A sampling of Halachos from Rambam, Hilchos Chametz u’Matzah, chapter 7.
§3:
וְצָרִיךְ לַעֲשׂוֹת שִׁנּוּי בַּלַּיְלָה הַזֶּה כְּדֵי שֶׁיִּרְאוּ הַבָּנִים וְיִשְׁאֲלוּ וְיֹאמְרוּ מַה נִּשְׁתַּנָּה הַלַּיְלָה הַזֶּה מִכָּל הַלֵּילוֹת עַד שֶׁיָּשִׁיב לָהֶם וְיֹאמַר לָהֶם כָּךְ וְכָךְ אֵרַע וְכָךְ וְכָךְ הָיָה. [...] אֵין לוֹ בֵּן אִשְׁתּוֹ שׁוֹאַלְתּוֹ. אֵין לוֹ אִשָּׁה שׁוֹאֲלִין זֶה אֶת זֶה מַה נִּשְׁתַּנָּה הַלַּיְלָה הַזֶּה. וַאֲפִלּוּ הָיוּ כֻּלָּן חֲכָמִים. הָיָה לְבַדּוֹ שׁוֹאֵל לְעַצְמוֹ מַה נִּשְׁתַּנָּה הַלַּיְלָה הַזֶּה:
He needs to make a distinction on this night so that the children will see, ask, and say, “Why is this night different from all other nights?” And he will say to them, “Such and such happened, and such and such was.” [...] If he doesn’t have a son, his wife asks him. If he doesn’t have a wife, [those in attendance] ask each other, “Why is this night different?” even if they are all sages. If he is alone, he asks himself, “Why is this night different?”
(Technically, Mah Nishtanah isn’t strictly required. What is required is that the general question be asked, even if not in those words: what’s the big deal about tonight? Cf. Pesachim 116a.)
§4:
וְצָרִיךְ לְהַתְחִיל בִּגְנוּת וּלְסַיֵּם בְּשֶׁבַח. כֵּיצַד. מַתְחִיל וּמְסַפֵּר שֶׁבַּתְּחִלָּה הָיוּ אֲבוֹתֵינוּ בִּימֵי תֶּרַח וּמִלְּפָנָיו כּוֹפְרִים וְטוֹעִין אַחַר הַהֶבֶל וְרוֹדְפִין אַחַר עֲבוֹדַת אֱלִילִים. וּמְסַיֵּם בְּדַת הָאֱמֶת שֶׁקֵּרְבָנוּ הַמָּקוֹם לוֹ וְהִבְדִּילָנוּ מֵהָאֻמּוֹת וְקֵרְבָנוּ לְיִחוּדוֹ. וְכֵן מַתְחִיל וּמוֹדִיעַ שֶׁעֲבָדִים הָיִינוּ לְפַרְעֹה בְּמִצְרַיִם וְכָל הָרָעָה שֶׁגְּמָלָנוּ וּמְסַיֵּם בַּנִּסִּים וּבַנִּפְלָאוֹת שֶׁנַּעֲשׂוּ לָנוּ וּבְחֵרוּתֵנוּ. וְהוּא שֶׁיִּדְרשׁ מֵ(דברים כו ה) "אֲרַמִּי אֹבֵד אָבִי" עַד שֶׁיִּגְמֹר כָּל הַפָּרָשָׁה. וְכָל הַמּוֹסִיף וּמַאֲרִיךְ בִּדְרַשׁ פָּרָשָׁה זוֹ הֲרֵי זֶה מְשֻׁבָּח:
He must begin with disgrace and conclude with praise. How so? He begins and tells over that originally our forefathers, in the days of Terach and earlier, were deniers, straying after vanity and chasing after idolatry. He concludes with the truth that Hashem has brought us close to him and separated us from the nations, bringing us close to his uniqueness. Likewise, he begins and acknowledges that we were slaves to Paroh in Mitzraim, and all the evil which he gave us, and he concludes with the miracles and wonders which were done for us and in our freedom. He expounds from “An Aramean destroyed my father” until he finishes the entire passage. Whoever adds and extends in expounding this passage, he is praiseworthy.
