diff --git a/backend/internal/pkg/apicompat/anthropic_responses_test.go b/backend/internal/pkg/apicompat/anthropic_responses_test.go
index c13e9985a..539ccda0d 100644
--- a/backend/internal/pkg/apicompat/anthropic_responses_test.go
+++ b/backend/internal/pkg/apicompat/anthropic_responses_test.go
@@ -146,7 +146,7 @@ func TestAnthropicToResponses_ToolUse(t *testing.T) {
 	assert.Equal(t, "Sunny, 72°F", items[3].Output)
 }
 
-func TestAnthropicToResponses_ThinkingIgnored(t *testing.T) {
+func TestAnthropicToResponses_ThinkingWithoutSignatureIgnored(t *testing.T) {
 	req := &AnthropicRequest{
 		Model:     "gpt-5.2",
 		MaxTokens: 1024,
@@ -162,7 +162,7 @@ func TestAnthropicToResponses_ThinkingIgnored(t *testing.T) {
 
 	var items []ResponsesInputItem
 	require.NoError(t, json.Unmarshal(resp.Input, &items))
-	// user + assistant(text only, thinking ignored) + user = 3
+	// user + assistant(text only, thinking without signature ignored) + user = 3
 	require.Len(t, items, 3)
 	assert.Equal(t, "assistant", items[1].Role)
 	// Assistant content should only have text, not thinking.
@@ -173,6 +173,30 @@ func TestAnthropicToResponses_ThinkingIgnored(t *testing.T) {
 	assert.Equal(t, "Hi!", parts[0].Text)
 }
 
+func TestAnthropicToResponses_ThinkingSignatureBecomesReasoning(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "grok-4.5",
+		MaxTokens: 1024,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"Hello"`)},
+			{Role: "assistant", Content: json.RawMessage(`[{"type":"thinking","thinking":"plan","signature":"enc-rs-1"},{"type":"text","text":"Hi!"},{"type":"tool_use","id":"toolu_1","name":"Bash","input":{"command":"ls"}}]`)},
+			{Role: "user", Content: json.RawMessage(`[{"type":"tool_result","tool_use_id":"toolu_1","content":"ok"}]`)},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	// user + reasoning + assistant text + function_call + function_call_output
+	require.GreaterOrEqual(t, len(items), 4)
+	assert.Equal(t, "reasoning", items[1].Type)
+	assert.Equal(t, "enc-rs-1", items[1].EncryptedContent)
+	assert.Equal(t, "assistant", items[2].Role)
+	assert.Equal(t, "function_call", items[3].Type)
+}
+
 func TestAnthropicToResponses_MaxTokensFloor(t *testing.T) {
 	req := &AnthropicRequest{
 		Model:     "gpt-5.2",
@@ -372,7 +396,8 @@ func TestResponsesToAnthropic_Reasoning(t *testing.T) {
 		Status: "completed",
 		Output: []ResponsesOutput{
 			{
-				Type: "reasoning",
+				Type:             "reasoning",
+				EncryptedContent: "enc-rs-roundtrip",
 				Summary: []ResponsesSummary{
 					{Type: "summary_text", Text: "Thinking about the answer..."},
 				},
@@ -390,10 +415,57 @@ func TestResponsesToAnthropic_Reasoning(t *testing.T) {
 	require.Len(t, anth.Content, 2)
 	assert.Equal(t, "thinking", anth.Content[0].Type)
 	assert.Equal(t, "Thinking about the answer...", anth.Content[0].Thinking)
+	assert.Equal(t, "enc-rs-roundtrip", anth.Content[0].Signature)
 	assert.Equal(t, "text", anth.Content[1].Type)
 	assert.Equal(t, "42", anth.Content[1].Text)
 }
 
