[Bug]: OPENCODE_OTLP_PROTOCOL=http/protobuf` actually sends OTLP/JSON, breaks Arize Phoenix and other proto-only collectors

[anthonyhull-ford]

Description

When OPENCODE_OTLP_PROTOCOL=http/protobuf is set, the plugin sends spans/metrics/logs as JSON (Content-Type: application/json), not protobuf. Collectors that only accept OTLP/HTTP-protobuf (notably Arize Phoenix, which ingests on the same port as its UI) reject every export with 415 Unsupported Media Type.

The env-var name promises one transport, the wire format on the network is a different one. gRPC works fine; only the HTTP path is affected.

Steps to reproduce

1. Run Phoenix locally:

   docker run --rm -p 6006:6006 -p 4317:4317 arizephoenix/phoenix:version-13.7.0

2. Configure opencode to use the plugin over HTTP:

   {
     "$schema": "https://opencode.ai/config.json",
     "plugin": ["@devtheops/opencode-plugin-otel"]
   }

   export OPENCODE_ENABLE_TELEMETRY=1
   export OPENCODE_OTLP_PROTOCOL=http/protobuf
   export OPENCODE_OTLP_ENDPOINT=http://localhost:6006
   opencode run "hello"

3. Watch Phoenix logs:

   POST /v1/traces  ->  415  Unsupported content type: application/json
   

   Equivalent reproduction without opencode (matches what the plugin sends):

   $ curl -sS -X POST -H 'content-type: application/json' \
       --data-binary '{"resourceSpans":[]}' \
       -w "\nstatus=%{http_code}\n" \
       http://127.0.0.1:6006/v1/traces
   Unsupported content type: application/json
   status=415

   And what Phoenix does accept:

   $ curl -sS -X POST -H 'content-type: application/x-protobuf' \
       --data-binary '' -w "\nstatus=%{http_code}\n" \
       http://127.0.0.1:6006/v1/traces
   status=200

Expected behavior

With OPENCODE_OTLP_PROTOCOL=http/protobuf, exports should hit :6006/v1/traces with Content-Type: application/x-protobuf and an OTLP-protobuf payload. Spans should land in Phoenix.

Actual behavior

Exports are sent as Content-Type: application/json with an OTLP/JSON body. Phoenix rejects every request with 415. No spans are visible in the UI.

Plugin version

0.9.0

Configuration

@devtheops/opencode-plugin-otel 0.9.0 (latest on npm)

opencode 1.14.39

arizephoenix/phoenix version-13.7.0

macOS 25.4 (darwin), Node via Bun 1.2.17

Relevant log output

Checklist

• I have searched existing issues to ensure this hasn't been reported
• I have redacted any sensitive information (API keys, tokens, etc.)