codegraph/docs/design/callback-edge-synthesis.md

Design + status: general callback / observer edge synthesis

Status: SHIPPED (the synthesizer in callback-synthesizer.ts is merged and on main). This doc records the original design. Motivation: close the dynamic-dispatch hole that static extraction leaves for observer / event-emitter / signal patterns, where a dispatcher invokes callbacks registered elsewhere through a shared store — so flows like "how does an update reach the screen" actually exist in the graph.

Update (2026-06-01): the codegraph_trace and codegraph_context MCP tools were since removed — codegraph_explore is the single surfacing tool now. Its "Flow" section (buildFlowFromNamedSymbols) and the codegraph_node trail surface these synthesized edges; the trace(a, b) notation below means "the a→b flow," which you now verify with codegraph_explore / probe-explore.mjs (the probe-trace.mjs / probe-context.mjs dev probes went away with the tools).

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TL;DR for a new session

We synthesize dispatcher → callback edges that static parsing misses. It works:

• Field observer (excalidraw Scene.onUpdate/triggerUpdate): synthesizes triggerUpdate → triggerRender. trace(mutateElement, triggerRender) now = 3 hops.
• EventEmitter (express on('mount', …)/emit('mount')): synthesizes use → onmount.
• Precision is high: excalidraw got 1 synthesized edge out of 27k (the correct one); node count moved +3 after Phase 3 (no explosion).

Files touched (all uncommitted on main):

• src/resolution/callback-synthesizer.ts — the whole-graph synthesis pass (Phase 1 + 2).
• src/resolution/index.ts — calls synthesizeCallbackEdges() at the end of resolveAndPersistBatched() (after base edges are persisted) + the import.
• src/extraction/tree-sitter.ts — visitFunctionBody now extracts named nested functions (Phase 3), so inline named handlers become linkable nodes.

How to reproduce / test:

npm run build
rm -rf /tmp/codegraph-corpus/excalidraw/.codegraph
( cd /tmp/codegraph-corpus/excalidraw && codegraph init -i )
# synthesized edges (provenance='heuristic', metadata.synthesizedBy in {callback,event-emitter,…,http-client,queue-job,event-bus}):
sqlite3 /tmp/codegraph-corpus/excalidraw/.codegraph/codegraph.db \
  "select s.name||' → '||t.name||'  '||coalesce(e.metadata,'') from edges e \
   join nodes s on e.source=s.id join nodes t on e.target=t.id where e.provenance='heuristic';"
# end-to-end flow (the synthesized edge shows up in explore's Flow section + node trail):
node scripts/agent-eval/probe-explore.mjs /tmp/codegraph-corpus/excalidraw "triggerUpdate triggerRender"

Probe scripts (dev-only, in scripts/agent-eval/): probe-node.mjs (symbol + trail), probe-explore.mjs (relevant source + the flow among named symbols). EventEmitter fixture lives at /tmp/cb-fixture/bus.js (ephemeral — recreate or move into __tests__/).

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Cross-tier channels (src/resolution/tier-synthesizer.ts, 2026-08-28)

The web's RN bridge: one pass, registered before the in-process emitter pass (the more specific edge wins a duplicate source>target pair in the merge), gated on JS-family files, never sourced from a test suite or a generated file. Three channels, each keyed on a literal on both sides, each edge kind:'calls', provenance:'heuristic', with synthesizedBy, channel (http | queue | event | socket), tier (client→server / server→client) when the direction is known, the event / queue / method / href it paired on, line + column of the call, and registeredAt = the other side (route registration, decorator, .on):

• http-client — fetch / $fetch / ofetch / axios / ky / got / useFetch / useSWR, <client>.get|post|…( where the receiver is a known client name or a binding made by axios.create(…) / ky.extend(…) (same file or the file it is imported from — resolveImportPath, since import mappings carry no resolved path), with a literal / template first argument (new URL('/x', base) and { url, method } configs read too) → the ONE route METHOD path in the index it denotes. A hole fills a :param / {id} / [id] / catch-all segment and never a literal one; a hole in front of the path (${API_URL}/users) matches by the route's tail; a line a framework resolver made a route node on is a registration, not a client call; a tie between routes is nothing. No fan-out cap — the match is exact.
• queue-job — <queue>.add('job', …) where the queue is named (new Queue('email'), @InjectQueue('email') x, in the file or its import) or queue-shaped → the @Process('job') method of the @Processor('email') class (a WorkerHost's process when there is none), new Worker('email', handler) (an inline handler → the enclosing function, else the enclosing constant), Bull's queue.process('job', handler). Most specific pairing wins (queue+job > job > the queue's default); an unnamed queue pairs only on a unique job name. Fan-out cap 6.
• event-bus — .emit|emitAsync('x') on a bus-shaped receiver (eventEmitter, bus, pubsub, …) → @OnEvent handlers, * / ** globs honoured; on a socket-shaped receiver (socket, io, server, client, .to(room), …) from a file without a socket server → @SubscribeMessage('x') and server-side socket.on('x') (client→server); from a file with one (@WebSocketGateway, io.on('connection'), new Server) → client-side socket.on('x', …), named or inline (→ the enclosing component) (server→client). Plain .on ↔ .emit stays the emitter pass's. Fan-out cap 6.

