The Kirkus Reviews observed, "The resolution is bittersweet but faintly hopeful—a fitting end for an unflinchingly gutsy series whose deftly drawn characters have always lived dangerously.

While Lizzie works on (and ultimately solves) the code, Gregor fights the Bane's armies, struggling all the while to cope with his emotions about Luxa and Sandwich's prophecy.

Though the humans routed the rats, Ares died in the Firelands of wounds inflicted by the Bane; Ripred is presumed dead; and Vikus has been partially paralyzed by a stroke brought on by his wife's death.

Angered by both a half-dead Ripred and Luxa's readiness to return to war, Gregor breaks Bartholomew of Sandwich's sword on his knees and pronounces the warrior officially dead.

The "Tree of Transmission" is a visual representation of the Underland equivalent of Morse code, i.e. a method of transmitting information over long distances using repeating patterns.

The gnawers' code is actually a cipher which uses "Gorger" (also an anagram of the word "Gregor"), with recurring letters removed to form "Gore" — easily remembered by the rat armies — to make it more difficult to crack.

[4] Gregor and the Code of Claw has been reviewed favorably by critics, yet its print version never reached the top of bestseller lists except as part of The Underland Chronicles.

A 2009 AudioFile review states, "Boehmer creates atmosphere by slightly modulating his voice to depict the darker tone of the story and the maturity of its theme.

A School Library Journal review praises the "realistic conclusion, including the loss of old friends and a message about the importance of peace and trust, [leaving] readers with a sense of optimism about the future of the Underland.

"[2] The novel's main criticisms have centered on its mature themes, which are much more prominent in the somber Gregor and the Code of Claw than in the series's other books.

Comparisons between The Underland Chronicles and Suzanne Collins's later Hunger Games trilogy cite parallels between this final novel more than any other, and note the series' many violent moments.

[13] One Kirkus reviewer comments that though this is "heavy stuff" for preteens, the novel is an excellent "read-alike" for series such as The Heroes of Olympus, which also possess more complex themes.