Yes it will lead to new bugs, but the older code probably contains a ton of bugs already and is probably difficult to update (original writers long gone, docs out of date etc), so this is probably the best way out of that situation, as long as proper support is given afterwards so they don't end up in the same situation a year later (this is the hard bit).
The ra-ra commentary from the CEO is somewhat off-putting, but the idea is not all bad.
Testing is still the long pull, even if you have an engineer with good expertise.
good thing they have unlimited token, now they would have 5 different model reviewing the work
What have been achieved using this port?
And how did you make sure the port is correct?
Followup question: did anyone read the automated test suite?
Silence.
The software rewrite is shelved.
The goal and objective is to deliver value for your user/customers; to the speed, reliability, and assurance requirements your market segment expects.
Just like you just need to ask the LLM to "make it bug free".
(At this point even I'm more sure if any of this is sarcasm.)
https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fv...
https://slimemoldtimemold.com/2026/08/05/a-stupid-idea-for-a...
Being a mess isn't a problem when you have more than 6 items context.
If history has taught us anything. It’s that the measure of a successful product is usually more to do with its sales people.
Cisco did provide updates via CCO and anonymous ftp server. They quite openly told that just download new image and test in your lab, and then if feature set did require licensing, please do it before deploying production. Trust was put on customers and they did very little to check and verify if customers did comply. Which is quite contrary to what for example Oracle practise is been.
>If history has taught us anything. It’s that the measure of a successful product is usually more to do with its sales people.
As someone who worked Cisco gear over thirty years I still remember why us networking did want cisco instead of many other brand devices and it hadn't anything to do with marketing and sales people did.
The Cisco IOS text based configuration was in early days of internet which made it so popular.
Many other manufacturers like HP, 3Com, Cabletron, Madge (IBM), Bridge, etc. did not get why we wanted and complained “give us textual configuration format like IOS has”.
With a plain text based configuration it was easy to adapt configuration from one model to have ready made templates for each model, upgrades and migration to new models were easier, you could use version controls (rcs,cvs, svn,... long before git) and then when RANCiD came it was easy to automate config backups, revert and rollback changes, pass audit with human readable configs and what was in production even long time after etc. In short text based configs made it possible to build large network scale configurations management tools.
IOS has compact, but quite easy to read config snippets made it much easier emailing others, posting examples newsgroups, opening TAC at Cisco CCO, books with examples, learning and teaching etc.
It took quite long before for example HP understood and switched text formats right after turn of century. Juniper seemed to understood right from beginning. But some like Fore, Nortel, Alcatel (ATM-switch makers) and many others did not understand before going out of business. Most of these others had either menus (3Com, HP) or a lousy CLI with obscure unique for just that device model and quite often not even option to print config or if they had not just with changes from default config but printing everything and providing no means to drop showing default. This made it manually while online very hard without external version control to even know what is current status and what had been changes.
BTW, this is still an issue with some manufacturers haven't understood and makes their devices much worse than they understand. Fortigate firewalls CLI is still like this. You can select a subsection of config to print, but it will no indication what differs from default configs. And the whole firewall config is hundreds of kilobytes. Good luck finding what's been changed unless you had saved default config when you started using that device!
When you have thousands to even tens of thousands of network devices in use as many were working you need to be rely that even when you encounter device failure and you don’t have same model any more available, you are able to adapt and find working fix configuring something else doing what is needed.
Binary and any non textual configurations may fit the purpose SOHO category devices, but when network grows even bit larger and configurations more complex there isn’t anything better than text base configs as far as us humans will need to configure and maintain those devices.
Cisco have had issues with software quality for sure, but it was Cisco IOS that was the reason it became so popular and we wanted to use it.
And we did put more weigh on manageability which that IOS text based configuration provided than for example speed some manufacturers got with their devices (Madge, etc.) and even when some Cisco switches dropped some Network World tests more packets we still wanted that manageability more and relied Cisco will fix those issues with software upgrades which it often did.
Juniper when it came end of 90's did understand this and made even better configuration system for Junos than IOS was and still is. Cisco have improved things with IOS-XR, but it's still not up to what Junos is currently.
e: fixed few typos and added that manageability chapter near end. Places where I did work none were always everything Cisco places. No, but either so called ABC (Anything But Cisco) places either. Cisco was and is still popular for those reasons mentioned above.
