diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..1e69fb0
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,17 @@
+# crossbar gate. `make gate` must pass before any task is called done. It needs no network.
+
+.PHONY: gate build smoke
+
+gate:
+	@test -z "$$(gofmt -l . 2>&1)" || { echo "gofmt: these files need formatting:"; gofmt -l .; exit 1; }
+	go vet ./...
+	go test -race -count=1 ./...
+	sh scripts/check-lines.sh
+	@echo "gate: ok"
+
+build:
+	go build -o bin/ ./cmd/...
+
+# Runs the whole thing against two fake upstreams. Task 05 brings the script.
+smoke: build
+	sh tools/smoke.sh
diff --git a/docs/implementer-log.md b/docs/implementer-log.md
index 90a7119..3abe907 100644
--- a/docs/implementer-log.md
+++ b/docs/implementer-log.md
@@ -5,5 +5,6 @@ owner fills in the Model column. The reviewer adds findings under "Reviews" once
 
 | Task | Date | Status | Gate runs | First gate | Deviations | Notes | Model |
 |---|---|---|---|---|---|---|---|
+| v0/01-module-gate-config | 2026-09-25 | done | 1 | pass | none | `go mod download` fetched the module (network available); gate passed on the first run. | ? |
 
 ## Reviews
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..ab77dc4
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module git.wntrmute.dev/kyle/crossbar
+
+go 1.26
+
+require github.com/BurntSushi/toml v1.6.0
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..f74b269
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/BurntSushi/toml v1.6.0 h1:dRaEfpa2VI55EwlIW72hMRHdWouJeRF7TPYhI+AUQjk=
+github.com/BurntSushi/toml v1.6.0/go.mod h1:ukJfTF/6rtPPRCnwkur4qwRxa8vTRFBF0uk2lLoLwho=
diff --git a/internal/config/config.go b/internal/config/config.go
new file mode 100644
index 0000000..1795333
--- /dev/null
+++ b/internal/config/config.go
@@ -0,0 +1,300 @@
+// Package config reads crossbar's TOML file: the hosts in front of which it
+// proxies, the model each serves, and the ordered routes that select them.
+//
+// The reader is strict. A misspelt key or a route naming a host that does not
+// exist is an *Error naming the offending field, returned at start-up rather
+// than surfaced later.
+package config
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/url"
+	"os"
+	"regexp"
+	"sort"
+	"strings"
+	"time"
+
+	"github.com/BurntSushi/toml"
+)
+
+// Duration is a time.Duration that TOML reads from a string such as "60s" or
+// "30m".
+type Duration struct{ time.Duration }
+
+// UnmarshalText implements encoding.TextUnmarshaler via time.ParseDuration.
+func (d *Duration) UnmarshalText(text []byte) error {
+	dt, err := time.ParseDuration(string(text))
+	if err != nil {
+		return err
+	}
+	d.Duration = dt
+	return nil
+}
+
+// Model is the per-model tuning carried by a host entry.
+type Model struct {
+	Parallel int `toml:"parallel"`
+}
+
+// Host names one upstream llama-server, the models it serves, and its weight.
+type Host struct {
+	BaseURL string           `toml:"base_url"`
+	Weight  float64          `toml:"weight"`
+	Models  map[string]Model `toml:"models"`
+}
+
+// Route is an ordered list of hosts to try, with an optional default model.
+type Route struct {
+	Hosts        []string `toml:"hosts"`
+	DefaultModel string   `toml:"default_model"`
+}
+
+// Config is the whole file: what to listen on, tuning, hosts and routes.
+type Config struct {
+	Listen       string           `toml:"listen"`
+	PollInterval Duration         `toml:"poll_interval"`
+	QueueMax     int              `toml:"queue_max"`
+	Hosts        map[string]Host  `toml:"hosts"`
+	Routes       map[string]Route `toml:"routes"`
+}
+
+// Error is a validation error naming the field it is about.
+type Error struct {
+	Field, Msg string
+}
+
+// Error implements the error interface.
