diff --git a/changelog.d/fixes/6726-kiro-adaptive-thinking-allowlist.md b/changelog.d/fixes/6726-kiro-adaptive-thinking-allowlist.md
new file mode 100644
index 0000000000..e31c103661
--- /dev/null
+++ b/changelog.d/fixes/6726-kiro-adaptive-thinking-allowlist.md
@@ -0,0 +1 @@
+- **fix(providers):** Kiro sent the adaptive-thinking `additionalModelRequestFields` envelope for `claude-sonnet-4.5`/`claude-haiku-4.5`, which Kiro/CodeWhisperer rejects upstream with a raw `[400]: additionalModelRequestFields is not supported for this model` ([#6576](https://github.com/diegosouzapw/OmniRoute/issues/6576)) — `buildKiroPayload()` (`open-sse/translator/request/openai-to-kiro.ts`) gated the field on the generic Anthropic-API `supportsReasoning()` capability flag, which is `true` for both models on Anthropic's direct API but does not reflect what Kiro's CodeWhisperer backend actually accepts; only `claude-sonnet-5` is confirmed adaptive-thinking-capable there. A new Kiro-specific allowlist (`supportsKiroAdaptiveThinking()` in `open-sse/translator/request/openai-to-kiro/adaptiveThinking.ts`) now gates the envelope instead. Regression guard: `tests/unit/repro-6576-kiro-thinking-unsupported-model.test.ts`.
diff --git a/open-sse/translator/request/openai-to-kiro.ts b/open-sse/translator/request/openai-to-kiro.ts
index d5d6d7f825..3da0a59340 100644
--- a/open-sse/translator/request/openai-to-kiro.ts
+++ b/open-sse/translator/request/openai-to-kiro.ts
@@ -5,12 +5,13 @@
 import { register } from "../registry.ts";
 import { FORMATS } from "../formats.ts";
 import { v4 as uuidv4, v5 as uuidv5 } from "uuid";
-import { capMaxOutputTokens, capThinkingBudget, supportsReasoning } from "@/lib/modelCapabilities";
+import { capMaxOutputTokens, capThinkingBudget } from "@/lib/modelCapabilities";
 import {
   parseToolInput,
   normalizeKiroToolSchema,
   serializeToolResultContent,
 } from "./openai-to-kiro/messageHelpers.ts";
+import { supportsKiroAdaptiveThinking } from "./openai-to-kiro/adaptiveThinking.ts";
 
