penguin-harness/packages/docs/content/omni-message.en.md

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The OmniMessage Protocol	One envelope, three message types, a five-value stop_reason — the unified protocol behind the SDK, the Trace and SSE, field by field.

OmniMessage is PenguinHarness's unified message protocol: the SDK yields it, the Trace stores it line by line, and the Server pushes it verbatim over SSE. What streams, what is stored and what the model sees are one structure — there is no second format between front end, back end and storage.

This page goes top-down: the envelope and the three message types first, then every payload field by field, then the protocol-wide semantics (streaming discipline, stop_reason, origin, fidelity fields). Type source: packages/core/src/omnimessage/types.ts.

The envelope

Every message shares one envelope; only the payload varies:

interface OmniMessage<P extends OmniPayload = OmniPayload> {
  timestamp: string;        // ISO 8601 UTC
  type: "session_meta" | "model_msg" | "event_msg";
  payload: P;
  origin?: string[];        // child-Session chain (outer→inner); absent = main Session
}

What each message type carries:

type	Meaning	Volume
session_meta	The full runtime configuration of one model context	exactly one per context
model_msg	Content inside the model context (text, thinking, tool calls and results)	the bulk
event_msg	Runtime events outside the context (approvals, usage, compaction, aborts)	alongside

session_meta

interface SessionMetaPayload {
  session_id: string;
  provider: string;                       // one half of the model-identity pair
  model_id: string;                       // the upstream request id sent to AgentHub
  model_context_window: number | string;
  system_prompt: string;                  // fully assembled, placeholders substituted
  tools: ToolDefinition[];                // the complete tool schema sent to the model
  thinking_level: string;                 // "default" when unconfigured
  agent_state: string;                    // absolute path of the Agent State
  workspace: string;                      // absolute path of the Workspace
}

interface ToolDefinition {
  name: string;
  description: string;
  parameters?: Record<string, unknown>;   // JSON Schema
}

On resume, the engine takes this Trace line as the runtime config — the model, system prompt and Workspace are immutable for the Session's lifetime. See Sessions & Traces.

model_msg: complete payloads

Seven content payloads, discriminated by payload.type. Shared optional fields: stop_reason (marks an abnormal terminal state) and fidelity (an opaque provider-fidelity payload, see below):

type Fidelity = Record<string, unknown>;  // opaque provider-fidelity payload (see below)

interface TextPayload {
  type: "text";
  role: "user" | "assistant";
  text: string;
  fidelity?: Fidelity;        // e.g. { phase } segment marker (GPT-5), { signature }
  stop_reason?: StopReason;
}

interface ThinkingPayload {
  type: "thinking";
  role: "assistant";
  thinking: string;
  fidelity?: Fidelity;        // required by some models to replay history
  stop_reason?: StopReason;
}

interface InlineThinkingPayload {
  type: "inline_thinking";
  role: "assistant";
  data: string;               // reasoning content in binary form
  mime_type: string;
  fidelity?: Fidelity;
  stop_reason?: StopReason;
}

interface ToolCallPayload {
  type: "tool_call";
  role: "assistant";
  name: string;
  arguments: string;          // arguments as a JSON string
  tool_call_id: string;
  fidelity?: Fidelity;
  stop_reason?: StopReason;
}

interface ToolCallOutputPayload {
  type: "tool_call_output";
  role: "user";
  output: string;
  images?: string[];          // data:<mime>;base64,… URLs (e.g. read_image results)
  tool_call_id: string;
  stop_reason?: StopReason;
}

interface ImageUrlPayload {
  type: "image_url";
  role: "user";
  image_url: string;          // web URL or base64 data URL
  stop_reason?: StopReason;
}

interface InlineDataPayload {
  type: "inline_data";
  role: "user" | "assistant";
  data: string;               // other binary content
  mime_type: string;
  fidelity?: Fidelity;
  stop_reason?: StopReason;
}

tool_call and tool_call_output pair strictly via tool_call_id; a turn's calls form one batch, and outputs are re-fed in the original call order (see The Agent Loop).

model_msg: streaming partials

Four partial_* payloads mirror their complete counterparts, carrying an event_type phase marker:

type StreamEventType = "start" | "delta" | "stop";

interface PartialTextPayload {
  type: "partial_text";
  role: "assistant";
  event_type: StreamEventType;
  text: string;                 // the text added by this fragment
  stop_reason?: StopReason;
}

interface PartialThinkingPayload {
  type: "partial_thinking";
  role: "assistant";
  event_type: StreamEventType;
  thinking: string;
  stop_reason?: StopReason;
}

interface PartialToolCallPayload {
  type: "partial_tool_call";
  role: "assistant";
  event_type: StreamEventType;
  name: string;
  arguments: string;            // incremental fragment of the arguments JSON
  tool_call_id: string;
  stop_reason?: StopReason;
}

interface PartialToolCallOutputPayload {
  type: "partial_tool_call_output";
  role: "user";
  event_type: StreamEventType;
  output: string;
  images?: string[];            // images are not incremental — one delta carries the whole set
  tool_call_id: string;
  stop_reason?: StopReason;
}

