diff --git a/server/tool/src/initializer.ts b/server/tool/src/initializer.ts
index fc4b51acb3..67d914865d 100644
--- a/server/tool/src/initializer.ts
+++ b/server/tool/src/initializer.ts
@@ -79,7 +79,6 @@ export interface UploadStep {
   type: 'upload'
   fromUrl: string
   contentType: string
-  size?: number
   resultVariable?: string
 }
 
@@ -142,12 +141,14 @@ export class WorkspaceInitializer {
       const id = uuid()
       const resp = await fetch(step.fromUrl)
       const buffer = Buffer.from(await resp.arrayBuffer())
-      await this.storageAdapter.put(this.ctx, this.wsUrl, id, buffer, step.contentType, step.size)
+      await this.storageAdapter.put(this.ctx, this.wsUrl, id, buffer, step.contentType, buffer.length)
       if (step.resultVariable !== undefined) {
         vars[step.resultVariable] = id
+        vars[`${step.resultVariable}_size`] = buffer.length
       }
     } catch (error) {
       logger.error('Upload failed', error)
+      throw error
     }
   }
 
@@ -297,7 +298,9 @@ export class WorkspaceInitializer {
           const matched = fieldRegexp.exec(value)
           if (matched === null) break
           const result = vars[matched[0]]
-          if (result !== undefined && typeof result === 'string') {
+          if (result === undefined || typeof result !== 'string') {
+            throw new Error(`Variable ${matched[0]} not found`)
+          } else {
             value = value.replaceAll(matched[0], result)
             fieldRegexp.lastIndex = 0
           }