Document the optional RulePrecheck rule interface

README.md

@@ -20,11 +20,12 @@ Use this as a template when you create your own checker.
    - [Step 6: Wire It Up (main.go)](#step-6-wire-it-up-maingo)
    - [Step 7: Create the Plugin Entrypoint](#step-7-create-the-plugin-entrypoint)
 5. [Optional: Standalone Human UI (`CheckerInteractive`)](#optional-standalone-human-ui-checkerinteractive)
-6. [Running the Checker](#running-the-checker)
-7. [Testing with curl](#testing-with-curl)
-8. [Deploying to happyDomain](#deploying-to-happydomain)
-9. [License & happyDomain compatibility](#license--happydomain-compatibility)
-10. [Going Further](#going-further)
+6. [Optional: Declaring rule prerequisites (`RulePrecheck`)](#optional-declaring-rule-prerequisites-ruleprecheck)
+7. [Running the Checker](#running-the-checker)
+8. [Testing with curl](#testing-with-curl)
+9. [Deploying to happyDomain](#deploying-to-happydomain)
+10. [License & happyDomain compatibility](#license--happydomain-compatibility)
+11. [Going Further](#going-further)
 
 ---
 
@@ -470,6 +471,59 @@ Returning an error from `ParseForm` re-renders the form with the error message d
 ---
 
 
+## Optional: Declaring rule prerequisites (`RulePrecheck`)
+
+Some rules cannot run unless the operator has supplied a prerequisite, typically an API key or auth token in an admin option. Without a way to advertise that, the UI shows the rule as if it were ready and the user only discovers the problem after a check returns an error. The SDK exposes an optional per-rule hook so the host can tell ahead of time which rules are usable with the current options.
+
+```go
+type RulePrecheck interface {
+    CheckRule
+    Precheck(ctx context.Context, opts CheckerOptions) error
+}
+```
+
+Return `nil` when the rule is ready to run; return an error describing the missing prerequisite otherwise. The error message is shown verbatim to the operator, so name the option they need to set (`"VirusTotal API key is not configured"` is much more useful than `"missing key"`).
+
+### Minimal implementation
+
+```go
+func (r *vtRule) Precheck(ctx context.Context, opts sdk.CheckerOptions) error {
+    if v, _ := opts["virustotal_api_key"].(string); v == "" {
+        return fmt.Errorf("VirusTotal API key is not configured")
+    }
+    return nil
+}
+```
+
+### How the host consumes it
+
+`GET /definition` is unchanged: it returns the static checker definition with no per-rule availability information. Whenever the host has a concrete set of options at hand (rendering the rule list with the user's admin/user settings, validating an edit, …) it issues a `POST /definition` carrying those options:
+
+```bash
+curl -X POST http://localhost:8080/definition \
+  -H "Content-Type: application/json" \
+  -d '{"options": {}}'
+```
+
+Response:
+
+```json
+{
+  "precheck_failures": {
+    "virustotal": "VirusTotal API key is not configured"
+  }
+}
+```
+
+Rules that pass precheck, or that do not implement `RulePrecheck`, are absent from the map. The UI uses this to grey out unconfigured rules and surface the message returned by `Precheck`.
+
+### Important: Precheck is advisory only
+
+The SDK never calls `Precheck` from `Collect` or `Evaluate`. If the user enables a rule whose precheck fails and the host runs `/collect` anyway, your `Collect` (or the rule itself) still has to short-circuit and surface its own error: precheck is purely a UI hint, not an admission gate.
+
+---
+
+
 ## Running the Checker
 
 ### Build and run locally