diff --git a/changelog.d/fixes/12920-aging-tool-result-block-order.md b/changelog.d/fixes/12920-aging-tool-result-block-order.md
new file mode 100644
index 00000000000..6f53f0e187f
--- /dev/null
+++ b/changelog.d/fixes/12920-aging-tool-result-block-order.md
@@ -0,0 +1 @@
+- **fix(compression):** progressive aging now appends its `[COMPRESSED:aging:…]` annotation after a turn's `tool_result` blocks instead of in front of them, so Anthropic no longer rejects aged conversations with "`tool_use` ids were found without `tool_result` blocks immediately after" ([#12920](https://github.com/diegosouzapw/OmniRoute/pull/12920)).
diff --git a/open-sse/services/compression/messageContent.ts b/open-sse/services/compression/messageContent.ts
index 5c3acb91e0d..47ea21ebde4 100644
--- a/open-sse/services/compression/messageContent.ts
+++ b/open-sse/services/compression/messageContent.ts
@@ -22,6 +22,12 @@ export function isTextBlock(value: unknown): value is TextBlock {
   );
 }
 
+export function isToolResultBlock(value: unknown): boolean {
+  return (
+    !!value && typeof value === "object" && (value as { type?: unknown }).type === "tool_result"
+  );
+}
+
 export function extractTextContent(content: ChatMessageLike["content"]): string {
   if (typeof content === "string") return content;
   if (!Array.isArray(content)) return "";
@@ -82,7 +88,14 @@ export function replaceTextContent(msg: ChatMessageLike, newText: string): ChatM
   });
 
   if (!replaced) {
-    return { ...msg, content: [{ type: "text", text: newText }, ...msg.content] };
+    // Anthropic requires every `tool_result` block to sit at the start of the
+    // user turn that answers a `tool_use`; a text block in front of them makes
+    // upstream reject the whole request with "tool_use ids were found without
+    // tool_result blocks immediately after" (#12890). Append the annotation in
+    // that case, and keep prepending everywhere else.
+    return msg.content.some(isToolResultBlock)
+      ? { ...msg, content: [...msg.content, { type: "text", text: newText }] }
+      : { ...msg, content: [{ type: "text", text: newText }, ...msg.content] };
   }
 
   return { ...msg, content };
diff --git a/tests/unit/compression/aging-tool-result-order-12890.test.ts b/tests/unit/compression/aging-tool-result-order-12890.test.ts
new file mode 100644
index 00000000000..bad0c3e0218
--- /dev/null
+++ b/tests/unit/compression/aging-tool-result-order-12890.test.ts
@@ -0,0 +1,63 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import {
+  replaceTextContent,
+  type ChatMessageLike,
+} from "../../../open-sse/services/compression/messageContent.ts";
+import { applyAging } from "../../../open-sse/services/compression/progressiveAging.ts";
+
+// ─── #12890 — aged tool_result turns must keep tool_result first ─────────────
+// The Anthropic Messages API requires the `tool_result` blocks answering a
+// `tool_use` to lead the following user message. Aging a tool-result-only user
+// turn used to prepend the `[COMPRESSED:aging:…]` annotation, producing
+// ["text", "tool_result"] and a 400 from upstream.
+
+function toolResultTurn(id: string): ChatMessageLike {
+  return {
+    role: "user",
+    content: [{ type: "tool_result", tool_use_id: id, content: "ls: 3 files" }],
+  };
+}
+
+function blockTypes(msg: unknown): string[] {
+  const content = (msg as ChatMessageLike).content;
+  return Array.isArray(content) ? content.map((b) => (b as { type?: string }).type ?? "") : [];
+}
+
+describe("aging a tool_result turn (#12890)", () => {
+  it("keeps tool_result first through applyAging", () => {
+    // distanceFromEnd of index 2 is 5 (> moderate: 3) → the fullSummary tier,
+    // which is where setContent/replaceTextContent injects the tag.
+    const messages: ChatMessageLike[] = [
+      { role: "user", content: "start the task" },
+      {
+        role: "assistant",
+        content: [{ type: "tool_use", id: "toolu_01", name: "bash", input: {} }],
+      },
+      toolResultTurn("toolu_01"),
+      { role: "assistant", content: "three files" },
+      { role: "user", content: "and now the second one" },
+      { role: "assistant", content: "done" },
+      { role: "user", content: "thanks" },
+      { role: "assistant", content: "you are welcome" },
+    ];
+
+    const { messages: aged } = applyAging(messages);
+    const types = blockTypes(aged[2]);
+
+    assert.deepEqual(types, ["tool_result", "text"], `got ${JSON.stringify(types)}`);
+    const annotation = (aged[2] as ChatMessageLike).content as Array<{ text?: string }>;
+    assert.match(annotation[1].text ?? "", /^\[COMPRESSED:aging:/);
+  });
+
+  it("still puts the annotation first when the turn carries no tool_result", () => {
+    const msg: ChatMessageLike = {
+      role: "user",
+      content: [{ type: "image", source: { foo: 1 } }],
+    };
+
+    const out = replaceTextContent(msg, "NEWTEXT");
+
+    assert.deepEqual(blockTypes(out), ["text", "image"]);
+  });
+});