Old runtimes meeting a newer session format now fail loud instead of misreading: version refusal names the direction (newer: upgrade the harness; older: no upgrade path) and points at the raw JSONL log, and an event type outside the generated known vocabulary refuses resume unless its envelope carries the new ignorable: true marker (default: required, so a forgotten marker over-refuses instead of silently resuming a gutted session). gen-persistence-catalog now also emits KNOWN_SESSION_EVENT_TYPES; SQLite stores the marker in a dedicated column (SCHEMA_VERSION 15). The versioning design (monotonic integer, n->n+1 upgrader chain, migrate-on-continue) is recorded in the session-log-version-mechanism Agent Note.
core/ — product API spine
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The session log, system-prompt assembly, tool registry, agent vocabulary, deployment-default model selection, and concrete loop that form the harness's default control spine. These are product packages — the stable surface plugins and consumers build against.
| Package | Role | ctx key |
|---|---|---|
scope/ |
Scoped-context registration primitive | library — no ctx key |
session/ |
Event-sourced session log and in-memory store | ctx.sessions |
system-prompt/ |
Prompt and tool-schema assembly registry | ctx.systemPrompt |
tools/ |
Scoped tool registry and execution pipeline | ctx.tools |
agent/ |
Agent interface, registry, and event vocabulary | ctx.agents |
agent-default-model/ |
Default model selection shared by Agent entry points | ctx.agentDefaultModel |
agent-loop/ |
Default concrete agent driver | ctx.agentLoop |
scope supplies the shared scoping primitive. agent owns the public contract, while agent-loop is its default implementation; extension plugins depend on the seam so the driver remains swappable. agent-default-model owns the deployment selection an Agent entry point uses only when a session has no selection of its own.
Runnable compositions belong to examples/agent-spine-demo; this group owns only the swappable spine pieces.
The subsystem reference — the package-by-package loop map, the Agent handle and its delivery/interception contracts — is docs/subsystems/core.md; the default runnable composition is examples/agent-spine-demo.