OpenSpec-aware Authoring (Claude Code plugin)

This skill is a shared sub-procedure invoked by the Chorus stage skills (proposal, develop, yolo) whenever the user wants spec-driven authoring through the OpenSpec CLI. It is opt-in:

• Activates when all three signals hold (see §1): CHORUS_OPENSPEC_MODE is not off, an openspec/ directory exists at the project root, and the openspec CLI is on PATH.
• Otherwise the calling skill falls back to its existing free-form behavior.

When you reach a point in proposal / develop / yolo where this skill is referenced, read the value of CHORUS_OPENSPEC_ACTIVE from the SessionStart context (see §1) and branch on it. Do not re-run the detection block — the SessionStart hook has already done it once for this session.

§1. Detection — already done at SessionStart

The Chorus plugin's SessionStart hook (bin/on-session-start.sh) computes CHORUS_OPENSPEC_ACTIVE once when the session opens and writes a ## OpenSpec Mode section into the plugin's injected context. The value of CHORUS_OPENSPEC_ACTIVE is 1 only when all three of these hold:

1. CHORUS_OPENSPEC_MODE is not set to off (explicit opt-out wins).
2. The project root contains an openspec/ directory (i.e. someone ran openspec init here).
3. The openspec CLI is on PATH.

Both signals (2) and (3) are required because the OpenSpec authoring path needs the working directory and the CLI — having one without the other leaves the workflow unrunnable. If signal (2) holds but (3) does not, the SessionStart hook surfaces a "OpenSpec repo detected — install with: npm i -g @fission-ai/openspec" hint to the user; the agent should pass this through if asked rather than silently choosing free-form.

How to read the value

You should already see something like this in your context (look for the ## OpenSpec Mode section near the top of the conversation):

## OpenSpec Mode

CHORUS_OPENSPEC_ACTIVE=1 (openspec/ directory + openspec CLI both present)

or:

## OpenSpec Mode

CHORUS_OPENSPEC_ACTIVE=0 (no openspec/ directory at /path/to/repo/openspec)

Branch:

• CHORUS_OPENSPEC_ACTIVE=1 → follow §3 (OpenSpec authoring).
• CHORUS_OPENSPEC_ACTIVE=0 → return to the calling skill's free-form path. Do not scaffold openspec/changes/. Do not add the slug line to the proposal description.

Manual fallback

If you're in a sub-shell, sub-agent, or session that did not see SessionStart context (e.g. you were spawned mid-session and the parent's context was not forwarded), reconstruct the value yourself with the same three checks:

bash
if [ "${CHORUS_OPENSPEC_MODE:-}" = "off" ]; then
  CHORUS_OPENSPEC_ACTIVE=0
elif [ ! -d "${CLAUDE_PROJECT_DIR:-$PWD}/openspec" ]; then
  CHORUS_OPENSPEC_ACTIVE=0
elif ! openspec --version >/dev/null 2>&1; then
  CHORUS_OPENSPEC_ACTIVE=0
else
  CHORUS_OPENSPEC_ACTIVE=1
fi

Use this only when SessionStart context is genuinely unavailable — duplicating the detection is wasteful when the hook already computed it.

§2. ⛔ Two non-negotiable rules

Both are enforced at review time. Both have caused incidents in past releases.

Rule 1 — Mirror via the wrapper, never re-type document content from agent output

Document/draft mirror calls (chorus_pm_add_document_draft, chorus_pm_update_document_draft, chorus_pm_update_document) MUST go through:

chorus-api.sh mcp-tool <tool_name> "$PAYLOAD"

chorus-api.sh is on PATH — call it by name.

with $PAYLOAD built using json_encode_file (defined in §3.4). Calling these tools directly from the agent's MCP harness with a hand-typed content field is a protocol violation for OpenSpec mode and will fail review. Reasons:

