Is Slug Sage Mode established?
The Naruto manga and official retrospectives checked for this guide do not establish a named Slug Sage Mode, a Shikkotsu Forest sage-training curriculum, or specific slug-sage transformations. Shikkotsu Forest and Katsuyu are established; a proposed mode built around them is a separate claim.
For the general mechanism of senjutsu, see the Sage Mode guide. The question here is narrower: what first-party material actually establishes about the forest, Katsuyu, healing, and Hashirama? An unknown detail should remain unknown rather than becoming an invented third training system.
What official material establishes
The three sage regions are named together in the published manga. Naming Mount Myoboku, Ryuchi Cave, and Shikkotsu Forest does not, by itself, demonstrate three matching transformations or training courses.
Tsunade and Sakura summon Katsuyu. The official Sakura retrospective describes her summoning a gigantic slug during the war, citing volume 70, chapter 676. The official Tsunade retrospective describes her use of Katsuyu to protect villagers during Pain’s attack. Those scenes establish the summon and its healing role. They do not supply an absolute measurement of Katsuyu’s whole body, immortality, or limitless splitting.
The other locations have separately documented teaching: Naruto trains with the toads at Mount Myoboku, while Kabuto learns from the White Snake Sage. Their examples are not evidence that Katsuyu has an equivalent curriculum.
Healing and stored chakra are not senjutsu
The Strength of a Hundred Seal stores accumulated chakra, signified by the forehead mark. Releasing that reserve supports the user’s techniques. Storage of chakra is not the same mechanism as gathering natural energy.
The official Tsunade retrospective describes Creation Rebirth healing and identifies her as Hashirama’s granddaughter. These facts establish her abilities and family relationship, not a slug-sage teaching lineage inherited from him. The seal, the healing technique, and the summon can be discussed on their own terms. Their use together does not prove that Tsunade or Sakura is performing Slug Sage Mode.
Hashirama’s Sage Mode is shown; his teacher is not
Hashirama’s Sage Mode is confirmed. The official Hashirama retrospective shows him activating it against Madara in volume 65, chapter 621, and continuing that clash in chapter 626. That column does not name a teacher, a forest curriculum, or a slug transformation.
Markings, regeneration, Wood Release, and Tsunade’s connection to Katsuyu can inspire a theory, but they do not identify Hashirama’s teacher. Treat the Shikkotsu explanation as interpretation. The demonstrated Sage Mode is canon; the teacher remains unspecified. This is the boundary of the checked official material, not a prediction about future stories.