Shared Memory and Blackboard Patterns

Two approaches coexist in 2026 multi-agent systems: the **message pool** (everyone sees everyone's messages, as in AutoGen GroupChat or MetaGPT) and the **blackboard with subscription** (agents subscribe to relevant events, as in Context-Aware MCP or the Matrix framework). Both are the only stateful part of a multi-agent system — which means both are where the interesting bugs live. The reference failure mode is **memory poisoning**: one agent hallucinates a "fact," other agents treat it as verified, and accuracy decays gradually in a way that is much harder to debug than an immediate crash. This lesson builds both structures from stdlib, injects a poisoning attack, and shows the three mitigations that actually work in production.