diff --git a/cmd/creator-web/responsive_test.go b/cmd/creator-web/responsive_test.go
index e908246..12248ee 100644
--- a/cmd/creator-web/responsive_test.go
+++ b/cmd/creator-web/responsive_test.go
@@ -32,3 +32,20 @@ func TestCreatorConsoleResponsiveGuardrails(t *testing.T) {
 		}
 	}
 }
+
+func TestChatResponsiveGuardrailsConstrainMobileGridItem(t *testing.T) {
+	css, err := staticFiles.ReadFile("static/assets/chat.css")
+	if err != nil {
+		t.Fatal(err)
+	}
+	stylesheet := string(css)
+	for _, guard := range []string{
+		".chat-view { width: 100%; min-width: 0;",
+		"@media (max-width: 640px)",
+		".chat-layout { height: auto; display: block; }",
+	} {
+		if !strings.Contains(stylesheet, guard) {
+			t.Fatalf("chat responsive guardrail missing: %s", guard)
+		}
+	}
+}
diff --git a/cmd/creator-web/static/assets/chat.css b/cmd/creator-web/static/assets/chat.css
index 3fc0dfe..b6fe4e4 100644
--- a/cmd/creator-web/static/assets/chat.css
+++ b/cmd/creator-web/static/assets/chat.css
@@ -29,7 +29,7 @@ textarea { resize: vertical; min-height: 84px; max-height: 240px; padding: 14px;
 .login-form button, .composer button { border: 0; color: #11150f; background: var(--signal); font-weight: 700; }
 .login-form button { width: 100%; height: 50px; margin-top: 18px; }
 #login-error { min-height: 20px; color: var(--bad); font-size: 13px; }
-.chat-view { height: 100vh; min-height: 0; display: grid; grid-template-rows: 64px minmax(0, 1fr); overflow: hidden; }
+.chat-view { width: 100%; min-width: 0; height: 100vh; min-height: 0; display: grid; grid-template-rows: 64px minmax(0, 1fr); overflow: hidden; }
 .chat-header { min-width: 0; display: flex; align-items: center; gap: 14px; padding: 0 22px; border-bottom: 1px solid var(--line); background: rgba(16, 23, 20, .98); }
 .chat-header a { color: var(--ink); text-decoration: none; font: 400 24px/1 var(--serif); }
 #active-focus { color: var(--good); }
diff --git a/dev/ACTIVE_TASK.json b/dev/ACTIVE_TASK.json
index 97dbd99..c842cd2 100644
--- a/dev/ACTIVE_TASK.json
+++ b/dev/ACTIVE_TASK.json
@@ -1,5 +1,5 @@
 {
-  "task_file": "dev/tasks/DEV-000288-production-evidence-revision.json",
-  "task_id": "DEV-000288",
+  "task_file": "dev/tasks/DEV-000289-chat-mobile-overflow-fix.json",
+  "task_id": "DEV-000289",
   "task_revision": 1
 }
diff --git a/dev/tasks/DEV-000289-chat-mobile-overflow-fix.json b/dev/tasks/DEV-000289-chat-mobile-overflow-fix.json
new file mode 100644
index 0000000..6e480da
--- /dev/null
+++ b/dev/tasks/DEV-000289-chat-mobile-overflow-fix.json
@@ -0,0 +1,23 @@
+{
+  "task_id": "DEV-000289",
+  "task_revision": 1,
+  "architecture_revision": "ADR-0018",
+  "base_repo_revision": "a2ff01050e5a2e22b29b85c700ff788d739e1c77",
+  "risk_tier": "E",
+  "intent": "Remove authenticated chat horizontal overflow at mobile viewport widths caused by intrinsic grid-item width.",
+  "requirement_ids": ["authenticated mobile UI acceptance", "no horizontal overflow at 390px"],
+  "allowed_change_scope": {"modules": ["creator-web"], "paths": ["dev/ACTIVE_TASK.json", "dev/tasks/DEV-000289-chat-mobile-overflow-fix.json", "cmd/creator-web/static/assets/chat.css", "cmd/creator-web/responsive_test.go"]},
+  "forbidden_change_scope": {"paths": ["architecture/**", "security/**", "database/**", "LLM-local/**", "modules/**", "adapters/**"]},
+  "allowed_architecture_delta": {"new_cross_module_dependencies": 0, "new_canonical_owners": 0, "new_privileges": 0, "new_third_party_dependencies": 0, "new_database_owner_roles": 0},
+  "contracts": {"consume": ["existing creator-web HTML/CSS surface"], "may_change": []},
+  "semantic_owner": "creator-web presentation",
+  "canonical_state": ["none"],
+  "state_transitions": ["mobile layout intrinsic width -> constrained viewport width"],
+  "side_effects_and_privileges": ["none"],
+  "persistence_and_migrations": ["none"],
+  "failure_retry_restart": ["CSS-only; no runtime state or recovery semantics changed"],
+  "assumptions": ["desktop layout must remain unchanged", "deep UI and load tests remain deferred"],
+  "acceptance_scenarios": ["authenticated chat has no horizontal overflow at 390px", "existing mobile media rules remain present", "desktop static layout remains unchanged"],
+  "required_tests": ["go test ./cmd/creator-web", "go vet ./cmd/creator-web", "preflight", "check-diff", "check-architecture", "independent", "git diff --check", "CI", "production health smoke", "one authenticated mobile smoke"],
+  "terminal_gap_states": ["BLOCKED_BY_SPEC_GAP", "BLOCKED_BY_ARCHITECTURE_GAP", "BLOCKED_BY_MISSING_CAPABILITY"]
+}