diff --git a/OTPKit/Sources/OTPKit/Core/Map/OTPMapProvider.swift b/OTPKit/Sources/OTPKit/Core/Map/OTPMapProvider.swift
index f1269e1..e5b5911 100644
--- a/OTPKit/Sources/OTPKit/Core/Map/OTPMapProvider.swift
+++ b/OTPKit/Sources/OTPKit/Core/Map/OTPMapProvider.swift
@@ -13,6 +13,16 @@ import SwiftUI
 /// Protocol defining the interface for map operations that OTPKit requires.
 /// Implementers of this protocol can provide their own map view (MKMapView, custom map, etc.)
 /// while allowing OTPKit to control map content and interactions.
+///
+/// Main-actor isolated because every call arrives from a main-actor context — from
+/// `MapCoordinator` inside the package, and from host UI code outside it — and because
+/// conformances drive UI. Conforming types inherit this isolation, so the members of a
+/// custom provider are main-actor isolated too.
+///
+/// Stating that here rather than leaving it implicit lets hosts building in the Swift 6
+/// language mode with main-actor default isolation conform directly, instead of opting
+/// the conformance out of isolation and hopping back in every method body.
+@MainActor
 public protocol OTPMapProvider: AnyObject {
 
     // MARK: - Route Display
diff --git a/OTPKit/Tests/SmokeTest.swift b/OTPKit/Tests/SmokeTest.swift
index 552e5e0..02f02d2 100644
--- a/OTPKit/Tests/SmokeTest.swift
+++ b/OTPKit/Tests/SmokeTest.swift
@@ -27,6 +27,9 @@ func testFixturesCreatePlace() {
     #expect(place.lon == -122.0)
 }
 
+// `MockMapProvider` conforms to `OTPMapProvider`, which is `@MainActor`, so the
+// mock picks up that isolation and the test has to run there too.
+@MainActor
 @Test("MockMapProvider - tracks addRoute calls")
 func mockMapProviderTracksRouteCalls() {
     let mockMap = MockMapProvider()
diff --git a/README.markdown b/README.markdown
index 6792b01..73b29b3 100644
--- a/README.markdown
+++ b/README.markdown
@@ -125,7 +125,7 @@ Both are Swift actors conforming to `APIService`; you can also implement `APISer
 
 - **Transport modes:** pass `enabledTransportModes` to `OTPConfiguration` (defaults to transit, walk, bike, car). Rental modes like `.bikeRental` are opt-in and need an OTP 2.x server with rental data.
 - **Theme:** pass an `OTPThemeConfiguration` to adjust colors.
-- **Map behavior:** implement `OTPMapProvider` to control exactly how routes and stops render on your map.
+- **Map behavior:** implement `OTPMapProvider` to control exactly how routes and stops render on your map. The protocol is `@MainActor`, so conformances inherit main-actor isolation — write your provider as main-actor isolated rather than opting the conformance out.
 
 ### Localization