diff --git a/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.test.ts b/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.test.ts
index 56e622e98e1..1501dfa913a 100644
--- a/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.test.ts
+++ b/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.test.ts
@@ -156,6 +156,26 @@ describe("createMantleAnthropicStreamFn", () => {
     expect(streamOptions.effort).toBe("high");
   });
 
+  it("maps Mythos Preview minimal reasoning to low effort", () => {
+    const model = createTestModel({
+      id: "anthropic.claude-mythos-preview",
+      name: "Claude Mythos Preview",
+      reasoning: true,
+      params: { canonicalModelId: "claude-mythos-preview" },
+    });
+    const deps = createTestDeps();
+    deps.stream.mockReturnValue({ kind: "anthropic-stream" } as never);
+
+    void createMantleAnthropicStreamFn(deps)(model, { messages: [] }, {
+      apiKey: "bedrock-bearer-token",
+      reasoning: "minimal",
+    });
+
+    const streamOptions = firstStreamOptions(deps);
+    expect(streamOptions.thinkingEnabled).toBe(true);
+    expect(streamOptions.effort).toBe("low");
+  });
+
   it("normalizes Mantle provider URLs to the Anthropic endpoint", () => {
     expect(resolveMantleAnthropicBaseUrl("https://bedrock-mantle.us-east-1.api.aws/v1")).toBe(
       "https://bedrock-mantle.us-east-1.api.aws/anthropic",
diff --git a/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.ts b/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.ts
index 395fc23ec84..9ce3e9434ba 100644
--- a/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.ts
+++ b/extensions/amazon-bedrock-mantle/mantle-anthropic.runtime.ts
@@ -48,9 +48,13 @@ function resolveMantleReasoning(
     return undefined;
   }
   const reasoning = options?.reasoning ?? (isClaudeMythosPreviewModel(model) ? "high" : undefined);
-  return isClaudeMythosPreviewModel(model) && (reasoning === "xhigh" || reasoning === "max")
-    ? "high"
-    : reasoning;
+  if (!isClaudeMythosPreviewModel(model)) {
+    return reasoning;
+  }
+  if (reasoning === "minimal") {
+    return "low";
+  }
+  return reasoning === "xhigh" || reasoning === "max" ? "high" : reasoning;
 }
 
 function mergeHeaders(