Mycelium is a MIT Claude Code plugin that structurally gates implementation on a discovery contract: the agent cannot create a new source file until four questions (what is the problem, who has it, what are you assuming, what would show you wrong) are answered into YAML the repo keeps. **The gate is hooks, not prose: it holds the agent's first source file until the brief exists, and the brief grades its own evidence (`validated: false`, `strength: speculation`), so the spec records how much is known, not only what is planned.** ## What it is A plugin installed through Claude Code's marketplace (`/plugin marketplace add haabe/mycelium`) from Håvard Bartnes (haabe), with the files and skills documented as porting to OpenCode, Codex, and Cursor. `/mycelium:start` sets up the `.claude/` state and asks the four questions once per repo, writing `.claude/canvas/purpose.yml` and `jobs-to-be-done.yml`, a decision log at `.claude/harness/decision-log.md`, and delivery diamonds at `.claude/diamonds/active.yml`. Sixty-three optional skills carry the method above the gate, grouped across six layers from purpose (L0) to market (L5) and named for their theories: Jobs To Be Done, Wardley maps, Cynefin, DORA, Opportunity Solution Trees. The standing cost is about six thousand tokens of context per session, always on. ## Status Active and shipping fast: 47 stars, 5 forks, created 2026-04-07, pushed 2026-10-16, as of 2026-10-16 (GitHub API). Pre-1.0 by the author's own framing, with schema and skill names moving between minor versions, and the release train runs in days: v0.326.0 on 2026-10-16 after four releases on 2026-10-15 alone. The adoption record is small and mostly first-hand: three of the steward's own projects, a three-person test cohort in May, and one stranger on record whose Lisp-interpreter canvas is public. **The independent footprint is empty, and that is this note's critical angle: a Hacker News search returns only the maintainer's own Show HN (2026-07-04, 1 point, 0 comments) plus an earlier post he says missed the Show HN tag, with no third-party coverage anywhere, so at 47 stars this admission rides the HermesGate-at-3-stars precedent rather than any membership floor.** Star History Chart ## Strengths - **The gate is structural**: harness hooks hold the agent's first new source file until the brief exists, which no advisory prompt convention can promise, and a repo that already has a dozen source files is asked once, ever. - **The artifact grades its own evidence**: `validated` and `strength` fields, with the public vibe-rant canvas marking its own idea at 0.15 confidence, give the spec an epistemic state the other members' artifacts lack. - The steward publishes his failures: the killed macOS file-viewer receipt, and the stated falsifier that you should uninstall it and say so if the brief never once changes what you build across three projects. - Leaving is clean: uninstall leaves the canvas as plain YAML that reads without the plugin, skip acks exist for untracked work, and the agent cannot write either without asking. ## Cautions - The four questions run once per repo, by the author's own admission: nothing brings you back in November to check the assumptions you wrote in August, so it buys one good discovery pass unless you supply the loop. - **Zero independent coverage**: one self-published Show HN thread, no write-ups anywhere, and an adoption record that is nearly all the steward's own, the profile that has kept thinner records out of other categories. - The gate leaks by design: scaffolder and clone commands still write files past it, hand-committed code is unchecked unless setup adds the git check, and code declared prototype is free. - Pre-1.0 churn is explicit, Claude Code turns auto-update on only for its own marketplaces by default, and the always-on token cost shows up on long sessions. ## Pricing Does not apply: MIT, free, no paid tier. The author states that if a business case exists underneath all this it is still worth giving away, and that he is not looking hard for one. ## Compared to - [Intent-Router](../intent-router/index.md): the nearest gate-kind member, and the difference is the scope of the contract; Intent-Router compiles one request into a typed spec and checks the delivery against it, Mycelium compiles the product's purpose into an evidence-graded brief and holds the first file on it, so choose Intent-Router for per-request clarification and Mycelium for repo-level discovery discipline. - [OpenSpec](../openspec/index.md): the brownfield spec-writing member; OpenSpec's delta proposals govern change to code that exists, Mycelium's canvas governs whether the thing should exist at all. - [BMad Method](../bmad-method/index.md): the sized-ceremony method; BMad runs a team process with roles and retrospectives, Mycelium is one solo founder's gate with 63 skills you may never invoke. ## Bottom line **Recommended for solo builders starting agent-built products who want the discovery questions structurally in the agent's way, at the price of ten minutes up front and about six thousand tokens a session.** Not for teams needing a stable interface (pre-1.0, schema moves between minors), throughput work with the decision already made (the author routes you elsewhere himself), or anyone who requires independent validation before adopting. ## Changes - 2026-10-16 - Created from the awesome-claude-code addition the cross-category scanner flagged, admitted under the Intent-Router gate-kind precedent (a spec contract is spec-driven even when tracker-adjacent), with the zero-coverage record as the critical angle. ## See also - [Intent-Router](../intent-router/index.md) - the nearest gate-kind member, request-scoped where Mycelium is product-scoped - [OpenSpec](../openspec/index.md) - the brownfield spec ledger, change-scoped where Mycelium is existence-scoped - [BMad Method](../bmad-method/index.md) - the team-sized method this solo gate contrasts with - [Spec Driven Development Feature Matrix](../spec-driven-development-feature-matrix/index.md) - the category comparison this note joins - [HermesGate](../../evaluation-review/hermes-gate/index.md) - the 3-star gate whose admission precedent this note rides ## References - https://api.github.com/repos/haabe/mycelium - 47 stars, 5 forks, MIT, topics including spec-driven-development, created 2026-04-07, pushed 2026-10-16, as of 2026-10-16 - https://raw.githubusercontent.com/haabe/mycelium/main/README.md - the gate design, artifact model, token cost, falsifier, six layers, porting, and when-to-use-something-else routing - https://raw.githubusercontent.com/haabe/mycelium/main/docs/get-started.md - install, the four questions, artifact paths, gate behavior, and the skip-ack files - https://raw.githubusercontent.com/haabe/mycelium/main/docs/skills/README.md - the 63-skill index, layer model, and theory attributions - https://api.github.com/repos/haabe/mycelium/releases - the release train: v0.326.0 on 2026-10-16, four releases on 2026-10-15 - https://github.com/haabe/vibe-rant - the steward's public canvas repository (0 stars, 1 commit) - https://raw.githubusercontent.com/haabe/vibe-rant/main/.claude/canvas/purpose.yml - the evidence fields (`validated: false`, `strength: speculation`) in the published artifact - https://raw.githubusercontent.com/dagfinndybvig/minilisp/main/.claude/canvas/gist.yml - the one stranger's public canvas, grounding the third-party-use record - https://raw.githubusercontent.com/hesreallyhim/awesome-claude-code/main/README.md - the listing entry naming the steward and the four-question gate - https://hn.algolia.com/api/v1/search?query=mycelium%20claude&hitsPerPage=8 - the footprint check: only the maintainer's Show HN at 1 point and 0 comments, the zero-coverage record