From 373e09f6049250dc5bb23ab9bd3a859e9fcd37d8 Mon Sep 17 00:00:00 2001
From: Maxence Maireaux <maxence@maireaux.fr>
Date: Thu, 4 Jun 2026 13:26:42 +0200
Subject: [PATCH] fix: reject parse errors before execution

---
 internal/mcp_impl/handlers.go      | 70 +++++++++++++++---------------
 internal/mcp_impl/handlers_test.go | 28 ++++++++++++
 numscript.go                       |  4 ++
 numscript_test.go                  |  9 ++++
 4 files changed, 77 insertions(+), 34 deletions(-)
 create mode 100644 internal/mcp_impl/handlers_test.go

diff --git a/internal/mcp_impl/handlers.go b/internal/mcp_impl/handlers.go
index 93139eb9..abe76b7b 100644
--- a/internal/mcp_impl/handlers.go
+++ b/internal/mcp_impl/handlers.go
@@ -95,45 +95,47 @@ func addEvalTool(s *server.MCPServer) {
 			`),
 		),
 	)
-	s.AddTool(tool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-		script, err := request.RequireString("script")
-		if err != nil {
-			return mcp.NewToolResultError(err.Error()), nil
-		}
+	s.AddTool(tool, handleEvalTool)
+}
 
-		parsed := parser.Parse(script)
-		if len(parsed.Errors) != 0 {
-			out := make([]string, len(parsed.Errors))
-			for index, err := range parsed.Errors {
-				out[index] = err.Msg
-			}
-			mcp.NewToolResultError(strings.Join(out, ", "))
-		}
+func handleEvalTool(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+	script, err := request.RequireString("script")
+	if err != nil {
+		return mcp.NewToolResultError(err.Error()), nil
+	}
 
-		balances, mcpErr := parseBalancesJson(request.GetArguments()["balances"])
-		if mcpErr != nil {
-			return mcpErr, nil
+	parsed := parser.Parse(script)
+	if len(parsed.Errors) != 0 {
+		out := make([]string, len(parsed.Errors))
+		for index, err := range parsed.Errors {
+			out[index] = err.Msg
 		}
+		return mcp.NewToolResultError(strings.Join(out, ", ")), nil
+	}
 
-		vars, mcpErr := parseVarsJson(request.GetArguments()["vars"])
-		if mcpErr != nil {
-			return mcpErr, nil
-		}
+	balances, mcpErr := parseBalancesJson(request.GetArguments()["balances"])
+	if mcpErr != nil {
+		return mcpErr, nil
+	}
 
-		out, iErr := interpreter.RunProgram(
-			ctx,
-			parsed.Value,
-			vars,
-			interpreter.StaticStore{
-				Balances: balances,
-			},
-			map[string]struct{}{},
-		)
-		if iErr != nil {
-			return mcp.NewToolResultError(iErr.Error()), nil
-		}
-		return mcp.NewToolResultJSON(*out)
-	})
+	vars, mcpErr := parseVarsJson(request.GetArguments()["vars"])
+	if mcpErr != nil {
+		return mcpErr, nil
+	}
+
+	out, iErr := interpreter.RunProgram(
+		ctx,
+		parsed.Value,
+		vars,
+		interpreter.StaticStore{
+			Balances: balances,
+		},
+		map[string]struct{}{},
+	)
+	if iErr != nil {
+		return mcp.NewToolResultError(iErr.Error()), nil
+	}
+	return mcp.NewToolResultJSON(*out)
 }
 
 func addCheckTool(s *server.MCPServer) {
diff --git a/internal/mcp_impl/handlers_test.go b/internal/mcp_impl/handlers_test.go
new file mode 100644
index 00000000..7398d96e
--- /dev/null
+++ b/internal/mcp_impl/handlers_test.go
@@ -0,0 +1,28 @@
+package mcp_impl
+
+import (
+	"context"
+	"testing"
+
+	"github.com/mark3labs/mcp-go/mcp"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHandleEvalToolRejectsParseErrors(t *testing.T) {
+	result, err := handleEvalTool(context.Background(), mcp.CallToolRequest{
+		Params: mcp.CallToolParams{
+			Arguments: map[string]any{
+				"script":   "send [COIN 100] (",
+				"balances": map[string]any{},
+				"vars":     map[string]any{},
+			},
+		},
+	})
+
+	require.NoError(t, err)
+	require.True(t, result.IsError)
+	require.NotEmpty(t, result.Content)
+	text, ok := result.Content[0].(mcp.TextContent)
+	require.True(t, ok)
+	require.Contains(t, text.Text, "mismatched input")
+}
diff --git a/numscript.go b/numscript.go
index e7fa0437..08cd5bdd 100644
--- a/numscript.go
+++ b/numscript.go
@@ -86,6 +86,10 @@ func (p ParseResult) RunWithFeatureFlags(
 	store Store,
 	featureFlags map[string]struct{},
 ) (ExecutionResult, InterpreterError) {
+	if len(p.parseResult.Errors) != 0 {
+		return ExecutionResult{}, p.parseResult.Errors[0]
+	}
+
 	if featureFlags == nil {
 		featureFlags = make(map[string]struct{})
 	}
diff --git a/numscript_test.go b/numscript_test.go
index b4da92d6..aef0c1da 100644
--- a/numscript_test.go
+++ b/numscript_test.go
@@ -58,6 +58,15 @@ func TestGetVarsNovars(t *testing.T) {
 	)
 }
 
+func TestRunRejectsParseErrors(t *testing.T) {
+	parseResult := numscript.Parse(`send [COIN 100] (`)
+	require.NotEmpty(t, parseResult.GetParsingErrors())
+
+	_, err := parseResult.Run(context.Background(), nil, interpreter.StaticStore{})
+	require.Error(t, err)
+	require.Equal(t, parseResult.GetParsingErrors()[0].Error(), err.Error())
+}
+
 func TestDoNotGetWorldBalance(t *testing.T) {
 	parseResult := numscript.Parse(`send [COIN 100] (
 	source = @world