+func (e *Error) Error() string {
+	return "config: " + e.Field + ": " + e.Msg
+}
+
+const (
+	DefaultPollInterval = 60 * time.Second
+	DefaultQueueMax     = 8
+	MinPollInterval     = time.Second
+)
+
+var routeName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
+
+// Load reads and parses the config file at path. An open failure is wrapped as
+// "config: …", the same shape as a decode failure.
+func Load(path string) (*Config, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("config: %w", err)
+	}
+	defer f.Close()
+	return Parse(f)
+}
+
+// Parse decodes TOML from r, applies defaults, and validates. A TOML syntax
+// error is returned wrapped as "config: …" and is not an *Error; an unknown
+// key is an *Error naming the first undecoded key in sorted order.
+func Parse(r io.Reader) (*Config, error) {
+	var c Config
+	md, err := toml.NewDecoder(r).Decode(&c)
+	if err != nil {
+		return nil, fmt.Errorf("config: %w", err)
+	}
+	if undecoded := md.Undecoded(); len(undecoded) > 0 {
+		keys := make([]string, 0, len(undecoded))
+		for _, k := range undecoded {
+			keys = append(keys, strings.Join(k, "."))
+		}
+		sort.Strings(keys)
+		return nil, &Error{Field: keys[0], Msg: "unknown key"}
+	}
+
+	if c.PollInterval.Duration == 0 {
+		c.PollInterval.Duration = DefaultPollInterval
+	}
+	if c.QueueMax == 0 {
+		c.QueueMax = DefaultQueueMax
+	}
+	if e := c.validate(); e != nil {
+		return nil, e
+	}
+	return &c, nil
+}
+
+// Serves reports whether the host exists and lists the model.
+func (c *Config) Serves(host, model string) bool {
+	h, ok := c.Hosts[host]
+	if !ok {
+		return false
+	}
+	_, ok = h.Models[model]
+	return ok
+}
+
+// IsError extracts an *Error from err, reporting whether one was present.
+func IsError(err error) (*Error, bool) {
+	var e *Error
+	if errors.As(err, &e) {
+		return e, true
+	}
+	return nil, false
+}
+
+// validate checks the config in a fixed order and writes defaults back into c.
+// The first problem wins; every problem is an *Error with a precise field.
+func (c *Config) validate() *Error {
+	if e := c.checkListen(); e != nil {
+		return e
+	}
+	if e := c.checkPoll(); e != nil {
+		return e
+	}
+	if e := c.checkQueue(); e != nil {
+		return e
+	}
+	if e := c.checkHosts(); e != nil {
+		return e
+	}
+	return c.checkRoutes()
+}
+
+func (c *Config) checkListen() *Error {
+	if c.Listen == "" {
+		return &Error{Field: "listen", Msg: "required, host:port"}
+	}
+	host, _, err := net.SplitHostPort(c.Listen)
+	if err != nil {
+		return &Error{Field: "listen", Msg: "must be host:port"}
+	}
+	if host == "" {
+		return &Error{Field: "listen", Msg: "host part required"}
+	}
+	if host == "0.0.0.0" || host == "::" {
+		return &Error{Field: "listen", Msg: "not an unspecified address"}
+	}
+	return nil
+}
+
+func (c *Config) checkPoll() *Error {
+	if c.PollInterval.Duration == 0 {
+		c.PollInterval.Duration = DefaultPollInterval
+	} else if c.PollInterval.Duration < MinPollInterval {
+		return &Error{Field: "poll_interval", Msg: "must be at least 1s"}
+	}
+	return nil
+}
+
+func (c *Config) checkQueue() *Error {
