Evidence compaction capability

EvidenceCompactionCapability keeps a multi-turn conversation from carrying every search result it ever produced. Every question adds its evidence to the history, so requests grow turn after turn, which degrades answers and can exceed a provider's limits.

Register it alongside an evidence capability:

from pydantic_ai import Agent
from haiku.rag.capabilities.compaction import create_capability as compaction
from haiku.rag.capabilities.rag import create_capability as rag

agent = Agent(
    "openai:gpt-5",
    capabilities=[rag(db_path="my.lancedb"), compaction()],
)

It exposes no tools and takes no configuration. Registering it is the only switch: leave it out and the transcript reaches the model untouched.

The host must carry the capability state between runs, alongside the message history: the capsule is built from what earlier questions recorded there. Given only a message history, every run starts from an empty record, and compaction refuses rather than replace evidence it cannot retain. See Compose an agent for the shape.

What it does

On each request, evidence from earlier questions is replaced by the evidence those questions actually cited. Cited text and cited page images are kept in full, grouped by the question that cited them, and stay citable by the same chunk ids. Evidence spanning more than one collection carries a Collection: line naming the one it came from. Every other earlier evidence return becomes a short receipt. The current question is untouched.

Compaction rewrites the request, never the stored history, so all_messages() still holds everything the run gathered.

This reduces what a request carries. It does not bound it: retained evidence still grows with the conversation. A host that needs more aggressive pruning can compact its own requests further, on the wire only.

Resuming a question

Resuming a question (deferred tool results, an interruption, a suspension) requires the host to carry the capability state from the run being resumed, alongside the message history. Without it the identity of the question in progress is unknowable, and the run fails rather than silently treating it as a new question.