Agent Action Handshake

Agent ↔ FSM-action handshake

This is the contract between an FSM action and the agent it invokes. Most "platform bug" misdiagnoses live in the gap between "how the FSM authors the action" and "what the agent prompt expects." Read this once, then ground every job/agent edit in it.

The four invocation kinds

Each path has the same outputTo contract but different tool surfaces and prompt-composition rules.

What the runtime auto-injects

Regardless of invocation kind, the LLM-side agent (any agent that's type: llm) sees:

• Platform tools — full PLATFORM_TOOL_NAMES set from packages/agent-sdk/src/platform-tools.ts (~35 entries spanning memory, artifacts, fs, shell/data, state, HITL, and flow-control like delegate). These inject on top of the agent's config.tools allowlist regardless of what's in tools:. See source for the exact set.
• complete tool — injected when the action declares outputTo. The LLM MUST call this to surface output (see "The complete contract" below).
• failStep tool — injected for inline type: llm FSM actions (see fsm-engine.ts case "llm"). The LLM calls this to deliberately fail the step with a structured reason. NOT injected on the type: agent → type: llm path that goes through from-llm.ts; an agent reachable through both shapes can't assume the tool is present.

What's NOT auto-injected:

• MCP server tools — those come from agents.<id>.config.tools (workspace-scope MCP only) or from the inline type: llm action's tools: array.

The complete contract

The single most important rule: if an action has outputTo, the agent's prompt MUST instruct the LLM to call complete with the structured output.

When the action is:

- type: agent
  agentId: stale-report-agent
  outputTo: response

The runtime injects a complete tool with input schema {response: string} (or richer if outputType is declared). The agent's prompt should end with:

When done, call the `complete` tool with:
  { "response": "<the full final text>" }
This is how the FSM captures your output. Do not stream the text as
your final assistant message — the runtime reads it from `complete`.

Without this instruction, the LLM streams prose, never calls complete, and the runtime throws:

LLM action with outputTo 'response' did not call complete

outputType and structured output

When you set outputType: SomeDoc, the runtime looks up fsm.documentTypes.SomeDoc for a JSON Schema and feeds it to the complete tool's input schema. The LLM must produce data matching the schema.

documentTypes:
  EmailDraft:
    type: object
    properties:
      to: { type: string }
      subject: { type: string }
      body: { type: string }
    required: [to, subject, body]

states:
  drafting:
    entry:
      - type: agent
        agentId: drafter
        outputTo: draft
        outputType: EmailDraft

The agent's prompt should know about to, subject, body — either from the prompt body or from a worked example.

inputFrom and document chaining

inputFrom reads a prior step's outputTo document and feeds it into the next step's input. String form chains one prior; array form concatenates multiple labeled.

states:
  draft:
    entry:
      - type: agent
        agentId: drafter
        outputTo: draft-doc
      - type: emit
        event: DONE
    on: { DONE: { target: review } }
  review:
    entry:
      - type: agent
        agentId: reviewer
        inputFrom: draft-doc       # reads from draft-doc

Bulky inputs are passed as artifactRefs plus a compact summary — the agent reads the full data via parse_artifact(<id>) if needed.

Pattern A vs Pattern B

Two job shapes:

• Pattern A (execution.strategy: sequential): list of agent IDs that run in order. Final agent's assistant text becomes the session output natively. Use when the work is a single LLM conversation, no document chaining needed, output is plain text.
• Pattern B (fsm:): state machine with named states, entry actions, and explicit outputTo documents. Required when you need branching, multi-step document flow, parallel actions, or HITL elicitation.

Don't migrate Pattern A → Pattern B "to get outputTo." The migration introduces the complete injection contract, which is the single most common source of empty-output failures. If a Pattern A job is producing empty output, fix the agent's prompt first.

Cross-references

• @friday/writing-workspace-jobs — FSM authoring shapes.
• @friday/workspace-api — agent CRUD.
• @friday/delegate-handoff — sibling contract for the parent ↔ delegate-child boundary.
• @friday/debugging-empty-output — what happens when this contract is violated.
• @friday/debugging-runtime-errors — did not call complete and related errors.