Some authorities permit adding instant coffee or tea to a Kli Sheni, but the stringent view requires a Kli Shlishi. The prohibition of bishul (cooking) applies, and adding granules directly under a Kli Rishon pour is universally forbidden; the sequence must be water first, then coffee.

Source: The Laws of Cooking on Shabbos by Nissan Dovid Dubov, p. 22

Ground coffee is kali habishul (easily cooked). Many poskim hold all foods are kali habishul unless otherwise designated (Mishnah Berurah 318:42), so a kli sheini would cook it. Rav Ovadiah Yosef (Yabiah Omer OC 8:35) ruled roasted coffee beans are already fully cooked, permitting a kli sheini for Sephardim. However, the Rema (318:5) is machmir for the view that bishul achar afiyah applies even in a kli sheini, so Ashkenazim may not brew ground coffee in a kli sheini.

Source: Daf HaKashrus 2014–2015 p.34; also Laws of Cooking on Shabbos (Dubov) p.22

Igros Moshe (OC 4:74:16) permits pouring hot water directly onto instant coffee in principle, though he himself is machmir to use a kli shlishi. Minchas Yitzchak (1:55) argues that since Nescafe dissolves in hot water, it should be viewed as a cold liquid rather than a dry food, making bishul achar bishul the relevant concern — and reheating liquid in a kli sheini is permitted (Shulchan Aruch HaRav 318:12).

Daf HaKashrus 2014–2015 (Daf HaShana 23), p.34

Splenda can be considered fully cooked (bishul achar bishul applies, not a new act of cooking). However, the concern raised by many poskim — discussed in Daf 23:7 (pg. 31) — is that an ingredient that dissolves in hot water should be treated as a liquid rather than a solid food. Since liquids in a kli sheini are subject only to bishul achar bishul (reheating already-cooked liquid), which is permitted, adding Splenda to a kli sheini is permitted. As Splenda also dissolves in coffee or tea, the same logic applies whether it is added to water, coffee, or tea.

Source: Daf HaKashrus 2014–2015 (Daf HaShana 23), p.46, by Rabbi Gavriel Price. Cross-references Daf 23:7 (pg.31) on dissolving-as-liquid principle.

Shut Az Nidberu (1:35–36) raises an additional reason for stringency: the industrial process of making instant coffee powder transforms the coffee from its cooked state into a roasted state. Therefore, when hot water is added, this constitutes a new act of bishul (cooking something that was roasted), which is biblically prohibited even in a kli sheini according to many opinions. This is distinct from the simple bishul achar bishul analysis.

Shut Az Nidberu 1:35–36. Unique rationale: manufacturing changes coffee from cooked→roasted, making re-hydration a new act of bishul.

Shemirat Shabbat Kehilchatah (1:55, footnote 157) permits adding milk to coffee on Shabbos. Pasteurized milk has already undergone a cooking process; therefore adding it to hot coffee does not constitute new bishul — it is reheating of an already-cooked liquid (bishul achar bishul for liquids), which is permitted.

SSK 1:55 fn.157.

Shemirat Shabbat Kehilchatah (1:49) rules leniently, permitting instant coffee in a kli sheini. This aligns with the position of Igros Moshe and Minchas Yitzchak. MB 318:47 notes that kali habishul is cooked even in a kli sheini, but instant coffee may be treated differently given its dissolved/liquid character.

SSK 1:49. Cross-reference MB 318:47 on kali habishul in kli sheini; Pri Megadim leans lenient for kli shlishi.

Shut Be'er Moshe (2:21) is stringent because adding hot water to instant coffee powder transforms it from an inedible state to an edible/drinkable state. This constitutes a meaningful act of bishul regardless of whether the vessel is a kli sheini or kli shlishi, since the forbidden action is the transformation of the substance itself.

Shut Be'er Moshe 2:21. Contrasts with Igros Moshe and SSK who permit kli sheini.

The method produces irui (pouring) from a kli sheni onto a kli shlishi. The poskim hold that a kli shlishi does not cook even kali habishul, and irui from a kli sheni is treated as a kli shlishi. This satisfies even the most stringent opinions (Be'er Moshe, Az Nidberu) since the water never directly contacts the coffee as a kli rishon or kli sheni pour. Sources cited: Kaf HaChaim 418:5, Rav Pa'alim 3:11, Ben Ish Chai (Bo 8), Mishnah Berurah (ibid.), Yabiah Omer 2:20.

Kitzur Shulchan Aruch — Shulchan HaMelech (Laniado). Cross-reference: Kaf HaChaim 418:5, Rav Pa'alim 3:11, Ben Ish Chai Bo 8, MB ibid., Yabiah Omer 2:20 — all cited as agreeing with this method.

Panim Meirot personally observed this practice in Ashkenazi communities without objection. Mor uKtzia and Ketzot HaShulchan hold that instant coffee powder (nems) is so finely processed that it is analogous to snow (שלג) — it dissolves rather than cooks, and therefore is not subject to bishul in a kli sheni. Az Nidberu (1:35-36, lenient side cited here) also permits. The dissolving-as-liquid argument (Minchas Yitzchak 1:55) is another basis: once dissolved it is a liquid subject only to bishul achar bishul which is permitted in kli sheni.

