diff --git a/AUTHORING.md b/AUTHORING.md
index e21e185..9f7bea0 100644
--- a/AUTHORING.md
+++ b/AUTHORING.md
@@ -54,6 +54,8 @@ Know this before writing anything:
 - Read settings under the names the agent gives them (see above). `${RCON_PORT}` when the setting is
   `RCONPort` silently ignores the owner's value; it arrives as `RCONPORT`.
 - Use `${VAR:-default}` for anything that may be unset.
+- Only use launch options the game's documentation lists, spelled as it spells them. Servers ignore options
+  they don't know, so an invented one (`-maxplayers` for Valheim) boots fine and its setting does nothing.
 - Quote text settings where they're used: `-name "${SERVER_NAME}"`. `ARGS="-name ${SERVER_NAME}"; ./server $ARGS`
   splits "GBRHosting - My Server" into words; build optional arguments in a bash array instead
   (`args=(-name "$SERVER_NAME"); [ -n "$PASSWORD" ] && args+=(-password "$PASSWORD"); ./server "${args[@]}"`).
diff --git a/cmd/gbrgames/build.go b/cmd/gbrgames/build.go
index 2bfcf4e..1d4a96b 100644
--- a/cmd/gbrgames/build.go
+++ b/cmd/gbrgames/build.go
@@ -138,7 +138,8 @@ func build(o buildOptions) (*buildResult, error) {
 		steamApp = o.variant.SteamAppID
 	}
 	d, err := newDrafter(draftJob{GamesDir: o.gamesDir, SchemaDir: o.schemaDir, Name: o.name, ID: o.id, Image: res.Image,
-		SteamApp: steamApp, Notes: variantNotes(o.variant, o.notes), Report: report, Provider: o.provider, Model: o.model, NoPull: true})
+		SteamApp: steamApp, Notes: variantNotes(o.variant, o.notes), Report: report, Provider: o.provider, Model: o.model, NoPull: true,
+		Docs: sourceText(o.variant.Sources)})
 	if err != nil {
 		return nil, err
 	}
@@ -370,3 +371,27 @@ func buildSummary(o buildOptions, res *buildResult) string {
 	}
 	return b.String()
 }
+
+// sourceText reads the pages research based the variant on, in full, so the drafter can check that every
+// launch option the model uses is documented there. Empty (check skipped) without a reader.
+func sourceText(sources []string) string {
+	reader := os.Getenv("READER_URL")
+	if reader == "" || len(sources) == 0 {
+		fmt.Println("[build] no research sources to check launch options against (READER_URL or sources missing)")
+		return ""
+	}
+	var b strings.Builder
+	read := 0
+	for _, s := range sources {
+		text, err := readPageLimit(reader, s, 200_000)
+		if err != nil {
+			fmt.Printf("[build] could not read %s: %v\n", s, err)
+			continue
+		}
+		read++
+		b.WriteString(text)
+		b.WriteString("\n")
+	}
+	fmt.Printf("[build] read %d of %d research sources to check launch options against\n", read, len(sources))
+	return b.String()
+}
diff --git a/cmd/gbrgames/draft.go b/cmd/gbrgames/draft.go
index dc62d25..0708e0a 100644
--- a/cmd/gbrgames/draft.go
+++ b/cmd/gbrgames/draft.go
@@ -82,6 +82,9 @@ type draftJob struct {
 	Report              *bootReport
 	Provider, Model     string
 	NoPull              bool
+	// Text of the pages research based the variant on; when set, every launch option in the startup
+	// command must appear in it (or in the image's scripts)
+	Docs string
 }
 
 // drafter holds one conversation with the model about one game, so later problems (a failed boot) can
@@ -200,6 +203,7 @@ func (d *drafter) check(answer string) []string {
 			problems = append(problems, savesOutsideVolumes(g, d.job.Report)...)
 			problems = append(problems, splitSettings(g)...)
 			problems = append(problems, portSettings(g)...)
+			problems = append(problems, undocumentedOptions(g, d.job.Docs, d.facts)...)