Almost every new product of theirs I’ve touched in the last 20 years has been trash, and even the R&S golden goose is being dragged in that direction with customer-hostile functionality like Smart Licensing and a general decline in quality.
And they've overstretched their product line to too many things, trying to be the "be all" company. Fortinet is making the same mistake, but they're earlier in that process.
I mean, it's not really a mistake on their parts, their primary goal is to get rich and they are succeeding.
Citation: I've been working with Cisco technologies since the mid-1990s. At my current job, we are a few weeks away from phasing out the last of our Cisco junk. And since people will ask: to Juniper/Mist and Arista.
When they started making smallish dedicated routers I remember opening one to see a dodgy backplane and a number of big boards plugged in held up only by minimal slides. The minute it got hot (which it did because no air management) those boards sagged in the middle and it just looked slapdash. The 3Com equivalents were always better built but Cisco had such mindshare it didn’t matter the products were a bit crap
https://www.cisco.com/c/en/us/support/docs/field-notices/743...
"Cisco Access Points That Are Running, Or Have Run, Releases 17.12.4, 17.12.5, 17.12.6, or 17.12.6a May Exhaust Flash Space, Preventing Upgrades"
Recovery path basically involves either factory resetting every single AP or up to, if it's already bricked, Cisco TAC + you having to console into every single bricked AP you have to pass a secret challenge/response at the bootloader to flash a recovery image :)
Otherwise he's just going to replicate all the legacy bugs and questionable design decisions.
Right? ...
[1] https://github.com/enterprisequalitycoding/fizzbuzzenterpris...
Anyone who has used the built in date picker knows why everyone rolls their own.
It's a bunch of text files in a repo with massive churn (all new version 5!), but somehow even the technical debt 'skill' (i.e. text file) has technical debt itself:
https://github.com/DietrichGebert/ponytail/blob/main/skills/...
> ...every ponytail shortcut is marked with a shortcut: comment ... (older code may say ponytail:).
It doesn’t matter if it’s satire. Developers seem to be so vulnerable to all these AI promises of ‘automating’ everything.
BTW it’s terrible. It gave two very hollow suggestions that did not solve the real problem. Hour wasted, angry colleagues, no code change.
Legacy code is typically a euphemism for old unmaintained code, no one knows how it works so it can be edited, and it probably doesn't align with current work practices.
Not to say a rewrite is necessary right, but avoiding rewrites of working code is how we ended up with more COBOL code than COBOL programmers in 2026.
I think people are assuming the worst but AIs most helpful contribution beyond churning out code is explaining the code base.
Who knows if they will do a good or bad job that's down to the engineers and not the tools imo.
Once AI came along, I became like Captain Picard (RIP) and would just sit back and let the AI write all unit tests. "Try harder, make sure coverage is 100%". It was very exhilarating to sit back and watch the models churn away, creating 100s of lines of unit tests. Of course, I checked them all manually before submission ;-)
That said, there's also research pointing to LLMs encoding existing bugs as test requirements: https://arxiv.org/abs/2607.05139
That CS PHD researcher, Konstantinou, in particular, has written a lot on this: https://arxiv.org/search/cs?searchtype=author&query=Konstant...
The issue with using "the code as the spec" is that humans can't read 100K lines of code in a day, can't hold the entire ball of unstructured mud in their brains all at once, and generally require hierarchical "structure".
Modern AIs just don't have these issues! Their context windows are big enough to encompass an entire codebase, so they can "see everything", all at once.
Each line of new code they write is compared against every relevant line of code in the original, which they treat as a spec.
My main issue in my experience is that that AIs tended to reproduce the legacy behaviour "bug for bug", which was not my intention.
Similarly, the AIs were hamstrung by external constraints: I typically prefer to leave the DB schema unchanged, which meant the torturous workarounds were needed for design errors in some stored procedures, etc.
The older the code, the bigger the issues with baked-in assumptions about transactions, stateful sessions, data types, string formats, etc.
This trips up AIs just as much as humans. You can't trivially maintain backwards compatibility and modernise the codebase! Those are conflicting goals, no matter what.
Surely we have more code than programmers for every language in existence?
-Works according to a defined specification
-Within the bounds of that specification, has a lot of code written to optimize speed, cost, latency, etc - all measureable goals
-Written in c, with all it's weakness
-Given limitless engineering work, some could be ported to better hardware
-the new system could be automatically compared to the old system for functional correctness
All those factors fit well with agent based development.