1. Token cost. Re-typing a multi-thousand-line markdown body through the LLM burns input + output tokens for every draft. The wrapper streams bytes through jq -Rs '.' — content never enters LLM context. A typical 3-doc proposal mirror via the script costs roughly zero content-tokens; via direct MCP it routinely costs 20k+.
2. Byte-equality. jq -Rs '.' is a byte-faithful encoder: backslashes, quotes, newlines, code-fence content, zero-width chars all survive. LLM re-emission has a non-zero failure rate on long markdown — table alignment drifts, fence escapes get "fixed", long URLs wrap. The exact byte-equality guarantee holds only on the wrapper path.
3. Single source of truth. With the wrapper, the local openspec/changes/<slug>/*.md is authoritative and Chorus is a mirror. With agent re-typing, authority splits between local file and whatever the LLM happened to output — a future diff cannot tell which one is correct.

Rule 2 — Halt on error via chorus_check_response

Every wrapper call must check three signals: wrapper exit code, "error": in body, empty body. Bare RC=$? is insufficient — the wrapper exits 0 on HTTP 401 (auth failure) with empty body, so a single-signal check silently misses the most common runtime failure. See §6 for the helper definition.

§3. OpenSpec mode authoring

3.1 Pick a slug

openspec/changes/<slug>/ is the local change folder. The slug must be:

• kebab-case (add-export-csv, not addExportCsv or add_export_csv),
• derived from the source Idea title,
• unique within openspec/changes/.

Record it for later steps:

bash
SLUG="add-export-csv"

3.2 Scaffold the change folder

bash
openspec new change "$SLUG" --description "<one-line idea summary>"

This creates openspec/changes/$SLUG/ with README.md and .openspec.yaml. Then author by hand:

| Local file | Purpose | Mirror as Document.type | |---|---|---| | proposal.md | Why + What Changes + Capabilities + Impact | prd | | design.md | Architecture, contracts, risks | tech_design | | specs/<capability>/spec.md | Delta spec (## ADDED Requirements + Scenarios) | spec (one draft per capability) | | tasks.md | OpenSpec tasks list | _(not mirrored — Chorus task drafts are source of truth)_ |

Use openspec instructions <artifact> --change "$SLUG" (artifacts: proposal, specs, design, tasks) for templates.

3.3 Spec file shape (verified against openspec instructions specs)

A delta spec lists one or more block headers — ## ADDED Requirements, ## MODIFIED Requirements, ## REMOVED Requirements, ## RENAMED Requirements — and within each, ### Requirement: entries. Mix freely in the same file; only include the blocks you actually need.

## ADDED Requirements

Append a brand-new Requirement to the long-term spec.

## ADDED Requirements

### Requirement: <name>
<requirement text — use SHALL / MUST for normative behavior>

#### Scenario: <name>
- **WHEN** <condition>
- **THEN** <expected outcome>

## MODIFIED Requirements

Whole-block replacement, not merge. Whatever you write here completely replaces the existing same-named Requirement in the long-term spec — title, description, and all scenarios. Half-writing it deletes the rest.

## MODIFIED Requirements

### Requirement: <existing name>
<full updated requirement text>

#### Scenario: <name>
- **WHEN** <condition>
- **THEN** <expected outcome>

#### Scenario: <other name>
- **WHEN** <condition>
- **THEN** <expected outcome>

Always include every scenario you want the post-archive spec to have, even ones that were already present and unchanged.

## REMOVED Requirements

Delete a Requirement from the long-term spec. The block under the heading is just the requirement name(s) you're removing — no scenarios needed.

## REMOVED Requirements

### Requirement: <existing name>

## RENAMED Requirements

Rename a Requirement's title. Body and scenarios are preserved as-is in the long-term spec; use MODIFIED instead if you need to change anything besides the title.

