Sometimes Phish, the four-piece, 40-year-old jam Band from Vermont, will be in the middle of an extended improvisation and building to one of its characteristic musical climaxes, when Trey Anastasio, the guitarist and frontman, will find a way to communicate to someone across the arena, a football field away, that the peak is not over just yet.

“I will in the subtlest way shake my head and say, ‘No, I’m not there,’” Anastasio said in an interview, “and from way back in the room he always gets this little message. I can take it around eight more bars, or four more bars, this peak, and he’ll make some incredible move right when we make the move. We’re, like, speaking to each other from a great distance. I don’t think anyone would notice this happening other than us.”

Last Friday night at Madison Square Garden, Anastasio’s interlocutor was standing behind five monitors and a Lighting control console, wearing a black T-shirt, jeans and Hoka sneakers. He controlled 100 base lighting looks — different permutations of 302 lights, with 79 colors preprogrammed for Phish — some mounted on 30 movable pieces of truss above the stage. It was his 1,752nd Phish show lighting the band.

What makes the job of Chris Kuroda, Phish’s lighting designer for every show save three since 1989, so difficult is the same thing that has made Phish such an object of obsession for its rabid fan base, who has yet again packed the Manhattan arena for a sold-out run, which concludes Friday and Saturday nights.

Namely: Every show is different. A five-minute song one night can last 20 another. There is no prewritten set list — the band decides on the fly what to play from its voluminous catalog (it performed 237 songs last year, according to its archivist). Kuroda must busk or punt, as it is called, clicking into every new look alongside the thoroughly unpredictable music.

“We don’t punt just because we want to,” Kuroda, 58, said in an interview. “It’s the only way to light this band.”

In an industry in which most concert lighting is automated — a series of cues triggered by a prerecorded click track — Kuroda’s ability to respond in real time makes him a unicorn.

“At trade shows, when he talks, they want to hear what he has to say, in part because of that improvisational nature,” said Michael S. Eddy, the editor of Projection, Lights & Staging News, an industry journal, who compared Kuroda to the longtime (and aptly named) Grateful Dead lighting director Candace Brightman. “He’s contributing artistically to the experience those fans want,” Eddy added.

Phish’s method of improvisation makes Kuroda’s task particularly challenging. Unlike, say, a jazz act, whose members might take turns soloing while the others keep the rhythm and backing chords, Phish’s four members — which also include Jon Fishman on drums, Mike Gordon on bass and Page McConnell on keyboards — strive to move together. Gordon has compared it to dozens of buffaloes suddenly turning in tandem mid-stampede.

Kuroda must make the lights change exactly when the band does, and reflect each jam’s mood. His fingers must match McConnell’s quirky chord-rhythms or Anastasio’s rapid-fire staccatos. He must do this musically, but in the form of light: angry reds, cool blues, slowly building pans, bursts of brightness, all on beat and in harmony.

“I can’t wait to hear something before I execute the lighting cue,” Kuroda said. “I need to execute the lighting cue anticipating what’s going to happen.”

Kuroda’s uncanny ability to accompany the band convincingly is the source of his appeal — yet another idiosyncratically locked-in thing about a group that commands deep devotion from its fan base.

“Anybody else who came and tried to do it, could do it,” Kuroda said. “But it wouldn’t look anything like what it looks like when I do it.”

The fan nickname for him, “CK5” — as in, the fifth member — is both exaggeration (the band cannot really see or respond to what the lights are doing while they are playing, Anastasio said) and a shortchanging of Kuroda’s deferential style.

“As grand as the lighting show is, it does not take you away from watching the band. That is still a musicians-first performance,” said Abigail Rosen Holmes, a designer who has worked with Kuroda on Phish’s show. “That’s so fundamental to the audience and who Phish is. He’s managed to become this fabulous extra element without taking away from the thing he’s lighting — in fact, to enhance it.”

Kuroda is sought well outside the world of Phish. He designs the lights for the New York Knicks and Rangers, and the Golden State Warriors. He has lit the Black Crowes, Aerosmith, R. Kelly, Ariana Grande and, a decade ago, Justin Bieber, a show which involved 2,900 lighting cues and no punting. (He used Phish’s color palette for Bieber’s Believe Tour. “I’m not saying I don’t add some Justin Bieber colors to that,” Kuroda noted. “Pink is a really good Justin Bieber color.”)

“He’s, I think, the most influential lighting designer on the planet,” said Michael Smalley, a lighting designer who has worked with Mariah Carey, Pitbull, and, in the jam-band space, the String Cheese Incident.

Kuroda, who grew up in Westchester County, N.Y., was a University of Vermont undergraduate and guitar student of Anastasio’s who schlepped gear to and from McConnell’s van before and after gigs for $20. At one show in southeastern New Hampshire, the person then-lighting the band asked Kuroda to cover for him briefly. After the set, Anastasio rushed over to compliment the lights during the intricate ballad “Fly Famous Mockingbird,” and Kuroda was gracefully credited. Phish soon hired him, and he took himself to school in Texas a few years into his tenure to beef up his skills.

“I never would have been a lighting designer had it not been for this band,” Kuroda said. “I’d probably be programming a computer in a cubicle.”

Around 30 Phish songs with particularly byzantine structures have their cues preprogrammed, though it is still Kuroda hitting the “GoTo” button through, say, “Fluffhead,” clicking each cue as the band hits each mark.

For every other song as well as all the jams, Kuroda is on his own. He has 100 base looks — combinations of colors, patterns and dynamic motions such as pans or tilts — and can manipulate each of them substantially, changing colors, speeding them up, slowing them down, changing a sprinkle into a wipe-fade, switching direction.

There is some method to the madness. Kuroda favors symmetry, though he has been tempted toward its opposite by the moving trusses, a recent addition. He uses stationary lights of varying colors and beam shapes (flared, straight) to create texture. He abjures audience blasters that flash tremendous amounts of light at the crowd. The band members rarely move much, but when they do, their lights do not follow them. Phish’s palette is very distinctive — you can almost smell the colors — with “a lot of saturation, a lot of pastel and not much in between,” as Kuroda put it.

“My whole mantra for 35 years is to keep everything looking organic and pure and not digital,” he said. “The gear is very digital, but we painstakingly try to make the digital look organic.”

When Phish got back together in 2009 after a four-plus-year pause, Kuroda was joined by a new programmer named Andrew Giffin, who also holds the title of assistant designer. After a quarter-century in Vermont, Kuroda and his wife, Rhia, who have a college-age daughter, moved to South Florida.

In the past decade, the rig has grown ever more complex. Kuroda’s newest toys are 144 tetras, few-foot-long bars mounted to the moving truss pieces that can light up, spin and arrange themselves into patterns.

Much as the band has stuck with juvenilia dreamed up in their 20s — synchronized trampoline-jumping, solos played on vacuum cleaners — Kuroda has sought to evolve the lights while maintaining the touch that won him his job 34 years ago.

“Chris is so good in, like, the Somerville Theater, when we were playing in front of 600 people,” Anastasio said. “So the trick over the years was finding a design that would fit on the scale that we’re at right now, that would not eat the show.”

Anastasio added, “You could put Phish in a room with a lamp and you could see how good Chris is.”