 		}
 	}
 	return problems
diff --git a/cmd/gbrgames/draftchecks.go b/cmd/gbrgames/draftchecks.go
index 5db39a0..61378f8 100644
--- a/cmd/gbrgames/draftchecks.go
+++ b/cmd/gbrgames/draftchecks.go
@@ -3,6 +3,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"path"
 	"regexp"
 	"sort"
 	"strconv"
@@ -248,3 +249,77 @@ func portNumber(v any) (int, bool) {
 	p, err := strconv.Atoi(strings.TrimSpace(s))
 	return p, err == nil && p > 0 && p < 65536
 }
+
+var (
+	optionToken = regexp.MustCompile(`(?:^|[\s("'])(-{1,2}[A-Za-z][A-Za-z0-9_.]*(?:-[A-Za-z0-9_.]+)*)`)
+	segmentSep  = regexp.MustCompile(`;|&&|\|\||\||\n`)
+	testOps     = map[string]bool{"eq": true, "ne": true, "lt": true, "gt": true, "le": true, "ge": true, "nt": true, "ot": true, "ef": true}
+	// Options of these are the shell's and its tools', not the game's
+	shellTools = map[string]bool{"mkdir": true, "cp": true, "mv": true, "ln": true, "rm": true, "chmod": true, "chown": true,
+		"wget": true, "curl": true, "tar": true, "unzip": true, "test": true, "[": true, "[[": true, "echo": true, "printf": true,
+		"sleep": true, "find": true, "sed": true, "grep": true, "cd": true, "export": true, "touch": true, "bash": true, "sh": true,
+		"cat": true, "head": true, "tail": true, "ls": true, "set": true, "ulimit": true, "env": true, "id": true}
+)
+
+// undocumentedOptions finds launch options in the startup command that none of the researched pages (nor
+// the image's own scripts) mention. Servers silently ignore options they don't know, so a made-up one
+// (Valheim has no -maxplayers) boots fine and passes every boot test while its setting does nothing.
+func undocumentedOptions(game map[string]any, docs string, facts *imageFacts) []string {
+	if strings.TrimSpace(docs) == "" {
+		return nil
+	}
+	startup, _ := game["defaultStartupCommand"].(string)
+	corpus := docs
+	if facts != nil {
+		for _, s := range facts.Scripts {
+			corpus += "\n" + s
+		}
+	}
+	seen := map[string]bool{}
+	var missing []string
+	for _, opt := range launchOptions(unwrapShell(startup)) {
+		name := strings.TrimLeft(opt, "-")
+		key := strings.ToLower(name)
+		if seen[key] {
+			continue
+		}
+		seen[key] = true
+		documented := regexp.MustCompile(`(?i)(^|[^A-Za-z0-9_])-{1,2}` + regexp.QuoteMeta(name) + `($|[^A-Za-z0-9_])`)
+		if !documented.MatchString(corpus) {
+			missing = append(missing, opt)
+		}
+	}
+	if len(missing) == 0 {
+		return nil
+	}
+	return []string{fmt.Sprintf("the startup command passes %s, which none of the pages researched for this game mention. "+
+		"Servers silently ignore options they don't know, so it boots fine but does nothing: remove it (and any setting that "+
+		"only feeds it), or spell it exactly as the documentation does", strings.Join(missing, ", "))}
+}
+
+// launchOptions lists the -options passed to the game, skipping the shell's own (mkdir -p, [ -n ... ]).
+func launchOptions(cmd string) []string {
+	var out []string
+	for _, seg := range segmentSep.Split(cmd, -1) {
+		fields := strings.Fields(strings.TrimLeft(strings.TrimSpace(seg), "({"))
+		i := 0
+		for i < len(fields) && (fields[i] == "exec" || fields[i] == "then" || fields[i] == "do" || fields[i] == "else" ||
+			(assignWord.MatchString(fields[i]) && !strings.Contains(fields[i], "=("))) {
+			i++
+		}
+		if i == len(fields) {
+			continue
+		}
+		if word := path.Base(fields[i]); shellTools[word] {
+			continue
+		}
+		for _, m := range optionToken.FindAllStringSubmatch(strings.Join(fields[i:], " "), -1) {
+			name := strings.TrimLeft(m[1], "-")
+			if len(name) < 2 || testOps[name] {
+				continue
+			}
+			out = append(out, m[1])
+		}
+	}
+	return out
+}
diff --git a/cmd/gbrgames/draftchecks_test.go b/cmd/gbrgames/draftchecks_test.go
index 148ea9f..83aedca 100644
--- a/cmd/gbrgames/draftchecks_test.go
+++ b/cmd/gbrgames/draftchecks_test.go
@@ -105,3 +105,47 @@ func TestPortSettings(t *testing.T) {
 		t.Errorf("got %v", p)
 	}
 }
+
+func TestUndocumentedOptionsCatchesMaxPlayers(t *testing.T) {
+	// PR #13's startup: every option is in Valheim's docs except -maxplayers
+	game := map[string]any{"defaultStartupCommand": `args=(-name "${SERVER_NAME:-GBRHosting - Valheim}" -port 2456 -world "${WORLD_NAME:-Dedicated}" -savedir /server); ` +
+		`[ -n "${SERVER_PASSWORD}" ] && args+=(-password "${SERVER_PASSWORD}"); [ "${CROSSPLAY}" = "true" ] && args+=(-crossplay); ` +
+		`[ -n "${MAX_PLAYERS}" ] && args+=(-maxplayers "${MAX_PLAYERS}"); ./valheim_server.x86_64 "${args[@]}"`}
+	docs := "Launch with `-name`, `-port`, `-world`, `-password`, `-savedir <path>` and `-crossplay` to allow consoles."
