/ost-builder

name: ost-builder
description: "Use to build or update an Opportunity Solution Tree from research data. Never from brainstorming."
metadata:
  instruction_budget: "47"
  framework_dependency: "mycelium"
  framework_dependency_note: "This skill is designed to run within the Mycelium framework (https://github.com/haabe/mycelium). Standalone use will skip the canvas state, theory gates, and harness behavior the skill assumes. Install: /plugin install mycelium@haabe-mycelium."

OST Builder Skill

Build and maintain Opportunity Solution Trees from research evidence.

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. Claude Code's Read-before-Write check requires the `Read` tool specifically — `cat`/`head`/`grep` via Bash do NOT satisfy it.

**Edit vs Write — different cost profiles** (verified 2026-05-14):

• **`Edit`** (exact-string replacement): `Read` with `limit: 1` satisfies the check at ~50 tokens. State-tracking is per-file, not per-byte — subsequent `Edit` calls work anywhere in the file. Use this for partial updates against large canvas files (e.g., `purpose.yml` at 800+ lines).
• **`Write`** (full replacement): do a **full Read** first. Write obliterates the file; you should see what you're about to replace. The `limit:1` shortcut is *not* appropriate here.

**ID-bearing entries — scan the ID space before assigning** (added 2026-05-15, v0.23.19): When adding a new component, opportunity, solution, or any other ID-bearing entry to a canvas file, run a Bash grep first to confirm the next ID in your prefix sequence is actually free:

grep -o "<prefix>-[0-9][0-9]*" .claude/canvas/<file>.yml | sort -u -t- -k2 -n | tail -3

Replace `<prefix>` with the canvas's ID prefix (`comp` for landscape, `opp` for opportunities, `sol` for solutions, `ht` for human-tasks, etc.). Then pick the next free integer, **matching the zero-padding already used in that file**. The sort is NUMERIC (`-t- -k2 -n`) rather than lexical, and that is not pedantry: a plain `sort -u` orders `ht-1` after `ht-080`, so on a canvas with inconsistent padding it reports the wrong maximum and the next ID collides. Verified on the dogfood repo 2026-08-13, where lexical sort returned `ht-1` as the highest human-task ID against an actual `ht-080`. `grep -o` is also deliberate: it matches IDs wherever they appear, including cross-references and prose, so an ID that was promised somewhere but not yet defined is not handed out twice. `validate_canvas.py` has a per-file duplicate-ID check (it reports `duplicate id '<id>'`) that catches the failure on CI, but a duplicate can persist in the working tree for days if CI isn't run between edit and discovery; that happened on 2026-05-15, when a duplicate ID was created in `landscape.yml`.

Original failure mode: anti-pattern #7 instance #5, 2026-05-09 — agent conflated Bash `head` with the Read tool, lost ~14k tokens to a Write-fail → remedial-full-Read → re-Write loop. The `limit:1` discipline (graduated 2026-05-14, v0.23.18) prevents the second-order cost where the agent *correctly* follows the rule but full-Reads every time. The ID-scan discipline (graduated 2026-05-15, v0.23.19) prevents the related class where the agent reads enough of the file to satisfy the Edit check but not enough to see existing ID assignments — kin to anti-pattern #8 (Stale State Read).

If this skill writes to multiple canvas files, register each one first (limit:1 for Edit-only paths; full Read for Write paths) AND ID-scan any prefix you intend to assign.

See `${CLAUDE_PLUGIN_ROOT}/engine/agent-operating-contract.md` *Canvas writes — Read before Write* for the canonical rule.

Workflow

Building a New OST

1. **Define the desired outcome** at the top of the tree. This comes from the north star metric or current strategic goal.

**If the outcome you are about to write is NOT a north-star input, say so in the root** via `north_star_input_ref: off_north_star` rather than leaving it blank. A blank reads as on-strategy by default, and an OST rooted on a metric the project does not steer by will faithfully optimise the wrong thing — the tree stays busy while the stuck thing stays stuck.

**When a second kind of outcome appears — typically the project's own users surfacing opportunities alongside internal/dogfood ones — add a SECOND ROOT rather than starting a second FILE.** Move `desired_outcome` to a `desired_outcomes:` list, give each root an `id`, and tag every opportunity with `rolls_up_to: <root id>`. A separate file breaks every `opportunities.yml#opp-NNN` reference pointing into this canvas, and is read by no script, gate or render — the built-not-wired failure. `validate_canvas.py` enforces that each `rolls_up_to` resolves; untagged opportunities in a multi-root file are an error, not a default-to-first.

2. **Review all research data**: Interview transcripts, behavioral data, analytics, observation notes — and, for a project with a public issue tracker, its open issues and discussions (v0.211.0). Issues are invited, structured statements of the opportunity space by the people who have it; the dogfood reader classified 15 of 53 open issues across two repos into actionable shapes on its first run (artifact requests, invited feedback, open questions, design decisions) while a Reddit scout of the same shape produced zero engage candidates in 46 runs. Honest limits: 72% of issues came back unclassified, so the tracker is research data for a human to read, not an automatic tree; and a reader is not a user.

3. **Extract opportunities** (unmet needs, pain points, desires):

• Each opportunity must cite at least 2 evidence sources.
• Each opportunity minted on or after 2026-09-15 carries `what_would_move_it`: a task id, a leaf id with a named test, a sweep the landing check runs, or a dated review (v0.2