The static-stub sentence over-generalized: `tools` and `ToolCallError` ARE bound at run time, and a model reading "everything below is a stub" could stop catching `ToolCallError`. State the boundary and pin both halves in the fixed-instruction assertions. UNPRINTABLE missed U+0085: it is Cc but not ECMAScript whitespace, so it survived the collapse and reached the docstring raw and invisible. Add it and scope the docstring to Cc, since the `\xNN` escape cannot address the Cf formatting characters that pass through by design. Record the backend PR's two runtime contracts -- inject only `tools` and `ToolCallError`, and bind the assembly-time language to the request -- in the Agent Note and at requireCodeRuntime.
core/ — product API spine
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The session log, system-prompt assembly, tool registry, agent vocabulary, 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-loop/ |
Default concrete agent driver | ctx.agentLoop |
scope supplies the shared scoping primitive. agent owns the public seam, while agent-loop is its default implementation; extension plugins depend on the seam so the driver remains swappable.
Runnable compositions belong to examples/agent-spine-demo; this group owns only the swappable spine pieces.