 /**
  * Anthropic's direct-provider `[1m]` context-1m beta suffix. Kiro is AWS
@@ -858,15 +859,14 @@ export function buildKiroPayload(model, body, stream, credentials) {
   // Thinking mode for Claude models on Kiro (ported from javargasm/pi-kiro).
   // Two coordinated signals steer reasoning on the CodeWhisperer surface:
   //   1. a `<thinking_mode>enabled</thinking_mode><max_thinking_length>N</...>`
-  //      directive prepended to the current user message — makes Claude emit its
-  //      reasoning INLINE as `<thinking>…</thinking>`, which the Kiro executor
-  //      splits back into the OpenAI `reasoning_content` channel (kiroThinking.ts);
+  //      directive prepended to the user message — makes Claude emit reasoning
+  //      INLINE, split back into `reasoning_content` by the executor (kiroThinking.ts);
   //   2. top-level `additionalModelRequestFields` (output_config.effort +
   //      thinking:{type:"adaptive"} + a clamped max_tokens), forwarded to AWS by
-  //      the Kiro executor's transformRequest allowlist — this is the graded
-  //      effort lever. Gated on models that advertise thinking support.
+  //      the Kiro executor's transformRequest allowlist — the graded effort lever,
+  //      gated on Kiro's adaptive-thinking allowlist (#6576), not supportsReasoning().
   const requestedEffort = resolveKiroEffort(body) || (modelRequestsThinking ? "high" : "");
-  const kiroEffort = supportsReasoning(normalizedModel) ? requestedEffort : "";
+  const kiroEffort = supportsKiroAdaptiveThinking(normalizedModel) ? requestedEffort : "";
   if (kiroEffort) {
     // `<thinking_mode>` / `<max_thinking_length>` are Kiro/CodeWhisperer prompt
     // conventions (NOT Anthropic API params); the length is a soft hint (the hard
diff --git a/open-sse/translator/request/openai-to-kiro/adaptiveThinking.ts b/open-sse/translator/request/openai-to-kiro/adaptiveThinking.ts
new file mode 100644
index 0000000000..ba8fefdc97
--- /dev/null
+++ b/open-sse/translator/request/openai-to-kiro/adaptiveThinking.ts
@@ -0,0 +1,19 @@
+/**
+ * Kiro/AWS CodeWhisperer only accepts the adaptive-thinking
+ * `additionalModelRequestFields` envelope for a narrow allowlist of models —
+ * NOT the same set the generic Anthropic-API capability table
+ * (`supportsReasoning()` in `@/lib/modelCapabilities`) marks as
+ * thinking-capable. That table is correct for Anthropic's own API, but Kiro
+ * rejects the field for `claude-sonnet-4.5` and `claude-haiku-4.5` with a raw
+ * upstream 400 (`additionalModelRequestFields is not supported for this
+ * model`, issue #6576) even though both ARE thinking-capable on Anthropic's
+ * direct API. Only `claude-sonnet-5` is confirmed to accept the adaptive
+ * envelope on Kiro today — keep this allowlist in sync with
+ * `open-sse/config/providers/registry/kiro/index.ts` if Kiro's catalog or
+ * upstream behavior changes.
+ */
+const KIRO_ADAPTIVE_THINKING_MODELS = new Set(["claude-sonnet-5"]);
+
+export function supportsKiroAdaptiveThinking(normalizedModel: string): boolean {
+  return KIRO_ADAPTIVE_THINKING_MODELS.has(normalizedModel);
+}
diff --git a/tests/unit/repro-6576-kiro-thinking-unsupported-model.test.ts b/tests/unit/repro-6576-kiro-thinking-unsupported-model.test.ts
new file mode 100644
index 0000000000..8c56090b33
--- /dev/null
+++ b/tests/unit/repro-6576-kiro-thinking-unsupported-model.test.ts
@@ -0,0 +1,64 @@
+// Repro probe for GitHub issue #6576.
+//
+// Kiro/CodeWhisperer rejects `additionalModelRequestFields` for
+// claude-sonnet-4.5 / claude-haiku-4.5 with a raw upstream 400:
+//   "[400]: additionalModelRequestFields is not supported for this model"
+//
+// buildKiroPayload() gates thinking injection on supportsReasoning(model),
+// which resolves from the GENERIC Anthropic-API capability data
+// (MODEL_SPECS["claude-sonnet-4-5-..."].supportsThinking === true,
+// MODEL_SPECS["claude-haiku-4-5-20251001"].supportsThinking === true).
+// That flag says nothing about whether the *Kiro/AWS CodeWhisperer*
+// backend accepts the adaptive-thinking additionalModelRequestFields
+// envelope for these specific models — only the newer adaptive-only
+// models (Opus 4.7/4.8, Sonnet 5, Fable 5) are proven to accept it there
+// (see the existing "drops temperature when thinking is enabled" tests).
+//
+// This test asserts the payload for claude-sonnet-4.5 (the reporter's own
+// model) must NOT carry additionalModelRequestFields when reasoning is
+// requested, matching what Kiro's upstream actually accepts. It currently
+// FAILS because buildKiroPayload has no Kiro-specific allowlist/exclusion
+// and blindly forwards the field whenever the generic capability flag says
+// supportsThinking:true.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { buildKiroPayload } = await import(
+  "../../open-sse/translator/request/openai-to-kiro.ts"
+);
+
+test("[repro #6576] buildKiroPayload must not attach additionalModelRequestFields for claude-sonnet-4.5 (Kiro rejects it)", () => {
+  const body = {
+    messages: [{ role: "user", content: "Calculate 51818+62218, and reply with result only." }],
+    reasoning_effort: "medium",
+    max_tokens: 2048,
+    stream: false,
+  };
+
+  const result = buildKiroPayload("claude-sonnet-4.5", body, false, null);
+
+  assert.equal(
+    result.additionalModelRequestFields,
+    undefined,
+    "additionalModelRequestFields must not be sent for claude-sonnet-4.5 — " +
+      "Kiro/CodeWhisperer rejects it upstream with " +
+      "'[400]: additionalModelRequestFields is not supported for this model' (issue #6576)"
+  );
+});
+
+test("[repro #6576] buildKiroPayload must not attach additionalModelRequestFields for claude-haiku-4.5 (Kiro rejects it)", () => {
+  const body = {
+    messages: [{ role: "user", content: "hi" }],
+    thinking: { type: "adaptive" },
+  };
+
+  const result = buildKiroPayload("claude-haiku-4.5", body, false, null);
+
+  assert.equal(
+    result.additionalModelRequestFields,
+    undefined,
+    "additionalModelRequestFields must not be sent for claude-haiku-4.5 — " +
+      "Kiro/CodeWhisperer rejects it upstream (issue #6576 comment by fenix007: " +
+      "9/9 production requests with reasoning params 400'd for this exact model)"
+  );
+});
diff --git a/tests/unit/translator-openai-to-kiro.test.ts b/tests/unit/translator-openai-to-kiro.test.ts
index 2b34de746b..4cf19efebb 100644
--- a/tests/unit/translator-openai-to-kiro.test.ts
+++ b/tests/unit/translator-openai-to-kiro.test.ts
@@ -1049,10 +1049,10 @@ test("buildKiroPayload accepts kr/* model ids without the [1m] suffix", () => {
 test("buildKiroPayload strips local Kiro selector suffixes before upstream", () => {
   const body = { messages: [{ role: "user", content: "Hello" }] };
 
