diff --git a/src/agents/pi-embedded-utils.test.ts b/src/agents/pi-embedded-utils.test.ts
index ab84a375d94..a7894452cec 100644
--- a/src/agents/pi-embedded-utils.test.ts
+++ b/src/agents/pi-embedded-utils.test.ts
@@ -461,6 +461,11 @@ File contents here`,
         text: "<antthinking>Some reasoning</antthinking>The actual answer.",
         expected: "The actual answer.",
       },
+      {
+        name: "antml namespaced thinking tag",
+        text: "<antml:thinking>This shows Robin Waslander DMing maintainers o...</antml:thinking>Actual reply.",
+        expected: "Actual reply.",
+      },
       {
         name: "final wrapper",
         text: "<final>\nAnswer\n</final>",
diff --git a/src/shared/text/reasoning-tags.test.ts b/src/shared/text/reasoning-tags.test.ts
index 59f22c9212e..e85ee56615c 100644
--- a/src/shared/text/reasoning-tags.test.ts
+++ b/src/shared/text/reasoning-tags.test.ts
@@ -50,6 +50,11 @@ describe("stripReasoningTagsFromText", () => {
         input: "X <antthinking>internal</antthinking> Y",
         expected: "X  Y",
       },
+      {
+        name: "strips antml namespaced thinking tags",
+        input: "Before <antml:thinking>secret</antml:thinking> after",
+        expected: "Before  after",
+      },
       {
         name: "strips multiple reasoning blocks",
         input: "<think>first</think>A<think>second</think>B",
@@ -194,6 +199,10 @@ describe("stripReasoningTagsFromText", () => {
         input: "A <THINK>hidden</THINK> <Thinking>also hidden</Thinking> B",
         expected: "A   B",
       },
+      {
+        input: "A <ANTML:THINKING hidden='1'>secret</ANTML:THINKING> B",
+        expected: "A  B",
+      },
     ] as const)("handles unicode/attributes/case-insensitive names: %j", (testCase) => {
       expectStrippedCase(testCase);
     });
diff --git a/src/shared/text/reasoning-tags.ts b/src/shared/text/reasoning-tags.ts
index fcf508b5724..62b73d5fe4f 100644
--- a/src/shared/text/reasoning-tags.ts
+++ b/src/shared/text/reasoning-tags.ts
@@ -2,9 +2,10 @@ import { findCodeRegions, isInsideCode } from "./code-regions.js";
 export type ReasoningTagMode = "strict" | "preserve";
 export type ReasoningTagTrim = "none" | "start" | "both";
 
-const QUICK_TAG_RE = /<\s*\/?\s*(?:think(?:ing)?|thought|antthinking|final)\b/i;
+const QUICK_TAG_RE = /<\s*\/?\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking|final)\b/i;
 const FINAL_TAG_RE = /<\s*\/?\s*final\b[^<>]*>/gi;
-const THINKING_TAG_RE = /<\s*(\/?)\s*(?:think(?:ing)?|thought|antthinking)\b[^<>]*>/gi;
+const THINKING_TAG_RE =
+  /<\s*(\/?)\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\b[^<>]*>/gi;
 
 function applyTrim(value: string, mode: ReasoningTagTrim): string {
   if (mode === "none") {