+	if c.QueueMax == 0 {
+		c.QueueMax = DefaultQueueMax
+	} else if c.QueueMax < 0 {
+		return &Error{Field: "queue_max", Msg: "must not be negative"}
+	}
+	return nil
+}
+
+func (c *Config) checkHosts() *Error {
+	if len(c.Hosts) == 0 {
+		return &Error{Field: "hosts", Msg: "at least one required"}
+	}
+	names := make([]string, 0, len(c.Hosts))
+	for name := range c.Hosts {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		h := c.Hosts[name]
+
+		baseField := fmt.Sprintf("hosts.%s.base_url", name)
+		u, err := url.Parse(h.BaseURL)
+		if err != nil {
+			return &Error{Field: baseField, Msg: err.Error()}
+		}
+		if u.Scheme != "http" && u.Scheme != "https" {
+			return &Error{Field: baseField, Msg: "scheme must be http or https"}
+		}
+		if u.Host == "" {
+			return &Error{Field: baseField, Msg: "host required"}
+		}
+		if u.RawQuery != "" {
+			return &Error{Field: baseField, Msg: "query not allowed"}
+		}
+		if u.Fragment != "" {
+			return &Error{Field: baseField, Msg: "fragment not allowed"}
+		}
+		h.BaseURL = strings.TrimRight(h.BaseURL, "/")
+
+		if h.Weight == 0 {
+			h.Weight = 1
+		} else if h.Weight < 0 {
+			return &Error{Field: fmt.Sprintf("hosts.%s.weight", name), Msg: "must not be negative"}
+		}
+
+		if len(h.Models) == 0 {
+			return &Error{Field: fmt.Sprintf("hosts.%s.models", name), Msg: "at least one required"}
+		}
+		models := make([]string, 0, len(h.Models))
+		for m := range h.Models {
+			models = append(models, m)
+		}
+		sort.Strings(models)
+		for _, m := range models {
+			pm := h.Models[m]
+			if pm.Parallel == 0 {
+				pm.Parallel = 1
+			} else if pm.Parallel < 0 {
+				return &Error{Field: fmt.Sprintf("hosts.%s.models.%s.parallel", name, m), Msg: "must not be negative"}
+			}
+			h.Models[m] = pm
+		}
+		c.Hosts[name] = h
+	}
+	return nil
+}
+
+func (c *Config) checkRoutes() *Error {
+	if len(c.Routes) == 0 {
+		return &Error{Field: "routes", Msg: "at least one required"}
+	}
+	names := make([]string, 0, len(c.Routes))
+	for name := range c.Routes {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		r := c.Routes[name]
+
+		if !routeName.MatchString(name) {
+			return &Error{Field: fmt.Sprintf("routes.%s", name), Msg: "must match [a-z0-9][a-z0-9-]*"}
+		}
+
+		hostsField := fmt.Sprintf("routes.%s.hosts", name)
+		if len(r.Hosts) == 0 {
+			return &Error{Field: hostsField, Msg: "at least one required"}
+		}
+		seen := make(map[string]bool, len(r.Hosts))
+		for _, h := range r.Hosts {
+			if seen[h] {
+				return &Error{Field: hostsField, Msg: "host listed twice"}
+			}
+			seen[h] = true
+			if _, ok := c.Hosts[h]; !ok {
+				return &Error{Field: hostsField, Msg: "unknown host"}
+			}
+		}
+
+		if r.DefaultModel != "" {
+			served := false
+			for _, h := range r.Hosts {
+				if _, ok := c.Hosts[h].Models[r.DefaultModel]; ok {
+					served = true
+					break
+				}
+			}
+			if !served {
+				return &Error{Field: fmt.Sprintf("routes.%s.default_model", name), Msg: "not served by any host in route"}
+			}
+		}
+	}
+	return nil
+}
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
