diff --git a/pkg/mcp/bridge.go b/pkg/mcp/bridge.go
index f1cb980..2b85313 100644
--- a/pkg/mcp/bridge.go
+++ b/pkg/mcp/bridge.go
@@ -63,7 +63,7 @@ func BridgeToAPI(svc *Service, bridge *api.ToolBridge) {
 			if err != nil {
 				// Body present + error = likely bad input (malformed JSON).
 				// No body + error = tool execution failure.
-				if len(body) > 0 && core.Contains(err.Error(), "unmarshal") {
+				if errors.Is(err, errInvalidRESTInput) {
 					c.JSON(http.StatusBadRequest, api.Fail("invalid_input", "Malformed JSON in request body"))
 					return
 				}
diff --git a/pkg/mcp/bridge_test.go b/pkg/mcp/bridge_test.go
index 87ad818..ae0a92b 100644
--- a/pkg/mcp/bridge_test.go
+++ b/pkg/mcp/bridge_test.go
@@ -165,13 +165,8 @@ func TestBridgeToAPI_Bad_InvalidJSON(t *testing.T) {
 	req.Header.Set("Content-Type", "application/json")
 	engine.ServeHTTP(w, req)
 
-	if w.Code != http.StatusInternalServerError {
-		// The handler unmarshals via RESTHandler which returns an error,
-		// but since it's a JSON parse error it ends up as tool_error.
-		// Check we get a non-200 with an error envelope.
-		if w.Code == http.StatusOK {
-			t.Fatalf("expected non-200 for invalid JSON, got 200")
-		}
+	if w.Code != http.StatusBadRequest {
+		t.Fatalf("expected 400 for invalid JSON, got %d: %s", w.Code, w.Body.String())
 	}
 
 	var resp api.Response[any]
diff --git a/pkg/mcp/registry.go b/pkg/mcp/registry.go
index c84b72d..c6a6baf 100644
--- a/pkg/mcp/registry.go
+++ b/pkg/mcp/registry.go
@@ -4,6 +4,8 @@ package mcp
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"reflect"
 	"time"
 
@@ -21,6 +23,9 @@ import (
 //	}
 type RESTHandler func(ctx context.Context, body []byte) (any, error)
 
+// errInvalidRESTInput marks malformed JSON bodies for the REST bridge.
+var errInvalidRESTInput = errors.New("invalid REST input")
+
 // ToolRecord captures metadata about a registered MCP tool.
 //
 //	for _, rec := range svc.Tools() {
@@ -53,9 +58,9 @@ func AddToolRecorded[In, Out any](s *Service, server *mcp.Server, group string,
 		if len(body) > 0 {
 			if r := core.JSONUnmarshal(body, &input); !r.OK {
 				if err, ok := r.Value.(error); ok {
-					return nil, err
+					return nil, fmt.Errorf("%w: %v", errInvalidRESTInput, err)
 				}
-				return nil, core.E("registry.RESTHandler", "failed to unmarshal input", nil)
+				return nil, fmt.Errorf("%w", errInvalidRESTInput)
 			}
 		}
 		// nil: REST callers have no MCP request context.