client: Honor IsError and extract JSON from tool text

#15

client/session_test.go

@@ -3,6 +3,7 @@ package client
 import (
 	"encoding/json"
 	"errors"
+	"strings"
 	"testing"
 
 	mcp "github.com/modelcontextprotocol/go-sdk/mcp"
@@ -42,6 +43,37 @@ func TestToolJSON(t *testing.T) {
 			res:  &mcp.CallToolResult{},
 			want: `{}`,
 		},
+		{
+			name: "wrapped JSON in prose",
+			res: &mcp.CallToolResult{
+				Content: []mcp.Content{&mcp.TextContent{Text: `here is data: {"symbol":"MU"} thanks`}},
+			},
+			want: `{"symbol":"MU"}`,
+		},
+		{
+			name: "one TextContent is JSON",
+			res: &mcp.CallToolResult{
+				Content: []mcp.Content{
+					&mcp.TextContent{Text: "note: "},
+					&mcp.TextContent{Text: `{"symbol":"MU"}`},
+				},
+			},
+			want: `{"symbol":"MU"}`,
+		},
+		{
+			name: "wrapped JSON array",
+			res: &mcp.CallToolResult{
+				Content: []mcp.Content{&mcp.TextContent{Text: `items: [1,{"a":2}] done`}},
+			},
+			want: `[1,{"a":2}]`,
+		},
+		{
+			name: "wrapped JSON with brace in string",
+			res: &mcp.CallToolResult{
+				Content: []mcp.Content{&mcp.TextContent{Text: `here {"msg":"say } hi"} x`}},
+			},
+			want: `{"msg":"say } hi"}`,
+		},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
@@ -76,12 +108,56 @@ func TestToolJSON_getError(t *testing.T) {
 	}
 }
 
+func TestToolJSON_isErrorText(t *testing.T) {
+	t.Parallel()
+	res := &mcp.CallToolResult{
+		IsError: true,
+		Content: []mcp.Content{&mcp.TextContent{Text: "historicals unavailable"}},
+	}
+	if res.GetError() != nil {
+		t.Fatal("GetError should be nil when IsError is set via field")
+	}
+	_, err := toolJSON(res)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if err.Error() != "historicals unavailable" {
+		t.Fatalf("%v", err)
+	}
+	if strings.Contains(err.Error(), "non-json tool result") {
+		t.Fatalf("got non-json wrapping: %v", err)
+	}
+}
+
+func TestToolJSON_isErrorEmpty(t *testing.T) {
+	t.Parallel()
+	_, err := toolJSON(&mcp.CallToolResult{IsError: true})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if err.Error() != "tool error" {
+		t.Fatalf("%v", err)
+	}
+	if strings.Contains(err.Error(), "non-json tool result") {
+		t.Fatalf("got non-json wrapping: %v", err)
+	}
+}
+
 func TestToolJSON_nonJSONText(t *testing.T) {
 	t.Parallel()
 	_, err := toolJSON(&mcp.CallToolResult{
 		Content: []mcp.Content{&mcp.TextContent{Text: "not json"}},
 	})
-	if err == nil || err.Error() != "non-json tool result" {
-		t.Fatalf("%v", err)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	msg := err.Error()
+	if !strings.Contains(msg, "non-json") {
+		t.Fatalf("missing non-json: %v", err)
+	}
+	for _, want := range []string{`isError=false`, `types=[text]`, `structured=nil`, `prefix="not json"`} {
+		if !strings.Contains(msg, want) {
+			t.Fatalf("missing %q in %v", want, err)
+		}
 	}
 }