diff --git a/cmd/dashboard/controller/mcp.go b/cmd/dashboard/controller/mcp.go
index 9788b56a..e35d6908 100644
--- a/cmd/dashboard/controller/mcp.go
+++ b/cmd/dashboard/controller/mcp.go
@@ -327,13 +327,12 @@ func handleToolsCall(c *gin.Context, req *jsonRPCRequest, tok *model.APIToken) {
 			})
 			return
 		}
+		// 错误结果不带 structuredContent：严格客户端会拿它去校验工具声明的
+		// outputSchema（要求 exit_code/stdout/... 等成功字段），缺字段就整条
+		// 响应报 -32602，把真正的 isError 文本掩盖掉。错误信息走 content[].text。
 		writeJSONRPCResult(c, req.ID, mcpToolCallResult{
 			Content: []mcpContent{{Type: "text", Text: errMsg}},
 			IsError: true,
-			StructuredContent: map[string]string{
-				"error_code": errCode,
-				"error":      errMsg,
-			},
 		})
 	}
 
diff --git a/cmd/dashboard/controller/mcp_capability.go b/cmd/dashboard/controller/mcp_capability.go
index e743f530..a7a137cd 100644
--- a/cmd/dashboard/controller/mcp_capability.go
+++ b/cmd/dashboard/controller/mcp_capability.go
@@ -27,13 +27,12 @@ func requireAgentSupportsMCP(server *model.Server) error {
 	return nil
 }
 
-// compareSemver 比较两个 "MAJOR.MINOR.PATCH[-suffix]" 字符串。
-// 返回 -1/0/1。无法解析时按字符串字典序比较，保证全序但可能不精确——
-// 对于 "agent 太老" 的快速失败用途已经足够。
+// compareSemver 比较两个 "MAJOR.MINOR.PATCH[-suffix]" 字符串，返回 -1/0/1。
+// semverParts 已剥掉可选的 "v" 前缀与 "-/+" 后缀，所以只按三段数字定序：
+// 数字段相等即视为相等版本。绝不能回退到字符串字典序——agent 上报 "2.1.0"
+// 而门槛常量是 "v2.1.0"，'2'(0x32) < 'v'(0x76) 会把相等版本误判为更旧，
+// 导致所有 agent 被错误地判为不支持 MCP。
 func compareSemver(a, b string) int {
-	if a == b {
-		return 0
-	}
 	aparts := semverParts(a)
 	bparts := semverParts(b)
 	for i := 0; i < 3; i++ {
@@ -44,12 +43,6 @@ func compareSemver(a, b string) int {
 			return 1
 		}
 	}
-	if a < b {
-		return -1
-	}
-	if a > b {
-		return 1
-	}
 	return 0
 }
 
diff --git a/cmd/dashboard/controller/mcp_capability_test.go b/cmd/dashboard/controller/mcp_capability_test.go
index b828c34b..41d04f91 100644
--- a/cmd/dashboard/controller/mcp_capability_test.go
+++ b/cmd/dashboard/controller/mcp_capability_test.go
@@ -50,3 +50,28 @@ func TestRequireAgentSupportsMCPDefersWhenAgentNeverReported(t *testing.T) {
 	require.NoError(t, requireAgentSupportsMCP(&model.Server{Host: nil}),
 		"Host==nil means agent never reported its build; defer the version decision to the CallAgent timeout layer")
 }
+
+func TestCompareSemverIgnoresVPrefixMismatch(t *testing.T) {
+	cases := []struct {
+		a, b string
+		want int
+	}{
+		{"2.1.0", "v2.1.0", 0},
+		{"v2.1.0", "2.1.0", 0},
+		{"2.1.0", "2.1.0", 0},
+		{"2.1.0", "v2.1.1", -1},
+		{"v2.1.2", "2.1.0", 1},
+		{"2.2.0", "v2.1.9", 1},
+	}
+	for _, c := range cases {
+		require.Equalf(t, c.want, compareSemver(c.a, c.b),
+			"compareSemver(%q,%q): the only difference is the optional 'v' prefix and/or numeric ordering; "+
+				"a bare lexical fallback wrongly orders %q < %q", c.a, c.b, c.a, c.b)
+	}
+}
+
+func TestRequireAgentSupportsMCPAcceptsBarePrefixReport(t *testing.T) {
+	agent := &model.Server{Host: &model.Host{Version: "2.1.0"}}
+	require.NoError(t, requireAgentSupportsMCP(agent),
+		"agents report Host.Version without the 'v' prefix (e.g. \"2.1.0\"); it must compare equal to MCPMinAgentVersion \"v2.1.0\" and be accepted")
+}
diff --git a/cmd/dashboard/controller/mcp_error_structured_test.go b/cmd/dashboard/controller/mcp_error_structured_test.go
new file mode 100644
index 00000000..fd457c20
--- /dev/null
+++ b/cmd/dashboard/controller/mcp_error_structured_test.go
@@ -0,0 +1,67 @@
+package controller
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/nezhahq/nezha/model"
+	"github.com/nezhahq/nezha/service/singleton"
+)
+
+// A tool error must not ship structuredContent: strict clients validate it
+// against the tool's outputSchema (which requires exec/fs result fields) and
+// reject the whole response with -32602, masking the real isError text.
+func TestServerExec_ErrorResult_OmitsStructuredContent(t *testing.T) {
+	cleanup, uid := setupMCPTest(t)
+	defer cleanup()
+
+	srv, _ := singleton.ServerShared.Get(7)
+	srv.SetTaskStream(&execErrorStream{errMsg: "agent disabled command execution"})
+
+	tok, _ := mkToken(t, uid, []string{model.ScopeServerExec}, nil)
+
+	c, w := mcpCallCtx(t, tok, uid, jsonRPCRequest{
+		JSONRPC: "2.0", ID: json.RawMessage("1"), Method: "tools/call",
+		Params: jsonObj(t, toolCallParams{
+			Name: "server.exec",
+			Arguments: jsonRaw(map[string]any{
+				"server_id":       7,
+				"cmd":             "whoami",
+				"timeout_seconds": 2,
+			}),
+		}),
+	})
+	mcpEndpoint(c)
+
+	_, tcr := decodeRPC(w)
+	require.NotNil(t, tcr)
+	require.True(t, tcr.IsError)
+	require.Contains(t, tcr.Content[0].Text, "agent disabled command execution",
+		"error text must carry the real cause")
+	require.Nil(t, tcr.StructuredContent,
+		"error responses must omit structuredContent so strict clients don't validate it against outputSchema and mask the real error")
+}
+
+func TestScopeDenied_OmitsStructuredContent(t *testing.T) {
+	cleanup, uid := setupMCPTest(t)
+	defer cleanup()
+
+	tok, _ := mkToken(t, uid, []string{model.ScopeServerRead}, nil)
+
+	c, w := mcpCallCtx(t, tok, uid, jsonRPCRequest{
+		JSONRPC: "2.0", ID: json.RawMessage("1"), Method: "tools/call",
+		Params: jsonObj(t, toolCallParams{
+			Name:      "server.exec",
+			Arguments: jsonRaw(map[string]any{"server_id": 7, "cmd": "echo"}),
+		}),
+	})
+	mcpEndpoint(c)
+
+	_, tcr := decodeRPC(w)
+	require.NotNil(t, tcr)
+	require.True(t, tcr.IsError)
+	require.Nil(t, tcr.StructuredContent,
+		"scope-denied error must also omit structuredContent")
+}