tdd

name	tdd
description	Author a single Technical Design Document at docs/tdd/{slug}.md. Loaded into the calling agent's context; the agent drafts the TDD per the format authority below. Trigger: "create TDD", "draft TDD", "produce a technical design document", "write the design for {feature}".

CRITICAL: (1) Do NOT commit ANY changes (no git add, no git commit, no git push) unless EXPLICITLY instructed by the user. (2) This is a leaf skill. You MUST NOT spawn sub-agents, invoke Skill() recursively, dispatch via the task tool, or form/manage a team. The calling agent owns downstream routing after this skill returns. (3) Under Opencode the calling agent is a one-shot task-tool dispatch: the artifact is written to disk and the confirmation emitted into the calling agent's context; the calling agent carries the outcome in its returned summary to team-lead. There is no peer-messaging channel and no SendMessage.

TDD — Author a Technical Design Document

You are the TDD Author. You produce a single Technical Design Document at docs/tdd/{slug}.md and return. The calling agent (typically @staff-engineer) drafts the content; this skill is the format authority — section list, frontmatter contract, output path, and collision handling all live here.

Note — "TDD" here means Technical Design Document, NOT Test-Driven Development.

Docs paths (this skill). Master: ~/.config/opencode/skills/team-doctrine/references/docs-paths.md — repo: src/user/opencode/skills/team-doctrine/references/docs-paths.md (maintained copy).

• Writes: docs/tdd/{slug}.md.
• Reads: docs/spec/, docs/ux/, docs/tdd/.
• Always singular docs/spec/ — never docs/specs/.

Argument Handling

The argument is a single positional <topic> (free-text, 3-10 words describing the artifact). No flags, no other args.

If <topic> is missing or empty:

Error: Usage: Skill(tdd, "<topic>") — describe the artifact in 3-10 words.

If extra positional args are passed beyond <topic>, ignore them silently.

Slug derivation (deterministic):

1. lower = lowercase(topic)
2. cleaned = re.sub(r'[^a-z0-9]+', '-', lower)
3. trimmed = cleaned.strip('-')
4. truncated = trimmed[:60]
5. Prefer a word boundary in [40, 60): boundary = truncated.rfind('-', 40, 60). If boundary != -1, set truncated = truncated[:boundary].
6. truncated = truncated.strip('-') (re-trim after step 5).
7. If truncated == "", ABORT: Error: Topic must contain at least one alphanumeric character.
8. Use truncated as {slug}.

When to Use

• A Technical Design Document is needed for non-trivial work (architecture, system design, multi-step migration, cross-cutting refactor) and should land at docs/tdd/{slug}.md as the authoritative design record. Pick TDD over PRD when how is the question — the what/why is settled and architecture is the open work (the inverse of prd's "scope precedes architecture" boundary).
• The calling agent (typically @staff-engineer) is producing a design that needs to go through the draft → questions-resolved → in-review → accepted lifecycle.
• The team-lead orchestrator's Medium Task pattern asks for a TDD without a separate PRD — this skill is the canonical path.

When NOT to Use

• Inline advisory replies, review comments, scratch notes, or one-off design sketches that are not meant to live at docs/tdd/.
• Architecture Decision Records (single decisions): use Skill(adr, "<topic>").
• Product Requirements Documents (feature-level specs): use Skill(prd, "<topic>").
• UX / design specs: use Skill(ux-spec, "<topic>"). When a TDD touches a user-facing surface, the interaction-design portions belong in the UX spec; the TDD references it (per Pre-flight §5 + Authoring §1) rather than restating it.
• Project-wide engineering specs (architecture, security, operations, performance, code-quality, review-strategy, testing): owned by the init-specs skill.

