diff --git a/open-sse/config/kiroConstants.js b/open-sse/config/kiroConstants.js
index e6408da1..f24cce04 100644
--- a/open-sse/config/kiroConstants.js
+++ b/open-sse/config/kiroConstants.js
@@ -171,7 +171,22 @@ export function resolveKiroThinkingBudget(body, headers, model) {
   return null;
 }
 
-export function extractKiroEffortLevel(body) {
+function parseClaudeVersion(model) {
+  if (typeof model !== "string") return null;
+  const normalized = model.toLowerCase().replace(/-/g, ".");
+  const match = normalized.match(/(?:^|[/.])claude(?:[/.][a-z]+)*[/.](\d+)(?:[/.](\d+))?(?:[/.]|$)/);
+  if (!match) return null;
+  return { major: Number(match[1]), minor: match[2] === undefined ? null : Number(match[2]) };
+}
+
+// Kiro effort tiers per model (kiro.dev docs + live additionalModelRequestFieldsSchema):
+// 4.6 Claude models cap at low|medium|high|max; 4.7+ add xhigh. Unknown models stay conservative.
+function kiroModelLacksXhigh(model) {
+  const v = parseClaudeVersion(model);
+  return !v || (v.major === 4 && v.minor !== null && v.minor <= 6);
+}
+
+export function extractKiroEffortLevel(body, model) {
   const effort =
     body?.output_config?.effort ??
     body?.reasoning_effort ??
@@ -179,7 +194,8 @@ export function extractKiroEffortLevel(body) {
   if (typeof effort !== "string") return null;
   const normalized = effort.toLowerCase();
   if (normalized === "none" || normalized === "off" || normalized === "disabled") return null;
-  if (normalized === "xhigh" || normalized === "max") return "high";
+  if (normalized === "xhigh") return kiroModelLacksXhigh(model) ? "high" : "xhigh";
+  if (normalized === "max") return "max";
   if (["low", "medium", "high"].includes(normalized)) return normalized;
   return null;
 }
@@ -199,10 +215,10 @@ function extractKiroGptEffortLevel(body) {
   return null;
 }
 
-export function buildKiroAdditionalModelRequestFields(body, effortPath = "output_config") {
+export function buildKiroAdditionalModelRequestFields(body, effortPath = "output_config", model) {
   const effort = effortPath === "reasoning"
     ? extractKiroGptEffortLevel(body)
-    : extractKiroEffortLevel(body);
+    : extractKiroEffortLevel(body, model);
   if (!effort) return undefined;
   if (effortPath === "reasoning") {
     // Mirrors Kiro CLI/KAS buildEffortRequestFields("reasoning") for GPT.
@@ -222,11 +238,9 @@ export function resolveKiroEffortPath(model) {
     return "reasoning";
   }
   if (!normalized.includes("claude")) return null;
-  const match = normalized.match(/(?:^|[/.])claude(?:[/.][a-z]+)*[/.](\d+)(?:[/.](\d+))?(?:[/.]|$)/);
-  if (!match) return null;
-  const [, majorText, minorText] = match;
-  const major = Number(majorText);
-  const minor = minorText === undefined ? null : Number(minorText);
+  const v = parseClaudeVersion(model);
+  if (!v) return null;
+  const { major, minor } = v;
   const dateSuffixMinor = minor !== null && minor >= 1000;
   // Kiro rejected additionalModelRequestFields on legacy 4.5 models in live smoke.
   // Default future Claude/Kiro models to supported so new model releases do not
@@ -248,7 +262,7 @@ export function usesKiroNativeGptEffort(body, model) {
 export function buildKiroAdditionalModelRequestFieldsForModel(body, model) {
   const effortPath = resolveKiroEffortPath(model);
   if (!effortPath) return undefined;
-  return buildKiroAdditionalModelRequestFields(body, effortPath);
+  return buildKiroAdditionalModelRequestFields(body, effortPath, model);
 }
 
 /**
diff --git a/open-sse/providers/thinkingLevels.js b/open-sse/providers/thinkingLevels.js
index 0b020100..5ecf2854 100644
--- a/open-sse/providers/thinkingLevels.js
+++ b/open-sse/providers/thinkingLevels.js
@@ -10,8 +10,8 @@ const L = {
   base: ["none", "low", "medium", "high"],                          // qwen, step, hunyuan, gemini-budget
   onOff: ["none", "thinking"],                                      // zai (binary), minimax (adaptive)
   openai: ["none", "minimal", "low", "medium", "high", "xhigh"],    // GPT-5.x / o-series (no "max")
-  levelMax: ["none", "low", "medium", "high", "max"],               // claude-adaptive, kimi
-  budgetX: ["none", "low", "medium", "high", "xhigh", "max"],       // claude-budget
+  levelMax: ["none", "low", "medium", "high", "max"],               // kimi
+  budgetX: ["none", "low", "medium", "high", "xhigh", "max"],       // claude-budget, claude-adaptive
   gemini: ["minimal", "low", "medium", "high"],                     // gemini-3 thinkingLevel (no disable)
   hiMax: ["none", "high", "max"],                                   // deepseek (low/med→high, xhigh→max)
 };
@@ -19,7 +19,7 @@ const L = {
 // thinkingFormat → valid selectable levels (source of truth for UI options).
 const FORMAT_LEVELS = {
   openai: L.openai,
-  "claude-adaptive": L.levelMax,
+  "claude-adaptive": L.budgetX,
   "claude-budget": L.budgetX,
   "gemini-level": L.gemini,
   "gemini-budget": L.base,
@@ -35,8 +35,13 @@ const FORMAT_LEVELS = {
 
 const CODEX_GPT_5_6_LEVELS = ["none", "minimal", "low", "medium", "high", "xhigh", "max"];
 
