diff --git a/Yattee/Core/AccountsManager.swift b/Yattee/Core/AccountsManager.swift
index 7960bd1d..4c2fbe70 100644
--- a/Yattee/Core/AccountsManager.swift
+++ b/Yattee/Core/AccountsManager.swift
@@ -639,14 +639,25 @@ final class AccountsManager {
         accountStatuses[accountID] ?? (identityScope.isResolved ? .credentialMissing : .checking)
     }
 
+    /// Asked to confirm an auth-shaped failure before anything is concluded
+    /// from it. `nil` — tests, and the window before wiring completes — means
+    /// nothing is concluded at all, which is the safe direction.
+    ///
+    /// Held here as a hook rather than as a dependency because the confirmation
+    /// needs an API client, and this manager deliberately knows nothing about
+    /// the network.
+    @ObservationIgnored var onSuspectedCredentialRejection: (@MainActor (Account) -> Void)?
+
     /// Records what a server call implies about the account that made it.
     ///
-    /// Deliberately conservative about rejection: a raw 401 marks the
-    /// credential only when no reverse-proxy basic auth is configured, because
-    /// otherwise the 401 may well have come from the proxy. The two-stage probe
-    /// that disambiguates the rest is Phase 2 work; until then an ambiguous
-    /// failure leaves the status alone rather than wrongly telling the user to
-    /// sign in again.
+    /// Nothing here rejects a credential. A raw 401 or 403 is *suspicion*: the
+    /// same statuses come back from a reverse proxy, from endpoints a valid
+    /// session may not touch, and — on Invidious — from a genuinely revoked
+    /// session, and the request that failed cannot tell them apart. It is
+    /// handed to `onSuspectedCredentialRejection`, which asks the question again
+    /// against an endpoint that exists to be authenticated; only that second
+    /// answer marks the account. Until it arrives the status is left alone,
+    /// rather than wrongly telling the user to sign in again.
     ///
     /// Shared by the subscription providers and the feed cache: a credential
     /// that expires during a feed-only refresh has to mark the account too, or
@@ -667,13 +678,31 @@ final class AccountsManager {
         case APIError.cancelled:
             return
         case APIError.unauthorized:
-            guard instance(for: account)?.usesBasicAuth == false else { return }
-            setStatus(.credentialRejected, for: account.id)
+            suspectRejection(of: account)
+        case let APIError.httpError(statusCode, _) where statusCode == 401 || statusCode == 403:
+            // 403 arrives untranslated on purpose: the shared HTTP mapping
+            // turns only 401 into `.unauthorized`, because unauthenticated call
+            // sites read 403 as "this instance is blocked" rather than "this
+            // login is". Both shapes mean the same thing here.
+            suspectRejection(of: account)
         default:
             return
         }
     }
 
+    /// Passes an auth-shaped failure on for confirmation, unless the instance
+    /// itself can explain it.
+    private func suspectRejection(of account: Account) {
+        // An instance behind HTTP Basic Auth answers 401 at the proxy for every
+        // request made to it, whichever account made it — so the status says
+        // nothing about this credential, and a probe would only be answered by
+        // the same proxy. An orphaned account, whose instance is unknown, is
+        // skipped by the same guard: there is nothing to probe against.
+        guard instance(for: account)?.usesBasicAuth == false else { return }
+
+        onSuspectedCredentialRejection?(account)
+    }
+
     // MARK: - Legacy migration
 
     /// Turns each pre-revamp single-login instance into an account.
diff --git a/Yattee/Core/AppEnvironment.swift b/Yattee/Core/AppEnvironment.swift
index 6b27cb5a..7535cfbf 100644
--- a/Yattee/Core/AppEnvironment.swift
+++ b/Yattee/Core/AppEnvironment.swift
@@ -56,6 +56,7 @@ final class AppEnvironment {
     let homeInstanceCache: HomeInstanceCache
     let invidiousAPI: InvidiousAPI
     let pipedAPI: PipedAPI
+    let credentialProbeService: CredentialProbeService
     let subscriptionAccountValidator: SubscriptionAccountValidator
     let dataContextValidator: DataContextValidator
     let playerControlsLayoutService: PlayerControlsLayoutService
@@ -213,6 +214,45 @@ final class AppEnvironment {
         let pipedAPI = PipedAPI(httpClient: client)
         self.pipedAPI = pipedAPI
 
+        // A 401 or 403 from an ordinary request is never allowed to reject a
+        // login on its own — it is re-asked here against an endpoint that
+        // exists to be authenticated, and only that answer marks the account.
