[tee8z]

Problem

Shared agents can delegate but can't report back, so a coordinator stalls until a human steps in. Staging DEV-2868 today: Brokkr handed off to three teammates (all admitted). Then:

• A teammate's @Brokkr reply is refused, agent handoff requires task ancestry (policy.rs). A plain agent mention has no task tags.
• A proper buzz agents handoff back to Brokkr counts against the budget (4 handoffs per human request, 2 admissions per agent). After a fan-out to three, only one result could ever wake Brokkr.

Design

A return is a handoff to the agent that sent its parent handoff: parent_receipt.requester == target. It's derived from the durable receipts the manager already keeps, so it needs no new event kind or tags, and the relay-side signature and ancestry checks are unchanged.

Rule	Why
Each handoff returns once	bounded fan-in
A return cannot itself be returned	no ping-pong; replying to a result with new work is an ordinary budgeted handoff
Returns don't count toward the 4-handoff or 2-per-agent limits	returns ≤ handoffs, so total work stays bounded
The first (human) receipt isn't returnable	only agent-to-agent delegation

• budget.rs: Receipt.returns: Option<EventId> records the handoff a return answers. validate() re-checks it on every admission:

  • it answers an earlier budgeted handoff
  • it reverses that handoff's direction
  • there's only one return per handoff

  The field is omitted when empty, so journals written before this load unchanged. A journal containing returns fails closed on an older manager (deny_unknown_fields).

• policy.rs: the ancestry walk was capped at 4 links. Each handoff and its return is a link, so the cap is now 2 × MAX_HANDOFFS (still bounded).

• CLI: buzz agents return --from <incoming-handoff> --message '…' addresses the sender automatically. It refuses anything that isn't a received handoff ("reply in the thread instead"). It shares the handoff code path.

• Harness: the shared-task team instructions now say to report results with buzz agents return, and that an @mention doesn't wake an agent.

• Docs: docs/shared-channel-agents.md and voltage/channel-agent-sharing.md.

Validation

• budget tests:
  • New a_coordinator_collects_every_result_without_spending_the_handoff_budget: fan out to 3, all 3 return; restart and retry; second return refused; a reply to a result is budgeted (4th fits, 5th refused); a non-delegator "return" is budgeted.
  • New a_forged_return_in_the_journal_is_refused, plus legacy journal load.
  • The existing test's second admission was in fact a return; it now expects Returned.
• policy test: two more round trips take the ancestry chain to 6 links. It fails against the old 4-link cap.
• CLI test: return addresses the coordinator with the right ancestry and refuses a human mention. The subcommand stability guards are updated.
• Full suites: buzz-sdk 342, buzz-cli 504, buzz-acp 1151, buzz-sandbox-reaper 141, all pass. clippy -D warnings and fmt are clean.

Rollout

The agent image (CLI and harness) and the manager image (budget) both change. Promote both digests: cd for the manager and gate, voltage-terraform for the worker template. End-to-end check: have a coordinator fan out to two shared agents in a test thread, and confirm both results wake it with no human message.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Agents can return results to the agent that delegated a task, waking it with the response.
  • Each handoff can receive one return. Returns cannot be returned and do not count toward handoff limits.
  • Plain mentions without a task handoff do not wake the delegating agent.
• Documentation
  • Updated shared-channel agent guidance with instructions for returning results and requesting follow-up work.

[coderabbitai]

Walkthrough

Shared-agent handoffs now support one return per handoff. Returns are validated without consuming handoff budget, and the ancestry limit accounts for handoffs and returns. The CLI adds a command to send a result to the delegating agent.

Changes: Shared-agent handoff returns

Layer / File(s)	Summary
Return receipts and budget validation crates/buzz-sdk/src/sharing/budget.rs, crates/buzz-sdk/src/sharing/budget/tests.rs	Receipts now record the handoff answered by a return. Validation checks return ownership and uniqueness. Returns do not count toward handoff limits; tests cover retries, invalid returns, and journal recovery.
CLI return command and shared-agent instructions crates/buzz-cli/src/commands/agents.rs, crates/buzz-cli/src/commands/agents_sharing_tests.rs, crates/buzz-cli/src/lib.rs, crates/buzz-acp/src/runtime.rs, docs/shared-channel-agents.md	The CLI adds agents return, which loads a task and sends its result to the source author. Tests check the published event and reject a human-authored mention. Runtime instructions and documentation describe return behavior.
Return-aware ancestry bounds crates/buzz-sdk/src/sharing/policy.rs, crates/buzz-sdk/src/sharing/policy/tests.rs, voltage/channel-agent-sharing.md	The ancestry walk allows up to twice the handoff limit. Tests cover additional handoff-and-return links, and the sharing contract documents the bound.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Return as AgentsCmd::Return
  participant Lookup as incoming_task
  participant Query as Event query
  participant Submit as send_handoff
  participant Publisher as Event submission
  Return->>Lookup: Load task by event ID
  Lookup->>Query: Query for matching kind 9 or 40002 event
  Query-->>Lookup: Return matching event
  Lookup-->>Return: Provide task
  Return->>Submit: Send result to source author
  Submit->>Publisher: Sign and submit handoff

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Possibly related PRs

• voltagecloud/buzz#115: Adds the shared-agent grant contract that this PR extends with handoff returns.
• voltagecloud/buzz#119: Defines task ownership and requester provenance used to validate and route returns.
• voltagecloud/buzz#110: Introduces shared-task provenance and handoff construction extended by this PR.

Suggested labels: human review wanted

Suggested reviewers: thebrandonlucas

Merge Risk: Minimal · up to 691e7

No confirmed merge-blocking issue is identified. Complete the planned end-to-end coordinator and shared-agent check before release.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: allowing a handoff target to return its result to the delegating agent.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 10 functions across 8 files. (2 skipped: 2 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Review info

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• Source mode: Auto
• Repository: voltagecloud/buzz
• Branch: feat/agent-handoff-return
• File: .coderabbit.yaml
• Commit: 691e7aa
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Commits

Reviewing files that changed from the base of the PR and between dbe955d and 691e7aa.

Files selected for processing: 10

• crates/buzz-acp/src/runtime.rs
• crates/buzz-cli/src/commands/agents.rs
• crates/buzz-cli/src/commands/agents_sharing_tests.rs
• crates/buzz-cli/src/lib.rs
• crates/buzz-sdk/src/sharing/budget.rs
• crates/buzz-sdk/src/sharing/budget/tests.rs
• crates/buzz-sdk/src/sharing/policy.rs
• crates/buzz-sdk/src/sharing/policy/tests.rs
• docs/shared-channel-agents.md
• voltage/channel-agent-sharing.md

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