diff --git a/packages/effect-acp/src/protocol.test.ts b/packages/effect-acp/src/protocol.test.ts
index cef04ffcd498..bbd1577cfa2e 100644
--- a/packages/effect-acp/src/protocol.test.ts
+++ b/packages/effect-acp/src/protocol.test.ts
@@ -8,6 +8,7 @@ import * as Schema from "effect/Schema";
 import * as Stream from "effect/Stream";
 import * as Ref from "effect/Ref";
 import { ChildProcess, ChildProcessSpawner } from "effect/unstable/process";
+import type * as RpcMessage from "effect/unstable/rpc/RpcMessage";
 
 import { it, assert } from "@effect/vitest";
 import * as NodeServices from "@effect/platform-node/NodeServices";
@@ -425,6 +426,44 @@ it.layer(NodeServices.layer)("effect-acp protocol", (it) => {
     }),
   );
 
+  it.effect("repairs a standards-compliant JSON-RPC error into a typed failure", () =>
+    Effect.gen(function* () {
+      const { stdio, input } = yield* makeInMemoryStdio();
+      const transport = yield* AcpProtocol.makeAcpPatchedProtocol({
+        stdio,
+        serverRequestMethods: new Set(),
+      });
+
+      const firstMessage = yield* Deferred.make<RpcMessage.FromServerEncoded>();
+      yield* transport.clientProtocol
+        .run(0, (message) => Deferred.succeed(firstMessage, message).pipe(Effect.asVoid))
+        .pipe(Effect.forkScoped);
+
+      // A plain `{code, message}` error, as a standards-compliant (non-Effect) ACP
+      // agent sends it: no `_tag: "Cause"` marker, so effect/rpc's codec boxes it as
+      // a `Die` unless `repairJsonRpcErrorExit` rewrites it back into a `Fail`.
+      yield* Queue.offer(
+        input,
+        encoder.encode(
+          `${encodeUnknownJsonString({
+            jsonrpc: "2.0",
+            id: 5,
+            error: { code: -32601, message: "Method not found" },
+          })}\n`,
+        ),
+      );
+
+      const message = yield* Deferred.await(firstMessage);
+      assert.equal(message._tag, "Exit");
+      const exit = (message as { readonly exit: { readonly _tag: string; readonly cause: any } })
+        .exit;
+      assert.equal(exit._tag, "Failure");
+      assert.deepEqual(exit.cause, [
+        { _tag: "Fail", error: { code: -32601, message: "Method not found" } },
+      ]);
+    }),
+  );
+
   it.effect("preserves numeric ids for inbound extension requests", () =>
     Effect.gen(function* () {
       const { stdio, input, output } = yield* makeInMemoryStdio();
diff --git a/packages/effect-acp/src/protocol.ts b/packages/effect-acp/src/protocol.ts
index cd6043f2db00..7ba9deb23f78 100644
--- a/packages/effect-acp/src/protocol.ts
+++ b/packages/effect-acp/src/protocol.ts
@@ -377,7 +377,7 @@ export const makeAcpPatchedProtocol = Effect.fn("makeAcpPatchedProtocol")(functi
       Effect.flatMap((pending) => {
         const pendingRequest = pending.get(String(message.requestId));
         if (!pendingRequest) {
-          return Queue.offer(clientQueue, message).pipe(Effect.asVoid);
+          return Queue.offer(clientQueue, repairJsonRpcErrorExit(message)).pipe(Effect.asVoid);
         }
         if (message.exit._tag === "Success") {
           return completeExtPendingSuccess(message.requestId, message.exit.value);
@@ -626,3 +626,30 @@ function isProtocolError(
     typeof value.message === "string"
   );
 }
+
+/**
+ * effect/rpc's ndjson codec only recognizes its own `_tag: "Cause"` marker on a
+ * JSON-RPC `error` as a typed failure; any other JSON-RPC error (i.e. one from a
+ * standards-compliant, non-Effect ACP agent) gets boxed as an opaque `Die`
+ * (see effect/unstable/rpc/RpcSerialization.js). Native Agent RPC responses
+ * (no `pendingRequest` tracked in `extPending`) skip this module's own
+ * request/response handling entirely, so that Die reaches `@effect/rpc`'s
+ * generic `Schema.Defect()` decoder and crashes instead of surfacing as the
+ * RPC's typed `error` schema. Rewriting the Die into a `Fail` here, while the
+ * defect is still the untouched raw object, lets it decode against the RPC's
+ * error schema like any other typed failure.
+ */
+function repairJsonRpcErrorExit(
+  message: RpcMessage.ResponseExitEncoded,
+): RpcMessage.ResponseExitEncoded {
+  if (message.exit._tag !== "Failure") return message;
+  let changed = false;
+  const cause = message.exit.cause.map((entry) => {
+    if (entry._tag === "Die" && isProtocolError(entry.defect)) {
+      changed = true;
+      return { _tag: "Fail" as const, error: entry.defect };
+    }
+    return entry;
+  });
+  return changed ? { ...message, exit: { ...message.exit, cause } } : message;
+}