§5:
כָּל מִי שֶׁלֹּא אָמַר שְׁלֹשָׁה דְּבָרִים אֵלּוּ בְּלֵיל חֲמִשָּׁה עָשָׂר לֹא יָצָא יְדֵי חוֹבָתוֹ וְאֵלּוּ הֵן. פֶּסַח מַצָּה וּמָרוֹר. פֶּסַח עַל שׁוּם שֶׁפָּסַח הַמָּקוֹם עַל בָּתֵּי אֲבוֹתֵינוּ בְּמִצְרַיִם שֶׁנֶּאֱמַר (שמות יב כז) "וַאֲמַרְתֶּם זֶבַח פֶּסַח הוּא לַה'" וְגוֹ'. מָרוֹר עַל שׁוּם שֶׁמֵּרְרוּ הַמִּצְרִיִּים אֶת חַיֵּי אֲבוֹתֵינוּ בְּמִצְרַיִם. מַצָּה עַל שָׁם שֶׁנִּגְאֲלוּ. וּדְבָרִים הָאֵלּוּ כֻּלָּן נִקְרָאִין הַגָּדָה:
Whoever doesn’t say these things on the night of the fifteenth has not fulfilled his obligation. These are they: Pesach, Matzah, and Marror. Pesach is because Hashem passed over the houses of our fathers in Mitzraim, as it says, “And you will say, ‘it is a Pesach sacrifice to Hashem, etc.’” Marror is because the Mitzri’im embittered the lives of our fathers in Mitzraim. Matzah is because we were redeemed. All of these things are called “Haggadah.”
(When the Rambam writes “all” of these things, it’s unclear to me whether he refers to Pesach, Matzah, and Marror, or to everything listed above, from §3.)
§6:
בְּכָל דּוֹר וָדוֹר חַיָּב אָדָם לְהַרְאוֹת אֶת עַצְמוֹ כְּאִלּוּ הוּא בְּעַצְמוֹ יָצָא עַתָּה מִשִּׁעְבּוּד מִצְרַיִם שֶׁנֶּאֱמַר (דברים ו כג) "וְאוֹתָנוּ הוֹצִיא מִשָּׁם" וְגוֹ'.
In every generation, a person is obligated to show himself as if he personally left, now, from the enslavement in Mitzraim, as it says, “And He took us from there.”
This doesn’t indicate that one is obligated to say this Mishnah, but it does indicate that one must show himself (Rambam has a different reading of the Mishnah than we do, apparently) as if he personally left. As such, it’s certainly prudent to say this Mishnah, to emphasize this point.
§7:
וְכָל אֶחָד וְאֶחָד בֵּין אֲנָשִׁים בֵּין נָשִׁים חַיָּב לִשְׁתּוֹת בַּלַּיְלָה הַזֶּה אַרְבָּעָה כּוֹסוֹת שֶׁל יַיִן. אֵין פּוֹחֲתִין לוֹ מֵהֶם.
Everyone, both men and women, are obligated to drink four cups of wine on this night, no fewer than that.
§10:
כָּל כּוֹס וְכוֹס מֵאַרְבָּעָה כּוֹסוֹת הַלָּלוּ מְבָרֵךְ עָלָיו בְּרָכָה בִּפְנֵי עַצְמָהּ. וְכוֹס רִאשׁוֹן אוֹמֵר עָלָיו קִדּוּשׁ הַיּוֹם. כּוֹס שֵׁנִי קוֹרֵא עָלָיו אֶת הַהַגָּדָה. כּוֹס שְׁלִישִׁי מְבָרֵךְ עָלָיו בִּרְכַּת הַמָּזוֹן. כּוֹס רְבִיעִי גּוֹמֵר עָלָיו אֶת הַהַלֵּל וּמְבָרֵךְ עָלָיו בִּרְכַּת הַשִּׁיר.
