--- title: Consent state reference description: How c15t saves choices, gates IAB vendors, hydrates server records and keeps browser tabs and storage in step. group: concepts lastModified: "2026-10-10T16:01:45+01:00" --- This reference covers the edge cases behind the model in [how consent works](/docs/concepts/how-consent-works). You need it when you build custom consent UI, run c15t across subdomains, or debug a choice that changes between tabs. ## Gate IAB vendors on the TC string Under an IAB policy, a script, network rule or iframe that names a `vendorId` or IAB purposes is an IAB target. It runs only while a confirmed TC string grants what it declares: purpose and vendor consent for `iabPurposes`, purpose and vendor legitimate interest for `iabLegIntPurposes`, and opt-ins for `iabSpecialFeatures`. Every category it names must also be free of restrictions, so GPC or strict scope blocks it. A refused category is the one exception. It does not block an IAB target that processes only on legitimate interest, after publisher restrictions, because the TCF lets that processing run without consent. The visitor's control for it is the objection, which the legitimate interest signals record. The category itself stays refused: its `effectivePermissions` entry is `false`, and targets that name only the category, or that also declare a consent purpose or special feature, stay blocked. ## Categories under an IAB policy Under an IAB policy the visitor chooses TCF purposes, and c15t derives each category from them when the choice is saved. A category is granted when the visitor consented to every purpose in it that a listed vendor processes on consent: |Category|TCF purposes| |--|--| |`marketing`|2, 3, 4| |`measurement`|7, 8, 9| |`experience`|5, 6| |`functionality`|10, 11| `necessary` is always granted. Purpose 1, device storage, decides no category. Accept All consents only to the purposes and special features that the vendors in your list declare, because the TCF Policies allow no consent signal for a purpose the visitor was not shown. A category whose purposes no listed vendor processes on consent therefore stays denied after Accept All, and so does every script, iframe or network rule gated on it. With a vendor list whose vendors declare only purposes 1 to 4 and 7, for example, `experience` and `functionality` stay denied. If your own scripts depend on such a category, do one of these: * Declare the processing. Add a custom vendor for it, with the purposes it uses, through the `customVendors` option of your IAB setup. The preference centre then lists those purposes, and Accept All and the purpose switches can grant the category. * Gate the script on a category that your listed vendors' purposes cover, or on `necessary` if it needs no consent. Check `effectivePermissions` after Accept All on a fresh visit to see which categories your vendor list leaves denied. ## When a choice is saved A save records the choice in the browser first and sends it to the backend afterwards. The stock banner, preference dialog and IAB surfaces in every framework adapter, and the `acceptAll()`, `rejectAll()` and `save()` methods of the browser and Astro clients, close without waiting for the backend. In order: 1. In the click task, the explicit choice and effective permissions change, `onChoiceRecorded` and `onPermissionsChanged` run, gated scripts, iframes and network rules follow the new permissions, and the surface leaves the active state. Its exit animation still plays. 2. In the next task, the choice is written to the cookie and localStorage. 3. After that write is queued, the request to the backend starts. 4. When the request settles, the kernel emits `command:save:completed`. The promise returned by `kernel.commands.save()`, React's `performAction()` and Svelte's `saveConsents()` resolves or rejects only then. 5. If the save turned off a category or vendor that was granted, the page reloads in the next task, after `onBeforeConsentRevocationReload` runs. Removing a script cannot stop code that already ran, so the reload starts a page with only permitted code. With several saves in flight, it waits for the last one. Set `reloadOnConsentRevoked: false` to handle revocation yourself. A failed request does not reopen the surface or roll the choice back. The kernel emits `command:error`, which reaches the `onError` callback in adapters that accept one, and queues the payload in localStorage. Where localStorage is unavailable, the queue lives in memory until the page unloads. The queue is replayed after the next successful initialization and when the browser comes back online, up to 10 attempts over 7 days. A replay carries the original action time and policy snapshot token, so the backend records when the visitor decided, and a duplicate submission resolves to the same consent record. A backend that signs policy snapshot tokens rejects a replay made after the token expires, which is 30 minutes by default for the self-hosted backend. A save still queued by then is recorded only in the browser. A replay that fails and stays queued emits `save:replayed` with `ok: false` but not `command:error`, since the save already reported its first failure. A save that leaves the queue without the backend recording it, because it ran out of attempts, waited more than 7 days or was refused, emits `command:error` once. Development builds also log a `[c15t]` console warning for each of these. IAB surfaces close in the click task too, but an IAB choice is recorded only after its TC string is encoded, which