diff --git a/Sources/LatrKit/Library/SavedLibrary.swift b/Sources/LatrKit/Library/SavedLibrary.swift
index e4317fa..2db3f17 100644
--- a/Sources/LatrKit/Library/SavedLibrary.swift
+++ b/Sources/LatrKit/Library/SavedLibrary.swift
@@ -159,26 +159,32 @@ public struct SavedLibrary: Sendable {
     }
 
     public func setState(ofSavedItemWithKey key: String, to state: SavedItemState) async throws {
-        guard let current = try await savedItem(withKey: key) else {
-            throw SavedLibraryError.itemNotFound
+        for attempt in 0 ... 1 {
+            guard let current = try await savedItem(withKey: key) else { throw SavedLibraryError.itemNotFound }
+            var next = current.value
+            next.state = state
+            do {
+                _ = try await repository.updateRecord(
+                    in: repositoryDID, collection: .savedItem, withKey: key,
+                    value: next, swapRecord: current.cid
+                )
+                return
+            } catch RepositoryClientError.conflict where attempt == 0 { continue }
+            catch RepositoryClientError.conflict { throw SavedLibraryError.conflict }
         }
-
-        var next = current.value
-        next.state = state
-        _ = try await repository.updateRecord(
-            in: repositoryDID,
-            collection: .savedItem,
-            withKey: key,
-            value: next
-        )
     }
 
     public func removeSavedItem(withKey key: String) async throws {
-        try await repository.deleteRecord(
-            in: repositoryDID,
-            collection: .savedItem,
-            withKey: key
-        )
+        for attempt in 0 ... 1 {
+            guard let current = try await savedItem(withKey: key) else { throw SavedLibraryError.itemNotFound }
+            do {
+                try await repository.deleteRecord(
+                    in: repositoryDID, collection: .savedItem, withKey: key, swapRecord: current.cid
+                )
+                return
+            } catch RepositoryClientError.conflict where attempt == 0 { continue }
+            catch RepositoryClientError.conflict { throw SavedLibraryError.conflict }
+        }
     }
 
     public func removeExternalSave(for url: String, includingWrapper: Bool = false) async throws {
diff --git a/Sources/LatrKit/Library/SavedLibraryError.swift b/Sources/LatrKit/Library/SavedLibraryError.swift
index 3eb429f..e1f1e17 100644
--- a/Sources/LatrKit/Library/SavedLibraryError.swift
+++ b/Sources/LatrKit/Library/SavedLibraryError.swift
@@ -3,4 +3,6 @@ import Foundation
 public enum SavedLibraryError: Error, Sendable {
     case invalidURL
     case itemNotFound
+    case conflict
+    case invalidStoredRecord(uri: String)
 }
diff --git a/Sources/LatrKit/Models/ExternalSave.swift b/Sources/LatrKit/Models/ExternalSave.swift
index 066b199..a702d9e 100644
--- a/Sources/LatrKit/Models/ExternalSave.swift
+++ b/Sources/LatrKit/Models/ExternalSave.swift
@@ -14,6 +14,7 @@ public struct ExternalSave: Codable, Sendable, Equatable {
     public var language: String?
     public var publishedAt: String?
     public var author: String?
