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Nodes vs. stages

A node is a step defined in the Graphviz file at author time. A stage is the runtime execution of a node. In a simple linear workflow, each node runs once and produces one stage. But when a workflow loops — for example, an implement-test-fix cycle — the same node can produce multiple stages within a single run. This distinction matters for observability and debugging: the workflow graph shows nodes, but the run timeline shows stages. Each stage records its own inputs, outputs, duration, and token usage.

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Node types

Every node’s Graphviz shape attribute determines its execution behavior. If no shape is specified, the node defaults to an agent.

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Start

Shape: Mdiamond The entry point of the workflow. Every workflow must have exactly one start node.

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Exit

Shape: Msquare The terminal node. When execution reaches exit, the workflow completes. Every workflow must have exactly one exit node.

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Agent

Shape: box (default) Runs an LLM with access to tools — bash, file editing, sub-agents — in an agentic loop. The agent works autonomously, calling tools as needed, until it decides the task is complete.
Key attributes: Fidelity levels: Fidelity can also be set at the graph level (default_fidelity) or on individual edges to control the transition between stages. See Context for the full reference on fidelity precedence, preamble construction, and thread integration. Thread ID: Setting thread_id on multiple nodes (e.g. thread_id="impl") groups them into a shared conversation thread, preserving context continuity across nodes as if they were part of the same session. This is an advanced feature typically used within subgraph clusters:

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Prompt

Shape: tab Makes a single LLM call with no tool use. Useful for analysis, summarization, generation, and lightweight reasoning where tools aren’t needed.
Prompt nodes accept the same attributes as agent nodes (prompt, reasoning_effort, max_tokens, etc.) but never invoke tools.

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Command

Shape: parallelogram Runs a shell script inside the configured sandbox and captures its output. The output is available to downstream nodes as context. This ensures command nodes execute in the same environment as agent nodes.

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Human

Shape: hexagon Pauses the workflow and waits for a person to choose a path. The outgoing edge labels define the available options:
In the web UI, human gates appear as interactive prompts. From the CLI, they appear as a menu.

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Wait

Shape: insulator Pauses the workflow for a configured duration before proceeding. Useful for rate limiting between API calls or waiting for external processes to complete.

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Conditional

Shape: diamond Routes execution to different edges based on conditions evaluated against the current run context:
Conditions support =, !=, &&, and context variable lookups (e.g. context.tests_passed=true). An edge with no condition acts as the default fallback.

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Parallel (fan-out)

Shape: component Fans out to execute multiple branches concurrently. Every branch runs in the same sandbox checkout and working directory, and the parallel node waits for every branch to finish.
Because the checkout is shared, file changes from one branch are immediately visible to the others. Concurrent writes can race or overwrite each other. Fabro does not isolate branch files, lock paths, detect conflicts, or warn about overlapping writes. Design branches to be read-only or assign each branch disjoint files and directories when deterministic workspace changes matter. For each branch’s first node, fidelity resolves from the fork-to-branch edge, then the branch node; otherwise it inherits the fork preamble unchanged. Fabro renders branch-specific preambles before fan-out from the fork snapshot. Branch-level full degrades to summary:high because concurrent branches cannot share sessions, and thread_id on a branch node or fork-to-branch edge is inert.

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Merge (fan-in)

Shape: tripleoctagon Converges parallel branches after all of them finish. Branch status and context updates are collected in the runtime context at parallel.results; Fabro does not create a parallel_results.json file in the checkout.
A fan-in node with a prompt synthesizes the collected results. It never chooses, restores, or merges a branch’s workspace state: all branches have already operated on the same checkout. Without a prompt, fan-in is only a convergence barrier.

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Common node attributes

These attributes can be set on any node type: Retry policies: If neither retry_policy nor max_retries is set, nodes default to 3 retries with standard backoff.