diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
index 5d2f340..072ae42 100644
--- a/.devcontainer/Dockerfile
+++ b/.devcontainer/Dockerfile
@@ -19,3 +19,24 @@ ENV PLAYWRIGHT_BROWSERS_PATH=/ms-playwright
 RUN npm install -g playwright \
     && playwright install --with-deps chromium \
     && chmod -R a+rx /ms-playwright
+
+# Tell dev container features where the target user's home directory is.
+# The devcontainer CLI passes `_REMOTE_USER_HOME`/`_CONTAINER_USER_HOME` to every
+# feature's install.sh, but envbuilder (used by Coder) only sets `_REMOTE_USER`
+# and `_CONTAINER_USER` -- it leaves the *_HOME variables empty. Features that
+# rely on them then break: e.g. the claude-code feature runs
+# `cp "$_REMOTE_USER_HOME/.local/bin/claude" /usr/local/bin/claude`, which under
+# envbuilder expands to `cp /.local/bin/claude ...` and fails the whole build.
+# Supplying the values here (as build ARGs, so they don't leak into the running
+# container's environment) keeps feature installs working on the envbuilder path.
+# ARGs declared here remain in scope for the feature install steps envbuilder
+# appends after this Dockerfile, but reset for the devcontainer CLI (which sets
+# these variables itself), so this is a no-op there.
+ARG _REMOTE_USER_HOME=/home/node
+ARG _CONTAINER_USER_HOME=/home/node
+
+# Default to the non-root `node` user. envbuilder (used by Coder) picks its target
+# user from the last `USER` directive when `containerUser`/`remoteUser` are not
+# applied from image metadata, so make the intended user explicit here to avoid
+# dropping into a root shell.
+USER node
diff --git a/.devcontainer/devcontainer.json b/.devcontainer/devcontainer.json
index ee38034..92b0684 100644
--- a/.devcontainer/devcontainer.json
+++ b/.devcontainer/devcontainer.json
@@ -31,6 +31,20 @@
   },
   "overrideFeatureInstallOrder": ["ghcr.io/devcontainers/features/rust"],
 
+  // Connect as the `node` user. Set this explicitly rather than relying on the
+  // base image's `remoteUser` metadata: envbuilder (used by Coder) does not honor
+  // image-metadata `remoteUser` and falls back to the image's `USER` (root),
+  // which would otherwise drop Coder workspaces into a root shell.
+  "remoteUser": "node",
+  "containerUser": "node",
+
+  // Don't remap the container user's UID/GID to match the host user. The sshd
+  // feature creates an `ssh` group at GID 1001, which collides with the host
+  // UID/GID (1001) used by CI runners. The CLI's automatic remap then fails with
+  // `groupmod: GID '1001' already exists`, breaking `devcontainer up`.
+  // (envbuilder ignores this key; it only affects the devcontainer CLI.)
+  "updateRemoteUserUID": false,
+
   // Features to add to the dev container. More info: https://containers.dev/features.
   // "features": {},
 
@@ -38,7 +52,9 @@
   // "forwardPorts": [],
 
   // Use 'postCreateCommand' to run commands after the container is created.
-  "postCreateCommand": "kubectl version --client --short 2>/dev/null || kubectl version --client 2>/dev/null",
+  // The sentinel file lets CI verify that the lifecycle actually ran to
+  // completion (both the devcontainer CLI and envbuilder honor this hook).
+  "postCreateCommand": "kubectl version --client 2>/dev/null || true; touch \"$HOME/.devcontainer-postcreate-done\"",
 
   // Configure tool-specific properties.
   "customizations": {
diff --git a/.github/workflows/build-containers.yml b/.github/workflows/build-containers.yml
index 24ac6ca..7e3afd2 100644
--- a/.github/workflows/build-containers.yml
+++ b/.github/workflows/build-containers.yml
@@ -4,12 +4,15 @@ on:
   pull_request:
     branches: [ main ]
 
+permissions:
+  contents: read
+
 jobs:
   build-dockerfile:
     name: Build Dockerfile
     runs-on: namespace-profile-devcontainer
     timeout-minutes: 20
-    
+
     steps:
     - name: Checkout code
       uses: actions/checkout@v6
@@ -23,11 +26,32 @@ jobs:
         docker build -t devcontainer-dockerfile .devcontainer/
         echo "Dockerfile build completed successfully"
 
