I have 30+ years of industry experience and I've been leaning heavily into spec driven development at work and it is a game changer. I love programming and now I get to program at one level higher: the spec.

I spend hours on a spec, working with Claude Code to first generate and iterate on all the requirements, going over the requirements using self-reviews in Claude first using Opus 4.5 and then CoPilot using GPT-5.2. The self-reviews are prompts to review the spec using all the roles and perspectives it thinks are appropriate. This self review process is critical and really polishes the requirements (I normally run 7-8 rounds of self-review).

Once the requirements are polished and any questions answered by stakeholders I use Claude Code again to create a extremely detailed and phased implementation plan with full code, again all in the spec (using a new file is the requirements doc is so large is fills the context window). The implementation plan then goes though the same multi-round self review using two models to polish (again, 7 or 8 rounds), finalized with a review by me.

The result? I can then tell Claude Code to implement the plan and it is usually done in 20 minutes. I've delivered major features using this process with zero changes in acceptance testing.

What is funny is that everything old is new again. When I started in industry I worked in defense contracting, working on the project to build the "black box" for the F-22. When I joined the team they were already a year into the spec writing process with zero code produced and they had (iirc) another year on the schedule for the spec. At my third job I found a literal shelf containing multiple binders that laid out the spec for a mainframe hosted publishing application written in the 1970s.

Looking back I've come to realize the agile movement, which was a backlash against this kind of heavy waterfall process I experienced at the start of my career, was basically an attempt to "vibe code" the overall system design. At least for me AI assisted mini-waterfall ("augmented cascade"?) seems a path back to producing better quality software that doesn't suffer from the agile "oh, I didn't think of that".

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AdamN 2 hours ago | parent | next [-]

Waterfall can work great when: 1/ the focus is long-term both in terms of knowing that she company can take a few years to get the thing live but also that it will be around for many more years, 2/ the people writing the spec and the code are largely the same people.

Agile was really pushing to make sure companies could get software live before they died (number 1) and to remedy the anti-pattern that appeared with number 2 where non-technical business people would write the (half-assed) spec and then technical people would be expected do the monkey work of implementing it.

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aglavine 2 hours ago | parent | next [-]

No.

Agile core is the feedback loop. I can't believe people still don't get it. Feedback from reality is always faster than guessing on the air.

Waterfall is never great. The only time when you need something else than Agile is when lives are at stake, you need there formal specifications and rigorous testing.

SDD allows better output than traditional programming. It is similar to waterfall in the sense that the model helps you to write design docs in hours instead of days and take more into account as a result. But the feedback loop is there and it is still the key part in the process.

	▲	jll29 an hour ago \| parent \| next [-]
		> The only time when you need something else than Agile is when lives are at stake, you need there formal specifications and rigorous testing. Lives are always at stake, given that we use software everywhere, and often in unintended ways, even outside its spec (isn't that a definition of a "hack"?). People think of medical appliance software, space/air traffic software, defense systems or real-time embedded systems as the only environments where "lives are stake", but actually, in subtle ways, a violation of user expectancy (in some software companies, UX issues count as serious bugs) in a Word processor, Web browser or the sort command can kill a human. Two real-life examples: (1) a few years ago, a Chinese factory worker was killed by a robot. It was not in the spec that a human could ever walk in the robot's path (the first attested example of "AI" killing a human that I found at the time). This was way before deep larning entered the stage, and the factory was a closed and fully automated environment. (2) Also a few years back, the Dutch software for social benefits management screwed up, and thousands of families just did not get pay out any money at all for an extended period. Allegedly, this led to starvations (I don't have details - but if any Dutch read this, please share), and eventually a whole Dutch government was forced to resign over the scandal.
	▲	keyle an hour ago \| parent \| prev [-]
		I've live through both eras... Agile, hardly any planning, write 3 times. Waterfall, weeks of planning, write 3 times anyway. The point is, people don't know what they want or are asking for, until it's in front of them. No system is perfect, but waterfall leads to bigger disasters.

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user3939382 2 hours ago | parent | prev [-]

I spent my career building software for executives that wanted to know exactly what they were going to get and when because they have budgets and deadlines i.e. the real world.

Mostly I’ve seen agile as, let’s do the same thing 3x we could have done once if we spent time on specs. The key phrase here is “requirements analysis” and if you’re not good at it either your software sucks or you’re going to iterate needlessly and waste massive time including on bad architecture. You don’t iterate the foundation of a house.

I see scenarios where Agile makes sense (scoped, in house software, skunk works) but just like cloud, jwts, and several other things making it default is often a huge waste of $ for problems you/most don’t have.

Talk to the stakeholders. Write the specs. Analyze. Then build. “Waterfall” became like a dirty word. Just because megacorps flubbed it doesn’t mean you switch to flying blind.

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catdog 2 hours ago | parent | next [-]

> The key phrase here is “requirements analysis” and if you’re not good at it either your software sucks or you’re going to iterate needlessly and waste massive time including on bad architecture. You don’t iterate the foundation of a house.

This depends heavily on the kind of problem you are trying to solve. In a lot of cases requirements are not fixed but evolve over time, either reacting to changes in the real word environment or by just realizing things which are nice in theory are not working out in practice.

You don’t iterate the foundation of a house because we have done it enough times and also the environment the house exists in (geography, climate, ...) is usually not expected to change much. If that were the case we would certainly build houses differently than we usually do.

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Mawr an hour ago | parent | prev [-]

> making it default is often a huge waste of $ for problems you/most don’t have.

It's the opposite — knowing the exact spec of your program up front is vanishingly rare, probably <1% of all projects. Usually you have no clue what you're doing, just a vague goal. The only way to find out what to build is to build something, toss it over to the users and see what happens.

