Codecov Report

❌ Patch coverage is 68.70748% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.85%. Comparing base (0de510d) to head (264f539).
⚠️ Report is 135 commits behind head on main.

@@            Coverage Diff             @@
##             main    #4047      +/-   ##
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- Coverage   69.13%   68.85%   -0.28%     
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  Files         462      462              
  Lines       46554    46720     +166     
==========================================
- Hits        32185    32171      -14     
- Misses      11897    11916      +19     
- Partials     2472     2633     +161     

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ok setting as draft for now. I know that is not essential but i was speeding up with pending tasks while the main code was reviewed...

@yrobla could you take a look at this draft PR and confirm if it’s still relevant?

@yrobla could you take a look at this draft PR and confirm if it’s still relevant?

well, we agreed to implement it at some point, but it was not critical. If you don't mind, i prefer to keep them as draft, to resume the work when needed.

So, I went through this pretty thoroughly and I have some thoughts. First off, the intent here is good... DoS protection for session management is something we'll want eventually. But there are some issues that I think need addressing before this can move forward.

I agree with Jeremy's two blockers, and I found a few more things on top of those.

The Manager is already doing too much

Jeremy flagged this and he's right. The Manager struct today has 4 fields and ~15 methods covering session lifecycle, caching, SDK adaptation, backend notification, and session decoration. That's already borderline. This PR would add three separate synchronization primitives (atomic.Int64, sync.Mutex, sync.RWMutex) each protecting different state subsets, plus a background goroutine. That's our own anti-pattern #3 in .claude/rules/vmcp-anti-patterns.md staring right at us.

The decomposition path is actually pretty clear:

• Session admission (global + per-client limits) should be its own type with its own lock and tests
• Idle reaping... we already have a TODO in RestorableCache (cache.go line 56) for exactly this. The cache already has onEvict callbacks. That's the natural home for it, not a new goroutine on Manager

Counter-drift bug in Generate()

OK this one is subtle but real. When storage fails in Generate(), the activeSessionCount decrement is conditional:

if sm.limits.MaxSessions > 0 {
    sm.activeSessionCount.Add(-1)
}

If you incremented the counter, you need to decrement it on failure. Period. Making the rollback conditional on a config value that could theoretically change between the increment and the error path means the counter can drift permanently. Once it drifts, the global limit rejects new sessions forever until restart. The fix is simple... make the decrement unconditional.

Also, the limited path increments before storage (reservation pattern) while the unlimited path increments after. That asymmetry is confusing and easy to break.

Per-client map entries stick around at zero

decrementPerClientCount decrements the count but never deletes the map entry when it hits 0. So every unique subject that ever connected leaves a permanent zero-valued entry in perClientCounts. Not a huge deal, but it's unbounded growth with no eviction. Easy fix.

The middleware splits admission logic across two layers

The sessionLimitMiddleware does an optimistic (non-atomic) check, then Generate() does the real atomic enforcement. This means:

• If the middleware rejects, fine, early exit
• If the middleware passes but Generate() rejects, Generate() returns "" and the SDK gets a cryptic failure

Pick one place for the check. Either the middleware owns it entirely (and returns a proper 503), or Generate() owns it entirely. Splitting it is anti-pattern #4 from our own rules.

Missing test coverage

The tests that are here use good patterns (timestamp back-dating instead of time.Sleep, nice). But there's no:

• Concurrent admission test (N goroutines racing against MaxSessions=M... that's the whole point of the atomic reservation pattern and it's untested)
• sessionLimitMiddleware test (HTTP 503, Retry-After header, existing sessions pass through)
• IdleSessionTimeout > SessionTTL clamping test

Codecov shows 68.7% patch coverage with 46 lines missing.

Small thing: wrong change type

The PR checks "Refactoring (no behavior change)" but adding session limits and idle timeout is a new feature. Not a big deal but worth fixing.

Bottom line

I think we should close this one out and reopen it as smaller, focused PRs once we've done the Manager decomposition. Something like:

1. PR to extract session admission into its own type
2. PR to implement the RestorableCache idle sweep (the TODO is already there waiting for us)
3. PR to wire the limits into the server config

That way each piece is testable and reviewable on its own. Thoughts?