+  build-envbuilder:
+    name: Build Envbuilder
+    runs-on: namespace-profile-devcontainer
+    timeout-minutes: 30
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v6
+
+    # Coder builds workspaces with envbuilder, NOT the devcontainer CLI. The two
+    # diverge on user selection, UID/GID remapping and feature install order, so
+    # the CLI job below can pass while a Coder workspace fails to start. This job
+    # exercises the envbuilder path and runs scripts/verify-devcontainer.sh as the
+    # post-build init script to assert: user is `node` (not root), tmux works, and
+    # the lifecycle (postCreateCommand) ran to completion. A non-zero exit from the
+    # init script fails the job.
+    - name: Build and verify with envbuilder
+      run: |
+        echo "Building dev container with envbuilder (Coder's builder)..."
+        scripts/test-envbuilder.sh
+
   build-devcontainer:
     name: Build Devcontainer
     runs-on: namespace-profile-devcontainer
     timeout-minutes: 30
-    
+
     steps:
     - name: Checkout code
       uses: actions/checkout@v6
@@ -39,8 +63,26 @@ jobs:
       run: |
         npm install -g @devcontainers/cli
 
-    - name: Build devcontainer
+    # `devcontainer build` only builds the image (Dockerfile + features); it does
+    # NOT create the container or run lifecycle commands such as postCreateCommand.
+    # Use `devcontainer up` so the container is actually started and the lifecycle
+    # commands run, which catches startup failures the plain build step misses.
+    - name: Start devcontainer
+      run: |
+        echo "Starting devcontainer (build + create + lifecycle commands)..."
+        devcontainer up --workspace-folder .
+        echo "Devcontainer started successfully"
+
+    - name: Smoke test devcontainer
+      run: |
+        echo "Running smoke test inside the devcontainer..."
+        devcontainer exec --workspace-folder . bash -lc 'echo "Devcontainer is up and exec works"'
+
+    - name: Tear down devcontainer
+      if: always()
       run: |
-        echo "Building devcontainer..."
-        devcontainer build --workspace-folder .
-        echo "Devcontainer build completed successfully"
\ No newline at end of file
+        container_id=$(docker ps -aq --filter "label=devcontainer.local_folder=${GITHUB_WORKSPACE}")
+        if [ -n "$container_id" ]; then
+          echo "Removing devcontainer ($container_id)..."
+          docker rm -f "$container_id"
+        fi
diff --git a/scripts/test-envbuilder.sh b/scripts/test-envbuilder.sh
new file mode 100755
index 0000000..6662818
--- /dev/null
+++ b/scripts/test-envbuilder.sh
@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+#
+# Build this dev container the same way Coder does (with envbuilder) and run the
+# verification checks, so you can reproduce and debug Coder-only startup issues
+# (e.g. "drops to root", "tmux won't run", "something isn't completing")
+# locally and read the full envbuilder build log.
+#
+# Coder builds workspaces with envbuilder, NOT the `@devcontainers/cli`. The two
+# diverge on user selection, UID/GID remapping and feature install order, so the
+# CLI can succeed while Coder fails. This script exercises the envbuilder path.
+#
+# Usage:
+#   scripts/test-envbuilder.sh
+#
+# Requirements:
+#   - docker
+#
+# Optional environment variables:
+#   ENVBUILDER_IMAGE   envbuilder image to use (default: ghcr.io/coder/envbuilder:latest)
+#   CACHE_DIR          host directory for the envbuilder layer cache (speeds up reruns)
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+envbuilder_image="${ENVBUILDER_IMAGE:-ghcr.io/coder/envbuilder:latest}"
+
+if ! command -v docker >/dev/null 2>&1; then
+  echo "error: docker is required but not found on PATH" >&2
+  exit 1
+fi
+
+# The repo is mounted into the envbuilder workspace. envbuilder discovers
