The control plane pins the bus's certificate, and keeps its password out of errors

The bus presents the mesh's own certificate, which names nothing a public verifier
accepts; the client verified by name and failed against a bus that was answering
("certificate is not valid for any names", 2026-09-28). It now pins the leaf's
fingerprint from MESH_BROKER_CERTIFICATE, as every host does. And a connection
error named the whole URL, password included — the address alone now.

internal/broker/jetstream.go

@@ -1,8 +1,14 @@
 package broker
 
 import (
+	"crypto/sha256"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/hex"
 	"errors"
 	"fmt"
+	"os"
+	"strings"
 	"time"
 
 	"github.com/nats-io/nats.go"
@@ -24,21 +30,58 @@ type JetStream struct {
 
 // Dial connects and returns the controller's JetStream handle.
 func Dial(url string, opts ...nats.Option) (*JetStream, error) {
-	// A name, because a connection nobody can identify in the server's own monitoring is one
-	// nobody can attribute a problem to.
 	opts = append(opts, nats.Name("mesh-controller"), nats.Timeout(10*time.Second))
+	// **Pinned, not named.** The bus presents the mesh's own certificate, which names nothing a
+	// public verifier would accept (design 25 §4: a host pins the server's exact certificate and
+	// checks nothing else, and so does this). Without this, the first connection failed with
+	// "certificate is not valid for any names" against a bus that was answering (2026-09-28).
+	if path := strings.TrimSpace(os.Getenv(CertificateVar)); path != "" {
+		pinned, err := pinnedTo(path)
+		if err != nil {
+			return nil, err
+		}
+		opts = append(opts, nats.Secure(pinned))
+	}
+	// The address in an error is the address alone. The URL carries this controller's password,
+	// and an error here is written on the assumption it will be logged.
+	where := BareAddress(url)
 	conn, err := nats.Connect(url, opts...)
 	if err != nil {
-		return nil, fmt.Errorf("connecting to the bus at %s: %w", url, err)
+		return nil, fmt.Errorf("connecting to the bus at %s: %w", where, err)
 	}
 	js, err := conn.JetStream()
 	if err != nil {
 		conn.Close()
-		return nil, fmt.Errorf("the bus at %s has no JetStream: %w", url, err)
+		return nil, fmt.Errorf("the bus at %s has no JetStream: %w", where, err)
 	}
 	return &JetStream{conn: conn, js: js}, nil
 }
 
+// pinnedTo is a TLS configuration that accepts exactly the certificate in the file and no other:
+// the leaf's SHA-256, compared on every handshake, with the name and the chain deliberately not
+// consulted — a self-signed certificate with no names is the ordinary case for a mesh's bus.
+func pinnedTo(path string) (*tls.Config, error) {
+	want, err := FingerprintOf(path)
+	if err != nil {
+		return nil, err
+	}
+	return &tls.Config{
+		InsecureSkipVerify: true, //nolint:gosec // replaced by the pin below, which is stricter
+		MinVersion:         tls.VersionTLS12,
+		VerifyPeerCertificate: func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+			if len(rawCerts) == 0 {
+				return errors.New("the bus presented no certificate")
+			}
+			sum := sha256.Sum256(rawCerts[0])
+			got := "sha256:" + hex.EncodeToString(sum[:])
+			if got != want {
+				return fmt.Errorf("the bus presented a certificate this mesh does not know (%s…), expected %s…", got[:23], want[:23])
+			}
+			return nil
+		},
+	}, nil
+}
+
 // Conn is the connection itself, for what the mesh keeps off JetStream on purpose — a heartbeat,
 // a tool call — where a lost message is answered by the next one or by a timeout the caller
 // already handles (design 25 §3).