# The 2020 WCF string-handle wall (2026-06-20)

Live-probing the local **Historian 2020** (WCF, port 32568) for HCAL roadmap M1
surfaced a clean structural boundary on what the pure-managed client can call. It
explains why R1.1/R1.4/R1.5 all fail and identifies the single RE target that
unblocks the rest of the M1 read surface.

## The dichotomy

Retrieval/Status/History ops split by the **type of their first (handle) parameter**:

| Handle type | Examples | Status on 2020 WCF |
|---|---|---|
| **`uint` client handle** (Open2 output) | `StartQuery2`, `GetNextQueryResultBuffer2`, `IsOriginalAllowed`, `GetTagInfosFromName`/`GetTagInfoFromName` (GetTgByNm), `GetSystemParameter`, `StartEventQuery`, `GetNextEventQueryResultBuffer`, `RegisterTags2`, `EnsureTags2`, `UpdateClientStatus3` | ✅ **work** — the proven read/browse/metadata/status-param/event/write surface |
| **`string` GUID handle** | `ExecuteSqlCommand` (ExeC), `StartTagQuery` (QTB), `QueryTag` (QTG), `GetHistorianInfo` (GETHI), `GetTagExtendedPropertiesFromName` (GetTepByNm), `GetTagInfosFromName2` (GetTgByNm2), `GetTagidsByTagnameAndSource` | ⛔ **blocked** — native error type 4, code **51 (InvalidParameter)** or **1 (Failure)** |

## Evidence (this probe + prior notes)

- **ExeC** → type 4 / code 51 for every handle variant (storageGuid, contextGuid).
  Matches `implementation-status.md` ~982 / ~1404 ("StartTagQuery depends on earlier
  native session/filter registration … do not wire through guessed calls").
- **GETHI** (`HistorianVersion` param query — the *exact* native request shape from
  `BuildGetHistorianInfoRequest`, with `Stat.GetV ×2` priming) → type 4 / code **1**
  for all five handle formats tried: storage-session GUID, context GUID, uint as
  decimal, uint as `X8` hex, uint as `0x`-hex. In the only place GETHI is used (the
  event-priming chain) its result is wrapped in `TryRun` and **discarded**, so there
  was never evidence it actually returns data from the managed client.
- **GetTepByNm / QTB / QTG / GetTgByNm2** all take a `string handle` → same family.

## Why

The string-handle ops are keyed off a **native-side session/filter registration**
that the C++ client performs but the managed replay does not reproduce. The uint
client handle is the Open2 session token the server already trusts; the string GUID
handle indexes a *different* per-service registration table that stays empty unless
the native priming is replicated faithfully. `Stat.GetV ×2` alone is insufficient.

## Consequence for the roadmap

Every remaining **M1 read** item is a string-handle op:

- R1.1 `ExecuteSqlCommandAsync` (ExeC) — blocked
- R1.4 `GetHistorianInfoAsync` (GETHI) — blocked
- R1.5 extended-property read (GetTepByNm) — blocked (string handle, confirmed)
- R1.6 localized-property read — same family

So **M1 read-surface completion on 2020 WCF is gated entirely behind one RE target:
the native session/filter registration for string-handle ops.** Reverse-engineer it
once and the whole family unlocks. Until then, the alternatives are:

1. **RE the registration** — instrument the native `CRetrievalConnectionWCF` /
   `CStatusConnectionWCF` priming between Open2 and the first successful string-handle
   call (capture-tier; the highest-leverage single RE task for M1).
2. **2023 R2 gRPC server** — these ops are first-class on the gRPC front door, where
   the handle/envelope differs and the registration wall may not apply.

Do **not** ship any string-handle op via guessed calls (project discipline:
"leave them throwing until evidence supports an implementation").