[opieter-aws]

Description

When a session-backed graph is serialized mid-execution, serialize_state runs from the per-node AfterNodeCallEvent hook — so it fires while sibling nodes in the same parallel batch are still executing. An in-flight sibling is in none of the terminal sets (completed / interrupted / failed), and _is_node_ready_for_resume treated any such untouched edge source as a bypassed dead branch. In a fan-in (AND-join), that dropped the in-flight parent's edge and marked the join node ready off its already-completed parent alone. On restore, the persisted frontier contained both the still-pending parent and the join node, so the join ran twice — its first run seeing incomplete inputs — duplicating side effects.

The fix distinguishes a live pending parent from a genuinely bypassed branch. _compute_pending_sources seeds from the nodes actually executing at serialize time (execution_status == EXECUTING) plus completed nodes, then walks traversable edges forward to collect every not-yet-completed node that will run on resume. A fan-in node stays out of the resume frontier while any such parent is outstanding. The executing-node seed is the authoritative in-flight signal and covers siblings with no completed ancestor (e.g. two concurrent entry points feeding one join), which forward reachability from completed nodes alone would miss.

Related Issues

Requirement for #3283

Documentation PR

N/A

Type of Change

Bug fix

Testing

Ran the multiagent unit suite (tests/strands/multiagent/), which covers the graph resume/serialize frontier logic, the interrupt-resume path, and the conditional-skip/bypass cases that must remain unaffected. Added regression coverage for both in-flight-sibling shapes (diamond with a shared root, and two independent entry points), each as a full serialize → deserialize → resume round-trip that captures a real mid-flight snapshot; both fail against the pre-fix code (frontier ['right', 'join'], join runs twice) and pass with the fix.

Ran the suite, ruff check, ruff format --check, and mypy on the changed files directly rather than via hatch run prepare, which fails in this environment building an unrelated optional cedar Rust dependency.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Assessment: Approve

Well-reasoned fix for a subtle mid-flight-serialize concurrency bug. The distinction between a live pending parent and a bypassed dead branch is correct, and the change is strictly more conservative — adding pending_sources to live_sources can only add edges to the AND-join check, so it can never mark a fan-in ready incorrectly. Verified locally: the 4 new tests fail against main (frontier ['right', 'join']) and pass with the fix; full test_graph.py suite (77) passes; ruff and mypy are clean.

Review Categories

• Correctness: Sound. The EXECUTING-status seed is valid because _compute_ready_nodes_for_resume runs only at serialize time (in-memory state), and it correctly covers entry-point siblings with no completed ancestor.
• Code quality: Extracting _is_edge_traversable removes the prior duplicated short-circuit logic; docstrings thoroughly capture the reasoning and tradeoffs.
• Testing: Strong — both fan-in shapes covered as full serialize → deserialize → resume round-trips with genuine mid-flight snapshots and call-count assertions; complements existing bypass/deadlock resume tests without duplication.
• API: Private methods only — no public surface change, so API bar-raising does not apply.

Nicely scoped change with regression coverage that provably guards against the double-run.