perf: speed up chat loading and resume

src/server/browserMessageProjection.ts

@@ -0,0 +1,59 @@
+import type { MessagePage, SessionUiEvent } from "../shared/apiTypes.js";
+
+/**
+ * Remove provider-only thinking data at the browser transport boundary. The
+ * runtime message remains unchanged because only affected messages and content
+ * blocks are copied.
+ */
+export function projectBrowserMessage(message: unknown): unknown {
+  if (!isRecord(message)) return message;
+  const originalContent = message["content"];
+  if (!isUnknownArray(originalContent)) return message;
+
+  const content = mapChanged(originalContent, (part) => {
+    if (!isRecord(part) || part["type"] !== "thinking" || !Object.hasOwn(part, "thinkingSignature")) return part;
+    const projected = { ...part };
+    delete projected["thinkingSignature"];
+    return projected;
+  });
+
+  return content === originalContent ? message : { ...message, content };
+}
+
+export function projectBrowserMessageResponse(response: unknown[] | MessagePage): unknown[] | MessagePage {
+  if (Array.isArray(response)) return mapChanged(response, projectBrowserMessage);
+  const messages = mapChanged(response.messages, projectBrowserMessage);
+  return messages === response.messages ? response : { ...response, messages };
+}
+
+export function projectBrowserSessionEvent(event: SessionUiEvent): SessionUiEvent {
+  if (event.type !== "message.end" || event.message === undefined) return event;
+  const message = projectBrowserMessage(event.message);
+  return message === event.message ? event : { ...event, message };
+}
+
+function mapChanged<T>(values: T[], project: (value: T) => T): T[] {
+  let projectedValues: T[] | undefined;
+  let index = 0;
+  for (const value of values) {
+    const projected = project(value);
+    if (projectedValues === undefined) {
+      if (projected === value) {
+        index += 1;
+        continue;
+      }
+      projectedValues = values.slice(0, index);
+    }
+    projectedValues.push(projected);
+    index += 1;
+  }
+  return projectedValues ?? values;
+}
+
+function isUnknownArray(value: unknown): value is unknown[] {
+  return Array.isArray(value);
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value);
+}