How can Iran seek to enforce some of the diplomatic terms it has made to the US?

TLDR: A signed treaty, ratified by the US Congress, would go a long way towards locking in mutually agreed to terms. Very unlikely to happen.

I know Iran has no reason to trust Trump and the US, but a dispassionate analysis of the events of the last 20 years shows that a big part of the problem was that there was no Congressional acknowledgment of the deals signed by US presidents, meaning that they could be reversed by the next incoming president.

That's not to downplay the diplomatic and trust catastrophe of Trump tearing up JCPOA (Joint Comprehensive Plan of Action, the agreement reached in 2015 sometimes referred to as the 'Iran nuclear deal') or waging this latest war during/on the heel of ongoing negotiations: I believe the US problem of Congress rarely ratifying is a major issue with US diplomacy and doubly so when people like Trump get elected. But a ratified treaty, unlikely though I believe it to be, would go a long way towards answering the question.

I don't see it as likely to happen however. Congress is so deadlocked that anything Trump wants, the Dems will deny. Meanwhile a POTUS has to "sell" a treaty to Congress and I can't see anything that Trump would want to sell, especially considering how little he has engaged with Congress since 2025, being something that Iran would find highly beneficial. To motivate Trump it would have to be a "win" and adversarial Congressional reviews of the terms would quickly nullify Trumpian spins of "win".

The parallels with Ukraine are not very strong. Europe has a large stake in Ukraine's future, or rather the cessation of Russian encroachment, and is in aggregate pretty close to Russia in terms of military power, nukes aside (and even counting nukes, it doesn't take many to glassify the big Russian cities). It is also right next door.

Iran presents no great reason for other countries to risk very much on its behalf. It is liked by few nations that matter much (the Houthis in Yemen are probably its most significant steadfast ally). Russia under Putin is hardly the most trustworthy nation, has done little for Venezuela or Cuba and seems to assist Iran more out of mutual convenience. China will take a hard-headed look at its own interests before committing to anything and those interests are just as well served by just buying Iranian oil. China doesn't mind the US losing influence and friends, but it doesn't have to put itself on the line to achieve that. Other countries are such minnows (esp. far from home) vs. the US far that they will either will sit this out or not be a credible counterbalance.

The UN? Quite beneficial and useful in many circumstances, including peacekeeping when both parties genuinely find it beneficial to disengage. Not so useful when either party is not acting in good faith. And doubly so when one of the parties is very powerful or has UN veto rights. Its record in Lebanon, for example, has been iffy to date (wrt constraining either Israel or Hezbollah).

The Gulf countries, if fully brought in, could somewhat achieve this goal, being important to the West and regional. But they have religious, political (and recent Iranian strike) reasons to sit this out as well. If anything, the general news coverage on their position - at least for some of them - seems to be: "we didn't want this war, but now that you started it, finish it and neutralize Iran, don't leave us with your mess".

As noted in a comment, "needing peacekeepers" is hardly the image Iran's rulers are likely to want to project. Not to mention that on-location peacekeepers would have a first row view at future Iranian "protest management methods", which they'd probably want to keep to themselves.

Iran will probably have to settle for something like the JCPOA, POTUS-only signatory, in practice.

The question would normally be: how likely is a future POTUS to tear up Trump's agreement? At a guess, probably not that likely: Democrats will not want to start another war with Iran, esp. not on Israel's behalf. While Republicans would find it hard to disavow Trump's legacy.

In the specific case of Trump however, it's not even sure he won't change his mind on his own agreement (which is somewhat Canada and Mexico's experience with USMCA, signed by Trump in 2020). Still, overall, I'd like to think American politicians other than this administration have learned from Trump's annulment of JCPOA that abrogating agreements without good reasons and without a good replacement is not as constructive as it might seem (but that thought may be naive, I admit).

Alternatively, a "frozen conflict"?

Barmar also makes a good point in a comment under their answer that it might just end up a la Korean War, a ceasefire that is only a ceasefire and remains a frozen conflict. That may be an even more likely ending to this war, although it by definition would not result in durable American commitments as asked in this question.

And how would that be "enforced" upon the US? As per this other answer, pretty much by Iran keeping the same cards as they currently have: the capacity for mayhem in the region. Ditto for a POTUS-only agreement.

p.s. As noted in a comment under the question, I am waiting to see what was really agreed to and I am rather skeptical that it is Iran's laundry list cited above. We know there is a 2 week ceasefire, during which negotiations are meant to take place and I would expect the final shape of the terms to become clear then, not now. Without that agreement what we have right now are Iranian demands and a temporary ceasefire with somewhat uncertain rules. Along with a US administration desperate to spin it into a "win". No more, no less.

p.p.s. Claims - see comments - that the war was already illegal according to US laws and that a ratified treaty wouldn't change anything are hogwash. Yes, Congress is supposed to authorize war, but this "isn't a war". We almost never have any wars left on the planet these days and POTUS is in charge of most foreign policy (treaty ratifications and war declarations aside). And in practice, for the last 80 yrs or so POTUS has largely "done their thing" by running undeclared wars under a "I am in charge of foreign policy" claim of authority. That's what War Powers Act of 1973 was supposed to fix, after Vietnam saw a even more massive "non-war". Didn't really happen, and there have been questionable cases since, even though this war dwarfs them all in magnitude. Whether this type of military adventure is legally, per US domestic law, within the rights of the President to decide or not is for SCOTUS to decide. However, not abiding by a ratified treaty would clearly be an abuse of presidential power.

what diplomatic measures or mechanism can be made by Iran to ensure the US (and Israel) honours these terms in the future?

