Lessons from building agentic dev workflows: let software own the control loop
mostly_deterministic · reddit · 2026-09-16
A long-form writeup on where deterministic software should end and agentic behavior begin in developer tooling, motivated by wanting harness-agnostic capability packaging, explicit per-stage model control, and deterministic guarantees after too many "I'm sorry, I should have…" moments.
Key principles:
- Keep repeatable workflows and invariants outside the conversational harness, using local MCP over stdio as the seam
- Use explicit context in tool parameters as a control surface across runs
- Treat deterministic vs agentic as a spectrum, not a binary — software-defined graphs can still contain judgment-heavy agentic nodes
- Software owns the control loop when topology is known; the conversational model owns it when deciding the next step is the hard part
- Model tools as ephemeral operations or durable workflows with explicit lifecycle (create/get/cancel/commit)
- Route stages to different models rather than one model owning everything
Implemented with FastMCP + LangGraph, though the author argues the patterns aren't framework-specific.
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