diff --git a/packages/server/scripts/reproduce-codex-history-string-limit.mts b/packages/server/scripts/reproduce-codex-history-string-limit.mts
new file mode 100644
index 0000000000..9c10cef73f
--- /dev/null
+++ b/packages/server/scripts/reproduce-codex-history-string-limit.mts
@@ -0,0 +1,91 @@
+// Opt-in diagnostic for #3905. Run from the repository root:
+// node --import tsx packages/server/scripts/reproduce-codex-history-string-limit.mts 520
+// Requires Codex on PATH and approximately 2 GiB of available memory.
+import { spawn } from "node:child_process";
+import { randomUUID } from "node:crypto";
+import { mkdtemp, mkdir, open, rm } from "node:fs/promises";
+import path from "node:path";
+import pino from "pino";
+import { CodexAppServerAgentSession } from "../src/server/agent/providers/codex-app-server-agent.js";
+
+const megabytes = Number(process.argv[2] ?? "520");
+if (!Number.isSafeInteger(megabytes) || megabytes < 1 || megabytes > 600) {
+  throw new Error("Expected an integer size between 1 and 600 MiB");
+}
+const root = await mkdtemp(path.join(process.cwd(), ".codex-history-repro-"));
+const home = path.join(root, "codex-home");
+const now = new Date();
+const timestamp = now.toISOString();
+const threadId = randomUUID();
+const turnId = randomUUID();
+const sessions = path.join(home, "sessions", ...timestamp.slice(0, 10).split("-"));
+await mkdir(sessions, { recursive: true });
+const rollout = await open(
+  path.join(sessions, `rollout-${timestamp.slice(0, 19).replaceAll(":", "-")}-${threadId}.jsonl`),
+  "w",
+);
+async function write(type: string, payload: unknown) {
+  await rollout.write(`${JSON.stringify({ timestamp, type, payload })}\n`);
+}
+await write("session_meta", {
+  id: threadId,
+  timestamp,
+  cwd: root,
+  originator: "codex_cli_rs",
+  cli_version: "0.159.0",
+  source: "cli",
+  model_provider: "openai",
+  history_mode: "legacy",
+});
+await write("event_msg", { type: "task_started", turn_id: turnId, model_context_window: 200000 });
+await write("event_msg", {
+  type: "user_message",
+  message: "Synthetic oversized history",
+  images: [],
+  local_images: [],
+  text_elements: [],
+});
+for (let index = 0; index < megabytes; index++) {
+  await write("event_msg", {
+    type: "agent_message",
+    message: "x".repeat(1024 * 1024),
+    phase: "commentary",
+  });
+}
+await write("event_msg", { type: "task_complete", turn_id: turnId, last_agent_message: "done" });
+await rollout.close();
+console.log(`Opening synthetic ${megabytes} MiB history through the real Codex app-server`);
+const session = new CodexAppServerAgentSession(
+  { provider: "codex", cwd: root },
+  { sessionId: threadId },
+  pino({ level: "silent" }),
+  async () =>
+    spawn(
+      "codex",
+      ["app-server", "--disable", "plugins", "--disable", "apps", "--disable", "remote_plugin"],
+      {
+        stdio: ["pipe", "pipe", "pipe"],
+        env: { ...process.env, CODEX_HOME: home },
+      },
+    ),
+  { codexHome: home },
+  false,
+  false,
+  false,
+  undefined,
+  "history",
+);
+try {
+  await session.connect();
+  let count = 0;
+  for await (const event of session.streamHistory()) if (event.type === "timeline") count++;
+  console.log(`History loaded: ${count} timeline rows`);
+} catch (error) {
+  // A recoverable session error is acceptable for the crash-isolation assertion.
+  console.log(
+    `History rejected without crashing: ${error instanceof Error ? error.message : String(error)}`,
+  );
+} finally {
+  await session.close();
+  await rm(root, { recursive: true, force: true, maxRetries: 10, retryDelay: 100 });
+}
diff --git a/packages/server/src/server/agent/providers/codex-history-string-limit.real.e2e.test.ts b/packages/server/src/server/agent/providers/codex-history-string-limit.real.e2e.test.ts
new file mode 100644
index 0000000000..8ee71aa1ce
--- /dev/null
+++ b/packages/server/src/server/agent/providers/codex-history-string-limit.real.e2e.test.ts
@@ -0,0 +1,50 @@
+import { execFile } from "node:child_process";
+import { mkdtemp, rm } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { expect, test } from "vitest";
+
+// Diagnostic for #3905; intentionally red on the affected baseline. Real Codex,
+// synthetic on-disk history, no model turns. Opt in because the oversized
+// case needs approximately 2 GiB of memory and over 520 MiB of temporary storage.
+const enabled = process.env.PASEO_CODEX_HISTORY_STRING_LIMIT_REPRO === "1";
+const script = fileURLToPath(
+  new URL("../../../../scripts/reproduce-codex-history-string-limit.mts", import.meta.url),
+);
+
+test.runIf(enabled).each([2, 520])(
+  "opening %i MiB of native Codex history does not crash its hosting process",
+  async (megabytes) => {
+    const root = await mkdtemp(path.join(process.cwd(), ".codex-history-test-"));
+    try {
+      const result = await new Promise<{ code: number; stdout: string; stderr: string }>(
+        (resolve) => {
+          const child = execFile(
+            process.execPath,
+            ["--import", "tsx", script, String(megabytes)],
+            {
+              cwd: root,
+              detached: true,
+              timeout: 60_000,
+              maxBuffer: 1024 * 1024,
+            },
+            (error, stdout, stderr) => {
+              try {
+                if (child.pid) process.kill(-child.pid, "SIGKILL");
+              } catch (killError) {
+                if ((killError as NodeJS.ErrnoException).code !== "ESRCH") throw killError;
+              }
+              resolve({ code: error ? Number(error.code ?? 1) : 0, stdout, stderr });
+            },
+          );
+        },
+      );
+      expect(result.code, result.stderr).toBe(0);
+      expect(result.stdout).toMatch(/History loaded:|History rejected without crashing:/);
+      if (megabytes === 2) expect(result.stdout).toContain("History loaded: 3 timeline rows");
+    } finally {
+      await rm(root, { recursive: true, force: true, maxRetries: 10, retryDelay: 100 });
+    }
+  },
+  90_000,
+);