diff --git a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
index bf5c23d5652..103258556b9 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
@@ -225,6 +225,56 @@ func TestChatCompletionsToResponses_WhitespaceOnlyBase64ImageURLSkipped(t *testi
 	assert.Equal(t, "Describe this", parts[0].Text)
 }
 
+func TestChatCompletionsToResponses_FileData(t *testing.T) {
+	content := `[{"type":"text","text":"Analyze this file"},{"type":"file","file":{"filename":"example-report.pdf","file_data":"data:application/pdf;base64,abc123"}}]`
+	req := &ChatCompletionsRequest{
+		Model: "gpt-4o",
+		Messages: []ChatMessage{
+			{Role: "user", Content: json.RawMessage(content)},
+		},
+	}
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[0].Content, &parts))
+	require.Len(t, parts, 2)
+	assert.Equal(t, "input_text", parts[0].Type)
+	assert.Equal(t, "Analyze this file", parts[0].Text)
+	assert.Equal(t, "input_file", parts[1].Type)
+	assert.Equal(t, "example-report.pdf", parts[1].Filename)
+	assert.Equal(t, "data:application/pdf;base64,abc123", parts[1].FileData)
+	assert.Empty(t, parts[1].FileURL)
+}
+
+func TestChatCompletionsToResponses_FileURL(t *testing.T) {
+	content := `[{"type":"file","file":{"filename":"report.pdf","file_url":"https://example.com/report.pdf"}}]`
+	req := &ChatCompletionsRequest{
+		Model: "gpt-4o",
+		Messages: []ChatMessage{
+			{Role: "user", Content: json.RawMessage(content)},
+		},
+	}
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 1)
+
+	var parts []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[0].Content, &parts))
+	require.Len(t, parts, 1)
+	assert.Equal(t, "input_file", parts[0].Type)
+	assert.Equal(t, "report.pdf", parts[0].Filename)
+	assert.Equal(t, "https://example.com/report.pdf", parts[0].FileURL)
+	assert.Empty(t, parts[0].FileData)
+}
+
 func TestChatCompletionsToResponses_SystemArrayContent(t *testing.T) {
 	req := &ChatCompletionsRequest{
 		Model: "gpt-4o",
diff --git a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
index 64ef5781585..338d88f8207 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
@@ -346,6 +346,29 @@ func convertChatContentPartsToResponses(parts []ChatContentPart) []ResponsesCont
 					ImageURL: p.ImageURL.URL,
 				})
 			}
+		case "file":
+			fileData := p.FileData
+			fileURL := p.FileURL
+			filename := p.Filename
+			if p.File != nil {
+				if fileData == "" {
+					fileData = p.File.FileData
+				}
+				if fileURL == "" {
+					fileURL = p.File.FileURL
+				}
+				if filename == "" {
+					filename = p.File.Filename
+				}
+			}
+			if fileData != "" || fileURL != "" {
+				responseParts = append(responseParts, ResponsesContentPart{
+					Type:     "input_file",
+					FileData: fileData,
+					FileURL:  fileURL,
+					Filename: filename,
+				})
+			}
 		}
 	}
 	return responseParts
diff --git a/backend/internal/pkg/apicompat/types.go b/backend/internal/pkg/apicompat/types.go
index f9cd5a1c7f9..164f73915e6 100644
--- a/backend/internal/pkg/apicompat/types.go
+++ b/backend/internal/pkg/apicompat/types.go
@@ -219,9 +219,12 @@ type ResponsesInputItem struct {
 
 // ResponsesContentPart is a typed content part in a Responses message.
 type ResponsesContentPart struct {
-	Type     string `json:"type"` // "input_text" | "output_text" | "input_image"
+	Type     string `json:"type"` // "input_text" | "output_text" | "input_image" | "input_file"
 	Text     string `json:"text,omitempty"`
 	ImageURL string `json:"image_url,omitempty"` // data URI for input_image
+	FileData string `json:"file_data,omitempty"` // data URI for input_file
+	FileURL  string `json:"file_url,omitempty"`  // URL for input_file
+	Filename string `json:"filename,omitempty"`  // original filename for input_file
 }
 
 // ResponsesTool describes a tool in the Responses API.
@@ -401,9 +404,15 @@ type ChatMessage struct {
 
 // ChatContentPart is a typed content part in a multi-modal message.
 type ChatContentPart struct {
-	Type     string        `json:"type"` // "text" | "image_url"
+	Type     string        `json:"type"` // "text" | "image_url" | "file"
 	Text     string        `json:"text,omitempty"`
 	ImageURL *ChatImageURL `json:"image_url,omitempty"`
+	File     *ChatFile     `json:"file,omitempty"`
+
+	// Optional flat file fields for compatibility with different client payloads.
+	FileData string `json:"file_data,omitempty"`
+	FileURL  string `json:"file_url,omitempty"`
+	Filename string `json:"filename,omitempty"`
 }
 
 // ChatImageURL contains the URL for an image content part.
@@ -412,6 +421,13 @@ type ChatImageURL struct {
 	Detail string `json:"detail,omitempty"` // "auto" | "low" | "high"
 }
 
+// ChatFile contains file metadata and source for a file content part.
+type ChatFile struct {
+	Filename string `json:"filename,omitempty"`
+	FileData string `json:"file_data,omitempty"`
+	FileURL  string `json:"file_url,omitempty"`
+}
+
 // ChatTool describes a tool available to the model.
 type ChatTool struct {
 	Type     string        `json:"type"` // "function"