fabro/docs/internal/fabro-event-schema-v2-concrete-shape.md

Fabro Event Schema V2: Concrete Shape

Date: 2026-04-09

Status: implemented

This document turns the settled design decisions from the event-schema discussion into a concrete wire-contract proposal.

It intentionally supersedes the earlier framing in fabro-event-schema-v2-proposal.md for:

• proposal 1: one canonical persisted log, not two truths
• proposal 2: formalize and generalize the existing since_seq replay contract, rather than inventing replay from scratch

Design Decisions Carried Forward

• one canonical persisted event log
• plain hand-coded Rust structs are the authoritative source of truth for the event contract
• RunEvent remains the canonical semantic event type
• seq remains outside RunEvent, in the store/API envelope
• replay stays built around ordered since_seq cursors
• typed Rust consumers matching on EventBody remain the primary consumer model
• the envelope widens only modestly for execution topology and tool-call correlation: stage_id, parallel_group_id, parallel_branch_id, tool_call_id
• existing durable event families stay broadly intact
• live token/delta noise does not become part of the durable persisted Rust event contract
• snapshots are out of scope for both the durable event contract and the attach API

Contract Source Of Truth

V2 does not adopt schema generation or a registry-first workflow.

The authoritative source of truth for the event contract should be plain, hand-coded Rust structs and enums that model the public wire shape directly.

Implications:

• the Rust event types are the canonical contract
• this document describes that contract and should stay aligned with the Rust types
• any TypeScript types, JSON Schema, or OpenAPI fragments are secondary artifacts, not the source of truth
• codegen is explicitly out of scope for the initial V2 implementation

Why Evolve The Current Model

V2 should evolve Fabro's existing event architecture rather than replace it with a generic event platform.

Earlier drafts of this document proposed a generic reducer contract, a larger ontology-first envelope, and a narrower replacement event catalog. V2 walks that back. The current code's boundary between internal workflow events, RunEvent, and EventEnvelope is stronger and simpler than it first appeared, so evolving that model is cheaper and clearer than replacing it.

The current code already has a strong separation of concerns:

• internal workflow/runtime events in fabro-workflow
• one canonical semantic RunEvent
• a store/API envelope that carries seq outside the event payload

That separation is worth preserving. The main V2 changes should be:

• modest envelope widening for execution topology
• cleanup and clarification of event-family boundaries
• keeping the durable event catalog semantic and typed

V2 should not introduce:

• a generic reducer contract based on entity_type / event_role
• canonical persisted token deltas
• snapshot events as a second truth layer

Capability Coverage Decisions

V2 is evolutionary over the current RunEvent surface. It keeps the existing durable event families broadly intact rather than replacing them with a new ontology.

The main additions are:

• stage_id in the envelope for concrete stage execution identity
• parallel_group_id in the envelope for one execution of a parallel node
• parallel_branch_id in the envelope for one branch inside a parallel execution
• tool_call_id in the envelope for agent tool lifecycle events that need a stable cross-family join key

Everything else should remain in typed EventBody props unless there is a strong cross-family reason to promote it. session_id already exists in the envelope today and stays as-is. tool_call_id is promoted now because agent.tool.* events already carry a stable tool-call identity that other durable families can reference when needed. turn_id is deferred because Fabro does not yet have a durable turn identity that spans the families that would need to join on it.

Exact Delta From Current Code

This is the implementation delta from the current Rust codebase, not the full history of how the design was reached.