The Steps view (ui-server/api/steps.ts) reads tier / channel before the languages in crossing(), so a hop between two TS files draws as a bridge (⇢ POST /api/users, a boundary like another screen) or an event (⇠ welcome); explore's Flow section labels them (context/index.ts, mcp/tools.ts). A Next 'use server' action needs no edge: steps.ts marks the call at request time from the directive. Validated on bradtraversy/proshop_mern (30 routes, 23 client→route edges, all correct on inspection, after Express mounts + chained router.route() landed) and nestjs/nest (sample/26-queues, sample/30-event-emitter); test __tests__/ui-steps-cross-tier.test.ts.

The hole

class Scene {
  private callbacks = new Set<Callback>();
  onUpdate(cb: Callback) { this.callbacks.add(cb); }          // REGISTRAR
  triggerUpdate() { for (const cb of this.callbacks) cb(); }  // DISPATCHER
}
this.scene.onUpdate(this.triggerRender);                      // REGISTRATION SITE

The runtime edge triggerUpdate → triggerRender does not exist statically: triggerUpdate's only literal call is cb() (anonymous). Measured: triggerUpdate's only callee was randomInteger; trace(triggerUpdate, triggerRender) returned no path.

Why it's a whole-graph pass, not a FrameworkResolver.resolve()

resolve(ref) answers "what does this named ref point to," one ref at a time. The callback edge has no ref to resolve (cb() is anonymous) and needs cross-file, multi-site correlation (registrar, registration, dispatcher). So it's a whole-graph pass after base resolution, language-level (any OO observer), living in src/resolution/callback-synthesizer.ts — not under frameworks/.

Sibling mechanism for the other dynamic-dispatch class — named attribute/ descriptor dispatch (e.g. django self._iterable_class(...)) — is the claimsReference hook (resolution/types.ts + resolution/index.ts pre-filter)

• a FrameworkResolver.resolve() (django ORM resolver in frameworks/python.ts). That one does fit resolve() because the ref is named. Both are part of the same coverage effort; see the "Related work" section.

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As-built algorithm (and where it diverged from the original design)

Field-observer channels (fieldChannelEdges, Phase 1)

1. Candidates by method/function name — registrar ^(on[A-Z]\w*|subscribe| addListener|addEventListener|register|watch|listen|addCallback)$; dispatcher contains (emit|trigger|notify|dispatch|fire|publish|flush).
2. Confirm by body (read via ctx.readFile + slice node lines): registrar has this.<F>.add|push|set(; dispatcher has for (… of [Array.from(]this.<F>) + a call, or this.<F>.forEach(.
3. Pairing — DIVERGENCE: the design said pair by class; the build pairs by same file + same field F (file as a class proxy — getting the containing class reliably was harder). Works for the common 1-class-per-file case; revisit for multi-class files.
4. Registrations: queries.getIncomingEdges(registrar.id, ['calls']) → for each, read the caller's source at the edge line and regex-recover the arg (<registrarName>\s*\(\s*(?:this\.)?(\w+)). DIVERGENCE: design preferred tree-sitter re-parse; build uses regex (named refs only — arrows/inline args are missed here).
5. Synthesize dispatcher → fn (getNodesByName(arg) → method|function). Capped at MAX_CALLBACKS_PER_CHANNEL = 40.