+	got := undocumentedOptions(game, docs, nil)
+	if len(got) != 1 || !strings.Contains(got[0], "passes -maxplayers,") {
+		t.Fatalf("want only -maxplayers flagged, got %v", got)
+	}
+	// Once removed, nothing is left to flag
+	game["defaultStartupCommand"] = strings.Replace(game["defaultStartupCommand"].(string), `[ -n "${MAX_PLAYERS}" ] && args+=(-maxplayers "${MAX_PLAYERS}"); `, "", 1)
+	if got := undocumentedOptions(game, docs, nil); len(got) != 0 {
+		t.Errorf("got %v", got)
+	}
+}
+
+func TestUndocumentedOptionsIgnoresShellAndUsesImageScripts(t *testing.T) {
+	game := map[string]any{"defaultStartupCommand": `mkdir -p /server/.config; cp -f /a /b; [ -n "$X" ] && [ "$Y" -eq 1 ] && ` +
+		`exec ./server --listen-port=7777 -ServerPlatform=ALL -logs`}
+	facts := &imageFacts{Scripts: map[string]string{"/start.sh": `exec "$@" -logs`}}
+	got := strings.Join(undocumentedOptions(game, "Use --listen-port and -ServerPlatform=ALL for crossplay.", facts), "\n")
+	if got != "" {
+		t.Errorf("shell options, documented ones and ones the image's script uses must pass, got %q", got)
+	}
+	// Without sources the check is off (draft run by hand, or no reader configured)
+	game["defaultStartupCommand"] = "./server -madeup"
+	if p := undocumentedOptions(game, "", nil); p != nil {
+		t.Errorf("got %v", p)
+	}
+	if p := undocumentedOptions(game, "unrelated page", nil); len(p) != 1 {
+		t.Errorf("want -madeup flagged, got %v", p)
+	}
+}
+
+func TestUndocumentedOptionsWholeWords(t *testing.T) {
+	// "-port" in the docs doesn't document "-portal", and "-port" isn't documented by "-portal"
+	game := map[string]any{"defaultStartupCommand": "./server -port 1 -portal x"}
+	got := strings.Join(undocumentedOptions(game, "options: -port", nil), "")
+	if !strings.Contains(got, "passes -portal,") || strings.Contains(got, "-port,") {
+		t.Errorf("got %q", got)
+	}
+}
diff --git a/cmd/gbrgames/research.go b/cmd/gbrgames/research.go
index 167fa7a..1ddfa1a 100644
--- a/cmd/gbrgames/research.go
+++ b/cmd/gbrgames/research.go
@@ -250,13 +250,18 @@ func dedupeHits(hits []searchHit) []searchHit {
 }
 
 func readPage(reader, pageURL string) (string, error) {
+	return readPageLimit(reader, pageURL, 9000)
+}
+
+// readPageLimit reads a page as text through the reader, cut to limit bytes.
+func readPageLimit(reader, pageURL string, limit int) (string, error) {
 	raw, err := httpGet(strings.TrimRight(reader, "/")+"/"+pageURL, 60*time.Second)
 	if err != nil {
 		return "", err
 	}
 	text := string(raw)
-	if len(text) > 9000 {
-		text = text[:9000] + "\n...(truncated)"
+	if len(text) > limit {
+		text = text[:limit] + "\n...(truncated)"
 	}
 	return text, nil
 }