diff --git a/backend/internal/pkg/antigravity/request_transformer.go b/backend/internal/pkg/antigravity/request_transformer.go
index f9b917fb17..9349e54ee8 100644
--- a/backend/internal/pkg/antigravity/request_transformer.go
+++ b/backend/internal/pkg/antigravity/request_transformer.go
@@ -532,8 +532,8 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			}
 			parts = append(parts, part)
 
-		case "image":
-			if block.Source != nil && block.Source.Type == "base64" {
+		case "image", "document":
+			if block.Source != nil && block.Source.Type == "base64" && strings.TrimSpace(block.Source.Data) != "" {
 				parts = append(parts, GeminiPart{
 					InlineData: &GeminiInlineData{
 						MimeType: block.Source.MediaType,
diff --git a/backend/internal/pkg/antigravity/request_transformer_test.go b/backend/internal/pkg/antigravity/request_transformer_test.go
index 99d61867c9..1aff9f5775 100644
--- a/backend/internal/pkg/antigravity/request_transformer_test.go
+++ b/backend/internal/pkg/antigravity/request_transformer_test.go
@@ -646,6 +646,20 @@ func TestGeminiToolConfig_DropsBuiltinsWhenClientFunctionsPresent(t *testing.T)
 	})
 }
 
+func TestBuildParts_DocumentBecomesInlineData(t *testing.T) {
+	content := `[
+		{"type":"text","text":"read this"},
+		{"type":"document","source":{"type":"base64","media_type":"application/pdf","data":"JVBERi0="}}
+	]`
+	parts, stripped, err := buildParts(json.RawMessage(content), map[string]string{}, true)
+	require.NoError(t, err)
+	require.False(t, stripped)
+	require.Len(t, parts, 2)
+	require.NotNil(t, parts[1].InlineData)
+	require.Equal(t, "application/pdf", parts[1].InlineData.MimeType)
+	require.Equal(t, "JVBERi0=", parts[1].InlineData.Data)
+}
+
 // TestToolConfigAlwaysPresent ensures toolConfig is always emitted, including for
 // reasoning models without any tools: upstream rejects requests that omit it.
 func TestToolConfigAlwaysPresent(t *testing.T) {
diff --git a/backend/internal/pkg/apicompat/responses_to_anthropic_invalid_blocks_test.go b/backend/internal/pkg/apicompat/responses_to_anthropic_invalid_blocks_test.go
index ed410f6851..d3bd29081b 100644
--- a/backend/internal/pkg/apicompat/responses_to_anthropic_invalid_blocks_test.go
+++ b/backend/internal/pkg/apicompat/responses_to_anthropic_invalid_blocks_test.go
@@ -109,7 +109,23 @@ func TestResponsesToAnthropic_UnknownItemTypeKeepsRecognizableText(t *testing.T)
 	require.NotContains(t, string(messages[0].Content), "drop me")
 }
 
-// user 消息的分片全部不可识别时，以前会退化成 content:""，Anthropic 拒收空内容消息。
+// data URI 形式的 input_file 要变成 Anthropic document，供后续 Gemini inlineData 使用。
+func TestResponsesToAnthropic_InputFileDataURIBecomesDocument(t *testing.T) {
+	messages := responsesToAnthropicMessages(t, `[
+		{"type":"message","role":"user","content":[
+			{"type":"input_text","text":"read this"},
+			{"type":"input_file","filename":"token.pdf","file_data":"data:application/pdf;base64,JVBERi0="}
+		]}
+	]`)
+
+	requireAnthropicMessagesAreSendable(t, messages)
+	require.Len(t, messages, 1)
+	require.Contains(t, string(messages[0].Content), `"type":"document"`)
+	require.Contains(t, string(messages[0].Content), `"media_type":"application/pdf"`)
+	require.Contains(t, string(messages[0].Content), `"data":"JVBERi0="`)
+}
+
+// 只有 file_id、没有 data URI 的 input_file 仍然无法转换，整条消息丢掉。
 func TestResponsesToAnthropic_UserMessageWithOnlyUnknownPartsIsDropped(t *testing.T) {
 	messages := responsesToAnthropicMessages(t, `[
 		{"type":"message","role":"user","content":[{"type":"input_file","file_id":"file_1"}]}
diff --git a/backend/internal/pkg/apicompat/responses_to_anthropic_request.go b/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
index 06d9669d29..69928591d3 100644
--- a/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
+++ b/backend/internal/pkg/apicompat/responses_to_anthropic_request.go
@@ -530,6 +530,14 @@ func convertResponsesUserToAnthropicContent(raw json.RawMessage) (json.RawMessag
 					Source: src,
 				})
 			}
+		case "input_file":
+			src := dataURIToAnthropicFileSource(p.FileData)
+			if src != nil {
+				blocks = append(blocks, AnthropicContentBlock{
+					Type:   "document",
+					Source: src,
+				})
+			}
 		}
 	}
 
@@ -617,6 +625,12 @@ func dataURIToAnthropicImageSource(dataURI string) *AnthropicImageSource {
 	}
 }
 
+// dataURIToAnthropicFileSource parses a data URI into a document source.
+// file_id-only parts are not convertible here and stay dropped.
+func dataURIToAnthropicFileSource(fileData string) *AnthropicImageSource {
+	return dataURIToAnthropicImageSource(fileData)
+}
+
 // mergeConsecutiveMessages merges consecutive messages with the same role
 // because Anthropic requires alternating user/assistant turns.
 func mergeConsecutiveMessages(messages []AnthropicMessage) []AnthropicMessage {