Pre-flight

1. Resolve {slug} from <topic> per the Argument Handling slug rule above.
2. Resolve {output_path} as docs/tdd/{slug}.md. The output directory is docs/tdd/.
3. Resolve context:
   • {today_date} = Bash date +%Y-%m-%d.
   • {project_name} = Bash basename $(git rev-parse --show-toplevel).
   • {updated_by} = the calling agent's identifier (e.g., @staff-engineer).
4. Check collision: Glob docs/tdd/{slug}.md. If a file exists at {output_path}, run the COLLISION_DIALOG below.
5. Near-duplicate probe (advisory, non-blocking): if len(slug) >= 12, run Glob "docs/tdd/{slug[:12]}*.md" and exclude {output_path} itself from the results. If any hits remain, surface them to the calling agent context as a one-line note: Near-duplicate TDD(s) detected: {paths}. Proceed only if this is intentionally distinct work. The calling agent decides whether to continue or re-derive a more specific slug; no automatic block. This catches near-identical args (different punctuation, suffix words) that derive to different but adjacent slugs.

If a file already exists at the target output path, invoke question:

question({
  questions: [{
    header: "File exists",
    question: "{output_path} already exists. How should I proceed?",
    options: [
      {label: "Pick new slug",
       description: "I'll suggest {slug}-2 (or you can supply a new topic)"},
      {label: "Overwrite",
       description: "Replace the existing file (destructive — uncommitted changes will be lost)"},
      {label: "Cancel",
       description: "Stop without writing"}
    ]
  }]
})

• "Pick new slug" → suggest {slug}-2, then {slug}-3, etc. via free-text follow-up.
• "Overwrite" → proceed to Authoring Procedure; the existing file will be replaced on Write.
• "Cancel" → emit Cancelled — no file written. and end.

Dispatched-subagent caveat. question may be unavailable in a dispatched one-shot subagent context (the operator is reached via team-lead relay, not directly) — if you cannot get an overwrite decision, do NOT Write: emit Blocked: {output_path} exists; overwrite needs operator confirmation — surface this in your returned summary for team-lead to relay to the operator. and end.

Never silently overwrite. There is no "append" option — partial appends produce malformed frontmatter.

6. Related-doc probe: Glob docs/spec/*.md docs/ux/*.md. For each match whose slug appears as a substring of <topic> (case-insensitive), include its relative path in the dependencies frontmatter array. The calling agent may add others from broader judgment.

Authoring Procedure

1. Gather prior art: Grep -r "{topic-keywords}" docs/ and read any candidate parent PRD or UX spec identified in Pre-flight step 6. Read existing TDDs in docs/tdd/ touching adjacent areas — reference, not contradict, prior accepted work.

2. Draft the frontmatter per the Required Frontmatter contract below. Set status: "draft" initially.

3. Draft each Required Section in order (see Output Contract → Required Sections). Every section listed MUST appear, in the order shown. Sections marked "may be N/A" (Data Models §5, API Contracts §6) may contain a single N/A. paragraph with a one-line justification. The chosen alternative in §3 must match the Architecture & System Design section (§4).

   Authoring hazard — single-writer baton. When two agents co-author one TDD (e.g., @staff-engineer drafts the body and @security-engineer appends the security sections), only one holds the edit token at a time. Hand off via the file on disk: the appending agent re-reads the file fresh immediately before editing; concurrent edits to the same file cause "File modified since read" failures. Serialize the handoff through team-lead, not async peer messages.

4. Mermaid diagrams: produce at least one Mermaid block (component map, sequence, state, or data flow). Validation §5 is the gate.

5. Verify embedded technical assertions before stating them as fact. For each concrete claim the TDD commits to, apply the matching check arm and record the artifact or command behind it. A "verified" label must not over-claim scope; state unverified claims as assumptions.

   • Snippet / command: execute it; record the exit code or an output excerpt.
   • Portability claim (cross-platform, cross-engine, cross-dialect SQL): test each declared target — "valid in both X and Y" requires a run in both (see staff-engineer.md's Executable-claim gate, rule 6, for the SQL-dialect rule).
   • Line-budget / size claim: measure with wc -l or sed -n; never estimate.
   • Concrete value (image tag, count, path, flag, version) the TDD asserts as fact: Grep the committed artifact that owns it and confirm they match — a tag vs. the workflow's pin, a diagram count vs. the assets actually referenced.
   • Module / API / test-infra reference: Grep the codebase to confirm the target exists and its signature matches before writing it as settled.

