Before: `send_file(to='parent')` from a sub-agent wrote the bytes to
the sub-agent's own session outbox, but agent-to-agent routing copied
only the content JSON — the target's inbound message referenced
`files: ['x.png']` but the bytes lived in a session directory the
target couldn't mount. Parent agents orchestrating sub-agents (e.g.
Design Team delegating illustration work to an Illustrator sub-agent
on Codex) received file-reference messages with nothing to forward.
Fix: on route, if the source's content has `files`, copy each referenced
file from `<source>/outbox/<src-msg-id>/` to
`<target>/inbox/<a2a-msg-id>/`, and emit `attachments` (the existing
formatter convention — see formatter.ts:223) with `localPath` relative
to `/workspace/`. The target formatter already renders these as
`[file: <name> — saved to /workspace/inbox/<a2a-id>/<name>]`, so the
target agent sees the path and can call `send_file(path=…, to=…)` to
forward onward.
Convention matches what session-manager.ts:256 already does for
base64-encoded channel-inbound attachments — same inbox layout, same
content shape. Nothing on the formatter/agent side needed to change.
## Scope
- `forwardAttachedFiles(source, target)` — pure-ish helper that copies
files and returns the attachments array.
- `forwardFileAttachments(msg, …)` — wraps the helper for the route
path: parses content, copies files if present, merges into any
existing `attachments`, re-serialises.
- `routeAgentMessage` — uses the rewritten content when writing the
target's inbound row.
- Log line now includes `forwardedFileCount` for observability.
Missing source files are skipped with a warning rather than killing
the route — a bad filename in a batch shouldn't drop the
accompanying text.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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