Add

• add stage_id: Option<String> to RunEvent
• add parallel_group_id: Option<String> to RunEvent
• add parallel_branch_id: Option<String> to RunEvent
• add tool_call_id: Option<String> to RunEvent
• add actor: Option<ActorRef> to RunEvent
• extend envelope extraction in stored_event_fields() to populate the new execution-topology fields when known
• extend envelope extraction in stored_event_fields() to populate tool_call_id on tool-lifecycle events when known
• update RunEvent serialization and parsing so the new optional envelope fields round-trip cleanly

Keep As-Is

• RunEvent remains the canonical semantic event type
• EventBody remains the typed tagged union of durable event families
• EventBody::Unknown remains the compatibility valve for unknown event names on read
• EventEnvelope remains the ordered outer wrapper with seq outside the event payload
• EventEnvelope.payload remains EventPayload, not RunEvent
• the internal/store EventEnvelope Rust type stays wrapped as { seq, payload }
• attach/replay remains exact ordered replay from since_seq, followed by live tailing
• current durable event families stay broadly intact
• live token/delta noise remains outside the durable persisted contract
• snapshots remain out of scope

Do Not Do

• do not inline seq into RunEvent
• do not introduce entity_type, entity_id, or event_role
• do not replace typed Rust consumers with a generic reducer model
• do not redesign the store envelope
• do not add snapshot events or attach-time synthetic snapshots
• do not persist token deltas or other live UI noise as durable RunEvents

Canonical Rust Shapes

V2 should model the public contract directly as hand-coded Rust types, following the existing architecture.

pub struct RunEvent {
    pub id: String,
    pub ts: DateTime<Utc>,
    pub run_id: RunId,
    pub node_id: Option<String>,
    pub node_label: Option<String>,
    pub stage_id: Option<String>,
    pub parallel_group_id: Option<String>,
    pub parallel_branch_id: Option<String>,
    pub session_id: Option<String>,
    pub parent_session_id: Option<String>,
    pub tool_call_id: Option<String>,
    pub actor: Option<ActorRef>,
    pub body: EventBody,
}

pub struct EventEnvelope {
    pub seq: u32,
    pub payload: EventPayload,
}

pub struct ActorRef {
    pub kind: ActorKind,
    pub id: Option<String>,
    pub display: Option<String>,
}

pub enum ActorKind {
    User,
    Agent,
    System,
}

RunEvent remains the semantic product event. EventEnvelope remains the ordered store/API wrapper. The store continues to persist validated JSON EventPayload, not typed RunEvent structs.

For wire JSON, EventEnvelope should serialize in flattened form so clients see:

{
  "seq": 4861,
  "id": "...",
  "ts": "...",
  "run_id": "...",
  "event": "...",
  "properties": { ... }
}

That flattening is a wire concern only. It does not move seq into RunEvent, and it does not change the internal/store Rust shape of EventEnvelope.

EventBody remains a hand-coded tagged enum serialized as:

{
  "event": "stage.completed",
  "properties": { "...": "..." }
}

V2 should also preserve the current unknown-event fallback shape:

EventBody::Unknown {
    name: String,
    properties: serde_json::Value,
}

This fallback already exists in the current code and should be kept.

Envelope Rules

• id, ts, run_id, and event are always present on the serialized RunEvent.
• seq is not part of RunEvent. It stays in the outer EventEnvelope.
• Optional envelope fields are omitted, never serialized as null.
• The existing top-level envelope fields remain:
  • node_id
  • node_label
  • session_id
  • parent_session_id
• V2 adds only these new optional envelope fields:
  • stage_id
  • parallel_group_id
  • parallel_branch_id
  • tool_call_id
• Other relationship identifiers stay inside typed properties.
• turn_id remains in typed properties; see the deferral decision in Capability Coverage Decisions.
• actor is optional. When present, it identifies the primary actor for the event.
• Set actor on human- or agent-initiated events where that identity matters to consumers. Example: run.cancel.requested should identify the user who initiated the cancel.
• Set actor on durable agent output when the producing session identity matters. Example: agent.message should identify the agent session.
• Omit actor for routine runtime events with no meaningful primary actor. Example: stage.started.

