OpenAI Is Testing “Luna Reserve,” a Codex Fallback Tier
The undocumented feature gives some Plus and Pro users a separate allowance after advanced-model access is constrained. RuntimeWire traced its model switching and metering logic inside OpenAI’s Windows client.
By Ryan Merket · Published · Updated
RUNTIMEWIRE INVESTIGATION — Scoop
Original reporting by RuntimeWire, based on reverse engineering.
Why it matters
Reserve would let OpenAI replace Codex's hard usage wall with a cheaper fallback, keeping developers active while turning premium capacity into an immediate upsell.
Reporting record
Finding
Codex contains a feature-gated “Luna Reserve” allowance that appears to switch eligible Plus and Pro users to a Luna-class reserve model after advanced-model limits are reached.
How we verified
Methods: reverse engineering.
Static analysis of the production-distributed Codex app.asar identified the model identifier gpt-reserve, feature gate reserve_enabled, luna_reserve banner logic, dedicated lifecycle analytics, eligibility checks for Plus/Pro accounts, and user-facing text describing a separate “Luna Reserve” allowance. The code preserves the user’s original model selection and restores it when Reserve ends.
Calculated the archive’s SHA-256 hash, parsed its ASAR index, and extracted the Electron main-process and renderer bundles. Searched the bundles for feature definitions, model identifiers, rate-limit handling, eligibility conditions, analytics events, and user-facing strings. Traced Luna Reserve from its server-controlled feature gate through account eligibility, rate-limit activation, model switching, UI presentation, and restoration of the original model. Compared findings against current official OpenAI documentation. No server interaction or feature-flag bypass was attempted.
Tested versions: Windows package 26.818.5229.0, embedded Codex app 26.818.41509, build 6962 (prod).
Reproduction
RuntimeWire independently reproduced the core finding.
Obtain the specified Microsoft Store build of Codex and hash app/resources/app.asar. Parse the ASAR and inspect the renderer bundle for gpt-reserve, reserve_enabled, luna_reserve, and CodexLunaReserveLifecycle. Confirm the surrounding code includes Plus/Pro eligibility checks, a separate gpt-reserve rate limit, model switching, and the user-facing string “Luna Reserve · {remaining}% remaining.” Live UI reproduction additionally requires server-side eligibility and rate-limit state.
File hashes
C5D839BC9B122B7EF2A2F0F45186B3E5895923DE5B6CEF5253C936FE670C0479
Company response
RuntimeWire requested comment; the company had not responded by publication time.
OpenAI has begun rolling out an undocumented fallback tier for Codex that can keep subscribers working after their regular advanced-model allowance runs low.
The feature is called Luna Reserve. It carries the internal model identifier gpt-reserve, maintains its own usage allowance and is designed to temporarily replace a user's selected model while preserving the settings needed to switch them back later.
Multiple images posted to X and reviewed by RuntimeWire show the feature surfacing in user accounts. One shows GPT-Reserve in Codex's model selector, directly beneath GPT-5.6 Luna and carrying the same visible "Fast and affordable agentic coding..." description. Others show gpt-reserve as a separately metered allowance alongside an account's ordinary weekly limit.
