diff --git a/Sources/Portico/MainLoop/GLibMainExecutor.swift b/Sources/Portico/MainLoop/GLibMainExecutor.swift
index a6d350a..790423f 100644
--- a/Sources/Portico/MainLoop/GLibMainExecutor.swift
+++ b/Sources/Portico/MainLoop/GLibMainExecutor.swift
@@ -216,6 +216,18 @@ nonisolated func portico_executor_destroy_notify(_ data: UnsafeMutableRawPointer
 }
 
 #if canImport(Glibc)
+
+/// `MainExecutor` refines `RunLoopExecutor & SerialExecutor` without adding
+/// further requirements, but Swift protocol refinement is one-directional:
+/// conforming to the two parent protocols separately does not implicitly
+/// satisfy the refined protocol, so the existential conversion to
+/// `any MainExecutor` below needs this explicit (requirement-free)
+/// conformance declaration. Scoped to Linux because `MainExecutor` carries a
+/// high Darwin `@available` floor that `GLibMainExecutor` should not
+/// inherit on Apple platforms, where it is used only as a plain
+/// `SerialExecutor` and never installed as the process main executor.
+extension GLibMainExecutor: MainExecutor {}
+
 // MARK: - Factory
 
 /// Installs ``GLibMainExecutor`` as the process main executor via the