ID Format Conventions

• run_id keeps Fabro's current format: an unprefixed ULID string.
• stage_id keeps Fabro's current format: "{node_id}@{visit}".
• node_id is the stable graph node identifier from the workflow definition.
• parallel_group_id should be the durable identity of one execution of a parallel node. The default format should be "{node_id}@{visit}".
• parallel_branch_id should be the durable identity of one branch within a parallel execution. The default format should be "{parallel_group_id}:{index}".
• Consumers should otherwise treat IDs as opaque strings.

Presence Expectations

• stage_id is present on events tied to a concrete stage execution.
• parallel_group_id is present on parallel.* events and on events emitted inside a parallel execution when that scope is known.
• parallel_branch_id is present on parallel.branch.* events and on nested events emitted inside a specific branch when that scope is known.
• session_id and parent_session_id keep their current meaning for forwarded agent/session activity.
• tool_call_id is present on agent.tool.* events and on other durable events that directly describe the same tool call.
• node_label remains in the envelope for display-oriented consumers.
• actor is expected on control actions and durable agent output when there is a meaningful user or agent identity to expose. It is usually omitted on routine runtime lifecycle events.

Consumer Model

Rust consumers should keep matching on RunEvent.body using typed EventBody variants.

This document does not adopt:

• entity_type
• entity_id
• event_role
• a generic reducer contract

External JSON consumers should continue to:

• match on "event"
• read event-specific values from "properties"
• read "seq" from the flattened outer event envelope on API/SSE responses
• use envelope metadata only for cross-cutting context such as stage, session, execution topology, and tool-call correlation

Replay Contract

Fabro keeps the current replay model:

• ordered events are stored as EventEnvelope { seq, payload }
• API/SSE serialization of EventEnvelope should flatten seq into the top-level JSON object returned to clients
• attach starts from since_seq
• the server replays exact persisted envelopes and then tails live envelopes while the run is active
• SSE keepalive comments are transport frames, not events

V2 does not introduce:

• run.snapshot
• session.snapshot
• API-level attach snapshots
• persisted snapshot events of any kind

The durable model remains simple: replay ordered events, no duplicate truth layer.

Implementation Checklist

An engineer implementing this proposal should make only these structural changes unless a later section explicitly says otherwise.

1. Update RunEvent to add:
   • stage_id
   • parallel_group_id
   • parallel_branch_id
   • tool_call_id
   • actor
2. Update RunEvent::to_value() and RunEvent parsing in run_event/mod.rs so the new envelope fields serialize and deserialize.
3. Extend StoredEventFields and stored_event_fields() in event.rs to populate:
   • stage_id
   • parallel_group_id
   • parallel_branch_id
   • tool_call_id on tool-lifecycle events
   • actor when there is a clear primary actor These values should come from the emitter's current execution context for stage and parallel scope, and from event-specific payloads for tool_call_id.
4. Leave EventEnvelope structurally unchanged:
   • seq: u32
   • payload: EventPayload
5. Update API/SSE envelope serialization so wire JSON is flattened:
   • top-level seq
   • then the RunEvent payload fields alongside it
   • no "payload": { ... } wrapper in JSON responses
6. Leave the replay/attach flow unchanged in behavior:
   • persisted replay from since_seq
   • live tail after replay
   • no snapshots
7. Keep the current EventBody family surface unless there is an explicit product reason to change a specific family.
8. Keep streaming-noise agent events out of durable RunEvent conversion.
9. Update the HTTP/API schema docs to reflect both:
   • new RunEvent envelope fields
   • flattened JSON serialization of EventEnvelope

EventBody And Property Model

V2 should keep the current hand-coded domain split for prop structs:

• run props in run.rs
• stage and checkpoint props in stage.rs
• agent props in agent.rs
• infra/setup props in infra.rs
• parallel/interview/git/misc props in misc.rs

That split is part of the design quality. V2 should keep adding hand-coded prop structs, not collapse everything into generic maps.

Durable Event Surface

V2 keeps the current durable family surface broadly intact.

