diff --git a/packages/provider-fx/src/server/adapter.test.ts b/packages/provider-fx/src/server/adapter.test.ts
new file mode 100644
index 0000000000000..d69d584602d8b
--- /dev/null
+++ b/packages/provider-fx/src/server/adapter.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, it } from "@effect/vitest";
+
+import { fxSubagentOutcome } from "./adapter.ts";
+
+describe("fx subagent replies", () => {
+  it.each(["", "1", "12", "123"])("preserves literal Unicode spelling ending in u%s", (digits) => {
+    for (const slashes of [1, 2]) {
+      const suffix = "\\".repeat(slashes) + "u" + digits;
+      const padding = "x".repeat(
+        200 - '{"ok":true,"result":"'.length - JSON.stringify(suffix).length + 2,
+      );
+      const reply = padding + suffix + "4 more text";
+      const whole = JSON.stringify({ ok: true, result: reply });
+      expect(fxSubagentOutcome(whole)).toEqual({ ok: true, result: reply });
+      expect(fxSubagentOutcome(Buffer.from(whole).subarray(0, 200).toString("utf8"))).toEqual({
+        ok: true,
+        result: padding + suffix + "…",
+      });
+    }
+  });
+
+  it.each([0, 1, 2])(
+    "recovers incomplete Unicode escapes after %i literal backslashes",
+    (slashes) => {
+      for (const digits of ["", "0", "00", "000"]) {
+        const prefix = "reply" + "\\".repeat(slashes * 2);
+        expect(fxSubagentOutcome('{"ok":true,"result":"' + prefix + "\\u" + digits)).toEqual({
+          ok: true,
+          result: "reply" + "\\".repeat(slashes) + "…",
+        });
+      }
+    },
+  );
+
+  it.each([
+    ['{"ok":true,"result":"hello', { ok: true, result: "hello…" }],
+    ['{"ok":false,"result":null}', { ok: false, result: null }],
+    ['{"ok":true,"result":"hello"', { ok: true, result: "hello" }],
+    ['{"unexpected":"body"}', undefined],
+    ['{"ok":true,"result":"\\q', { ok: true, result: null }],
+  ])("retains existing normalization for %s", (text, expected) => {
+    expect(fxSubagentOutcome(text)).toEqual(expected);
+  });
+});
diff --git a/packages/provider-fx/src/server/adapter.ts b/packages/provider-fx/src/server/adapter.ts
index 868b13f497de3..b48293f4fb986 100644
--- a/packages/provider-fx/src/server/adapter.ts
+++ b/packages/provider-fx/src/server/adapter.ts
@@ -102,14 +102,17 @@ const decodeJsonString = Schema.decodeUnknownOption(Schema.fromJsonString(Schema
  * fx clips every tool result to 199 bytes, so a long sub-agent reply arrives
  * as cut-off `{"ok":…,"result":"…` JSON. Keeps the readable prefix and marks the cut.
  */
-function fxSubagentOutcome(
+export function fxSubagentOutcome(
   text: string,
 ): { readonly ok: boolean; readonly result: string | null } | undefined {
   const whole = decodeFxSubagentResult(text);
   if (Option.isSome(whole)) return whole.value;
   const clipped = /^\{"ok":(true|false),"result":"((?:[^"\\]|\\.)*)/.exec(text);
   if (clipped === null) return undefined;
-  const prefix = decodeJsonString(`"${(clipped[2] ?? "").replace(/\\u[0-9a-fA-F]{0,3}$/, "")}"`);
+  const fragment = clipped[2] ?? "";
+  const prefix = decodeJsonString(`"${fragment}"`).pipe(
+    Option.orElse(() => decodeJsonString(`"${fragment.replace(/\\u[0-9a-fA-F]{0,3}$/, "")}"`)),
+  );
   const cut = text[clipped[0].length] !== '"';
   return {
     ok: clipped[1] === "true",