handle http errors / rpc errors

Author: burmudar

internal/mcp/mcp_request.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	"strings"
 
 	"github.com/sourcegraph/src-cli/internal/api"
 
@@ -14,22 +15,33 @@ import (
 
 const McpURLPath = ".api/mcp/v1"
 
+type jsonRPCError struct {
+	Code    int    `json:"code"`
+	Message string `json:"message"`
+}
+
 func FetchToolDefinitions(ctx context.Context, client api.Client) (map[string]*ToolDef, error) {
 	resp, err := doJSONRPC(ctx, client, "tools/list", nil)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to list tools from mcp endpoint")
 	}
+	defer resp.Body.Close()
+
 	data, err := readSSEResponseData(resp)
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to read list tools SSE response")
 	}
 
 	var rpcResp struct {
 		Result json.RawMessage `json:"result"`
+		Error  *jsonRPCError   `json:"error"`
 	}
 	if err := json.Unmarshal(data, &rpcResp); err != nil {
 		return nil, errors.Wrap(err, "failed to unmarshal JSON-RPC response")
 	}
+	if rpcResp.Error != nil {
+		return nil, errors.Newf("MCP tools/list failed: %d %s", rpcResp.Error.Code, rpcResp.Error.Message)
+	}
 
 	return loadToolDefinitions(rpcResp.Result)
 }
@@ -70,9 +82,21 @@ func doJSONRPC(ctx context.Context, client api.Client, method string, params any
 		return nil, err
 	}
 	req.Header.Add("Content-Type", "application/json")
-	req.Header.Add("Accept", "*/*")
+	req.Header.Add("Accept", "application/json, text/event-stream")
 
-	return client.Do(req)
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, 4096))
+		resp.Body.Close()
+		return nil, errors.Newf("MCP endpoint %s returned %d: %s",
+			McpURLPath, resp.StatusCode, strings.TrimSpace(string(body)))
+	}
+
+	return resp, nil
 }
 
 func DecodeToolResponse(resp *http.Response) (map[string]json.RawMessage, error) {
@@ -86,16 +110,18 @@ func DecodeToolResponse(resp *http.Response) (map[string]json.RawMessage, error)
 	}
 
 	jsonRPCResp := struct {
-		Version string `json:"jsonrpc"`
-		ID      int    `json:"id"`
-		Result  struct {
+		Result struct {
 			Content           []json.RawMessage          `json:"content"`
 			StructuredContent map[string]json.RawMessage `json:"structuredContent"`
 		} `json:"result"`
+		Error *jsonRPCError `json:"error"`
 	}{}
 	if err := json.Unmarshal(data, &jsonRPCResp); err != nil {
 		return nil, errors.Wrapf(err, "failed to unmarshal MCP JSON-RPC response")
 	}
+	if jsonRPCResp.Error != nil {
+		return nil, errors.Newf("MCP tools/call failed: %d %s", jsonRPCResp.Error.Code, jsonRPCResp.Error.Message)
+	}
 
 	return jsonRPCResp.Result.StructuredContent, nil
 }