Add download button for training artifacts

Add a backend endpoint to download artifact files and directories
(zipped on-the-fly). The frontend artifacts modal now shows both
Download and Copy Path buttons for each artifact.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: shreyaskarnik

frontend/src/api/client.ts

@@ -79,3 +79,7 @@ export function fetchArtifacts(runId: number): Promise<ArtifactsResponse> {
 export function fetchVersion(): Promise<{ version: string }> {
   return request('/api/version')
 }
+
+export function getArtifactDownloadUrl(runId: number, artifactName: string): string {
+  return `/runs/${runId}/artifacts/${encodeURIComponent(artifactName)}/download`
+}

frontend/src/components/modals/ArtifactsModal.tsx

@@ -72,12 +72,21 @@ export function ArtifactsModal() {
                     <span>{formatFileSize(artifact.size)}</span>
                   </div>
                 </div>
-                <button
-                  className="artifact-download"
-                  onClick={() => copyPath(artifact.path)}
-                >
-                  {copiedPath === artifact.path ? 'Copied!' : 'Copy Path'}
-                </button>
+                <div className="artifact-actions">
+                  <a
+                    className="artifact-download"
+                    href={api.getArtifactDownloadUrl(selectedRunId!, artifact.name)}
+                    download
+                  >
+                    Download
+                  </a>
+                  <button
+                    className="artifact-copy"
+                    onClick={() => copyPath(artifact.path)}
+                  >
+                    {copiedPath === artifact.path ? 'Copied!' : 'Copy Path'}
+                  </button>
+                </div>
               </div>
             ))
           )}

frontend/src/styles.css

@@ -729,7 +729,14 @@ body {
     letter-spacing: 0.5px;
 }
 
-.artifact-download {
+.artifact-actions {
+    display: flex;
+    gap: var(--space-xs);
+    flex-shrink: 0;
+}
+
+.artifact-download,
+.artifact-copy {
     padding: var(--space-xs) var(--space-sm);
     background: var(--ui-element-background);
     border: 1px solid var(--ui-element-border);
@@ -739,9 +746,13 @@ body {
     cursor: pointer;
     transition: all 0.15s;
     white-space: nowrap;
+    text-decoration: none;
+    display: inline-flex;
+    align-items: center;
 }
 
-.artifact-download:hover {
+.artifact-download:hover,
+.artifact-copy:hover {
     background: var(--ui-element-background-hover);
     border-color: var(--ui-element-border-hover);
 }

src/vespaembed/web/app.py

@@ -1,14 +1,16 @@
+import io
 import json
 import os
 import re
 import subprocess
 import sys
 import time
+import zipfile
 from pathlib import Path
 from typing import Optional
 
 from fastapi import FastAPI, File, Form, HTTPException, UploadFile
-from fastapi.responses import HTMLResponse
+from fastapi.responses import FileResponse, HTMLResponse, StreamingResponse
 from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel, Field, field_validator
 
@@ -639,3 +641,50 @@ async def get_run_artifacts(run_id: int):
         )
 
     return {"artifacts": artifacts, "output_dir": str(output_dir)}
+
+
+@app.get("/runs/{run_id}/artifacts/{artifact_name}/download")
+async def download_artifact(run_id: int, artifact_name: str):
+    """Download an artifact file or directory (as zip) for a training run."""
+    run = get_run(run_id)
+    if not run:
+        raise HTTPException(status_code=404, detail="Run not found")
+
+    output_dir = Path(run["output_dir"])
+    if not output_dir.exists():
+        raise HTTPException(status_code=404, detail="Output directory not found")
+
+    artifact_path = output_dir / artifact_name
+    if not artifact_path.exists():
+        raise HTTPException(status_code=404, detail="Artifact not found")
+
+    # Security: ensure the artifact is within the output directory
+    try:
+        artifact_path.resolve().relative_to(output_dir.resolve())
+    except ValueError:
+        raise HTTPException(status_code=403, detail="Access denied")
+
+    if artifact_path.is_file():
+        return FileResponse(
+            path=str(artifact_path),
+            filename=artifact_path.name,
+            media_type="application/octet-stream",
+        )
+
+    # Directory: stream as zip
+    def iter_zip():
+        buffer = io.BytesIO()
+        with zipfile.ZipFile(buffer, "w", zipfile.ZIP_DEFLATED) as zf:
+            for file_path in sorted(artifact_path.rglob("*")):
+                if file_path.is_file():
+                    arcname = file_path.relative_to(artifact_path)
+                    zf.write(file_path, arcname)
+        buffer.seek(0)
+        yield buffer.read()
+
+    zip_filename = f"{artifact_name}.zip"
+    return StreamingResponse(
+        iter_zip(),
+        media_type="application/zip",
+        headers={"Content-Disposition": f'attachment; filename="{zip_filename}"'},
+    )