Scaffold Cost Check

Measure the token cost of Mycelium's stable scaffolding — the context surface loaded at SessionStart and on routine canvas/memory reads — so claims about "negligible overhead" are auditable. Datadog's State of AI Engineering (2026) reports ~69% of input tokens across production agents are system prompts; this skill makes Mycelium's analogous number visible.

When to Use

• First-time audit on a project to compare actual scaffold cost against any "~6K negligible" claim in landscape/positioning.
• Periodic refresh via /mycelium:framework-health (see Wiring below).
• Before adding a new canvas file or harness doc that joins the stable load surface, to surface the marginal cost.

Preflight: Read target canvas file(s) before any Write/Edit

Hard rule. Before issuing Write or Edit against any .claude/canvas/*.yml, use the Read tool on that file in this session. This skill only WRITES if explicitly asked to persist its output to dora-metrics.yml#apex.scaffold_token_estimate; default behavior is print-only.

Workflow

1. Inventory the stable scaffold surfaces in the current project's plugin cache (or fall back to .claude/ for legacy installs):
   • CLAUDE.md (project-local; the dispatcher)
   • ${CLAUDE_PLUGIN_ROOT}/engine/ (all .md and .yml)
   • ${CLAUDE_PLUGIN_ROOT}/harness/ (all .md)
   • AGENTS.md (project-local, if present — the router for non-Claude-Code runtimes; not a plugin file. It lives at a repo root and is never packaged, so resolving it through the plugin cache always missed and the row silently vanished from the table via the "if present" clause. Report 0 / absent explicitly when the project has none, rather than omitting the row — an omitted row reads as "not part of the surface" instead of "not there")
   • .claude/canvas/ (project state — not framework, but it ALSO joins the load surface; report separately)
   • .claude/memory/ (corrections.md, patterns.md, MEMORY.md)
2. Sum bytes per surface via wc -c.
3. Convert bytes → tokens via the 4-bytes-per-token heuristic (±15% for English markdown across OpenAI/Anthropic tokenizers). Document the heuristic + the uncertainty band in the output.
4. Render the output block (see Output below).
5. Optional: persist to canvas if the user passes --write. Path: .claude/canvas/dora-metrics.yml#apex.scaffold_token_estimate. Follow the Postflight discipline below.

Postflight: Verify-After-Write (write-narration-verification discipline)

If --write was passed, before claiming "✅ persisted to dora-metrics.yml", use the Read tool on dora-metrics.yml after the edit to confirm apex.scaffold_token_estimate.total_tokens actually carries the new number. AP#7 instance #18 (2026-06-05) is the worked failure mode this discipline prevents.

Output

## Mycelium Scaffold Cost Audit

**Method**: byte-count ÷ 4 (heuristic; ±15% for English markdown).
**Measured**: <date>

| Surface | Bytes | Est. tokens |
|---|---|---|
| CLAUDE.md (project) | <b> | <t> |
| Plugin engine/ | <b> | <t> |
| Plugin harness/ | <b> | <t> |
| AGENTS.md | <b> | <t> |
| Canvas (.claude/canvas/) | <b> | <t> |
| Memory (.claude/memory/) | <b> | <t> |
| **Total stable surface** | <B> | **<T>** |

**Honest framing**: compare against any documented "~6K negligible" or similar claim in landscape/positioning. If actual is materially above (>2×), surface as a positioning correction.

**Goodhart pair**: scaffold_token_estimate ↔ first-pass-success-rate. Cutting scaffold to lower the number is only a win if first-pass quality doesn't drop. Track the pair.

**Sources**:
- [S5] Datadog, *State of AI Engineering* (2026): ~69% input tokens are system prompts across production agents.
- [S2] Faros, *Harness Engineering* (2026): staged measurement plan — start with metrics whose raw data exists.

Canvas Output (only if --write)

Target: .claude/canvas/dora-metrics.yml#apex.scaffold_token_estimate

apex:
  scaffold_token_estimate:
    claude_md_tokens: <n>
    engine_docs_tokens: <n>
    harness_docs_tokens: <n>
    agents_md_tokens: <n>
    canvas_tokens: <n>
    memory_tokens: <n>
    total_tokens: <n>
    method: "byte-count ÷ 4 (heuristic; ±15%)"
    last_measured: <date>
    compared_against_claim: "<found / not found>; <within / above / below>"

Decision Log (MANDATORY if --write)

If persisting to canvas, append a decision-log entry referencing this skill's run, the measured total, and the comparison-against-prior-claim outcome. Source-tag with [S5] Datadog.

Wiring

• Standalone: invoke as /mycelium:scaffold-cost-check (print-only) or /mycelium:scaffold-cost-check --write (persist).
• Trend tracking via /mycelium:framework-health: that skill's "Goodhart pair" check may reference this surface once a baseline exists. Not auto-invoked in v0.39.19 (sequenced for next minor); operator invokes manually until then.

Rules

• Print-only by default; --write is opt-in.
• Heuristic-tokens carry ±15% explicitly in the output — do not narrate as exact.
• If the actual number is materially above a prior documented "negligible" claim, surface as a positioning correction back to landscape.yml or wherever the prior claim lived. Source-tag [S5].

Theory Citations

• Datadog 2026 ([S5]): production runtime telemetry — system prompts dominate input tokens; scaffolding cost is real and frequently unmeasured.
• Faros 2026 ([S2]): staged measurement plan — compute from existing raw data before adding new instrumentation.
• Goodhart's Law: a token-count metric becomes a target once named; the counter-metric (first-pass-success) is required to prevent quality degradation in pursuit of token reduction.