On each of these four cups, we make an individual blessing on it. On the first cup, he says Kiddush. On the second cup, he reads the Haggadah. On the third cup, he bentches. On the fourth cup, he finishes Hallel and recites Yehalelucha.
The identity of Birchas HaShir is disputed in the Gemara (Pesachim 118a), whether this refers to Yehalelucha or Nishmas. In practice, we say both in Hallel; however, in 8:10, the Rambam explicitly identifies it as Yehalelucha.
What about the rest of the Seder? In chapter 8, the Rambam gives the basic rundown of the Seder. Some notable parts:
§1-2:
וְאַחַר כָּךְ מְבָרֵךְ עַל נְטִילַת יָדַיִם וְנוֹטֵל יָדָיו. [...] מַתְחִיל וּמְבָרֵךְ בּוֹרֵא פְּרִי הָאֲדָמָה וְלוֹקֵחַ יָרָק וּמְטַבֵּל אוֹתוֹ בַּחֲרֹסֶת וְאוֹכֵל כְּזַיִת הוּא וְכָל הַמְסֻבִּין עִמּוֹ כָּל אֶחָד וְאֶחָד אֵין אוֹכֵל פָּחוֹת מִכְּזַיִת. וְאַחַר כָּךְ עוֹקְרִין הַשֻּׁלְחָן מִלִּפְנֵי קוֹרֵא הַהַגָּדָה לְבַדּוֹ. וּמוֹזְגִין הַכּוֹס הַשֵּׁנִי וְכָאן הַבֵּן שׁוֹאֵל. וְאוֹמֵר הַקּוֹרֵא מַה נִּשְׁתַּנָּה הַלַּיְלָה הַזֶּה מִכָּל הַלֵּילוֹת שֶׁבְּכָל הַלֵּילוֹת אֵין אָנוּ מַטְבִּילִין אֲפִלּוּ פַּעַם אַחַת וְהַלַּיְלָה הַזֶּה שְׁתֵּי פְּעָמִים. שֶׁבְּכָל הַלֵּילוֹת אָנוּ אוֹכְלִין חָמֵץ וּמַצָּה וְהַלַּיְלָה הַזֶּה כֻּלּוֹ מַצָּה. שֶׁבְּכָל הַלֵּילוֹת אָנוּ אוֹכְלִין בְּשַׂר צָלִי שָׁלוּק וּמְבֻשָּׁל וְהַלַּיְלָה הַזֶּה כֻּלּוֹ צָלִי. שֶׁבְּכָל הַלֵּילוֹת אָנוּ אוֹכְלִין שְׁאָר יְרָקוֹת וְהַלַּיְלָה הַזֶּה מְרוֹרִים. שֶׁבְּכָל הַלֵּילוֹת אָנוּ אוֹכְלִין בֵּין יוֹשְׁבִין בֵּין מְסֻבִּין וְהַלַּיְלָה הַזֶּה כֻּלָּנוּ מְסֻבִּין:
After [Kiddush], he makes an Al Netilas Yadayim and washes his hands. [...] He begins and makes a Borei Pri Ha’adamah, and he takes a vegetable and dips it into Charoses and eats a Kezayis - he and all those seated with him. Nobody may eat less than a Kezayis. Afterwards, we remove the table only from the one who recites the Haggadah; we pour the second cup, and here the son asks, “Why is this night different from all other nights? [...]”
The Rambam is clearly implying that Urchatz and Karpas are performed in order that the children ask; while this is not his suggestion in 7:3 regarding how to do this, it’s for that reason that it’s done. This is borne out by Pesachim 116a as well:
מה נשתנה הלילה הזה מכל הלילות שבכל הלילות אנו מטבילין פעם אחת הלילה הזה שתי פעמים: מתקיף לה רבא אטו כל יומא לא סגיא דלא מטבלא חדא זימנא אלא אמר רבא הכי קתני שבכל הלילות אין אנו חייבין לטבל אפילו פעם אחת הלילה הזה שתי פעמים מתקיף לה רב ספרא חיובא לדרדקי
”Why is this night different from all other nights? For on all nights we dip once, but tonight we dip twice.” Rava challenges this: “Do you think every day a person manages to dip once?!” Rather, Rava said, “This is how it should be taught: ‘For on all nights we are not obligated to dip once, but tonight, we dip twice.’” Rav Safra challenged this: “Is there an obligation for the children?!”