can wait for the TCF library to load. If that local step records nothing, for example because the vendor list failed to load, the surface comes back so the visitor can try again. Every adapter leaves the same surface after a save: the banner while the policy still owes a choice or a notice, and nothing once no prompt is owed, while the policy is still loading, or after it failed to resolve. A banner the visitor reopened closes too. A save that records nothing new, such as an unchanged selection, closes the surface when it resolves successfully. Opening or closing a surface, or starting another save, before then leaves the surface as the visitor set it. Closing the dialog without saving, with Escape or the adapter's close call, leaves the same surface a save would. A visitor who opens preferences from the banner and backs out still owes a choice, so the banner comes back. Once a choice is recorded, closing the dialog leaves nothing open. ## Preserve records during hydration Server helpers return records with policy information and evaluation time. Forward that configuration intact. Copying an allowed category into a receipt would invent a grant and lose its original confirmation time. Valid v2 records can be read without a startup rewrite. The next explicit action writes the v3 format. The upgrade guides for [Next.js](/docs/frameworks/next/upgrade-v3#keep-visitors-existing-choices), [React](/docs/frameworks/react/upgrade-v3#keep-visitors-existing-choices) and [JavaScript](/docs/frameworks/javascript/upgrade-v3#keep-visitors-existing-choices) cover how v3 treats v2 records. ## Keep open tabs in step Tabs and windows on the same origin (scheme, host and port) share the consent cookie and localStorage. When a visitor rejects in one tab, every other open tab on that origin applies the rejection without a reload. Scripts and features gated on `effectivePermissions` lose permission, subscribers are notified once and `onPermissionsChanged` fires. Clearing records in one tab returns the others to the active policy's defaults: an opt-in policy denies optional categories and shows the prompt again. Tabs on another subdomain that shares the consent cookie do not share localStorage, so the browser sends them no `storage` event. They pick up the change on their next focus or visibility change, or when you reconcile yourself as described in [Reconcile yourself or turn it off](#reconcile-yourself-or-turn-it-off). The `storage` event only exists for localStorage. When localStorage is unavailable (blocked by the browser, a sandboxed frame or a privacy mode) and c15t stores in the cookie alone, another tab's change arrives only on the next focus or visibility change, or when you call `runtime.reconcileStorage()` or `persistence.reconcile()`. c15t does not poll the cookie or use a `BroadcastChannel` for this. Browser persistence reads stored records again at these moments: |Moment|What happens| |--|--| |Another tab or window on the same origin changes a c15t localStorage key|Reconciles on the `storage` event| |The page becomes visible again|Reconciles on `visibilitychange`| |The window regains focus|Reconciles on `focus`| |The network reconnects|No storage read| Several triggers in quick succession run one reconciliation in a later task. Reconnecting does not read storage, because going online changes nothing in browser storage. On reconnect the kernel retries saves the backend did not accept and a failed initialization. Save retries do not write the cookie or localStorage. A write that follows initialization obeys the rules in [Ordering with pending writes](#ordering-with-pending-writes). Every adapter that mounts browser persistence does this: the React, Next.js and TanStack Start providers, Vue, Svelte, Astro, the script tag and `createConsentRuntime`. With `persistence: false` nothing is stored or read. ### What a reconciliation applies Stored records are merged into the ones in memory: * Category decisions merge per category. Each category keeps the decision with the newer confirmation time, so a tab that only changed marketing never reverts another tab's newer measurement decision. * The notice dismissal and the vendor record are single decisions. A stored one at least as new as the one in memory replaces it; an older one is ignored. * When two tabs record decisions in the same millisecond, the one stored first wins in both tabs. * Tabs keep one subject. A stored subject is considered only when the record that carries it changed, never on focus alone. It replaces a subject the tab generated on its first save or copied from storage, so a tab that opened before another tab stored a subject joins it. A subject id the server resolved (at init, from a prefetch or in a save response) is replaced only by a strictly newer stored choice, and an identity set with `identify()` is never replaced. * Under an IAB policy, the TC string follows the reconciled choice. See [How the TC string is reconciled](#how-the-tc-string-is-reconciled). * A record removed from readable storage since this tab last saw it present is cleared, and the active policy decides again. A record this tab never saw in storage, such as a receipt merged from the server after `identify()` or a choice seeded while storage was blocked, stays. * Blocked storage, or bytes that do not decode, change nothing. A failed read never grants a category. Expiry is not decided here. The applied records are evaluated at the time of the read, the same way as at startup. ### How the TC string is reconciled Under an IAB policy, a tab adopts the TC string another tab stored, unless that TC string conflicts with the reconciled choice. A conflicting TC string is withdrawn, and `__tcfapi` reports no consent until the next save. Vendors never receive a stale TC string: c15t withdraws it, or holds it back, before `__tcfapi` publishes. 