+    public var unknownFields: [String: JSONValue]
 
     enum CodingKeys: String, CodingKey {
         case type = "$type"
@@ -41,7 +42,8 @@ public struct ExternalSave: Codable, Sendable, Equatable {
         image: String? = nil,
         language: String? = nil,
         publishedAt: String? = nil,
-        author: String? = nil
+        author: String? = nil,
+        unknownFields: [String: JSONValue] = [:]
     ) {
         self.type = LexiconCollection.external.identifier
         self.url = url
@@ -55,6 +57,33 @@ public struct ExternalSave: Codable, Sendable, Equatable {
         self.language = language
         self.publishedAt = publishedAt
         self.author = author
+        self.unknownFields = unknownFields
+    }
+
+    public init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        type = try c.decode(String.self, forKey: .type); url = try c.decode(String.self, forKey: .url)
+        normalizedUrl = try c.decode(String.self, forKey: .normalizedUrl); fingerprint = try c.decode(String.self, forKey: .fingerprint)
+        createdAt = try c.decode(String.self, forKey: .createdAt); title = try c.decodeIfPresent(String.self, forKey: .title)
+        excerpt = try c.decodeIfPresent(String.self, forKey: .excerpt); site = try c.decodeIfPresent(String.self, forKey: .site)
+        image = try c.decodeIfPresent(String.self, forKey: .image); language = try c.decodeIfPresent(String.self, forKey: .language)
+        publishedAt = try c.decodeIfPresent(String.self, forKey: .publishedAt); author = try c.decodeIfPresent(String.self, forKey: .author)
+        let dynamic = try decoder.container(keyedBy: AnyCodingKey.self)
+        let known = Set(CodingKeys.allCases.map(\.rawValue))
+        unknownFields = try dynamic.allKeys.reduce(into: [:]) { result, key in
+            if !known.contains(key.stringValue) { result[key.stringValue] = try dynamic.decode(JSONValue.self, forKey: key) }
+        }
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var c = encoder.container(keyedBy: CodingKeys.self)
+        try c.encode(type, forKey: .type); try c.encode(url, forKey: .url); try c.encode(normalizedUrl, forKey: .normalizedUrl)
+        try c.encode(fingerprint, forKey: .fingerprint); try c.encode(createdAt, forKey: .createdAt)
+        try c.encodeIfPresent(title, forKey: .title); try c.encodeIfPresent(excerpt, forKey: .excerpt); try c.encodeIfPresent(site, forKey: .site)
+        try c.encodeIfPresent(image, forKey: .image); try c.encodeIfPresent(language, forKey: .language)
+        try c.encodeIfPresent(publishedAt, forKey: .publishedAt); try c.encodeIfPresent(author, forKey: .author)
+        var dynamic = encoder.container(keyedBy: AnyCodingKey.self)
+        for (key, value) in unknownFields { try dynamic.encode(value, forKey: AnyCodingKey(stringValue: key)!) }
     }
 
     /// Best available human-readable title for display.
@@ -74,3 +103,5 @@ public struct ExternalSave: Codable, Sendable, Equatable {
         return "Saved link"
     }
 }
+
+extension ExternalSave.CodingKeys: CaseIterable {}
diff --git a/Sources/LatrKit/Models/SavedItem.swift b/Sources/LatrKit/Models/SavedItem.swift
index f7d1863..53cf7a7 100644
--- a/Sources/LatrKit/Models/SavedItem.swift
+++ b/Sources/LatrKit/Models/SavedItem.swift
@@ -15,6 +15,7 @@ public struct SavedItem: Codable, Sendable, Equatable {
     public var previewSite: String?
     public var previewImage: String?
     public var previewAuthor: String?
