Summary

When running docker mcp gateway run with a before interceptor (exec, docker, or http), the gateway can fail with:

json: unsupported type: func(mcp.CloseSSEStreamArgs)

This prevents before-interceptors from working (notably on streaming/sse transports).

Environment

• Repo: docker/mcp-gateway
• Version: v0.37.0 (commit cc7998a51493a26bd9e51e1f7fb25cee60fc03a5)
• Transport: streaming (also seen on sse)
• Host: Docker Desktop (macOS)

Steps to reproduce

1. Start the gateway with a before exec interceptor:

docker mcp gateway run --port 8080 --transport streaming \
  --interceptor 'before:exec:echo Arguments=$(jq -r ".params.arguments") >&2'

2. Connect a client and invoke any tool call (tools/call).

Expected behavior

• The before interceptor receives JSON and can read .params.arguments.
• The call proceeds (passthrough) unless the interceptor returns a non-empty JSON response.

Actual behavior

• Gateway fails with an error like:

executing interceptor: marshalling request: json: unsupported type: func(mcp.CloseSSEStreamArgs)

Root cause

In pkg/interceptors/interceptors.go, ToMiddleware() attempts to serialize the full mcp.Request for before interceptors:

message, err := json.Marshal(req)

On streaming/SSE transports, the mcp.Request may contain internal callback functions (e.g. func(mcp.CloseSSEStreamArgs)), which are not JSON-serializable.

Proposed fix

For "before" interceptors, marshal only the serializable tool-call payload (matching the documented/observed interceptor request shape):

{ "method": "tools/call", "params": { "name": "...", "arguments": { ... } } }

Patch sketch:

var payload any
if callReq, ok := req.(*mcp.CallToolRequest); ok && callReq.Params != nil {
  payload = map[string]any{"method": "tools/call", "params": callReq.Params}
} else {
  payload = map[string]any{"method": "tools/call", "params": map[string]any{}}
}
message, err := json.Marshal(payload)

This preserves compatibility with interceptors using jq -r ".params.arguments".

Workaround

No reliable workaround other than disabling before interceptors.