The streaming discipline

Every streamed segment follows one timing rule, with the complete message immediately after the stop:

partial_text(start) → partial_text(delta) → … → partial_text(stop) → text (complete)
                      └── concatenation of all deltas ≡ the complete message ──┘
                          (truncation applies to both alike)

Renderers can therefore paint deltas incrementally and swap in the complete message in place; the Trace records only complete messages, never fragments. Interface implementations close their structures internally and never leak an unclosed fragment upward. PartialAggregator (aggregate.ts) ships a ready-made aggregator.

event_msg

Eight event payloads, all listed field by field:

interface RequestBeginPayload {
  type: "request_begin";
}

interface RequestEndPayload {
  type: "request_end";
  status: StopReason;         // "completed" is the mechanical commit criterion for replay
}

interface ApprovalDecisionPayload {
  type: "approval_decision";
  decision: "allow" | "deny";
  tool_call_id: string;       // pairs with the approved tool_call — the audit record
}

interface TokenUsagePayload {
  type: "token_usage";
  session: TokenCounts;       // Session cumulative
  request: TokenCounts;       // this Request
}

interface TokenCounts {
  cache_read: number;
  cache_write: number;
  output: number;
  total: number;
}

type CompactionReason = "context" | "turns" | "manual";
type CompactionMode = "summarize" | "discard";

interface CompactionBeginPayload {
  type: "compaction_begin";
  reason: CompactionReason;
  mode: CompactionMode;
  context: number;            // context tokens at trigger time
  turns: number;              // cumulative turns at trigger time
}

interface CompactionEndPayload {
  type: "compaction_end";
  reason: CompactionReason;
  mode: CompactionMode;
  status: StopReason;
}

interface AbortPayload {
  type: "abort";
  reason?: string | null;
}

interface SubagentPayload {
  type: "subagent";
  session_id: string;         // pointer in the parent Trace to a direct child Session
}

stop_reason

A five-value enum used across messages and interface results (LLMOutcome.status uses the same set — see Core Interfaces):

type StopReason = "completed" | "failed" | "aborted" | "timeout" | "malformed";

Value	Meaning	Engine reaction
completed	finished normally	continue
aborted	user interrupt	stop, hand back to the user
timeout	LLM timeout / lost connection	LLM side only: auto-reconnect within the run
malformed	parse failure / truncated stream	LLM side only: auto-reconnect within the run
failed	other non-retryable error	stop, hand back to the user

Errors never cross an interface boundary as exceptions — they are messages. See The Agent Loop.

origin: the Subagent chain

origin serves Subagents: when a child Session's messages are forwarded to the parent, each hop prepends one child Session id (outer→inner), and renderers route messages into the right nested card by the chain:

// message from the main Session: no origin
{ timestamp: "…", type: "model_msg", payload: { type: "text", … } }

// message from a one-level Subagent: origin = [child Session id]
{ timestamp: "…", type: "model_msg", origin: ["session-2026-07-18-…-a1b2c3d4"], payload: { … } }

origin-tagged messages are not written to the parent Trace — the child Session has its own Trace, and the parent keeps only the subagent pointer event.

Provider-fidelity fields

Provider-specific wire data travels in a single optional field, fidelity — an arbitrary JSON object the LLM client records to reproduce the original message on replay: thinking signatures, phase segment labels, GPT-5 encrypted reasoning, the OpenAI-compatible upstream reasoning field name:

// Claude: a thinking block closed by its signature
{ type: "thinking", thinking: "…", fidelity: { signature: "EqQBCkYIBxgCKkB…" } }

// GPT-5: encrypted reasoning (empty thinking text, fidelity only)
{ type: "thinking", thinking: "", fidelity: { id: "rs_0d3…", encrypted_content: "gAAAA…" } }

// OpenAI-compatible: the upstream field the reasoning text came from
{ type: "thinking", thinking: "…", fidelity: { reasoning_field: "reasoning_content" } }

The payload is opaque to PenguinHarness: it passes through and persists verbatim end to end — some models require it byte-for-byte when history is replayed, and any rewriting (or loss) would break compatibility. This is one of the preconditions for lossless Session recovery from the Trace.

Three jobs, one protocol

Surface	Subset used
SDK boundary (session.run output)	complete model_msg + streaming partial_* + all event_msg
Trace on disk	session_meta + complete model_msg + all event_msg (no partials, no origin-tagged messages)
Server SSE stream	same as the SDK boundary, verbatim single-line JSON — see Server API

How messages travel along these surfaces — and every ordering guarantee — is covered on Message Flow & Ordering.

Builders and guards

@prismshadow/penguin-core exports all types, a builder per message kind (builders.ts: userText, assistantText, toolCall, toolCallOutput, partialText, tokenUsage, withOrigin, emptyTokenCounts, addTokenCounts, …) and runtime guards (isCompleteModelMessage, isPartialPayload, isModelMessage, isEventMessage, isSessionMeta):

import { userText, isCompleteModelMessage } from "@prismshadow/penguin-core";

const prompt = userText("List the files in the current directory");
// { timestamp: "…", type: "model_msg", payload: { type: "text", role: "user", text: "…" } }