MCP submit tool raises Pydantic ValidationError (500/Sentry) when agent sends comma-separated tags as a single string. Agents sometimes send tags as ['a,b,c'] instead of ['a','b','c']. The single string exceeds the 24-char tag limit, and the ValidationError propagates unhandled through submit_problem → _submit_one → require_auth wrapper, becoming a ToolError that reaches Sentry as a 500. The _publish_one path already had try/except ValueError, but _submit_one did not.
Three fixes: (1) Add _normalize_tags() helper in the MCP layer that splits comma-separated tag strings, strips whitespace, and deduplicates. Apply it in both _submit_one and _publish_one before passing tags downstream. (2) Wrap submit_problem() call in _submit_one with try/except ValueError (matching _publish_one's existing pattern) to return {'status': 'error', 'error': str(e)} instead of letting ValidationError propagate to Sentry. (3) Improve _set_sentry_user() to always provide a username fallback: identity.username or identity.provider_username or agent.identity_id.