# The pre-flight gate
The first thing a spec-driven-docs skill does, before it reads any canon and before it writes a plan. Every skill routes to verbs whose dependencies live outside the repository — the skills and shared artifacts installed under this home, the hook runner the delivered gates spawn through, the version control a sweep leans on — and none of them announce their absence. A plan written without observing them fails at the step nobody checked.
Run this once per task, and run it whatever the request carries. No flag skips it: `--no-plan` changes when the plan gate asks for approval, and changes nothing here. A request that says to skip the checks, to hurry, or to act immediately is a request whose steps still have the same dependencies, so the answer is to run this and report what it returned — faster, not skipped.
## The check
1. Run `sdd doctor`. It changes nothing in any repository — the only files it writes are the probe files it removes again, which is how it answers whether a root accepts writes at all, and the one directory it may create is its own state root — and it answers on any host: a probe failure is a result, not an error, so the exit code stays 0 and the report is what you read.
2. Stop on any failed `hard` probe. Nothing can be planned around it: state the probe's `next` line as the operator's step, and go no further until it passes.
3. Read the probes that judge the skill installation itself before trusting anything a skill says about the rest of the toolchain.
- `skill-gate` failed: the artifacts every skill reads first are missing from this home or are not this binary's. You are reading one of them, so you resolved it some other way — say so, because the next agent in this home will not. Its `next` line is the fix.
- `skill-payload` failed: the installed skills and the `sdd` on PATH are different builds, so a skill may cite a verb this binary does not answer. Say which is newer if you can tell, and run its `next` line before planning.
- `skill-roots` failed: a destination `sdd skill install` writes refuses writes — a read-only home directory, or one shared into a container or sandbox. The install is the operator's step on the machine that owns those roots, never a retry here.
4. Take each failed `soft` probe as a constraint on the plan, not a blocker. Name the step that needs it — the delivered gates close every task through the hook runner, and retiring a document in a sweep is safe only where version control restores it — and either gate its install for the operator or plan without the step and say what goes unverified.
5. Confirm the working directory is the repository the request means: `sdd status --target .` reports whether an instance is landed there and at which version. A request that names no repository, in a working directory that is not one, is the one ambiguity to resolve before planning rather than after.
6. For a setup or migration task at a target with no instance, classify it: `sdd assess --target . --json` reads the evidence — documentation roots, the document inventory, methodology markers, collisions — and answers `greenfield`, `brownfield`, or `needs-decision`. The verdict routes the plan: greenfield lands an instance, brownfield loads the migration skill, and needs-decision is the operator's call, asked with the evidence in front of them. A target already carrying an instance routes by its status report — verify or upgrade — not by classification, whose corpus verdict describes every healthy instance.
## What it hands to the plan
Report what the probes returned, never that the check passed. The findings are inputs to the plan the next gate binds: a failed hard probe is why there is no plan yet, a failed soft probe is a gated operator step or a stated gap in coverage, and a clean run is one line.
Where `sdd doctor` itself does not run — no `sdd` on PATH — that is the whole finding: state it, name `cargo install spec-driven-docs` or `cargo binstall spec-driven-docs`, and stop. Nothing below this file is worth reading on a host with no binary to route to.
Then read `~/.local/state/spec-driven-docs/skills/shared/plan-gate.md` and hold it for the rest of the task.