fix(trace-store): merge _update span patches on FileSystemTraceStore.load

When a fresh FileSystemTraceStore opens a directory written by a prior
process, load() calls appendSpan() for every NDJSON row — including the
{ ...patch, _update: true } fragments that updateSpan writes. Those
fragments lack kind / runId / name / model, so the reader ended up with
two spans per id (full row + fragment), breaking any consumer that
re-opens the store cross-process (e.g. tax-agent's canonical eval OTLP
converter, which then reported 0 spans to the trace analyst).

The runs branch already handled this via appendRun-then-updateRun
fallback. Mirror the same pattern for spans: detect \_update and route
to updateSpan, merging into the prior span. New regression test reads
the dir from a second FileSystemTraceStore instance and asserts the
patch fields are applied without losing kind / runId / name.

Author: Drew Stone

src/trace/store.ts

@@ -268,7 +268,25 @@ export class FileSystemTraceStore implements TraceStore {
               await store.updateRun(record.runId, record)
             }
           } else if (base === 'spans') {
-            await store.appendSpan(record)
+            // `updateSpan` appends an `_update: true` patch row instead of
+            // rewriting the original span. On reload we must collapse those
+            // patches into the prior span — otherwise a fresh
+            // FileSystemTraceStore reading the same dir reports duplicate
+            // spans (one full, one fragment with no runId/kind/name), which
+            // breaks any downstream consumer that re-opens the store
+            // cross-process (e.g. the canonical eval's OTLP converter).
+            if (record?._update) {
+              try {
+                await store.updateSpan(record.spanId, record)
+              } catch {
+                // Patch row arrived before the original — should not happen
+                // with locked append order, but fall through to append so we
+                // don't lose data.
+                await store.appendSpan(record)
+              }
+            } else {
+              await store.appendSpan(record)
+            }
           } else if (base === 'events') {
             await store.appendEvent(record)
           } else if (base === 'artifacts') {

tests/trace-store.test.ts

@@ -139,6 +139,37 @@ describe('FileSystemTraceStore', () => {
     expect(spans).toHaveLength(1)
     expect(spans[0].endedAt).toBe(2)
   })
+
+  it('cross-process reload merges _update span patches into the original span — regression: a fresh FileSystemTraceStore opened on a written dir reported each span twice (the original row + the patch fragment with no runId/kind/name), which broke OTLP conversion downstream (canonical eval analyst saw 0 spans)', async () => {
+    const dir = await makeDir()
+
+    // First process: write run, append a full LLM span, then patch it via updateSpan.
+    const writer = new FileSystemTraceStore({ dir })
+    await writer.appendRun(makeRun('r1'))
+    await writer.appendSpan({
+      runId: 'r1',
+      spanId: 's1',
+      kind: 'llm',
+      model: 'claude',
+      messages: [{ role: 'user', content: 'hi' }],
+      name: 'turn-1',
+      startedAt: 1,
+    })
+    await writer.updateSpan('s1', { endedAt: 5, status: 'ok', output: 'merged-output' } as Partial<Span>)
+
+    // Second process: fresh store reads the dir from scratch (forces load()).
+    const reader = new FileSystemTraceStore({ dir })
+    const spans = await reader.spans({ runId: 'r1' })
+    expect(spans).toHaveLength(1)
+    const [s] = spans
+    expect(s.spanId).toBe('s1')
+    expect(s.runId).toBe('r1')           // must survive the merge (patch row has no runId)
+    expect(s.kind).toBe('llm')           // must survive the merge (patch row has no kind)
+    expect(s.name).toBe('turn-1')        // must survive the merge (patch row has no name)
+    expect(s.endedAt).toBe(5)            // applied from patch
+    expect(s.status).toBe('ok')          // applied from patch
+    expect((s as { output?: string }).output).toBe('merged-output')
+  })
 })
 
 describe('TraceEmitter', () => {