What causes "no codec for this MimeType" in pion/webrtc?

Pass a supported MimeType to WithCodec: 'video/vp8', 'video/vp9', or 'video/av1'

no codec for this MimeType

Error message

no codec for this MimeType

What it means

errNoSuchCodec is returned by ivfwriter.NewWith when the WithCodec option receives a MimeType that the IVF writer does not support. The writer only knows how to map a small set of media types (VP8, VP9, AV1) to its internal codec enum, and any other value (including empty string) hits the default branch. It signals misconfiguration of the writer, not a runtime data problem.

Source

Thrown at pkg/media/ivfwriter/ivfwriter.go:23

package ivfwriter

import (
	"encoding/binary"
	"errors"
	"io"
	"os"

	"github.com/pion/rtp"
	"github.com/pion/rtp/codecs"
	"github.com/pion/rtp/codecs/av1/obu"
)

var (
	errFileNotOpened        = errors.New("file not opened")
	errInvalidNilPacket     = errors.New("invalid nil packet")
	errCodecUnset           = errors.New("codec is unset")
	errCodecAlreadySet      = errors.New("codec is already set")
	errNoSuchCodec          = errors.New("no codec for this MimeType")
	errInvalidMediaTimebase = errors.New("invalid media timebase")
)

type (
	codec int

	// IVFWriter is used to take RTP packets and write them to an IVF on disk.
	IVFWriter struct {
		ioWriter     io.Writer
		count        uint64
		seenKeyFrame bool

		codec codec

		timebaseDenominator uint32
		timebaseNumerator   uint32
		firstFrameTimestamp uint32
		clockRate           uint64

View on GitHub (pinned to 8c25dc09fa)

Solutions

Pass a supported MimeType to WithCodec: 'video/vp8', 'video/vp9', or 'video/av1'
Check the string for typos, extra spaces, or parameters; compare against the constants the writer's switch matches
If codec support is unknown at runtime, validate the MimeType against a whitelist before constructing the writer
Upgrade the library if you need a newer codec the switch does not cover yet

Example fix

// before
w, err := ivfwriter.NewWith(buf, ivfwriter.WithCodec("video/H264")) // errNoSuchCodec
// after
w, err := ivfwriter.NewWith(buf, ivfwriter.WithCodec("video/VP8"))

Defensive patterns

Strategy: validation

Validate before calling

var supported = map[string]bool{"video/vp8": true, "video/vp9": true, "video/av1": true}
if !supported[strings.ToLower(mimeType)] {
    return fmt.Errorf("unsupported IVF codec: %q", mimeType)
}

Type guard

func isIVFSupportedCodec(mt string) bool {
    switch mt {
    case "video/vp8", "video/vp9", "video/av1":
        return true
    }
    return false
}

Prevention

Always pass an explicit, non-empty MimeType to WithCodec
Whitelist codecs before constructing the writer
Use the same constant strings as the SDP/media layer to avoid case/spacing drift
Add a unit test constructing the writer with each codec you ship

When it happens

Trigger: Calling ivfwriter.NewWith(w, ivfwriter.WithCodec(...)) with an empty string, an unsupported MimeType (e.g. 'video/h264', 'video/ VP8' with wrong case), or a misspelled type, so the switch over MimeType falls to default and returns errNoSuchCodec.

Common situations: Copy-pasting an SDP mime string including parameters, passing an audio codec (e.g. Opus) to a video-only IVF writer, forgetting WithCodec so an empty codec is probed, or using a codec added in a newer spec than the library supports.

Related errors

AI-assisted analysis of pion/webrtc@8c25dc09fa (2026-09-03). Data as JSON: /api/errors/5bfe9ad69c4fe420. Report an issue: GitHub.