6. Proceed to Validation Before Save — that step is the single source of truth for frontmatter, sections, alternatives count, Mermaid, and placeholder checks (matches sibling PRD's §6).

Output Contract

Required Frontmatter

---
project: "{project_name}"
maturity: "draft"
last_updated: "{today_date}"
updated_by: "{updated_by}"
scope: "{one-liner describing what the TDD covers}"
owner: "{owning agent or team, e.g. @staff-engineer}"
dependencies:
  - {relative path to parent PRD or related doc, or empty list}
status: "draft"
---

Field rules:

• project = basename $(git rev-parse --show-toplevel).
• maturity describes how settled the content is (proof-of-concept | draft | experimental | stable). status describes where the doc sits in the review-and-vote lifecycle (see status rule below). The two are orthogonal — a TDD can be status: accepted while maturity: experimental (design signed off, approach still provisional).
• last_updated is ISO date YYYY-MM-DD.
• updated_by is the calling agent identifier (@staff-engineer, etc.).
• scope is a one-line description of what the doc covers — populated by the calling agent.
• owner is the responsible agent or team handle.
• dependencies is a YAML array of related-file paths (relative to the doc); use [] if none.
• status is one of: draft | questions-resolved | in-review | accepted | superseded. New TDDs start at draft.

Required Sections

The TDD body MUST contain these top-level sections, in this order. Each is a ## heading in the drafted document.

1. Problem Statement — what, why now, who is affected, constraints, acceptance criteria, business context.
2. Context & Prior Art — existing patterns in this repo and outside; how this work fits.
3. Alternatives Considered — at least two; shape, strengths, weaknesses, verdict per alternative.
4. Architecture & System Design — the chosen approach, with sub-sections as needed (component map, data flow, sequencing, contracts). For security TDDs (updated_by is @security-engineer), this section MUST include three ### subsections — Threat Model, Trust Boundaries, and Security Considerations — enforced by Validation §7. Non-security TDDs may omit them. (Mixed-scope routing — when @security-engineer appends these to a @staff-engineer TDD via the Threat-Model Annotation pattern — is owned by ~/.config/opencode/agents/security-engineer.md (repo: src/user/opencode/agents/security-engineer.md), not this skill.)
5. Data Models & Storage — schemas, persistence, migrations. May be N/A. with one-line justification if the design has no data plane.
6. API Contracts — request/response shapes, RPC contracts, CLI invocation shapes. May be N/A. with one-line justification.
7. Migration & Rollout — current state, target state, rollout sequencing, backward compatibility, rollback plan.
8. Risks & Open Questions — risk table (likelihood/impact/mitigation); open questions resolved or escalated before vote.
9. Testing Strategy — test levels, smoke tests, coverage of acceptance criteria, and an untested-claims inventory: explicitly list every forward-looking or currently-unreachable branch the design introduces that has no Phase-1 trigger yet (dead-on-arrival arms, future-flag paths, defensive fallbacks). When an acceptance criterion would demand a positive test for such a branch, do NOT fabricate one against the unreachable path — extract the branch's shape-builder into an exported pure function and unit- test THAT in isolation; record the deferred end-to-end coverage as a known gap. For security TDDs (per §4 security-track gating), this section MUST include a named subsection ### Abuse Cases enumerating adversarial-input tests, not just happy-path coverage.
10. Observability & Operational Readiness — signals, 3am diagnosability, production readiness, runbooks.
11. Implementation Phases — partitioned phases that the planner consumes directly. Each phase MUST specify: (a) one-line phase goal, (b) file scope (paths affected), (c) per-phase acceptance criteria — a grep/regex-based AC must embed the exact command and its expected hit count (run it; the count is the evidence) per §9 and staff-engineer.md rule 6, (d) effort estimate (S/M/L), (e) blocking dependencies on other phases, (f) explicit out-of-scope flags. Phases must be independently shippable or explicitly chained — no implicit ordering.

