fix(authbridge): Render MCP-protocol error frame on outbound deny

When an outbound gate (IBAC, etc.) rejects an MCP JSON-RPC request,
the proxy used to emit a transport-level 4xx/5xx with a non-MCP body.
The agent's MCP client sees that as a session break instead of a
single failed tool call.

This change adds an MCP-aware rejection helper for both proxy-sidecar
(httpx.WriteRejectionForRequest) and envoy-sidecar
(rejectFromActionForRequest) listeners. When pctx.Extensions.MCP
carries a real JSON-RPC method + non-nil id, the deny renders as
HTTP 200 with a JSON-RPC 2.0 error frame echoing the original id and
carrying the violation's reason/code as error.message and
error.data.error. JSON-RPC notifications (no id), non-MCP traffic,
and nil pctx all fall through to today's HTTP-level shape unchanged.

Wired in at the outbound request reject sites in forwardproxy
(HTTP and CONNECT) and extproc (header- and body-phase outbound).
Inbound and response-phase rejects keep the existing shape.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Kelly Abuelsaad <kaymar@gmail.com>

Author: kellyaa

authbridge/authlib/listener/extproc/mcpreject_test.go

@@ -0,0 +1,110 @@
+package extproc
+
+import (
+	"encoding/json"
+	"testing"
+
+	corev3 "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
+	extprocv3 "github.com/envoyproxy/go-control-plane/envoy/service/ext_proc/v3"
+	typev3 "github.com/envoyproxy/go-control-plane/envoy/type/v3"
+
+	"github.com/kagenti/kagenti-extensions/authbridge/authlib/pipeline"
+)
+
+// pctxWithMCP builds the minimum pctx needed to trigger the JSON-RPC
+// rendering path on the extproc listener.
+func pctxWithMCP(id any) *pipeline.Context {
+	return &pipeline.Context{
+		Extensions: pipeline.Extensions{
+			MCP: &pipeline.MCPExtension{
+				Method: "tools/call",
+				RPCID:  id,
+			},
+		},
+	}
+}
+
+func ibacBlocked() pipeline.Action {
+	return pipeline.DenyStatus(403, "ibac.blocked", "intent does not align")
+}
+
+// immediateBody fishes the ImmediateResponse body out of a
+// ProcessingResponse — saves boilerplate in every assertion below.
+func immediateBody(t *testing.T, resp *extprocv3.ProcessingResponse) (typev3.StatusCode, []byte, []*corev3.HeaderValueOption) {
+	t.Helper()
+	imm, ok := resp.GetResponse().(*extprocv3.ProcessingResponse_ImmediateResponse)
+	if !ok {
+		t.Fatalf("response is not ImmediateResponse: %T", resp.GetResponse())
+	}
+	var hs []*corev3.HeaderValueOption
+	if imm.ImmediateResponse.Headers != nil {
+		hs = imm.ImmediateResponse.Headers.SetHeaders
+	}
+	return imm.ImmediateResponse.Status.Code, imm.ImmediateResponse.Body, hs
+}
+
+// TestRejectFromActionForRequest_MCPRequest verifies the extproc
+// listener emits an HTTP 200 ImmediateResponse with a JSON-RPC error
+// body when the rejected request was MCP — the envoy-sidecar twin of
+// the forwardproxy fix.
+func TestRejectFromActionForRequest_MCPRequest(t *testing.T) {
+	resp := rejectFromActionForRequest(ibacBlocked(), pctxWithMCP(float64(7)))
+	status, body, headers := immediateBody(t, resp)
+
+	if status != typev3.StatusCode_OK {
+		t.Fatalf("status = %v, want OK (200) — JSON-RPC errors travel over a 200 transport", status)
+	}
+	gotCT := false
+	for _, h := range headers {
+		if h.Header.Key == "content-type" && string(h.Header.RawValue) == "application/json" {
+			gotCT = true
+		}
+	}
+	if !gotCT {
+		t.Errorf("content-type header missing or wrong: %v", headers)
+	}
+	var parsed map[string]any
+	if err := json.Unmarshal(body, &parsed); err != nil {
+		t.Fatalf("body is not JSON: %v\n%s", err, body)
+	}
+	if parsed["jsonrpc"] != "2.0" {
+		t.Errorf("jsonrpc = %v, want 2.0", parsed["jsonrpc"])
+	}
+	if id, _ := parsed["id"].(float64); id != 7 {
+		t.Errorf("id = %v, want 7", parsed["id"])
+	}
+	errObj, _ := parsed["error"].(map[string]any)
+	if errObj["code"] != float64(-32000) {
+		t.Errorf("error.code = %v, want -32000", errObj["code"])
+	}
+	if errObj["message"] != "intent does not align" {
+		t.Errorf("error.message = %v, want IBAC reason", errObj["message"])
+	}
+}
+
+// TestRejectFromActionForRequest_NonMCPFallsBack: non-MCP outbound
+// denials keep today's transport-level error shape. Asserts the
+// status mirrors the violation, not 200.
+func TestRejectFromActionForRequest_NonMCPFallsBack(t *testing.T) {
+	resp := rejectFromActionForRequest(ibacBlocked(), &pipeline.Context{})
+	status, _, _ := immediateBody(t, resp)
+	if status != typev3.StatusCode_Forbidden {
+		t.Fatalf("status = %v, want Forbidden (403) — non-MCP request should keep HTTP-level shape", status)
+	}
+}
+
+// TestRejectFromActionForRequest_NotificationFallsBack: JSON-RPC
+// notifications (id == nil) get no JSON-RPC response by spec, so we
+// fall through to the HTTP-level shape rather than echoing a null id.
+func TestRejectFromActionForRequest_NotificationFallsBack(t *testing.T) {
+	pctx := &pipeline.Context{
+		Extensions: pipeline.Extensions{
+			MCP: &pipeline.MCPExtension{Method: "tools/call", RPCID: nil},
+		},
+	}
+	resp := rejectFromActionForRequest(ibacBlocked(), pctx)
+	status, _, _ := immediateBody(t, resp)
+	if status != typev3.StatusCode_Forbidden {
+		t.Fatalf("status = %v, want Forbidden (403) — notification should keep HTTP-level shape", status)
+	}
+}

