Closes J's 2026-05-02 multi-call observability gap: a single
/v1/iterate session with N retries used to surface in Langfuse as
N+1 disconnected traces (one per /v1/chat hop + one for the iterate
request itself), with no parent/child linkage. Operators couldn't
scroll the retry chain in one trace tree to spot where grounding
failed.
## Wire-level change
- New header constant `shared.TraceIDHeader = "X-Lakehouse-Trace-Id"`
- `langfuseMiddleware` honors the header on inbound requests: if
set, reuses that trace id instead of minting a new one. Stashes
the trace id on the request context so handlers can attach
application-level child spans.
- `validatord.chatCaller` forwards the header to chatd. Every chat
hop in an iterate session lands as a child of the parent trace.
## Application-level spans
- `validator.IterateConfig` gains `Tracer` (optional callback).
When wired, each iteration attempt emits one Langfuse span
via `validator.AttemptSpan`:
Name: iterate.attempt[N]
Input: { iteration, model, provider, prompt }
Output: { verdict, raw, error }
Level: WARNING when verdict != accepted
- `validatord.iterTracer` is the production hook — bridges
`validator.Tracer` → `langfuse.Client.Span`.
- `IterateRequest`/`IterateResponse`/`IterateFailure` gain
`TraceID`; each `IterateAttempt` gains `SpanID`. The /v1/iterate
caller can pivot from the JSON response straight into the
Langfuse trace tree.
## What an operator sees post-cutover
GET /v1/iterate {kind=fill, prompt=...} → Trace TR-1
├─ http.request span (from middleware)
├─ iterate.attempt[0] span (validator.Iterate emit)
│ input: prompt+model
│ output: { verdict: validation_failed, error: ..., raw }
├─ chatd /v1/chat call (X-Lakehouse-Trace-Id: TR-1)
│ ├─ http.request span (chatd middleware)
│ └─ chatd-internal spans (existing)
├─ iterate.attempt[1] span
└─ ...
All in one Langfuse trace tree, not N+1 separate traces.
## Hallucinated-worker safety net is unchanged
The /v1/iterate flow's hard correctness gate is still
FillValidator + WorkerLookup. Phantom candidate IDs raise
ValidationError::Consistency which 422s and forces the iteration
loop to retry. The trace-id propagation is the OBSERVABILITY layer
on top — it makes the existing safety net's outcomes visible per-call,
not a replacement for it.
## Verification
- internal/validator: 4 new tests
- TestIterate_TracerEmitsSpanPerAttempt — span/attempt count + SpanID
- TestIterate_NoTraceIDSkipsTracer — no orphan spans without trace_id
- TestIterate_ChatCallerReceivesTraceID — propagation contract
- (existing iterate tests updated for new ChatCaller signature)
- internal/shared: 1 new test
- TestLangfuseMiddleware_HonorsTraceIDHeader — cross-service linkage
- cmd/validatord: existing HTTP tests still PASS via the dual-shape
UnmarshalJSON contract.
- validatord_smoke.sh: 5/5 PASS through gateway :3110 (unchanged).
- Full go test ./... green across 33 packages.
## Architecture invariant added
STATE_OF_PLAY "DO NOT RELITIGATE" gains a paragraph documenting
the X-Lakehouse-Trace-Id header contract + the iterate.attempt[N]
span emission. Future-Claude won't re-propose "wire trace-id
propagation" — the header IS the wiring.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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