Tape and model context

An application needs a complete, trustworthy history; a model needs only a bounded, relevant input. Bub keeps the two apart: the Tape is the durable record, and the model context is one projection of that record.

One record, two views

• The Tape (write): appends TapeEntry records in order — conversation messages, tool calls, tool results, anchors, and business events recorded explicitly. tape.query() returns the raw entries.
• TapeContext (read): selects the entries this request needs and converts them into model messages. tape.read_messages() returns the projection, not the raw record.

The default projection, default_tape_context(), reads the entries after the most recent anchor and rebuilds the conversation and tool round-trips. Entries marked context=False are excluded before selection and never enter the model context.

Append-only, but not an immutable audit log

Normal recording only appends; tape.reset() clears the record and writes a new session/start anchor.

Anchors and handoffs

An anchor marks a cognitive transition, such as finishing information gathering or starting a new task. A handoff appends a new anchor, moving the start of the default context forward while keeping every earlier record.

A handoff changes only the default read boundary: it generates no summary, deletes no history, and does not automatically inject the anchor’s state into the next request. When you need a summary, generate one explicitly and write it into later model input. For usage, see SDK: anchors and handoffs and the app’s built-in tape.handoff command.

Where data belongs

Home	Lifetime	Good for
state / TurnState	One Run or one Turn	Scratch values shared within a request
metadata	One Run	Observability data such as a request ID; never enters model input
Tape entries	Decided by storage; may span processes	Conversation, tool round-trips, and business facts written explicitly
Anchor state	Kept with the history	Phase snapshots
Application storage	Decided by the app	Session settings restored by load_state

core.Agent uses an in-memory store by default. See SDK: Tape and memory for persistence, forks, and concurrency, and the Storage integration for plugging in a store backend.