ADR-026 AttachmentWriter Host Boundary (E2A)¶

Intent¶

Freeze the host-side policy boundary for raw payload preservation used by protocol adapters.

The adapter contract remains minimal, but host implementations of AttachmentWriter must enforce a consistent safety policy before runtime rollout.

Scope¶

In-scope: - host-enforced payload size caps - media-type allowlist / validation - explicit redaction boundary ownership - error taxonomy for attachment persistence failures

Out-of-scope: - runtime implementation of the host writer (E2B) - workflow changes - release-lane integration changes

Host policy contract (v1)¶

A production AttachmentWriter implementation must enforce: - a hard maximum payload size before persistence - media-type validation against an explicit allowlist or contract rule - no direct adapter-managed filesystem writes - no raw payload contents in logs

Redaction boundary¶

Adapters do not perform redaction.

Redaction, rejection, or secret-handling policy is owned by the host layer implementing AttachmentWriter.

Error taxonomy¶

The host writer contract must distinguish at least: - measurement/contract failures - oversize payload - unsupported or invalid media type - invalid persistence contract inputs - infrastructure failures - storage write failure - unavailable attachment backend

Required output shape¶

Successful writes must produce a stable digest-backed reference containing: - sha256 - size_bytes - media_type

Non-goals¶

• no adapter mapping behavior changes in this freeze slice
• no workflow changes
• no crates.io publication changes