diff --git a/internal/domain/instance/endpoint.go b/internal/domain/instance/endpoint.go
new file mode 100644
index 0000000..15648e2
--- /dev/null
+++ b/internal/domain/instance/endpoint.go
@@ -0,0 +1,89 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package instance contains the pure domain model of a registered PostgreSQL instance.
+// It does not depend on Kubernetes types, database drivers or credential providers.
+package instance
+
+import (
+	"errors"
+	"net/netip"
+	"regexp"
+)
+
+// TLSMode is an explicit transport policy, not a driver-specific default.
+type TLSMode string
+
+const (
+	TLSDisable    TLSMode = "disable"
+	TLSRequire    TLSMode = "require"
+	TLSVerifyCA   TLSMode = "verify-ca"
+	TLSVerifyFull TLSMode = "verify-full"
+)
+
+// EndpointValues carries explicit, effective values across the application boundary.
+// Defaults are supplied by the API/application mapping, never silently by the domain.
+type EndpointValues struct {
+	Host               string
+	HostAddr           string
+	Port               int
+	ManagementDatabase string
+	TLSMode            TLSMode
+}
+
+// Endpoint is an immutable connection target. Equality compares its declared values,
+// not physical server identity. Its zero value is invalid; aggregate construction
+// must Validate incoming endpoints, even if callers bypass NewEndpoint.
+type Endpoint struct {
+	values EndpointValues
+}
+
+var identifier = regexp.MustCompile(`^[a-z][a-z0-9_]{0,62}$`)
+
+func NewEndpoint(values EndpointValues) (Endpoint, error) {
+	endpoint := Endpoint{values: values}
+	if err := endpoint.Validate(); err != nil {
+		return Endpoint{}, err
+	}
+	return endpoint, nil
+}
+
+// Values returns a copy, without exposing mutable state.
+func (e Endpoint) Values() EndpointValues { return e.values }
+
+// Validate checks local invariants only; it does not resolve DNS or perform IO.
+// Errors intentionally omit input values.
+func (e Endpoint) Validate() error {
+	if e.values.Host == "" {
+		return errors.New("endpoint host is required")
+	}
+	address, err := netip.ParseAddr(e.values.HostAddr)
+	if err != nil || address.Zone() != "" {
+		return errors.New("endpoint host address must be an IPv4 or IPv6 address")
+	}
+	if e.values.Port < 1 || e.values.Port > 65535 {
+		return errors.New("endpoint port must be between 1 and 65535")
+	}
+	if !identifier.MatchString(e.values.ManagementDatabase) {
+		return errors.New("endpoint management database must be a valid PostgreSQL identifier")
+	}
+	switch e.values.TLSMode {
+	case TLSDisable, TLSRequire, TLSVerifyCA, TLSVerifyFull:
+		return nil
+	default:
+		return errors.New("endpoint TLS mode must be explicitly supported")
+	}
+}
diff --git a/internal/domain/instance/endpoint_test.go b/internal/domain/instance/endpoint_test.go
new file mode 100644
index 0000000..d24ea82
--- /dev/null
+++ b/internal/domain/instance/endpoint_test.go
@@ -0,0 +1,118 @@
+/*
+Copyright 2026.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package instance_test
+
+import (
+	"strings"
+	"testing"
+
+	"git.ddupan.top/panxiao81/postgresql-tenant-operator/internal/domain/instance"
+)
+
+func validEndpoint() instance.EndpointValues {
+	return instance.EndpointValues{
+		Host: "postgres.home.arpa", HostAddr: "192.0.2.10", Port: 5432,
+		ManagementDatabase: "postgres", TLSMode: instance.TLSVerifyFull,
+	}
+}
+
+// Acceptance: docs/domain-instance.md §2, explicit values and no implicit TLS downgrade.
+func TestEndpointRejectsInvalidValues(t *testing.T) {
+	cases := []struct {
+		name   string
+		change func(*instance.EndpointValues)
+	}{
+		{"empty host", func(v *instance.EndpointValues) { v.Host = "" }},
+		{"missing address", func(v *instance.EndpointValues) { v.HostAddr = "" }},
+		{"DNS instead of IP", func(v *instance.EndpointValues) { v.HostAddr = "postgres.home.arpa" }},
+		{"invalid IP", func(v *instance.EndpointValues) { v.HostAddr = "192.0.2.999" }},
+		{"address with port", func(v *instance.EndpointValues) { v.HostAddr = "192.0.2.10:5432" }},
+		{"scoped address", func(v *instance.EndpointValues) { v.HostAddr = "fe80::1%eth0" }},
+		{"zero port", func(v *instance.EndpointValues) { v.Port = 0 }},
+		{"negative port", func(v *instance.EndpointValues) { v.Port = -1 }},
+		{"large port", func(v *instance.EndpointValues) { v.Port = 65536 }},
+		{"empty database", func(v *instance.EndpointValues) { v.ManagementDatabase = "" }},
+		{"uppercase database", func(v *instance.EndpointValues) { v.ManagementDatabase = "Postgres" }},
+		{"leading digit", func(v *instance.EndpointValues) { v.ManagementDatabase = "1postgres" }},
+		{"punctuation", func(v *instance.EndpointValues) { v.ManagementDatabase = "post-gres" }},
+		{"NUL", func(v *instance.EndpointValues) { v.ManagementDatabase = "post\x00gres" }},
+		{"long identifier", func(v *instance.EndpointValues) { v.ManagementDatabase = strings.Repeat("a", 64) }},
+		{"missing TLS mode", func(v *instance.EndpointValues) { v.TLSMode = "" }},
+		{"unsupported TLS mode", func(v *instance.EndpointValues) { v.TLSMode = "prefer" }},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			values := validEndpoint()
+			tc.change(&values)
+			endpoint, err := instance.NewEndpoint(values)
+			if err == nil {
+				t.Fatal("invalid endpoint accepted")
+			}
+			if endpoint != (instance.Endpoint{}) {
+				t.Fatal("constructor returned a partial endpoint on failure")
+			}
+		})
+	}
+}
+
+func TestEndpointPreservesValidValues(t *testing.T) {
+	for _, mode := range []instance.TLSMode{
+		instance.TLSDisable, instance.TLSRequire, instance.TLSVerifyCA, instance.TLSVerifyFull,
+	} {
+		for _, address := range []string{"192.0.2.10", "2001:db8::10"} {
+			for _, port := range []int{1, 65535} {
+				values := validEndpoint()
+				values.TLSMode, values.HostAddr, values.Port = mode, address, port
+				values.ManagementDatabase = "a" + strings.Repeat("_", 62)
+				endpoint, err := instance.NewEndpoint(values)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if endpoint.Values() != values {
+					t.Fatal("constructor changed explicit values")
+				}
+				if err := endpoint.Validate(); err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+	}
+}
+
+func TestEndpointIsAnImmutableComparableValue(t *testing.T) {
+	values := validEndpoint()
+	endpoint, err := instance.NewEndpoint(values)
+	if err != nil {
+		t.Fatal(err)
+	}
+	same, err := instance.NewEndpoint(values)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if endpoint != same {
+		t.Fatal("identical endpoint values must compare equal")
+	}
+	values.Host = "changed.example"
+	snapshot := endpoint.Values()
+	snapshot.Host = values.Host
+	if endpoint.Values().Host == snapshot.Host {
+		t.Fatal("caller mutated endpoint through a copy")
+	}
+	if err := (instance.Endpoint{}).Validate(); err == nil {
+		t.Fatal("zero endpoint must not be valid")
+	}
+}