+func TestResponsesToAnthropic_StreamEmitsThinkingSignature(t *testing.T) {
+	state := NewResponsesEventToAnthropicState()
+	var all []AnthropicStreamEvent
+
+	appendAll := func(events []AnthropicStreamEvent) {
+		all = append(all, events...)
+	}
+
+	appendAll(ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.added",
+		OutputIndex: 0,
+		Item:        &ResponsesOutput{Type: "reasoning", ID: "rs_1"},
+	}, state))
+	appendAll(ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:         "response.reasoning_summary_text.delta",
+		OutputIndex:  0,
+		Delta:        "thinking...",
+		SummaryIndex: 0,
+	}, state))
+	// summary.done must not close the thinking block before encrypted_content arrives
+	appendAll(ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:         "response.reasoning_summary_text.done",
+		OutputIndex:  0,
+		SummaryIndex: 0,
+	}, state))
+	appendAll(ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.done",
+		OutputIndex: 0,
+		Item: &ResponsesOutput{
+			Type:             "reasoning",
+			ID:               "rs_1",
+			EncryptedContent: "enc-stream-1",
+			Status:           "completed",
+		},
+	}, state))
+
+	var sawSignature bool
+	for _, ev := range all {
+		if ev.Type == "content_block_delta" && ev.Delta != nil && ev.Delta.Type == "signature_delta" {
+			assert.Equal(t, "enc-stream-1", ev.Delta.Signature)
+			sawSignature = true
+		}
+	}
+	require.True(t, sawSignature, "expected signature_delta with encrypted_content")
+}
+
 func TestResponsesToAnthropic_Incomplete(t *testing.T) {
 	resp := &ResponsesResponse{
 		ID:     "resp_inc",
@@ -785,12 +857,26 @@ func TestStreamingReasoning(t *testing.T) {
 	assert.Equal(t, "thinking_delta", events[0].Delta.Type)
 	assert.Equal(t, "Let me think...", events[0].Delta.Thinking)
 
-	// reasoning done
+	// summary.done keeps thinking open until output_item.done (for signature)
 	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
 		Type: "response.reasoning_summary_text.done",
 	}, state)
-	require.Len(t, events, 1)
-	assert.Equal(t, "content_block_stop", events[0].Type)
+	require.Len(t, events, 0)
+
+	events = ResponsesEventToAnthropicEvents(&ResponsesStreamEvent{
+		Type:        "response.output_item.done",
+		OutputIndex: 0,
+		Item: &ResponsesOutput{
+			Type:             "reasoning",
+			EncryptedContent: "enc-rs-stream",
+			Status:           "completed",
+		},
+	}, state)
+	require.Len(t, events, 2)
+	assert.Equal(t, "content_block_delta", events[0].Type)
+	assert.Equal(t, "signature_delta", events[0].Delta.Type)
+	assert.Equal(t, "enc-rs-stream", events[0].Delta.Signature)
+	assert.Equal(t, "content_block_stop", events[1].Type)
 }
 
 func TestStreamingIncomplete(t *testing.T) {
diff --git a/backend/internal/pkg/apicompat/anthropic_to_responses.go b/backend/internal/pkg/apicompat/anthropic_to_responses.go
index e2011bee0..dfbe7b5e0 100644
--- a/backend/internal/pkg/apicompat/anthropic_to_responses.go
+++ b/backend/internal/pkg/apicompat/anthropic_to_responses.go
@@ -257,7 +257,9 @@ func anthropicUserToResponses(raw json.RawMessage) ([]ResponsesInputItem, error)
 // anthropicAssistantToResponses handles an Anthropic assistant message.
 // Text content → assistant message with output_text parts.
 // tool_use blocks → function_call items.
-// thinking blocks → ignored (OpenAI doesn't accept them as input).
+// thinking blocks with signature → reasoning items (encrypted_content) so
+// multi-turn Grok/Codex prompt cache can reuse prior reasoning prefixes.
+// thinking without signature remains ignored (not accepted as plain text input).
 func anthropicAssistantToResponses(raw json.RawMessage) ([]ResponsesInputItem, error) {
 	// Try plain string.
 	var s string
@@ -277,6 +279,25 @@ func anthropicAssistantToResponses(raw json.RawMessage) ([]ResponsesInputItem, e
 
 	var items []ResponsesInputItem
 
+	// Preserve turn order: reasoning → assistant text → tool calls. xAI/Codex
+	// multi-turn cache and tool continuations expect reasoning before the
+	// assistant message that followed it.
+	for _, b := range blocks {
+		if b.Type != "thinking" {
+			continue
+		}
+		sig := strings.TrimSpace(b.Signature)
+		// Only replay provider ciphertext. Skip GPT/Codex-style gAAAA blobs and
+		// empty placeholders — xAI returns 400 on decrypt for foreign signatures.
+		if sig == "" || strings.HasPrefix(sig, "gAAAA") {
+			continue
+		}
+		items = append(items, ResponsesInputItem{
+			Type:             "reasoning",
+			EncryptedContent: sig,
+		})
+	}
+
 	// Text content → assistant message with output_text content parts.
 	text := extractAnthropicTextFromBlocks(blocks)
 	if text != "" {
diff --git a/backend/internal/pkg/apicompat/responses_to_anthropic.go b/backend/internal/pkg/apicompat/responses_to_anthropic.go
index 20b8ac029..129fff08d 100644
--- a/backend/internal/pkg/apicompat/responses_to_anthropic.go
+++ b/backend/internal/pkg/apicompat/responses_to_anthropic.go
@@ -3,6 +3,7 @@ package apicompat
 import (
 	"encoding/json"
 	"fmt"
+	"strings"
 	"time"
 )
 