+        let probeService = CredentialProbeService(
+            accountsManager: accounts,
+            credentialReader: { [weak accounts, weak invidiousCreds, weak pipedCreds] account in
+                guard let type = accounts?.instance(for: account)?.type else { return nil }
+                switch type {
+                case .invidious: return invidiousCreds?.credential(for: account)
+                case .piped: return pipedCreds?.credential(for: account)
+                default: return nil
+                }
+            },
+            transport: { instance, credential in
+                do {
+                    switch instance.type {
+                    case .invidious:
+                        // GET /api/v1/auth/subscriptions — the cheapest call
+                        // that does nothing except require a session.
+                        _ = try await invidiousAPI.subscriptions(instance: instance, sid: credential)
+                    case .piped:
+                        _ = try await pipedAPI.subscriptions(instance: instance, authToken: credential)
+                    default:
+                        return .indeterminate
+                    }
+                    return .authenticated
+                } catch {
+                    return CredentialProbeService.classify(error)
+                }
+            }
+        )
+        self.credentialProbeService = probeService
+
+        // Weak: the manager is owned for the life of the app and the probe
+        // service holds it back, so a strong capture here would be a cycle.
+        accounts.onSuspectedCredentialRejection = { [weak probeService] account in
+            probeService?.probeSuspectedRejection(for: account)
+        }
+
         // Initialize SubscriptionService with all required dependencies
         self.subscriptionService = SubscriptionService(
             dataManager: dm,
@@ -726,6 +766,13 @@ final class AppEnvironment {
         subscriptionService.discardProvider(for: account.id)
         HomeInstanceCache.shared.clear(instanceID: account.instanceID, contentType: .feed)
 
+        // Reached by both ways a credential is replaced — the user signing in
+        // again here, and a replacement arriving over iCloud Keychain that the
+        // fingerprint digest spots — so the probe service's per-account dedup
+        // is retired here rather than at either call site. A verdict earned by
+        // the old secret must never delay asking about the new one.
+        credentialProbeService.credentialDidChange(for: account.id)
+
         if accountsManager.currentContext == .account(account.id) {
             SubscriptionFeedCache.shared.handleAccountChange()
         }
diff --git a/Yattee/Services/Credentials/CredentialProbeService.swift b/Yattee/Services/Credentials/CredentialProbeService.swift
new file mode 100644
index 00000000..6c6f6564
--- /dev/null
+++ b/Yattee/Services/Credentials/CredentialProbeService.swift
@@ -0,0 +1,233 @@
+//
+//  CredentialProbeService.swift
+//  Yattee
+//
+//  Confirms — or refuses to confirm — that a server really rejected an
+//  account's credential.
+//
+
+import Foundation
+
+/// Turns an auth-shaped failure into a verdict about the credential itself.
+///
+/// A raw 401 or 403 from an ordinary account request is not evidence: Invidious
+/// answers 403 `{"error":"Request must be authenticated"}` for a revoked
+/// session, but also for endpoints an otherwise valid session may not touch,
+/// and a reverse proxy in front of either backend answers 401 for reasons that
+/// have nothing to do with the account. Acting on the raw status would tell
+/// users with a perfectly good login to sign in again — and drop their reads to
+/// local data — on the strength of one unlucky request.
+///
+/// So the failure only *suspects* rejection. This service asks the question
+/// again, deliberately, against an endpoint that exists purely to be
+/// authenticated (the subscriptions list), and only that answer is allowed to
+/// set `credentialRejected`. Everything else the probe can return maps to a
+/// state that lifts by itself.
