Let a nested behavior without a start node run from On start (#230)

A behavior built as a plain behavior, with On start and no Behavior
start, did nothing when another behavior called it through Run
behavior. The node only showed a warning. Such a behavior should run
as a call too, without the author having to rebuild its entry.

- New callEntries() in nested.ts returns the Behavior start node or,
  when there is none, every On start node. callProblem uses it too.
- GraphInterpreter.call() follows all entries one after the other,
  in graph order, in the same frame and with the same variables.
- Behavior end still returns what reaches it. A call through On start
  has no inputs, because only the start node declares them.
- It is a fallback only. With a Behavior start present, On start
  nodes stay out of the call.
- The node warning now reads "No start or On start node - does
  nothing" and only shows when neither exists.
- Keep and local state machines behave as before.

Tests:
- ./check.sh: 163 files, 2396 tests, CHECK EXIT 0 (before 2392).
- 4 new unit tests in tests/core/nested-behavior.test.ts: the flow
  after On start runs, the return value arrives, several On start
  nodes run in graph order, a start node keeps On start out.
- e2e 19: ALL PASSED, 35 checks. Only the expected warning text
  changed, the check sum stays at 1385.
- Counter-checks: without the fallback 3 tests fail. With only the
  first On start exactly the order test fails.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/graph/GraphInterpreter.ts

@@ -1,6 +1,6 @@
 import { edgesInto, edgesOutOf, type Graph, type GraphNode } from './Graph';
 import { type GraphLookup, NO_LOOKUP } from './lookup';
-import { BEHAVIOR_END, BEHAVIOR_START, resolveDef, terminalNode } from './nested';
+import { BEHAVIOR_END, callEntries, resolveDef, terminalNode } from './nested';
 import type { PropertyBag } from '../model/Properties';
 import type { NodeContext } from './NodeContext';
 import {
@@ -159,46 +159,47 @@ export class GraphInterpreter {
   }
 
   /**
-   * Runs another behavior from its start node to its end node and returns what the end node was
-   * handed. The nested graph gets its own frame and a fresh copy of its variable defaults, so it is
-   * a call and not an inlining: nothing of it survives the return but the values it hands back –
-   * and the variables it keeps, which wait for the next call from the same site.
+   * Runs another behavior from its start node (or, lacking one, its `On start` nodes) to its end
+   * node and returns what the end node was handed. The nested graph gets its own frame and a fresh
+   * copy of its variable defaults, so it is a call and not an inlining: nothing of it survives the
+   * return but the values it hands back – and the variables it keeps, which wait for the next call
+   * from the same site.
    */
   private call(call: CallResult, site: string, ctx: NodeContext, depth: number): Record<string, Value> {
     const program = this.lookup.behavior(call.behaviorId);
     if (!program) return {};
     if (this.callStack.includes(call.behaviorId)) throw new Error(`Behavior "${program.name}" runs itself`);
-    const start = terminalNode(program.graph, BEHAVIOR_START);
-    if (!start) return {};
+    const entries = callEntries(program.graph);
+    if (entries.length === 0) return {};
     const sites = [...this.callSites, site].join('/');
     const kept = program.kept ?? [];
     const variables = resumedVariables(program.defaults, this.keptCalls.get(sites), kept);
     if (kept.length > 0) this.keptCalls.set(sites, variables);
     const outer = this.frame;
     this.frame = emptyFrame(program.graph);
-    this.frame.outputCache.set(start.id, { ...call.inputs });
-    this.record(start.id, call.inputs);
+    for (const entry of entries) {
+      this.frame.outputCache.set(entry.id, { ...call.inputs });
+      this.record(entry.id, call.inputs);
+    }
     this.callStack.push(call.behaviorId);
     this.callSites.push(site);
     this.calledBehaviors.add(call.behaviorId);
-    try {
-      this.trace?.onNode(start.id);
     // Fresh states every call, and nothing of the caller's variables or states. What it does to a
     // local machine therefore dies with the call and is not reported; a `global` machine is the
     // run's, so moving that one is a move like any other.
-      this.follow(
-        start,
-        'exec',
-        {
+    const inner: NodeContext = {
       ...ctx,
       variables,
       states: {},
       stateChanged: (machineId, from, to) => {
         if (machineId in ctx.shared.states) ctx.stateChanged(machineId, from, to);
       },
-        },
-        depth + 1,
-      );
+    };
+    try {
+      for (const entry of entries) {
+        this.trace?.onNode(entry.id);
+        this.follow(entry, 'exec', inner, depth + 1);
+      }
       const end = terminalNode(program.graph, BEHAVIOR_END);
       return (end && this.frame.outputCache.get(end.id)) ?? {};
     } finally {