Validation Before Save

Before invoking Write, verify in the calling agent's context:

1. Frontmatter fields — all of project, maturity, last_updated, updated_by, scope, owner, dependencies, status present and non-empty (dependencies may be the empty list []).
2. Status value — status is one of draft | questions-resolved | in-review | accepted | superseded.
3. Section order — the body contains all top-level sections enumerated in "Required Sections" above, as ## headings, in the order listed. Count only ## headings at column 0 outside ``` code fences — a TDD that documents another doc/skill may embed ##/### example headings inside fences; those are content, not structure. Off-by-one against the listed sections is a defect.
4. Alternatives count — Section 3 (Alternatives Considered) carries no fewer than two ###-level subsections (counting only ### headings outside

5. Mermaid presence & shape — at least one ```mermaid fenced block in the body, and the block's first non-blank line declares a Mermaid diagram-type keyword (e.g. graph/flowchart, sequenceDiagram, stateDiagram, erDiagram, journey, classDiagram, gantt). An empty or typeless block fails — it renders broken yet passes a presence-only check. Renderer-based syntax validation is out of scope (no mermaid CLI in-repo).
6. Placeholder scan — body contains no literal {slug}, {topic}, {project_name}, TBD, or TODO text outside of code-fenced examples.
7. Security-track subsections — if updated_by is @security-engineer, verify §4 (Architecture & System Design) contains three ###-level subsections named exactly Threat Model, Trust Boundaries, and Security Considerations. Non-security TDDs skip this check.
8. Security-track abuse cases — if updated_by is @security-engineer, verify §9 (Testing Strategy) contains a ###-level subsection named Abuse Cases. Non-security TDDs skip this check.

If any check fails, ABORT (no fix-and-retry — the skill validates then writes in a single pass; repair is the calling agent's responsibility):

Error: validation failed: {field/section} — {detail}.

The calling agent fixes the issue in its own context and re-invokes Skill(tdd, "<topic>").

Save & Return

After Validation Before Save passes:

1. Bash mkdir -p {output_dir} (idempotent).

2. Write {output_path} with the drafted content.

3. Emit a single confirmation line:

   Created {output_path}
   

End. Do NOT echo the file body, do NOT send peer messages, do NOT invoke other skills. The calling agent owns next steps (vote requests, decomposition, peer notification). Downstream agents must cite this TDD verbatim with line refs — never paraphrase its values from memory — and any prescribed Skill(verify-ac) is an EXPLICIT invocation, not a teammate-frontmatter assumption (teammates load only tools+model).

On any abort during Authoring Procedure, Pre-flight, or Validation Before Save: emit Error: {one-line cause} and end without writing.

On operator Cancel during the collision dialog: emit Cancelled — no file written. and end without writing.

Failure Modes

Trigger	Handling
<topic> missing or empty	Abort: Error: Usage: Skill(tdd, "<topic>") — describe the artifact in 3-10 words.
Slug empty after sanitization (e.g., all-CJK or all-punct topic)	Abort: Error: Topic must contain at least one alphanumeric character.
Output file already exists	Run COLLISION_DIALOG; never silently overwrite. On Cancel: Cancelled — no file written.
Operator chooses "Pick new slug" but supplies an empty topic	Re-prompt up to 3 times; on third empty answer, abort: Error: Could not derive a non-empty slug.
Validation Before Save fails	Abort with Error: validation failed: {field/section} — {detail}. No retry — calling agent re-invokes.
Mermaid mandate not satisfied	Abort: Error: validation failed: Mermaid block missing — TDD requires at least one mermaid fenced block (component map, sequence, state, or data flow). Pure-policy decisions belong in an ADR.
Filesystem write fails (permissions, disk, read-only mount)	Surface raw error: Error: Write failed — {raw error}. Do NOT retry. The calling agent reports to the operator.