Since Karpas is only meant for the children to ask, it seems strange to refer to it as an obligation. What we see from here is that while Karpas specifically isn’t mandatory, something to the effect of getting the children to ask is.
I should also note that the Rambam did not seem to hold of Yachatz or Ha Lachma Anya, as he jumps from Karpas straight to Mah Nishtanah.
It should go without saying that Matzah and Marror are obligatory (the latter Rabbinically nowadays, but still obligatory), but the Rambam discusses them in 7:12-13 and 8:6-8. At the end of §8, he writes:
וְחוֹזֵר וְכוֹרֵךְ מַצָּה וּמָרוֹר וּמְטַבֵּל בַּחֲרֹסֶת וְאוֹכְלָן בְּלֹא בְּרָכָה זֵכֶר לַמִּקְדָּשׁ:
He goes back and wraps Matzah and Marror and dips it in Charoses and eats them without a Beracha, as a remembrance of the Mikdash.
It’s unclear to me from this whether the Rambam holds Korech is an absolute obligation or not.
§9:
וְאַחַר כָּךְ נִמְשָׁךְ בַּסְּעֻדָּה וְאוֹכֵל כָּל מַה שֶּׁהוּא רוֹצֶה לֶאֱכל וְשׁוֹתֶה כָּל מַה שֶּׁהוּא רוֹצֶה לִשְׁתּוֹת. וּבָאַחֲרוֹנָה אוֹכֵל מִבְּשַׂר הַפֶּסַח אֲפִלּוּ כְּזַיִת וְאֵינוֹ טוֹעֵם אַחֲרָיו כְּלָל. וּבַזְּמַן הַזֶּה אוֹכֵל כְּזַיִת מַצָּה וְאֵינוֹ טוֹעֵם אַחֲרֶיהָ כְּלוּם. כְּדֵי שֶׁיִּהְיֶה הֶפְסֵק סְעֻדָּתוֹ וְטַעַם בְּשַׂר הַפֶּסַח אוֹ הַמַּצָּה בְּפִיו שֶׁאֲכִילָתָן הִיא הַמִּצְוָה:
Afterwards, he drags out the meal and eats whatever he wants to eat and drinks whatever he wants to drink. At the end, he eats from the meat of the Pesach, even a Kezayis, and he doesn’t taste anything after it. Nowadays, he eats a Kezayis of Matzah, and he doesn’t taste anything after it. This is so that there be a distinction from the meal, as the taste of the meat of the Pesach or the Matzah in his mouth that he ate them is the Mitzvah.
In summary, then, the Rambam holds that the following are mandatory:
- Kadeish
- Urchatz and Karpas, or some equivalent so that the children will ask
- Maggid
- Mah Nishtanah or some equivalent for questions
- Avadim Hayinu
- Mitechilah
- Arami Oved Avi; not necessarily the Sifra, as we do, but at least some exposition on it, the longer, the better. The ten plagues are included in our exposition on Arami Oved Avi, as an explanation of the final verse in the passage.
- Pesach, Matzah, and Marror
- Not necessarily B’chol dor v’Dor, but certainly the spirit of it
- The second cup and its Berachos
- Rachtzah and Motzi Matzah
- Marror
- Maybe Korech?
- Shulchan Orech
- Tzafun
- Barech
- Hallel
The Rambam does quote Ha Lachma Anya, the story in Bnei Brak, the four sons, and the passage of B’chol dor v’Dor in his text of Maggid; however, as per the above, these are not obligatory. Further, Yachatz, Nirtzah, hiding the Afikoman, and Dayeinu are not discussed at all in these sections.