## RENAMED Requirements

### Requirement: <old name> -> <new name>

Critical formatting rules (verified):

• Scenarios MUST use exactly 4 hashtags (#### Scenario:). 3 hashtags or a bullet list silently fail validation.
• Every ### Requirement: under ADDED or MODIFIED MUST have at least one #### Scenario:.
• MODIFIED blocks MUST include the full updated content — they overwrite, not patch.
• Use SHALL / MUST for normative requirements; avoid should / may.
• The merge into openspec/specs/<capability>/spec.md happens at openspec archive time (§3.9), not at proposal time. While the proposal is in flight, Chorus only sees the delta file as one spec Document — there is no half-merged state for the skill to reason about.

Optional:

bash
openspec validate "$SLUG"

3.4 Helper: json_encode_file

Define once at the top of the authoring session. With jq available it streams the file into a JSON string; the fallback matches chorus-api.sh's own escaping when jq is missing.

bash
json_encode_file() {
  local _path="$1"
  if command -v jq >/dev/null 2>&1; then
    jq -Rs '.' < "$_path"
  else
    local _content
    _content=$(cat "$_path")
    _content=${_content//\\/\\\\}
    _content=${_content//\"/\\\"}
    _content=${_content//$'\n'/\\n}
    printf '"%s"' "$_content"
  fi
}

Round-trip verification is exact: a trailing newline difference is real drift and MUST NOT be normalized or ignored.

3.5 Create the proposal container with the slug provenance line

Use the regular chorus_pm_create_proposal MCP tool (no wrapper required for this single call — the description is short, the LLM-emitted version is fine). The description must carry exactly one line:

OpenSpec change slug: <slug>

• on its own line (no other text on that line),
• literal prefix OpenSpec change slug: (capital O, capital S, single space after colon),
• no trailing punctuation,
• value matches the slug passed to openspec new change.

This line is machine-grep-able by future runs of this skill and by the §3.9 archive trigger.

3.6 Mirror each document draft via the wrapper

> Rule 1 reminder: these calls go through chorus-api.sh, not direct MCP. The agent must not retype the document body.

Define the halt-on-error helper from §6 once at the top, then run one call per file:

bash
# chorus-api.sh is on PATH — no absolute path needed.
# PRD draft
CONTENT=$(json_encode_file "openspec/changes/$SLUG/proposal.md")
PAYLOAD=$(cat <<JSON
{
  "proposalUuid": "$PROPOSAL_UUID",
  "type": "prd",
  "title": "PRD: $HUMAN_TITLE",
  "content": $CONTENT
}
JSON
)
RESULT=$(chorus-api.sh mcp-tool chorus_pm_add_document_draft "$PAYLOAD")
RC=$?
chorus_check_response "chorus_pm_add_document_draft (prd)" "$RC" "$RESULT"
PRD_DRAFT_UUID=$(printf '%s' "$RESULT" | grep -o '"draftUuid"[[:space:]]*:[[:space:]]*"[^"]*"' | head -1 | sed 's/.*"\([^"]*\)"$/\1/')

Repeat with type: "tech_design" for design.md, and one call per capability with type: "spec" for each specs/<capability>/spec.md. Do not mirror tasks.md — Chorus task drafts (created via the chorus_pm_add_task_draft MCP tool, no wrapper needed) are the source of truth for tasks.

> Why parsing uses printf '%s' "$RESULT" | grep not echo "$RESULT" | jq: echo interprets backslash sequences inside the captured JSON, turning embedded \n into a real newline. jq then aborts with Invalid string: control characters from U+0000 through U+001F must be escaped. printf '%s' emits the captured bytes verbatim. Same pattern applies to all wrapper-result parsing in this skill.