new file mode 100644
index 0000000..9069c36
--- /dev/null
+++ b/internal/config/config_test.go
@@ -0,0 +1,191 @@
+package config_test
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"git.wntrmute.dev/kyle/crossbar/internal/config"
+)
+
+func TestGoodFile(t *testing.T) {
+	c, err := config.Load(filepath.Join("testdata", "good.toml"))
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.Listen != "100.64.0.9:7777" {
+		t.Errorf("Listen = %q", c.Listen)
+	}
+	if c.PollInterval.Duration != 5*time.Second {
+		t.Errorf("PollInterval = %v", c.PollInterval.Duration)
+	}
+	if c.QueueMax != 4 {
+		t.Errorf("QueueMax = %d", c.QueueMax)
+	}
+	alpha := c.Hosts["alpha"]
+	if alpha.BaseURL != "http://alpha.example:11434" {
+		t.Errorf("trailing slash not stripped: %q", alpha.BaseURL)
+	}
+	if alpha.Weight != 2 {
+		t.Errorf("alpha.Weight = %v", alpha.Weight)
+	}
+	if alpha.Models["ornith-1.5-35b-a3b"].Parallel != 4 || alpha.Models["small-9b"].Parallel != 6 {
+		t.Errorf("alpha.Models = %+v", alpha.Models)
+	}
+	beta := c.Hosts["beta"]
+	if beta.Weight != 1 {
+		t.Errorf("beta.Weight default = %v, want 1", beta.Weight)
+	}
+	if beta.Models["ornith-1.5-35b-a3b"].Parallel != 1 {
+		t.Errorf("beta parallel default = %d, want 1", beta.Models["ornith-1.5-35b-a3b"].Parallel)
+	}
+	r := c.Routes["opencode-a"]
+	if len(r.Hosts) != 2 || r.Hosts[0] != "alpha" || r.Hosts[1] != "beta" {
+		t.Errorf("route hosts = %v", r.Hosts)
+	}
+	if r.DefaultModel != "ornith-1.5-35b-a3b" {
+		t.Errorf("DefaultModel = %q", r.DefaultModel)
+	}
+	if c.Routes["hermes-x"].DefaultModel != "" {
+		t.Errorf("hermes-x DefaultModel should be empty")
+	}
+	if !c.Serves("alpha", "small-9b") || c.Serves("beta", "small-9b") || c.Serves("nope", "m") {
+		t.Errorf("Serves is wrong")
+	}
+}
+
+func TestDefaults(t *testing.T) {
+	c, err := config.Parse(strings.NewReader(`
+listen = "127.0.0.1:1"
+[hosts.a]
+base_url = "http://a:1"
+models = { "m" = { } }
+[routes.r]
+hosts = ["a"]
+`))
+	if err != nil {
+		t.Fatalf("Parse: %v", err)
+	}
+	if c.PollInterval.Duration != config.DefaultPollInterval {
+		t.Errorf("PollInterval default = %v", c.PollInterval.Duration)
+	}
+	if c.QueueMax != config.DefaultQueueMax {
+		t.Errorf("QueueMax default = %d", c.QueueMax)
+	}
+}
+
+func TestBadFiles(t *testing.T) {
+	cases := []struct{ file, field string }{
+		{"bad-listen.toml", "listen"},
+		{"bad-unknown-host.toml", "routes.r.hosts"},
+		{"bad-default-model.toml", "routes.r.default_model"},
+		{"bad-unknown-key.toml", "lease_idle"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.file, func(t *testing.T) {
+			_, err := config.Load(filepath.Join("testdata", tc.file))
+			if err == nil {
+				t.Fatalf("want error")
+			}
+			e, ok := config.IsError(err)
+			if !ok {
+				t.Fatalf("want *config.Error, got %T: %v", err, err)
+			}
+			if e.Field != tc.field {
+				t.Errorf("Field = %q, want %q (%v)", e.Field, tc.field, err)
+			}
+			if !strings.HasPrefix(err.Error(), "config: "+tc.field+": ") {
+				t.Errorf("Error() = %q", err.Error())
+			}
+		})
+	}
+}