+    public var unknownFields: [String: JSONValue]
 
     enum CodingKeys: String, CodingKey {
         case type = "$type"
@@ -44,7 +45,8 @@ public struct SavedItem: Codable, Sendable, Equatable {
         previewExcerpt: String? = nil,
         previewSite: String? = nil,
         previewImage: String? = nil,
-        previewAuthor: String? = nil
+        previewAuthor: String? = nil,
+        unknownFields: [String: JSONValue] = [:]
     ) {
         self.type = LexiconCollection.savedItem.identifier
         self.subjectUri = subjectUri
@@ -59,5 +61,41 @@ public struct SavedItem: Codable, Sendable, Equatable {
         self.previewSite = previewSite
         self.previewImage = previewImage
         self.previewAuthor = previewAuthor
+        self.unknownFields = unknownFields
+    }
+
+    public init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        type = try c.decode(String.self, forKey: .type)
+        subjectUri = try c.decode(String.self, forKey: .subjectUri)
+        savedAt = try c.decode(String.self, forKey: .savedAt)
+        state = try c.decodeIfPresent(SavedItemState.self, forKey: .state)
+        tags = try c.decodeIfPresent([String].self, forKey: .tags)
+        note = try c.decodeIfPresent(String.self, forKey: .note)
+        lastOpenedAt = try c.decodeIfPresent(String.self, forKey: .lastOpenedAt)
+        linkedWebUrl = try c.decodeIfPresent(String.self, forKey: .linkedWebUrl)
+        previewTitle = try c.decodeIfPresent(String.self, forKey: .previewTitle)
+        previewExcerpt = try c.decodeIfPresent(String.self, forKey: .previewExcerpt)
+        previewSite = try c.decodeIfPresent(String.self, forKey: .previewSite)
+        previewImage = try c.decodeIfPresent(String.self, forKey: .previewImage)
+        previewAuthor = try c.decodeIfPresent(String.self, forKey: .previewAuthor)
+        let dynamic = try decoder.container(keyedBy: AnyCodingKey.self)
+        let known = Set(CodingKeys.allCases.map(\.rawValue))
+        unknownFields = try dynamic.allKeys.reduce(into: [:]) { result, key in
+            if !known.contains(key.stringValue) { result[key.stringValue] = try dynamic.decode(JSONValue.self, forKey: key) }
+        }
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var c = encoder.container(keyedBy: CodingKeys.self)
+        try c.encode(type, forKey: .type); try c.encode(subjectUri, forKey: .subjectUri); try c.encode(savedAt, forKey: .savedAt)
+        try c.encodeIfPresent(state, forKey: .state); try c.encodeIfPresent(tags, forKey: .tags); try c.encodeIfPresent(note, forKey: .note)
+        try c.encodeIfPresent(lastOpenedAt, forKey: .lastOpenedAt); try c.encodeIfPresent(linkedWebUrl, forKey: .linkedWebUrl)
+        try c.encodeIfPresent(previewTitle, forKey: .previewTitle); try c.encodeIfPresent(previewExcerpt, forKey: .previewExcerpt)
+        try c.encodeIfPresent(previewSite, forKey: .previewSite); try c.encodeIfPresent(previewImage, forKey: .previewImage); try c.encodeIfPresent(previewAuthor, forKey: .previewAuthor)
+        var dynamic = encoder.container(keyedBy: AnyCodingKey.self)
+        for (key, value) in unknownFields { try dynamic.encode(value, forKey: AnyCodingKey(stringValue: key)!) }
     }
 }
+
+extension SavedItem.CodingKeys: CaseIterable {}
diff --git a/Sources/LatrKit/Repository/RepositoryClient.swift b/Sources/LatrKit/Repository/RepositoryClient.swift
index 5cbaabf..9447182 100644
--- a/Sources/LatrKit/Repository/RepositoryClient.swift
+++ b/Sources/LatrKit/Repository/RepositoryClient.swift
@@ -24,12 +24,32 @@ public protocol RepositoryClient: Sendable {
         in repository: String,
         collection: LexiconCollection,
         withKey key: String,
-        value: some Encodable & Sendable
+        value: some Encodable & Sendable,
+        swapRecord: String?
     ) async throws -> UpdateRecordResponse
 
     func deleteRecord(
         in repository: String,
         collection: LexiconCollection,
-        withKey key: String
+        withKey key: String,
+        swapRecord: String?
