diff --git a/open-sse/services/claudeCodeObfuscation.ts b/open-sse/services/claudeCodeObfuscation.ts
index 3a8423cbf1..92c7c86d6e 100644
--- a/open-sse/services/claudeCodeObfuscation.ts
+++ b/open-sse/services/claudeCodeObfuscation.ts
@@ -87,9 +87,18 @@ export function obfuscateInBody(body: Record<string, unknown>): void {
       if (typeof content === "string") {
         msg.content = obfuscateSensitiveWords(content);
       } else if (Array.isArray(content)) {
-        for (const block of content as Array<Record<string, unknown>>) {
-          if (typeof block.text === "string") {
-            block.text = obfuscateSensitiveWords(block.text);
+        // Anthropic verifies a signature over a thinking turn. Mutating a text
+        // sibling in that same turn invalidates it and makes the next request
+        // fail with `Invalid signature in thinking block`.
+        const blocks = content as Array<Record<string, unknown>>;
+        const hasSignedThinking = blocks.some(
+          (block) => block?.type === "thinking" || block?.type === "redacted_thinking"
+        );
+        if (!hasSignedThinking) {
+          for (const block of blocks) {
+            if (typeof block.text === "string") {
+              block.text = obfuscateSensitiveWords(block.text);
+            }
           }
         }
       }
diff --git a/open-sse/services/systemTransforms.ts b/open-sse/services/systemTransforms.ts
index 813bd5350f..e7542619e3 100644
--- a/open-sse/services/systemTransforms.ts
+++ b/open-sse/services/systemTransforms.ts
@@ -341,9 +341,17 @@ function applyObfuscateWords(body: RequestBody, op: ObfuscateWordsOp): void {
       if (typeof content === "string") {
         msg.content = obfuscateWithList(content, words);
       } else if (Array.isArray(content)) {
-        for (const block of content as Array<Record<string, unknown>>) {
-          if (typeof block.text === "string") {
-            block.text = obfuscateWithList(block.text, words);
+        // A signed Anthropic thinking turn covers its text siblings too. Leave
+        // the entire turn byte-for-byte intact so its signature remains valid.
+        const blocks = content as Array<Record<string, unknown>>;
+        const hasSignedThinking = blocks.some(
+          (block) => block?.type === "thinking" || block?.type === "redacted_thinking"
+        );
+        if (!hasSignedThinking) {
+          for (const block of blocks) {
+            if (typeof block.text === "string") {
+              block.text = obfuscateWithList(block.text, words);
+            }
           }
         }
       }
diff --git a/tests/unit/claude-code-obfuscation.test.ts b/tests/unit/claude-code-obfuscation.test.ts
new file mode 100644
index 0000000000..e426d2113c
--- /dev/null
+++ b/tests/unit/claude-code-obfuscation.test.ts
@@ -0,0 +1,30 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { obfuscateInBody } = await import("../../open-sse/services/claudeCodeObfuscation.ts");
+
+test("obfuscateInBody preserves a turn that carries signed thinking", () => {
+  const body = {
+    messages: [
+      {
+        role: "assistant",
+        content: [
+          { type: "thinking", thinking: "private", signature: "signed-by-anthropic" },
+          { type: "text", text: "I used opencode to inspect it." },
+        ],
+      },
+    ],
+  };
+
+  obfuscateInBody(body);
+  assert.equal((body.messages[0].content[1] as { text: string }).text, "I used opencode to inspect it.");
+});
+
+test("obfuscateInBody still obfuscates an unsigned normal text turn", () => {
+  const body = {
+    messages: [{ role: "assistant", content: [{ type: "text", text: "I used opencode." }] }],
+  };
+
+  obfuscateInBody(body);
+  assert.notEqual((body.messages[0].content[0] as { text: string }).text, "I used opencode.");
+});
diff --git a/tests/unit/system-transforms.test.ts b/tests/unit/system-transforms.test.ts
index ab3cbafb4f..cf1f7391e0 100644
--- a/tests/unit/system-transforms.test.ts
+++ b/tests/unit/system-transforms.test.ts
@@ -145,6 +145,25 @@ test("obfuscate_words with empty list is a no-op", () => {
   assert.equal((body.system[0] as { text: string }).text, "opencode");
 });
 
+test("obfuscate_words preserves a message turn with signed thinking", () => {
+  const body = {
+    messages: [
+      {
+        role: "assistant",
+        content: [
+          { type: "thinking", thinking: "private", signature: "signed-by-anthropic" },
+          { type: "text", text: "opencode remains unchanged" },
+        ],
+      },
+    ],
+  };
+  applyTransformPipeline(body, [{ kind: "obfuscate_words", words: ["opencode"] }]);
+  assert.equal(
+    (body.messages[0].content[1] as { text: string }).text,
+    "opencode remains unchanged"
+  );
+});
+
 // ────────────────────────────────────────────────────────────────────────────
 // Pipeline ordering: drop paragraph then obfuscate what survives
 // ────────────────────────────────────────────────────────────────────────────