diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 2332f0e..97ecea0 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -44,6 +44,25 @@ jobs:
       - name: Setup Quarto
         uses: quarto-dev/quarto-actions/setup@v2
 
+      # q2 has no `debug` library, so the modules find their siblings another
+      # way there. The suite renders a vendored copy with q2 when it is on the
+      # PATH, which this step provides. HTML needs no pandoc of its own. The
+      # release is pinned, so a new q2 changes the job only through a commit.
+      - name: Set up q2
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          set -euo pipefail
+          q2_dir="${RUNNER_TEMP}/q2"
+          mkdir -p -- "${q2_dir}/bin"
+          gh release download v0.34.0 -R quarto-dev/q2 \
+            -p 'q2-*-linux_amd64.tar.gz' -p 'checksums.sha256' -D "${q2_dir}"
+          cd -- "${q2_dir}"
+          sha256sum --check --ignore-missing --strict checksums.sha256
+          tar -xzf q2-*-linux_amd64.tar.gz -C "${q2_dir}/bin"
+          printf '%s\n' "${q2_dir}/bin" >> "${GITHUB_PATH}"
+          "${q2_dir}/bin/q2" --version
+
       # The suite loads and executes every module it finds on disk, which
       # subsumes a parse check, so there is no separate one.
       - name: Run the tests
diff --git a/CHANGELOG.md b/CHANGELOG.md
index b86fa0f..63a33b3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- fix: Load sibling modules on q2, which has no `debug` library. An extension that vendors the modules no longer stops there with a shortcode script error. (#12)
+
 ## 2.5.0 (2026-09-12)
 
 - feat: Add `Checker:format`, which checks one output format's options against the `formats` section and returns what they resolve to. It needs Quarto Wizard 3.6.0 or newer, whose `validate_format` reads the top level of the metadata where Quarto puts them.
diff --git a/modules/content-extraction.lua b/modules/content-extraction.lua
index fd7b55b..59f3554 100644
--- a/modules/content-extraction.lua
+++ b/modules/content-extraction.lua
@@ -10,9 +10,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load required modules
diff --git a/modules/html.lua b/modules/html.lua
index ecb4870..d947a3c 100644
--- a/modules/html.lua
+++ b/modules/html.lua
@@ -10,9 +10,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load string module for escape_html and escape_attribute
diff --git a/modules/metadata.lua b/modules/metadata.lua
index 7bc55c1..79a3a6e 100644
--- a/modules/metadata.lua
+++ b/modules/metadata.lua
@@ -10,9 +10,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load required modules
diff --git a/modules/pandoc-helpers.lua b/modules/pandoc-helpers.lua
index f90919b..5213f8e 100644
--- a/modules/pandoc-helpers.lua
+++ b/modules/pandoc-helpers.lua
@@ -10,9 +10,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load string module for is_empty
diff --git a/modules/paths.lua b/modules/paths.lua
index e00670d..3012c7b 100644
--- a/modules/paths.lua
+++ b/modules/paths.lua
@@ -10,9 +10,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load string module for is_empty
diff --git a/modules/schema-check.lua b/modules/schema-check.lua
index 9fe2c17..f16da4e 100644
--- a/modules/schema-check.lua
+++ b/modules/schema-check.lua
@@ -36,9 +36,15 @@ local M = {}
 --- @param filename string The sibling module filename (e.g., 'string.lua')
 --- @return table The loaded module
 local function load_sibling(filename)
+  local name = filename:gsub('%.lua$', '')
+  if not (debug and debug.getinfo) then
+    -- q2 has no `debug` library, and its `require` looks in the directory of
+    -- the module being loaded first.
+    return require(name)
+  end
   local source = debug.getinfo(1, 'S').source:sub(2)
   local dir = source:match('(.*[/\\])') or ''
-  return require((dir .. filename):gsub('%.lua$', ''))
+  return require(dir .. name)
 end
 
 --- Load required modules
diff --git a/tests/run.lua b/tests/run.lua
index 032ddd0..735c5c1 100644
--- a/tests/run.lua
+++ b/tests/run.lua
@@ -109,6 +109,86 @@ for _, name in ipairs(names) do
   end
 end
 