     ) async throws
 }
+
+public extension RepositoryClient {
+    func updateRecord(
+        in repository: String, collection: LexiconCollection, withKey key: String,
+        value: some Encodable & Sendable
+    ) async throws -> UpdateRecordResponse {
+        try await updateRecord(in: repository, collection: collection, withKey: key, value: value, swapRecord: nil)
+    }
+
+    func deleteRecord(in repository: String, collection: LexiconCollection, withKey key: String) async throws {
+        try await deleteRecord(in: repository, collection: collection, withKey: key, swapRecord: nil)
+    }
+}
+
+public enum RepositoryClientError: Error, Sendable, Equatable {
+    case conflict
+    case invalidStoredRecord(uri: String)
+}
diff --git a/Sources/LatrKit/Utilities/JSONValue.swift b/Sources/LatrKit/Utilities/JSONValue.swift
new file mode 100644
index 0000000..b7af7ad
--- /dev/null
+++ b/Sources/LatrKit/Utilities/JSONValue.swift
@@ -0,0 +1,36 @@
+import Foundation
+
+public enum JSONValue: Codable, Sendable, Equatable {
+    case string(String), integer(Int), double(Double), boolean(Bool), object([String: JSONValue]), array([JSONValue]), null
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        if container.decodeNil() { self = .null }
+        else if let value = try? container.decode(Bool.self) { self = .boolean(value) }
+        else if let value = try? container.decode(Int.self) { self = .integer(value) }
+        else if let value = try? container.decode(Double.self) { self = .double(value) }
+        else if let value = try? container.decode(String.self) { self = .string(value) }
+        else if let value = try? container.decode([JSONValue].self) { self = .array(value) }
+        else { self = .object(try container.decode([String: JSONValue].self)) }
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var container = encoder.singleValueContainer()
+        switch self {
+        case let .string(value): try container.encode(value)
+        case let .integer(value): try container.encode(value)
+        case let .double(value): try container.encode(value)
+        case let .boolean(value): try container.encode(value)
+        case let .object(value): try container.encode(value)
+        case let .array(value): try container.encode(value)
+        case .null: try container.encodeNil()
+        }
+    }
+}
+
+struct AnyCodingKey: CodingKey {
+    let stringValue: String
+    let intValue: Int? = nil
+    init?(stringValue: String) { self.stringValue = stringValue }
+    init?(intValue: Int) { return nil }
+}
diff --git a/Sources/LatrKit/XRPC/LatrXRPCContracts.swift b/Sources/LatrKit/XRPC/LatrXRPCContracts.swift
new file mode 100644
index 0000000..0bde3a0
--- /dev/null
+++ b/Sources/LatrKit/XRPC/LatrXRPCContracts.swift
@@ -0,0 +1,45 @@
+import Foundation
+
+public struct LatrXRPCErrorBody: Codable, Sendable, Equatable { public let error: String; public let message: String }
+public struct LatrListItemsParameters: Codable, Sendable, Equatable { public let limit: Int; public let cursor: String?; public init(limit: Int, cursor: String? = nil) { self.limit = limit; self.cursor = cursor } }
+public struct LatrListItemsOutput: Codable, Sendable { public let records: [RepositoryRecord<SavedItem>]; public let cursor: String? }
+public struct LatrSaveURLInput: Codable, Sendable, Equatable { public let url: String; public init(url: String) { self.url = url } }
+public struct LatrSaveSubjectInput: Codable, Sendable, Equatable { public let subjectUri: String; public let linkedWebUrl: String?; public init(subjectUri: String, linkedWebUrl: String? = nil) { self.subjectUri = subjectUri; self.linkedWebUrl = linkedWebUrl } }
+public struct LatrSetStateInput: Codable, Sendable, Equatable { public let itemRkey: String; public let state: SavedItemState; public init(itemRkey: String, state: SavedItemState) { self.itemRkey = itemRkey; self.state = state } }
+public struct LatrDeleteItemInput: Codable, Sendable, Equatable { public let itemRkey: String; public init(itemRkey: String) { self.itemRkey = itemRkey } }
+public struct LatrSimpleOK: Codable, Sendable, Equatable { public let ok: Bool }
