The Remote Config keys you add after your release path closes are invisible to the build already on devices

The four switches I planted in the last build

This is also an exercise in restraint. Every switch you add is another branch to read and another surface to verify. I settled on four.

1. Per-feature kill switch

Rather than hiding a whole screen, this disables a feature and falls back to a substitute. Taking the disabled set as an array of strings means new features do not require touching the read path. For naming and granularity, I reused the approach from Before Your Feature Flags Turn Into a Junkyard — Governing Naming, Staged Rollout, and Kill Switches Across Six Apps.

2. Minimum supported build number

Anything below this gets a notice. It also lays the groundwork for cutting old builds loose once the freeze lifts. Version-string comparison behaves inconsistently across implementations, so I standardized on integer build numbers. The traps in string comparison are measured in When 1.10.0 Gets Locked Out: Measuring Four Version Comparison Approaches for Forced Updates.

3. Asset delivery base URL

Being able to swap the origin absorbs CDN incidents and path restructures without an app update. I only accept strings that start with https://.

4. Notice copy

A slot for "we're aware of an issue" or "a fix is on the way." It is the switch most likely to matter during a freeze and the one most likely to be forgotten.

Typed keys versus a single payload

Conventional advice favors individually typed parameters. They read clearly in the console and the type is enforced there.

Under a freeze, that shape works against you. When a new angle comes up, you cannot add a new key that anyone will read. So I folded all four into one JSON string behind a single key and made the app parse it loosely. The idea is to keep the expressive power inside the build that is already on devices.

That last drawback turns straight into a requirement. If you fold everything into one payload, the parser must never throw. An unreadable config has to fall back to defaults and let the app start normally. Get that wrong and a bad config push becomes a launch failure.

Here is the parser I ended up with. Unknown keys are dropped and mistyped values fall back.

export const FLAG_DEFAULTS = {
  schema: 1,
  killedFeatures: [],      // feature keys to disable
  minSupportedBuild: 0,    // show a notice below this
  assetBaseUrl: "https://assets.example.net/v1",
  notice: null,            // { id, title, body, url } | null
};
 
const isPlainObject = (v) =>
  typeof v === "object" && v !== null && !Array.isArray(v);
 
// Priority one: even an unreadable config must still let the app start
export function parseFlags(raw) {
  if (typeof raw !== "string" || raw.length === 0)
    return { ...defaults(), source: "default" };
 
  let obj;
  try {
    obj = JSON.parse(raw);
  } catch {
    return { ...defaults(), source: "parse-error" };
  }
  if (!isPlainObject(obj)) return { ...defaults(), source: "shape-error" };
  // Bumping schema drops every older build back to defaults — a deliberate escape hatch
  if (obj.schema !== FLAG_DEFAULTS.schema)
    return { ...defaults(), source: "schema-mismatch" };
 
  const out = { ...defaults(), source: "remote" };
  if (Array.isArray(obj.killedFeatures)) {
    out.killedFeatures = obj.killedFeatures.filter((k) => typeof k === "string");
  }
  if (Number.isInteger(obj.minSupportedBuild) && obj.minSupportedBuild >= 0) {
    out.minSupportedBuild = obj.minSupportedBuild;
  }
  if (typeof obj.assetBaseUrl === "string" && /^https:\/\//.test(obj.assetBaseUrl)) {
    out.assetBaseUrl = obj.assetBaseUrl;
  }
  if (isPlainObject(obj.notice) && typeof obj.notice.id === "string") {
    out.notice = {
      id: obj.notice.id,
      title: typeof obj.notice.title === "string" ? obj.notice.title : "",
      body: typeof obj.notice.body === "string" ? obj.notice.body : "",
      url:
        typeof obj.notice.url === "string" && /^https:\/\//.test(obj.notice.url)
          ? obj.notice.url
          : null,
    };
  }
  return out;
}

source comes back with the result so a debug menu can show where the current config came from. During a freeze, the first fork in any investigation is whether a setting is not working or simply has not arrived.

Running seven inputs through it locally produced this. With mixed types, non-string entries were dropped, the string "412" fell back to 0, and an http:// base URL was rejected.

Sharing the defaults object poisoned the second read

My first version returned defaults by spreading { ...FLAG_DEFAULTS }. That is a shallow copy, so the array still pointed at the same instance.

In that state, if a caller pushes anything onto the killedFeatures it received, every subsequent "default" is contaminated. I checked, and the pushed string came back as the default on the next read.

defaults after mutation: ["accidental"]
second parse: ["accidental"]

In day-to-day operation this stays invisible. As long as config is arriving normally, the default array gets replaced and thrown away. It only surfaces on the run where the fetch failed and you fell back to defaults — in other words, on the exact path you most wanted to protect. Hit that during a freeze, when you are leaning on remote config, and a feature you never disabled stays in the disabled set.

The fix is to freeze the defaults and hand out a fresh instance every time.

const FLAG_DEFAULTS = Object.freeze({
  schema: 1,
  killedFeatures: Object.freeze([]),
  minSupportedBuild: 0,
  assetBaseUrl: "https://assets.example.net/v1",
  notice: null,
});
 
// However callers treat the result, the next default stays clean
function defaults() {
  return {
    ...FLAG_DEFAULTS,
    killedFeatures: [...FLAG_DEFAULTS.killedFeatures], // a fresh array on every call
    notice: null,
  };
}

Object.freeze is there so that code accidentally writing to the defaults themselves fails loudly. ES modules are evaluated in strict mode, so a push onto the frozen array raises a TypeError on the spot — I confirmed it throws rather than being silently ignored. With configuration bugs, failing fast beats failing without a sound.