@@ -32,10 +33,14 @@ func ResponsesToAnthropic(resp *ResponsesResponse, model string) *AnthropicRespo
 					summaryText += s.Text
 				}
 			}
-			if summaryText != "" {
+			// Always surface encrypted_content as thinking.signature so Claude
+			// Code / multi-turn clients can send it back. Signature-only
+			// thinking blocks are valid when the model omits a visible summary.
+			if summaryText != "" || strings.TrimSpace(item.EncryptedContent) != "" {
 				blocks = append(blocks, AnthropicContentBlock{
-					Type:     "thinking",
-					Thinking: summaryText,
+					Type:      "thinking",
+					Thinking:  summaryText,
+					Signature: item.EncryptedContent,
 				})
 			}
 		case "message":
@@ -177,7 +182,10 @@ type ResponsesEventToAnthropicState struct {
 	CurrentToolName     string
 	CurrentToolArgs     string
 	CurrentToolHadDelta bool
-	HasToolCall         bool
+	// PendingThinkingSignature is filled from reasoning.encrypted_content and
+	// emitted as signature_delta before the thinking block is closed.
+	PendingThinkingSignature string
+	HasToolCall              bool
 
 	// OutputIndexToBlockIdx maps Responses output_index → Anthropic content block index.
 	OutputIndexToBlockIdx map[int]int
@@ -228,7 +236,10 @@ func ResponsesEventToAnthropicEvents(
 		"response.reasoning_text.delta":
 		return resToAnthHandleReasoningDelta(evt, state)
 	case "response.reasoning_summary_text.done":
-		return resToAnthHandleBlockDone(state)
+		// Keep the thinking block open until response.output_item.done.
+		// Grok/Codex attach encrypted_content on the finished reasoning item;
+		// closing early would drop signature_delta and break multi-turn cache.
+		return nil
 	// response.done 是 Realtime/WS 与项目透传路径使用的终止别名；
 	// 普通 Responses HTTP SSE 的公开终止事件仍以 response.completed 为主。
 	case "response.completed", "response.done", "response.incomplete", "response.failed":
@@ -359,6 +370,7 @@ func resToAnthHandleOutputItemAdded(evt *ResponsesStreamEvent, state *ResponsesE
 		state.OutputIndexToBlockIdx[evt.OutputIndex] = idx
 		state.ContentBlockOpen = true
 		state.CurrentBlockType = "thinking"
+		state.PendingThinkingSignature = strings.TrimSpace(evt.Item.EncryptedContent)
 
 		events = append(events, AnthropicStreamEvent{
 			Type:  "content_block_start",
@@ -542,6 +554,13 @@ func resToAnthHandleOutputItemDone(evt *ResponsesStreamEvent, state *ResponsesEv
 		return resToAnthHandleWebSearchDone(evt, state)
 	}
 