src/core/graph/nested.ts

@@ -20,6 +20,7 @@ export const BEHAVIOR_END = 'BehaviorEnd';
 export const RUN_BEHAVIOR = 'RunBehavior';
 export const VARIABLES = 'Variables';
 export const PROPERTIES = 'Properties';
+export const ON_START = 'OnStart';
 
 export const BEHAVIOR_PARAM = 'behavior';
 export const VARIABLES_PARAM = 'variablesAsset';
@@ -92,6 +93,15 @@ export function terminalNode(graph: Graph, type: string): GraphNode | undefined
   return graph.nodes.find((n) => n.type === type);
 }
 
+/**
+ * Where a call into `graph` begins: its start node, or – for a behavior that has none – every
+ * `On start` in it, in graph order, as when the behavior starts on its own (#230).
+ */
+export function callEntries(graph: Graph): GraphNode[] {
+  const start = terminalNode(graph, BEHAVIOR_START);
+  return start ? [start] : graph.nodes.filter((n) => n.type === ON_START);
+}
+
 export function isTerminalType(type: string): boolean {
   return type === BEHAVIOR_START || type === BEHAVIOR_END;
 }
@@ -267,7 +277,7 @@ export function callProblem(node: GraphNode, lookup: GraphLookup, ownerId = ''):
   if (id === ownerId) return 'A behavior cannot run itself';
   const program = lookup.behavior(id);
   if (!program) return 'That behavior is gone';
-  if (!terminalNode(program.graph, BEHAVIOR_START)) return 'No start node – does nothing';
+  if (callEntries(program.graph).length === 0) return 'No start or On start node – does nothing';
   return null;
 }
 

src/core/graph/nodes/flow/RunBehavior.ts

@@ -4,16 +4,17 @@ import { BEHAVIOR_END, BEHAVIOR_PARAM, BEHAVIOR_START, declaredPorts, RUN_BEHAVI
 import { exec } from '../shared';
 
 /**
- * Runs another behavior's graph in place, from its start node to its end node. Its ports are those
- * the named behavior declares; the interpreter does the call, because only it can hold a second
- * graph's pulse.
+ * Runs another behavior's graph in place, from its start node (or its `On start` nodes, #230) to
+ * its end node. Its ports are those the named behavior declares; the interpreter does the call,
+ * because only it can hold a second graph's pulse.
  */
 export const RunBehavior: NodeDef = {
   type: RUN_BEHAVIOR,
   label: 'Run behavior',
   category: 'flow',
   description:
-    'Runs another behavior as a step of this one, passing values in and taking results out. A ' +
+    'Runs another behavior as a step of this one, passing values in and taking results out. It ' +
+    'begins at the Behavior start node, or at On start if that behavior has none. A ' +
     'variable marked Keep in that behavior carries on from the last call of this node on this ' +
     'object; the others start at their defaults every time.',
   inputs: [exec()],

tests/core/nested-behavior.test.ts

@@ -198,7 +198,7 @@ describe('a call that cannot do anything says so', () => {
     call.params[BEHAVIOR_PARAM] = 'gone';
     expect(problem()).toBe('That behavior is gone');
     call.params[BEHAVIOR_PARAM] = makeBehavior(fixture, 'Empty').id;
-    expect(problem()).toBe('No start node – does nothing');
+    expect(problem()).toBe('No start or On start node – does nothing');
     call.params[BEHAVIOR_PARAM] = callee.id;
     expect(problem()).toBe(null);
     // Only a call can be dead this way; no other node type may be marked.
@@ -245,6 +245,78 @@ describe('a call that cannot do anything says so', () => {
   });
 });
 
