diff --git a/backend/internal/service/openai_gateway_forward.go b/backend/internal/service/openai_gateway_forward.go
index 0484ea7d36..bb13dd5989 100644
--- a/backend/internal/service/openai_gateway_forward.go
+++ b/backend/internal/service/openai_gateway_forward.go
@@ -46,6 +46,16 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 	if normalized {
 		body = normalizedBody
 	}
+	// 在分流到 passthrough / Codex transform / 原生 ChatCompletions 之前统一修正
+	// 显式为 null 的工具 Schema type，否则 upstream 的 400 会被归一成可重试的 502，
+	// 同一份坏定义在账号池里反复重放。
+	sanitizedToolBody, toolSchemaSanitized, toolSchemaErr := sanitizeOpenAIResponsesToolParameterTypes(body)
+	if toolSchemaErr != nil {
+		return nil, fmt.Errorf("sanitize OpenAI Responses tool parameters: %w", toolSchemaErr)
+	}
+	if toolSchemaSanitized {
+		body = sanitizedToolBody
+	}
 	if account.IsOpenAIOAuth() && isOpenAIResponsesLiteHeader(c.GetHeader(responsesLiteHeader)) {
 		liteBody, changed, liteErr := normalizeOpenAIResponsesLiteToolsPayload(body)
 		if liteErr != nil {
diff --git a/backend/internal/service/openai_responses_tool_schema.go b/backend/internal/service/openai_responses_tool_schema.go
new file mode 100644
index 0000000000..af9811e2cb
--- /dev/null
+++ b/backend/internal/service/openai_responses_tool_schema.go
@@ -0,0 +1,92 @@
+package service
+
+import (
+	"fmt"
+
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+const (
+	// 工具定义在多轮历史里最多再嵌套一层 tools，留出余量后截断，避免畸形请求体
+	// 造成无界递归。
+	openAIResponsesToolSchemaMaxDepth = 4
+	// JSON Schema 里 type 只能是字符串或字符串数组；显式 null 无论哪个方言都非法，
+	// 补成 object 与 upstream 对该工具的实际期望一致。
+	openAIResponsesToolSchemaFallbackType = "object"
+)
+
+// sanitizeOpenAIResponsesToolParameterTypes 修正请求体中显式为 null 的
+// tools[].parameters.type。
+//
+// Codex Desktop 内置的 automation_update 工具会带 parameters.type = null，
+// OpenAI 直接回 400 invalid_function_parameters，而网关把该状态归一成可重试的
+// 502 upstream_error；该工具定义又会沉进多轮历史，导致之后每一轮继续失败并在
+// 账号池里反复重放同一份坏 Schema。
+//
+// 只修正显式 null：缺失 type 的 Schema 本身合法（等价于不约束），补写会收窄
+// 客户端语义，因此保持原样。
+func sanitizeOpenAIResponsesToolParameterTypes(body []byte) ([]byte, bool, error) {
+	if len(body) == 0 {
+		return body, false, nil
+	}
+
+	paths := make([]string, 0, 2)
+	collectOpenAIResponsesToolSchemaNullTypePaths(gjson.GetBytes(body, "tools"), "tools", 0, &paths)
+	if input := gjson.GetBytes(body, "input"); input.IsArray() {
+		index := 0
+		input.ForEach(func(_, item gjson.Result) bool {
+			itemPath := fmt.Sprintf("input.%d.tools", index)
+			index++
+			if item.IsObject() {
+				collectOpenAIResponsesToolSchemaNullTypePaths(item.Get("tools"), itemPath, 0, &paths)
+			}
+			return true
+		})
+	}
+	if len(paths) == 0 {
+		return body, false, nil
+	}
+
+	sanitized := body
+	for _, path := range paths {
+		next, err := sjson.SetBytes(sanitized, path, openAIResponsesToolSchemaFallbackType)
+		if err != nil {
+			return body, false, fmt.Errorf("normalize %s: %w", path, err)
+		}
+		sanitized = next
+	}
+	return sanitized, true, nil
+}
+
+// collectOpenAIResponsesToolSchemaNullTypePaths 收集一个 tools 数组里所有需要修正的
+// parameters.type 路径。不按 tool type 过滤：null 的 schema type 在 function、
+// custom 以及任何 hosted 工具上都同样非法。
+func collectOpenAIResponsesToolSchemaNullTypePaths(tools gjson.Result, basePath string, depth int, paths *[]string) {
+	if depth > openAIResponsesToolSchemaMaxDepth || !tools.IsArray() {
+		return
+	}
+	index := 0
+	tools.ForEach(func(_, tool gjson.Result) bool {
+		toolPath := fmt.Sprintf("%s.%d", basePath, index)
+		index++
+		if !tool.IsObject() {
+			return true
+		}
+		// Responses 形态用顶层 parameters，ChatCompletions 形态用 function.parameters，
+		// 两种都可能出现在 Responses 请求里（见 normalizeCodexTools）。