Run

• run.created
• run.started
• run.submitted
• run.starting
• run.running
• run.removing
• run.cancel.requested
• run.pause.requested
• run.unpause.requested
• run.paused
• run.unpaused
• run.rewound
• run.completed
• run.failed
• run.notice

Stage And Prompt

• stage.started
• stage.completed
• stage.failed
• stage.retrying
• stage.prompt
• prompt.completed

Parallel

• parallel.started
• parallel.branch.started
• parallel.branch.completed
• parallel.completed

Interview / Human Input

• interview.started
• interview.completed
• interview.timeout
• interview.interrupted

Checkpoint

• checkpoint.completed
• checkpoint.failed

Agent Durable Events

• agent.session.started
• agent.session.ended
• agent.processing.end
• agent.input
• agent.message
• agent.tool.started
• agent.tool.completed
• agent.error
• agent.warning
• agent.loop.detected
• agent.steering.injected
• agent.compaction.started
• agent.compaction.completed
• agent.llm.started
• agent.llm.first_output
• agent.llm.retry
• agent.sub.spawned
• agent.sub.completed
• agent.sub.failed
• agent.sub.closed
• agent.mcp.ready
• agent.mcp.failed
• agent.failover

Git

• git.commit
• git.push
• git.branch
• git.worktree.added
• git.worktree.removed
• git.fetch
• git.reset

Infra And Execution

• sandbox.*
• setup.*
• cli.ensure.* (legacy only)
• command.*
• agent.cli.*
• pull_request.*
• artifact.captured
• ssh.ready
• subgraph.*
• edge.selected
• loop.restart
• retro.*

Explicitly Non-Durable Streaming Noise

The current boundary that keeps live token/delta noise out of RunEvent should remain in place.

These stay outside the durable persisted contract:

• agent.output.replace
• agent.text.delta
• agent.reasoning.delta
• agent.tool.output.delta

(agent.skill.expanded was previously listed here. No such event exists — the AgentEvent::SkillExpanded variant was removed, and slash-skill expansion is reported through the durable agent.skill.activated with source == "slash".)

If Fabro needs those for UI, they belong in a separate transient stream, not in the canonical persisted Rust event contract.

Example Shapes

Flattened Wire JSON

{
  "seq": 4861,
  "id": "evt_01JSE1N7RJD1NW2JSDT3W0YQ92",
  "ts": "2026-04-08T16:21:11.106Z",
  "run_id": "01JSE1M0Q0P8P6KQW9Q6D58Q0E",
  "event": "agent.tool.completed",
  "stage_id": "code@1",
  "node_id": "code",
  "node_label": "Code",
  "session_id": "ses_child",
  "tool_call_id": "call_1",
  "parent_session_id": "ses_parent",
  "properties": {
    "tool_name": "read_file",
    "output": {
      "summary": "Read docs-internal/events-strategy.md"
    },
    "is_error": false,
    "visit": 1
  }
}

In Rust, EventEnvelope still remains { seq, payload: EventPayload }. The example above is only the flattened API/SSE JSON form of that envelope.

Practical Guidance

• Preserve the current one-time canonicalization boundary from internal Event to external RunEvent.
• Keep RunEvent semantic and typed. Do not turn it into a generic reducer envelope.
• Keep seq outside the event payload.
• Widen the envelope only modestly: stage_id, parallel_group_id, parallel_branch_id, and tool_call_id.
• Keep session_id as the existing top-level session field.
• Keep event-specific detail inside typed props.
• Preserve EventBody::Unknown as the compatibility valve for unknown event names on read.
• Do not store token deltas or other live UI noise as durable RunEvents.
• Do not add snapshot events or attach-time synthetic snapshots.
• When adding a new durable event, update the current Rust boundary cleanly:
  • internal Event
  • event_name()
  • envelope extraction
  • EventBody
  • typed props
  • affected consumers

Open Follow-Up

• correlation_id-style cross-entity grouping remains deferred until Fabro has a concrete consumer and explicit propagation rules