diff --git a/.sandcastle/main.ts b/.sandcastle/main.ts
index 124e09d..9e1e202 100644
--- a/.sandcastle/main.ts
+++ b/.sandcastle/main.ts
@@ -14,6 +14,10 @@
  *   AGENTIC_BASE_BRANCH   PR base branch (default: main)
  *   AGENTIC_MODEL         claudeCode model (default: claude-sonnet-4-6)
  *   SANDCASTLE_IMAGE      inner image (default: sandcastle:local)
+ *   SANDCASTLE_CONTAINER_UID  inner image's non-root UID (default: 1000, sandcastle's
+ *                             own default — only needed when a custom image renames its
+ *                             user to a different UID, e.g. the dotnet SDK base's 1654)
+ *   SANDCASTLE_CONTAINER_GID  matching GID (default: 1000)
  *   AGENTIC_CONCURRENCY   max parallel sandboxes (default: 1, serial)
  *   SMEE_URL              smee.io channel URL (enables event-driven mode)
  *   WEBHOOK_SECRET        shared HMAC secret for X-Hub-Signature-256 validation
@@ -603,6 +607,12 @@ async function main(): Promise<void> {
 
   const issues = new IssueSource(repo);
   const tier = (process.env.AGENTIC_TIER ?? "claude") as "claude" | "local";
+  const containerUid = process.env.SANDCASTLE_CONTAINER_UID
+    ? Number(process.env.SANDCASTLE_CONTAINER_UID)
+    : undefined;
+  const containerGid = process.env.SANDCASTLE_CONTAINER_GID
+    ? Number(process.env.SANDCASTLE_CONTAINER_GID)
+    : undefined;
   const runner = new SandboxRunner({
     imageName: process.env.SANDCASTLE_IMAGE,
     model: process.env.AGENTIC_MODEL,
@@ -611,6 +621,8 @@ async function main(): Promise<void> {
     localImageName: process.env.SANDCASTLE_OPENCODE_IMAGE,
     cwd: repoRoot,
     network,
+    containerUid,
+    containerGid,
   });
 
   const reviewer = new ReviewerAdapter({
diff --git a/examples/dotnet-project/orchestrator.env b/examples/dotnet-project/orchestrator.env
index ab60b10..6245c37 100644
--- a/examples/dotnet-project/orchestrator.env
+++ b/examples/dotnet-project/orchestrator.env
@@ -10,6 +10,12 @@ GH_TOKEN=
 #   docker build -f .sandcastle/Dockerfile.dotnet -t sandcastle-dotnet:local .sandcastle
 SANDCASTLE_IMAGE=sandcastle-dotnet:local
 
+# mcr.microsoft.com/dotnet/sdk:8.0 ships its own non-root user at uid/gid 1654
+# (not sandcastle's default of 1000) — Dockerfile.dotnet renames it but keeps
+# the id, so the runner must be told to expect 1654 instead of assuming 1000.
+SANDCASTLE_CONTAINER_UID=1654
+SANDCASTLE_CONTAINER_GID=1654
+
 # Target repo is NOT the one `gh` detects from the working directory here —
 # this recipe validates against a small, purpose-built dotnet fixture repo,
 # so AGENTIC_REPO is set explicitly rather than left to the default.