3.7 Editing a draft after the first mirror

Local file changes propagate via chorus_pm_update_document_draft — same wrapper, same json_encode_file, same halt check.

bash
CONTENT=$(json_encode_file "openspec/changes/$SLUG/proposal.md")
PAYLOAD=$(cat <<JSON
{
  "proposalUuid": "$PROPOSAL_UUID",
  "draftUuid": "$PRD_DRAFT_UUID",
  "content": $CONTENT
}
JSON
)
RESULT=$(chorus-api.sh mcp-tool chorus_pm_update_document_draft "$PAYLOAD")
RC=$?
chorus_check_response "chorus_pm_update_document_draft" "$RC" "$RESULT"

3.8 Editing a Document after proposal approval

Once the proposal is approved, drafts materialize into Documents with their own UUIDs. To keep openspec/changes/$SLUG/ and the Chorus Document in sync, mirror file edits via chorus_pm_update_document:

bash
CONTENT=$(json_encode_file "openspec/changes/$SLUG/specs/<capability>/spec.md")
PAYLOAD=$(cat <<JSON
{
  "documentUuid": "$SPEC_DOCUMENT_UUID",
  "content": $CONTENT
}
JSON
)
RESULT=$(chorus-api.sh mcp-tool chorus_pm_update_document "$PAYLOAD")
RC=$?
chorus_check_response "chorus_pm_update_document" "$RC" "$RESULT"

To re-derive $SPEC_DOCUMENT_UUID from a fresh shell, look it up via chorus_get_documents for the proposal's project and match by title + type. Re-derive $SLUG by grepping the proposal's description for ^OpenSpec change slug: .

3.9 Archive after the last task is verified

When the LAST task of an OpenSpec-mode idea is admin-verified via chorus_admin_verify_task, the plugin's PostToolUse hook (bin/on-post-verify-task.sh) injects an additionalContext reminder containing the literal substring openspec archive <slug> so you can act without re-reading the slug.

The hook is read-only; you (the agent) perform the archive:

1. Run archive locally. Use --yes for non-interactive mode. Do NOT pass --skip-specs (defeats the mirror-back) or --no-validate (lets malformed deltas corrupt cumulative specs).

bash openspec archive "$SLUG" --yes

This moves openspec/changes/$SLUG/ under openspec/changes/archive/<date>-<slug>/ and emits/updates openspec/specs/<capability>/spec.md for each capability. (Run openspec archive --help against your installed version to confirm the current flag set — flags can drift between releases.)

1. Mirror each updated openspec/specs/<capability>/spec.md back to the matching post-approval Chorus Document (§3.8 contract). chorus_get_documents only supports projectUuid + type server-side filters; filter by title client-side. One chorus_pm_update_document call per capability.

1. Halt on any error from openspec archive or chorus_pm_update_document. Print stderr verbatim, post a comment on the proposal recording the failure (chorus_add_comment with targetType: "proposal", targetUuid: <proposalUuid>), then stop. No retry. Matches §6 "no silent errors." (Comment on the proposal, not the idea: the failure is in archiving proposal-derived specs, and proposals can be inputType: "document" with no idea attached.)

1. Confirm success. Resolve each matching Document UUID and run

verify-document-roundtrip.sh <local-spec-path> <document-uuid>. This performs exact-byte comparison and metadata-only mismatch diagnostics. Do not replace it with recursive jq, head, command substitution, or newline normalization.

Strict opt-in: if the verified task is not the last of its idea, OR the proposal description carries no OpenSpec change slug: <slug> line, OR the local shell has no openspec CLI, the hook exits 0 silently and no archive reminder is injected. Existing free-form behavior is preserved.

§4. Fallback authoring (no openspec)

When detection puts the agent in fallback mode (CHORUS_OPENSPEC_ACTIVE=0), this skill is a no-op. Return to the calling skill's free-form path:

• No openspec/changes/ folder is created or referenced.
• No OpenSpec change slug: … line is added to the proposal description.
• Document drafts are authored via direct MCP chorus_pm_add_document_draft calls with inline content — same as before this skill existed.
• Rule 1 (wrapper-only mirror) does not apply — there is no local file source of truth.
• The §3.9 archive hook does nothing (no slug → silent exit).

