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Eleven OSS agent runtimes share one recurring pattern of 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 which are dead ends, and this field guide sorts the agents by archetype so the right one is easier to pick.