+public struct LatrSaveResult: Codable, Sendable, Equatable { public let ok: Bool; public let kind: String; public let subjectUri: String?; public let linkedWebUrl: String?; public let storage: String? }
+
+public enum LatrPayloadValidationError: Error, Sendable, Equatable { case invalidLimit; case invalidURL; case invalidATURI; case emptyRecordKey; case exceedsUTF8Limit(field: String, maximum: Int) }
+public enum LatrPayloadValidator {
+    public static func validate(_ value: LatrListItemsParameters) throws { guard (1 ... 100).contains(value.limit) else { throw LatrPayloadValidationError.invalidLimit } }
+    public static func validateURL(_ value: String, field: String = "url") throws {
+        guard value.utf8.count <= 8192 else { throw LatrPayloadValidationError.exceedsUTF8Limit(field: field, maximum: 8192) }
+        guard let url = URL(string: value), let scheme = url.scheme?.lowercased(), scheme == "http" || scheme == "https" else { throw LatrPayloadValidationError.invalidURL }
+    }
+    public static func validateATURI(_ value: String) throws { guard value.utf8.count <= 8192, value.hasPrefix("at://"), value.split(separator: "/").count >= 5 else { throw LatrPayloadValidationError.invalidATURI } }
+    public static func validateRecordKey(_ value: String) throws { guard !value.isEmpty else { throw LatrPayloadValidationError.emptyRecordKey } }
+}
+
+public struct LatrXRPCClient: Sendable {
+    public let transport: any LatrXRPCTransport
+    public init(transport: any LatrXRPCTransport) { self.transport = transport }
+    public func listItems(_ parameters: LatrListItemsParameters) async throws -> LatrListItemsOutput {
+        try LatrPayloadValidator.validate(parameters)
+        var query = [URLQueryItem(name: "limit", value: String(parameters.limit))]
+        if let cursor = parameters.cursor { query.append(URLQueryItem(name: "cursor", value: cursor)) }
+        return try JSONDecoder().decode(LatrListItemsOutput.self, from: try await transport.send(method: .listItems, parameters: query, body: nil))
+    }
+    public func saveURL(_ input: LatrSaveURLInput) async throws -> LatrSaveResult {
+        try LatrPayloadValidator.validateURL(input.url)
+        return try await procedure(.saveURL, input, as: LatrSaveResult.self)
+    }
+    public func setState(_ input: LatrSetStateInput) async throws -> LatrSimpleOK {
+        try LatrPayloadValidator.validateRecordKey(input.itemRkey)
+        return try await procedure(.setState, input, as: LatrSimpleOK.self)
+    }
+    private func procedure<Input: Encodable & Sendable, Output: Decodable>(_ method: LatrXRPCMethod, _ input: Input, as: Output.Type) async throws -> Output {
+        let data = try JSONEncoder().encode(input)
+        return try JSONDecoder().decode(Output.self, from: try await transport.send(method: method, parameters: [], body: data))
+    }
+}
diff --git a/Sources/LatrKit/XRPC/LatrXRPCMethod.swift b/Sources/LatrKit/XRPC/LatrXRPCMethod.swift
new file mode 100644
index 0000000..1afd3c3
--- /dev/null
+++ b/Sources/LatrKit/XRPC/LatrXRPCMethod.swift
@@ -0,0 +1,32 @@
+import Foundation
+
+public struct LatrXRPCMethod: Hashable, Sendable {
+    public enum Kind: String, Sendable { case query, procedure }
+    public let nsid: String
+    public let kind: Kind
+    public let requiresApplicationCredential: Bool
+    public var verb: String { kind == .query ? "GET" : "POST" }
+
+    public static let listItems = Self("link.latr.saved.listItems", .query)
+    public static let getItem = Self("link.latr.saved.getItem", .query)
+    public static let saveURL = Self("link.latr.saved.saveUrl", .procedure)
+    public static let saveSubject = Self("link.latr.saved.saveSubject", .procedure)
+    public static let setState = Self("link.latr.saved.setState", .procedure)
+    public static let deleteItem = Self("link.latr.saved.deleteItem", .procedure)
+    public static let migrateLegacy = Self("link.latr.saved.migrateLegacy", .procedure)
+    public static let getOpenGraph = Self("link.latr.preview.getOpenGraph", .query)
+    public static let resolveURL = Self("link.latr.discovery.resolveUrl", .query)
+    public static let authProbe = Self("link.latr.auth.probe", .query)