Btw, many of them are true in general to embedded systems.
Then consider that not all vendors implement the standards identically, because the standards can be a bit woolly and in some cases followed rather than lead live implementations.
There will be a bunch of code that looks useless, but is actually there because of specific interoperability scenarios.
I wonder whether AI is any good at looking at that code,running the system, and documenting those edge cases?
Lots of this stuff should work in theory, you only find out that it doesn't in practice when you try it on real hardware. And there are thousands of bits of hardware.
This whole thing sounds like a deluded vanity project birthed in an ivory tower.
But yes, fact it uses a bunch of very well documented standards with multiple implementations gonna help massively
The whole design where you need to get a magical list of IDs to decode the metrics and the list is not even shipped with device is entirely asinine
There are also tight time budgets (basically SLAs) that must be met along with memory considerations (both in SW and on the NPU/HBM), not to mention how the device needs to respond to load and various failure scenarios.
if any hardware does not confirm to specs, thats on them and its actually a reason to pull out and replace legacy hw
Your rewritten codebase will presumably look much cleaner and more maintainable that what it's aiming to replace, but it's easy to miss the purpose/necessity of all of the old "cruft" which need not be just poor coding but rather later enhancements not anticipated by the original design whose implementation was therefore crufty.
Does anyone here work at Cisco, and have any insight as to why a rewrite is considered necessary and might be expected to succeed?
A rewrite has historically been a huge mistake in many situations, some of which I've personally had the agony of witnessing firsthand, powerless to stop it. But AI completely changes the calculus because it doesn't waste significant developer time or energy crawling back to the previous working state, it just does it while you go get coffee. And this I've found to be a huge benefit, that I can continue making my codebases nicer to work with and better documented/integrated with bugs stamped out for free. There are upsides to a rewrite, and they're now much more likely to be worth it as the downsides are automated away.
AI has no magic powers to figure out what the “API/etc contracts” are in a nasty old codebase. This problem is fundamentally hard and not always well defined.
AI is also (at least for now) not consistently good at writing tests that test the behavior that matters. Recent models seem to be getting slightly less bad, but again, this is a hard problem.
I’ve personally used AI to rewrite gnarly code where the goal is to have exactly identical results to the old code. GPT-5.6 Sol could do this correctly with some extra help guiding it to rest the right things, and I haven’t tried this particular task on a newer model.
But ancient code like in the article is probably an enormous buggy mess full of accidental behaviors and implementing protocols in ways that do not fully match the spec. Some, but not all, of the mismatches will be cases where the spec is wrong. Sure, an AI can probably generate a new implementation with a similar set of bugs, but is this what is actually wanted? Also, depending on the gnarly set of bugs, a clean implementation may be unable to reproduce the bugs, leading to a messy implementation instead.
With the current sota models code is free.
But there is pretty big risk of false positives and negatives; some "bugs" are design decisions that were not documented well/at all, and inevitably some new ones will appear when it reasoned something wrong.
An LLM doesn't have enough context to know what you so and don't want. If you give it call logs for every function that was called and how, maybe, but a seasoned engineer will have more context around what the company needs.
Honestly though, it's pretty clear the AI rewrites work, we have a ton of high profile examples. It's just a holdover attitude about rewrites from before AI. In reality, AI is going to make the code better when it rewrites
Such as?
You don't get to pick what parts of the API you're porting during something like that, it has to work the same or some of the ecosystem around it will break.
Obviously this fails if this is a large org with many teams and microservices, but for a leaner org this isn’t really a problem.
I realize this thread is about Cisco so my comment doesn’t apply here haha.
And AI is really good at all the stages here.
AI makes these projects significantly more feasible, at least in my domain. We have some critical-path software I wouldn't touch with such an approach, but the vast majority of our code is internal and/or supporting tooling.
For those cases, burning it down and rewriting via AI tooling has proven far more productive than anything we have attempted in the past. This is for old technical debt laden stuff everyone has been afraid to touch for well over a decade.
It's not perfect, but it moves the needle where before a lot of such projects were stuck on the backburner for years.
It certainly introduces new bugs and such, but at least this way forward momentum is happening vs. just being stuck in decision/analysis paralysis forever.
This also assumes some very senior SME folks are driving the process. I would not expect similar results with junior or even mid-level folks in these specific domains.
One of the reasons to avoid rewriting has long been that the time spent rewriting code that works is time not spent writing code to meet new business needs. In other words, it's usually a huge opportunity cost, rarely balanced out by sufficient gains to be net positive.
