Two north stars

The twelve factors

From humanlayer/12-factor-agents. Software constraints, not model constraints.

1. Factor 01

   Natural language to tool calls

   Map user language onto structured intent. The model proposes; code dispatches.

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2. Factor 02

   Own your prompts

   Prompts are source code. Diff them, test them, do not inherit a black box.

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3. Factor 03

   Own your context window

   Build the working set explicitly. The window is a product, not a log.

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4. Factor 04

   Tools are structured outputs

   A tool call is JSON. The ACI — names, schemas, errors — is the real prompt.

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5. Factor 05

   Unify execution and business state

   One serializable blob. The ticket is the agent state.

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6. Factor 06

   Launch, pause, resume

   Boring APIs for long-running work. Crashes must not lose the thread.

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7. Factor 07

   Contact humans with tool calls

   Approval and missing info are tools named ask_human, not a side channel.

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8. Factor 08

   Own your control flow

   The model picks among legal edges. Your code owns the graph.

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9. Factor 09

   Compact errors into context

   Observations should be the smallest string that still enables the next correct action.

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10. Factor 10

    Small, focused agents

    Few tools, narrow goal, short context. Generalists orchestrate; specialists act.

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11. Factor 11

    Trigger from anywhere

    Meet users in mail, chat, tickets, cron. The loop does not live in one text box.

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12. Factor 12

    Stateless reducer

    (state, event) → (newState, effects). The call is pure given the context you built.

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Anthropic’s catalog

Workflows first. An autonomous agent last, and only when the path cannot be named. Simplicity, transparency of plan, and a tested ACI.