+    public static let listClients = Self("link.latr.developer.listClients", .query, false)
+    public static let createClient = Self("link.latr.developer.createClient", .procedure, false)
+    public static let deleteClient = Self("link.latr.developer.deleteClient", .procedure, false)
+    public static let listKeys = Self("link.latr.developer.listKeys", .query, false)
+    public static let createKey = Self("link.latr.developer.createKey", .procedure, false)
+    public static let revokeKey = Self("link.latr.developer.revokeKey", .procedure, false)
+    public static let getUsage = Self("link.latr.developer.getUsage", .query, false)
+    public static let all: [Self] = [.listItems, .getItem, .saveURL, .saveSubject, .setState, .deleteItem, .migrateLegacy, .getOpenGraph, .resolveURL, .authProbe, .listClients, .createClient, .deleteClient, .listKeys, .createKey, .revokeKey, .getUsage]
+
+    private init(_ nsid: String, _ kind: Kind, _ requiresApplicationCredential: Bool = true) {
+        self.nsid = nsid; self.kind = kind; self.requiresApplicationCredential = requiresApplicationCredential
+    }
+}
diff --git a/Sources/LatrKit/XRPC/LatrXRPCTransport.swift b/Sources/LatrKit/XRPC/LatrXRPCTransport.swift
new file mode 100644
index 0000000..2034343
--- /dev/null
+++ b/Sources/LatrKit/XRPC/LatrXRPCTransport.swift
@@ -0,0 +1,42 @@
+import Foundation
+#if canImport(FoundationNetworking)
+import FoundationNetworking
+#endif
+
+public typealias LatrXRPCHeaderProvider = @Sendable (LatrXRPCMethod, String, URL) async throws -> [String: String]
+
+public protocol LatrXRPCTransport: Sendable {
+    func send(method: LatrXRPCMethod, parameters: [URLQueryItem], body: Data?) async throws -> Data
+}
+
+public enum LatrXRPCTransportError: Error, Sendable, Equatable {
+    case invalidURL, invalidResponse, http(status: Int, error: String?, message: String?)
+}
+
+public struct URLSessionLatrXRPCTransport: LatrXRPCTransport, Sendable {
+    public let baseURL: URL
+    public let session: URLSession
+    public let headerProvider: LatrXRPCHeaderProvider
+
+    public init(baseURL: URL, session: URLSession = .shared, headerProvider: @escaping LatrXRPCHeaderProvider) {
+        self.baseURL = baseURL; self.session = session; self.headerProvider = headerProvider
+    }
+
+    public func send(method: LatrXRPCMethod, parameters: [URLQueryItem] = [], body: Data? = nil) async throws -> Data {
+        var components = URLComponents(url: baseURL.appending(path: "xrpc/\(method.nsid)"), resolvingAgainstBaseURL: false)
+        components?.queryItems = parameters.isEmpty ? nil : parameters
+        guard let url = components?.url else { throw LatrXRPCTransportError.invalidURL }
+        var request = URLRequest(url: url)
+        request.httpMethod = method.verb
+        request.setValue("application/json", forHTTPHeaderField: "Accept")
+        if let body { request.httpBody = body; request.setValue("application/json", forHTTPHeaderField: "Content-Type") }
+        for (name, value) in try await headerProvider(method, method.verb, url) { request.setValue(value, forHTTPHeaderField: name) }
+        let (data, response) = try await session.data(for: request)
+        guard let http = response as? HTTPURLResponse else { throw LatrXRPCTransportError.invalidResponse }
+        guard (200 ... 299).contains(http.statusCode) else {
+            let decoded = try? JSONDecoder().decode(LatrXRPCErrorBody.self, from: data)
+            throw LatrXRPCTransportError.http(status: http.statusCode, error: decoded?.error, message: decoded?.message)
+        }
+        return data
+    }
+}
diff --git a/Tests/LatrKitTests/InMemoryRepository.swift b/Tests/LatrKitTests/InMemoryRepository.swift
index e063810..4e1c919 100644
--- a/Tests/LatrKitTests/InMemoryRepository.swift
+++ b/Tests/LatrKitTests/InMemoryRepository.swift
@@ -57,7 +57,8 @@ final class InMemoryRepository: RepositoryClient, @unchecked Sendable {
         in repository: String,
         collection: LexiconCollection,
         withKey key: String,
-        value: some Encodable & Sendable
+        value: some Encodable & Sendable,
+        swapRecord: String?
