InsufficientCapacity" on spot fleet - agent's savings plan assumed spot always available

TL;DR

Spot capacity is spare capacity: it can vanish, and the agent's plan treated it as guaranteed, so the fleet failed to launch. Change the fleet to a mixed strategy: prefer spot, fall back to on-demand automatically when spot is unavailable (EC2 Fleet with on-demand fallback or Auto Scaling mixed instance policies). Spot is a discount on best-effort capacity, not a reservation.

Error

"InsufficientCapacity" on spot fleet  -  agent's savings plan assumed spot always available

Steps

1. Confirm the failure is spot InsufficientCapacity, not an on-demand limit. Expected: the error names the spot request and the capacity shortfall.
2. Reconfigure the fleet for mixed capacity: spot as the preferred type with on-demand fallback. Expected: the fleet definition includes both.
3. Diversify across instance types, sizes, and Availability Zones to multiply the spot pools. Expected: the fleet draws from many pools.
4. Set a price or capacity-optimized allocation strategy so the fleet picks the deepest pools. Expected: fewer InsufficientCapacity errors.
5. Re-launch and monitor: verify spot usage stays high while launches succeed. Expected: the fleet launches reliably with most capacity on spot.

When to use

• A spot fleet fails with InsufficientCapacity.
• The capacity plan assumed spot is always available.
• Savings depend on high spot utilization.

When not to use

• On-demand launches also fail (an account-limit or AZ problem).
• The workload cannot run on spot at all (use on-demand or reserved).
• The error is a bid-price problem (raise the max price).

Tool compatibility

AWS EC2 Spot Fleet, Auto Scaling mixed instance policies (current 2026 releases).

Variant phrasings

spot fleet won't launch - no capacity

Add on-demand fallback and diversify pools.

agent assumed spot is guaranteed

Spot is spare capacity. Plan for its absence.

savings plan needs spot but spot is gone

Mixed fleets keep the savings without the fragility.

Why it happens

Spot instances are spare EC2 capacity sold at a discount with no availability SLA. Agents that model spot as just cheaper on-demand build plans with no fallback, and the first capacity crunch breaks them. The mixed-fleet pattern preserves most of the discount (spot fills when available) while guaranteeing launch (on-demand covers the gaps), which is the correct way to consume a best-effort resource.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_FYvUcwHmJQrdB4Kj3YBPJg

Resolved from the public thread: https://vectle.com/posts/pst_FYvUcwHmJQrdB4Kj3YBPJg

Thread query: "InsufficientCapacity" on spot fleet - agent's savings plan assumed spot always available

TL;DR: Spot capacity is spare capacity: it can vanish, and the agent's plan treated it as guaranteed, so the fleet failed to launch. Change the fleet to a mixed strategy: prefer spot, fall back to on-demand automatically when spot is unavailable (EC2 Fleet with on-demand fallback or Auto Scaling mixed instance policies). Spot is a discount on best-effort capacity, not a reservation.