diff --git a/go/adk/pkg/models/base.go b/go/adk/pkg/models/base.go
index ea22a3d639..b4bddc45e7 100644
--- a/go/adk/pkg/models/base.go
+++ b/go/adk/pkg/models/base.go
@@ -116,6 +116,19 @@ func (t *headerTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	return t.base.RoundTrip(req)
 }
 
+// nonNilFunctionCallArgs returns args, substituting an empty map for nil.
+// A no-argument FunctionCall reloads from the session store with nil Args
+// (omitempty drops the empty map), and some providers reject null or "null".
+//
+// TODO: remove once the pinned go-genai fixes
+// https://github.com/googleapis/go-genai/issues/920.
+func nonNilFunctionCallArgs(args map[string]any) map[string]any {
+	if args == nil {
+		return map[string]any{}
+	}
+	return args
+}
+
 // parametersJsonSchemaToMap converts a genai.FunctionDeclaration.ParametersJsonSchema value
 // to map[string]any. ParametersJsonSchema is typed as `any` and can hold:
 //   - map[string]any (rare — only if someone constructs it manually)
diff --git a/go/adk/pkg/models/bedrock.go b/go/adk/pkg/models/bedrock.go
index e7d8701753..049ff64a52 100644
--- a/go/adk/pkg/models/bedrock.go
+++ b/go/adk/pkg/models/bedrock.go
@@ -675,22 +675,10 @@ func convertGenaiContentsToBedrockMessages(contents []*genai.Content, nameMap ma
 				if sanitized, ok := nameMap[callName]; ok {
 					callName = sanitized
 				}
-				// Bedrock requires toolUse.input to be a JSON object. A tool call
-				// with no arguments arrives here with a nil Args map (genai's
-				// FunctionCall.Args is `json:"args,omitempty"`, so an empty map is
-				// dropped when the event is persisted to the session store and
-				// reloaded as nil). NewLazyDocument(nil) serializes to `null`, which
-				// Bedrock rejects with "ValidationException: Malformed input request"
-				// ("The value at messages.N.content.M.toolUse.input is empty").
-				// Coerce nil to an empty object so no-argument tool calls round-trip.
-				args := part.FunctionCall.Args
-				if args == nil {
-					args = map[string]any{}
-				}
 				toolUse := types.ToolUseBlock{
 					ToolUseId: aws.String(sanitizeBedrockToolID(part.FunctionCall.ID, idMap, &idCounter)),
 					Name:      aws.String(callName),
-					Input:     document.NewLazyDocument(args),
+					Input:     document.NewLazyDocument(nonNilFunctionCallArgs(part.FunctionCall.Args)),
 				}
 				contentBlocks = append(contentBlocks, &types.ContentBlockMemberToolUse{
 					Value: toolUse,
diff --git a/go/adk/pkg/models/openai_adk.go b/go/adk/pkg/models/openai_adk.go
index 89665545f1..59040741e1 100644
--- a/go/adk/pkg/models/openai_adk.go
+++ b/go/adk/pkg/models/openai_adk.go
@@ -269,7 +269,7 @@ func genaiContentsToOpenAIMessages(contents []*genai.Content, config *genai.Gene
 			toolCalls := make([]openai.ChatCompletionMessageToolCallUnionParam, 0, len(functionCalls))
 			var toolResponseMessages []openai.ChatCompletionMessageParamUnion
 			for _, fc := range functionCalls {
-				argsJSON, _ := json.Marshal(fc.Args)
+				argsJSON, _ := json.Marshal(nonNilFunctionCallArgs(fc.Args))
 				toolCall := openai.ChatCompletionMessageFunctionToolCallParam{
 					ID:   fc.ID,
 					Type: constant.Function(openAIToolTypeFunction),
diff --git a/go/adk/pkg/models/openai_adk_test.go b/go/adk/pkg/models/openai_adk_test.go
index dde5959be8..7352c00ca5 100644
--- a/go/adk/pkg/models/openai_adk_test.go
+++ b/go/adk/pkg/models/openai_adk_test.go
@@ -220,6 +220,19 @@ func TestGenaiContentsToOpenAIMessages(t *testing.T) {
 			t.Errorf("len(messages) = %d, want 1", len(msgs))
 		}
 	})
+
+	t.Run("nil args (replay regression) encode as empty object", func(t *testing.T) {
+		contents := []*genai.Content{
+			{Role: "model", Parts: []*genai.Part{{FunctionCall: &genai.FunctionCall{ID: "call_1", Name: "ping", Args: nil}}}},
+		}
+		msgs, _ := genaiContentsToOpenAIMessages(contents, nil)
+		if len(msgs) == 0 || msgs[0].OfAssistant == nil || len(msgs[0].OfAssistant.ToolCalls) != 1 {
+			t.Fatalf("messages = %#v, want first message to be assistant with 1 tool call", msgs)
+		}
+		if got := msgs[0].OfAssistant.ToolCalls[0].GetFunction().Arguments; got != `{}` {
+			t.Errorf("arguments = %q, want {}", got)
+		}
+	})
 }
 