+		for _, suffix := range []string{"parameters", "function.parameters"} {
+			params := tool.Get(suffix)
+			if !params.IsObject() {
+				continue
+			}
+			if typ := params.Get("type"); typ.Exists() && typ.Type == gjson.Null {
+				*paths = append(*paths, toolPath+"."+suffix+".type")
+			}
+		}
+		// 历史输入里的工具定义会再嵌套一层 tools（upstream 报错路径形如
+		// input[234].tools[0].tools[3].parameters）。
+		collectOpenAIResponsesToolSchemaNullTypePaths(tool.Get("tools"), toolPath+".tools", depth+1, paths)
+		return true
+	})
+}
diff --git a/backend/internal/service/openai_responses_tool_schema_test.go b/backend/internal/service/openai_responses_tool_schema_test.go
new file mode 100644
index 0000000000..59b22801c5
--- /dev/null
+++ b/backend/internal/service/openai_responses_tool_schema_test.go
@@ -0,0 +1,188 @@
+package service
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
+)
+
+// issue #5364 的最小复现体：Codex Desktop 内置 automation_update 带
+// parameters.type = null，upstream 回 400 invalid_function_parameters。
+func TestSanitizeOpenAIResponsesToolParameterTypes_TopLevelFunctionTool(t *testing.T) {
+	body := []byte(`{
+		"model": "gpt-5.6-sol",
+		"input": "Reply with OK.",
+		"stream": false,
+		"tools": [
+			{
+				"type": "function",
+				"name": "automation_update",
+				"description": "Update an automation.",
+				"parameters": {"type": null, "properties": {}}
+			}
+		]
+	}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "tools.0.parameters.type").String())
+	// 只改 type，工具其余定义原样保留。
+	require.Equal(t, "automation_update", gjson.GetBytes(sanitized, "tools.0.name").String())
+	require.Equal(t, "Update an automation.", gjson.GetBytes(sanitized, "tools.0.description").String())
+	require.True(t, gjson.GetBytes(sanitized, "tools.0.parameters.properties").IsObject())
+	// 请求体其余字段不受影响。
+	require.Equal(t, "gpt-5.6-sol", gjson.GetBytes(sanitized, "model").String())
+	require.Equal(t, "Reply with OK.", gjson.GetBytes(sanitized, "input").String())
+}
+
+// 合法 Schema 必须原样返回：changed=false 且字节不变，避免无谓重写打散
+// prompt cache 前缀。
+func TestSanitizeOpenAIResponsesToolParameterTypes_ValidSchemaUntouched(t *testing.T) {
+	body := []byte(`{"tools":[{"type":"function","name":"ok","parameters":{"type":"object","properties":{}}}]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.False(t, changed)
+	require.Equal(t, string(body), string(sanitized))
+}
+
+// 缺失 type 的 Schema 本身合法（等价于不约束），不得补写——补写会收窄客户端语义。
+func TestSanitizeOpenAIResponsesToolParameterTypes_MissingTypeNotInvented(t *testing.T) {
+	body := []byte(`{"tools":[{"type":"function","name":"ok","parameters":{"properties":{}}}]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.False(t, changed)
+	require.False(t, gjson.GetBytes(sanitized, "tools.0.parameters.type").Exists())
+}
+
+// 多轮历史：工具定义沉进 input 后，upstream 报错路径形如
+// input[N].tools[i].tools[j].parameters，两层都要修。
+func TestSanitizeOpenAIResponsesToolParameterTypes_NestedHistoryTools(t *testing.T) {
+	body := []byte(`{
+		"input": [
+			{"type": "message", "role": "user", "content": "hi"},
+			{
+				"type": "additional_tools",
+				"role": "developer",
+				"tools": [
+					{
+						"type": "namespace",
+						"name": "codex_app",
+						"tools": [
+							{"type": "function", "name": "noop", "parameters": {"type": "object"}},
+							{"type": "function", "name": "automation_update", "parameters": {"type": null}}
+						]
+					},
+					{"type": "function", "name": "outer", "parameters": {"type": null}}
+				]
+			}
+		]
+	}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "input.1.tools.0.tools.1.parameters.type").String())
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "input.1.tools.1.parameters.type").String())
+	// 原本合法的兄弟条目保持不变。