+///
+/// The transport and the credential lookup are injected so tests can script the
+/// answer without a server, and so this file stays free of any API-client
+/// knowledge.
+@MainActor
+final class CredentialProbeService {
+    /// What a probe concluded about the credential it was given.
+    enum Verdict: Equatable {
+        /// The server accepted it — the original failure was about the
+        /// operation, not the login.
+        case authenticated
+        /// The server refused it. The only verdict that marks an account.
+        case rejected
+        /// The server could not be asked. Says nothing about the credential.
+        case unreachable
+        /// The probe produced an answer that proves nothing either way — a
+        /// decoding failure, a 500, a cancelled task. Status is left alone.
+        case indeterminate
+    }
+
+    /// Performs one authenticated request with the given credential.
+    typealias Transport = @MainActor (Instance, String) async -> Verdict
+
+    /// The credential an account currently holds, or `nil` when there is none
+    /// to be read — including while the identity scope is unresolved, which is
+    /// "cannot tell yet" rather than "signed out".
+    typealias CredentialReader = @MainActor (Account) -> String?
+
+    private let accountsManager: AccountsManager
+    private let credentialReader: CredentialReader
+    private let transport: Transport
+
+    /// How long an *answered* probe stands before another may run for the same
+    /// account. A failing feed refresh can produce several auth-shaped failures
+    /// in a row; without this each one would cost a round trip to say the same
+    /// thing. Injectable so tests do not have to wait a real minute.
+    private let cooldown: TimeInterval
+
+    /// Probes currently in flight, keyed by account.
+    ///
+    /// Exposed because it is also the only way to await one: the probe is
+    /// deliberately fire-and-forget from the caller's side — `recordServerOutcome`
+    /// cannot become async — so tests need a handle to join on.
+    private(set) var probeTasks: [UUID: Task<Void, Never>] = [:]
+
+    /// When a probe last produced an answer *about the credential the account
+    /// still holds*. A verdict dropped as stale never lands here: it says
+    /// nothing about what is in place now, so it may not delay asking.
+    private var lastProbeFinished: [UUID: Date] = [:]
+
+    /// Bumped every time an account's credential is replaced.
+    ///
+    /// A probe carries the value it was spawned under and may only speak while
+    /// it still matches. Cancellation alone would not do: the transport may not
+    /// notice it for a while, and comparing the credential is not enough either
+    /// — a replacement can happen to be the same string, and the comparison
+    /// races with the gates being cleared.
+    private var generations: [UUID: Int] = [:]
+
+    init(
+        accountsManager: AccountsManager,
+        credentialReader: @escaping CredentialReader,
+        transport: @escaping Transport,
+        cooldown: TimeInterval = 60
+    ) {
+        self.accountsManager = accountsManager
+        self.credentialReader = credentialReader
+        self.transport = transport
+        self.cooldown = cooldown
+    }
+
+    // MARK: - Probing
+
+    /// Asks the server whether this account's credential is still good.
+    ///
+    /// Returns immediately; the verdict is applied to `AccountsManager` when the
+    /// answer arrives, and only if it still describes the same credential on the
+    /// same account.
+    func probeSuspectedRejection(for account: Account) {
+        guard probeTasks[account.id] == nil else { return }
+
+        if let finished = lastProbeFinished[account.id],
+           Date().timeIntervalSince(finished) < cooldown {
+            return
+        }
+
+        // No credential to test. `refreshAccountStatuses` owns that verdict —
+        // it can tell a missing credential from an unresolved identity scope,
+        // and this service deliberately cannot.
+        guard let credential = credentialReader(account) else { return }
+
+        // An account whose instance has not arrived (or is already gone) has
+        // nothing to probe against.
