diff --git a/pkg/agent/loop.go b/pkg/agent/loop.go
index 570ff6cd5..0f794386d 100644
--- a/pkg/agent/loop.go
+++ b/pkg/agent/loop.go
@@ -600,21 +600,24 @@ func (al *AgentLoop) runLLMIteration(ctx context.Context, agent *AgentInstance,
 		}
 		for _, tc := range normalizedToolCalls {
 			argumentsJSON, _ := json.Marshal(tc.Arguments)
+			// Copy ExtraContent to ensure thought_signature is persisted for Gemini 3
+			extraContent := tc.ExtraContent
 			thoughtSignature := ""
 			if tc.Function != nil {
 				thoughtSignature = tc.Function.ThoughtSignature
 			}
 
 			assistantMsg.ToolCalls = append(assistantMsg.ToolCalls, providers.ToolCall{
-				ID:        tc.ID,
-				Type:      "function",
-				Name:      tc.Name,
-				Arguments: tc.Arguments,
+				ID:   tc.ID,
+				Type: "function",
+				Name: tc.Name,
 				Function: &providers.FunctionCall{
 					Name:             tc.Name,
 					Arguments:        string(argumentsJSON),
 					ThoughtSignature: thoughtSignature,
 				},
+				ExtraContent:     extraContent,
+				ThoughtSignature: thoughtSignature,
 			})
 		}
 		messages = append(messages, assistantMsg)
diff --git a/pkg/providers/openai_compat/provider.go b/pkg/providers/openai_compat/provider.go
index d894d98ce..6bc43a470 100644
--- a/pkg/providers/openai_compat/provider.go
+++ b/pkg/providers/openai_compat/provider.go
@@ -22,6 +22,8 @@ type UsageInfo = protocoltypes.UsageInfo
 type Message = protocoltypes.Message
 type ToolDefinition = protocoltypes.ToolDefinition
 type ToolFunctionDefinition = protocoltypes.ToolFunctionDefinition
+type ExtraContent = protocoltypes.ExtraContent
+type GoogleExtra = protocoltypes.GoogleExtra
 
 type Provider struct {
 	apiKey         string
@@ -145,6 +147,11 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 						Name      string `json:"name"`
 						Arguments string `json:"arguments"`
 					} `json:"function"`
+					ExtraContent *struct {
+						Google *struct {
+							ThoughtSignature string `json:"thought_signature"`
+						} `json:"google"`
+					} `json:"extra_content"`
 				} `json:"tool_calls"`
 			} `json:"message"`
 			FinishReason string `json:"finish_reason"`
@@ -169,6 +176,12 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 		arguments := make(map[string]interface{})
 		name := ""
 
+		// Extract thought_signature from Gemini/Google-specific extra content
+		thoughtSignature := ""
+		if tc.ExtraContent != nil && tc.ExtraContent.Google != nil {
+			thoughtSignature = tc.ExtraContent.Google.ThoughtSignature
+		}
+
 		if tc.Function != nil {
 			name = tc.Function.Name
 			if tc.Function.Arguments != "" {
@@ -179,11 +192,23 @@ func parseResponse(body []byte) (*LLMResponse, error) {
 			}
 		}
 
-		toolCalls = append(toolCalls, ToolCall{
-			ID:        tc.ID,
-			Name:      name,
-			Arguments: arguments,
-		})
+		// Build ToolCall with ExtraContent for Gemini 3 thought_signature persistence
+		toolCall := ToolCall{
+			ID:               tc.ID,
+			Name:             name,
+			Arguments:        arguments,
+			ThoughtSignature: thoughtSignature,
+		}
+
+		if thoughtSignature != "" {
+			toolCall.ExtraContent = &ExtraContent{
+				Google: &GoogleExtra{
+					ThoughtSignature: thoughtSignature,
+				},
+			}
+		}
+
+		toolCalls = append(toolCalls, toolCall)
 	}
 
 	return &LLMResponse{
diff --git a/pkg/providers/protocoltypes/types.go b/pkg/providers/protocoltypes/types.go
index 53ebaee53..b7e7062b9 100644
--- a/pkg/providers/protocoltypes/types.go
+++ b/pkg/providers/protocoltypes/types.go
@@ -1,11 +1,21 @@
 package protocoltypes
 
 type ToolCall struct {
-	ID        string                 `json:"id"`
-	Type      string                 `json:"type,omitempty"`
-	Function  *FunctionCall          `json:"function,omitempty"`
-	Name      string                 `json:"name,omitempty"`
-	Arguments map[string]interface{} `json:"arguments,omitempty"`
+	ID               string                 `json:"id"`
+	Type             string                 `json:"type,omitempty"`
+	Function         *FunctionCall          `json:"function,omitempty"`
+	Name             string                 `json:"name,omitempty"`
+	Arguments        map[string]interface{} `json:"arguments,omitempty"`
+	ThoughtSignature string                 `json:"-"` // Internal use only
+	ExtraContent     *ExtraContent          `json:"extra_content,omitempty"`
+}
+
+type ExtraContent struct {
+	Google *GoogleExtra `json:"google,omitempty"`
+}
+
+type GoogleExtra struct {
+	ThoughtSignature string `json:"thought_signature,omitempty"`
 }
 
 type FunctionCall struct {
diff --git a/pkg/providers/types.go b/pkg/providers/types.go
index c4a9de58a..e783e6348 100644
--- a/pkg/providers/types.go
+++ b/pkg/providers/types.go
@@ -14,6 +14,8 @@ type UsageInfo = protocoltypes.UsageInfo
 type Message = protocoltypes.Message
 type ToolDefinition = protocoltypes.ToolDefinition
 type ToolFunctionDefinition = protocoltypes.ToolFunctionDefinition
+type ExtraContent = protocoltypes.ExtraContent
+type GoogleExtra = protocoltypes.GoogleExtra
 
 type LLMProvider interface {
 	Chat(ctx context.Context, messages []Message, tools []ToolDefinition, model string, options map[string]interface{}) (*LLMResponse, error)