ai-workflow-course/modules/06-branches-sandboxes-for-experiments

Module 6 — Branches: Sandboxes for Experiments

A branch is a disposable copy of your project where the AI can try anything — and main never finds out unless you decide it should. This is what turns "let the agent attempt something bold" from a gamble into a one-line decision: keep it or throw it away.

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Prerequisites

• Module 2 — Version Control as a Safety Net. You can init, commit, read git diff/git log/git status, and git restore an unwanted change. Branches build directly on commits: a branch is just a label on the commit history you already understand.
• Module 4 — Getting the AI Out of the Browser. The AI now edits your real files directly from your editor. That's exactly the capability that makes branches matter — you're about to let it edit files fast and confidently, and you want a wall around the blast radius.
• Module 5 — Commit the AI's Config, Not Just the Code. Your committed instructions file travels with the branch automatically, so an agent working on a branch inherits the same setup. (You'll see this for free in the lab — nothing to do, just notice it.)

Module 2's git restore undoes uncommitted changes back to your last checkpoint. This module is the next size up: isolating a whole line of committed work so you can keep or discard it as a unit.

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Learning objectives

By the end of this module you can:

1. Create a branch, switch between branches, and explain what a branch actually is (a movable pointer, not a copy of your files).
2. Let an AI make a bold, multi-commit change on a branch while main stays untouched and runnable.
3. Decide the experiment's fate in one command: merge it into main to keep it, or delete the branch to throw it away with zero trace.
4. Read a merge conflict — the <<<<<<</=======/>>>>>>> markers — and resolve it deliberately, including handing the conflict to the AI to resolve.
5. Tell the difference between a fast-forward merge and a merge commit, and know which one you just got.

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Key concepts

What a branch actually is

Strip the mystique and a branch is a named, movable pointer to a commit. That's the whole definition. Your commit history is a chain of snapshots (Module 2); a branch is a sticky label that points at one of them and moves forward every time you commit on it.

When you ran git init -b main in Module 2, Git made one branch for you automatically — named main (the -b main is what guaranteed that name; in this course your repo is always on main). Every commit you made moved the main label forward. You were "on a branch" the entire time without thinking about it.

The thing that surprises people coming from an ops background: creating a branch copies nothing. There's no second folder, no duplicated files, no disk cost worth mentioning. Git just writes a new label pointing at the same commit you're standing on. That's why branches are cheap enough to be disposable — and disposable is exactly the property we want.

git branch                       # list branches; the * marks the one you're on
git switch -c experiment         # create a branch called "experiment" and switch to it
git switch main                  # switch back to main
git branch -d experiment         # delete a branch you've already merged
git branch -D experiment         # FORCE-delete a branch, merged or not (the "throw it away" button)

Naming note. git switch (create/move between branches) and git restore (the Module 2 undo) were split out of the older, overloaded git checkout command. You'll still see git checkout -b experiment everywhere online — it does the same thing as git switch -c experiment. Both work; this module uses switch/restore because they say what they mean.

The reframe: a branch is a sandbox you can blow away

You already have the instinct for this. A branch is the Git equivalent of a scratch VM you can snapshot and roll back, a staging environment nobody depends on, a feature-flag you can rip out. You spin one up precisely because you're about to do something you might regret, and you want a clean way to make it never have happened.

In Module 2 the safety net was "commit, then restore if the AI makes a mess." That's perfect for a single bad edit. But some experiments are bigger than one edit — "rewrite the storage layer," "try a totally different CLI structure," "add a feature that touches four files." Those take several commits to even evaluate, and you don't want that half-finished, possibly-broken work sitting on main. A branch gives the whole experiment its own track:

main:         A───B───C                  (always runnable; this is your "known good")
                       \
experiment:             D───E───F         (the AI's bold attempt, however messy)

While you're on experiment, main is frozen at C — runnable, shippable, untouched. The AI can leave experiment in a smoking crater at F and main doesn't care. When you're done you make one decision:

• Keep it: merge experiment into main (C gains D, E, F).
• Kill it: delete experiment. D, E, F evaporate. main is still exactly C, as if the experiment never happened.

That "kill it, no trace" path is the one this module exists for. It's the difference between "I have to carefully undo everything the AI did" and "I delete the branch."

Switching branches changes your files

Here's the part that feels like magic the first time. When you git switch to another branch, Git rewrites the files in your folder to match that branch. Switch to experiment and the AI's half-built feature appears in your editor. Switch back to main and it vanishes — your files are back to commit C. Same folder, different contents, instantly.

This is why you can't switch with uncommitted changes lying around that would be clobbered: Git stops you, because switching would silently throw work away. The fix is the Module 2 habit — commit (or stash) before you switch. On a branch, "commit often" pays off again: each commit is a safe point to switch away from.

