Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add rusel95/ios-agent-skills --skill gcd-operationsgit clone --depth 1 https://github.com/rusel95/ios-agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/rusel95/ios-agent-skills/gcd-operations)<a href="https://agentmods.dev/skills/rusel95/ios-agent-skills/gcd-operations"><img src="https://agentmods.dev/badge/skills/rusel95/ios-agent-skills/gcd-operations/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/rusel95/ios-agent-skills/gcd-operations"><img src="https://agentmods.dev/badge/skills/rusel95/ios-agent-skills/gcd-operations.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00165 | $0.02442 |
| Opus 5.5 | $0.00066 | $0.00977 |
| Sonnet 5.5 | $0.00033 | $0.00488 |
| Haiku 4.5 | $0.00016 | $0.00244 |
Grade A, and why
gcd-operations scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 175 lines — stays where its author put it; the contents beside it link to each section on GitHub.
GCD & OperationQueue Concurrency
Enterprise-grade concurrency skill for Grand Central Dispatch and OperationQueue. Opinionated: prescribes serial queues by default, target queue hierarchies, labeled queues, NSLock/OSAllocatedUnfairLock for synchronization, barrier-based reader-writer on custom concurrent queues, and OperationQueue for dependency graphs. Apple has not deprecated any core GCD APIs — GCD remains appropriate for parallelism, system I/O, and performance-critical paths. This skill covers correct usage, deadly bug prevention, and coexistence with Swift Concurrency.
Concurrency Layers
Application Layer -> OperationQueue for dependency graphs, cancellation, throttling.
Dispatch Layer -> Serial queues for state protection, concurrent+barrier for R/W.
Synchronization -> NSLock / OSAllocatedUnfairLock for short critical sections.
System I/O -> DispatchSource (timers, file monitoring), DispatchIO (non-blocking file I/O).
Thread Pool -> GCD manages threads. App targets 3-4 well-defined queue subsystems.
Quick Decision Trees
"Which queue type should I use?"
Do you need mutual exclusion (protecting shared state)?
+-- YES -> Serial queue (default). Simplest, safest.
+-- NO -> Is this independent parallel work (image processing, batch transforms)?
+-- YES -> Concurrent queue (custom) with barrier for any writes.
+-- NO -> Do you need dependency graphs, cancellation, or throttling?
+-- YES -> OperationQueue with maxConcurrentOperationCount.
+-- NO -> Serial queue. When in doubt, serial.
"Which lock should I use?"
Which lock?
+-- iOS 16+ -> OSAllocatedUnfairLock (Apple's safe wrapper)
+-- iOS 18+ / Swift 6 -> Mutex from Synchronization framework
+-- Any iOS -> NSLock (safe, simple, recommended by Apple DTS)
NEVER: os_unfair_lock as direct Swift stored property (memory corruption).
NEVER: DispatchSemaphore as a mutex (no priority donation).
"Which QoS should I use?"
What ships with it
12 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
- README.md 6.5 KB
- references/data-races.md 6.5 KB
- references/deadlocks.md 5.1 KB
- references/debugging.md 7.0 KB
- references/dispatch-primitives.md 6.9 KB
- references/migration.md 8.0 KB
- references/operation-queue.md 7.3 KB
- references/queue-creation.md 5.5 KB
- references/refactoring-workflow.md 11 KB
- references/rules.md 2.8 KB
- references/thread-explosion.md 6.1 KB
- references/thread-safety.md 7.6 KB
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 175 lines · 165 tokens per session scan A 172783882d7e
gcd-operations is a skill published in the GitHub repository rusel95/ios-agent-skills (13 stars, last pushed 11d ago), licensed MIT. It adds 165 tokens to every session and 2,442 once invoked, about $0.0007 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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