Adding more Claude subagents made my pipeline slower past 6 — but the real problem wasn't concurrency at all.
When I finally looked at the cost logs for my ad analytics SaaS, every task was hitting Sonnet: renaming files, formatting Slack messages, parsing JSON, and interpreting 12-campaign performance reports. All the same model. Sonnet 4.5 runs $3/M input and $15/M output tokens. Haiku 3.5 is $0.80/$4. Same tokens, 3-4x cost difference based purely on model choice.
I split tasks into three tiers — Haiku for format/parse/extract work with no judgment needed, Sonnet for pattern recognition and multi-step tool use, Opus for architectural decisions (currently one worker out of twelve, run manually). The routing decision itself is made by Haiku classifying the incoming task in ~100 tokens, which costs roughly $0.00008 per call — noise compared to the savings from avoiding a wrong-model assignment.
The counter-intuitive finding: task complexity mattered less than context length. I expected complex tasks to need Sonnet. What I actually found was that Haiku handled surprisingly hard work just fine when context was compressed under 2,000 tokens — and fell apart on simple tasks when context ballooned past 5,000. So context length is now the first branch in my router, not task type.