-  const result = buildKiroPayload("claude-opus-4.8-thinking-agentic", body, true, {});
+  const result = buildKiroPayload("claude-sonnet-5-thinking-agentic", body, true, {});
   assert.equal(
     result.conversationState.currentMessage.userInputMessage.modelId,
-    "claude-opus-4.8",
+    "claude-sonnet-5",
     "local -thinking/-agentic aliases must not be forwarded to Kiro"
   );
   assert.equal(
@@ -1121,7 +1121,7 @@ test("buildKiroPayload enables thinking mode for Claude models via reasoning_eff
     max_tokens: 64000,
   };
 
-  const result = buildKiroPayload("claude-opus-4.8", body, false, null);
+  const result = buildKiroPayload("claude-sonnet-5", body, false, null); // only Kiro model accepting adaptive thinking (#6576)
 
   assert.ok(result.additionalModelRequestFields, "additionalModelRequestFields must be set");
   assert.deepEqual(result.additionalModelRequestFields.thinking, {
@@ -1149,7 +1149,7 @@ test("buildKiroPayload drops temperature when thinking is enabled", () => {
     temperature: 0.5,
   };
 
-  const result = buildKiroPayload("claude-opus-4.8", body, false, null);
+  const result = buildKiroPayload("claude-sonnet-5", body, false, null);
 
   assert.ok(result.additionalModelRequestFields, "thinking must be enabled");
   assert.equal(
@@ -1180,7 +1180,7 @@ test("buildKiroPayload maps body.thinking budget_tokens to effort level", () =>
     thinking: { type: "enabled", budget_tokens: 50000 },
   };
 
-  const result = buildKiroPayload("claude-opus-4.7", body, false, null);
+  const result = buildKiroPayload("claude-sonnet-5", body, false, null);
 
   assert.ok(result.additionalModelRequestFields, "thinking must be enabled from budget_tokens");
   assert.equal(result.additionalModelRequestFields.output_config.effort, "high");
@@ -1212,7 +1212,7 @@ test("buildKiroPayload maps reasoning_effort to the same Kiro effort level (no +
 
 test("buildKiroPayload reads effort from Anthropic output_config.effort", () => {
   const result = buildKiroPayload(
-    "claude-opus-4.8",
+    "claude-sonnet-5",
     { messages: [{ role: "user", content: "hard" }], output_config: { effort: "xhigh" } },
     false,
     null
@@ -1224,7 +1224,7 @@ test("buildKiroPayload reads effort from Anthropic output_config.effort", () =>
 
 test("buildKiroPayload defaults adaptive thinking (no effort) to high", () => {
   const result = buildKiroPayload(
-    "claude-opus-4.8",
+    "claude-sonnet-5",
     { messages: [{ role: "user", content: "hard" }], thinking: { type: "adaptive" } },
     false,
     null
@@ -1239,7 +1239,7 @@ test("buildKiroPayload defaults adaptive thinking (no effort) to high", () => {
 
 test("buildKiroPayload drops both temperature and top_p when thinking is enabled", () => {
   const result = buildKiroPayload(
-    "claude-opus-4.8",
+    "claude-sonnet-5",
     {
       messages: [{ role: "user", content: "hard" }],
       reasoning_effort: "high",