+        guard let instance = accountsManager.instance(for: account) else { return }
+
+        let accountID = account.id
+        let generation = generations[accountID, default: 0]
+
+        probeTasks[accountID] = Task { [weak self] in
+            let verdict = await self?.transport(instance, credential)
+
+            guard let self else { return }
+            defer {
+                // Only if this probe is still the one on record. A credential
+                // change takes the slot away and a fresh probe may already be
+                // using it; clearing it here would strand that one's dedup.
+                if self.generations[accountID, default: 0] == generation {
+                    self.probeTasks[accountID] = nil
+                }
+            }
+
+            guard let verdict else { return }
+
+            // The world may have moved while the probe was out. A verdict about
+            // a credential that has since been replaced — or read under an
+            // identity scope that has since flipped, which makes the reader
+            // return `nil` — describes nothing that is still true, and applying
+            // it would reject a login the user just fixed.
+            guard self.generations[accountID, default: 0] == generation,
+                  let current = self.accountsManager.account(id: accountID),
+                  self.credentialReader(current) == credential
+            else {
+                LoggingService.shared.debug(
+                    "Dropped a credential probe verdict — the account or its credential changed",
+                    category: .general,
+                    details: "accountID=\(accountID)"
+                )
+                return
+            }
+
+            // Stamped only here, on the one path where an answer about the
+            // credential now in place was actually obtained. A dropped verdict
+            // describes a secret that is gone, so it may not silence the first
+            // probe of its replacement — the login the user just fixed is
+            // exactly the one whose rejection has to be found quickly. Probe
+            // storms are held off by the in-flight guard regardless.
+            self.lastProbeFinished[accountID] = Date()
+
+            self.apply(verdict, to: accountID)
+        }
+    }
+
+    /// Retires everything this service knows about an account's *previous*
+    /// credential.
+    ///
+    /// Both gates in `probeSuspectedRejection` are keyed by account alone and
+    /// are checked before the credential is even read, so without this a
+    /// replacement inherits the old secret's dedup: a finished probe would
+    /// silence the new login's first failure for the whole cooldown, and one
+    /// still in flight would silence it until that round trip returned. The
+    /// replacement is the login the user just fixed — a rejection of *it* is
+    /// the one worth finding immediately.
+    ///
+    /// The in-flight probe is cancelled as well as disowned, so a network call
+    /// nobody will listen to is not left running.
+    func credentialDidChange(for accountID: UUID) {
+        generations[accountID, default: 0] += 1
+        lastProbeFinished[accountID] = nil
+        probeTasks.removeValue(forKey: accountID)?.cancel()
+    }
+
+    private func apply(_ verdict: Verdict, to accountID: UUID) {
+        switch verdict {
+        case .authenticated:
+            // The credential works; whatever failed was about the request.
+            accountsManager.setStatus(.signedIn, for: accountID)
+        case .rejected:
+            LoggingService.shared.info(
+                "A credential probe confirmed the server rejects this account's login",
+                category: .general,
+                details: "accountID=\(accountID)"
+            )
+            accountsManager.setStatus(.credentialRejected, for: accountID)
+        case .unreachable:
+            // Recorded rather than ignored so reads fall back to local data,
+            // and lifted again by the manager's own retry timer.
+            accountsManager.setStatus(.unreachable, for: accountID)
+        case .indeterminate:
+            break
+        }
+    }
+
+    // MARK: - Classification
+
+    /// Maps a probe's outcome onto a verdict.
+    ///
+    /// `nil` means the request succeeded. Only the two auth statuses reject:
+    /// 403 is included because Invidious answers it — not 401 — for a session
+    /// that has been revoked, and it reaches here as a plain `httpError`
+    /// because the global 403 mapping in `HTTPClient` is shared with
+    /// unauthenticated call sites that read it as "blocked", not "signed out".
+    static func classify(_ error: Error?) -> Verdict {
+        guard let error else { return .authenticated }
+
+        switch error {
+        case APIError.unauthorized:
+            return .rejected
+        case let APIError.httpError(statusCode, _) where statusCode == 401 || statusCode == 403:
+            return .rejected
+        case APIError.timeout, APIError.noConnection:
+            return .unreachable
+        default:
+            // Cancellation included: a probe that was torn down never asked the
+            // question, so it may not answer it either.
