vLLM (and Nous Hermes portal) only accept a single system message.
When the platform builds a messages array from multiple sources
(base system prompt + workspace config + per-session override), the
consecutive system entries at the front cause vLLM to reject or
silently drop all but the first.
Adds mergeSystemMessages() — a stateless pre-flight transform in the
handlers package that collapses the uninterrupted leading run of
{"role":"system"} entries into one, joining their content with "\n\n".
Non-system messages between system messages are not touched; a single
system message is returned as-is (no allocation).
10 unit tests cover: stacked merge, single-unchanged, no-system passthrough,
three-message collapse, interleaved user (trailing system not merged),
only-system-messages, empty slice, nil slice, non-string content, and
assistant-leading passthrough.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
e0d674089f