Building a Coding Agent from Scratch: What's Really Under the Hood

Agents such as coding assistants require more than an LLM wrapper, relying on tool access, context management, orchestration, durable state, and reliable interaction patterns. The post describes a proof-of-concept durable coding-agent REPL built with Agentspan and Conductor OSS, in which an LLM can read and write files, execute shell commands, receive user messages, and reply continuously through a long-running workflow. Agentspan compiles the agent into a Conductor workflow, isolates concurrent sessions with unique run domains, streams workflow activity through Server-Sent Events, and uses a workflow message queue to durably deliver new prompts when the agent is waiting. It distinguishes queued messages for new tasks from signals, which inject instructions such as course corrections into the agent’s context on its next reasoning turn without interrupting its current work. The client uses separate threads to keep a single SSE connection open while handling terminal input, avoiding replay issues, and an enhanced version provides tools for launching, monitoring, and stopping background processes such as development servers. Session execution IDs can be stored and resumed later, restoring the agent’s conversation history and state while re-registering its workers, illustrating how the framework handles operational concerns so developers can focus on agent behavior and tools.

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