One folder, one branch at a time. Switching swaps the whole folder between branches, which means you can only have one branch checked out at once. The moment you want two branches live simultaneously — say, two agents working in parallel without overwriting each other's files — you've hit the limit of branches alone. That's exactly what Module 7 (Worktrees) solves: multiple working directories from one repo. Branches are the concept; worktrees are how you run several at once. Keep that in your back pocket.

Merging: keeping the experiment

Merging takes the commits from one branch and brings them into another. You switch to the branch you want to receive the work (usually main), then merge the other branch in:

git switch main
git merge experiment

There are two outcomes, and it's worth knowing which you got:

• Fast-forward. If main hasn't moved since you branched (it's still at C), Git doesn't need to do anything clever — it just slides the main label forward to F. The history stays a straight line. This is the common case for a solo experiment.
• Merge commit. If main did move on (someone — or you — committed to main while experiment was off doing its thing), the two lines of history have diverged. Git stitches them together with a new commit that has two parents. You'll be dropped into an editor to confirm the merge message; save and close it.

You don't choose between these — Git picks based on whether the branches diverged. You just need to recognize them in git log --oneline --graph, where a fast-forward is a straight line and a merge commit is a visible fork-and-join.

After a successful merge, the branch has done its job. Delete it:

git branch -d experiment         # -d refuses if it's NOT fully merged — a safety check

Discarding: killing the experiment

This is the payoff. The AI tried something bold on the branch, you looked at it, and you don't want it. You don't undo anything. You don't restore file by file. You switch away and delete the branch:

git switch main                  # your files snap back to known-good main
git branch -D experiment         # -D force-deletes even though it was never merged

That's it. The experiment is gone. main never changed. git log on main shows no sign it ever happened. The whole bold attempt cost you one branch and one delete.

This is the mental shift the module is selling: when discarding is this cheap, you stop being precious about what you let the AI try. Risky refactor? Branch it. Want to compare two approaches? A branch each, keep the winner, delete the loser. The branch is the unit of "maybe."

Merge conflicts: when two changes collide

Most merges just work — Git is good at combining changes that touch different lines. A conflict happens only when two branches changed the same lines in different ways, and Git refuses to guess which one you meant. It stops the merge and marks the collision inside the file so you can decide:

<<<<<<< HEAD
    print("usage: python cli.py [add <title> | list | done <index> | stats]")
=======
    print("usage: python cli.py [add <title> | list | done <index> | purge]")
>>>>>>> experiment

Read it like this:

• <<<<<<< HEAD to ======= is your current branch's version (the branch you're merging into — main, here).
• ======= to >>>>>>> experiment is the incoming branch's version.
• Both markers and the divider are real text Git inserted into your file. Resolving means editing the file so it contains the version you want and deleting all three marker lines.

You're not picking a side mechanically — you're deciding what the line should say. Often that's one side, sometimes it's a blend of both (here: a usage string that lists both stats and purge). Then you tell Git the conflict is settled:

# edit the file: remove the markers, leave the correct content
git add cli.py                   # marks this file's conflict as resolved
git commit                       # completes the merge (opens an editor for the merge message)

git status during a conflict is your map — it lists every file still "unmerged." When that list is empty and you've git add-ed them all, you commit and the merge is done. If you panic mid-conflict, git merge --abort rewinds you to before the merge, no harm done.

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The AI angle

Everything above is standard Git. Here's why it matters more in an AI-assisted workflow, not less:

• The branch is the blast-radius container for an autonomous attempt. An agent editing your files directly (Module 4) is fast and confident — including when it's confidently wrong across four files. On main, cleaning that up is a chore. On a branch, you delete the branch. The riskier and more autonomous the AI work, the more a branch earns its keep — which is why this concept underpins everything in Unit 5, where agents run with far less supervision.
• "Throw it away" is the feature, not the failure. With copy-paste, a rejected AI attempt still cost you the manual work of pasting it in and the manual work of ripping it back out. With a branch, a rejected attempt costs nothing — git branch -D and it's as if it never happened. That flips the economics: you can let the AI try things you'd never risk if undoing were expensive.
• Compare, don't commit-and-hope. Ask the AI for approach A on one branch and approach B on another. Run both. Keep the winner, delete the loser. You're using branches as cheap A/B experiments on implementation — something that's painful without them and trivial with them.
• Conflicts are a great place to put the AI to work. A merge conflict is a small, perfectly bounded reasoning task: here are two versions of the same lines and the surrounding code — produce the correct combined version. The AI can see both sides and the intent. You still decide whether its resolution is right (it can absolutely merge two changes into something that satisfies neither), but "explain this conflict and propose a resolution" is one of the highest-hit-rate uses of an editor-integrated agent. You'll do exactly this in the lab.