+	// Capture encrypted_content on reasoning item done (often only present here).
+	if evt.Item.Type == "reasoning" {
+		if sig := strings.TrimSpace(evt.Item.EncryptedContent); sig != "" {
+			state.PendingThinkingSignature = sig
+		}
+	}
+
 	if state.ContentBlockOpen {
 		return closeCurrentBlock(state)
 	}
@@ -663,13 +682,30 @@ func closeCurrentBlock(state *ResponsesEventToAnthropicState) []AnthropicStreamE
 		return nil
 	}
 	idx := state.ContentBlockIndex
+	var events []AnthropicStreamEvent
+	// Emit signature_delta before stop so Claude clients retain encrypted
+	// reasoning for the next turn (required for Grok multi-turn cache).
+	if state.CurrentBlockType == "thinking" {
+		if sig := strings.TrimSpace(state.PendingThinkingSignature); sig != "" {
+			events = append(events, AnthropicStreamEvent{
+				Type:  "content_block_delta",
+				Index: &idx,
+				Delta: &AnthropicDelta{
+					Type:      "signature_delta",
+					Signature: sig,
+				},
+			})
+		}
+		state.PendingThinkingSignature = ""
+	}
 	state.ContentBlockOpen = false
 	state.ContentBlockIndex++
 	state.CurrentToolName = ""
 	state.CurrentToolArgs = ""
 	state.CurrentToolHadDelta = false
-	return []AnthropicStreamEvent{{
+	events = append(events, AnthropicStreamEvent{
 		Type:  "content_block_stop",
 		Index: &idx,
-	}}
+	})
+	return events
 }
diff --git a/backend/internal/pkg/apicompat/types.go b/backend/internal/pkg/apicompat/types.go
index 54b2adf3d..6031eb41d 100644
--- a/backend/internal/pkg/apicompat/types.go
+++ b/backend/internal/pkg/apicompat/types.go
@@ -60,6 +60,9 @@ type AnthropicContentBlock struct {
 
 	// type=thinking
 	Thinking string `json:"thinking,omitempty"`
+	// Signature carries provider encrypted reasoning (e.g. xAI encrypted_content)
+	// so multi-turn Claude clients can round-trip it back on subsequent turns.
+	Signature string `json:"signature,omitempty"`
 
 	// type=image
 	Source *AnthropicImageSource `json:"source,omitempty"`
@@ -247,6 +250,9 @@ type ResponsesInputItem struct {
 	Role    string          `json:"role,omitempty"`
 	Content json.RawMessage `json:"content,omitempty"` // string or []ResponsesContentPart
 
+	// type=reasoning (multi-turn replay of encrypted reasoning)
+	EncryptedContent string `json:"encrypted_content,omitempty"`
+
 	// type=function_call
 	CallID    string `json:"call_id,omitempty"`
 	Name      string `json:"name,omitempty"`
diff --git a/backend/internal/service/openai_gateway_grok.go b/backend/internal/service/openai_gateway_grok.go
index dec30c3bd..680b34bbe 100644
--- a/backend/internal/service/openai_gateway_grok.go
+++ b/backend/internal/service/openai_gateway_grok.go
@@ -201,15 +201,125 @@ func isGrokInvalidEncryptedContentResponse(statusCode int, body []byte) bool {
 		return false
 	}
 
-	code := gjson.GetBytes(body, "code")
-	message := gjson.GetBytes(body, "error")
-	if code.Type != gjson.String || message.Type != gjson.String ||
-		!strings.EqualFold(strings.TrimSpace(code.String()), "invalid-argument") {
+	// xAI has used both flat and nested error envelopes:
+	//   {"code":"invalid-argument","error":"Could not decrypt the provided encrypted_content."}
+	//   {"error":{"message":"Could not decrypt the provided encrypted_content."}}
+	code := strings.TrimSpace(gjson.GetBytes(body, "code").String())
+	message := ""
+	errNode := gjson.GetBytes(body, "error")
+	switch {
+	case errNode.Type == gjson.String:
+		message = errNode.String()
+	case errNode.IsObject():
+		message = firstNonEmpty(errNode.Get("message").String(), errNode.Get("error").String())
+		if code == "" {
+			code = strings.TrimSpace(errNode.Get("code").String())
+		}
+	default:
+		message = gjson.GetBytes(body, "message").String()
+	}
+	normalizedMessage := strings.ToLower(strings.TrimSpace(message))
+	if normalizedMessage == "" {
 		return false
 	}
 