I do think AI does challenge this prior, as the time needed to do a rewrite shrinks dramatically. It's not zero, of course, but it now becomes worth asking if it's worth doing, rather than defaulting to avoiding rewrites in nearly all cases.
Until one day you leave and the next guy comes and says “what is all this shit”
A critical security firmware update once brought down redundant pair of firewalls we used because the bug didn’t manifest until a counter buffer overflowed months after application. Three hours into the outage the tech power cycles the thing and our services came back to life.
Cisco is only doing this because some other alternative is more expensive. They had more than a decade to get started.
We had to turn off a bunch of on-by-default features in the system I'm deploying now because IOS matches too aggressively and decides that arbitrary UDP datagrams are DHCP requests or PTP packets that need rewriting (and then dropping when they turn out to not be DHCP/PTP). I have to think that an AI rewrite with a today-good model would recognize the lax checks and do them better.
Idiocracy (2006): https://en.wikipedia.org/wiki/Idiocracy
yeah, i think they'll survive whatever the outcome
The main takeaway though is we had one person, in a single month, do the equivalent of at least a couple years of work for a small team. We view bugs and performance issues as a currently inevitable part of the process and we are dealing with them with models just like we do with humans on every other project already. Overall though it is representing an absolutely massive speedup. If you work in an environment where the models make egregiously more mistakes than your human counterparts then I am quite jealous.
https://www.reddit.com/r/programming/comments/eaqgk/java_is_...
"XML is like violence: if it doesn't solve your problem, you aren't using enough of it." -Chris Maden
https://www.reddit.com/r/ProgrammerHumor/comments/1mbgih/xml...
"XML is a giant step in no direction at all" -Erik Naggum
https://quotefancy.com/quote/1643080/Erik-Naggum-XML-is-a-gi...
Not clear to me what he's doing. Re-implement switching? Simplify features? Reduce LoC? Add test coverage? Either way, AI in that world will be helpful. I'm sure it'll spot all kinds of problems in opaque drivers that are hard to follow from MF specs.
rewrite can be done to quickly refactor and/or close the feature parity, improve the UI/UX for end customer, add missing features or integration needs
Demonstrates fanatical devotion to capital’s golden idol - “yes, all of the code; yes, unlimited budget”. Doesn’t actually cost the company anything - only one engineer off task, any nuggets of value can be harvested as it progresses but any costs or big changes can be deferred until completion of the impossibly large open-ended mandate (ie never), one guy won’t have the personal bandwidth to spend so much that the “unlimited” part actually matters, and in case of emergency it only costs six months severance to make it all go away. It even rescues a fellow national from the ignominy of being employed so far below his previous station, there’s so much to like about it. Hmm? The code? What about it?
I see you haven’t met my coworkers and their fanatical use of parallel “workflows”
Tell me you don’t use agents effectively without telling me you don’t use agents effectively.
The feeling of coding like that is a bit like realizing that an elevator is 10cm of aluminum and a 100 meters of empty space under your feet.
After the first few bouts of existential dread you get uses to it.
Lightly manually edit for speech rec failures.
LLM takes that and converts it into a much much denser, much more structured technical spec in markdown. This is issue 1 in the repo.
I edit this directly extensively, adding detail, removing misconceptions, adding sections with more dictated text.
LLM takes this and further densifies it, closer to an actual implementor’s spec. I carefully note what got added, removed, edited in this step.
I edit this carefully and extensively again. Some removals get added back, some additions get removed. This is the first commit and closes issue 1.
I then have an LLM write an implementation plan and basic project scaffolding. Makefiles, smoke tests, logging, test framework, DI. I edit the plan, and the plan is the second commit. An LLM uses both plus scaffolding plus my repo policy documents (global) to write a README. We’re still single threaded.
An agent files the first n top level implementation tickets. These refer to and sometimes quote any interface contracts in the spec or implementation plan. These usually map 1-to-1 to a module/directory. These sometimes have stub commits to create the module directory and commit interface definitions. Some larger modules have a README in their module directory if their purpose or structure or inputs or outputs are complicated or non-obvious.
Parallel agents write commit level issues that are the task items for each top level implementation ticket. One agent per module.
Then, spawn parallel agents with detailed global instructions, the full spec+plan, and the full readme, and the full code from the module it is working in, plus the issue text. Its output is a single commit on the feature branch and a PR that closes out one feature ticket.
