diff --git a/internal/cache/signature_cache.go b/internal/cache/signature_cache.go
index dee1b13b..ee8ad0b2 100644
--- a/internal/cache/signature_cache.go
+++ b/internal/cache/signature_cache.go
@@ -3,6 +3,7 @@ package cache
 import (
 	"crypto/sha256"
 	"encoding/hex"
+	"fmt"
 	"sync"
 	"time"
 )
@@ -94,7 +95,7 @@ func purgeExpiredSessions() {
 
 // CacheSignature stores a thinking signature for a given session and text.
 // Used for Claude models that require signed thinking blocks in multi-turn conversations.
-func CacheSignature(sessionID, text, signature string) {
+func CacheSignature(modelName, sessionID, text, signature string) {
 	if sessionID == "" || text == "" || signature == "" {
 		return
 	}
@@ -102,7 +103,7 @@ func CacheSignature(sessionID, text, signature string) {
 		return
 	}
 
-	sc := getOrCreateSession(sessionID)
+	sc := getOrCreateSession(fmt.Sprintf("%s#%s", modelName, sessionID))
 	textHash := hashText(text)
 
 	sc.mu.Lock()
@@ -116,12 +117,12 @@ func CacheSignature(sessionID, text, signature string) {
 
 // GetCachedSignature retrieves a cached signature for a given session and text.
 // Returns empty string if not found or expired.
-func GetCachedSignature(sessionID, text string) string {
+func GetCachedSignature(modelName, sessionID, text string) string {
 	if sessionID == "" || text == "" {
 		return ""
 	}
 
-	val, ok := signatureCache.Load(sessionID)
+	val, ok := signatureCache.Load(fmt.Sprintf("%s#%s", modelName, sessionID))
 	if !ok {
 		return ""
 	}
diff --git a/internal/translator/antigravity/claude/antigravity_claude_request.go b/internal/translator/antigravity/claude/antigravity_claude_request.go
index 87772d8c..5b6ffe22 100644
--- a/internal/translator/antigravity/claude/antigravity_claude_request.go
+++ b/internal/translator/antigravity/claude/antigravity_claude_request.go
@@ -69,6 +69,7 @@ func deriveSessionID(rawJSON []byte) string {
 // Returns:
 //   - []byte: The transformed request data in Gemini CLI API format
 func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _ bool) []byte {
+	enableThoughtTranslate := true
 	rawJSON := bytes.Clone(inputRawJSON)
 
 	// Derive session ID for signature caching
@@ -132,27 +133,34 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 					if contentTypeResult.Type == gjson.String && contentTypeResult.String() == "thinking" {
 						// Use GetThinkingText to handle wrapped thinking objects
 						thinkingText := thinking.GetThinkingText(contentResult)
-						// signatureResult := contentResult.Get("signature")
-						// clientSignature := ""
-						// if signatureResult.Exists() && signatureResult.String() != "" {
-						// 	clientSignature = signatureResult.String()
-						// }
 
 						// Always try cached signature first (more reliable than client-provided)
 						// Client may send stale or invalid signatures from different sessions
 						signature := ""
 						if sessionID != "" && thinkingText != "" {
-							if cachedSig := cache.GetCachedSignature(sessionID, thinkingText); cachedSig != "" {
+							if cachedSig := cache.GetCachedSignature(modelName, sessionID, thinkingText); cachedSig != "" {
 								signature = cachedSig
 								// log.Debugf("Using cached signature for thinking block")
 							}
 						}
 
-						// NOTE: We do NOT fallback to client signature anymore.
-						// Client signatures from Claude models are incompatible with Antigravity/Gemini API.
-						// When switching between models (e.g., Claude Opus -> Gemini Flash), the Claude
-						// signatures will cause "Corrupted thought signature" errors.
-						// If we have no cached signature, the thinking block will be skipped below.
+						// Fallback to client signature only if cache miss and client signature is valid
+						if signature == "" {
+							signatureResult := contentResult.Get("signature")
+							clientSignature := ""
+							if signatureResult.Exists() && signatureResult.String() != "" {
+								arrayClientSignatures := strings.SplitN(signatureResult.String(), "#", 2)
+								if len(arrayClientSignatures) == 2 {
+									if modelName == arrayClientSignatures[0] {
+										clientSignature = arrayClientSignatures[1]
+									}
+								}
+							}
+							if cache.HasValidSignature(clientSignature) {
+								signature = clientSignature
+							}
+							// log.Debugf("Using client-provided signature for thinking block")
+						}
 
