fix(plugin-sdk): guard provider stream error formatting

src/plugin-sdk/provider-stream-shared.test.ts

@@ -199,6 +199,43 @@ describe("createPayloadPatchStreamWrapper", () => {
 });
 
 describe("createPlainTextToolCallCompatWrapper", () => {
+  it("emits sanitized errors when stream normalization sees hostile failures", async () => {
+    const hostileError = new Error("stream failed");
+    Object.defineProperty(hostileError, "message", {
+      get() {
+        throw new Error("message denied");
+      },
+    });
+    const baseStreamFn: StreamFn = () =>
+      (async function* () {
+        yield* [];
+        throw hostileError;
+      })() as never;
+    const wrapped = createPlainTextToolCallCompatWrapper(baseStreamFn);
+    const events: unknown[] = [];
+
+    for await (const event of wrapped(
+      {} as never,
+      { tools: [{ name: "read" }] } as never,
+      {},
+    ) as AsyncIterable<unknown>) {
+      events.push(event);
+    }
+
+    expect(events).toEqual([
+      {
+        type: "error",
+        reason: "error",
+        error: {
+          role: "assistant",
+          content: [],
+          stopReason: "error",
+          errorMessage: "Unknown provider stream error",
+        },
+      },
+    ]);
+  });
+
   it("promotes standalone text tool calls into tool-call stream events", async () => {
     const baseStreamFn: StreamFn = () =>
       createEventStream([

src/plugin-sdk/provider-stream-shared.ts

@@ -173,6 +173,17 @@ function normalizeProviderDoneMessage(
   return promotedMessage ? { kind: "promoted", message: promotedMessage } : undefined;
 }
 
+function describeProviderStreamError(error: unknown): string {
+  try {
+    if (error instanceof Error) {
+      return error.message || error.name || "Unknown provider stream error";
+    }
+    return String(error);
+  } catch {
+    return "Unknown provider stream error";
+  }
+}
+
 function wrapPlainTextToolCallStream(
   source: ReturnType<StreamFn>,
   context: Parameters<StreamFn>[1],
@@ -220,7 +231,7 @@ function wrapPlainTextToolCallStream(
           role: "assistant",
           content: [],
           stopReason: "error",
-          errorMessage: error instanceof Error ? error.message : String(error),
+          errorMessage: describeProviderStreamError(error),
         },
       });
     } finally {