-	normalizedMessage := strings.ToLower(message.String())
-	return strings.Contains(normalizedMessage, "decrypt") && strings.Contains(normalizedMessage, "encrypted_content")
+	if strings.EqualFold(code, "invalid_encrypted_content") {
+		return true
+	}
+	// Keep the official xAI flat-code gate so unrelated 400s are not retried.
+	if !strings.EqualFold(code, "invalid-argument") && code != "" {
+		return false
+	}
+	// Nested OpenAI-style envelopes may omit top-level code; require decrypt text.
+	if code == "" && !strings.Contains(normalizedMessage, "decrypt") {
+		return false
+	}
+	return strings.Contains(normalizedMessage, "encrypted_content") &&
+		(strings.Contains(normalizedMessage, "decrypt") ||
+			strings.Contains(normalizedMessage, "unmodified"))
+}
+
+// requestHasGrokEncryptedReasoning reports whether the outbound Responses body
+// still carries reasoning.encrypted_content that can be stripped for retry.
+func requestHasGrokEncryptedReasoning(body []byte) bool {
+	input := gjson.GetBytes(body, "input")
+	if !input.Exists() {
+		return false
+	}
+	items := input.Array()
+	if input.IsObject() {
+		items = []gjson.Result{input}
+	}
+	for _, item := range items {
+		if strings.TrimSpace(item.Get("type").String()) != "reasoning" {
+			continue
+		}
+		enc := item.Get("encrypted_content")
+		if enc.Exists() && enc.Type != gjson.Null && strings.TrimSpace(enc.String()) != "" {
+			return true
+		}
+	}
+	return false
+}
+
+type grokEncryptedContentStripRetriedKey struct{}
+
+func markGrokEncryptedContentStripRetried(ctx context.Context) context.Context {
+	return context.WithValue(ctx, grokEncryptedContentStripRetriedKey{}, true)
+}
+
+func grokEncryptedContentStripRetried(ctx context.Context) bool {
+	v, _ := ctx.Value(grokEncryptedContentStripRetriedKey{}).(bool)
+	return v
+}
+
+// stripAnthropicThinkingSignatures removes thinking.signature from Claude
+// history so a different Grok OAuth account can accept multi-turn tool
+// continuations after decrypt failures. Returns ok=false when nothing changed.
+func stripAnthropicThinkingSignatures(body []byte) ([]byte, bool) {
+	if len(body) == 0 || !bytes.Contains(body, []byte(`"signature"`)) {
+		return body, false
+	}
+	var req map[string]any
+	if err := json.Unmarshal(body, &req); err != nil {
+		return body, false
+	}
+	messages, ok := req["messages"].([]any)
+	if !ok || len(messages) == 0 {
+		return body, false
+	}
+	changed := false
+	for _, rawMsg := range messages {
+		msg, ok := rawMsg.(map[string]any)
+		if !ok {
+			continue
+		}
+		content, ok := msg["content"].([]any)
+		if !ok {
+			continue
+		}
+		for _, rawBlock := range content {
+			block, ok := rawBlock.(map[string]any)
+			if !ok {
+				continue
+			}
+			if typ, _ := block["type"].(string); typ != "thinking" {
+				continue
+			}
+			if _, has := block["signature"]; has {
+				delete(block, "signature")
+				changed = true
+			}
+		}
+	}
+	if !changed {
+		return body, false
+	}
+	out, err := json.Marshal(req)
+	if err != nil {
+		return body, false
+	}
+	return out, true
 }
 
 func trimGrokInvalidEncryptedContentRetryBody(body []byte) ([]byte, bool, error) {
diff --git a/backend/internal/service/openai_gateway_grok_cache.go b/backend/internal/service/openai_gateway_grok_cache.go
index de52115f1..a7bb29f8e 100644
--- a/backend/internal/service/openai_gateway_grok_cache.go
+++ b/backend/internal/service/openai_gateway_grok_cache.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"regexp"
 	"strings"
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/xai"
@@ -14,12 +15,47 @@ import (
 
 const (
 	grokConversationIDHeader         = "X-Grok-Conv-Id"
+	claudeCodeSessionHeader          = "X-Claude-Code-Session-Id"
 	grokClientToolCacheOptInHeader   = "X-Sub2API-Grok-Client-Tool-Cache"
 	grokFreeCacheNativeToolsJSON     = `[{"type":"web_search"},{"type":"x_search"}]`
 	grokFreeCacheDisabledToolChoice  = "none"
 	grokClientToolCacheOptInExtraKey = "grok_client_tool_cache_enabled"
 )
 