Realistically, while the IRI is in place (since you mentioned their list of demands), none other than what they did already: blockade the Hormuz again and shoot missiles at most neighboring countries (especially oil & gas infrastructure) if someone tries to reopen it by force.

The ceasefire has de facto come down just to the US not bombing Iran in return for them opening the Hormuz. Everyone claims victory but/because very little else was actually agreed upon.

If there were to be a major regime change in Iran (beyond 'replaced Khamenei with Khamenei' level), the new regime might able to trust the US more, but only if they gave in on the major US demands: removal of all nuclear material, dismantling most their missiles & drones etc. If Syria is any indication, Israel would probably just bomb (again) Iran in case of a regime change, to remove much of the remaining weapons during the chaos. So, some of that may be 'automatic' in such a scenario.

P.S. I've only read the other answers now, after writing that. If you want more of the Korea analogy, Iran retaining the ability to close the strait is their equivalent of DPRK being able to inflict large amounts of damage on Seoul, even without nukes.

P.P.S. Iran has been asking for some kind of guarantees from Russia & China, but it's very unlikely to get them, I think. (Quoting this because those 'live update' links tend to go bad.)

At a press conference in Beijing early Wednesday, Abdolreza Rahmani Fazli, Iran’s ambassador to China, requested that its two closest allies and the world body help guarantee Tehran’s long-term security as part of the recent deal to end hostilities between U.S and Iran.

The Islamic Republic has made similar requests in the past but they have not come to fruition.

When asked if China would be willing to guarantee such security, foreign ministry spokesperson Mao Ning demurred Wednesday, telling reporters: “We hope that all parties will resolve their disputes through dialogue and negotiation.”

This is quite unlike the Korea situation, where the ceasefire stability also came from the mutual defense treaties that Russia and China have with the North.

Finally, as you mentioned Ukraine, Pro-Russia accounts (and even US these days) often like to portray the resolution to the war in Ukraine as "make a deal and take an L(oss) now, or take a bigger L(oss) in the future" (like when Trump says 'Russia has all the cards' etc.) The US-Israel alliance basically tells the same to IRI. (Trump literally said the US has all the card vis-à-vis of Iran.) Of course, Ukraine receives much more external help, but has less Hormuz-type leverage over Russia (even with all the attacks on tankers in the Black Sea or even in the Mediterranean), so the two situations aren't that similar beyond this superficial gradient level. Also, the Hormuz only affects the US indirectly, via global oil price, but its GCC allies are directly affected.

By actually getting the nuke

Saddam, North Korea, Iran itself and most clearly Gaddaffis Lybia have well proven that in diplomacy (without subservience) with the US, only actually having WMDs counts.

US treaties and especially non-formalised promises cannot be trusted, this was true even before the more erratic current US presidency. As was proven by Gaddafi giving up his WMDs and being toppled soon after despite explicit US promises. And North Korea remaining untouched despite all the rethoric threatening to change that.

Sure, having WMDs won't stop the US from later applying sanctions (like on North Korea), but by threatening to close the strait again Iran (now unattackable) would have sufficient leverage to avoid actually being sanctioned. After all, besides kicking out US (and british) oil interests, what has Iran done that the US doesn't happily tolerate in it's allies?

Israel would surely throw a fit but what are they going to do? Attack a country capable of actually fighting back? Not gonna happen.

Leveraging this leverage in favour of Irans allies would be a complex political process but at least as or more promising than any alternatives.

How would this happen in reality?

It most likely wouldn't, that's the flaw in this answer. The one actual solution to Iran's issues is the one that Israel and the US will never accept for differing reasons. Israel not because that was the proclaimed point of all their anti-Iran actions (including this very attack) and the US not because it would a) signal impotence (if it happens against their will); b) be an admission that they cannot be trusted, an absolute no-go and/or c) be a massive betrayal of Israel.

So above-board obtaining, aka as part of the negotiations is out of the question.

(Semi) secret avenues are similarily thin. If they could obtain one natively they would've done so ages ago for obvious reasons (note that this as always includes a sufficiently reliable delivery vehicle). So we need an external power with a nuke or two to spare. Whether the device will be handed over or nuclear troops of that nation are stationed in Iran is largely irrelevant, what counts is Iran's ability to "press the button" (or at least the international perception that they have this ability). But once again we are at an impasse. The list of nations having a nuke is short and the subsection that don't hugely fear US reprisals is even shorter, namely North Korea, China and Russia.