1. The `@c15t/iab` module loads the TC string the other tab stored, with its purpose, vendor and special-feature selections, so `__tcfapi` and the preference controls show the choice now in force. Selections the visitor changed in this tab without saving are kept. 2. A missing or older stored TC string leaves the current one in place, unless the current one conflicts with the reconciled choice. Then it is withdrawn. 3. A category denied after the TC string was saved conflicts if the TC string grants any of its purposes. A partial selection saved through IAB, which records its category as denied because not every purpose is granted, keeps its TC string. 4. A TC string confirmed before the choice's newest decision no longer describes it and is withdrawn, unless a newer receipt replaces it. That receipt is adopted even when its TC string is identical, because TC strings round their time to the day and custom-vendor selections live only in the receipt. The receipt's expiry then applies. 5. The TC string and its receipt (`euconsent-v2`, `c15t-iab-authority-v1`) belong to one origin. After a save on a sibling subdomain that shares the consent cookie, this subdomain reports no consent through `__tcfapi` until it saves again. 6. A tab reloads the TC string when another tab on the same origin stores a new one. This covers a save in the same millisecond, or one that changed only vendors. 7. When two tabs save in the same millisecond with different selections, the more restrictive TC string wins in both, so a revoked vendor is never advertised again. If each grants something the other denies, neither is published until the next save, and the stored receipt is removed so a page opened later does not restore it. 8. When another tab removes the receipt or clears localStorage, this tab withdraws its TC string too, because a page opened now would find none. A receipt this tab was still decoding is not installed. 9. The removal after a tie checks that the stored receipt is the one it read. localStorage has no conditional removal, so a receipt another tab stored a moment earlier can still be removed. Every tab then withdraws its TC string until the next save, so the race only ever withholds consent. ### Clearing records across tabs `clearRecords()` also drops every save queued for replay, with or without browser persistence, so nothing the visitor decided before the clear reaches the backend afterwards. `clearRecords()` removes every record and then stores the time of the clear, the clear epoch, under its own key: `c15t-epoch` in localStorage and a cookie of the same name (`-epoch` with a custom `storageKey`). Clearing never removes it. Every consent record written afterwards also records the epoch it was written under. A decision confirmed before the epoch was made before the clear, so it is void wherever it turns up. A decision stamped in the very millisecond of the clear counts only if it comes from a tab that had already seen that clear, so a tab's own choice right after its own clear stands while another tab's decision in the same millisecond does not. A stored record left with no decisions after the clear is void too, subject included. * A tab that reconciles only after another tab cleared and saved again drops its pre-clear decisions instead of merging them back. Its subject from before the clear is dropped too. * A tab that missed the clear writes only decisions it made after it. Its queued write of an earlier decision is discarded, so it cannot bring back a cleared record. * Browser hydration and server reads (`readStoredRecordsFromCookieHeader`, used by the Next.js, TanStack Start, Nuxt, SvelteKit and Astro helpers) apply the same rule, so a server render agrees with the browser. Records from before any clear, including v2 and legacy records, read as epoch 0 and are unaffected. A corrupt epoch, or one that cannot be read, also reads as 0: it voids nothing, so a failed read never grants a category. A consent record whose own epoch field is corrupt is kept, and only its epoch is ignored. Each clear moves the epoch forward, even when the device clock went back, so a later clear never lets earlier decisions back in. Two clears in the same millisecond therefore leave the epoch a millisecond ahead, and a decision saved in that millisecond is void. An epoch up to one hour ahead of the clock is kept, as it is when the clock was set back after a clear. Until the clock catches up, decisions saved in that window are void too. An epoch more than an hour ahead is treated as corrupt and reads as 0, so a clear never writes one: after the clock went back more than an hour, the new epoch is capped at an hour ahead of the clock. That cap is a known limit. A cleared record carries times from before the clock went back, and a runtime that missed the clear can write those times back. The capped epoch is lower than them, so once the clock has recovered, such a decision counts again. Leaving the epoch uncapped does not help: every tab whose clock is still behind reads it as corrupt, which voids nothing. Times alone cannot order a clear against decisions stamped by a clock that went back more than an hour. This changes the stored format. After a clear, the consent cookie carries `&e=