fix(engine,condition): stop starving a downstream node after a single-branch condition

[graycyrus]

Bug: a single-branch condition silently starves its downstream node

Two coupled bugs made a downstream node (e.g. an agent with input_context:"=item") receive null when its predecessor was a condition:

A1 — engine drops items on a single-branch condition (engine.rs, HandlerRouting::Plain single-edge branch). A condition with only a true edge was lowered as an unconditional add_edge (ignoring from_port), so the successor ran on every execution — but collect_input port-matches true vs the emitted false and drops the items, so the successor ran with empty input. Fix: when the single outgoing edge's from_port != "main", lower it as a conditional edge with an END fallback — a true-only condition now acts as a filter (runs with items on true, terminates to END on false).

A2 — condition node didn't resolve =-expressions (nodes/control_flow/condition.rs). field was read as a raw literal key, so field:"=item.assignee" did get("=item.assignee") → always None → always false. Fix: if field is_expression(), resolve it and check the resolved value's truthiness; non-expression fields keep the literal-key path (backward compatible).

Tests

condition_single_true_edge_filters_on_false, condition_single_true_edge_item_flows_through, expression_in_field_resolves_and_checks_truthiness. Full suite green (cargo test 304 lib + 12 doctests, --all-features e2e all pass).

Summary by CodeRabbit

• New Features

  • Condition steps now accept either a plain field name or an expression-based field value, making routing decisions more flexible.
  • Downstream steps now only run when a condition actually takes the matching branch, preventing unintended execution on skipped paths.

• Bug Fixes

  • Fixed cases where a single outgoing “true” or similar branch could incorrectly continue as if it were unconditional.
  • Prevented empty or null inputs from appearing in downstream branches when a condition is not met.

• Tests

  • Added regression coverage for conditional routing and branch behavior with per-item flows.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 196494bc-12a0-49d8-8edb-614069c1d8d0

📥 Commits

Reviewing files that changed from the base of the PR and between eb4865d and 48e51f4.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (2)

• src/engine.rs
• src/nodes/control_flow/condition.rs

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📝 Walkthrough

Walkthrough

Graph lowering in engine.rs now emits conditional edges (instead of unconditional edges) for nodes with a single outgoing edge on a non-"main" port, routing based on the emitted port or terminating at END. ConditionNode's field evaluation now supports =-prefixed expressions resolved via expression scope, alongside existing key lookup. New tests cover both changes.

Changes

Condition Node Filtering Fix

Layer / File(s)	Summary
Expression-based field evaluation src/nodes/control_flow/condition.rs	ConditionNode.execute detects =-prefixed expression fields, resolves them via expression scope, and truthiness-checks the result; plain string fields still use direct key lookup with missing keys treated as null. New test verifies routing for non-empty, empty, and missing resolved values.
Conditional lowering for single non-main edges src/engine.rs	Graph lowering now emits conditional edges (based on the node's emitted port) instead of unconditional edges when a node has exactly one outgoing edge on a non-"main" port, terminating at END on mismatch. New regression tests verify filter-like condition nodes correctly gate downstream execution, including with upstream fan-out.

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

Sequence Diagram(s)

sequenceDiagram
  participant Engine
  participant ConditionNode
  participant Successor

  Engine->>ConditionNode: execute(item)
  ConditionNode->>ConditionNode: resolve field (expression or key lookup)
  ConditionNode-->>Engine: emitted port ("true"/"false")
  Engine->>Engine: compare emitted port to edge.from_port
  alt port matches edge
    Engine->>Successor: run with item
  else port does not match
    Engine->>Engine: terminate to END
  end

Loading

Poem

A hop, a check, a truthy field,
No more phantom runs revealed,
Ports now matched with careful care,
Filters filter, fair and square,
This rabbit thumps with pure delight! 🐰✅

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

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