diff --git a/backend/internal/service/openai_gateway_grok.go b/backend/internal/service/openai_gateway_grok.go
index 1906b83780..8bd1d456dc 100644
--- a/backend/internal/service/openai_gateway_grok.go
+++ b/backend/internal/service/openai_gateway_grok.go
@@ -185,6 +185,10 @@ func patchGrokResponsesBody(body []byte, upstreamModel string) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	out, err = sanitizeGrokReasoningNullContent(out)
+	if err != nil {
+		return nil, err
+	}
 	out, err = sanitizeGrokResponsesTools(out)
 	if err != nil {
 		return nil, err
@@ -304,6 +308,35 @@ func sanitizeGrokResponsesInput(body []byte) ([]byte, error) {
 	return sjson.SetRawBytes(body, "input", encoded)
 }
 
+// sanitizeGrokReasoningNullContent 删除 reasoning 项中的 "content": null。
+// xAI 的 untagged enum 反序列化器拒收该字段，返回 422。
+func sanitizeGrokReasoningNullContent(body []byte) ([]byte, error) {
+	input := gjson.GetBytes(body, "input")
+	if !input.Exists() || !input.IsArray() {
+		return body, nil
+	}
+
+	items := input.Array()
+	changed := false
+	for i := len(items) - 1; i >= 0; i-- {
+		item := items[i]
+		if strings.TrimSpace(item.Get("type").String()) != "reasoning" {
+			continue
+		}
+		contentResult := item.Get("content")
+		if contentResult.Exists() && contentResult.Type == gjson.Null {
+			var err error
+			body, err = sjson.DeleteBytes(body, fmt.Sprintf("input.%d.content", i))
+			if err != nil {
+				return nil, err
+			}
+			changed = true
+		}
+	}
+	_ = changed
+	return body, nil
+}
+
 var grokResponsesSupportedToolTypes = map[string]struct{}{
 	"code_execution":     {},
 	"code_interpreter":   {},
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index b45e0bccc0..3b64cbc16b 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -1771,3 +1771,70 @@ func TestFailoverOpenAIUpstreamHTTPErrorUsesOnlyGrokRateLimitPolicy(t *testing.T
 	require.Equal(t, 1, repo.rateLimitedCalls)
 	require.Zero(t, repo.tempUnschedCalls)
 }
+
+func TestPatchGrokResponsesBody_StripsReasoningContentNull(t *testing.T) {
+	t.Parallel()
+
+	body := []byte(`{
+		"model": "grok-latest",
+		"input": [
+			{"type":"message","role":"user","content":[{"type":"input_text","text":"hi"}]},
+			{"type":"reasoning","summary":[{"type":"summary_text","text":"thinking..."}],"content":null,"encrypted_content":null},
+			{"type":"message","role":"assistant","content":[{"type":"output_text","text":"Hello!"}]}
+		]
+	}`)
+
+	patched, err := patchGrokResponsesBody(body, "grok-4.5")
+	require.NoError(t, err)
+	require.True(t, json.Valid(patched))
+
+	input := gjson.GetBytes(patched, "input")
+	require.True(t, input.IsArray())
+
+	items := input.Array()
+	require.Len(t, items, 3)
+
+	reasoning := items[1]
+	require.Equal(t, "reasoning", reasoning.Get("type").String())
+	require.True(t, reasoning.Get("summary").Exists(), "summary should be preserved")
+	require.False(t, reasoning.Get("content").Exists(), "content: null should be stripped")
+}
+
+func TestPatchGrokResponsesBody_KeepsReasoningContentNonNull(t *testing.T) {
+	t.Parallel()
+
+	body := []byte(`{
+		"model": "grok-latest",
+		"input": [
+			{"type":"reasoning","summary":[{"type":"summary_text","text":"ok"}],"content":"real content"}
+		]
+	}`)
+
+	patched, err := patchGrokResponsesBody(body, "grok-4.5")
+	require.NoError(t, err)
+
+	reasoning := gjson.GetBytes(patched, "input.0")
+	require.Equal(t, "real content", reasoning.Get("content").String(), "non-null content must not be stripped")
+}
+
+func TestPatchGrokResponsesBody_MultipleReasoningContentNull(t *testing.T) {
+	t.Parallel()
+
+	body := []byte(`{
+		"model": "grok-latest",
+		"input": [
+			{"type":"reasoning","summary":[{"type":"summary_text","text":"r1"}],"content":null},
+			{"type":"message","role":"user","content":"hi"},
+			{"type":"reasoning","summary":[{"type":"summary_text","text":"r2"}],"content":null}
+		]
+	}`)
+
+	patched, err := patchGrokResponsesBody(body, "grok-4.5")
+	require.NoError(t, err)
+
+	items := gjson.GetBytes(patched, "input").Array()
+	require.Len(t, items, 3)
+
+	require.False(t, items[0].Get("content").Exists())
+	require.False(t, items[2].Get("content").Exists())
+}
diff --git a/backend/internal/service/openai_gateway_request_body.go b/backend/internal/service/openai_gateway_request_body.go
index d56b11fdcc..b47c2c8638 100644
--- a/backend/internal/service/openai_gateway_request_body.go
+++ b/backend/internal/service/openai_gateway_request_body.go
@@ -136,12 +136,20 @@ func sanitizeEncryptedReasoningInputItem(item any) (next any, changed bool, keep
 		return item, false, true
 	}
 
