feat(files): update save_file tool description and enforce size limit for base64 payloads

internal/tools/files.go

@@ -16,6 +16,11 @@ import (
 	thoughttypes "git.warky.dev/wdevs/amcs/internal/types"
 )
 
+// maxBase64ToolBytes is the maximum base64 payload accepted by save_file via
+// the MCP tool interface. For larger files use POST /files (binary) and pass
+// the returned amcs://files/{id} URI as content_uri instead.
+const maxBase64ToolBytes = 10 << 20 // 10 MB of base64 ≈ 7.5 MB decoded
+
 type FilesTool struct {
 	store    *store.DB
 	sessions *session.ActiveProjects
@@ -75,6 +80,11 @@ func (t *FilesTool) Save(ctx context.Context, req *mcp.CallToolRequest, in SaveF
 	if uri != "" && b64 != "" {
 		return nil, SaveFileOutput{}, errInvalidInput("provide content_uri or content_base64, not both")
 	}
+	if len(b64) > maxBase64ToolBytes {
+		return nil, SaveFileOutput{}, errInvalidInput(
+			"content_base64 exceeds the 10 MB MCP tool limit; upload the file via POST /files and pass the returned amcs://files/{id} URI as content_uri instead",
+		)
+	}
 
 	var content []byte
 	var mediaTypeFromSource string