+// Opus/Sonnet 4.6 lack xhigh (Anthropic + Kiro docs) — keep the 4-level+max set.
+const CLAUDE_NO_XHIGH = ["none", "low", "medium", "high", "max"];
+
 // Model-name pattern overrides (glob, first match wins) — more precise than format default.
 const PATTERN_THINKING = [
+  { pattern: "*claude*4.6*", levels: CLAUDE_NO_XHIGH },
+  { pattern: "*claude*4-6*", levels: CLAUDE_NO_XHIGH },
   { provider: "codex", pattern: "*gpt-6*", levels: CODEX_GPT_5_6_LEVELS },
   { provider: "codex", pattern: "*gpt-5.6-sol*", levels: [...CODEX_GPT_5_6_LEVELS, "ultra"] },
   { provider: "codex", pattern: "*gpt-5.6-terra*", levels: [...CODEX_GPT_5_6_LEVELS, "ultra"] },
diff --git a/open-sse/translator/concerns/thinkingUnified.js b/open-sse/translator/concerns/thinkingUnified.js
index 5ce73b32..4bcacd3b 100644
--- a/open-sse/translator/concerns/thinkingUnified.js
+++ b/open-sse/translator/concerns/thinkingUnified.js
@@ -271,7 +271,9 @@ function applyFormat(fmt, body, cfg, caps, supportedLevels, display) {
       if (canDisable) body.thinking = { type: "adaptive", ...(display ? { display } : {}) };
       else delete body.thinking;
       const level = toLevel(eff);
-      body.output_config = { effort: level === "xhigh" || level === "auto" ? "high" : level };
+      // xhigh is model-gated (Opus/Sonnet 4.6 reject it) — clamp when not advertised.
+      body.output_config = { effort: level === "auto" ? "high"
+        : level === "xhigh" && !supportedLevels?.includes("xhigh") ? "high" : level };
       break;
     }
     case "claude-budget": {
diff --git a/tests/unit/thinking-levels-kiro.test.js b/tests/unit/thinking-levels-kiro.test.js
index edb3b7aa..b1ff2acb 100644
--- a/tests/unit/thinking-levels-kiro.test.js
+++ b/tests/unit/thinking-levels-kiro.test.js
@@ -1,5 +1,7 @@
 import { describe, it, expect } from "vitest";
 import { getThinkingLevels } from "../../open-sse/providers/thinkingLevels.js";
+import { buildKiroAdditionalModelRequestFieldsForModel } from "../../open-sse/config/kiroConstants.js";
+import { applyThinking } from "../../open-sse/translator/concerns/thinkingUnified.js";
 
 describe("getThinkingLevels for Kiro", () => {
   it("does not advertise native intensity for legacy Kiro models", () => {
@@ -9,6 +11,25 @@ describe("getThinkingLevels for Kiro", () => {
 
   it("advertises native levels for supported Kiro models", () => {
     expect(getThinkingLevels("kiro", "claude-sonnet-5")).toContain("high");
+    expect(getThinkingLevels("kiro", "claude-sonnet-5")).toContain("xhigh");
+    expect(getThinkingLevels("kiro", "claude-sonnet-5")).toContain("max");
     expect(getThinkingLevels("kiro", "gpt-5.6-sol")).toContain("xhigh");
   });
+
+  it("omits xhigh on 4.6 models (upstream rejects it there)", () => {
+    for (const model of ["claude-opus-4.6", "claude-opus-4-6", "claude-sonnet-4.6"]) {
+      expect(getThinkingLevels("kiro", model)).not.toContain("xhigh");
+      expect(getThinkingLevels("kiro", model)).toContain("max");
+    }
+  });
+
+  it("passes xhigh/max through on the wire for 4.7+, clamps xhigh on 4.6", () => {
+    const xhigh = { output_config: { effort: "xhigh" } };
+    expect(buildKiroAdditionalModelRequestFieldsForModel(xhigh, "claude-sonnet-5")?.output_config?.effort).toBe("xhigh");
+    expect(buildKiroAdditionalModelRequestFieldsForModel({ output_config: { effort: "max" } }, "claude-opus-4.6")?.output_config?.effort).toBe("max");
+    expect(buildKiroAdditionalModelRequestFieldsForModel(xhigh, "claude-opus-4.6")?.output_config?.effort).toBe("high");
+    // Anthropic-wire path: suffix override sends real xhigh on 4.7+, high on 4.6.
+    expect(applyThinking("claude", "claude-opus-5.5(xhigh)", { messages: [] }, "claude").output_config?.effort).toBe("xhigh");
+    expect(applyThinking("claude", "claude-opus-4.6(xhigh)", { messages: [] }, "claude").output_config?.effort).toBe("high");
+  });
 });