Audit the keys against the shipped tag, not your working tree

Reconciling "keys in the console" against "keys the code reads" is worth setting up early. But if a freeze is on the horizon, pointing the audit at the wrong tree inverts its meaning.

Reading HEAD and concluding that a key is referenced tells you nothing if that code has not shipped. What you need to audit is the tag that is currently on devices.

import { readFileSync, readdirSync, statSync } from "node:fs";
import { join, extname } from "node:path";
 
const SRC = process.argv[2] ?? "src";          // point this at a checkout of the shipped tag
const EXPORT = process.argv[3] ?? "console-export.json";
const KEY_RE =
  /remoteConfig\.get(?:String|Boolean|Number|Value)\(\s*["'`]([\w.-]+)["'`]\s*\)/g;
 
function walk(dir, acc = []) {
  for (const name of readdirSync(dir)) {
    const p = join(dir, name);
    if (statSync(p).isDirectory()) walk(p, acc);
    else if ([".ts", ".tsx", ".js", ".jsx"].includes(extname(p))) acc.push(p);
  }
  return acc;
}
 
const referenced = new Map();
for (const file of walk(SRC)) {
  for (const m of readFileSync(file, "utf8").matchAll(KEY_RE)) {
    if (!referenced.has(m[1])) referenced.set(m[1], []);
    referenced.get(m[1]).push(file);
  }
}
 
const declared = new Set(
  Object.keys(JSON.parse(readFileSync(EXPORT, "utf8")).parameters ?? {})
);
const missing = [...referenced.keys()].filter((k) => !declared.has(k));
const orphan = [...declared].filter((k) => !referenced.has(k));
 
console.log(`referenced ${referenced.size} / declared ${declared.size}`);
for (const k of missing)
  console.log(`  x not in console: ${k}  (${referenced.get(k).join(", ")})`);
for (const k of orphan) console.log(`  ! nobody reads: ${k}`);
 
process.exitCode = missing.length > 0 ? 1 : 0;

On a small sample it prints something like this.

referenced 4 / declared 4
  x not in console: enable_new_gallery  (src/useFlags.ts)
  ! nobody reads: winter_campaign

Equal counts with mismatched contents show up immediately. The important part of the workflow is to expand the shipped tag into a separate tree — git worktree add ../shipped v2.1.0 — and hand the script that src. During a freeze the input to this audit is frozen too, and the fact that the referenced key list cannot change is precisely the definition of what you can still touch.

One nuance: the nobody reads side normally means dead keys to clean up. Just before a freeze it can mean the opposite — a key you placed ahead of time whose reader has not shipped yet.

Fetch and activate timing is unforgiving during a freeze

Saving a value does not push it to devices. With Firebase Remote Config, the default minimum fetch interval in production is 12 hours. Shortening it is a development affordance; use it in production and you run into throttling.

There is also a second route, real-time Remote Config. According to the official documentation, while the app holds an update listener open, it is told when a newer version exists on the server and fetches automatically, bypassing the minimum fetch interval. The connection stays open only while the app is in the foreground. The same rule as the rest of this article applies: if the build on devices has no listener, this route does not exist. It is worth checking before your last pre-freeze build whether your SDK supports it (the docs list Apple platforms from v10.7.0; I have not been able to confirm React Native Firebase myself).

Fetch and activate are also separate operations. Fetching at launch and activating immediately often misses that session entirely, taking effect from the next cold start instead. Combine that with a cautious staged rollout starting at 5% and you get a window where a config you believe you have shipped is live on nobody's device.

That window is what hurts during a freeze. When you can still ship builds, a slow config rollout can be overtaken by a hotfix. With the release path closed, there is nothing to overtake it with. I settled on this order:

1. Save the current values to a local JSON file before changing anything
2. Roll out under a test-device condition first and confirm the debug menu shows source flipping to remote
3. Widen to everyone, then verify on a real device after at least one cold start
4. Record when it actually took effect and use that number for the next estimate

Step four looks trivial and pays off the longer a freeze runs. Measuring once how long your own app takes between saving and taking effect prevents the panicked double-push. The flag-side operational design carries straight over from Making Feature Flags Survive Production: Kill Switches and Gradual Rollouts in Rork Max.

Backing out of a config you should not have shipped

If the parser never throws, malformed JSON just falls back to defaults. The harder case is a payload that is syntactically valid and semantically wrong — a feature key that should not have gone into killedFeatures, say.

You have two ways back. Paste in the values you saved beforehand, or deliberately bump schema so every device retreats to defaults at once. The second looks blunt, but when your defaults are the safe side, it is the fastest reliable escape hatch available. That is exactly why the parser above drops to defaults on a schema mismatch instead of ignoring the field.

Being able to retreat first and fix calmly afterwards makes a large difference to how a freeze feels.

What is left when the freeze lifts

Switches you planted ahead of time stay in the code after the deadline passes. The ones you never used become branch debt. Once a normal release path returns, rerun the key audit against HEAD and clear out whatever shows up under nobody reads.

This inventory knocked down several assumptions I had been carrying about remote config being a general-purpose escape hatch. What you can move is decided by the code sitting on devices, not by the settings screen. I only noticed I had that direction backwards partway through the recount, and since it had been backwards for a long stretch, the recount was worth doing.

Start by expanding your shipped tag into a separate tree and printing the list of referenced keys. Those names are the entire control surface you would be left with on the day your updates stop.