+# `.devcontainer/devcontainer.json` automatically. After the build + lifecycle
+# commands complete, it runs ENVBUILDER_INIT_SCRIPT as the target user; we point
+# that at the verification script (resolved from the mounted workspace).
+workspace_folder="/workspaces/devcontainer"
+init_script="bash ${workspace_folder}/scripts/verify-devcontainer.sh"
+
+docker_args=(
+  run --rm
+  -e "ENVBUILDER_WORKSPACE_FOLDER=${workspace_folder}"
+  -e "ENVBUILDER_INIT_SCRIPT=${init_script}"
+  -v "${repo_root}:${workspace_folder}"
+)
+
+# Optionally persist the layer cache between runs for faster iteration.
+if [ -n "${CACHE_DIR:-}" ]; then
+  mkdir -p "${CACHE_DIR}"
+  docker_args+=(-v "${CACHE_DIR}:/cache" -e "ENVBUILDER_LAYER_CACHE_DIR=/cache")
+fi
+
+docker_args+=("${envbuilder_image}")
+
+echo "=== Running envbuilder ($envbuilder_image) on $repo_root ==="
+echo "docker ${docker_args[*]}"
+exec docker "${docker_args[@]}"
diff --git a/scripts/verify-devcontainer.sh b/scripts/verify-devcontainer.sh
new file mode 100755
index 0000000..f2148c4
--- /dev/null
+++ b/scripts/verify-devcontainer.sh
@@ -0,0 +1,88 @@
+#!/usr/bin/env bash
+#
+# Verify that the built dev container started up correctly.
+#
+# This runs *inside* the container after the build and lifecycle commands have
+# completed. It is used both as the envbuilder `ENVBUILDER_INIT_SCRIPT` (the
+# source of truth for how Coder builds this workspace) and can be run by hand
+# inside any container started from this dev container.
+#
+# It asserts the things that have previously broken on Coder/envbuilder:
+#   1. the effective user is `node`, not root,
+#   2. tmux is on PATH and can start a session that reads ~/.tmux.conf,
+#   3. the lifecycle ran to completion (postCreateCommand sentinel exists).
+#
+# Any failed check exits non-zero so CI fails loudly instead of silently
+# dropping you into a broken shell.
+
+set -euo pipefail
+
+failures=0
+
+fail() {
+  echo "FAIL: $*" >&2
+  failures=$((failures + 1))
+}
+
+pass() {
+  echo "PASS: $*"
+}
+
+echo "=== Verifying dev container startup ==="
+echo "whoami: $(whoami)"
+echo "id: $(id)"
+echo "HOME: ${HOME:-<unset>}"
+
+# 1. Effective user must be `node`, not root.
+if [ "$(id -un)" = "node" ]; then
+  pass "running as expected user 'node'"
+else
+  fail "expected to run as 'node' but running as '$(id -un)' (uid=$(id -u))"
+fi
+
+# 2. tmux must be on PATH and able to start a session that reads ~/.tmux.conf.
+if command -v tmux >/dev/null 2>&1; then
+  pass "tmux is on PATH ($(command -v tmux))"
+
+  session="verify-$$"
+  tmux_conf="${HOME}/.tmux.conf"
+  tmux_err="$(mktemp)"
+
+  if [ -f "$tmux_conf" ]; then
+    pass "tmux config present at $tmux_conf"
+  else
+    fail "tmux config missing at $tmux_conf"
+  fi
+
+  # Start a detached session, explicitly loading the user's config so a broken
+  # config (or a missing HOME) fails the check rather than silently starting a
+  # default session.
+  if tmux -f "$tmux_conf" new-session -d -s "$session" 'sleep 5' 2>"$tmux_err"; then
+    if tmux has-session -t "$session" 2>/dev/null; then
+      pass "tmux started a session using $tmux_conf"
+    else
+      fail "tmux session '$session' did not stay alive"
+    fi
+    tmux kill-session -t "$session" 2>/dev/null || true
+  else
+    fail "tmux could not start a session: $(cat "$tmux_err" 2>/dev/null)"
+  fi
+  rm -f "$tmux_err"
+else
+  fail "tmux is not on PATH"
+fi
+
+# 3. The lifecycle (postCreateCommand) must have completed.
+sentinel="${HOME}/.devcontainer-postcreate-done"
+if [ -f "$sentinel" ]; then
+  pass "lifecycle sentinel present ($sentinel)"
+else
+  fail "lifecycle sentinel missing ($sentinel); postCreateCommand did not complete"
+fi
+
+echo "=== Verification complete ==="
+if [ "$failures" -ne 0 ]; then
+  echo "$failures check(s) failed." >&2
+  exit 1
+fi
+echo "All checks passed."