diff --git a/internal/api/openai.go b/internal/api/openai.go
index df8a551..a7b1112 100644
--- a/internal/api/openai.go
+++ b/internal/api/openai.go
@@ -245,11 +245,18 @@ func chatMessageContent(message chatMessage) (aistudio.Content, error) {
 		if !json.Valid(arguments) {
 			return aistudio.Content{}, fmt.Errorf("tool call %q arguments must be JSON", call.Function.Name)
 		}
+		signature := call.ExtraContent.Google.ThoughtSignature
+		if signature == "" {
+			// A client that does not echo extra_content (every ordinary
+			// OpenAI client, litellm included) still gets its round trip:
+			// the signature was kept when this call was emitted.
+			signature = thoughtSignatureFor(call.ID)
+		}
 		parts = append(parts, aistudio.Part{FunctionCall: &aistudio.FunctionCall{
 			ID:               call.ID,
 			Name:             call.Function.Name,
 			Arguments:        arguments,
-			ThoughtSignature: call.ExtraContent.Google.ThoughtSignature,
+			ThoughtSignature: signature,
 		}})
 	}
 	return aistudio.Content{Role: role, Parts: parts}, nil
@@ -684,6 +691,7 @@ func (s *server) streamChatCompletion(w http.ResponseWriter, r *http.Request, re
 				return nil
 			}
 			call := event.ToolCall
+			rememberThoughtSignature(call.ID, call.ThoughtSignature)
 			toolCall := map[string]any{
 				"index": toolIndex,
 				"id":    call.ID,
@@ -804,6 +812,7 @@ func openAIFinishReason(reason string, hasTools bool) string {
 }
 
 func openAIToolCallOutput(calls []aistudio.FunctionCall) []map[string]any {
+	rememberThoughtSignatures(calls)
 	output := make([]map[string]any, 0, len(calls))
 	for _, call := range calls {
 		item := map[string]any{
diff --git a/internal/api/thoughtsig.go b/internal/api/thoughtsig.go
new file mode 100644
index 0000000..3c9c32b
--- /dev/null
+++ b/internal/api/thoughtsig.go
@@ -0,0 +1,75 @@
+package api
+
+// Gemini 3 refuses a functionCall that comes back in history without the
+// thought signature it was issued with:
+//
+//	AI Studio GenerateContent 返回 HTTP 400、协议错误码 3:
+//	[original: beyond::dependency::INVALID_ARGUMENT]
+//	Function call is missing a thought signature. (qos=CRITICAL_PLUS)
+//
+// The signature does reach the client - this proxy hands it over as
+// extra_content.google.thought_signature on every tool call - but no ordinary
+// OpenAI client (OpenAI SDKs, litellm, most agent frameworks) echoes unknown
+// fields back. By the next turn of a tool round trip the signature is gone,
+// so every agent loop dies on the second request while both tool-call checks
+// pass.
+//
+// Keep it server-side instead, keyed by the tool call id the client saw, and
+// put it back when the assistant message comes home without one. A client that
+// does echo extra_content still wins: chatMessageContent only consults this
+// store when the field is missing, and a call this process never emitted still
+// falls through to the skip_thought_signature_validator placeholder in
+// internal/aistudio/request.go.
+
+import (
+	"sync"
+
+	"github.com/Mag1cFall/AIStudio2API/internal/aistudio"
+)
+
+// thoughtSignatureLimit bounds the store. Tool call ids are unique per call
+// and a client only ever echoes back ids from the recent past, so a plain FIFO
+// cap is enough - it keeps a long-lived process from growing one entry per
+// tool call forever.
+const thoughtSignatureLimit = 1024
+
+var thoughtSignatures = struct {
+	mu      sync.Mutex
+	entries map[string]string
+	order   []string
+}{entries: make(map[string]string)}
+
+func rememberThoughtSignature(id, signature string) {
+	if id == "" || signature == "" {
+		return
+	}
+	thoughtSignatures.mu.Lock()
+	defer thoughtSignatures.mu.Unlock()
+	if _, seen := thoughtSignatures.entries[id]; !seen {
+		thoughtSignatures.order = append(thoughtSignatures.order, id)
+	}
+	thoughtSignatures.entries[id] = signature
+	for len(thoughtSignatures.order) > thoughtSignatureLimit {
+		oldest := thoughtSignatures.order[0]
+		thoughtSignatures.order = thoughtSignatures.order[1:]
+		delete(thoughtSignatures.entries, oldest)
+	}
+}
+
+func thoughtSignatureFor(id string) string {
+	if id == "" {
+		return ""
+	}
+	thoughtSignatures.mu.Lock()
+	defer thoughtSignatures.mu.Unlock()
+	return thoughtSignatures.entries[id]
+}
+
+// rememberThoughtSignatures stores every signed call of one model response.
+// Calls the model did not sign are skipped, so a parallel response can leave
+// some ids unknown - that is the model's choice, not something to invent here.
+func rememberThoughtSignatures(calls []aistudio.FunctionCall) {
+	for _, call := range calls {
+		rememberThoughtSignature(call.ID, call.ThoughtSignature)
+	}
+}
diff --git a/internal/api/thoughtsig_test.go b/internal/api/thoughtsig_test.go
new file mode 100644
index 0000000..3dc330c
--- /dev/null
+++ b/internal/api/thoughtsig_test.go
@@ -0,0 +1,43 @@
+package api
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestChatMessageContentRestoresStoredThoughtSignature(t *testing.T) {
+	rememberThoughtSignature("call_sig_1", "sig-abc")
+
+	var message chatMessage
+	raw := `{"role":"assistant","content":null,"tool_calls":[{"id":"call_sig_1","type":"function","function":{"name":"get_weather","arguments":"{\"city\":\"Berlin\"}"}}]}`
+	if err := json.Unmarshal([]byte(raw), &message); err != nil {
+		t.Fatal(err)
+	}
+	content, err := chatMessageContent(message)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := content.Parts[0].FunctionCall.ThoughtSignature; got != "sig-abc" {
+		t.Fatalf("stored signature not restored: got %q", got)
+	}
+
+	message.ToolCalls[0].ExtraContent.Google.ThoughtSignature = "sig-from-client"
+	content, err = chatMessageContent(message)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := content.Parts[0].FunctionCall.ThoughtSignature; got != "sig-from-client" {
+		t.Fatalf("client-supplied signature must win: got %q", got)
+	}
+}
+
+func TestThoughtSignatureStoreIsBounded(t *testing.T) {
+	for i := 0; i < thoughtSignatureLimit+10; i++ {
+		rememberThoughtSignature(string(rune('a'+i%26))+string(rune(i)), "s")
+	}
+	thoughtSignatures.mu.Lock()
+	defer thoughtSignatures.mu.Unlock()
+	if len(thoughtSignatures.entries) > thoughtSignatureLimit {
+		t.Fatalf("store grew past its limit: %d", len(thoughtSignatures.entries))
+	}
+}