+// Claude Code metadata.user_id often ends with _session_<uuid>.
+var claudeCodeSessionSuffixPattern = regexp.MustCompile(`_session_([a-f0-9-]+)$`)
+
+// extractClaudeCodeSessionID resolves the Claude Code conversation id from
+// headers or Anthropic/OpenAI-compatible payload metadata.
+func extractClaudeCodeSessionID(c *gin.Context, body []byte) string {
+	if c != nil {
+		if seed := strings.TrimSpace(c.GetHeader(claudeCodeSessionHeader)); seed != "" {
+			return seed
+		}
+	}
+	return extractClaudeCodeSessionIDFromPayload(body)
+}
+
+func extractClaudeCodeSessionIDFromPayload(body []byte) string {
+	if len(body) == 0 {
+		return ""
+	}
+	userID := strings.TrimSpace(gjson.GetBytes(body, "metadata.user_id").String())
+	if userID == "" {
+		return ""
+	}
+	if matches := claudeCodeSessionSuffixPattern.FindStringSubmatch(userID); len(matches) >= 2 {
+		return matches[1]
+	}
+	// Claude Code may embed JSON: {"session_id":"..."}
+	if len(userID) > 0 && userID[0] == '{' {
+		if sid := strings.TrimSpace(gjson.Get(userID, "session_id").String()); sid != "" {
+			return sid
+		}
+	}
+	return ""
+}
+
 // resolveGrokCacheIdentity derives one stable, tenant-isolated routing identity
 // for xAI's server-side prompt cache. The returned value is safe to expose to
 // the upstream: it never contains the client's raw session identifier.
@@ -68,7 +104,13 @@ func resolveGrokCacheIdentity(c *gin.Context, body []byte, explicitKey, upstream
 func explicitGrokCacheSeed(c *gin.Context, body []byte, explicitKey string) string {
 	seed := ""
 	if c != nil {
-		seed = strings.TrimSpace(c.GetHeader("session_id"))
+		// Claude Code session is the most stable multi-turn identity for
+		// /v1/messages → Grok bridges. Prefer it over generic session headers
+		// so prompt cache routing matches CPA behavior.
+		seed = extractClaudeCodeSessionID(c, body)
+		if seed == "" {
+			seed = strings.TrimSpace(c.GetHeader("session_id"))
+		}
 		if seed == "" {
 			seed = strings.TrimSpace(c.GetHeader("conversation_id"))
 		}
@@ -76,6 +118,9 @@ func explicitGrokCacheSeed(c *gin.Context, body []byte, explicitKey string) stri
 			seed = strings.TrimSpace(c.GetHeader(grokConversationIDHeader))
 		}
 	}
+	if seed == "" && len(body) > 0 {
+		seed = extractClaudeCodeSessionIDFromPayload(body)
+	}
 	if seed == "" && len(body) > 0 {
 		seed = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
 	}
diff --git a/backend/internal/service/openai_gateway_grok_cache_test.go b/backend/internal/service/openai_gateway_grok_cache_test.go
index 034ba4843..2255f4a2b 100644
--- a/backend/internal/service/openai_gateway_grok_cache_test.go
+++ b/backend/internal/service/openai_gateway_grok_cache_test.go
@@ -140,6 +140,28 @@ func TestResolveGrokCacheIdentityExplicitHeaderPriority(t *testing.T) {
 	require.Equal(t, want, got)
 }
 
+func TestResolveGrokCacheIdentityPrefersClaudeCodeSession(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	c := newGrokCacheTestContext(701)
+	c.Request.Header.Set(claudeCodeSessionHeader, "cc-session-abc")
+	c.Request.Header.Set("session_id", "session-key")
+	body1 := []byte(`{"model":"grok","input":"turn-1"}`)
+	body2 := []byte(`{"model":"grok","input":"turn-2-different-user-text"}`)
+
+	first := resolveGrokCacheIdentity(c, body1, "", "grok-4.5")
+	second := resolveGrokCacheIdentity(c, body2, "unrelated-explicit", "grok-4.5")
+	require.NotEmpty(t, first)
+	require.Equal(t, first, second, "same Claude Code session must keep stable Grok cache identity across turns")
+
+	// metadata.user_id JSON form used by Claude Code clients
+	metaBody := []byte(`{"model":"grok","metadata":{"user_id":"{\"session_id\":\"meta-session-xyz\"}"},"input":"hi"}`)
+	metaOnly := newGrokCacheTestContext(702)
+	metaID := resolveGrokCacheIdentity(metaOnly, metaBody, "", "grok-4.5")
+	require.NotEmpty(t, metaID)
+	metaBody2 := []byte(`{"model":"grok","metadata":{"user_id":"{\"session_id\":\"meta-session-xyz\"}"},"input":"later turn"}`)
+	require.Equal(t, metaID, resolveGrokCacheIdentity(metaOnly, metaBody2, "", "grok-4.5"))
+}
+
 func TestResolveGrokCacheIdentityFailsClosedWithoutAPIKeyContext(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	c := newGrokCacheTestContext(0)
diff --git a/backend/internal/service/openai_gateway_grok_test.go b/backend/internal/service/openai_gateway_grok_test.go
index 4bc6eb9b0..235098d12 100644
--- a/backend/internal/service/openai_gateway_grok_test.go
+++ b/backend/internal/service/openai_gateway_grok_test.go
@@ -1654,11 +1654,6 @@ func TestForwardGrokResponsesInvalidEncryptedContentRecoveryDoesNotOvermatch(t *
 			requestBody:  `{"model":"grok","input":[{"type":"reasoning","encrypted_content":"cipher"}],"stream":false}`,
 			responseBody: `{"code":"invalid-argument","error":"The provided encrypted_content is invalid."}`,
 		},
-		{
-			name:         "nested OpenAI error shape",
-			requestBody:  `{"model":"grok","input":[{"type":"reasoning","encrypted_content":"cipher"}],"stream":false}`,
-			responseBody: `{"code":"invalid-argument","error":{"message":"Could not decrypt the provided encrypted_content."}}`,
-		},
 		{
 			name:         "request has no encrypted reasoning",
 			requestBody:  `{"model":"grok","input":[{"type":"message","role":"user","content":"hi"}],"stream":false}`,
@@ -1697,6 +1692,44 @@ func TestForwardGrokResponsesInvalidEncryptedContentRecoveryDoesNotOvermatch(t *
 	}
 }
 