     ) async throws -> UpdateRecordResponse {
         let uri = "at://\(repository)/\(collection.identifier)/\(key)"
         let json = try JSONEncoder().encode(value)
@@ -68,7 +69,8 @@ final class InMemoryRepository: RepositoryClient, @unchecked Sendable {
     func deleteRecord(
         in repository: String,
         collection: LexiconCollection,
-        withKey key: String
+        withKey key: String,
+        swapRecord: String?
     ) async throws {
         store.removeValue(forKey: storeKey(collection: collection, key: key))
     }
diff --git a/Tests/LatrKitTests/SubjectPreviewResolverTests.swift b/Tests/LatrKitTests/SubjectPreviewResolverTests.swift
index 37c818a..d328e5f 100644
--- a/Tests/LatrKitTests/SubjectPreviewResolverTests.swift
+++ b/Tests/LatrKitTests/SubjectPreviewResolverTests.swift
@@ -132,7 +132,8 @@ private struct MockRepository: RepositoryClient {
         in repository: String,
         collection: LexiconCollection,
         withKey key: String,
-        value: some Encodable & Sendable
+        value: some Encodable & Sendable,
+        swapRecord: String?
     ) async throws -> UpdateRecordResponse {
         UpdateRecordResponse(uri: "at://\(repository)/\(collection.identifier)/\(key)")
     }
@@ -140,6 +141,7 @@ private struct MockRepository: RepositoryClient {
     func deleteRecord(
         in repository: String,
         collection: LexiconCollection,
-        withKey key: String
+        withKey key: String,
+        swapRecord: String?
     ) async throws {}
 }
diff --git a/Tests/LatrKitTests/XRPCContractTests.swift b/Tests/LatrKitTests/XRPCContractTests.swift
new file mode 100644
index 0000000..fa3372f
--- /dev/null
+++ b/Tests/LatrKitTests/XRPCContractTests.swift
@@ -0,0 +1,26 @@
+import Foundation
+import Testing
+@testable import LatrKit
+
+@Test func xrpcDescriptorsHaveStableVerbsAndCredentialPolicy() {
+    #expect(LatrXRPCMethod.all.count == 17)
+    #expect(LatrXRPCMethod.listItems.verb == "GET")
+    #expect(LatrXRPCMethod.saveURL.verb == "POST")
+    #expect(!LatrXRPCMethod.listClients.requiresApplicationCredential)
+}
+
+@Test func recordsPreserveUnknownFieldsAcrossMutationRoundTrip() throws {
+    let data = Data(#"{"$type":"link.latr.saved.item","subjectUri":"at://did:plc:test/app.bsky.feed.post/abc","savedAt":"2026-08-13T00:00:00Z","future":{"enabled":true}}"#.utf8)
+    var item = try JSONDecoder().decode(SavedItem.self, from: data)
+    item.state = .archived
+    let encoded = try JSONEncoder().encode(item)
+    let object = try #require(JSONSerialization.jsonObject(with: encoded) as? [String: Any])
+    #expect((object["future"] as? [String: Bool])?["enabled"] == true)
+}
+
+@Test func validationCountsUTF8Bytes() {
+    let oversized = String(repeating: "😀", count: 2_049)
+    #expect(throws: LatrPayloadValidationError.exceedsUTF8Limit(field: "url", maximum: 8192)) {
+        try LatrPayloadValidator.validateURL(oversized)
+    }
+}