+describe('a called behavior without a start node begins at On start (#230)', () => {
+  /** A caller whose On start runs `callee` and logs "after" once the call is done. */
+  function logCaller(fixture: ProjectFixture, callee: BehaviorAsset) {
+    const caller = makeBehavior(fixture, 'Caller');
+    const edit = graphEditor(fixture, caller);
+    const onStart = edit.addNode('OnStart');
+    const call = edit.addNode(RUN_BEHAVIOR, { [BEHAVIOR_PARAM]: callee.id });
+    edit.wire([
+      [onStart, 'exec', call, 'exec'],
+      [call, 'exec', edit.addNode('Log', { message: 'after' }), 'exec'],
+    ]);
+    return { caller, call };
+  }
+
+  it('runs the flow behind its On start, and the call is not marked dead', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Plain');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'sub' }), 'exec');
+    const { caller, call } = logCaller(fixture, callee);
+
+    expect(callProblem(call, graphLookup(fixture.project), caller.id)).toBe(null);
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['sub', 'after']);
+  });
+
+  it('still hands back what its end node was given', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Seven');
+    const cEdit = graphEditor(fixture, callee);
+    const onStart = cEdit.addNode('OnStart');
+    const end = cEdit.addNode(BEHAVIOR_END);
+    fixture.stack.execute(new AddPortCommand(callee.id, end.id, { key: 'out1', kind: 'number' }));
+    cEdit.wire([
+      [onStart, 'exec', end, 'exec'],
+      [cEdit.addNode('Number', { value: 7 }), 'value', end, 'out1'],
+    ]);
+    const caller = makeBehavior(fixture, 'Caller', ['result']);
+    const edit = graphEditor(fixture, caller);
+    const call = edit.addNode(RUN_BEHAVIOR, { [BEHAVIOR_PARAM]: callee.id });
+    const set = edit.variables(['result'], { write: true });
+    edit.wire([
+      [edit.addNode('OnStart'), 'exec', call, 'exec'],
+      [call, 'exec', set, 'exec'],
+      [call, 'out1', set, 'result'],
+    ]);
+
+    expect(runOnStart(fixture, caller, { result: 0 }).ctx.variables.result).toBe(7);
+  });
+
+  it('runs every On start, in graph order, as the behavior would on its own', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Twice');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'one' }), 'exec');
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'two' }), 'exec');
+    const { caller } = logCaller(fixture, callee);
+
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['one', 'two', 'after']);
+  });
+
+  it('is only a fallback: with a start node, its On start stays out of the call', () => {
+    const fixture = makeProject();
+    const callee = makeBehavior(fixture, 'Both');
+    const cEdit = graphEditor(fixture, callee);
+    cEdit.connect(cEdit.addNode(BEHAVIOR_START), 'exec', cEdit.addNode('Log', { message: 'start' }), 'exec');
+    cEdit.connect(cEdit.addNode('OnStart'), 'exec', cEdit.addNode('Log', { message: 'on start' }), 'exec');
+    const { caller } = logCaller(fixture, callee);
+
+    expect(runOnStart(fixture, caller).ctx.logs).toEqual(['start', 'after']);
+  });
+});
+
 describe('declared ports', () => {
   it('renaming one rewrites the edges in this graph and in every calling one', () => {
     const fixture = makeProject();

tools/e2e/19-nested-behavior.mjs

@@ -252,7 +252,7 @@ export async function nestedBehavior(t) {
   const problem = await evaluate(`__gm.graph.nodeProblem(${JSON.stringify(emptyCall?.id ?? '')})`);
   check(
     'and the node says why it does nothing, instead of failing silently',
-    problem === 'No start node – does nothing',
+    problem === 'No start or On start node – does nothing',
     `problem=${JSON.stringify(problem)}`,
   );
   check(