iMessage enforces strict per-line limits to protect deliverability. Every dedicated line has caps on new inbound conversations per minute, maximum active users, and message throughput — exceed any of them and the line degrades. Functionality drops. Messages stop delivering.
Until now, managing that was on you.
Agent teams building on iMessage had to estimate traffic, provision lines ahead of time, and build their own load-balancing logic to spread users across them. Before a launch, the math was always a guess: buy too few lines and you risk delivery failures on the day that matters most; buy too many and you're paying for capacity you never use.
We saw this pattern over and over — developers spending engineering time on infrastructure plumbing instead of the product they're actually building.
Managed auto-scaling and load balancing
Starting today, Spectrum handles both problems for you.
Auto-scale
Spectrum now handles line scaling automatically. The system samples usage data, user behavior, and per-line health every minute. When a line approaches its capacity ceiling, Spectrum provisions a new dedicated line before deliverability is affected — no manual intervention, no downtime.
Here's what the system evaluates on each sampling cycle:
Active user count — how many users are currently assigned to the line
Inbound growth rate — how many new users hit the line in the last minute
Line health signals — delivery success rate, queue depth, and response latency
When any of those signals cross the threshold, a new line is provisioned and added to the routing pool within seconds. Your existing users stay on their current line. New users are routed to the fresh one.
Auto-scale is turned on by default for every Business plan project. You don't configure thresholds, watch dashboards, or pre-buy lines for launch day. Your line pool grows and shrinks with actual traffic — not your best guess of what traffic might look like.
This is especially useful for launches and campaigns. Instead of over-provisioning lines weeks in advance and paying for idle capacity, you start with what you need and let Spectrum handle the rest. If your launch drives 10× the expected traffic, new lines spin up automatically. If traffic drops after the spike, you're not stuck with unused lines.
Load-balanced routing
Paired with auto-scale is a new load-balancing endpoint. Instead of writing your own routing logic, call a single API to get the best line for the next user:
The response returns the optimal dedicated iMessage line, balanced by active user count and recent growth:
The isBestAvailable flag tells you whether a genuinely healthy line was found. When it's false, the returned line is the least-saturated fallback — it holds more than 500 active users or grew by more than 10 users in the last minute. That signal lets you decide how to handle edge cases: queue the user, alert your team, or proceed anyway.
In your Spectrum server, route new conversations to the returned line with per-phone routing:
Stop managing infrastructure. Start shipping.
Before today, scaling an iMessage agent meant provisioning lines, writing balancing logic, monitoring health, and hoping your estimates held up on launch day.
Now it means calling one endpoint.
Auto-scale keeps your lines healthy. The routing API keeps your users on the right one. Together, they give iMessage agent developers a Vercel-like experience — you don't think about the infra, you just build what matters.
