fix: preserve unbounded heartbeat compatibility

[khaliqgant]

Summary

• keep nullable node.heartbeat.load decoding in place for the coordinated Relaycast rollout
• temporarily emit legacy numeric load: 0 when max_agents == 0, so deployed Relaycast engines accept the whole heartbeat
• add a producer serialization regression test covering load and active_agents
• correct the changelog: remove the unshipped claim from 11.4.2 and describe the compatibility fix under Unreleased

Root cause

Relay #1445 began omitting load for unbounded providers. Fleet E2E pins Relaycast v6.0.1, which rejects heartbeats without a numeric load. Spawn and brief delivery still complete, but the entire heartbeat is discarded, so active_agents and load never update. This is the same wire-contract boundary addressed by Relaycast PR #307; it is a product compatibility defect, not a stale harness assertion.

The decisive comparison used the same local Node 22.14.0, Relaycast commit 58a9342, and release broker build:

• 1ae49812 (post-feat: verify AgentWorkforce persona spawns #1464, pre-Report unbounded node load as unmeasured #1445): Fleet E2E exit 0, 14/14 passed
• 0b43dbe3 (post-both): Fleet E2E exit 1, 12/14 passed; five-agent heartbeat and least-loaded placement both timed out with last=null

Validation

• patched Fleet E2E against pinned Relaycast v6.0.1: 3 consecutive passes / 0 failures, exit codes 0, 0, 0, 14/14 each
• full broker suite: exit 0; 868 unit tests passed, 4 ignored; all broker integration tests passed
• focused heartbeat tests: exit 0; 5 passed
• cargo fmt --all -- --check: exit 0
• git diff --check: exit 0

Release sequencing

Relaycast PR #307 must merge and deploy before Relay can safely switch unbounded heartbeat load back to omitted/null. Until then, numeric emission preserves active-agent and liveness updates across currently deployed engines without weakening Fleet E2E.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 2d358aaa-106f-4f2f-a04e-0a15e9dba548

📥 Commits

Reviewing files that changed from the base of the PR and between 0b43dbe and a1c2c5c.

📒 Files selected for processing (3)

• CHANGELOG.md
• crates/broker/src/fleet_wire.rs
• crates/broker/src/node_control.rs

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📝 Walkthrough

Walkthrough

Unbounded-capacity fleet heartbeats now report load: 0.0 while preserving active_agents. Documentation and changelog entries describe compatibility with older engines and the temporary numeric-load requirement.

Changes

Fleet heartbeat compatibility

Layer / File(s)	Summary
Heartbeat load compatibility contract crates/broker/src/fleet_wire.rs, CHANGELOG.md	The documentation and changelog describe numeric load reporting during the rollout.
Numeric load generation and validation crates/broker/src/node_control.rs	Unbounded heartbeats emit 0.0, and tests verify the serialized load and active agent count.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• AgentWorkforce/relay#1139: Updates the same heartbeat load handling and fleet load publication logic.
• AgentWorkforce/relay#1445: Revises optional NodeHeartbeat.load behavior and unbounded heartbeat generation.
• AgentWorkforce/relay#1473: Directly addresses Fleet heartbeat compatibility for unbounded nodes.

Suggested reviewers: willwashburn

Poem

A rabbit checks the heartbeat flow,
Zero load makes old engines glow.
Active agents still march in line,
The wire stays steady, clear, and fine.
Compatibility hops ahead! 🐇

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary heartbeat compatibility fix.
Description check	✅ Passed	The description explains the change, root cause, release sequencing, and validation results; it omits the template headings for Test Plan and Screenshots.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/fleet-heartbeat-regression

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.