+io.stdout:write('# loading from an extension\n')
+
+--- The modules as an extension vendors them, in a directory whose path has a
+--- space. A shortcode loads every module that has siblings and prints what
+--- each one returned. quarto-cli has the `debug` library and q2 does not, so
+--- each renders the project with its own way to find the siblings.
+do
+  -- Read from disk, so a module that starts to load a sibling is probed too.
+  local with_siblings = {}
+  for _, name in ipairs(names) do
+    local handle = assert(io.open(here .. 'modules/' .. name .. '.lua', 'r'))
+    if handle:read('a'):find('load_sibling(', 1, true) then
+      with_siblings[#with_siblings + 1] = name
+    end
+    handle:close()
+  end
+  check(#with_siblings == 6, 'six modules load a sibling', table.concat(with_siblings, ', '))
+  local function quote(text)
+    return "'" .. text:gsub("'", "'\\''") .. "'"
+  end
+  local function write(path, text)
+    local handle = assert(io.open(path, 'w'))
+    handle:write(text)
+    handle:close()
+  end
+  pandoc.system.with_temporary_directory('lua-modules', function(tmp)
+    local project = tmp .. '/with space'
+    local vendor = project .. '/_extensions/probe/_vendor/quarto-lua-modules'
+    pandoc.system.make_directory(vendor, true)
+    os.execute('cp ' .. quote(here .. 'modules') .. '/*.lua ' .. quote(vendor))
+    local lines, expected = { 'local loaded = {}' }, {}
+    for _, name in ipairs(with_siblings) do
+      lines[#lines + 1] = string.format(
+        "loaded[#loaded + 1] = '%s=' .. type(require(quarto.utils.resolve_path('_vendor/quarto-lua-modules/%s.lua'):gsub('%%.lua$', '')))",
+        name, name)
+      expected[#expected + 1] = name .. '=table'
+    end
+    -- A sibling bound to the wrong table still loads, so one call goes through
+    -- `string.lua`, which the standard library of the same name cannot answer.
+    lines[#lines + 1] = "local html = require(quarto.utils.resolve_path('_vendor/quarto-lua-modules/html.lua'):gsub('%.lua$', ''))"
+    lines[#lines + 1] = "local ok, header = pcall(html.raw_header, 2, 'T', 'a&b')"
+    lines[#lines + 1] = "loaded[#loaded + 1] = 'escaped=' .. tostring(ok and header:find('a&b', 1, true) ~= nil)"
+    expected[#expected + 1] = 'escaped=true'
+    expected = table.concat(expected, ',')
+    lines[#lines + 1] = "return { probe = function() return pandoc.Str(table.concat(loaded, ',')) end }"
+    write(project .. '/_extensions/probe/probe.lua', table.concat(lines, '\n') .. '\n')
+    write(project .. '/_extensions/probe/_extension.yml',
+      'title: Probe\nversion: 1.0.0\ncontributes:\n  shortcodes:\n    - probe.lua\n')
+    write(project .. '/_quarto.yml', 'project:\n  type: default\n')
+    write(project .. '/doc.qmd', '---\ntitle: Probe\nformat: html\n---\n\n{{< probe >}}\n')
+
+    local function read(path)
+      local handle = io.open(path, 'r')
+      if not handle then
+        return ''
+      end
+      local text = handle:read('a')
+      handle:close()
+      return text
+    end
+
+    --- The rendered page, and the engine's output for the failure detail.
+    local function render(command)
+      os.remove(project .. '/doc.html')
+      os.execute('cd ' .. quote(project) .. ' && ' .. command .. ' render doc.qmd > render.log 2>&1')
+      return read(project .. '/doc.html'), read(project .. '/render.log')
+    end
+
+    for _, engine in ipairs({ 'quarto', 'q2' }) do
+      local description = engine .. ' loads every module and its siblings from a vendored path with a space'
+      if os.execute(engine .. ' --version > /dev/null 2>&1') then
+        local page, output = render(engine)
+        check(page:find(expected, 1, true) ~= nil, description, output:sub(-600))
+      else
+        io.stdout:write('skip ', description, ': ', engine, ' is not on the PATH\n')
+      end
+    end
+  end)
+end
+
 io.stdout:write('# exports\n')
 
 --- Checked both ways. `EXPECTED` is hand-maintained on purpose, because these