Harden prompt base-message materialization against traps

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

currentState.md

@@ -142,6 +142,7 @@ HyperAgent exposes a TypeScript SDK for browser automation with three primary pa
 - Added an additional open-tab fallback path: when the tab array becomes unreadable (e.g. trapped `length`), prompt assembly now still emits the current tab line instead of a blank/no-tabs summary.
 - Added constructor regression coverage for trap-prone `llm` config getters, ensuring fallback failure paths stay deterministic and readable.
 - Hardened CDP frame-filter URL normalization to support protocol-relative and scheme-less frame URLs while avoiding path-only false positives in host-based ad-domain detection.
+- Hardened prompt base-message materialization with trap-safe array reads so malformed/trap-prone seed message arrays no longer crash message assembly and readable entries are preserved.
 - Expanded top-level package exports for key workflow/config types at `@hyperbrowser/agent`.
 - Removed stale script entry (`build-dom-tree-script`) and improved README usage docs.
 - Added canonical single-action debug writer helper (`writePerformDebug`) while preserving deprecated alias compatibility.

src/agent/messages/builder.test.ts

@@ -252,6 +252,98 @@ describe("buildAgentStepMessages", () => {
     expect(joined).toContain("=== Final Goal ===");
   });
 
+  it("continues when base message array length getter traps", async () => {
+    const page = createFakePage("https://example.com/current", [
+      "https://example.com/current",
+    ]);
+    const trappedBaseMessages = new Proxy(
+      [{ role: "system", content: "seed message" }],
+      {
+        get: (target, prop, receiver) => {
+          if (prop === "length") {
+            throw new Error("base message length trap");
+          }
+          return Reflect.get(target, prop, receiver);
+        },
+      }
+    ) as unknown as Parameters<typeof buildAgentStepMessages>[0];
+
+    const messages = await buildAgentStepMessages(
+      trappedBaseMessages,
+      [],
+      "task",
+      page,
+      {
+        elements: new Map(),
+        domState: "dom",
+        xpathMap: {},
+        backendNodeMap: {},
+      },
+      undefined,
+      []
+    );
+
+    const joined = messages
+      .map((message) =>
+        typeof message.content === "string" ? message.content : ""
+      )
+      .join("\n");
+    expect(joined).toContain("=== Final Goal ===");
+    expect(joined).not.toContain("seed message");
+  });
+
+  it("keeps readable base messages when base message entry getter traps", async () => {
+    const page = createFakePage("https://example.com/current", [
+      "https://example.com/current",
+    ]);
+    const trappedBaseMessages = new Proxy(
+      [
+        { role: "system", content: "trapped message" },
+        { role: "system", content: "safe message" },
+      ],
+      {
+        get: (target, prop, receiver) => {
+          if (prop === "0") {
+            throw new Error("base message item trap");
+          }
+          return Reflect.get(target, prop, receiver);
+        },
+      }
+    ) as unknown as Parameters<typeof buildAgentStepMessages>[0];
+
+    const messages = await buildAgentStepMessages(
+      trappedBaseMessages,
+      [],
+      "task",
+      page,
+      {
+        elements: new Map(),
+        domState: "dom",
+        xpathMap: {},
+        backendNodeMap: {},
+      },
+      undefined,
+      []
+    );
+
+    expect(messages).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          role: "system",
+          content: "safe message",
+        }),
+      ])
+    );
+    expect(messages).not.toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          role: "system",
+          content: "trapped message",
+        }),
+      ])
+    );
+  });
+
   it("ignores unreadable step array entries when index getter traps", async () => {
     const page = createFakePage("https://example.com/current", [
       "https://example.com/current",

src/agent/messages/builder.ts

@@ -213,6 +213,24 @@ function safeReadArrayItem<T>(value: unknown, index: number): T | undefined {
   }
 }
 
+function materializeSafeBaseMessages(
+  baseMessages: HyperAgentMessage[]
+): HyperAgentMessage[] {
+  const total = safeArrayLength(baseMessages);
+  if (total === 0) {
+    return [];
+  }
+
+  const normalizedMessages: HyperAgentMessage[] = [];
+  for (let index = 0; index < total; index += 1) {
+    const message = safeReadArrayItem<HyperAgentMessage>(baseMessages, index);
+    if (typeof message !== "undefined") {
+      normalizedMessages.push(message);
+    }
+  }
+  return normalizedMessages;
+}
+
 function getBoundedVariables(variables: HyperVariable[]): {
   visibleVariables: HyperVariable[];
   omittedCount: number;
@@ -439,7 +457,7 @@ export const buildAgentStepMessages = async (
   screenshot: string | undefined,
   variables: HyperVariable[]
 ): Promise<HyperAgentMessage[]> => {
-  const messages = [...baseMessages];
+  const messages = materializeSafeBaseMessages(baseMessages);
   const normalizedSteps = materializeSafeSteps(steps);
 
   // Add the final goal section