+            return .indeterminate
+        }
+    }
+}
diff --git a/YatteeTests/CredentialProbeTests.swift b/YatteeTests/CredentialProbeTests.swift
new file mode 100644
index 00000000..a37683c2
--- /dev/null
+++ b/YatteeTests/CredentialProbeTests.swift
@@ -0,0 +1,486 @@
+//
+//  CredentialProbeTests.swift
+//  YatteeTests
+//
+//  The two-stage rule: an auth-shaped failure only ever *suspects* rejection,
+//  and nothing but a deliberate probe may confirm it.
+//
+
+import Testing
+import Foundation
+@testable import Yattee
+
+/// A probe service wired to a scripted transport instead of a server.
+///
+/// Holds the credential in a mutable box so a test can replace it mid-probe,
+/// and can park every probe inside the transport so the window between "the
+/// request failed" and "the probe answered" can be inspected — that window is
+/// where the whole design lives.
+@MainActor
+private final class ProbeHarness {
+    let instances: InstancesManager
+    let instance: Instance
+    let accounts: AccountsManager
+    let account: Account
+    private(set) var service: CredentialProbeService!
+
+    /// What the credential reader reports right now.
+    var credential: String? = "sid-original"
+
+    /// What the next probe concludes.
+    var verdict: CredentialProbeService.Verdict = .authenticated
+
+    /// Per-credential answers, for the tests where two probes overlap and each
+    /// has to conclude something different.
+    var verdictsByCredential: [String: CredentialProbeService.Verdict] = [:]
+
+    private(set) var transportCalls = 0
+    private(set) var probedCredentials: [String] = []
+
+    /// While true, every probe suspends inside the transport until `release()`.
+    var holdsProbesOpen = false
+    private var parked: [(credential: String, continuation: CheckedContinuation<Void, Never>)] = []
+
+    init(
+        type: InstanceType = .invidious,
+        usesBasicAuth: Bool = false,
+        cooldown: TimeInterval = 0,
+        unreachableRetryInterval: TimeInterval = 60
+    ) {
+        instances = InstancesManager()
+        instance = Instance(
+            type: type,
+            url: URL(string: "https://probe-\(UUID().uuidString).example.com")!,
+            usesBasicAuth: usesBasicAuth
+        )
+        instances.add(instance)
+
+        accounts = AccountsManager(
+            identityScope: CredentialIdentityScope(fixedScope: "identity-A"),
+            instancesManager: instances,
+            defaults: UserDefaults(suiteName: "yattee.tests.\(UUID().uuidString)")!,
+            unreachableRetryInterval: unreachableRetryInterval
+        )
+        account = accounts.addAccount(instanceID: instance.id)
+        accounts.setStatus(.signedIn, for: account.id)
+
+        service = CredentialProbeService(
+            accountsManager: accounts,
+            credentialReader: { [weak self] _ in self?.credential },
+            transport: { [weak self] _, credential in
+                guard let self else { return .indeterminate }
+                return await self.runProbe(credential)
+            },
+            cooldown: cooldown
+        )
+
+        accounts.onSuspectedCredentialRejection = { [weak self] account in
+            self?.service.probeSuspectedRejection(for: account)
+        }
+    }
+
+    /// Instances live in the shared defaults, so each harness has to take its
+    /// own back out again.
+    func tearDown() {
+        instances.remove(instance)
+    }
+
+    var status: AccountStatus {
+        accounts.status(for: account.id)
+    }
+
+    var probeTask: Task<Void, Never>? {
+        service.probeTasks[account.id]
+    }
+
+    func record(_ error: Error?) {
+        accounts.recordServerOutcome(error, for: account)
+    }
+
+    private func runProbe(_ credential: String) async -> CredentialProbeService.Verdict {
+        transportCalls += 1
+        probedCredentials.append(credential)
+
+        if holdsProbesOpen {
+            await withCheckedContinuation { parked.append((credential, $0)) }
+        }
+
+        return verdictsByCredential[credential] ?? verdict
+    }
+
+    /// Lets parked probes finish — the one probing `credential`, or every one
+    /// of them when it is `nil`, which also stops new probes being parked.