+func TestForwardGrokResponsesInvalidEncryptedContentRecoveryNestedErrorShape(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	body := []byte(`{"model":"grok","input":[{"type":"reasoning","encrypted_content":"cipher"},{"type":"message","role":"user","content":"hi"}],"stream":false}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
+
+	account := &Account{
+		ID:          4538,
+		Name:        "grok-api-key",
+		Platform:    PlatformGrok,
+		Type:        AccountTypeAPIKey,
+		Concurrency: 1,
+		Credentials: map[string]any{"api_key": "token", "base_url": "https://api.x.ai/v1"},
+	}
+	upstream := &httpUpstreamRecorder{responses: []*http.Response{
+		{
+			StatusCode: http.StatusBadRequest,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"code":"invalid-argument","error":{"message":"Could not decrypt the provided encrypted_content."}}`)),
+		},
+		{
+			StatusCode: http.StatusOK,
+			Header:     http.Header{"Content-Type": []string{"application/json"}},
+			Body:       io.NopCloser(strings.NewReader(`{"id":"resp_ok","output":[],"usage":{"input_tokens":1,"output_tokens":1}}`)),
+		},
+	}}
+	svc := &OpenAIGatewayService{httpUpstream: upstream}
+
+	result, err := svc.forwardGrokResponses(context.Background(), c, account, body, "grok", false, time.Now())
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Len(t, upstream.requests, 2)
+	require.True(t, gjson.GetBytes(upstream.bodies[0], "input.0.encrypted_content").Exists())
+	require.False(t, gjson.GetBytes(upstream.bodies[1], "input.0.encrypted_content").Exists())
+}
+
 func TestForwardGrokResponsesInvalidEncryptedContentRetryFailureIsTerminal(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
diff --git a/backend/internal/service/openai_gateway_messages.go b/backend/internal/service/openai_gateway_messages.go
index db9e95a8f..d21cba5c5 100644
--- a/backend/internal/service/openai_gateway_messages.go
+++ b/backend/internal/service/openai_gateway_messages.go
@@ -1,10 +1,12 @@
 package service
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"strings"
 	"sync/atomic"
@@ -60,6 +62,18 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 	anthropicDigestChain := ""
 	anthropicMatchedDigestChain := ""
 	compatPromptCacheInjected := false
+	// Grok is outside the gpt-5/codex compat injector, but Claude Code still
+	// carries a stable session id. Prefer that as the Grok prompt-cache seed so
+	// multi-turn /v1/messages traffic can hit xAI's server-side cache.
+	if promptCacheKey == "" && account.Platform == PlatformGrok {
+		if sessionSeed := extractClaudeCodeSessionID(c, body); sessionSeed != "" {
+			promptCacheKey = sessionSeed
+			compatPromptCacheInjected = true
+		} else if sessionSeed := promptCacheKeyFromAnthropicMetadataSession(&anthropicReq); sessionSeed != "" {
+			promptCacheKey = sessionSeed
+			compatPromptCacheInjected = true
+		}
+	}
 	if promptCacheKey == "" && shouldAutoInjectPromptCacheKeyForCompat(upstreamModel) {
 		promptCacheKey = promptCacheKeyFromAnthropicMetadataSession(&anthropicReq)
 		if promptCacheKey == "" {
@@ -319,9 +333,56 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 	if account.Proxy != nil {
 		proxyURL = account.Proxy.URL()
 	}
-	resp, err := s.httpUpstream.Do(upstreamReq, proxyURL, account.ID, account.Concurrency)
-	if err != nil {
