diff --git a/guard/guard.go b/guard/guard.go
index e8152687..42aaa490 100644
--- a/guard/guard.go
+++ b/guard/guard.go
@@ -252,7 +252,7 @@ var targetGuidance = map[string]string{
 // always final (an approval can still arrive, a lookup can succeed later), so
 // it does not end with "This operation is forbidden."
 func TargetBlockMessage(res intercept.TargetResult) string {
-	reason := res.Reason
+	reason := capBlockReason(res.Reason)
 	if res.Host != "" && !strings.Contains(reason, res.Host) {
 		target := res.Host
 		if res.Port != 0 {
@@ -277,7 +277,27 @@ func TargetBlockMessage(res intercept.TargetResult) string {
 // model-facing tool_result carries this framing.
 func systemBlockMessage(reason string) string {
 	return "[ARTEX platform control, not target defense] This call was intercepted by the platform. " +
-		"Reason: " + reason + ". This operation is forbidden."
+		"Reason: " + capBlockReason(reason) + ". This operation is forbidden."
+}
+
+// maxBlockReasonRunes bounds the reason a block message carries back to the
+// model. Rule messages are operator text and judge or target reasons can embed
+// command input, so a reason has no natural bound; 4000 runes keeps every
+// realistic reason whole while one oversized reason cannot flood the context.
+// Only the model-facing text is capped: Interceptor.Log stores the raw reason.
+const maxBlockReasonRunes = 4000
+
+// capBlockReason returns s unchanged when it has at most maxBlockReasonRunes
+// runes, and otherwise its first maxBlockReasonRunes runes and a marker.
+func capBlockReason(s string) string {
+	n := 0
+	for i := range s {
+		if n == maxBlockReasonRunes {
+			return s[:i] + " …(truncated)"
+		}
+		n++
+	}
+	return s
 }
 
 var reBlocked = regexp.MustCompile(`(?i)\b(403|forbidden|waf|blocked|rate.?limit|429|captcha|denied)\b`)
diff --git a/guard/guard_test.go b/guard/guard_test.go
index 03f2b2c1..f4d58014 100644
--- a/guard/guard_test.go
+++ b/guard/guard_test.go
@@ -6,6 +6,7 @@ import (
 	"strings"
 	"testing"
 	"time"
+	"unicode/utf8"
 
 	"github.com/Autumn-27/artex/db"
 	"github.com/Autumn-27/artex/intercept"
@@ -150,4 +151,65 @@ func TestTargetOnlyGuardAppliesBuiltinDenyRules(t *testing.T) {
 	}
 }
 
+// A deny rule with an oversized message reaches the model capped, with the
+// platform framing and the guidance intact.
+func TestDenyRuleReasonIsCapped(t *testing.T) {
+	ic := intercept.New(nil)
+	ic.SetRulesForTest([]db.InterceptRule{
+		{ID: 1, Name: "Huge message", Enabled: true, Priority: 50,
+			MatchTarget: "tool_input", MatchType: "string", Action: "deny",
+			Pattern: "whoami", Message: strings.Repeat("日", 20000)},
+	})
+	block, msg := pre(NewWithInterceptor(ic), context.Background(), "Bash", `{"command":"whoami"}`)
+	if !block {
+		t.Fatal("the deny rule did not block")
+	}
+	limit := maxBlockReasonRunes + utf8.RuneCountInString(systemBlockMessage("")+" …(truncated)")
+	if n := utf8.RuneCountInString(msg); n > limit {
+		t.Fatalf("message has %d runes, want at most %d", n, limit)
+	}
+	if !utf8.ValidString(msg) ||
+		!strings.HasPrefix(msg, "[ARTEX platform control, not target defense] This call was intercepted by the platform. Reason: ") ||
+		!strings.HasSuffix(msg, strings.Repeat("日", maxBlockReasonRunes)+" …(truncated). This operation is forbidden.") {
+		t.Errorf("capped message lost its framing or guidance: %q...%q", msg[:120], msg[len(msg)-120:])
+	}
+}
+
+// A target block with an oversized reason is capped the same way, and keeps the
+// target and the guidance of its code.
+func TestTargetBlockReasonIsCapped(t *testing.T) {
+	res := intercept.TargetResult{Code: intercept.TargetDenied, Host: "app.example.com", Port: 443,
+		Reason: strings.Repeat("é", 20000)}
+	msg := TargetBlockMessage(res)
+	empty := res
+	empty.Reason = ""
+	limit := maxBlockReasonRunes + utf8.RuneCountInString(TargetBlockMessage(empty)+" …(truncated)")
+	if n := utf8.RuneCountInString(msg); n > limit {
+		t.Fatalf("message has %d runes, want at most %d", n, limit)
+	}
+	if !utf8.ValidString(msg) ||
+		!strings.HasPrefix(msg, "[ARTEX platform control, not target defense] TARGET_DENIED: target app.example.com:443: ") ||
+		!strings.HasSuffix(msg, strings.Repeat("é", maxBlockReasonRunes)+" …(truncated). "+targetGuidance[intercept.TargetDenied]) {
+		t.Errorf("capped message lost its target or guidance: %q...%q", msg[:120], msg[len(msg)-120:])
+	}
+}
+
+// A reason within the cap passes through byte for byte.
+func TestShortBlockReasonUnchanged(t *testing.T) {
+	for _, s := range []string{"", "Destructive command rejected", strings.Repeat("日", maxBlockReasonRunes)} {
+		if got := capBlockReason(s); got != s {
+			t.Errorf("capBlockReason changed a reason of %d runes", utf8.RuneCountInString(s))
+		}
+	}
+	if got, want := systemBlockMessage("Destructive command rejected"),
+		"[ARTEX platform control, not target defense] This call was intercepted by the platform. Reason: Destructive command rejected. This operation is forbidden."; got != want {
+		t.Errorf("systemBlockMessage = %q, want %q", got, want)
+	}
+	res := intercept.TargetResult{Code: intercept.TargetDenied, Host: "app.example.com", Port: 443, Reason: "the request was denied"}
+	if got, want := TargetBlockMessage(res),
+		"[ARTEX platform control, not target defense] TARGET_DENIED: target app.example.com:443: the request was denied. "+targetGuidance[intercept.TargetDenied]; got != want {
+		t.Errorf("TargetBlockMessage = %q, want %q", got, want)
+	}
+}
+
 var _ = hook.PreToolUse