Statechart Agent Skill

Statechart

The chart shows how a system moves between states, in the words of the person who reads it. The scripts call that person the reader, as in readerActions and reader steps. The reader decides whether the behavior is right. The reader can be a PM who reads a product flow, an engineer who reads a protocol or a service, or a reviewer who reads a design. Label each state with what the reader can observe or cares about, such as "Paying" in an app or "Lease held, write pending" in a lock service. Label each step with the event that moves the system, such as "tap Pay" in an app, "the request reads the count" in a service, or "the lease expires" in a lock service. Keep variable and action names out of labels.

Run node <skill dir>/scripts/statechart.mjs --help for the commands and for how to read a chart. Work in a scratch folder outside the repo. The command writes its files next to its input. The first run installs TLC and Java with mise, which downloads about 305 MB and needs the network. The help's TLC section tells how to use a TLC and Java that you have.

Pick the direction

Use from-spec when a TLA+ model exists, or when the code or design is concrete enough to model. Use from-chart when there is no model, and the people who own the behavior should agree on it before anyone builds or changes it.

If the code already has a TLA+ spec, start from a copy of that spec. Add the variables that the labels and rules need, and keep the spec's existing invariants. To audit the code against that spec, make the copy model what the code does, and pass the spec unchanged as --against. Check it with the config that its authors say it passes, and pass that config as --against-config unless it has the spec's file name. One mapping labels both models, so label states and steps from variables that both have. A label that reads a variable that only the copy has must test "name" in v first, and label the spec's states another way. When a model describes code, tie every variable and action to code that a reviewer can open, and take its name from that code. Make each action one indivisible step in the code. Put the file and line of the code that checks each condition of an action in a comment on that condition, as in \* pay.ts:120.

Spec to chart

• Choose the chart's states. When the reader cares about something that a label would hide, put it in the label, such as "Paying, charged twice" when the screen still says "Paying" after a second charge. Start with few states. Split a state when the split separates two bugs, or when the reader must see a step that leaves the label the same, such as a request that reads a count, because the chart does not draw such a step.
• Write each rule that the reader agrees to as one invariant, with the reader's sentence as its comment. A rule can be about any state in the model: what a screen shows, what a request returns, a stored value, or a message between parts. When the model describes code, the rule also names the code that can change that state. If the model has no variable for what a rule is about, add the variable. Do not drop the rule. For a rule that applies once everything settles, define Settled == ~ENABLED System, where System is the disjunction of the actions that happen without the reader. For a rule that applies at a narrower point, such as after each request finishes, write a predicate for that point and make the rule hold where it is true. For a goal that the system must be able to reach, write a rule that says the goal never happens, and pass that rule with --expect-broken. The path that breaks it is then a way to the goal. The chart still draws the goal as a state that breaks a rule, and stories.md tells it as a bug, so name each goal rule in the hand-off. Report each other broken rule as a bug. Never weaken a rule to make the chart pass.
• Run from-spec, then fill in the mapping file it writes. Put the labels in state and step. When the chart must separate what the reader does from what happens around the reader, list the reader's actions in readerActions by action name. Leave the list empty when every action counts the same. When the overview is too large to read, add --cover. The overview then keeps only the shortest path to each settled state, plus every bug path, and the command lists what it leaves out. When two bugs end in the same state for different reasons, add cause and make it name each reason. To color the states by what the reader cares about, such as "Order shipped", return that group from group. The legend names the groups, and in the PNG a state that breaks a rule is red in place of its group color.
• If there is a proposed fix, model it and pass that model as --fixed.
• Render each chart with --png and look at every PNG. Check that each numbered path reads in order. If a label reads like code, fix state or step and run the command again.
• When the model describes code, replay each bug's story against the real code, as a test or a live repro, one step per model action. Drive only entry points that a user, a client, or another part of the system reaches, and assert only what they can observe. A test that asserts a flag that it sets, or that searches the source text, proves nothing. When a run proves part of the story and reading the code proves the rest, say which part each proves. When the test copies code that it cannot import, give the commit and line range of the copy, and list what the copy leaves out. If a bug does not reproduce, assume the model is wrong until you show otherwise. Fix the model, not the rule. For a design with no code yet, the TLC trace is the evidence, and the hand-off must say so.

Chart to spec

• Draw the chart that the reader agrees to. Use one region for each part that changes independently, such as a device's connection and the job that the device runs. Add one %% never line per rule, with the reader's sentence. Draw an event that stops or restarts a part, such as "the app closes" or "the process restarts", as an edge to the state that comes next. Run render CHART.mmd to get a PNG of the chart for the reader.
• Run from-chart. If a rule breaks, the agreed chart has a design bug. Take the bug to the people who agreed to the chart.
• Add the real code's guards and variables to the spec that from-chart wrote, and run from-chart again. For each difference it prints, ask the reader whether the code is wrong or the chart needs a correction.

Hand off

Give the PNG and the Mermaid source of each chart to the person who must decide. Add one line per bug with the rule it breaks, its steps as the chart numbers them, whether it reproduced, and whether the proposed fix fixes it. A broken PROPERTY has no path on the chart, so give the TLC counterexample that stories.md names for it. With --against, also say whether the spec already lists it: in the configs that stories.md names, in the spec's notes on what the model leaves out, or not at all. When the model describes code, do not report that a rule holds while the command lists conditions with no file and line. Take every count you quote from summary.json. Attach stories.md and the command that regenerates everything. Say what the model leaves out.