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Hands-on lab

Lab language: shell (Git commands), driving the tasks-app from Modules 1–2 with your editor-integrated AI from Module 4.

You'll do three things: let the AI try a bold change on a branch, decide its fate, and then deliberately create and resolve a merge conflict — using the AI to help resolve it.

You'll need:

• The tasks-app Git repo from Module 2 (committed, clean working tree — run git status and make sure it says "nothing to commit").
• Your editor-integrated AI from Module 4.
• Git (you've had it since Module 2).

Throughout, "ask your AI" now means your editor-integrated agent (Module 4) editing the files directly — no more copy-paste. After it edits, you still read git diff before committing. That habit doesn't go away; the branch just decides how much damage a bad diff can do.

Part A — Branch it and let the AI go bold

1. Confirm you're on main and clean, then create an experiment branch and switch to it:

   cd ~/workflow-course/tasks-app
   git switch main
   git status                       # must be clean
   git switch -c experiment/priorities
   git branch                       # the * is now on experiment/priorities
   

2. Give the AI a deliberately bold task — the kind you'd hesitate to run straight on main:

   "Add task priorities (low/medium/high) to this app. Store a priority on each task, let me set it when adding (add "thing" --priority high), show it in list, and sort list so high priority comes first. Change whatever files you need to."

   Let it edit tasks.py and cli.py freely. This is a multi-file change — exactly the kind that's nerve-wracking on main and relaxed on a branch.

3. Review and commit the experiment on the branch:

   git diff                         # read what it actually changed
   python cli.py add "ship module 6" --priority high
   python cli.py add "water plants" --priority low
   python cli.py list               # see if priorities work and sort
   git add .
   git commit -m "Add task priorities (experiment)"
   

4. Now prove the isolation. Switch back to main and watch the feature disappear:

   git switch main
   python cli.py list               # no priorities — main is exactly as you left it
   

   Your bold change exists only on the branch. main never saw it. Sit with that for a second — that's the whole point.

Part B — Decide its fate

Pick the path that matches reality. Do at least one; ideally do Path 2 (discard) on this experiment so you feel how clean it is, then re-run Part A and do Path 1 (keep) so you've done both.

Path 1 — Keep it (merge):

git switch main
git merge experiment/priorities      # likely a fast-forward: main slides up to the branch
git log --oneline --graph            # see the history; straight line = fast-forward
python cli.py list                   # the feature is now on main
git branch -d experiment/priorities  # branch did its job; -d is the safe delete

Path 2 — Throw it away (discard):

git switch main                      # files snap back to known-good main
git branch -D experiment/priorities  # force-delete the unmerged branch
git log --oneline                    # no trace of the experiment on main
python cli.py list                   # main is untouched, exactly as before

Notice what you did not do in Path 2: no file-by-file restore, no manual undo, no hunting through diffs. You deleted a label and the entire experiment was gone. That's the economics shift — bold AI attempts become free to reject.

Part C — Create a merge conflict and resolve it with the AI

Now the skill everyone fears and nobody should. You'll engineer a guaranteed conflict by having two branches change the same line in different ways, then resolve it.

Starting state. By now your tasks-app has accumulated commands from earlier modules, so your usage: line is longer than the bare [add <title> | list | done <index>] you started with — and that's fine. This lab works regardless of what's on that line, because the collision is just "two branches each appended a different new command to the same usage line." To make it reproduce even on a carried-forward app, we deliberately add two commands you haven't built yet — stats and purge. (Any two brand-new commands would do; the point is the same line, edited two ways.) The marker examples below show the shape; your real markers will carry your fuller usage string.

1. Make sure you're on a clean main. Create the first branch and have the AI add a stats command:

   git switch main
   git switch -c feature/stats
   

   Ask the AI: "Add a stats command to cli.py that prints how many tasks are total, done, and pending, and update the usage string to include it." Then:

   git diff                         # confirm it edited the usage line + added the command
   git add . && git commit -m "Add stats command"
   

2. Switch back to main and create a different branch that touches the same usage line:

   git switch main
   git switch -c feature/purge
   

   Ask the AI: "Add a purge command to cli.py that removes all completed (done) tasks, and update the usage string to include it." Then:

   git diff                         # it also edited the usage line — this is the collision to come
   git add . && git commit -m "Add purge command"
   

   Both branches changed the same usage: line, each adding a different command to it. Git will not be able to auto-merge that line.

