diff --git a/ClaudeMeter/Models/API/UsageAPIResponse.swift b/ClaudeMeter/Models/API/UsageAPIResponse.swift
index b1bea04..7efc153 100644
--- a/ClaudeMeter/Models/API/UsageAPIResponse.swift
+++ b/ClaudeMeter/Models/API/UsageAPIResponse.swift
@@ -49,42 +49,35 @@ enum MappingError: LocalizedError {
 /// Extension to map API response to domain model
 extension UsageAPIResponse {
     func toDomain() throws -> UsageData {
-        // Configure ISO8601 formatter with proper options
         let iso8601Formatter = ISO8601DateFormatter()
         iso8601Formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
 
-        // Parse reset dates (must be present and valid)
-        let sessionResetDate: Date
-        let weeklyResetDate: Date
-
-        guard let sessionResetString = fiveHour.resetsAt,
-              let parsedDate = iso8601Formatter.date(from: sessionResetString) else {
-            throw MappingError.missingCriticalField(field: "fiveHour.resetsAt")
-        }
-        sessionResetDate = parsedDate
-
-        guard let weeklyResetString = sevenDay.resetsAt,
-              let parsedDate = iso8601Formatter.date(from: weeklyResetString) else {
-            throw MappingError.missingCriticalField(field: "sevenDay.resetsAt")
-        }
-        weeklyResetDate = parsedDate
-
-        // Handle optional sonnet usage
-        let sonnetLimit: UsageLimit? = sevenDaySonnet.flatMap { sonnet in
-            let sonnetResetDate: Date
-
-            if let sonnetResetString = sonnet.resetsAt,
-               let parsedDate = iso8601Formatter.date(from: sonnetResetString) {
-                sonnetResetDate = parsedDate
-            } else {
-                // Default to 7 days in the future if no reset date
-                sonnetResetDate = Date().addingTimeInterval(7 * 24 * 3600)
-            }
+        // The Claude API legitimately returns `resets_at: null` for windows with no usage in
+        // them yet (e.g. a freshly created session window or a client account that hasn't been
+        // used today). In that case fall back to the end of the rolling window so the UI shows
+        // a sensible "Resets in …" hint instead of refusing the response. We only treat a
+        // present-but-malformed date string as a hard error.
+        let sessionResetDate = try parseResetDate(
+            from: fiveHour.resetsAt,
+            field: "fiveHour.resetsAt",
+            formatter: iso8601Formatter,
+            fallback: Constants.Pacing.sessionWindow
+        )
+        let weeklyResetDate = try parseResetDate(
+            from: sevenDay.resetsAt,
+            field: "sevenDay.resetsAt",
+            formatter: iso8601Formatter,
+            fallback: Constants.Pacing.weeklyWindow
+        )
 
-            return UsageLimit(
-                utilization: sonnet.utilization,
-                resetAt: sonnetResetDate
+        let sonnetLimit: UsageLimit? = try sevenDaySonnet.flatMap { sonnet -> UsageLimit? in
+            let sonnetResetDate = try parseResetDate(
+                from: sonnet.resetsAt,
+                field: "sevenDaySonnet.resetsAt",
+                formatter: iso8601Formatter,
+                fallback: Constants.Pacing.weeklyWindow
             )
+            return UsageLimit(utilization: sonnet.utilization, resetAt: sonnetResetDate)
         }
 
         return UsageData(
@@ -100,4 +93,19 @@ extension UsageAPIResponse {
             lastUpdated: Date()
         )
     }
+
+    private func parseResetDate(
+        from raw: String?,
+        field: String,
+        formatter: ISO8601DateFormatter,
+        fallback: TimeInterval
+    ) throws -> Date {
+        guard let raw else {
+            return Date().addingTimeInterval(fallback)
+        }
+        guard let date = formatter.date(from: raw) else {
+            throw MappingError.missingCriticalField(field: field)
+        }
+        return date
+    }
 }
diff --git a/ClaudeMeterTests/UsageServiceTests.swift b/ClaudeMeterTests/UsageServiceTests.swift
index f5e1ca0..88f08ea 100644
--- a/ClaudeMeterTests/UsageServiceTests.swift
+++ b/ClaudeMeterTests/UsageServiceTests.swift
@@ -181,15 +181,59 @@ final class UsageServiceTests: XCTestCase {
         assertDate(usageData.weeklyUsage.resetAt, equalsIso8601String: TestConstants.weeklyResetDateString)
     }
 
-    func test_usageFetch_withInvalidPayload_surfacesInvalidResponse() async throws {
+    func test_usageFetch_withMissingResetAt_fillsInFallbackWindow() async throws {
+        // The Claude API returns `resets_at: null` for windows with no usage in them yet
+        // (typical for an account that hasn't been used today). The mapper should fall
+        // back to "now + window duration" rather than refusing the response.
         let responseData = try makeUsageResponseData(
-            sessionUtilization: TestConstants.sessionPercentage,
+            sessionUtilization: 0,
             weeklyUtilization: TestConstants.weeklyPercentage,
             sessionResetAt: nil,
             weeklyResetAt: TestConstants.weeklyResetDateString,
             sonnetUtilization: nil,
             sonnetResetAt: nil
         )
+        let networkService = NetworkServiceStub(responseData: responseData)
+        let cacheRepository = CacheRepositoryFake()
+        let keychainRepository = KeychainRepositoryFake()
+        let settingsRepository = SettingsRepositoryFake()
+
+        let service = UsageService(
+            networkService: networkService,
+            cacheRepository: cacheRepository,
+            keychainRepository: keychainRepository,
+            settingsRepository: settingsRepository
+        )
+
+        try await keychainRepository.save(
+            sessionKey: TestConstants.sessionKeyValue,
+            account: "default"
+        )
+        var settings = AppSettings.default
+        settings.cachedOrganizationId = UUID(uuidString: TestConstants.organizationUUIDString)
+        try await settingsRepository.save(settings)
+
+        let usageData = try await service.fetchUsage(forceRefresh: true)
+
+        XCTAssertEqual(usageData.sessionUsage.utilization, 0)
+        // Reset date should be in the future, roughly one session window from now.
+        XCTAssertGreaterThan(usageData.sessionUsage.resetAt.timeIntervalSinceNow, 0)
+        XCTAssertLessThanOrEqual(
+            usageData.sessionUsage.resetAt.timeIntervalSinceNow,
+            Constants.Pacing.sessionWindow + 5
+        )
+    }
+
+    func test_usageFetch_withMalformedResetAt_surfacesInvalidResponse() async throws {
+        // A non-null but unparseable date string is still treated as a hard error.
+        let responseData = try makeUsageResponseData(
+            sessionUtilization: TestConstants.sessionPercentage,
+            weeklyUtilization: TestConstants.weeklyPercentage,
+            sessionResetAt: "not-a-date",
+            weeklyResetAt: TestConstants.weeklyResetDateString,
+            sonnetUtilization: nil,
+            sonnetResetAt: nil
+        )
 
         let networkService = NetworkServiceStub(responseData: responseData)
         let cacheRepository = CacheRepositoryFake()