authbridge/authlib/listener/extproc/server.go

@@ -499,7 +499,7 @@ func (s *Server) handleOutbound(stream extprocv3.ExternalProcessor_ProcessServer
 	if action.Type == pipeline.Reject {
 		s.recordOutboundReject(pctx, action)
 		s.OutboundPipeline.RunFinish(ctx, pctx, pipeline.OutcomeFromContext(pctx))
-		return rejectFromAction(action), nil
+		return rejectFromActionForRequest(action, pctx), nil
 	}
 
 	s.recordOutboundSession(pctx)
@@ -551,7 +551,7 @@ func (s *Server) handleOutboundBody(stream extprocv3.ExternalProcessor_ProcessSe
 	if action.Type == pipeline.Reject {
 		s.recordOutboundReject(pctx, action)
 		s.OutboundPipeline.RunFinish(ctx, pctx, pipeline.OutcomeFromContext(pctx))
-		return rejectFromAction(action), nil
+		return rejectFromActionForRequest(action, pctx), nil
 	}
 
 	s.recordOutboundSession(pctx)
@@ -851,6 +851,71 @@ func replaceTokenResponse(token string) *extprocv3.ProcessingResponse {
 	}
 }
 
+// rejectFromActionForRequest is the MCP-aware sibling of rejectFromAction.
+// When pctx carries an MCP JSON-RPC request shape (Method + non-nil RPCID),
+// the response is an HTTP 200 carrying a JSON-RPC 2.0 error frame so the
+// caller's MCP client surfaces this as one failed tool call rather than a
+// transport break. All other shapes fall through to rejectFromAction.
+func rejectFromActionForRequest(action pipeline.Action, pctx *pipeline.Context) *extprocv3.ProcessingResponse {
+	if pctx != nil && pctx.Extensions.MCP != nil &&
+		pctx.Extensions.MCP.Method != "" && pctx.Extensions.MCP.RPCID != nil {
+		body := mcpRejectionBody(action, pctx.Extensions.MCP.RPCID)
+		return &extprocv3.ProcessingResponse{
+			Response: &extprocv3.ProcessingResponse_ImmediateResponse{
+				ImmediateResponse: &extprocv3.ImmediateResponse{
+					Status: &typev3.HttpStatus{Code: typev3.StatusCode(http.StatusOK)},
+					Body:   body,
+					Headers: &extprocv3.HeaderMutation{SetHeaders: []*corev3.HeaderValueOption{{
+						Header: &corev3.HeaderValue{Key: "content-type", RawValue: []byte("application/json")},
+					}}},
+				},
+			},
+		}
+	}
+	return rejectFromAction(action)
+}
+
+// JSON-RPC 2.0 server-error code; see authlib/listener/httpx/render.go for
+// rationale on -32000 specifically.
+const jsonRPCServerError = -32000
+
+func mcpRejectionBody(action pipeline.Action, id any) []byte {
+	v := action.Violation
+	message := "request rejected"
+	var data map[string]any
+	if v != nil {
+		if v.Reason != "" {
+			message = v.Reason
+		}
+		data = map[string]any{}
+		if v.Code != "" {
+			data["error"] = v.Code
+		}
+		if v.PluginName != "" {
+			data["plugin"] = v.PluginName
+		}
+		if v.Description != "" {
+			data["description"] = v.Description
+		}
+		if len(v.Details) > 0 {
+			data["details"] = v.Details
+		}
+		if len(data) == 0 {
+			data = nil
+		}
+	}
+	body, _ := json.Marshal(map[string]any{
+		"jsonrpc": "2.0",
+		"id":      id,
+		"error": map[string]any{
+			"code":    jsonRPCServerError,
+			"message": message,
+			"data":    data,
+		},
+	})
+	return body
+}
+
 // rejectFromAction turns a pipeline Reject into an Envoy ImmediateResponse,
 // preserving the plugin's status/headers/body. Replaces the old
 // denyResponse helper which hardcoded {"error":...,"message":...} at each

