diff --git a/internal/summary/presenter.go b/internal/summary/presenter.go
index 662d469..285daf9 100644
--- a/internal/summary/presenter.go
+++ b/internal/summary/presenter.go
@@ -80,11 +80,14 @@ func BuildSummaryMessage(header string, highlights []Highlight, digests []Channe
 		embed := &discordgo.MessageEmbed{
 			Title:       title,
 			Description: description,
-			URL:         link,
 			Color:       color,
 		}
-		if msg := findMessageByURL(digests, link); msg != nil {
-			embed.Author = &discordgo.MessageEmbedAuthor{Name: msg.Author, IconURL: msg.AvatarURL}
+		// モデルが幻覚した Discord 外 URL をリンク化しない
+		if isDiscordMessageLink(link) {
+			embed.URL = link
+			if msg := findMessageByURL(digests, link); msg != nil {
+				embed.Author = &discordgo.MessageEmbedAuthor{Name: msg.Author, IconURL: msg.AvatarURL}
+			}
 		}
 		embeds = append(embeds, embed)
 	}
@@ -96,6 +99,11 @@ func BuildSummaryMessage(header string, highlights []Highlight, digests []Channe
 	return header, embeds
 }
 
+func isDiscordMessageLink(link string) bool {
+	return strings.HasPrefix(link, "https://discord.com/channels/") ||
+		strings.HasPrefix(link, "https://discordapp.com/channels/")
+}
+
 func findMessageByURL(digests []ChannelDigest, url string) *MessageDigest {
 	for i := range digests {
 		for j := range digests[i].Messages {
diff --git a/internal/summary/presenter_test.go b/internal/summary/presenter_test.go
index 2aba3af..4acf20d 100644
--- a/internal/summary/presenter_test.go
+++ b/internal/summary/presenter_test.go
@@ -139,6 +139,17 @@ func TestBuildSummaryMessage(t *testing.T) {
 		}
 	})
 
+	t.Run("non-discord link is not linked", func(t *testing.T) {
+		highlights := []Highlight{{Title: "t", Description: "d", Link: "https://evil.example.com/x"}}
+		_, embeds := BuildSummaryMessage("h", highlights, digests)
+		if len(embeds) != 1 {
+			t.Fatalf("highlight should be kept, embeds = %d", len(embeds))
+		}
+		if embeds[0].URL != "" || embeds[0].Author != nil {
+			t.Errorf("external URL should not be linked: %+v", embeds[0])
+		}
+	})
+
 	t.Run("incomplete highlights are dropped", func(t *testing.T) {
 		highlights := []Highlight{{Title: "", Description: "d", Link: "l"}}
 		_, embeds := BuildSummaryMessage("h", highlights, nil)
diff --git a/internal/summary/prompt.go b/internal/summary/prompt.go
index 56c3966..049ed6f 100644
--- a/internal/summary/prompt.go
+++ b/internal/summary/prompt.go
@@ -3,18 +3,23 @@ package summary
 import (
 	"fmt"
 	"strings"
+	"time"
 )
 
-func buildSummaryPrompt(digests []ChannelDigest, maxHighlights int) string {
+func buildSummaryPrompt(digests []ChannelDigest, maxHighlights int, lookback time.Duration) string {
 	if len(digests) == 0 {
 		return ""
 	}
 	if maxHighlights <= 0 {
 		maxHighlights = 3
 	}
+	lookbackHours := int(lookback.Hours())
+	if lookbackHours <= 0 {
+		lookbackHours = 24
+	}
 