-		return nil, s.handleOpenAIUpstreamTransportError(ctx, c, account, err, false)
+	// Grok may reject encrypted reasoning replayed under a different OAuth
+	// account/cache identity. Match forwardGrokResponses: one strip+retry before
+	// treating the 400 as a hard failure / failover trigger.
+	var resp *http.Response
+	for attempt := 0; ; attempt++ {
+		if attempt > 0 {
+			if account.Platform != PlatformGrok {
+				break
+			}
+			upstreamCtxRetry, releaseRetry := detachUpstreamContext(ctx)
+			upstreamReq, err = buildGrokResponsesRequest(upstreamCtxRetry, c, account, responsesBody, token, grokCacheIdentity, s.cfg)
+			releaseRetry()
+			if err != nil {
+				return nil, fmt.Errorf("build grok retry request: %w", err)
+			}
+		}
+		resp, err = s.httpUpstream.Do(upstreamReq, proxyURL, account.ID, account.Concurrency)
+		if err != nil {
+			return nil, s.handleOpenAIUpstreamTransportError(ctx, c, account, err, false)
+		}
+		if account.Platform != PlatformGrok || attempt > 0 || resp.StatusCode != http.StatusBadRequest {
+			break
+		}
+		respBody := s.readUpstreamErrorBody(resp)
+		if resp.Body != nil {
+			_ = resp.Body.Close()
+		}
+		// Prefer explicit decrypt errors; also strip once on any 400 when the
+		// outbound body still carries reasoning.encrypted_content (account
+		// switch often returns opaque "Upstream error: 400").
+		shouldStrip := isGrokInvalidEncryptedContentResponse(resp.StatusCode, respBody) ||
+			requestHasGrokEncryptedReasoning(responsesBody)
+		if !shouldStrip {
+			resp.Body = io.NopCloser(bytes.NewReader(respBody))
+			break
+		}
+		retryBody, changed, trimErr := trimGrokInvalidEncryptedContentRetryBody(responsesBody)
+		if trimErr != nil {
+			return nil, fmt.Errorf("prepare Grok invalid encrypted_content retry: %w", trimErr)
+		}
+		if !changed {
+			resp.Body = io.NopCloser(bytes.NewReader(respBody))
+			break
+		}
+		responsesBody = retryBody
+		logger.L().Info("openai messages: retrying after stripping invalid Grok encrypted_content",
+			zap.Int64("account_id", account.ID),
+			zap.Bool("cache_identity_present", strings.TrimSpace(grokCacheIdentity) != ""),
+			zap.String("upstream_error_preview", truncateOpenAIWSLogValue(string(respBody), 240)),
+		)
 	}
 	defer func() { _ = resp.Body.Close() }()
 
@@ -348,6 +409,19 @@ func (s *OpenAIGatewayService) ForwardAsAnthropic(
 			)
 			return s.ForwardAsAnthropic(ctx, c, account, body, promptCacheKey, defaultMappedModel)
 		}
+		// Grok account-switched history often fails decrypt; strip encrypted
+		// reasoning once at the client-body level so failover accounts can accept
+		// the multi-turn tool continuation instead of cascading 400s.
+		if account.Platform == PlatformGrok &&
+			isGrokInvalidEncryptedContentResponse(resp.StatusCode, respBody) &&
+			!grokEncryptedContentStripRetried(ctx) {
+			if strippedBody, ok := stripAnthropicThinkingSignatures(body); ok {
+				logger.L().Info("openai messages: stripping thinking signatures for Grok failover retry",
+					zap.Int64("account_id", account.ID),
+				)
+				return s.ForwardAsAnthropic(markGrokEncryptedContentStripRetried(ctx), c, account, strippedBody, promptCacheKey, defaultMappedModel)
+			}
+		}
 		if foErr := s.failoverOpenAIUpstreamHTTPError(ctx, c, account, resp, respBody, upstreamMsg, upstreamModel); foErr != nil {
 			return nil, foErr
 		}