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "input.1.tools.0.tools.0.parameters.type").String())
+	require.Equal(t, "hi", gjson.GetBytes(sanitized, "input.0.content").String())
+}
+
+// ChatCompletions 形态的工具（{type:"function", function:{...}}）同样可能出现在
+// Responses 请求里，见 normalizeCodexTools。
+func TestSanitizeOpenAIResponsesToolParameterTypes_ChatCompletionsShape(t *testing.T) {
+	body := []byte(`{"tools":[{"type":"function","function":{"name":"legacy","parameters":{"type":null}}}]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "tools.0.function.parameters.type").String())
+	require.Equal(t, "legacy", gjson.GetBytes(sanitized, "tools.0.function.name").String())
+}
+
+// 索引映射：只有坏条目被改，前后兄弟条目按原下标保持不变。
+func TestSanitizeOpenAIResponsesToolParameterTypes_OnlyOffendingIndexRewritten(t *testing.T) {
+	body := []byte(`{"tools":[
+		{"type":"function","name":"a","parameters":{"type":"object"}},
+		{"type":"function","name":"b","parameters":{"type":null}},
+		{"type":"function","name":"c","parameters":{"type":"object"}}
+	]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+
+	require.NoError(t, err)
+	require.True(t, changed)
+	require.Equal(t, "a", gjson.GetBytes(sanitized, "tools.0.name").String())
+	require.Equal(t, "b", gjson.GetBytes(sanitized, "tools.1.name").String())
+	require.Equal(t, "c", gjson.GetBytes(sanitized, "tools.2.name").String())
+	require.Equal(t, "object", gjson.GetBytes(sanitized, "tools.1.parameters.type").String())
+	require.Equal(t, 3, int(gjson.GetBytes(sanitized, "tools.#").Int()))
+}
+
+// 畸形/非常规形态不得 panic，且一律按不变处理。
+func TestSanitizeOpenAIResponsesToolParameterTypes_MalformedShapesAreNoOps(t *testing.T) {
+	cases := []struct {
+		name string
+		body string
+	}{
+		{"empty_body", ``},
+		{"no_tools", `{"model":"gpt-5.6-sol","input":"hi"}`},
+		{"tools_null", `{"tools":null}`},
+		{"tools_object", `{"tools":{"type":"function"}}`},
+		{"tool_is_string", `{"tools":["freeform"]}`},
+		{"parameters_is_string", `{"tools":[{"type":"function","parameters":"nope"}]}`},
+		{"parameters_null", `{"tools":[{"type":"function","parameters":null}]}`},
+		{"input_string", `{"input":"hi","tools":[]}`},
+		{"input_item_not_object", `{"input":["hi"]}`},
+		{"type_already_array", `{"tools":[{"type":"function","parameters":{"type":["object","null"]}}]}`},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes([]byte(tc.body))
+			require.NoError(t, err)
+			require.False(t, changed)
+			require.Equal(t, tc.body, string(sanitized))
+		})
+	}
+}
+
+// 递归深度守卫：超深嵌套只做截断，不递归到栈溢出，也不报错。
+func TestSanitizeOpenAIResponsesToolParameterTypes_DepthGuard(t *testing.T) {
+	tool := map[string]any{"type": "function", "name": "deep", "parameters": map[string]any{"type": nil}}
+	for i := 0; i < 12; i++ {
+		tool = map[string]any{"type": "namespace", "tools": []any{tool}}
+	}
+	body, err := json.Marshal(map[string]any{"tools": []any{tool}})
+	require.NoError(t, err)
+
+	require.NotPanics(t, func() {
+		_, _, sanitizeErr := sanitizeOpenAIResponsesToolParameterTypes(body)
+		require.NoError(t, sanitizeErr)
+	})
+}
+
+// 输出必须是合法 JSON，且除目标字段外与输入等价。
+func TestSanitizeOpenAIResponsesToolParameterTypes_OutputStaysValidJSON(t *testing.T) {
+	body := []byte(`{"model":"gpt-5.5","tool_choice":"none","store":false,"tools":[{"type":"function","name":"automation_update","parameters":{"type":null,"properties":{}}}]}`)
+
+	sanitized, changed, err := sanitizeOpenAIResponsesToolParameterTypes(body)
+	require.NoError(t, err)
+	require.True(t, changed)
+
+	var decoded map[string]any
+	require.NoError(t, json.Unmarshal(sanitized, &decoded))
+	require.Equal(t, "gpt-5.5", decoded["model"])
+	require.Equal(t, "none", decoded["tool_choice"])
+	require.Equal(t, false, decoded["store"])
+}