+
+func TestBadValues(t *testing.T) {
+	base := `
+listen = %q
+poll_interval = %q
+[hosts.a]
+base_url = %q
+weight = %v
+models = { "m" = { parallel = %d } }
+[routes.%s]
+hosts = ["a"]
+`
+	cases := []struct {
+		name                     string
+		listen, poll, url, route string
+		weight                   float64
+		parallel                 int
+		field                    string
+	}{
+		{"empty listen", "", "5s", "http://a:1", "r", 1, 1, "listen"},
+		{"no port", "127.0.0.1", "5s", "http://a:1", "r", 1, 1, "listen"},
+		{"v6 any", "[::]:7", "5s", "http://a:1", "r", 1, 1, "listen"},
+		{"poll too short", "127.0.0.1:7", "500ms", "http://a:1", "r", 1, 1, "poll_interval"},
+		{"ftp url", "127.0.0.1:7", "5s", "ftp://a:1", "r", 1, 1, "hosts.a.base_url"},
+		{"no host", "127.0.0.1:7", "5s", "http://", "r", 1, 1, "hosts.a.base_url"},
+		{"query", "127.0.0.1:7", "5s", "http://a:1/v1?x=1", "r", 1, 1, "hosts.a.base_url"},
+		{"negative weight", "127.0.0.1:7", "5s", "http://a:1", "r", -1, 1, "hosts.a.weight"},
+		{"negative parallel", "127.0.0.1:7", "5s", "http://a:1", "r", 1, -2, "hosts.a.models.m.parallel"},
+		{"route name", "127.0.0.1:7", "5s", "http://a:1", "Bad_Name", 1, 1, "routes.Bad_Name"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			text := fmt.Sprintf(base, tc.listen, tc.poll, tc.url, tc.weight, tc.parallel, tc.route)
+			_, err := config.Parse(strings.NewReader(text))
+			if err == nil {
+				t.Fatalf("want error for %s", tc.name)
+			}
+			e, ok := config.IsError(err)
+			if !ok {
+				t.Fatalf("want *config.Error, got %T: %v", err, err)
+			}
+			if e.Field != tc.field {
+				t.Errorf("Field = %q, want %q (%v)", e.Field, tc.field, err)
+			}
+		})
+	}
+}
+
+func TestMissingSections(t *testing.T) {
+	for _, tc := range []struct{ name, text, field string }{
+		{"no hosts", "listen = \"127.0.0.1:7\"\n[routes.r]\nhosts = [\"a\"]\n", "hosts"},
+		{"no routes", "listen = \"127.0.0.1:7\"\n[hosts.a]\nbase_url = \"http://a:1\"\nmodels = { \"m\" = { } }\n", "routes"},
+		{"host without models", "listen = \"127.0.0.1:7\"\n[hosts.a]\nbase_url = \"http://a:1\"\n[routes.r]\nhosts = [\"a\"]\n", "hosts.a.models"},
+		{"route without hosts", "listen = \"127.0.0.1:7\"\n[hosts.a]\nbase_url = \"http://a:1\"\nmodels = { \"m\" = { } }\n[routes.r]\n", "routes.r.hosts"},
+		{"host twice", "listen = \"127.0.0.1:7\"\n[hosts.a]\nbase_url = \"http://a:1\"\nmodels = { \"m\" = { } }\n[routes.r]\nhosts = [\"a\", \"a\"]\n", "routes.r.hosts"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := config.Parse(strings.NewReader(tc.text))
+			e, ok := config.IsError(err)
+			if !ok {
+				t.Fatalf("want *config.Error, got %v", err)
+			}
+			if e.Field != tc.field {
+				t.Errorf("Field = %q, want %q", e.Field, tc.field)
+			}
+		})
+	}
+}
+
+func TestNotTOML(t *testing.T) {
+	_, err := config.Parse(strings.NewReader("listen = [unterminated"))
+	if err == nil {
+		t.Fatal("want error")
+	}
+	if _, ok := config.IsError(err); ok {
+		t.Errorf("a syntax error is not a validation Error")
+	}
+	if !strings.HasPrefix(err.Error(), "config: ") {