Collected in Dor HaMelaktim Shabbat 5, siman 318 se'if 14, pp.61-63. Cross-reference: Minchas Yitzchak 1:55, Igros Moshe OC 4:74:16 (also lenient).

Multiple poskim (Menorah HaTehorah, Torat Shabbat, Shevitat HaShabbat, Nishmat Shabbat, Machazeh Eliyahu, Maor HaShabbat, Birkat Yitzchak, Divrei Binyahu) hold that instant coffee/sugar/cocoa are kali habishul and cooking them even in a kli sheni constitutes bishul. Additionally, some raise the concern of bishul achar afiyah (cooking after roasting) since instant coffee has been roasted in manufacturing. The permitted method according to the stringent view is: place coffee+sugar in the cup first (empty/dry), then pour the hot water — this is irui (pouring) from a kli sheni onto a kli shlishi, which is permitted even according to stringent opinions.

Collected in Dor HaMelaktim Shabbat 5, siman 318 se'if 14, pp.61-63 (section לד). All stringent sources agree that the water-first method (irui from kli sheni onto kli shlishi) is acceptable.

GRSh"Z distinguishes: (1) The basic instant coffee in kli sheni is permitted on the grounds that the nems dissolves like snow (שלג), not a solid food being cooked. (2) However, he notes the concern raised by Az Nidberu 1:35-36 — the manufacturing process roasts the coffee, so adding water could constitute bishul achar afiyah (cooking after roasting), which some hold is biblically prohibited. (3) Regarding a Shabbos clock drip coffee maker: the water actively transforms the coffee from an unusable state to a drinkable state, which he views as more problematic than simply placing nems into already-hot water in a kli sheni.

Cited in Dor HaMelaktim Shabbat 5, pp.62-63. Cross-reference: Az Nidberu 1:35-36 (roasting argument), HaPardess Vol.1 No.74 (Shabbos clock coffee maker), Minchas Yitzchak 1:55 (dissolving argument).

Pouring hot liquid from a kli rishon can cook (bishul) the sugar; decanting liquid from sediment, or separating yolk from white, is selection (borer) of food from waste or of two foods from each other.

The Mishnah Berurah (216:16) references the heat found in freshly ground coffee. Rav Ferrell clarifies in the name of Rav Schachter that this heat is not residual external heat but is generated by the friction of grinding. Rav Schachter's consequent ruling: friction-generated heat carries the full chumros of a Keli Rishon. This has practical ramifications for: (1) basar b'chalav — if a hot coffee grinder contacts dairy residue, it may have keli rishon-level absorption implications; (2) Shabbos — grinding coffee fresh may produce keli rishon-status heat with consequences for items subsequently placed in the ground coffee; (3) general bishul — friction heat is not treated as a lesser heat source but fully equivalent to direct flame heating.

Cross-references: MB 216:16 (heat of ground coffee); the previous entry on ground coffee and Pesach/kemach concerns (Shtilerman/Peri Megadim). This ruling is transmitted orally from Rav Schachter and recorded in Ferrell's footnote 312 — treat as a b'shem omro rather than a published teshuvah.

Tzovea (coloring) is a Shabbos melacha. When dark coffee or tea is added to water, it colors the water. Shaar HaTziyun (318:65) says one should add coffee/tea first before the water. Igros Moshe OC 4:74:18 discusses this in the context of bishul. Shevet HaLevi 9:71 and Betzel HaChochma 2:49 also address this. However, the primary concern of bishul may override the tzovea discussion.

Halachically Speaking Vol.4 Issue 19. Cites Shaar HaTziyun 318:65, Igros Moshe OC 4:74:18, Shevet HaLevi 9:71, Betzel HaChochma 2:49.

Shut Darkei Yitzchak: Unground coffee beans have no Shabbos use and are muktzeh machmat gufo regardless of whether they are roasted. Once roasted AND ground, there is a machloket: those who hold it is muktzeh reason that there is no further preparation possible on Shabbos (it cannot be cooked on Shabbos); those who hold it is not muktzeh may rely on the view that bishul achar afiyah does not apply (roasted coffee can still be "cooked" — brewed — on Shabbos in a permitted way, giving it a potential Shabbos use). Footnote קכב cites Shlomi Yehudah ch.8:12 fn.22 on this machloket regarding bishul achar afiyah for roasted coffee.

Shut Darkei Yitzchak im Haarot Chayei Moshe (Kardi). The bishul achar afiyah question is the hinge: if roasted coffee cannot be halachically "cooked" further, it may be muktzeh; if it can (e.g. brewing counts as bishul), it has a potential use and may not be muktzeh.

The question involves several issues: (1) whether pre-setting a filter coffee maker before Shabbos constitutes hatmana (insulation/delayed cooking) or bishul; (2) whether the Shabbos clock (שעון-שבת) is an acceptable halachic mechanism for activating appliances on Shabbat; (3) the shema yachateh concern — since the filter coffee maker maintains a fixed temperature and the user has no access to adjust it, this concern does not apply. The coffee grounds and water are placed before Shabbos; the machine simply executes the pre-set action. The Shabbos clock itself is characterized as a separate and independent halachic unit (חלק עצמו).

HaPardess Vol.1 No.74 (Sept 1999), siman 254 — R. Meir A. Cohen, New York. Article page 16 of the issue (PDF page 18 on HebrewBooks req=52419).