Who actually sent the Do Not Answer message?
Not the Trisolaran leadership. The warning came from Listener 1379, a low-ranking monitor at the very bottom of Trisolaran society. This is the single most misremembered fact from book one. Readers recall the triple "Do not answer! Do not answer! Do not answer!" but assume it was an official broadcast or some strategic bait from the Trisolaran command. It was neither. It was one individual's private choice, made against his own civilization.
That detail matters because it completes a mirror Liu Cixin builds into the first book: Earth has Ye Wenjie betraying humanity, and Trisolaris has Listener 1379 betraying Trisolaris. Both first causes are lonely people who had given up on their own species.
Why was monitoring such a hopeless job on Trisolaris?
Because on Trisolaris, listening for alien signals meant a life spent on nothing. Their civilization ran thousands of monitoring stations scanning the sky, but almost none of them ever caught anything. Being assigned to monitoring duty was close to a life sentence of meaninglessness.
Listener 1379 was one of those forgotten men, until the day he received the sun-amplified broadcast sent from the Red Coast Base. The message carried language, music, and a description of a blue world. For someone who had wasted his whole life in silence, it was less an intelligence intercept than a revelation that something gentler than Trisolaris might exist out there.
Why did Listener 1379 warn Earth instead of reporting it?
Because he understood exactly what the message meant, and he could not bear it. A world with a single stable sun was the one thing his dying civilization needed, and reporting Earth's signal would doom that world instantly.
The logic of his warning is the part most readers skip past. Earth's first broadcast revealed only the direction of the source, not the distance. Trisolaris knew the signal came from the direction of the sun, but not where along that line the sun actually sat. As long as Earth stayed silent, the coordinate stayed blurred. But a single reply would let Trisolaris triangulate the exact position, and invasion would follow automatically.
So he transmitted the warning himself: do not answer, or your world will be located and occupied. He gambled his civilization's secret and his own life on whether a stranger a few light-years away would keep quiet. He had never seen Earth or a human being. What he was protecting was an abstraction, a civilization he judged might be more worth saving than his own. It is the first act of cross-civilizational kindness in the whole series, and its price was treason.
What happened to Listener 1379 in the end?
His warning failed, and so did he. Ye Wenjie, who received it, did not stay silent. She pressed the transmit key and handed over Earth's coordinates on purpose. Listener 1379 had bet that humanity would cherish its own world; Ye Wenjie had lost all faith in humanity and bet that aliens would be better. Two people, four light-years apart, made opposite choices that together lit the fuse for the entire trilogy.
His unauthorized act was discovered by the Trisolaran leadership, and there was no mercy in their logic. A stable world is prey, and the invasion fleet launched regardless of his plea. That is why Listener 1379 is best understood as the Trisolaran mirror of Ye Wenjie, and the darker joke is that both idealists were wrong. He overestimated how much humans valued their own world; she overestimated the morality of the aliens. Both betrayed their own civilizations, and together they pushed two worlds into the dark forest. The universe gave Earth one warning, and Earth answered.
Is Listener 1379 the same as the Trisolaran leadership or the Princeps?
No. This is worth stating plainly because the confusion is so common. The Princeps and the Trisolaran command wanted Earth's coordinates and ordered the invasion. Listener 1379 acted alone, in direct defiance of them, and was punished for it. Attributing the "Do Not Answer" warning to Trisolaran strategy or to the leadership gets the story exactly backwards: the warning was the one voice inside Trisolaris arguing against invasion, and it lost.