I tested opus 5 full context gathering against a large codebase and here are its shortcomings

Some context: I tried opus 5 when it came out. Was not impressed at first. The slimming of the system prompt means I have to fix my claude.md and agents files before it’s usable. My codebase is about 500,000 lines of code and 100,000 lines of markdown. I architected it as well as I could (I was a sr frontend dev) and have been consistent about cleanups and refactors but it’s now on the edge where my agents are starting to drift a little with my current memory stack.

How I tested it: I had a fable session I’ve been getting into the weeds with write a list of questions for opus 5.

Then I informed a fresh opus session i was going to test it and had it write what it knew at different steps - quick first look \- overall look \- read every file with a subagent/full context

I had it note what surprised it during full context that should have been found earlier which produced some nuggets. It took about 10% of my current session limit and 2% of my weekly just for it to get full context on the max plan.

Then I gave it the test. No tools. Then had fable check the answers, fable also had the wrong answers for a few but it also hadn’t done a full context refresh before writing the test.

Opus missed 2. They were both similarly wrong and related to the discovery subagents it used. On the questions that it said “I’m didnt see the actual file but I think the answer is” it got those right enough. Both the answers it got wrong had no hedge and was confident in them because the subagents had supplied it with the answers specifically. My take away- if it hedges I’ll trust it’s directionally right. No hedge is suspect.

I didn’t baseline against what it would have gotten before I improved the system prompt and added project context files. I’m sure the results of that would have shown opus flailing wildly because it hadn’t looked at any files and would have only been useful if I was trying to disingenuously dunk on it online.

Here’s the tunings I added to my Claude file for opus 5:

**1. Prefer enumerations to counts** in derived docs. **2. Mark provenance.** **3. Delegation needs a verification pass before durable publication.**

**What have you added to your agents file for opus 5?**