§5. Document type mapping (reference table)

| Local file | Chorus Document.type | Mirrored? | |---|---|---| | openspec/changes/<slug>/proposal.md | prd | yes | | openspec/changes/<slug>/design.md | tech_design | yes | | openspec/changes/<slug>/specs/<capability>/spec.md | spec | yes (one draft per capability) | | openspec/changes/<slug>/tasks.md | _(not mapped)_ | no — Chorus task drafts are source of truth |

prd, tech_design, spec are pre-existing valid Document.type values — no schema change required.

§6. Failure visibility — the chorus_check_response helper

There is a known wrapper edge case: when the server returns HTTP 4xx (e.g. 401 from a bad CHORUS_API_KEY), chorus-api.sh mcp-tool captures the JSON-RPC error body internally, pipes it through a .result.content[]? jq filter that produces no output when .result is absent, and exits 0 with empty stdout. A bare RC=$? check would not halt on this — the most common runtime failure mode would be invisible.

Define this helper once at the top of the authoring session and use it after every wrapper call:

bash
chorus_check_response() {
  local _tool="$1"
  local _rc="$2"
  local _body="$3"
  local _has_error=0
  local _is_empty=0

  local _trimmed
  _trimmed=$(printf '%s' "$_body" | tr -d ' \t\n\r')
  [ -z "$_trimmed" ] && _is_empty=1

  if [ "$_is_empty" -eq 0 ]; then
    if command -v jq >/dev/null 2>&1; then
      if printf '%s' "$_body" | jq -e 'try ([.. | objects | has("error")] | any) catch false' >/dev/null 2>&1; then
        _has_error=1
      fi
    else
      printf '%s' "$_body" | grep -qE '"error"[[:space:]]*:' && _has_error=1
    fi
  fi

  if [ "$_rc" -ne 0 ] || [ "$_has_error" -eq 1 ] || [ "$_is_empty" -eq 1 ]; then
    echo "ERROR: $_tool failed (exit=$_rc, error_in_body=$_has_error, empty_body=$_is_empty)" >&2
    echo "Output: $_body" >&2
    [ "$_rc" -ne 0 ] && exit "$_rc" || exit 1
  fi
}

Anti-patterns — do not:

• Collapse to || true.
• Redirect stderr to /dev/null.
• Bury the wrapper call inside a pipeline (masks $?).
• Skip capturing $RESULT into a variable; the helper needs the body.
• Use only if [ "$RC" -ne 0 ]; then ... — that misses the HTTP-error path.

Minimal call site shape:

bash
RESULT=$(chorus-api.sh mcp-tool <tool_name> "$PAYLOAD")
RC=$?
chorus_check_response "<tool_name>" "$RC" "$RESULT"
# ...if we reach here, the call succeeded; parse RESULT and continue.

This is project-wide policy: no silent errors.

§7. Quick reference checklist

When invoked from a stage skill (proposal / develop / yolo):

1. Read CHORUS_OPENSPEC_ACTIVE from the ## OpenSpec Mode section in the SessionStart context (§1). If it isn't there, fall back to the manual probe in §1.
2. If CHORUS_OPENSPEC_ACTIVE=0 → return to caller's free-form path (§4).
3. Otherwise:

a. Pick $SLUG (§3.1). b. openspec new change "$SLUG" (§3.2). c. Author proposal.md, design.md, specs/<capability>/spec.md (§3.2–§3.3). Mix ADDED / MODIFIED / REMOVED / RENAMED blocks as needed; remember MODIFIED overwrites the whole Requirement. d. Optional: openspec validate "$SLUG". e. chorus_pm_create_proposal (direct MCP) with the OpenSpec change slug: $SLUG line in description (§3.5). f. Define json_encode_file, chorus_check_response helpers. (chorus-api.sh is on PATH — no $API variable needed.) g. For each row in §5 with "yes" — mirror via chorus-api.sh mcp-tool chorus_pm_add_document_draft (§3.6). Record each $DRAFT_UUID. h. On any failed chorus_check_response — halt, surface the error, do NOT proceed.

1. Edits before approval → §3.7. Edits after approval → §3.8.
2. Last task verified → hook fires → run §3.9 archive flow.