fix(adk/bedrock): coerce nil tool-call args to empty object

go/adk/pkg/models/bedrock.go

@@ -647,10 +647,22 @@ 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(part.FunctionCall.Args),
+					Input:     document.NewLazyDocument(args),
 				}
 				contentBlocks = append(contentBlocks, &types.ContentBlockMemberToolUse{
 					Value: toolUse,

go/adk/pkg/models/bedrock_test.go

@@ -187,6 +187,44 @@ func TestConvertGenaiContentsToBedrockMessages(t *testing.T) {
 	}
 }
 
+// TestConvertGenaiContentsNoArgToolInput verifies that a tool call with no
+// arguments serializes toolUse.input as an empty JSON object rather than null.
+// A no-argument FunctionCall arrives here with a nil Args map because genai's
+// FunctionCall.Args is `json:"args,omitempty"`, so the empty map is dropped when
+// the event is persisted to the session store and reloaded. Bedrock Converse
+// rejects a null input with "ValidationException: Malformed input request"
+// ("The value at messages.N.content.M.toolUse.input is empty").
+func TestConvertGenaiContentsNoArgToolInput(t *testing.T) {
+	for _, args := range []map[string]any{nil, {}} {
+		contents := []*genai.Content{
+			{
+				Role: "model",
+				Parts: []*genai.Part{
+					{FunctionCall: &genai.FunctionCall{ID: "call_noargs", Name: "datetime_get_current_time", Args: args}},
+				},
+			},
+		}
+		msgs, _ := convertGenaiContentsToBedrockMessages(contents, nil, nil)
+		if len(msgs) != 1 || len(msgs[0].Content) != 1 {
+			t.Fatalf("args=%v: expected 1 message with 1 content block, got %d messages", args, len(msgs))
+		}
+		tu, ok := msgs[0].Content[0].(*types.ContentBlockMemberToolUse)
+		if !ok {
+			t.Fatalf("args=%v: want *ContentBlockMemberToolUse, got %T", args, msgs[0].Content[0])
+		}
+		if tu.Value.Input == nil {
+			t.Fatalf("args=%v: toolUse.Input is nil; Bedrock requires a JSON object", args)
+		}
+		got := documentToMap(tu.Value.Input)
+		if got == nil {
+			t.Fatalf("args=%v: toolUse.Input serialized to null; want an empty JSON object {}", args)
+		}
+		if len(got) != 0 {
+			t.Errorf("args=%v: expected empty input object, got %v", args, got)
+		}
+	}
+}
+
 // TestConvertGenaiToolsToBedrock verifies schema conversion for all three tool
 // sources: genai.Schema (declaration-based), map[string]any (MCP), and
 // *jsonschema.Schema (functiontool.New).