From 466bfbe78c205e2d06b9f85f0fb88ef00ca2cf07 Mon Sep 17 00:00:00 2001
From: Peter Steinberger <steipete@gmail.com>
Date: Sat, 30 May 2026 07:57:15 +0200
Subject: [PATCH] fix(codex): cap native reserve for small windows

---
 .../src/app-server/startup-binding.test.ts    | 35 +++++++++++++++++++
 .../codex/src/app-server/startup-binding.ts   | 12 ++++++-
 2 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/extensions/codex/src/app-server/startup-binding.test.ts b/extensions/codex/src/app-server/startup-binding.test.ts
index fda0cff125f..0817a44c354 100644
--- a/extensions/codex/src/app-server/startup-binding.test.ts
+++ b/extensions/codex/src/app-server/startup-binding.test.ts
@@ -112,6 +112,41 @@ describe("Codex app-server startup binding", () => {
     expect(savedBinding).toBeUndefined();
   });
 
+  it("caps the default native reserve so small context windows keep prompt budget", async () => {
+    const sessionFile = path.join(tempDir, "session.jsonl");
+    const workspaceDir = path.join(tempDir, "workspace");
+    const agentDir = path.join(tempDir, "agent");
+    await writeExistingBinding(sessionFile, workspaceDir, { dynamicToolsFingerprint: "[]" });
+    await writeSessionRecord(sessionFile, { totalTokens: 100 });
+    const rolloutDir = path.join(agentDir, "codex-home", "sessions");
+    await fs.mkdir(rolloutDir, { recursive: true });
+    await fs.writeFile(
+      path.join(rolloutDir, "rollout-thread-existing.jsonl"),
+      `${JSON.stringify({
+        payload: {
+          type: "token_count",
+          info: {
+            last_token_usage: {
+              total_tokens: 100,
+            },
+            model_context_window: 16_000,
+          },
+        },
+      })}\n`,
+    );
+
+    const binding = await rotateOversizedCodexAppServerStartupBinding({
+      binding: await readCodexAppServerBinding(sessionFile),
+      sessionFile,
+      agentDir,
+      config: undefined,
+    });
+
+    expect(binding?.threadId).toBe("thread-existing");
+    const savedBinding = await readCodexAppServerBinding(sessionFile);
+    expect(savedBinding?.threadId).toBe("thread-existing");
+  });
+
   it("honors shorthand byte units for native rollout limits", async () => {
     const sessionFile = path.join(tempDir, "session.jsonl");
     const workspaceDir = path.join(tempDir, "workspace");
diff --git a/extensions/codex/src/app-server/startup-binding.ts b/extensions/codex/src/app-server/startup-binding.ts
index 1d504cd2a44..da568e562e0 100644
--- a/extensions/codex/src/app-server/startup-binding.ts
+++ b/extensions/codex/src/app-server/startup-binding.ts
@@ -13,6 +13,8 @@ import { clearCodexAppServerBinding, type CodexAppServerThreadBinding } from "./
 // thread that is already too close to the server-side window for the next turn.
 const CODEX_APP_SERVER_NATIVE_THREAD_FALLBACK_MAX_TOKENS = 300_000;
 const CODEX_APP_SERVER_NATIVE_THREAD_DEFAULT_RESERVE_TOKENS = 20_000;
+const CODEX_APP_SERVER_NATIVE_THREAD_MIN_PROMPT_BUDGET_TOKENS = 8_000;
+const CODEX_APP_SERVER_NATIVE_THREAD_MIN_PROMPT_BUDGET_RATIO = 0.5;
 const CODEX_APP_SERVER_BYTE_UNITS: Record<string, number> = {
   b: 1,
   k: 1024,
@@ -250,7 +252,15 @@ function resolveCodexAppServerNativeThreadTokenFuse(params: {
       : 0;
   const contextWindow =
     params.modelContextWindow ?? CODEX_APP_SERVER_NATIVE_THREAD_FALLBACK_MAX_TOKENS;
-  return Math.max(1, contextWindow - params.reserveTokens - projectedTurnTokens);
+  const minPromptBudget = Math.min(
+    CODEX_APP_SERVER_NATIVE_THREAD_MIN_PROMPT_BUDGET_TOKENS,
+    Math.max(1, Math.floor(contextWindow * CODEX_APP_SERVER_NATIVE_THREAD_MIN_PROMPT_BUDGET_RATIO)),
+  );
+  const effectiveReserveTokens = Math.min(
+    params.reserveTokens,
+    Math.max(0, contextWindow - minPromptBudget),
+  );
+  return Math.max(1, contextWindow - effectiveReserveTokens - projectedTurnTokens);
 }
 
 function maxFiniteNumber(values: Array<number | undefined>): number | undefined {