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Eleven OSS agent runtimes, one recurring pattern: memory, sandboxing, skills-vs-MCP, tool governance, and transport. Only Letta solved memory, only smolagents solved sandboxing, and the 2026 vendor SDKs are quietly absorbing the rest into hosting platforms.

Fifteen open-source deep-research agents reduce to four architecture archetypes. Two of the four are dead ends. Here is the field guide for picking.