diff --git a/src/agents/pi-embedded-subscribe.e2e-harness.ts b/src/agents/pi-embedded-subscribe.e2e-harness.ts
index 53fc38233f4..12138fefbcd 100644
--- a/src/agents/pi-embedded-subscribe.e2e-harness.ts
+++ b/src/agents/pi-embedded-subscribe.e2e-harness.ts
@@ -13,6 +13,7 @@ export const THINKING_TAG_CASES = [
   { tag: "thinking", open: "<thinking>", close: "</thinking>" },
   { tag: "thought", open: "<thought>", close: "</thought>" },
   { tag: "antthinking", open: "<antthinking>", close: "</antthinking>" },
+  { tag: "antml:thinking", open: "<antml:thinking>", close: "</antml:thinking>" },
 ] as const;
 
 export function createStubSessionHarness(): {
diff --git a/src/agents/pi-embedded-subscribe.ts b/src/agents/pi-embedded-subscribe.ts
index e257abf58b5..24b4bdb4718 100644
--- a/src/agents/pi-embedded-subscribe.ts
+++ b/src/agents/pi-embedded-subscribe.ts
@@ -36,7 +36,8 @@ import type { SubscribeEmbeddedPiSessionParams } from "./pi-embedded-subscribe.t
 import { formatReasoningMessage, stripDowngradedToolCallText } from "./pi-embedded-utils.js";
 import { hasNonzeroUsage, normalizeUsage, type UsageLike } from "./usage.js";
 
-const THINKING_TAG_SCAN_RE = /<\s*(\/?)\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
+const THINKING_TAG_SCAN_RE =
+  /<\s*(\/?)\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/gi;
 const FINAL_TAG_SCAN_RE = /<\s*(\/?)\s*final\s*>/gi;
 const log = createSubsystemLogger("agent/embedded");
 
diff --git a/src/agents/pi-embedded-utils.test.ts b/src/agents/pi-embedded-utils.test.ts
index 2c69c9614bd..01cf81e386f 100644
--- a/src/agents/pi-embedded-utils.test.ts
+++ b/src/agents/pi-embedded-utils.test.ts
@@ -806,6 +806,20 @@ describe("promoteThinkingTagsToBlocks", () => {
     expect(types).toContain("text");
   });
 
+  it("splits antml namespaced thinking tags into thinking blocks", () => {
+    const msg = makeAssistantMessage({
+      role: "assistant",
+      content: [{ type: "text", text: "<antml:thinking>hidden</antml:thinking>Visible" }],
+      timestamp: Date.now(),
+    });
+
+    promoteThinkingTagsToBlocks(msg);
+    expect(msg.content).toEqual([
+      { type: "thinking", thinking: "hidden" },
+      { type: "text", text: "Visible" },
+    ]);
+  });
+
   it("does not crash on undefined content entries", () => {
     const msg = makeAssistantMessage({
       role: "assistant",
diff --git a/src/agents/pi-embedded-utils.ts b/src/agents/pi-embedded-utils.ts
index 666716ed20b..67b4b6adb47 100644
--- a/src/agents/pi-embedded-utils.ts
+++ b/src/agents/pi-embedded-utils.ts
@@ -189,8 +189,8 @@ export function splitThinkingTaggedText(text: string): ThinkTaggedSplitBlock[] |
   if (!trimmedStart.startsWith("<")) {
     return null;
   }
-  const openRe = /<\s*(?:think(?:ing)?|thought|antthinking)\s*>/i;
-  const closeRe = /<\s*\/\s*(?:think(?:ing)?|thought|antthinking)\s*>/i;
+  const openRe = /<\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/i;
+  const closeRe = /<\s*\/\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/i;
   if (!openRe.test(trimmedStart)) {
     return null;
   }
@@ -198,7 +198,7 @@ export function splitThinkingTaggedText(text: string): ThinkTaggedSplitBlock[] |
     return null;
   }
 
-  const scanRe = /<\s*(\/?)\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
+  const scanRe = /<\s*(\/?)\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/gi;
   let inThinking = false;
   let cursor = 0;
   let thinkingStart = 0;
@@ -299,7 +299,7 @@ export function extractThinkingFromTaggedText(text: string): string {
   if (!text) {
     return "";
   }
-  const scanRe = /<\s*(\/?)\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
+  const scanRe = /<\s*(\/?)\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/gi;
   let result = "";
   let lastIndex = 0;
   let inThinking = false;
@@ -324,8 +324,8 @@ export function extractThinkingFromTaggedStream(text: string): string {
     return closed;
   }
 
-  const openRe = /<\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
-  const closeRe = /<\s*\/\s*(?:think(?:ing)?|thought|antthinking)\s*>/gi;
+  const openRe = /<\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/gi;
+  const closeRe = /<\s*\/\s*(?:(?:antml:)?(?:think(?:ing)?|thought)|antthinking)\s*>/gi;
   const openMatches = [...text.matchAll(openRe)];
   if (openMatches.length === 0) {
     return "";