+    func release(credential: String? = nil) {
+        guard let credential else {
+            holdsProbesOpen = false
+            let waiting = parked
+            parked = []
+            waiting.forEach { $0.continuation.resume() }
+            return
+        }
+
+        let matching = parked.filter { $0.credential == credential }
+        parked.removeAll { $0.credential == credential }
+        matching.forEach { $0.continuation.resume() }
+    }
+
+    /// Waits until the transport has been entered at least `count` times.
+    func waitForTransport(count: Int) async throws {
+        for _ in 0 ..< 200 {
+            if transportCalls >= count { return }
+            try await Task.sleep(for: .milliseconds(5))
+        }
+        Issue.record("The transport was never entered \(count) time(s)")
+    }
+}
+
+@Suite("Credential probe", .serialized)
+@MainActor
+struct CredentialProbeTests {
+    // MARK: - The raw status never decides
+
+    @Test("A 403 asks the server again instead of rejecting the login")
+    func forbiddenNeverRejectsDirectly() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 403, message: "Request must be authenticated"))
+
+        // Invidious answers exactly this for a revoked session — and for
+        // several things that are not one. The failed request may not conclude
+        // anything by itself.
+        #expect(harness.status == .signedIn)
+
+        try await harness.waitForTransport(count: 1)
+        #expect(harness.status == .signedIn)
+
+        let task = try #require(harness.probeTask)
+        harness.release()
+        await task.value
+
+        // Only the probe's own answer marks the account.
+        #expect(harness.status == .credentialRejected)
+    }
+
+    @Test("An unauthorized error goes through the probe as well")
+    func unauthorizedRoutesThroughTheProbe() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdict = .rejected
+
+        harness.record(APIError.unauthorized)
+
+        // A bare 401 used to reject on the spot. A reverse proxy answers it too.
+        #expect(harness.status == .signedIn)
+
+        try await harness.waitForTransport(count: 1)
+        let task = try #require(harness.probeTask)
+        harness.release()
+        await task.value
+
+        #expect(harness.status == .credentialRejected)
+    }
+
+    // MARK: - Verdicts
+
+    @Test("A confirmed rejection makes the selected context fall back to local")
+    func confirmedRejectionEngagesTheFallback() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.verdict = .rejected
+
+        let validator = DataContextValidator(
+            accountsManager: harness.accounts,
+            instancesManager: harness.instances,
+            toastManager: ToastManager()
+        )
+        harness.accounts.setContext(.account(harness.account.id))
+        #expect(validator.validity == .valid)
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        await harness.probeTask?.value
+
+        #expect(harness.status == .credentialRejected)
+
+        // The point of the whole exercise: reads stop going to a server that
+        // will not answer them, without the user's choice being thrown away.
+        #expect(validator.validity == .temporarilyUnavailable(.credentialRejected))
+        #expect(validator.effectiveContext == .local)
+        #expect(!validator.allowsServerMutations)
+        #expect(harness.accounts.currentContext == .account(harness.account.id))
+    }
+
+    @Test("A probe the server accepts leaves the account signed in")
+    func acceptedProbeKeepsTheAccountUsable() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.verdict = .authenticated
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        await harness.probeTask?.value
+
+        // The credential is fine; the operation that failed was not allowed.
+        #expect(harness.transportCalls == 1)
+        #expect(harness.status == .signedIn)
+    }
+
+    @Test("A probe that cannot reach the server is retried, not written off")
+    func unreachableProbeLiftsItself() async throws {
+        let harness = ProbeHarness(unreachableRetryInterval: 0.05)
+        defer { harness.tearDown() }
+        harness.verdict = .unreachable
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        await harness.probeTask?.value
+
+        #expect(harness.status == .unreachable)
+
+        // Nothing about the credential was learned, so the state has to expire
+        // on its own — the same auto-expiry every unreachable account gets.
