Edit an existing plan

Change a .arch file without breaking what already works. If archlang is not on the path, write npx -y @chanmeng666/archlang in its place. Never run a bare arch: on Linux, macOS and Git Bash that name is the coreutils command.

This skill is a workflow, not a language reference. Run archlang spec for the language and archlang <cmd> --help for a command's flags.

Scope

• Run the archlang CLI commands below and no other program.
• Change the one .arch file the user asked you to edit (plan.arch below). If the user has not named it, ask. Besides it, write only the drawing the CLI renders and the .bak copy that --backup keeps.
• The user's explicit instructions take priority over these steps.

Steps

1. Record the starting state. Run archlang validate plan.arch --json and archlang describe plan.arch --select rooms,totals --json. Keep both results: they are what you compare against at the end. If the plan already has errors, tell the user before you edit.
2. Read only what the change touches. archlang describe plan.arch --room <ids> --json returns those rooms with the doors, windows and furniture that touch them. archlang describe plan.arch --select freedom --json says which positions the author typed (absolute) and which the compiler derives. Change a typed number; a derived one moves by itself when the thing it depends on moves.
3. Read the language for the forms you will write. archlang spec.
4. Make the smallest edit. Change the statements the request names and leave the rest as they are: same ids, same order, same comments. Do not reformat the file and do not rewrite statements you were not asked to change. When a room changes size, check the walls, doors, windows and neighbouring rooms that share its edges.
5. Compile. archlang compile plan.arch -o plan.svg --json. If ok is false, preview the mechanical fixes with archlang fix plan.arch --dry-run --json and read the diff before you apply them with archlang fix plan.arch --backup.
6. Move furniture out of the way if needed. If the edit pushed a piece into a wall, a doorway or a door swing, archlang repair plan.arch -o plan.arch moves it. Read unresolved for the pieces it could not move.
7. Verify the change and nothing else. Run the two commands from step 1 again. The rooms you meant to change differ as intended. Every other room has the same area and the same connections. No diagnostic is new unless the request causes it; if one is, tell the user.
8. Refresh the sheet if the drawing grew. archlang finish plan.arch --dry-run --json shows what is missing. If it reports that the drawing no longer fits the authored paper or scale, archlang finish plan.arch --reissue may replace those two statements and nothing else.

When another agent may edit the same file

Pass the file's current revision as --expected-hash <hash>, as in archlang fix plan.arch --expected-hash <hash> --json. The --json result of each command that rewrites the file (fix, finish, repair -o plan.arch, fmt --write) says which hash that is: source_hash is the file as the command read it, and written_hash the file once the command has written it. A result that hands you rewritten text instead (repair --json, a --dry-run, or the MCP repair, fix and finish tools) gives output_hash, the file's revision once you write that text to it unchanged. archlang fmt plan.arch --json reports source_hash without writing. If the file changed since, the command writes nothing and exits 4 with E_STALE_SOURCE: read the file again, redo your edit on what is there now, and use the new hash. This narrows the window for a clash; it is not a lock.

Report to the user

• The statements you changed, as a short diff.
• Before and after for the facts the request is about (for example the room's area).
• Any new warning, and whether the request causes it.

When something goes wrong

• Exit code 2 is a fault in the source: read the diagnostic, do not retry unchanged. 3 is a wrong flag: run archlang <cmd> --help.
• archlang explain <CODE> --json explains a diagnostic code.
• If the request needs a new door or window and you are unsure where it can go, archlang suggest plan.arch --json returns candidates with the reason for each.