 	var builder strings.Builder
-	builder.WriteString("あなたはDiscordサーバーの編集者です。以下は過去24時間に投稿されたメッセージ一覧です。\n")
+	fmt.Fprintf(&builder, "あなたはDiscordサーバーの編集者です。以下は過去%d時間に投稿されたメッセージ一覧です。\n", lookbackHours)
 	fmt.Fprintf(&builder, "最も注目すべき出来事を最大%d件選び、日本語の夕刊ハイライトを作成してください。\n", maxHighlights)
 	builder.WriteString("出力は必ず次のJSON配列のみとし、整形や説明文を入れないでください。\n")
 	builder.WriteString("例: [{\"title\":\"見出し\",\"emoji\":\"\",\"description\":\"本文\",\"link\":\"https://discord.com/channels/...\",\"color\":\"#b0b0b0\"}]\n")
diff --git a/internal/summary/prompt_test.go b/internal/summary/prompt_test.go
index bc55227..b5e9266 100644
--- a/internal/summary/prompt_test.go
+++ b/internal/summary/prompt_test.go
@@ -17,11 +17,14 @@ func TestBuildSummaryPrompt(t *testing.T) {
 		},
 	}}
 
-	prompt := buildSummaryPrompt(digests, 3)
+	prompt := buildSummaryPrompt(digests, 3, 24*time.Hour)
 
 	if !strings.Contains(prompt, "最大3件") {
 		t.Errorf("prompt should mention max highlights: %q", prompt)
 	}
+	if !strings.Contains(prompt, "過去24時間") {
+		t.Errorf("prompt should mention lookback hours")
+	}
 	if !strings.Contains(prompt, "# general") {
 		t.Errorf("prompt should contain channel header")
 	}
@@ -36,11 +39,11 @@ func TestBuildSummaryPrompt(t *testing.T) {
 }
 
 func TestBuildSummaryPromptEdgeCases(t *testing.T) {
-	if got := buildSummaryPrompt(nil, 3); got != "" {
+	if got := buildSummaryPrompt(nil, 3, 24*time.Hour); got != "" {
 		t.Errorf("empty digests should return empty prompt, got %q", got)
 	}
 	digests := []ChannelDigest{{Name: "c", Messages: []MessageDigest{{Author: "a", Content: "x"}}}}
-	if got := buildSummaryPrompt(digests, 0); !strings.Contains(got, "最大3件") {
+	if got := buildSummaryPrompt(digests, 0, 0); !strings.Contains(got, "最大3件") {
 		t.Errorf("non-positive maxHighlights should fall back to 3")
 	}
 }
diff --git a/internal/summary/service.go b/internal/summary/service.go
index 57de690..5017d38 100644
--- a/internal/summary/service.go
+++ b/internal/summary/service.go
@@ -126,7 +126,7 @@ func (s *Service) Generate(ctx context.Context, req Request) (Result, error) {
 			summaryCtx = context.Background()
 		}
 
-		prompt := buildSummaryPrompt(collectorResult.Digests, s.Config.MaxHighlights)
+		prompt := buildSummaryPrompt(collectorResult.Digests, s.Config.MaxHighlights, s.Config.Lookback)
 		if prompt == "" {
 			s.info(ctx, "Gemini呼び出しスキップ (プロンプトなし)")
 		} else {
diff --git a/internal/summary/types.go b/internal/summary/types.go
index 98bad35..ea70a28 100644
--- a/internal/summary/types.go
+++ b/internal/summary/types.go
@@ -65,6 +65,8 @@ type Config struct {
 	MessageCharBudget int
 	MaxHighlights     int
 	MaxAttempts       int
+	// Lookback はプロンプトの「過去N時間」表記に使う (収集期間と揃える)
+	Lookback time.Duration
 }
 
 // Summarizer は収集メッセージの要約を生成し、構造化ハイライトの生 JSON を返す。
diff --git a/main.go b/main.go
index b25a74f..e401d38 100644
--- a/main.go
+++ b/main.go
@@ -86,6 +86,7 @@ func run() error {
 			MessageCharBudget: cfg.Summary.MessageCharBudget,
 			MaxHighlights:     cfg.Summary.MaxHighlights,
 			MaxAttempts:       cfg.Summary.MaxAttempts,
+			Lookback:          cfg.Summary.Lookback,
 		},
 	}