fix: reduce modern MCP file content payloads

Author: huong8373tt-beep

pkg/github/repositories.go

@@ -1024,7 +1024,7 @@ func GetFileContents(t translations.TranslationHelperFunc) inventory.ServerTool
 			InputSchema: schema,
 		},
 		scopes.PublicRead(scopes.Repo),
-		func(ctx context.Context, deps ToolDependencies, _ *mcp.CallToolRequest, input GetFileContentsInput) (*mcp.CallToolResult, *RepositoryContentsOutput, error) {
+		func(ctx context.Context, deps ToolDependencies, request *mcp.CallToolRequest, input GetFileContentsInput) (*mcp.CallToolResult, *RepositoryContentsOutput, error) {
 			owner := input.Owner
 			if owner == "" {
 				return utils.NewToolResultError("missing required parameter: owner"), nil, nil
@@ -1101,7 +1101,7 @@ func GetFileContents(t translations.TranslationHelperFunc) inventory.ServerTool
 				if fallbackUsed {
 					successNote = fmt.Sprintf(" Note: the provided ref '%s' does not exist, default branch '%s' was used instead.", originalRef, rawOpts.Ref)
 				}
-				const maxContentSize = 1024 * 1024 // 1MB
+				maxContentSize := repositoryContentSizeLimit(request)
 
 				read, respInspect, err := inspectRepositoryFile(ctx, client, owner, repo, ref, path, fileContent)
 				if err != nil {
@@ -1232,6 +1232,24 @@ func recordDirContentsFieldsUsage(ctx context.Context, deps ToolDependencies, fu
 	recordFieldsUsageFor(ctx, deps, "get_file_contents", full, filtered, sentBytes)
 }
 
+const (
+	legacyRepositoryContentSizeLimit = 1024 * 1024
+	modernRepositoryContentSizeLimit = 448 * 1024
+)
+
+// repositoryContentSizeLimit accounts for modern typed output mirroring file
+// content in both MCP content blocks and structuredContent.
+func repositoryContentSizeLimit(request *mcp.CallToolRequest) int {
+	if request == nil || request.Params == nil {
+		return legacyRepositoryContentSizeLimit
+	}
+	version, _ := request.Params.Meta[mcp.MetaKeyProtocolVersion].(string)
+	if inventory.ProtocolEraForVersion(version) == inventory.ProtocolEraModern {
+		return modernRepositoryContentSizeLimit
+	}
+	return legacyRepositoryContentSizeLimit
+}
+
 // ForkRepository creates a tool to fork a repository.
 func ForkRepository(t translations.TranslationHelperFunc) inventory.ServerTool {
 	return NewTool[ForkRepositoryInput, *ForkRepositoryOutput](

pkg/github/repository_contents_protocol_test.go

@@ -0,0 +1,80 @@
+package github
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/github/github-mcp-server/v2/pkg/inventory"
+	"github.com/github/github-mcp-server/v2/pkg/translations"
+	"github.com/google/go-github/v92/github"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetFileContentsModernProtocolUsesTransportSafeLimit(t *testing.T) {
+	const fileName = "medium-file.txt"
+	fileSHA := strings.Repeat("b", 40)
+
+	mockedClient := MockHTTPClientWithHandlers(map[string]http.HandlerFunc{
+		GetReposGitRefByOwnerByRepoByRef: mockResponse(t, http.StatusOK, "{\"ref\": \"refs/heads/main\", \"object\": {\"sha\": \"\"}}"),
+		GetReposByOwnerByRepo:            mockResponse(t, http.StatusOK, "{\"name\": \"repo\", \"default_branch\": \"main\"}"),
+		"GET /repos/owner/repo/git/trees/refs/heads/main": mockResponse(t, http.StatusOK, &github.Tree{
+			Entries: []*github.TreeEntry{{
+				Path: new(fileName),
+				Mode: new("100644"),
+				Type: new("blob"),
+				SHA:  new(fileSHA),
+			}},
+		}),
+		GetReposContentsByOwnerByRepoByPath: func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+			fileContent := map[string]any{
+				"name":         fileName,
+				"path":         fileName,
+				"sha":          fileSHA,
+				"type":         "file",
+				"size":         500 * 1024,
+				"download_url": "https://raw.githubusercontent.com/owner/repo/main/" + fileName,
+			}
+			contentBytes, err := json.Marshal(fileContent)
+			require.NoError(t, err)
+			_, err = w.Write(contentBytes)
+			require.NoError(t, err)
+		},
+	})
+
+	deps := BaseDeps{Client: mustNewGHClient(t, mockedClient)}
+	tool := GetFileContents(translations.NullTranslationHelper)
+	request := createMCPRequest(map[string]any{
+		"owner": "owner",
+		"repo":  "repo",
+		"path":  fileName,
+		"ref":   "refs/heads/main",
+	})
+	request.Params.Meta = mcp.Meta{mcp.MetaKeyProtocolVersion: inventory.ProtocolVersionMultiRoundTrip}
+
+	result, err := tool.Handler(deps)(ContextWithDeps(context.Background(), deps), &request)
+	require.NoError(t, err)
+	require.False(t, result.IsError)
+	require.Len(t, result.Content, 2)
+
+	resourceLink, ok := result.Content[1].(*mcp.ResourceLink)
+	require.True(t, ok, "modern clients should receive a ResourceLink before the 1 MiB wire limit")
+	require.Equal(t, "repo://owner/repo/refs/heads/main/contents/"+fileName, resourceLink.URI)
+	require.Equal(t, fileName, resourceLink.Name)
+}
+
+func TestRepositoryContentSizeLimit(t *testing.T) {
+	modernRequest := createMCPRequest(map[string]any{})
+	modernRequest.Params.Meta = mcp.Meta{mcp.MetaKeyProtocolVersion: inventory.ProtocolVersionMultiRoundTrip}
+
+	legacyRequest := createMCPRequest(map[string]any{})
+	legacyRequest.Params.Meta = mcp.Meta{mcp.MetaKeyProtocolVersion: "2025-11-25"}
+
+	require.Equal(t, modernRepositoryContentSizeLimit, repositoryContentSizeLimit(&modernRequest))
+	require.Equal(t, legacyRepositoryContentSizeLimit, repositoryContentSizeLimit(&legacyRequest))
+	require.Equal(t, legacyRepositoryContentSizeLimit, repositoryContentSizeLimit(nil))
+}