diff --git a/pkg/cmd/betafile.go b/pkg/cmd/betafile.go
index f5564fb..eb65c9c 100644
--- a/pkg/cmd/betafile.go
+++ b/pkg/cmd/betafile.go
@@ -6,6 +6,7 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"strings"
 
 	"github.com/anthropics/anthropic-cli/internal/apiquery"
 	"github.com/anthropics/anthropic-cli/internal/requestflag"
@@ -15,6 +16,21 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
+const managedAgentsBeta = "managed-agents-2026-04-01"
+
+// addManagedAgentsBetaForFiles appends the managed-agents beta header when the
+// request targets a session-scoped file (scope-id prefixed with "sesn_").
+// Session-scoped file operations require this header in addition to the
+// files-api beta; without it the API returns 404.
+func addManagedAgentsBetaForFiles(cmd *cli.Command, options []option.RequestOption) []option.RequestOption {
+	if cmd.IsSet("scope-id") {
+		if scopeID, ok := cmd.Value("scope-id").(string); ok && strings.HasPrefix(scopeID, "sesn_") {
+			return append(options, option.WithHeaderAdd("anthropic-beta", managedAgentsBeta))
+		}
+	}
+	return options
+}
+
 var betaFilesList = cli.Command{
 	Name:    "list",
 	Usage:   "List Files",
@@ -163,6 +179,7 @@ func handleBetaFilesList(ctx context.Context, cmd *cli.Command) error {
 	if err != nil {
 		return err
 	}
+	options = addManagedAgentsBetaForFiles(cmd, options)
 
 	params := anthropic.BetaFileListParams{}
 
@@ -276,6 +293,10 @@ func handleBetaFilesDownload(ctx context.Context, cmd *cli.Command) error {
 	if err != nil {
 		return err
 	}
+	// Download takes only a file ID, so we can't tell from flags whether the
+	// target is session-scoped. Send the managed-agents beta unconditionally;
+	// it is harmless on non-session files and required on session files.
+	options = append(options, option.WithHeaderAdd("anthropic-beta", managedAgentsBeta))
 
 	params := anthropic.BetaFileDownloadParams{}
 
@@ -316,6 +337,7 @@ func handleBetaFilesRetrieveMetadata(ctx context.Context, cmd *cli.Command) erro
 	if err != nil {
 		return err
 	}
+	options = append(options, option.WithHeaderAdd("anthropic-beta", managedAgentsBeta))
 
 	params := anthropic.BetaFileGetMetadataParams{}