Russia risks renewed (and/or decisive) American support for Ukraine, and relies on a complicated, impotent US-internal state on the matter for the somewhat ongoing negotiations for that war (not to mention that by giving Iran a nuke, suddenly US peacekeepers for Ukraine would likely become a real proposal, something Russia desperately does not want).

North Korea seems plausible at first:The US is more or less already as hostile to them as possible (in large parts because they have nukes), but they don't have that many, so the cost of handing over a couple (one isn't enough) to Iran is huge.

That leaves us with China. China merely risks a(n intensification of the) trade war and informal hostilities. Which is tolerable, so maybe maybe maybe they could do it. But they are committed to nonproliferation so I'd expect severe internal resistance to such a move. They are nevertheless the most likely source of this actually effective guarantee though. Not likely, just more likely than all the others.

And with these large costs we come to the main point: Why would anyone? What's to gain? Iran isn't situated well enough financially (especially not after the US/Israeli destruction), can't unilaterally reliably provide oil (as the embargo to Cuba shows, the US can stop shipments whenever they want) and doesn't really have much more to offer except the capability of throwing the world economy into chaos. And if anyone would want that, why intervene in the first place?

So tl:dr: Iran will have to be happy with some not-really-guaranteeting appearance of "guarantees", because it will not get any actual ones in the negotiations because the one that would be real and effective is too costly for any potential provider and absolutely off-limits for the attacking side.

Note: I somewhat handwaved the "handing over a nuke vs stationing nuclear armed troops in Iran" topic because it simply doesn't matter and comes with it's own lengthy list of pros and cons. Handing over the nuke has far higher (primarily political) costs for the donor nation while stationed foreign troops would go against Iran's unalignment principles and might put the nuclear deterrents reliability up to debate. Also it doesn't change the broader arguments made here, just the in-detail cost-benefit analyses, but not the actual outcome. If the nuclear threat is credible, it's credible, if not, it may as well not be there.

Edit for clarity: Iran doesn't need a nuke specifically. Just any credible WMD, although nukes are the strongest variant in terms of medial/political/diplomatic impact. I also include the availability of a decently robust delivery option to at least Israel. OP asked (indirectly) for "diplomatic means" to which I say that getting a nuke (or alternative) is as much a diplomatic endeavour as it is a physical/military one.

These two conditions - a guarantee that [country] will not be attacked again; a permanent end to the war, not just a ceasefire - is quite similar to the conditions Russia/Ukraine want to end their war. So it's a reasonable guess that Iran is seeking something similar to what Ukraine is seeking, which is peacekeepers from more powerful countries. The idea is that if there are French & UK (and possibly other countries') soldiers on the ground, then a new invasion by Russia would trigger retaliation from these nuclear-armed countries.

Presumably Iran would seek peacekeepers from China/Russia, maybe Pakistan/India.

NB: This is speculative right now; no details have emerged (that I have seen, at least). Iran is also not Ukraine - thus far it is fighting mostly alone, and it has historically refrained from joining military alliances. They might be looking for something else than foreign troops on their soil.

Edit: Iran has apparently asked China for security guarantees. China has not and apparently is not likely to accept, although they did guarantee the security of Iran's diplomats.

Underscoring the risks, Pakistani officials reportedly told the Guardian that while Islamabad acted as mediator in recent talks, China functioned as “guarantor,” promising Iranian officials they wouldn’t be assassinated during any future negotiations. It’s unclear how Beijing would offer such an assurance, or the motivation for doing so given the repercussions should something then go wrong.

...

Bringing Iran to the negotiation table, however, is only the first step to end hostilities. Tehran is seeking security guarantees that it won’t be attacked again, and has publicly asked Beijing to play such a role — a request analysts say China is unlikely to accept.

At this point in time, this is just a broad framework of the goals of all the parties in this conflict. They're being used to justify a two-week ceasefire, during which time all the details will hopefully be fleshed out.

It seems premature at this time to speculate on the precise ways in which any of the provisions will be enforced, or even if they'll all still exist in the same form at the end of the negotiations. This is just a starting point, not a full plan.

Also, phrases like "permanent end to the war" are somewhat meaningless. There hasn't actually been a war declared by either party, although many in the public and media refer to this as a war (the same is true for the war between Russia and Ukraine). So how can we say when the war has officially ended, and whether future hostilities are part of the same or a different war?

To answer your basic question, though, there aren't really many enforcement methods in international relations. Many consider the attacks from the US and Israel that started this conflict to be war crimes, because they believe there was no imminent threat from Iran, but that didn't prevent the war and there's good reason to believe no one will ever be punished for it. Peacekeepers can be deployed, but it will be hard to find neutral nations to act in this capacity, and they may be ineffective against a superpower like the US. Sanctions are common, but how useful has that been against Russia regarding the Ukraine War?