diff --git a/src/Instance.hxx b/src/Instance.hxx
index cb85aa979..189aabab4 100644
--- a/src/Instance.hxx
+++ b/src/Instance.hxx
@@ -194,7 +194,7 @@ struct Instance final
 #endif
 
 #ifdef ENABLE_SQLITE
-	bool HasStickerDatabase() noexcept {
+	bool HasStickerDatabase() const noexcept {
 		return sticker_database != nullptr;
 	}
 #endif
diff --git a/src/decoder/DecoderPlugin.hxx b/src/decoder/DecoderPlugin.hxx
index 7e68ef7c1..d4887477c 100644
--- a/src/decoder/DecoderPlugin.hxx
+++ b/src/decoder/DecoderPlugin.hxx
@@ -143,34 +143,34 @@ struct DecoderPlugin {
 		 scan_stream(_scan_stream) {}
 
 	constexpr auto WithInit(bool (*_init)(const ConfigBlock &block),
-				void (*_finish)() noexcept = nullptr) noexcept {
+				void (*_finish)() noexcept = nullptr) const noexcept {
 		auto copy = *this;
 		copy.init = _init;
 		copy.finish = _finish;
 		return copy;
 	}
 
-	constexpr auto WithContainer(std::forward_list<DetachedSong> (*_container_scan)(Path path_fs)) noexcept {
+	constexpr auto WithContainer(std::forward_list<DetachedSong> (*_container_scan)(Path path_fs)) const noexcept {
 		auto copy = *this;
 		copy.container_scan = _container_scan;
 		return copy;
 	}
 
 	constexpr auto WithProtocols(std::set<std::string> (*_protocols)() noexcept,
-				     void (*_uri_decode)(DecoderClient &client, const char *uri)) noexcept {
+				     void (*_uri_decode)(DecoderClient &client, const char *uri)) const noexcept {
 		auto copy = *this;
 		copy.protocols = _protocols;
 		copy.uri_decode = _uri_decode;
 		return copy;
 	}
 
-	constexpr auto WithSuffixes(const char *const*_suffixes) noexcept {
+	constexpr auto WithSuffixes(const char *const*_suffixes) const noexcept {
 		auto copy = *this;
 		copy.suffixes = _suffixes;
 		return copy;
 	}
 
-	constexpr auto WithMimeTypes(const char *const*_mime_types) noexcept {
+	constexpr auto WithMimeTypes(const char *const*_mime_types) const noexcept {
 		auto copy = *this;
 		copy.mime_types = _mime_types;
 		return copy;
diff --git a/src/input/cache/Manager.cxx b/src/input/cache/Manager.cxx
index c5dd21283..d94a4912e 100644
--- a/src/input/cache/Manager.cxx
+++ b/src/input/cache/Manager.cxx
@@ -75,7 +75,7 @@ InputCacheManager::Flush() noexcept
 }
 
 bool
-InputCacheManager::IsEligible(const InputStream &input) noexcept
+InputCacheManager::IsEligible(const InputStream &input) const noexcept
 {
 	assert(input.IsReady());
 
diff --git a/src/input/cache/Manager.hxx b/src/input/cache/Manager.hxx
index b8dbb1e0b..81492a97a 100644
--- a/src/input/cache/Manager.hxx
+++ b/src/input/cache/Manager.hxx
@@ -98,7 +98,7 @@ private:
 	 * Check whether the given #InputStream can be stored in this
 	 * cache.
 	 */
-	bool IsEligible(const InputStream &input) noexcept;
+	bool IsEligible(const InputStream &input) const noexcept;
 
 	void Remove(InputCacheItem &item) noexcept;
 	void Delete(InputCacheItem *item) noexcept;
diff --git a/src/output/SharedPipeConsumer.hxx b/src/output/SharedPipeConsumer.hxx
index a33d33c18..119b1e90c 100644
--- a/src/output/SharedPipeConsumer.hxx
+++ b/src/output/SharedPipeConsumer.hxx
@@ -66,7 +66,7 @@ public:
 		return *pipe;
 	}
 
-	bool IsInitial() {
+	bool IsInitial() const {
 		return chunk == nullptr;
 	}
 
diff --git a/src/playlist/PlaylistPlugin.hxx b/src/playlist/PlaylistPlugin.hxx
index c62ecd762..be3644caa 100644
--- a/src/playlist/PlaylistPlugin.hxx
+++ b/src/playlist/PlaylistPlugin.hxx
@@ -85,32 +85,32 @@ struct PlaylistPlugin {
 		:name(_name), open_stream(_open_stream) {}
 
 	constexpr auto WithInit(bool (*_init)(const ConfigBlock &block),
-				void (*_finish)() noexcept = nullptr) noexcept {
+				void (*_finish)() noexcept = nullptr) const noexcept {
 		auto copy = *this;
 		copy.init = _init;
 		copy.finish = _finish;
 		return copy;
 	}
 
-	constexpr auto WithSchemes(const char *const*_schemes) noexcept {
+	constexpr auto WithSchemes(const char *const*_schemes) const noexcept {
 		auto copy = *this;
 		copy.schemes = _schemes;
 		return copy;
 	}
 
-	constexpr auto WithSuffixes(const char *const*_suffixes) noexcept {
+	constexpr auto WithSuffixes(const char *const*_suffixes) const noexcept {
 		auto copy = *this;
 		copy.suffixes = _suffixes;
 		return copy;
 	}
 
-	constexpr auto WithMimeTypes(const char *const*_mime_types) noexcept {
+	constexpr auto WithMimeTypes(const char *const*_mime_types) const noexcept {
 		auto copy = *this;
 		copy.mime_types = _mime_types;
 		return copy;
 	}
 
-	constexpr auto WithAsFolder(bool value=true) noexcept {
+	constexpr auto WithAsFolder(bool value=true) const noexcept {
 		auto copy = *this;
 		copy.as_folder = value;
 		return copy;
diff --git a/src/util/OptionParser.hxx b/src/util/OptionParser.hxx
index 3625dedd4..8371fa76f 100644
--- a/src/util/OptionParser.hxx
+++ b/src/util/OptionParser.hxx
@@ -49,7 +49,7 @@ public:
 		int index;
 		const char *value;
 
-		constexpr operator bool() noexcept {
+		constexpr operator bool() const noexcept {
 			return index >= 0;
 		}
 	};