# @deepseek-ai/dsh-token-meter

Replay-aware token measurement through the singleton `ctx.tokenMeter` service. It advances one isolated fold per session from the durable log, so compaction and other pressure-sensitive plugins can share accounting without depending on `CompactService`.

## Configuration

| Key | Default | Contract |
|---|---:|---|
| `contextWindow` | `128000` | Positive integer service-wide context capacity. |

The estimator intentionally uses one fixed heuristic: four characters per token plus structural overhead for roles, blocks, and request-envelope fields. `contextWindow` is the only deployment setting. Direct construction validates it; Loader mounts first apply the package's Schemastery shape validation. Unrecognized top-level keys are rejected.

## Measurement contract

`ctx.tokenMeter` directly exposes two operations:

- `measure(session, requestHeader?)` returns request pressure and the current priced surface at one consumed-log revision.
- `estimateMessage(message)` prices one message with the fixed heuristic.

`measure()` synchronizes once and returns one detached, deeply immutable snapshot. `totalTokens` is request-and-response pressure, while `surfaceTokens` is the surface-only heuristic total and equals the sum of `nodes[].tokens`. A `requestHeader` override affects pressure fields only; the surface fields still describe the current session. Every call clones the positional nodes, so measurement is O(surface).

The fold tracks full request-header snapshots, step boundaries, surface appends and replacements, successful assistant messages, provider usage, and assistant-chunk provenance. Provider usage is reused only when the latest successful call's canonical request envelope matches the measured envelope and its total is no lower than that call's full heuristic anchor; a later success replaces the earlier anchor. Otherwise the complete current envelope and surface are estimated. Surface changes remain signed relative to a matching anchor, including negative deltas after shrinking replacements.

Usage accounting sums disjoint input, cache-read, cache-write, and output buckets; reasoning is not added again. Every successful call records an assistant anchor, including content-less calls. An explicit empty provenance list means a known empty provider stream, while absent legacy provenance conservatively treats the durable assistant output as provider output.

## Composition

```yaml
- name: '@deepseek-ai/dsh-token-meter'
- name: '@deepseek-ai/dsh-compact-basic'
```

Both plugins have usable defaults. A deployment with a different capacity configures the meter once:

```yaml
- name: '@deepseek-ai/dsh-token-meter'
  config:
    contextWindow: 32768
```

## Model Experience

Indirectly, through consumers such as `dsh-compact-basic`; the service itself adds no prompt, message, schema, tool, or model call.

#### KV Cache effect

No direct invalidation; the named consumer owns any request-prefix changes.

## Known Limitations and Deferred Work

- **The fixed heuristic is approximate** — content without reusable provider usage is priced by character count plus structural overhead, not an exact provider tokenizer or request serializer.
- **Every measurement clones the current surface** — coherent immutable snapshots make reads O(surface), including below-threshold pressure checks.
- **Provider usage is only reusable for an identical canonical envelope** — prompt, prefix, tools, provider, model, or call-config changes deliberately fall back to full heuristic estimation.
- **Legacy provenance is conservative** — assistant messages without `sourceEventSeqs` cannot distinguish provider output from listener rewrites, so the fold avoids claiming a known empty or exact chunk stream.
