Codex Didn't Get Slower — It Started Acting Like a Full Engineer
Agentic coding tools are shipping into production editors now, and the default behavior is shifting from "assist" to "own." Developers who don't adjust their prompting and session habits will pay a latency tax on every small change — turning a two-minute fix into a twenty-minute audit.
A routine image-reference fix that once finished in minutes now triggers a cascade of review, verification, and quality-check steps that can run for hours — even after the code change is already applied. The shift stems from Codex evolving from a prompt-driven code assistant into an agent that treats every request as a full engineering task: analyze boundaries, scan the project, split work, modify, verify, review, and re-verify.
Plan Mode no longer means "output a plan only"; it means "plan first, then execute." The underlying workflow has become agentic regardless of the mode toggle, so even small requests inherit heavyweight process overhead. Long-running sessions compound the problem by accumulating massive context that the agent feels obligated to reconcile.
The practical fix is deliberate scope management. Explicitly instructing Codex to avoid global scans, skip unrelated modules, and prioritize direct modification over exhaustive review brings response times back down. The larger lesson is that agent capability is not free: matching the workflow's depth to the task's complexity is now a skill developers must learn.
Codex's Plan Mode now behaves like a mislabeled feature — users who select it expecting a read-only plan get a full agent execution instead, which erodes trust in the mode toggle.
The agent workflow adds genuine engineering rigor, but applying it indiscriminately to trivial changes mirrors the organizational dysfunction of over-processed teams: a one-line fix shouldn't require a design review.
Session-length management is becoming a first-class performance concern. The convenience of a single long-running conversation directly conflicts with the latency cost of ballooning context, and the tool offers no obvious mitigation.
The shift from "code generator" to "task owner" changes the success metric. A 15-minute agent run that delivers a verified, working result may beat a 5-second generation that still needs 30 minutes of manual fixing — but only if the task warrants the overhead.
Prompt engineering now includes a new dimension: telling the AI what NOT to do. Negative constraints — "don't scan," "don't refactor," "don't expand scope" — are becoming as important as positive instructions.
I thought my relay station was just too slow; turns out the slowdown is Codex's own fault.
I now add a line at the end every time: don't use agent, just make the quick fix directly.