+		t.Errorf("Error() = %q", err.Error())
+	}
+}
+
+func TestMissingFile(t *testing.T) {
+	if _, err := config.Load(filepath.Join("testdata", "does-not-exist.toml")); err == nil {
+		t.Fatal("want error")
+	}
+}
diff --git a/internal/config/testdata/bad-default-model.toml b/internal/config/testdata/bad-default-model.toml
new file mode 100644
index 0000000..4e1cc32
--- /dev/null
+++ b/internal/config/testdata/bad-default-model.toml
@@ -0,0 +1,9 @@
+listen = "127.0.0.1:7777"
+
+[hosts.alpha]
+base_url = "http://alpha.example:11434"
+models   = { "m" = { } }
+
+[routes.r]
+hosts         = ["alpha"]
+default_model = "not-served"
diff --git a/internal/config/testdata/bad-listen.toml b/internal/config/testdata/bad-listen.toml
new file mode 100644
index 0000000..15e0e6d
--- /dev/null
+++ b/internal/config/testdata/bad-listen.toml
@@ -0,0 +1,8 @@
+listen = "0.0.0.0:7777"
+
+[hosts.alpha]
+base_url = "http://alpha.example:11434"
+models   = { "m" = { } }
+
+[routes.r]
+hosts = ["alpha"]
diff --git a/internal/config/testdata/bad-unknown-host.toml b/internal/config/testdata/bad-unknown-host.toml
new file mode 100644
index 0000000..fd5aa92
--- /dev/null
+++ b/internal/config/testdata/bad-unknown-host.toml
@@ -0,0 +1,8 @@
+listen = "127.0.0.1:7777"
+
+[hosts.alpha]
+base_url = "http://alpha.example:11434"
+models   = { "m" = { } }
+
+[routes.r]
+hosts = ["alpha", "gamma"]
diff --git a/internal/config/testdata/bad-unknown-key.toml b/internal/config/testdata/bad-unknown-key.toml
new file mode 100644
index 0000000..e3e94ec
--- /dev/null
+++ b/internal/config/testdata/bad-unknown-key.toml
@@ -0,0 +1,9 @@
+listen = "127.0.0.1:7777"
+lease_idle = "30m"
+
+[hosts.alpha]
+base_url = "http://alpha.example:11434"
+models   = { "m" = { } }
+
+[routes.r]
+hosts = ["alpha"]
diff --git a/internal/config/testdata/good.toml b/internal/config/testdata/good.toml
new file mode 100644
index 0000000..92ad98e
--- /dev/null
+++ b/internal/config/testdata/good.toml
@@ -0,0 +1,19 @@
+listen        = "100.64.0.9:7777"
+poll_interval = "5s"
+queue_max     = 4
+
+[hosts.alpha]
+base_url = "http://alpha.example:11434/"
+weight   = 2.0
+models   = { "ornith-1.5-35b-a3b" = { parallel = 4 }, "small-9b" = { parallel = 6 } }
+
+[hosts.beta]
+base_url = "https://beta.example:8081"
+models   = { "ornith-1.5-35b-a3b" = { } }
+
+[routes.opencode-a]
+hosts         = ["alpha", "beta"]
+default_model = "ornith-1.5-35b-a3b"
+
+[routes.hermes-x]
+hosts = ["beta"]
diff --git a/scripts/check-lines.sh b/scripts/check-lines.sh
new file mode 100644
index 0000000..c8e113e
--- /dev/null
+++ b/scripts/check-lines.sh
@@ -0,0 +1,14 @@
+#!/bin/sh
+# No Go source file over 400 lines. Reports every offender, then fails.
+set -u
+limit=400
+bad=0
+for f in $(find . -name '*.go' -not -path './.git/*' -not -path './vendor/*'); do
+    n=$(wc -l < "$f") || { echo "check-lines: cannot read $f" >&2; exit 2; }
+    if [ "$n" -gt "$limit" ]; then
+        echo "check-lines: $f has $n lines (limit $limit)"
+        bad=1
+    fi
+done
+[ "$bad" -eq 0 ] || exit 1
+echo "check-lines: ok"