diff --git a/src/conversion.ts b/src/conversion.ts
index 1356fc5..1ef6efe 100644
--- a/src/conversion.ts
+++ b/src/conversion.ts
@@ -112,9 +112,9 @@ const FALLBACK_SAMPLE_RATE = 48000;
  */
 export class Conversion {
 	/** The input file. */
-	input: Input;
+	readonly input: Input;
 	/** The output file. */
-	output: Output;
+	readonly output: Output;
 
 	/** @internal */
 	_options: ConversionOptions;
@@ -168,9 +168,9 @@ export class Conversion {
 	_lastProgress = 0;
 
 	/** The list of tracks that are included in the output file. */
-	utilizedTracks: InputTrack[] = [];
+	readonly utilizedTracks: InputTrack[] = [];
 	/** The list of tracks from the input file that have been discarded, alongside the discard reason. */
-	discardedTracks: {
+	readonly discardedTracks: {
 		/** The track that was discarded. */
 		track: InputTrack;
 		/** The reason for discarding the track. */
diff --git a/src/custom-coder.ts b/src/custom-coder.ts
index e45656c..83c82f5 100644
--- a/src/custom-coder.ts
+++ b/src/custom-coder.ts
@@ -9,11 +9,11 @@ import { AudioSample, VideoSample } from './sample';
  */
 export abstract class CustomVideoDecoder {
 	/** The input video's codec. */
-	codec!: VideoCodec;
+	readonly codec!: VideoCodec;
 	/** The input video's decoder config. */
-	config!: VideoDecoderConfig;
+	readonly config!: VideoDecoderConfig;
 	/** The callback to call when a decoded VideoSample is available. */
-	onSample!: (sample: VideoSample) => unknown;
+	readonly onSample!: (sample: VideoSample) => unknown;
 
 	/** Returns true iff the decoder can decode the given codec configuration. */
 	// eslint-disable-next-line @typescript-eslint/no-unused-vars
@@ -38,11 +38,11 @@ export abstract class CustomVideoDecoder {
  */
 export abstract class CustomAudioDecoder {
 	/** The input audio's codec. */
-	codec!: AudioCodec;
+	readonly codec!: AudioCodec;
 	/** The input audio's decoder config. */
-	config!: AudioDecoderConfig;
+	readonly config!: AudioDecoderConfig;
 	/** The callback to call when a decoded AudioSample is available. */
-	onSample!: (sample: AudioSample) => unknown;
+	readonly onSample!: (sample: AudioSample) => unknown;
 
 	/** Returns true iff the decoder can decode the given codec configuration. */
 	// eslint-disable-next-line @typescript-eslint/no-unused-vars
@@ -67,11 +67,11 @@ export abstract class CustomAudioDecoder {
  */
 export abstract class CustomVideoEncoder {
 	/** The codec with which to encode the video. */
-	codec!: VideoCodec;
+	readonly codec!: VideoCodec;
 	/** Config for the encoder. */
-	config!: VideoEncoderConfig;
+	readonly config!: VideoEncoderConfig;
 	/** The callback to call when an EncodedPacket is available. */
-	onPacket!: (packet: EncodedPacket, meta?: EncodedVideoChunkMetadata) => unknown;
+	readonly onPacket!: (packet: EncodedPacket, meta?: EncodedVideoChunkMetadata) => unknown;
 
 	/** Returns true iff the encoder can encode the given codec configuration. */
 	// eslint-disable-next-line @typescript-eslint/no-unused-vars
@@ -96,11 +96,11 @@ export abstract class CustomVideoEncoder {
  */
 export abstract class CustomAudioEncoder {
 	/** The codec with which to encode the audio. */
-	codec!: AudioCodec;
+	readonly codec!: AudioCodec;
 	/** Config for the encoder. */
-	config!: AudioEncoderConfig;
+	readonly config!: AudioEncoderConfig;
 	/** The callback to call when an EncodedPacket is available. */
-	onPacket!: (packet: EncodedPacket, meta?: EncodedAudioChunkMetadata) => unknown;
+	readonly onPacket!: (packet: EncodedPacket, meta?: EncodedAudioChunkMetadata) => unknown;
 
 	/** Returns true iff the encoder can encode the given codec configuration. */
 	// eslint-disable-next-line @typescript-eslint/no-unused-vars
diff --git a/src/media-sink.ts b/src/media-sink.ts
index 269526d..d79bcc9 100644
--- a/src/media-sink.ts
+++ b/src/media-sink.ts
@@ -705,8 +705,11 @@ class VideoDecoderWrapper extends DecoderWrapper<VideoSample> {
 		if (MatchingCustomDecoder) {
 			// @ts-expect-error "Can't create instance of abstract class 🤓"
 			this.customDecoder = new MatchingCustomDecoder() as CustomVideoDecoder;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.codec = codec;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.config = decoderConfig;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.onSample = (sample) => {
 				if (!(sample instanceof VideoSample)) {
 					throw new TypeError('The argument passed to onSample must be a VideoSample.');
@@ -1151,8 +1154,11 @@ class AudioDecoderWrapper extends DecoderWrapper<AudioSample> {
 		if (MatchingCustomDecoder) {
 			// @ts-expect-error "Can't create instance of abstract class 🤓"
 			this.customDecoder = new MatchingCustomDecoder() as CustomAudioDecoder;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.codec = codec;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.config = decoderConfig;
+			// @ts-expect-error It's technically readonly
 			this.customDecoder.onSample = (sample) => {
 				if (!(sample instanceof AudioSample)) {
 					throw new TypeError('The argument passed to onSample must be an AudioSample.');
diff --git a/src/media-source.ts b/src/media-source.ts
index 49fdc67..70d7a4c 100644
--- a/src/media-source.ts
+++ b/src/media-source.ts
@@ -382,8 +382,11 @@ class VideoEncoderWrapper {
 			if (MatchingCustomEncoder) {
 				// @ts-expect-error "Can't create instance of abstract class 🤓"
 				this.customEncoder = new MatchingCustomEncoder() as CustomVideoEncoder;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.codec = this.encodingConfig.codec;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.config = encoderConfig;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.onPacket = (packet, meta) => {
 					if (!(packet instanceof EncodedPacket)) {
 						throw new TypeError('The first argument passed to onPacket must be an EncodedPacket.');
@@ -949,8 +952,11 @@ class AudioEncoderWrapper {
 			if (MatchingCustomEncoder) {
 				// @ts-expect-error "Can't create instance of abstract class 🤓"
 				this.customEncoder = new MatchingCustomEncoder() as CustomAudioEncoder;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.codec = this.encodingConfig.codec;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.config = encoderConfig;
+				// @ts-expect-error It's technically readonly
 				this.customEncoder.onPacket = (packet, meta) => {
 					if (!(packet instanceof EncodedPacket)) {
 						throw new TypeError('The first argument passed to onPacket must be an EncodedPacket.');