Repository navigation
Remote MCP: StreamableHTTPClientTransport missing Accept: text/event-stream header (406 from SSE-over-POST servers) #15103
Description
Summary
Remote MCP servers that use SSE-over-POST (streamable HTTP with SSE response format) fail to connect because opencode's StreamableHTTPClientTransport doesn't send Accept: text/event-stream in the request headers.
Reproduction
- Configure a remote MCP server that requires
Accept: application/json, text/event-stream:
{
"mcp": {
"braintrust": {
"type": "remote",
"url": "https://api.braintrust.dev/mcp",
"headers": { "Authorization": "Bearer ..." }
}
}
}- Restart opencode
- MCP shows as
failed ... Loading
Root Cause
The server returns 406 Not Acceptable when StreamableHTTPClientTransport POSTs without Accept: text/event-stream. The fallback to SSEClientTransport also fails because the server returns 405 Method Not Allowed on GET (it doesn't support legacy EventSource SSE).
# StreamableHTTP (POST, Accept: application/json only) → 406
# Legacy SSE (GET, EventSource) → 405
The server works when both accept types are sent:
curl -X POST https://api.braintrust.dev/mcp \
-H "Accept: application/json, text/event-stream" \
-H "Content-Type: application/json" \
-d '{"jsonrpc":"2.0","method":"initialize",...}'
# → 200 OK, SSE responseExpected Behavior
StreamableHTTPClientTransport should include Accept: text/event-stream alongside application/json per the MCP spec, allowing SSE-over-POST servers to respond correctly.
Workaround
Use mcp-remote as a local proxy:
{
"braintrust": {
"type": "local",
"command": ["npx", "-y", "mcp-remote", "https://api.braintrust.dev/mcp",
"--header", "Authorization: Bearer ...", "--transport", "http-only"]
}
}Related Issues
- MCP client doesn't send Accept header required by Context7 (causes 406 Not Acceptable error) #6972 — Context7 406 (same root cause)
- Remote MCP servers using Server-Sent Events (SSE) fail to start with 'UnknownError Server error' #834 — SSE-only servers failing
- Remote MCP should negotiate HTTP/Streamable before deprecated SSE #6242 — StreamableHTTP/SSE negotiation
Activity
github-actions commentedon Feb 25, 2026
This issue might be a duplicate of an existing issue. Please check:
Accept: application/json, text/event-streamheader, causing 406 Not Acceptable errors from remote MCP serversIt looks like you've already identified this connection in the "Related Issues" section. Consider adding your findings (the specific Braintrust reproduction case and the SSE-over-POST failure mode) as a comment on #6972 to consolidate the discussion there.
dmmulroy commentedon Feb 25, 2026
Closing — misdiagnosis. The MCP SDK (
@modelcontextprotocol/sdk) correctly setsAccept: application/json, text/event-streaminStreamableHTTPClientTransport.send(). The header is set after mergingrequestInit.headers, so user-provided headers can't override it.The actual root cause was
{env:BRAINTRUST_API_KEY}silently resolving to""(env var not exported in shell), causing auth failures. Transport failures were logged atdebuglevel only, making the real error invisible.