+        try await Task.sleep(for: .seconds(0.5))
+        #expect(harness.status == .signedIn)
+    }
+
+    // MARK: - What must not be probed
+
+    @Test("An instance behind basic auth is never probed")
+    func basicAuthInstanceIsNotProbed() async throws {
+        let harness = ProbeHarness(usesBasicAuth: true)
+        defer { harness.tearDown() }
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 401, message: nil))
+        harness.record(APIError.unauthorized)
+
+        try await Task.sleep(for: .milliseconds(50))
+
+        // The proxy answers 401 for every request to this instance whoever
+        // makes it, and would answer the probe identically.
+        #expect(harness.transportCalls == 0)
+        #expect(harness.probeTask == nil)
+        #expect(harness.status == .signedIn)
+    }
+
+    @Test("Errors that are not about authentication start no probe")
+    func nonAuthErrorsDoNotProbe() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 500, message: "Internal Server Error"))
+        #expect(harness.probeTask == nil)
+
+        harness.record(APIError.decodingError("unexpected shape"))
+        #expect(harness.probeTask == nil)
+
+        try await Task.sleep(for: .milliseconds(50))
+        #expect(harness.transportCalls == 0)
+        #expect(harness.status == .signedIn)
+    }
+
+    // MARK: - Staleness and dedup
+
+    @Test("A verdict about a credential that has since been replaced is dropped")
+    func replacedCredentialDropsTheVerdict() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        try await harness.waitForTransport(count: 1)
+
+        // A replacement arriving over iCloud Keychain — or the user signing in
+        // again — while the probe was out. The answer describes the old secret.
+        harness.credential = "sid-replacement"
+
+        let task = try #require(harness.probeTask)
+        harness.release()
+        await task.value
+
+        #expect(harness.probedCredentials == ["sid-original"])
+        #expect(harness.status == .signedIn)
+    }
+
+    @Test("A dropped verdict never silences the replacement credential's first probe")
+    func droppedVerdictStartsNoCooldown() async throws {
+        let harness = ProbeHarness(cooldown: 60)
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        try await harness.waitForTransport(count: 1)
+
+        harness.credential = "sid-replacement"
+        let dropped = try #require(harness.probeTask)
+        harness.release()
+        await dropped.value
+        #expect(harness.status == .signedIn)
+
+        // The replacement can be rejected too, and nothing has asked about it
+        // yet — the login the user just fixed is exactly the one whose failure
+        // has to be found quickly, not a minute later.
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        let second = try #require(harness.probeTask)
+        await second.value
+
+        #expect(harness.transportCalls == 2)
+        #expect(harness.probedCredentials == ["sid-original", "sid-replacement"])
+        #expect(harness.status == .credentialRejected)
+    }
+
+    @Test("Replacing the credential retires the cooldown the old one earned")
+    func credentialChangeClearsTheCooldown() async throws {
+        let harness = ProbeHarness(cooldown: 60)
+        defer { harness.tearDown() }
+        harness.verdict = .authenticated
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        await harness.probeTask?.value
+        #expect(harness.transportCalls == 1)
+
+        // Both gates are keyed by account alone and are checked before the
+        // credential is even read, so the new login would otherwise inherit the
+        // old one's silence for a full minute.
+        harness.credential = "sid-replacement"
+        harness.service.credentialDidChange(for: harness.account.id)
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        let second = try #require(harness.probeTask)
+        await second.value
+
+        #expect(harness.transportCalls == 2)
+        #expect(harness.probedCredentials == ["sid-original", "sid-replacement"])
+        #expect(harness.status == .credentialRejected)
+    }
+
+    @Test("A probe left over from a replaced credential is disowned, not merely stale")
+    func credentialChangeInvalidatesAProbeInFlight() async throws {
+        let harness = ProbeHarness(cooldown: 60)
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdictsByCredential = [
+            "sid-original": .rejected,
+            "sid-replacement": .authenticated
+        ]
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        try await harness.waitForTransport(count: 1)
+        let stale = try #require(harness.probeTask)
+
+        harness.credential = "sid-replacement"
+        harness.service.credentialDidChange(for: harness.account.id)
+
+        // Disowned and cancelled: nobody is going to listen to its answer.