 func TestApplyOpenAIConfig(t *testing.T) {
diff --git a/go/adk/pkg/models/openai_responses.go b/go/adk/pkg/models/openai_responses.go
index a39810dd58..9ef4eb45d0 100644
--- a/go/adk/pkg/models/openai_responses.go
+++ b/go/adk/pkg/models/openai_responses.go
@@ -124,7 +124,7 @@ func genaiContentsToResponsesInput(contents []*genai.Content, config *genai.Gene
 				))
 			}
 			for _, fc := range functionCalls {
-				argsJSON, _ := json.Marshal(fc.Args)
+				argsJSON, _ := json.Marshal(nonNilFunctionCallArgs(fc.Args))
 				input = append(input, responses.ResponseInputItemParamOfFunctionCall(
 					string(argsJSON),
 					fc.ID,
diff --git a/go/adk/pkg/models/openai_responses_test.go b/go/adk/pkg/models/openai_responses_test.go
index bbc9e1b0b4..920155084b 100644
--- a/go/adk/pkg/models/openai_responses_test.go
+++ b/go/adk/pkg/models/openai_responses_test.go
@@ -59,6 +59,19 @@ func TestGenaiContentsToResponsesInput(t *testing.T) {
 			t.Fatalf("output = %q, want 3", got)
 		}
 	})
+
+	t.Run("nil args (replay regression) encode as empty object", func(t *testing.T) {
+		contents := []*genai.Content{
+			{Role: "model", Parts: []*genai.Part{{FunctionCall: &genai.FunctionCall{ID: "call_1", Name: "ping", Args: nil}}}},
+		}
+		input, _ := genaiContentsToResponsesInput(contents, nil)
+		if len(input) == 0 || input[0].OfFunctionCall == nil {
+			t.Fatalf("input = %#v, want function_call item", input)
+		}
+		if got := input[0].OfFunctionCall.Arguments; got != `{}` {
+			t.Errorf("arguments = %q, want {}", got)
+		}
+	})
 }
 
 func TestGenaiToolsToResponsesTools(t *testing.T) {
diff --git a/go/adk/pkg/models/sapaicore_adk.go b/go/adk/pkg/models/sapaicore_adk.go
index 96d92c644b..6ed53d9965 100644
--- a/go/adk/pkg/models/sapaicore_adk.go
+++ b/go/adk/pkg/models/sapaicore_adk.go
@@ -211,7 +211,7 @@ func genaiContentsToOrchTemplate(contents []*genai.Content, config *genai.Genera
 			toolCalls := make([]map[string]any, 0, len(functionCalls))
 			var toolResponses []map[string]any
 			for _, fc := range functionCalls {
-				argsJSON, _ := json.Marshal(fc.Args)
+				argsJSON, _ := json.Marshal(nonNilFunctionCallArgs(fc.Args))
 				tc := map[string]any{
 					"type": "function",
 					"function": map[string]any{
diff --git a/go/adk/pkg/models/sapaicore_adk_test.go b/go/adk/pkg/models/sapaicore_adk_test.go
index 39a199ceb5..2f85887982 100644
--- a/go/adk/pkg/models/sapaicore_adk_test.go
+++ b/go/adk/pkg/models/sapaicore_adk_test.go
@@ -152,6 +152,27 @@ func TestGenaiContentsToOrchTemplate_ToolCall(t *testing.T) {
 	if toolCalls[0]["id"] != "call_1" {
 		t.Errorf("tool_calls[0].id = %v, want call_1", toolCalls[0]["id"])
 	}
+
+	t.Run("nil args (replay regression) encode as empty object", func(t *testing.T) {
+		contents := []*genai.Content{
+			{Role: "model", Parts: []*genai.Part{{FunctionCall: &genai.FunctionCall{ID: "call_1", Name: "ping", Args: nil}}}},
+		}
+		msgs, _ := genaiContentsToOrchTemplate(contents, nil)
+		if len(msgs) == 0 {
+			t.Fatal("expected at least 1 message")
+		}
+		toolCalls, ok := msgs[0]["tool_calls"].([]map[string]any)
+		if !ok || len(toolCalls) == 0 {
+			t.Fatalf("tool_calls = %v, want non-empty slice", msgs[0]["tool_calls"])
+		}
+		fn, ok := toolCalls[0]["function"].(map[string]any)
+		if !ok {
+			t.Fatalf("tool_calls[0].function = %v, want map", toolCalls[0]["function"])
+		}
+		if fn["arguments"] != `{}` {
+			t.Errorf("tool_calls[0].function.arguments = %v, want {}", fn["arguments"])
+		}
+	})
 }
 
 func TestGenaiContentsToOrchTemplate_FunctionResponse(t *testing.T) {