3. Merge them and watch it conflict. Merge feature/stats into feature/purge (you're on feature/purge):

   git merge feature/stats
   

   Git stops with a conflict and tells you which file is unmerged. Confirm:

   git status                       # cli.py listed under "Unmerged paths"
   

4. Open cli.py and find the conflict markers around the usage line (your usage string will be longer — it carries the commands from earlier modules — but the collision is exactly this: both branches appended a different new command to it):

   <<<<<<< HEAD
       print("usage: python cli.py [add <title> | list | done <index> | purge]")
   =======
       print("usage: python cli.py [add <title> | list | done <index> | stats]")
   >>>>>>> feature/stats
   

   (The command bodies for stats and purge touch different lines, so Git merged those cleanly on its own — the only collision is the usage string both branches edited.)

5. Resolve it with the AI. With your editor-integrated agent, this is its sweet spot. Ask:

   "cli.py has a merge conflict on the usage line. I want the final version to list BOTH the stats and purge commands. Resolve the conflict and remove the markers."

   It should produce a single, marker-free line listing both commands, e.g.:

       print("usage: python cli.py [add <title> | list | done <index> | stats | purge]")
   

   Verify its work — this is the part the AI can get subtly wrong. A conflict resolver can confidently drop one side, leave a stray marker, or "blend" the lines into something that runs but means the wrong thing. Read the result and run it:

   git diff                         # check ONLY what you intended changed; no markers remain
   python cli.py                    # run with no args — see the merged usage string
   python cli.py stats              # both commands actually work
   python cli.py purge
   

6. Tell Git the conflict is settled and complete the merge:

   git add cli.py
   git commit                       # opens an editor for the merge message; save and close
   git log --oneline --graph        # see the fork-and-join: this is a merge commit
   

   You just resolved a real merge conflict. The marker syntax is identical no matter the file or the project — once you can read those three lines, conflicts stop being scary and become a five-minute chore.

Guaranteed-conflict generator. AI edits are nondeterministic, so if the agent didn't touch the same line on both branches and you didn't get a conflict in step 3, run the helper script to manufacture one deterministically, then practice steps 4–6 on it. Copy it into your tasks-app first (the course's lab scripts live in the course repo, not in tasks-app — see Module 4's You'll need), then run it from inside the repo:

cp /path/to/modules/06-branches-sandboxes-for-experiments/lab/make-conflict.sh .
bash make-conflict.sh

It creates two branches that both edit the same line of README.md, leaving you mid-conflict with on-screen instructions. The resolution mechanic is identical to the code case above.

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Where it breaks

The honest limits, so you don't over-trust the sandbox:

• A branch isolates files in the repo, nothing else. Switching branches rewrites your tracked files — it does not roll back a database the app wrote to, files Git is ignoring, running processes, or anything outside version control. If your AI experiment ran a migration or wrote to tasks.json (which the Module 2 .gitignore excludes), deleting the branch won't undo that. The sandbox is the repo, not the world. (Real environment isolation is a later problem — containers, Module 16.)
• Branches are local until you push them. Everything in this module lives on your laptop. A branch isn't shared, backed up, or visible to anyone else until there's a remote — that's Module 8. Right now git branch -D deletes work that exists nowhere else, permanently. Treat an unpushed branch as exactly as fragile as the rest of your local-only repo.
• The AI can resolve a conflict into something plausible and wrong. It sees both sides and the intent, which makes it good at this — but "good" isn't "trusted." A resolution that runs cleanly can still mean the wrong thing (silently keeping the worse of two changes, or merging two behaviors into one that satisfies neither). The git diff + run-it check in the lab isn't optional ceremony; it's the actual safeguard. Reviewing AI output is its own discipline — Module 10.
• Long-lived branches drift and conflict harder. The longer a branch lives away from main, the more main moves underneath it and the gnarlier the eventual merge. The defense is the same as "commit often": branch small, merge soon, delete promptly. A branch that's been open for three weeks is a future conflict, not a sandbox.
• Force-delete (-D) and merge --abort are sharp. -D discards unmerged commits with no confirmation; --abort throws away an in-progress resolution. Both are exactly what you want at the right moment and a foot-gun at the wrong one. Know which one you're reaching for.

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Check for understanding

You're done when:

• You created a branch, let the AI make a multi-file change on it, and confirmed main was untouched by switching back and seeing the change vanish.
• You have discarded an experiment with git branch -D and confirmed main shows no trace, and you have merged one in and seen it land on main.
• You can explain, in one sentence, why creating a branch costs essentially nothing (it's a movable pointer, not a copy).
• You deliberately created a merge conflict, read the <<<<<<</=======/>>>>>>> markers, resolved it (with the AI's help) to a marker-free file that runs, and completed the merge with git add + git commit.
• You can name the limit: a branch isolates tracked files, not your database, ignored files, or the outside world.

When "let the agent try something wild" feels like a one-line decision instead of a risk assessment, you've got it. Module 7 takes the next step: running several of these branches live at the same time in separate working directories, so multiple agents can work in parallel without colliding.