Think normally" }]
+      };
+
+      const result = openaiToKiroRequest("kiro/gpt-5.6-terra", body, true, {});
+
+      expect(result.additionalModelRequestFields).toEqual({
+        reasoning: { effort: "medium" },
+      });
+    });
+
+    it("does not forward unsupported GPT-5.6 effort values", () => {
+      const body = {
+        reasoning: { effort: "ultra" },
+        messages: [{ role: "user", content: "Unknown effort" }]
+      };
+
+      const result = openaiToKiroRequest("gpt-5.6-luna", body, true, {});
+
+      expect(result.additionalModelRequestFields).toBeUndefined();
+    });
+
     it("does not send additionalModelRequestFields for legacy Kiro model ids", () => {
       const body = {
         reasoning_effort: "high",