This spawns an adversarial reviewer which has detailed agent instructions as well as our code style guides and repo policies. The scaffolding includes tests and linting and checks. Any failures spawn rework agents until the feature branch is up to snuff.
I review the feature branch diff and merge it, having read the review and rework comments on the PR. I frequently add comments about the implementation (“don’t do x, do y”) and add WIP: back to loop it back for more rework. I don’t see anything that isn’t green, formatted, passes linting, passes tests, and a (dumb) bot thinks is ready to land. Most of the time, it is. Sometimes, it did it the wrong way and I correct it.
The more modular the code, the easier it is to parallelize (obviously). It is ridiculously easy to build large software quickly now, provided it is sufficiently modular so the implementation doesn’t have to cross modules very much. The detailed spec and implementation plan up front helps a lot with this. It’s a lot like writing multithreaded programs really.
Any bugs discovered are tackled with strict TDD, one commit with a failing test, one bugfix commit that must be green and not touch the test.
Individual modules must conform to good contracts, meet best OOP practices, and fit into context. Too big and they get broken down further. Context size is actually a useful constraint for code structure here.
This workflow, the code standards, the styleguide, the linter settings, the fmt settings (obviously only applicable to non-Go as the Go fmt is not configurable), everything is global and stored in another repo and is vendored into each AI managed project repo. It has a fair bit of “I shouldn’t have to say this, but…” stuff (in the style of “fire codes are written in blood” - each warning sign came from experience).
My results are quite good. It’s as the saying goes, much like managing a team of junior devs. It is easy to end up with a LOT of parallel implementors, reviewers, reworkers, and tons of CI, but it’s still so much vastly cheaper than humans that it doesn’t matter.
Someone will make a LOT of money wringing out the inefficiencies from this workflow, because I think the “agent as replacement for third-world contracted junior dev” model is the one almost everyone is using right now.
Fortune's source was the CEO himself:
> “I’ve given him unlimited tokens because he’s going to refactor code,” Patel told Fortune at a small roundtable dinner. “So I’m like, you can go, you can go nuts.”
I think that Fortune story is better than the Tom's Hardware one. It ties the Cisco anecdote into a larger trend of some people deliberately hiring younger, less experienced developers for their AI projects - because they are more likely to be "AI native".
I frequently find myself in conversations with people who are resistant to AI and turn out to have a very skewed idea of what it can and cannot do.
“If you don’t hire the early-in-career [workers], and you just let your workforce continue the way that they were, where do fresh ideas get injected? How do you challenge the status quo?” he said."
-------------------
This would be an interesting approach for a bleeding edge startup that's betting big on producing something novel or going bankrupt. If you're going to bet the company on one guy and some AI, that's bold. He'd better have some serious chops and not be merely a slick salesman.
However, Cisco is an infrastructure company. Status quo is what most people want from an infrastructure company. You don't want your Cisco stuff to suddenly start doing new, innovative things that require a complete redux of everything built on top of it. This, of course, assumes that having one guy do a refactor of your entire code-base produces something "innovative" rather than a cavalcade of snafu's.
It's probably not the best time to be holding CSCO.
it's the testing in real world usage to shake out all the edge cases and bugs that's hard and that you can't just write unit tests for
I think it is a reasonable risk to encounter these bugs again in order to get a more clean and documented codebase. Especially when asking AI to plan and A/B release is also trivial.
I’d love to know what internal services and tools that are included in this, as well as any deliverables that shipped as software products to customers.
If it’s legacy code for IOS firmware, however, I’d be very worried.
The ability to clean room Adobe products came at about the same time Claude's RE abilities suddenly improved.. Hmm lets think about this a little more.
Anthropic sucked the bones dry of the public domain, literature and all of opensource. It seems likely that Anthropic RL'd Claude into being a great reverse engineer because it needed more training data.
I believe that Anthropic turned Claude into a digital software shredder and started decomping proprietary code, like adobes.
So the reason Claude can suddenly clean room Photoshop isnt because it became a more capable programmer, but rather, the Adobe code is literally in its training data now.
Ciscos internal legacy code is not going to be in Claude's data (yet).
The decision to increase Claudes ability to RE is brilliant, because now everyone is doing the source code decomp for them, and sending them fresh data from previously closed source repositories of code.
Thanks for the very thought provoking reply tho!!!!!!!!!!!