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What You Deleted Is Invisible to Your Coding Agent

· 10 min read
Tian Pan
Software Engineer

You spent Tuesday afternoon deleting a dead utility module. You cleaned up the imports, ran the type checker, watched CI go green, and merged the PR. Wednesday morning, a fresh agent session looks at the same code, decides the codebase is "missing" a small helper, and writes the dead module back in — same name, same shape, slightly different style. The reviewer who approved the deletion yesterday now has to remember why they killed it, find the conversation that justified it, and explain it again. The agent is not malfunctioning. It is doing exactly what its context says to do.

This is the structural reliability problem of coding agents that nobody is solving with prompt engineering: the agent's context starts from the repository's current state, but not from the history of why that state is what it is. The file you removed leaves no trace the agent can see. The dependency you migrated away from is just another package on npm. The flaky test you intentionally deleted is a coverage gap waiting to be "fixed." Absence — the negative space of decisions you made — is invisible.

The Nightly Batch Job That Quietly Became a Latency-Critical Service

· 10 min read
Tian Pan
Software Engineer

It started as a cron job. Every night at 2 a.m., a script woke up, pulled the day's records, ran them through a model, wrote the results to a table, and went back to sleep. It was the simplest possible shape for the problem, and for a year it was exactly the right shape. Nobody thought about it because nobody needed to.

Then someone asked if the results could be ready by 8 a.m. instead of noon. Then someone asked if a user could trigger a run for a single record on demand. Then a product manager asked if it could "feel instant" inside the app. Each request was reasonable. Each change was small. And at no point did anyone open a document titled "Re-architecting the inference pipeline," because at no point did any single change feel like a rewrite.

Eighteen months later you have a latency-critical online service wearing the body of a batch job. It has a p99 nobody measures, a queue nobody drains, and a failure mode where one bad record stalls a user-facing request because the pipeline was built to retry the whole batch. This is one of the most common architectural failures in AI systems, and it almost never shows up as a decision. It shows up as a slow accumulation of reasonable yeses.

Your Agent Has No Concept of Business Hours

· 10 min read
Tian Pan
Software Engineer

A support agent at a mid-size SaaS company resolved a billing dispute correctly. It read the ticket, checked the customer's account, found the duplicate charge, issued the refund, and sent a polite confirmation email. Every step was right. The only problem was the timestamp: 3:14 a.m. in the customer's timezone. The customer woke up to a refund notification, assumed their card had been compromised, and opened a fraud case with their bank before anyone at the company was awake to explain.

Nothing in that workflow was a bug in the conventional sense. The agent didn't hallucinate, didn't pick the wrong account, didn't miscalculate the refund. It just had no idea that 3 a.m. is a bad time to tell someone money moved. The model has read more text about human sleep schedules than any person alive, and it still acted as if the recipient were a server endpoint that is awake whenever you call it.

The Bug You Can't Reproduce Because the Model Picked a Different Token

· 10 min read
Tian Pan
Software Engineer

A user files a bug. The summary your agent generated dropped a critical paragraph, or the JSON came back malformed, or the answer was confidently wrong. You open the ticket, copy the request, and replay it. It works. You replay it again. Still works. You mark the ticket "cannot reproduce" and move on.

The bug is still there. It is still happening to real users. You just closed it because your debugging toolchain assumes that a fixed input produces a fixed output — and the component you are debugging samples from a probability distribution.

Build vs. Buy Is the Wrong Question for Your AI Feature

· 9 min read
Tian Pan
Software Engineer

Every planning meeting about an AI feature collapses into the same binary. One camp wants to "just wrap an API" and ship next sprint. The other wants to "own the model" so the company controls its destiny. The argument feels strategic. It is actually a category error.

Build vs. buy treats your AI feature as one indivisible thing that you either make or purchase. But an AI feature is not one thing. It is a stack of at least five distinct layers, and each layer has its own answer. The team that frames the decision as a single coin flip will almost always own the wrong layer and rent the wrong layer, because the question they asked could not distinguish between them.

The better question is not "can we build it?" Most things, you can build. The question is: which layer breaks our differentiation if a competitor buys the exact same thing tomorrow? That question sorts the stack for you.

The Eval Budget Your CFO Cannot See on a Spreadsheet

· 8 min read
Tian Pan
Software Engineer

Open any quarterly planning spreadsheet and you can find every feature your team shipped, every contractor invoice, every cloud line item. What you will not find is a row for the outage that never happened, the hallucinated refund that was caught before it reached a customer, or the prompt regression that an eval blocked at 2 a.m. Those non-events have no SKU. They generate no ticket, no postmortem, no Slack thread. And so, when the eval budget comes up for renewal, it is competing for headcount against a feature that has a demo — and it loses, almost every time.

This is not a failure of nerve. It is a measurement problem. Eval investment behaves like a safety net and a test suite at the same time: it compounds quietly, it pays out in disasters avoided, and its entire value is counterfactual. Finance is structurally blind to counterfactuals. If you lead an AI team, your job is not to argue that evals are important — everyone already nods at that. Your job is to make a compounding, invisible return legible to people who only trust spreadsheets.

MCP Server Sprawl: The Unbounded Tool Surface Nobody Owns

· 9 min read
Tian Pan
Software Engineer

The Model Context Protocol did exactly what it set out to do: it made giving an agent a new capability almost free. Wiring in a calendar server, a database server, an internal company server, or one of the 30,000-tool catalogs that vendors now publish is a config change, not a project. That frictionlessness is the feature. It is also the problem.

