Optional Dependencies

A dependency can be switched on and off by a variable. The condition is written on the edge — deps="fix?USE_AGENT" — not on the block it points at, so fix stays an ordinary block that any other block may still depend on unconditionally, or run directly — running it ignores every condition that points at it (its own deps= conditions still apply).

Entry	The edge exists when
`fix?USE_AGENT`	the `bool` variable `USE_AGENT` is `true`
`fix?!USE_AGENT`	it is `false`
`fix?PROVIDER=claude`	`PROVIDER` equals `claude` (any variable type)
`fix?!PROVIDER=claude`	`PROVIDER` is anything else
`fix?USE_AGENT!`	like the first, plus the usual sync `!`

An edge whose condition is false is simply not there: the dependency is not run for this block and does not count towards its freshness, so flipping the value makes the block stale. If the variable is computed (from=), its source runs first and the edge is decided afterwards.

The two variables below are the whole "configuration" of this canvas. Neither block asks for them through env= — a condition is enough for meshfox to resolve the variable (and to ask for it, when it has no default) as soon as a chain reaches the edge.

Convert

convert always runs. The repair step in front of it is optional:

echo "converting the (possibly repaired) input"

Try it from the repository root:

meshfox examples/optional-deps.canvas.md run convert                       # only "convert"
meshfox examples/optional-deps.canvas.md run --set USE_AGENT=true convert  # "fix" first
meshfox examples/optional-deps.canvas.md run repair fix                    # "fix" regardless of USE_AGENT

Files and variables follow the same edge

A condition on a deps= entry covers everything that makes this block depend on the target, not only the explicit edge. If fix declared outputs="fixed.md" and convert declared inputs="fixed.md", the producer would normally be pulled in automatically; with deps="fix?USE_AGENT" it is pulled in only while USE_AGENT is true, and while it is false the inputs="fixed.md" declaration is ignored altogether (the file need not exist). The same goes for a variable computed by fix (meshfox:var from="fix") that convert takes through env=: it is simply absent while the edge is off, so branch on the flag, not on whether the file or variable happens to exist.

Repair

In a real canvas fix would be interpreter="@agent" (add tty for an interactive session) — see the agent recipe in the README. Here it only prints what it would do, so the example runs anywhere. Its own dependencies pick the provider's set-up step with a comparison, so only the chosen one runs:

echo "checking that the claude CLI is configured"

echo "checking that the codex CLI is configured"

echo "asking $PROVIDER to repair the input"