-	_, hasEncryptedContent := inputItem["encrypted_content"]
-	if !hasEncryptedContent {
-		return item, false, true
+	if _, has := inputItem["encrypted_content"]; has {
+		delete(inputItem, "encrypted_content")
+		changed = true
 	}
 
-	delete(inputItem, "encrypted_content")
+	// xAI 422: "content": null 导致 untagged enum 反序列化失败
+	if v, has := inputItem["content"]; has && v == nil {
+		delete(inputItem, "content")
+		changed = true
+	}
+
+	if !changed {
+		return item, false, true
+	}
 	if len(inputItem) == 1 {
 		return nil, true, false
 	}
diff --git a/backend/internal/service/openai_gateway_request_body_reasoning_test.go b/backend/internal/service/openai_gateway_request_body_reasoning_test.go
new file mode 100644
index 0000000000..679d245884
--- /dev/null
+++ b/backend/internal/service/openai_gateway_request_body_reasoning_test.go
@@ -0,0 +1,106 @@
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestTrimOpenAIEncryptedReasoningItems_ContentNull(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "grok-4.5",
+		"input": []any{
+			map[string]any{"type": "message", "role": "user", "content": "hi"},
+			map[string]any{
+				"type":              "reasoning",
+				"summary":          []any{map[string]any{"type": "summary_text", "text": "thinking..."}},
+				"content":          nil,
+				"encrypted_content": nil,
+			},
+			map[string]any{"type": "message", "role": "assistant", "content": "Hello!"},
+		},
+	}
+
+	changed := trimOpenAIEncryptedReasoningItems(reqBody)
+	require.True(t, changed)
+
+	input := reqBody["input"].([]any)
+	require.Len(t, input, 3)
+
+	reasoning := input[1].(map[string]any)
+	assert.Equal(t, "reasoning", reasoning["type"])
+	assert.NotNil(t, reasoning["summary"])
+	_, hasContent := reasoning["content"]
+	assert.False(t, hasContent, "content: null should be stripped")
+	_, hasEncrypted := reasoning["encrypted_content"]
+	assert.False(t, hasEncrypted, "encrypted_content should be stripped")
+}
+
+func TestTrimOpenAIEncryptedReasoningItems_ContentNullOnly(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "grok-4.5",
+		"input": []any{
+			map[string]any{
+				"type":    "reasoning",
+				"summary": []any{map[string]any{"type": "summary_text", "text": "ok"}},
+				"content": nil,
+			},
+		},
+	}
+
+	changed := trimOpenAIEncryptedReasoningItems(reqBody)
+	require.True(t, changed)
+
+	input := reqBody["input"].([]any)
+	require.Len(t, input, 1)
+
+	reasoning := input[0].(map[string]any)
+	_, hasContent := reasoning["content"]
+	assert.False(t, hasContent, "content: null should be stripped even without encrypted_content")
+}
+
+func TestTrimOpenAIEncryptedReasoningItems_ContentNonNull(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "grok-4.5",
+		"input": []any{
+			map[string]any{
+				"type":    "reasoning",
+				"summary": []any{map[string]any{"type": "summary_text", "text": "ok"}},
+				"content": "some actual content",
+			},
+		},
+	}
+
+	changed := trimOpenAIEncryptedReasoningItems(reqBody)
+	assert.False(t, changed, "non-null content should not be stripped")
+
+	input := reqBody["input"].([]any)
+	reasoning := input[0].(map[string]any)
+	assert.Equal(t, "some actual content", reasoning["content"])
+}
+
+func TestTrimOpenAIEncryptedReasoningItems_NoReasoningItems(t *testing.T) {
+	reqBody := map[string]any{
+		"model": "grok-4.5",
+		"input": []any{
+			map[string]any{"type": "message", "role": "user", "content": "hi"},
+		},
+	}
+
+	changed := trimOpenAIEncryptedReasoningItems(reqBody)
+	assert.False(t, changed)
+}
+
+func TestTrimOpenAIEncryptedReasoningItems_ContentNullDropsBareSkeleton(t *testing.T) {
+	reqBody := map[string]any{
+		"input": []any{
+			map[string]any{"type": "reasoning", "content": nil},
+		},
+	}
+
+	changed := trimOpenAIEncryptedReasoningItems(reqBody)
+	require.True(t, changed)
+	_, hasInput := reqBody["input"]
+	assert.False(t, hasInput, "bare reasoning skeleton should be dropped, emptying input")
+}