diff --git a/tests/entrypoint_test.sh b/tests/entrypoint_test.sh
new file mode 100755
index 0000000..e8cb987
--- /dev/null
+++ b/tests/entrypoint_test.sh
@@ -0,0 +1,145 @@
+#!/bin/bash
+
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+
+fail() {
+  echo "FAIL: $1" >&2
+  exit 1
+}
+
+make_fake_codex() {
+  local dir="$1"
+  local exit_code="$2"
+  local message="$3"
+
+  cat > "$dir/codex" <<SH
+#!/bin/sh
+echo "$message"
+exit $exit_code
+SH
+  chmod +x "$dir/codex"
+}
+
+encoded_json() {
+  printf '%s' "$1" | base64 | tr -d '\n'
+}
+
+run_entrypoint() {
+  local tmpdir="$1"
+  shift
+
+  env \
+  PATH="$tmpdir:$PATH" \
+  CODEX_HOME="$tmpdir/codex-home" \
+  GITHUB_OUTPUT="$tmpdir/github-output" \
+  "$@" \
+  bash "$ROOT_DIR/entrypoint.sh" > "$tmpdir/stdout" 2> "$tmpdir/stderr"
+}
+
+assert_status() {
+  local actual="$1"
+  local expected="$2"
+
+  [[ "$actual" -eq "$expected" ]] || fail "expected status $expected, got $actual"
+}
+
+test_missing_oauth() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 0 "unused"
+
+  set +e
+  run_entrypoint "$tmpdir" MODEL="gpt-test" PROMPT="hello"
+  local status="$?"
+  set -e
+
+  assert_status "$status" 1
+  grep -q "OAUTH is required" "$tmpdir/stderr" || fail "missing OAUTH error"
+  rm -rf "$tmpdir"
+}
+
+test_missing_model() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 0 "unused"
+
+  set +e
+  run_entrypoint "$tmpdir" OAUTH="$(encoded_json '{}')" PROMPT="hello"
+  local status="$?"
+  set -e
+
+  assert_status "$status" 1
+  grep -q "MODEL is required" "$tmpdir/stderr" || fail "missing MODEL error"
+  rm -rf "$tmpdir"
+}
+
+test_invalid_base64() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 0 "unused"
+
+  set +e
+  run_entrypoint "$tmpdir" OAUTH="not-base64" MODEL="gpt-test" PROMPT="hello"
+  local status="$?"
+  set -e
+
+  assert_status "$status" 1
+  grep -q "valid base64" "$tmpdir/stderr" || fail "invalid base64 error"
+  rm -rf "$tmpdir"
+}
+
+test_non_object_json() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 0 "unused"
+
+  set +e
+  run_entrypoint "$tmpdir" OAUTH="$(encoded_json '[]')" MODEL="gpt-test" PROMPT="hello"
+  local status="$?"
+  set -e
+
+  assert_status "$status" 1
+  grep -q "JSON object" "$tmpdir/stderr" || fail "non-object JSON error"
+  rm -rf "$tmpdir"
+}
+
+test_success_output() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 0 "codex ok"
+
+  run_entrypoint "$tmpdir" OAUTH="$(encoded_json '{}')" MODEL="gpt-test" PROMPT="hello"
+
+  grep -q "status=completed" "$tmpdir/github-output" || fail "completed status output"
+  grep -q "codex ok" "$tmpdir/github-output" || fail "codex output"
+  [[ ! -e "$tmpdir/codex-home/auth.json" ]] || fail "auth.json was not cleaned up"
+  rm -rf "$tmpdir"
+}
+
+test_failure_output() {
+  local tmpdir
+  tmpdir="$(mktemp -d)"
+  make_fake_codex "$tmpdir" 7 "codex failed"
+
+  set +e
+  run_entrypoint "$tmpdir" OAUTH="$(encoded_json '{}')" MODEL="gpt-test" PROMPT="hello"
+  local status="$?"
+  set -e
+
+  assert_status "$status" 7
+  grep -q "status=failed" "$tmpdir/github-output" || fail "failed status output"
+  grep -q "codex failed" "$tmpdir/github-output" || fail "failed codex output"
+  [[ ! -e "$tmpdir/codex-home/auth.json" ]] || fail "auth.json was not cleaned up after failure"
+  rm -rf "$tmpdir"
+}
+
+test_missing_oauth
+test_missing_model
+test_invalid_base64
+test_non_object_json
+test_success_output
+test_failure_output
+
+echo "entrypoint tests passed"