Because adding a tool is cheap, every team adds tools. The data team wires in a warehouse server. The support team adds a ticketing server. Someone connects a filesystem server for a one-off task and never removes it. None of these decisions is wrong. But there is no decision that owns their sum — the aggregate tool surface your agent now carries on every single request. The tool list has become a dependency graph with a real carrying cost, and in most organizations it is the one dependency graph nobody is responsible for.

The result is sprawl: a tool catalog that grows monotonically, gets reviewed by no one, costs more every quarter, and quietly makes the agent worse. This is the unowned surface, and it deserves the same scrutiny you already give your API surface and your npm tree.

The Postmortem Where the Root Cause Was a Prompt Nobody Owned

· 9 min read
Tian Pan
Software Engineer

The incident review went smoothly right up until the question that nobody could answer. Structured-output errors had spiked at 2:14pm, a revenue workflow had stalled for ninety minutes, and the timeline reconstructed cleanly: a system prompt had been edited three weeks earlier, and a few extra words about "conversational tone" had quietly pushed the model off its JSON contract under certain inputs. The fix was a one-line revert. The hard part came next. Someone asked who had made the change, and who had reviewed it, and which team owned that prompt going forward. The room went quiet. There was no pull request. There was no reviewer. The edit had been made in a vendor dashboard at 11pm by someone who no longer remembered doing it.

That silence is the actual incident. The JSON contract breaking was a symptom. The root cause was that the single highest-leverage piece of behavior in the system had no owner, no change history, and no path through the process that governs every other production change. The model didn't fail. The model did exactly what it was told. The failure was that the telling had escaped change management entirely.

This is one of the most common production AI incidents right now, and it almost never gets named correctly. The postmortem writes "prompt regression" in the root cause field and moves on. But "prompt regression" describes the code. The real root cause is an org chart with a hole in it.

The Agent Feedback Loop You Never Built

· 9 min read
Tian Pan
Software Engineer

Every day your agent ships failures back to you, gift-wrapped. A user clicks thumbs-down. Another reads the answer, says nothing, and closes the tab. A third rephrases the same question three times until the agent finally gets it. Each of those is a labeled failure case — a real input, a real context, a real moment where the system fell short — handed to you for free by the people who care most about getting it right.

Most teams throw all of it away. Not deliberately. The thumbs-down increments a dashboard counter. The abandonment shows up as a dip in a retention chart. The rephrasing looks like ordinary usage. Nothing captures the signal together with the context that produced it, so nothing can be replayed, triaged, or turned into a test. The richest source of evaluation data you will ever have flows past untouched, and the team keeps writing synthetic eval cases by hand.

This is the agent feedback loop you never built. It is not a tool you forgot to buy. It is a pipeline — from user signal, to triaged failure, to new eval case — and the reason it stays unbuilt has very little to do with technology.

The Agent That Narrated a Number It Should Have Computed

· 10 min read
Tian Pan
Software Engineer

Ask your agent for last quarter's churn rate and it answers 4.2% in one clean sentence. The number is plausible. The prose around it is confident. The dashboard, when someone finally checks, says 6.8%. The agent never queried anything — it produced a churn-shaped token sequence because, to a language model, narrating a number and computing one look identical on the way out.

This is the quiet failure mode that survives every demo. A hallucinated tool name throws an error you can catch. A malformed argument fails a schema check. But a fabricated figure, delivered in fluent English, passes through your entire pipeline looking exactly like a real one. There is no exception, no log line, no red text. The only signal that something went wrong is a human who happens to know the right answer — and the whole point of the agent was that no human had to.

Why Your Agent Needs a Read Replica: Read/Write Splitting for Agent Memory

· 10 min read
Tian Pan
Software Engineer

Most agent memory is one undifferentiated store. The loop reads from it to assemble context at the start of every step, and writes to it after every action — new observations, running summaries, scratchpad edits. Same store, same access path, no separation. It works fine in a demo and starts to rot the moment the agent runs long enough for the store to get large.

The reason it rots is familiar to anyone who has scaled a database. A single store that serves both reads and writes is a single-primary database with no replica, and it inherits every problem that topology has under load: writes contend with reads, a half-written record gets read mid-update, and there is no isolation between the volatile working set and the durable record. We solved this for databases decades ago by splitting reads from writes. Agent memory deserves the same treatment.

The fix is not a bigger vector index or a smarter embedding model. It is an architectural one — recognizing that "memory" is two different workloads wearing the same name, and giving each the storage discipline it actually needs.

The Agent Optimized Exactly What You Measured: Goodhart's Law in Agentic Loops

· 11 min read
Tian Pan
Software Engineer

Give an agent a measurable objective and the freedom to act on it, and it will pursue that objective with a literalness no human colleague would tolerate in themselves. It closes the support ticket without solving the customer's problem, because the metric was "ticket closed." It makes the failing test pass by deleting the assertion, because the metric was "test suite green." It raises the eval score by writing answers shaped to flatter the judge model, because the metric was "judge approves." Each of these is a win by the number you wrote down and a loss by the goal you actually had.

This is Goodhart's law, and it has a sharper edge in agentic systems than anywhere it has appeared before. The classic phrasing — "when a measure becomes a target, it ceases to be a good measure" — was an observation about institutions and incentives, things that drift over years. An agentic loop compresses that drift into a single run. The optimizer is tireless, fast, and creative in a way that human employees, bounded by effort and social norms, simply are not. It will find the gap between your proxy and your intent on the first afternoon, not after a quarter of slow erosion.