EventEmitter channels (eventEmitterEdges, Phase 2)

• File-oriented scan (ctx.getAllFiles() + readFile, substring pre-filter on .emit(/.on(/etc). ON_RE = \.(?:on|once|addListener)\(\s*['"]([^'"]+)['"]\s*,\s* (?:function\s+(\w+)|(?:this\.)?(\w+)); EMIT_RE = \.(?:emit|fire|dispatchEvent)\(\s*['"]([^'"]+)['"].
• Dispatcher = enclosing function of the emit('e') call (enclosingFn finds the tightest function/method/component node containing the line). Handler = getNodesByName of the on-handler name.
• Correlate by event-name literal; synthesize dispatcher → handler.
• Precision — DIVERGENCE: design proposed receiver-type matching; build uses an event fan-out cap (EVENT_FANOUT_CAP = 6) — skip events with >6 handlers or dispatchers (generic names like error/change would over-link without type info).

Provenance — DIVERGENCE

Edge.provenance is a fixed enum ('tree-sitter'|'scip'|'heuristic'), so synthesized edges use provenance: 'heuristic' + metadata: { synthesizedBy: 'callback'| 'event-emitter', via/event/field }. The design's 'callback-synthesis' provenance and high/medium/low confidence tiers were NOT implemented — the fan-out cap + registrar-name uniqueness + named-only handlers are the precision guards instead.

Phase 3 — inline callback extraction (tree-sitter.ts)

The real blocker for EventEmitter on real repos: inline handlers (on('mount', function onmount(){})) weren't nodes, so nothing could link to them. Root cause: visitFunctionBody walked through nested functions without extracting them. Fix: in visitForCallsAndStructure, when a body node is a functionType and extractName returns a real name, call extractFunction (which extracts it and walks its own body) and return. Named only — anonymous arrows fall through to the existing recursion (so their inner calls stay attributed to the enclosing fn). This bounded it: excalidraw +3 nodes, no explosion, no regression.

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Validation results (actual)

Repo	Result
excalidraw	1 synthesized edge triggerUpdate → triggerRender (of 27,214); trace(mutateElement, triggerRender) = 3 hops; nodes 9,286 → 9,289
express	after Phase 3: use → onmount {event-emitter, event:"mount"} (onmount now extracted at application.js:109)
/tmp/cb-fixture/bus.js	tick → handleRefresh, persist → handleSave (named-method EventEmitter handlers)
excalidraw / express	no Phase-1 regression; node counts stable

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Remaining work (prioritized for the next session)

1. Anonymous-arrow handlers — on('e', () => foo()) still produce no edge (no node, intentionally not extracted in Phase 3). The fix is synthesizer link-through-body: parse the arrow's body and link dispatcher → (calls inside the arrow). Highest remaining recall win; handles the most common modern callback shape.
2. Wire into resolveAndPersist (incremental sync) — synthesis currently runs only in resolveAndPersistBatched (full index). Incremental re-index won't refresh synthesized edges.
3. Receiver-type matching for EventEmitter precision (replace/augment the fan-out cap) — use type_of edges so x.emit('change') only links to y.on('change', fn) when x,y are the same type. Lets the fan-out cap relax.
4. Tree-sitter arg recovery (replace the regex in field-channel Stage 4) — robust for arrows, multi-arg, line-wrapped calls.
5. Single-callback fields (this.onChange = cb; … this.onChange()) — scalar-store variant of the field observer; not built.
6. Broad precision/recall audit — run across the full corpus; tally synthesized edges per repo, spot-check, confirm no explosion on EventEmitter-heavy repos.
7. Tests + CHANGELOG — the fixture is a ready vitest case for the synthesizer; add extractor tests for Phase 3 (named-nested-fn extraction; confirm other languages unaffected — the change is in the shared walker), resolver tests for the django side.

Edge cases / model

• Over-approximation across instances is accepted (reachability, not instance precision). unregister/off ignored.
• Synthesized edges are additive — never replace static edges; tooling can filter on provenance='heuristic' + metadata.synthesizedBy.

Related work (same coverage effort)

This is one half of closing dynamic-dispatch coverage. The other artifacts on main:

• Named attribute/descriptor resolver: claimsReference (resolution/types.ts, pre-filter in resolution/index.ts) + django ORM resolver (frameworks/python.ts, _iterable_class → ModelIterable.__iter__).
• Retrieval/UX changes (separate from coverage): explore whole-small-file + glue fixes, the explore Flow section (buildFlowFromNamedSymbols), and node-with-trail — all in src/mcp/tools.ts. (codegraph_trace / codegraph_context were later removed; explore is the one surfacing tool.)
• Full investigation context + findings: auto-memory project_codegraph_read_displacement (why coverage — not prompting/hooks/new-tools — is the lever for getting agents to use codegraph over Read).