diff --git a/AGENTS.md b/AGENTS.md
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index 00000000..9ecacedf
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@@ -0,0 +1,130 @@
+# django-bootstrap4 — Agent Guide
+
+Bootstrap 4 template tags and filters for Django, by [Zostera](https://github.com/zostera).
+
+## Maintenance mode
+
+This package is in **maintenance mode** — Bootstrap 4 has been superseded by Bootstrap 5. Only bug fixes and security updates are accepted. Do not add new features or enhancements. Suggest Bootstrap 5 (`django-bootstrap5`) for new work.
+
+Bootstrap 4 is EOL — no new releases are expected, but if they occur we will evaluate supporting them. Bootstrap 4 docs: https://getbootstrap.com/docs/4.6/
+
+## Related packages
+
+These packages share tooling and conventions — changes in one often mirror to others:
+
+- `https://github.com/zostera/django-bootstrap3` — Bootstrap 3 for Django
+- `https://github.com/zostera/django-bootstrap4` — Bootstrap 4 for Django (this package)
+- `https://github.com/zostera/django-bootstrap5` — Bootstrap 5 for Django
+- `https://github.com/zostera/django-icons` — Icons for Django
+- `https://github.com/zostera/django-marina` — Django extensions by Zostera
+
+Config files (justfile, tox.ini, pyproject.toml, etc.) are kept in sync across packages.
+AGENTS.md is **not** synced — each package has its own.
+
+## Setup
+
+Requires [uv](https://github.com/astral-sh/uv) and [just](https://github.com/casey/just).
+
+```
+just install    # install deps from uv.lock
+just upgrade    # upgrade all deps
+```
+
+Never invoke `python`, `pip`, or `ruff` directly. All commands go through `just`, which delegates to `uv run` (venv) or `uvx` (ephemeral tools like ruff, twine, check-manifest).
+
+`uv.lock` is fully generated — never manually resolve merge conflicts in it. On conflict: accept either side, then run `just upgrade` to regenerate.
+
+## Key commands
+
+```
+just test           # run tests (single Python/Django version)
+just test-cov       # run tests with coverage report
+just tests          # run full tox matrix (all Python × Django combos)
+just lint           # check formatting and style (ruff)
+just format         # auto-fix formatting and style
+just build          # build + packaging checks (preflight before release)
+just docs           # build Sphinx documentation
+just example        # run the example Django project
+just version        # print current package version
+```
+
+## Code style
+
+- **Formatter/linter**: ruff (line length 120)
+- **Docstrings**: pydocstyle D2xx/D4xx rules; D1xx (missing docstring) is ignored
+- `ruff check --fix` auto-fixes isort, pyupgrade, and some flake8 issues
+- `F8` (unused names) is not auto-fixed — fix manually
+
+Run `just lint` before committing. CI enforces it.
+
+## Package structure
+
+```
+src/bootstrap4/
+    __about__.py        version string
+    __init__.py         exports __version__
+    bootstrap.py        settings access and URL helpers (css_url, javascript_url, etc.)
+    components.py       alert renderers
+    exceptions.py       custom exceptions
+    forms.py            form and field rendering functions
+    models.py           model helpers
+    renderers.py        field and form renderer classes
+    text.py             text utilities
+    utils.py            template and HTML utilities
+    widgets.py          custom form widgets
+    templatetags/
+        bootstrap4.py   all template tags and filters
+    templates/bootstrap4/
+        *.html          component templates (messages, pagination, field errors, etc.)
+        widgets/        widget templates
+```
+
+## Testing
+
+**Test runner is Django's test runner, not pytest.** Use `manage.py test` or `just test`.
+
+```
+tests/
+    app/                minimal Django project used as test harness
+    smoke_test.py       import smoke test (run against built wheel/tarball)
+    test_alerts.py
+    test_buttons.py
+    test_forms.py
+    test_formsets.py
+    test_media.py
+    test_messages.py
+    test_paginator.py
+    test_settings.py
+    test_templates.py
+    test_utils.py
+    test_version.py
+```
+
+Test matrix (tox) — not a full grid:
+
+| Python  | Django versions         |
+|---------|-------------------------|
+| 3.10    | 4.2, 5.2                |
+| 3.11    | 4.2, 5.2                |
+| 3.12    | 4.2, 5.2, 6.0, main     |
+| 3.13    | 4.2, 5.2, 6.0, main     |
+| 3.14    | 5.2, 6.0, main          |
+
+Target the matrix when adding features; avoid Django-version-specific code paths where possible.
+
+## CI
+
+GitHub Actions runs on every push and PR:
+- `ci.yml` — lint + full tox matrix
+- `release.yml` — publishes to PyPI on version tags
+
+`just lint` must pass before committing — CI enforces it and will fail the PR.
+
+## Release process
+
+1. Update `CHANGELOG.md` and bump `version` in `pyproject.toml`
+2. Commit and push to `main`
+3. `just build` — builds wheel + tarball, runs packaging checks, and smoke-tests both against an isolated env
+4. `just release-tag` — creates and pushes the version tag; GitHub Actions publishes to PyPI
+
+`just release-tag` requires: clean working directory AND current branch is `main`. It will fail otherwise.
diff --git a/src/bootstrap4/utils.py b/src/bootstrap4/utils.py
index 82472e3e..b79589de 100644
--- a/src/bootstrap4/utils.py
+++ b/src/bootstrap4/utils.py
@@ -119,7 +119,7 @@ def url_replace_param(url, name, value):
     params = parse_qs(url_components.query)
 
     if value is None:
-        del params[name]
+        params.pop(name, None)
     else:
         params[name] = value