Identifier used by OpenRouter to group related requests together.
Useful any time multiple requests should share an observability
grouping (e.g. a conversation, an agent workflow, a batch job, or a CI
run). Equivalent to setting the x-session-id HTTP header on the
underlying request. OpenRouter rejects values longer than 128
characters.
Falls back to the OPENROUTER_SESSION_ID environment variable when
unset, so callers can group all requests from a process without
threading the value through application code. Empty strings are
treated as unset.
Example: "conv-2026-04-30-abc"
See https://openrouter.ai/docs/guides/features/broadcast/overview
