diff --git a/doc/resources/evolution.md b/doc/resources/evolution.md
index 9756bc3..758a5e1 100644
--- a/doc/resources/evolution.md
+++ b/doc/resources/evolution.md
@@ -1166,3 +1166,13 @@ toolsrv uses `log/slog` (Go stdlib) with a `"svc": "toolsrv"` attribute.
 This is intentionally separate from olliesrv's `ollie/log` package — they
 are different programs with different lifecycles.
 
+### Open Problem: Concurrency
+
+Tool execution in toolsrv is capable of running in the background (`proc/new.bg`),
+but foreground blocking execution (`proc/new`) remains the default. The reason:
+**concurrency is an unsolved problem.** When multiple tools run simultaneously,
+the agent must reason about interleaved results, partial state, and ordering —
+LLMs handle this poorly. Background processes exist for cases where the user
+or agent explicitly opts in (long-running builds, watchers), but the default
+serial execution is a deliberate constraint, not a limitation of the architecture.
+