+        #expect(harness.probeTask == nil)
+        #expect(stale.isCancelled)
+
+        // The replacement's own failure is examined at once rather than waiting
+        // out a round trip that is about a secret nobody holds any more.
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        let fresh = try #require(harness.probeTask)
+        #expect(fresh != stale)
+        try await harness.waitForTransport(count: 2)
+
+        harness.release(credential: "sid-original")
+        await stale.value
+
+        // The late rejection belongs to the old secret and may not be spoken —
+        // nor may finishing evict the probe that has taken its place.
+        #expect(harness.status == .signedIn)
+        #expect(harness.probeTask == fresh)
+
+        harness.release(credential: "sid-replacement")
+        await fresh.value
+
+        #expect(harness.transportCalls == 2)
+        #expect(harness.probedCredentials == ["sid-original", "sid-replacement"])
+        #expect(harness.status == .signedIn)
+        #expect(harness.probeTask == nil)
+    }
+
+    @Test("Repeated failures while a probe is in flight cost one round trip")
+    func concurrentTriggersDedupe() async throws {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+        harness.holdsProbesOpen = true
+        harness.verdict = .rejected
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        try await harness.waitForTransport(count: 1)
+
+        // A feed refresh produces a burst of these.
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        harness.record(APIError.unauthorized)
+
+        let task = try #require(harness.probeTask)
+        harness.release()
+        await task.value
+
+        #expect(harness.transportCalls == 1)
+        #expect(harness.status == .credentialRejected)
+    }
+
+    @Test("A finished probe stands for the cooldown")
+    func cooldownSuppressesAnImmediateReprobe() async throws {
+        let harness = ProbeHarness(cooldown: 60)
+        defer { harness.tearDown() }
+        harness.verdict = .authenticated
+
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        await harness.probeTask?.value
+        #expect(harness.transportCalls == 1)
+
+        // The next failed request arrives moments later and asks the same
+        // question; the answer cannot have changed.
+        harness.record(APIError.httpError(statusCode: 403, message: nil))
+        #expect(harness.probeTask == nil)
+
+        try await Task.sleep(for: .milliseconds(50))
+        #expect(harness.transportCalls == 1)
+        #expect(harness.status == .signedIn)
+    }
+
+    // MARK: - Classification
+
+    @Test("Only the two authentication statuses reject")
+    func classifyMapsOutcomes() {
+        #expect(CredentialProbeService.classify(nil) == .authenticated)
+
+        #expect(CredentialProbeService.classify(APIError.unauthorized) == .rejected)
+        #expect(CredentialProbeService.classify(
+            APIError.httpError(statusCode: 401, message: nil)
+        ) == .rejected)
+        // The one the shared HTTP mapping leaves untranslated.
+        #expect(CredentialProbeService.classify(
+            APIError.httpError(statusCode: 403, message: "Request must be authenticated")
+        ) == .rejected)
+
+        #expect(CredentialProbeService.classify(APIError.timeout) == .unreachable)
+        #expect(CredentialProbeService.classify(APIError.noConnection) == .unreachable)
+
+        // A probe that never completed answers nothing.
+        #expect(CredentialProbeService.classify(APIError.cancelled) == .indeterminate)
+        #expect(CredentialProbeService.classify(
+            APIError.httpError(statusCode: 500, message: nil)
+        ) == .indeterminate)
+    }
+
+    // MARK: - Regression
+
+    @Test("A successful request still lifts a rejection")
+    func successLiftsARejection() {
+        let harness = ProbeHarness()
+        defer { harness.tearDown() }
+
+        harness.accounts.setStatus(.credentialRejected, for: harness.account.id)
+        harness.record(nil)
+
+        #expect(harness.status == .signedIn)
+    }
+}