authbridge/authlib/listener/forwardproxy/server.go

@@ -260,7 +260,12 @@ func (s *Server) handleRequest(w http.ResponseWriter, r *http.Request) {
 
 		if action.Type == pipeline.Reject {
 			s.recordOutboundReject(pctx, action)
-			httpx.WriteRejection(w, action)
+			// Render as a JSON-RPC error frame when the rejected
+			// request was MCP JSON-RPC, so the agent's MCP client
+			// surfaces this as one failed tool call rather than a
+			// transport break. Falls through to plain HTTP-level
+			// rejection for non-MCP traffic.
+			httpx.WriteRejectionForRequest(w, action, pctx)
 			return
 		}
 	}
@@ -748,7 +753,12 @@ func (s *Server) handleConnect(w http.ResponseWriter, r *http.Request) {
 		action := s.OutboundPipeline.Run(r.Context(), pctx)
 		if action.Type == pipeline.Reject {
 			s.recordOutboundReject(pctx, action)
-			httpx.WriteRejection(w, action)
+			// Render as a JSON-RPC error frame when the rejected
+			// request was MCP JSON-RPC, so the agent's MCP client
+			// surfaces this as one failed tool call rather than a
+			// transport break. Falls through to plain HTTP-level
+			// rejection for non-MCP traffic.
+			httpx.WriteRejectionForRequest(w, action, pctx)
 			return
 		}
 	}

authbridge/authlib/listener/httpx/render.go

@@ -4,6 +4,7 @@
 package httpx
 
 import (
+	"encoding/json"
 	"net/http"
 
 	"github.com/kagenti/kagenti-extensions/authbridge/authlib/pipeline"
@@ -28,3 +29,87 @@ func WriteRejection(w http.ResponseWriter, action pipeline.Action) {
 	w.WriteHeader(status)
 	_, _ = w.Write(body)
 }
+
+// WriteRejectionForRequest renders a Reject the same way as
+// WriteRejection EXCEPT when the rejected request was a JSON-RPC
+// request that an MCP-aware parser already classified — in that case
+// it writes a JSON-RPC 2.0 error frame at HTTP 200 with the original
+// id echoed back, so the caller's MCP client surfaces this as a
+// failed tool call rather than a transport break.
+//
+// The MCP-shape detection is conservative: we only rewrite when
+// pctx.Extensions.MCP is populated with a non-empty Method AND a
+// non-nil RPCID. JSON-RPC notifications (no id) deliberately fall
+// through to plain WriteRejection — by spec the client expects no
+// response, so emitting a JSON-RPC error frame would violate the
+// notification contract.
+//
+// All non-MCP requests fall through to WriteRejection, so this is a
+// safe drop-in replacement at any call site.
+func WriteRejectionForRequest(w http.ResponseWriter, action pipeline.Action, pctx *pipeline.Context) {
+	if !shouldRenderMCPError(pctx) {
+		WriteRejection(w, action)
+		return
+	}
+	writeMCPRejection(w, action, pctx.Extensions.MCP.RPCID)
+}
+
+func shouldRenderMCPError(pctx *pipeline.Context) bool {
+	if pctx == nil || pctx.Extensions.MCP == nil {
+		return false
+	}
+	mcp := pctx.Extensions.MCP
+	if mcp.Method == "" || mcp.RPCID == nil {
+		return false
+	}
+	return true
+}
+
+// JSON-RPC 2.0 error code for application errors. -32000..-32099 is
+// the "implementation-defined server-error" range reserved by the
+// spec; -32000 is the conventional generic value used when there's
+// no protocol-level reason for a more specific code. Authbridge's
+// denials are policy decisions outside the JSON-RPC layer, so the
+// generic server-error code fits — operators read the human reason
+// and structured details, not the numeric code, to understand what
+// happened.
+const jsonRPCServerError = -32000
+
+func writeMCPRejection(w http.ResponseWriter, action pipeline.Action, id any) {
+	v := action.Violation
+	message := "request rejected"
+	var data map[string]any
+	if v != nil {
+		if v.Reason != "" {
+			message = v.Reason
+		}
+		data = map[string]any{}
+		if v.Code != "" {
+			data["error"] = v.Code
+		}
+		if v.PluginName != "" {
+			data["plugin"] = v.PluginName
+		}
+		if v.Description != "" {
+			data["description"] = v.Description
+		}
+		if len(v.Details) > 0 {
+			data["details"] = v.Details
+		}
+		if len(data) == 0 {
+			data = nil
+		}
+	}
+	body, _ := json.Marshal(map[string]any{
+		"jsonrpc": "2.0",
+		"id":      id,
+		"error": map[string]any{
+			"code":    jsonRPCServerError,
+			"message": message,
+			"data":    data,
+		},
+	})
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write(body)
+}