Existing Codebases

Getting Deeper

You already know Build from small projects. Here, you will use it in a specific version of Django: first for one bounded command change, then for three related stories defined by one BMad Spec. The two exercises demonstrate the one-session and epic-sized planning paths.

1. Check Out the Exact Django Version

Clone Django 5.2.4 into a new directory, confirm that you have the expected source code, and create a branch for the exercise:

git clone --depth 1 --branch 5.2.4 https://github.com/django/django.git bmad-django
cd bmad-django
git rev-parse HEAD
git switch -c bmad-getting-deeper

git rev-parse HEAD should print:

c941d0deec0ea08a30670be0fac879f2372f071b

2. Set Up Django for Editing

Set up Python 3.12, install your Django checkout so the example app uses it, and create a small Django project next to the repository:

uv python install 3.12
uv venv --python 3.12
uv pip install -e .
mkdir ../bmad-django-app
uv run django-admin startproject tutorial_project ../bmad-django-app

3. Check the Starting Behavior

Confirm that JSON output is not yet available:

uv run python ../bmad-django-app/manage.py diffsettings --output=json

The command ends with this error:

manage.py diffsettings: error: argument --output: invalid choice: 'json' (choose from hash, unified)

4. Install BMad

Install BMad Method with the skills CLI, then let the bmad skill set up the project:

npx skills add bmad-code-org/BMAD-METHOD

Open Claude Code in this directory and ask the bmad skill to run bmad setup. Restart Claude Code once the skills are installed.

Tell Git to ignore the BMad files and uv lockfile created for this tutorial:

cat >> .git/info/exclude <<'EOF'
/_bmad/
/_bmad-output/
/.claude/
/uv.lock
EOF

5. Build It

Open your coding tool from the repository root. For Claude Code, run:

claude

/bmad-build Add JSON output support to django-admin diffsettings. Preserve
the existing output formats, add focused tests, and update the command
documentation. Leave the implementation in the working tree for local
inspection.

Build asks any questions it needs before it writes a plan. Answer according to your own preferences for the new JSON output. There is no single required JSON design for this exercise.

Build presents a plan and waits for you to approve it or ask for changes. Once approved, it builds and reviews the change, handles its findings, and shows you the result. Keep this exercise about JSON output for diffsettings; filtering, redaction, and CI behavior belong in the next exercise.

Build ends with a short summary and offers the next steps. Continue with the manual checks below before asking it to create a PR.

6. See It Work

Back in your shell, run Django’s diffsettings tests:

uv run python tests/runtests.py admin_scripts.tests.DiffSettings --verbosity 1

The tests should pass.

Now run the command again:

uv run python ../bmad-django-app/manage.py diffsettings --output=json

Look through the JSON and compare it with the choices you made with Build.

7. You Built It

Congratulations, you’ve now added something useful to a complex open-source codebase.

8. Write a Spec for the Larger Change

The next change needs three Build runs. /bmad-forge-idea can help you decide what to build. /bmad-advanced-elicitation can help you improve a draft. You do not need either here because the requirements are already clear. Send them straight to BMad Spec:

/bmad-spec Create a spec named diffsettings-audit and break it into
exactly three stories in this order: filters, redaction, then CI status.

Read the current diffsettings implementation, focused tests, and command
documentation before writing the spec. Keep every existing output format
and the JSON design already approved. Add repeatable --include and --exclude
shell-glob filters. Include patterns are OR, and exclusions always win. Add
repeatable --redact shell-glob masks that replace current and default values
with [REDACTED] without changing whether a difference exists. Add
--fail-on-difference, which exits 1 when differences remain after filtering and
0 otherwise. Each story adds focused tests and updates the existing command
documentation. Do not add another Django documentation file or an external
service. Use diffsettings-audit as the spec folder slug.

BMad Spec first has you pick or create an initiative, then writes _bmad-output/initiative-<slug>/spec-diffsettings-audit/spec-diffsettings-audit.md. Hand it to bmad-ticket to create epic-diffsettings-audit and its three ordered entries in tickets.toml. Read the spec and entries and answer any questions. Continue when they match the requirements above; note the epic folder the skill creates.

9. Build the Three Stories

Run Build once for each story, in order. Complete each Build run before moving to the next one. Every run uses the same spec. You will run these stories attentively because they establish how filtering, redaction, and exit behavior fit together. Later epics with stable, repeated patterns may be better candidates for automation.

Story 1: Filters

/bmad-build Implement story 1, filters, from
the epic-diffsettings-audit ticket tree.

After Build finishes, observe the result:

uv run python ../bmad-django-app/manage.py diffsettings \
  --include=DATABASES --include=DEBUG --include=SECRET_KEY \
  --exclude=DATABASES
printf 'exit: %s\n' "$?"

The output has DEBUG and SECRET_KEY, but no DATABASES, followed by exit: 0. The include patterns are combined, while the exclusion wins.

Story 2: Redaction

/bmad-build Implement story 2, redaction, from
the epic-diffsettings-audit ticket tree.

Observe the unified output:

uv run python ../bmad-django-app/manage.py diffsettings \
  --output=unified --include=SECRET_KEY --redact='SECRET*'
printf 'exit: %s\n' "$?"

The secret does not appear. Both sides of the difference are masked:

- SECRET_KEY = [REDACTED]
+ SECRET_KEY = [REDACTED]
exit: 0

Story 3: CI Status

/bmad-build Implement story 3, CI status, from
the epic-diffsettings-audit ticket tree.

Observe a difference that remains after filtering:

uv run python ../bmad-django-app/manage.py diffsettings \
  --include=DEBUG --fail-on-difference
printf 'exit: %s\n' "$?"

The DEBUG difference remains visible, and the command finishes with exit: 1.

10. See the Whole Change Work

Now combine the three stories in one observation:

uv run python ../bmad-django-app/manage.py diffsettings \
  --output=json --include=DEBUG --include=SECRET_KEY --exclude=DEBUG \
  --redact='SECRET*' --fail-on-difference
printf 'exit: %s\n' "$?"

The JSON contains only SECRET_KEY. Every current or default value exposed by the JSON shape you chose earlier is [REDACTED]; neither original value appears. The underlying values still differ, so the final line is exit: 1.

The first exercise gave Build one bounded change directly. This exercise gave three separate Build runs one spec. Filtering, redaction, and CI status still work together at the end. You have extended a mature Django command, and the final result still does what you asked for at the start.

If you want several perspectives on the result, /bmad-party-mode is an optional final step. You do not need it to finish this tutorial.

11. Review the Epic

Run Retrospective against the epic:

/bmad-retrospective epic-diffsettings-audit

Retrospective reads the epic’s tickets.toml and joined plans, then checks the integrated result against its requirements and source spec. It writes epic-diffsettings-audit-retrospective.md directly in the epic folder. Review its evidence, acceptance verdict, and proposed follow-up work. Keep the plans: they own status and historical evidence.

12. Keep Building

Now install BMad in your own repository, then use the bmad-build skill to make a change you want. See Build a Change for the attended path. Use Choose a Planning Path to decide when a change needs a spec, automation, or the full project flow.