diff --git a/cmd/chat-cli/api/client.go b/cmd/chat-cli/api/client.go
index 95141b5..01ce582 100644
--- a/cmd/chat-cli/api/client.go
+++ b/cmd/chat-cli/api/client.go
@@ -4,6 +4,7 @@ package api
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -12,6 +13,13 @@ import (
 	"time"
 )
 
+var (
+	// ErrHTTPStatus is returned when the server responds with an error status code.
+	ErrHTTPStatus = errors.New("HTTP error")
+	// ErrUnexpectedFormat is returned when the response format is unexpected.
+	ErrUnexpectedFormat = errors.New("unexpected format")
+)
+
 // Client wraps HTTP calls to the chat server API.
 type Client struct {
 	BaseURL    string
@@ -64,7 +72,7 @@ func (c *Client) do(method, path string, body any) ([]byte, error) {
 	}
 
 	if resp.StatusCode >= 400 {
-		return data, fmt.Errorf("HTTP %d: %s", resp.StatusCode, string(data))
+		return data, fmt.Errorf("%w: %d: %s", ErrHTTPStatus, resp.StatusCode, string(data))
 	}
 
 	return data, nil
@@ -150,7 +158,7 @@ func (c *Client) PollMessages(afterID string, timeout int) ([]Message, error) {
 	}
 
 	if resp.StatusCode >= 400 {
-		return nil, fmt.Errorf("HTTP %d: %s", resp.StatusCode, string(data))
+		return nil, fmt.Errorf("%w: %d: %s", ErrHTTPStatus, resp.StatusCode, string(data))
 	}
 
 	// The server may return an array directly or wrapped.
@@ -218,7 +226,7 @@ func (c *Client) GetMembers(channel string) ([]string, error) {
 			return nil, err
 		}
 		// Extract member names from whatever format.
-		return nil, fmt.Errorf("unexpected members format: %s", string(data))
+		return nil, fmt.Errorf("%w: members: %s", ErrUnexpectedFormat, string(data))
 	}
 
 	return members, nil
diff --git a/internal/models/auth_token.go b/internal/models/auth_token.go
index 2b8b4b9..c2c3fd1 100644
--- a/internal/models/auth_token.go
+++ b/internal/models/auth_token.go
@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"errors"
 	"time"
 )
 
@@ -23,5 +22,5 @@ func (t *AuthToken) User(ctx context.Context) (*User, error) {
 		return ul.GetUserByID(ctx, t.UserID)
 	}
 
-	return nil, errors.New("user lookup not available")
+	return nil, ErrUserLookupNotAvailable
 }
diff --git a/internal/models/channel_member.go b/internal/models/channel_member.go
index e98cf7f..59586c7 100644
--- a/internal/models/channel_member.go
+++ b/internal/models/channel_member.go
@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"errors"
 	"time"
 )
 
@@ -23,7 +22,7 @@ func (cm *ChannelMember) User(ctx context.Context) (*User, error) {
 		return ul.GetUserByID(ctx, cm.UserID)
 	}
 
-	return nil, errors.New("user lookup not available")
+	return nil, ErrUserLookupNotAvailable
 }
 
 // Channel returns the full Channel for this membership.
@@ -32,5 +31,5 @@ func (cm *ChannelMember) Channel(ctx context.Context) (*Channel, error) {
 		return cl.GetChannelByID(ctx, cm.ChannelID)
 	}
 
-	return nil, errors.New("channel lookup not available")
+	return nil, ErrChannelLookupNotAvailable
 }
diff --git a/internal/models/model.go b/internal/models/model.go
index b65c6e8..ca5c7bc 100644
--- a/internal/models/model.go
+++ b/internal/models/model.go
@@ -6,6 +6,14 @@ package models
 import (
 	"context"
 	"database/sql"
+	"errors"
+)
+
+var (
+	// ErrUserLookupNotAvailable is returned when the user lookup interface is not set.
+	ErrUserLookupNotAvailable = errors.New("user lookup not available")
+	// ErrChannelLookupNotAvailable is returned when the channel lookup interface is not set.
+	ErrChannelLookupNotAvailable = errors.New("channel lookup not available")
 )
 
 // DB is the interface that models use to query the database.
diff --git a/internal/models/session.go b/internal/models/session.go
index e6aaf94..295def2 100644
--- a/internal/models/session.go
+++ b/internal/models/session.go
@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"errors"
 	"time"
 )
 
@@ -23,5 +22,5 @@ func (s *Session) User(ctx context.Context) (*User, error) {
 		return ul.GetUserByID(ctx, s.UserID)
 	}
 
-	return nil, errors.New("user lookup not available")
+	return nil, ErrUserLookupNotAvailable
 }