What causes "proto: wrong wireType = %d for field Timeseries" in thanos-io/thanos?

Regenerate both sides from the same types.proto so Timeseries is field 5, wire type 2

proto: wrong wireType =

Error message

proto: wrong wireType = %d for field Timeseries

What it means

Field 5 (Timeseries) in remote-write v2 Request is a repeated message, which must be length-delimited (wire type 2). The custom Unmarshal returns this error when the bytes use another wire type for it, i.e. the stream does not conform to the schema.

Solutions

Regenerate both sides from the same types.proto so Timeseries is field 5, wire type 2
Validate payload with protoc --decode_raw before decoding
Re-send the request after fixing the encoder
Check for proxies/middleware altering request bodies

Example fix

// before: mixed schema versions
senderReq.Marshal() // old schema
// after: pin both services to the same generated package version
require github.com/prometheus/prometheus/prompb/io/prometheus/write/v2 v0.x.y (same in sender & receiver)

Defensive patterns

Strategy: validation

Try / catch

if err := req.Unmarshal(body); err != nil { return errors.Wrap(err, "rw2 decode") }

Prevention

Ensure encoder and decoder generate from the identical .proto file
Avoid middleware that mutates protobuf bodies
Use round-trip tests in CI for the v2 write path

When it happens

Trigger: Unmarshal on a payload where field 5 is encoded as varint/fixed instead of length-delimited — schema mismatch between sender and receiver, byte corruption, or wrong message type decoded.

Common situations: Sender built from a different prompb version, intermediary transforming the body, corrupted transfer, or hand-written test data with wrong encoding.

Understand the failure class

Background: "cannot parse invalid wire-format data", "cannot unmarshal", "failed unmarshalling": protobuf unmarshal errors explained — this error's family across 10 libraries.

Related errors

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/ebb3d46e6e0515a2. Report an issue: GitHub.

Appendix: source

Thrown at pkg/store/storepb/prompb/io/prometheus/write/v2/custom.go:75

					break
				}
			}
			intStringLen := int(stringLen)
			if intStringLen < 0 {
				return ErrInvalidLengthTypes
			}
			postIndex := iNdEx + intStringLen
			if postIndex < 0 {
				return ErrInvalidLengthTypes
			}
			if postIndex > l {
				return io.ErrUnexpectedEOF
			}
			m.Symbols = append(m.Symbols, unsafe.String(unsafe.SliceData(dAtA[iNdEx:postIndex]), len(dAtA[iNdEx:postIndex])))
			iNdEx = postIndex
		case 5:
			if wireType != 2 {
				return fmt.Errorf("proto: wrong wireType = %d for field Timeseries", wireType)
			}
			var msglen int
			for shift := uint(0); ; shift += 7 {
				if shift >= 64 {
					return ErrIntOverflowTypes
				}
				if iNdEx >= l {
					return io.ErrUnexpectedEOF
				}
				b := dAtA[iNdEx]
				iNdEx++
				msglen |= int(b&0x7F) << shift
				if b < 0x80 {
					break
				}
			}
			if msglen < 0 {
				return ErrInvalidLengthTypes

View on GitHub (pinned to 35b8b99117)