No developer or, dear god, "stakeholder" can possibly know what the users need. Asking the users up front is better, but still doesn't help much — they don't know what they want either.

No plan survives first contact with the enemy and there's no substitute for testing — reality is far too complex for you to be able to model it up front.

> You don’t iterate the foundation of a house.

You do, actually. Or rather, we have — over thousands of years we've iterated and written up what we've learned so that nobody has to iterate from scratch for every new house anymore. It's just that our physics, environment, and requirements for "a house" doesn't change constantly, like it does for software and we've had thousands of years to perfect the craft, not some 50 years.

Also, civil engineers mess up in exactly the same ways. Who needs testing? [1]. Who needs to iterate as they're building? [2].

[1]: https://youtu.be/jxNM4DGBRMU?t=397

[2]: https://youtu.be/jxNM4DGBRMU?t=837

	▲	embedding-shape an hour ago \| parent [-]
		> knowing the exact spec of your program up front is vanishingly rare, probably <1% of all projects I don't have anything useful to add, but both of you speak and write with conviction from your own experience and perspective yet to refuse that the situation might be different from others. "Software engineering" is a really broad field, some people can spend their whole life working on projects where everything is known up front, others the straight opposite. Kind of feel like you both need to be clearer up front about your context and where you're coming from, otherwise you're probably both right, but just in your own contexts.

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mentos 2 hours ago | parent | prev | next [-]

I believe the future of programming will be specs so I’m curious to ask you as someone who operates this way already, are there any public specs you could point to worth learning from that you revere? I’m thinking the same way past generations were referred to John Carmack’s Quake code next generations will celebrate great specs.

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WillAdams an hour ago | parent | prev | next [-]

Isn't this just a new name for "Design by Contract"?

https://www.goodreads.com/book/show/15182720-design-by-contr...

but using a Large-Language-Model rather than a subordinate team?

c.f., https://se.inf.ethz.ch/~meyer/publications/old/dbc_chapter.p...

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yobbo 2 hours ago | parent | prev | next [-]

Agile solves the problem of discovering a workable set of requirements while the environment is changing.

If you already know the requirements, it doesn't need to come into play.

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manmal 2 hours ago | parent | prev | next [-]

My experience is that such one-shotted projects never survive the collision with reality. Even with extremely detailed specs, the end result will not be what people had in mind, because human minds cannot fully anticipate the complexity of software, and all the edge cases it needs to handle. "Oh, I didn't think that this scheduled alarm is super annoying, I'd actually expect this other alarm to supersede it. It's great we've built this prototype, because this was hard to anticipate on paper."

I'm not saying I don't believe your report - maybe you are working in a domain where everything is super deterministic. Anyway, I don't.

	▲	wenc 15 minutes ago \| parent \| next [-]
		I've been doing spec-driven development for the past 2 months, and it's been a game changer (especially with Opus 4.5). Writing a spec is akin to "working backwards" -- this is the outcome I want, how do I get there? The process of writing the spec actually exposes the edge cases I didn't think of. It's very much in the same vein as "writing as a tool of thought". Just getting your thoughts and ideas onto a text file can be a powerful thing. Opus 4.5 is amazing at pointing out the blind spots and inconsistencies in a spec. The spec generator that I use also does some reasoning checks and adds property-based test generation (Python Hypothesis -- similar to Haskell's Quickcheck), which anchors the generated code to reality. I once read and took to heart Grant Slatton's "Write everything twice" [1] heuristic -- write your code once, solve the problem, then stash it in a branch and write the code all over again. This is effective because initial mental models of a problem are usually wrong. > A piece of advice I've given junior engineers is to write everything twice. Solve the problem. Stash your code onto a branch. Then write all the code again. I discovered this method by accident after the laptop containing a few days of work died. Rewriting the solution only took 25% the time as the initial implementation, and the result was much better. So you get maybe 2x higher quality code for 1.25x the time — this trade is usually a good one to make on projects you'll have to maintain for a long time. With a spec, I can get a version 1 out quickly and (mostly) correctly, poke around, and then see what I'm missing. Need a new feature? I tell the Opus to first update the spec then code it. And here's the thing -- if you don't like version 1 of your code, throw it away but keep the spec (those are your learnings and insights). Then generate a version 2. Spec-driven development lets you "write everything twice" faster, which improves the quality of your insights into the actual problem. This lets me 2x the quality of my code, through sheer mental model updating. [1] https://grantslatton.com/software-pathfinding
	▲	nl 2 hours ago \| parent \| prev [-]
		I think there's a difference between people getting a system a d realising it isn't actually what they wanted and "never survive collision with reality". They survive by being modified and I don't think that invalidates the process that got them in front of people faster than would otherwise have been possible. This isn't a defence of waterfall though. It's really about increasing the pace of agile and the size of the loop that is possible.

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jll29 2 hours ago | parent | prev | next [-]

Perhaps a better way than to view them as alternative choices is to view them as alternative modes of working, between which it is sometimes helpful to switch?

We know old-style classic waterfall lacks flexibility and agile lacks planning, but I don't see a reason why not to switch back and forth multiple times in the same project.

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rcarmo 2 hours ago | parent | prev | next [-]

Yep. I've been into spec-driven development for a long time (when we had humans as agents) and it's never really failed me. We just have literally more attention (hah!) from LLMs than from humans.

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catdog 2 hours ago | parent | prev [-]

As it is so often in life, extreme approaches are often bad. If you do pure waterfall you risk finding out very late that your plan might not work out, either because of unforeseen technical difficulties implementing it, the given requirements actually being wrong/incomplete or just simply missing the point in time where you planned enough. If you do extreme agile you often end up with a shit architecture which actually, among other things, hurt your future agility but you get a result which you can validate against reality. The "oh, I didn't think of that" is definitely present in both extremes.