 						// Store for subsequent tool_use in the same message
 						if cache.HasValidSignature(signature) {
@@ -167,6 +175,7 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 						// Converting to text would break this requirement
 						if isUnsigned {
 							// log.Debugf("Dropping unsigned thinking block (no valid signature)")
+							enableThoughtTranslate = false
 							continue
 						}
 
@@ -394,7 +403,7 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 	}
 
 	// Map Anthropic thinking -> Gemini thinkingBudget/include_thoughts when type==enabled
-	if t := gjson.GetBytes(rawJSON, "thinking"); t.Exists() && t.IsObject() {
+	if t := gjson.GetBytes(rawJSON, "thinking"); enableThoughtTranslate && t.Exists() && t.IsObject() {
 		if t.Get("type").String() == "enabled" {
 			if b := t.Get("budget_tokens"); b.Exists() && b.Type == gjson.Number {
 				budget := int(b.Int())
diff --git a/internal/translator/antigravity/claude/antigravity_claude_response.go b/internal/translator/antigravity/claude/antigravity_claude_response.go
index 1672a835..c32918d6 100644
--- a/internal/translator/antigravity/claude/antigravity_claude_response.go
+++ b/internal/translator/antigravity/claude/antigravity_claude_response.go
@@ -73,6 +73,7 @@ func ConvertAntigravityResponseToClaude(_ context.Context, _ string, originalReq
 			SessionID:        deriveSessionID(originalRequestRawJSON),
 		}
 	}
+	modelName := gjson.GetBytes(requestRawJSON, "model").String()
 
 	params := (*param).(*Params)
 
@@ -139,13 +140,13 @@ func ConvertAntigravityResponseToClaude(_ context.Context, _ string, originalReq
 						// log.Debug("Branch: signature_delta")
 
 						if params.SessionID != "" && params.CurrentThinkingText.Len() > 0 {
-							cache.CacheSignature(params.SessionID, params.CurrentThinkingText.String(), thoughtSignature.String())
+							cache.CacheSignature(modelName, params.SessionID, params.CurrentThinkingText.String(), thoughtSignature.String())
 							// log.Debugf("Cached signature for thinking block (sessionID=%s, textLen=%d)", params.SessionID, params.CurrentThinkingText.Len())
 							params.CurrentThinkingText.Reset()
 						}
 
 						output = output + "event: content_block_delta\n"
-						data, _ := sjson.Set(fmt.Sprintf(`{"type":"content_block_delta","index":%d,"delta":{"type":"signature_delta","signature":""}}`, params.ResponseIndex), "delta.signature", thoughtSignature.String())
+						data, _ := sjson.Set(fmt.Sprintf(`{"type":"content_block_delta","index":%d,"delta":{"type":"signature_delta","signature":""}}`, params.ResponseIndex), "delta.signature", fmt.Sprintf("%s#%s", modelName, thoughtSignature.String()))
 						output = output + fmt.Sprintf("data: %s\n\n\n", data)
 						params.HasContent = true
 					} else if params.ResponseType == 2 { // Continue existing thinking block if already in thinking state
@@ -372,7 +373,7 @@ func resolveStopReason(params *Params) string {
 //   - string: A Claude-compatible JSON response.
 func ConvertAntigravityResponseToClaudeNonStream(_ context.Context, _ string, originalRequestRawJSON, requestRawJSON, rawJSON []byte, _ *any) string {
 	_ = originalRequestRawJSON
-	_ = requestRawJSON
+	modelName := gjson.GetBytes(requestRawJSON, "model").String()
 
 	root := gjson.ParseBytes(rawJSON)
 	promptTokens := root.Get("response.usageMetadata.promptTokenCount").Int()
@@ -437,7 +438,7 @@ func ConvertAntigravityResponseToClaudeNonStream(_ context.Context, _ string, or
 		block := `{"type":"thinking","thinking":""}`
 		block, _ = sjson.Set(block, "thinking", thinkingBuilder.String())
 		if thinkingSignature != "" {
-			block, _ = sjson.Set(block, "signature", thinkingSignature)
+			block, _ = sjson.Set(block, "signature", fmt.Sprintf("%s#%s", modelName, thinkingSignature))
 		}
 		responseJSON, _ = sjson.SetRaw(responseJSON, "content.-1", block)
 		thinkingBuilder.Reset()