# Openspec Aware

> OpenSpec-mode authoring for Chorus PM workflows in Claude Code. The default whenever OpenSpec is usable; consumes the resolved `## Spec Mode` (never re-detects). Scaffolds `openspec/changes/<slug>/` on disk, and mirrors Markdown files into Chorus document drafts via `chorus mcp call --arg-file` (bash `chorus-api.sh` wrapper as fallback). Required reading for the proposal, develop, and yolo skills. When OpenSpec is not the resolved mode, this skill no-ops and the caller follows the resolved mode (spec-lite or free-form).

- Skill: `chorus-aidlc/openspec-aware-3` (Agent Skill)
- Install (CLI): `npx skillmds@latest add chorus-aidlc/openspec-aware-3`
- Raw SKILL.md: https://api.skillmd.com/api/skills/chorus-aidlc/openspec-aware-3/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- License: AGPL-3.0
- Author: chorus-aidlc (https://skillmd.com/u/chorus-aidlc)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/chorus-aidlc/openspec-aware-3

---


# 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](https://github.com/Fission-AI/OpenSpec). 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 follows the resolved `SPEC_MODE` — **spec-lite** (the default when OpenSpec isn't usable) or free-form (`=off`).

> **See also — `spec-lite` (the lightweight fallback):** OpenSpec (this skill) stays the **default whenever it is usable** (`openspec/` dir + CLI, not disabled). When OpenSpec is absent or disabled — or when `CHORUS_SPEC_MODE=lite` is set explicitly — the mode resolves to **spec-lite**: a Chorus-native, git-tracked model with a durable local spec `.chorus/specs/<slug>/spec.md` per capability (edited in place, **never synced**), plus one dated folder per change effort `.chorus/specs/<slug>/<YYYY-MM-DD>-<change-slug>/` of plain-markdown docs named by Document type (`prd.md`, `tech_design.md`, …), those dated-folder docs each mirrored 1:1 into a persistent Chorus Document via the same `--arg-file` transport, with no CLI and no strict validation. A session that resolves to OpenSpec follows everything below unchanged. See the `spec-lite` skill (`skills/spec-lite/SKILL.md`).

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`) resolves the active spec mode once when the session opens and writes a `## Spec Mode` section into the plugin's injected context. When the resolved mode is a usable OpenSpec, that section also carries a `CHORUS_OPENSPEC_ACTIVE=1` line. `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 `## Spec Mode` section near the top of the conversation). This skill is only loaded when the mode resolved to a usable OpenSpec, so you should see:

```
## Spec Mode

CHORUS_SPEC_MODE=openspec (explicit; openspec/ directory + openspec CLI both present)

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

Branch:

- `CHORUS_OPENSPEC_ACTIVE=1` → follow §3 (OpenSpec authoring).
- No `CHORUS_OPENSPEC_ACTIVE=1` line (the section shows `CHORUS_SPEC_MODE=lite` / `off`, or you were spawned without SessionStart context) → this skill is a no-op; return to the calling skill's path. **Do not** scaffold `openspec/changes/`. **Do not** add the slug line to the proposal description. (Reconstruct the value with the manual fallback below if you genuinely have no context.)

### Manual fallback (no SessionStart context)

If you're in a sub-shell, sub-agent, or session that did not see the `## Spec Mode` section (e.g. spawned mid-session without the parent's context), **do not hand-roll the detection** — source the *same* resolver the hook uses, so there is one computation of the mode:

```bash
# The resolver sits beside the plugin hooks; ${CLAUDE_PLUGIN_ROOT} is the plugin
# root Claude Code exports (the same var hooks.json uses).
RESOLVER="${CLAUDE_PLUGIN_ROOT}/bin/resolve-spec-mode.sh"
if [ -f "$RESOLVER" ]; then
  . "$RESOLVER"   # sets SPEC_MODE (lite|openspec|off), SPEC_FAIL (non-empty ⇒ halt),
                  # and CHORUS_OPENSPEC_ACTIVE (1 only for a usable openspec)
else
  echo "resolve-spec-mode.sh not found; set CHORUS_SPEC_MODE explicitly and re-launch" >&2
fi
```

Then: if `SPEC_FAIL` is non-empty, halt and surface it; if `CHORUS_OPENSPEC_ACTIVE=1` follow §3; otherwise this skill is a no-op — return to the caller, which follows the resolved `SPEC_MODE` (spec-lite or free-form). If you genuinely cannot locate the helper, set `CHORUS_SPEC_MODE` explicitly and re-launch rather than guessing — never re-derive the rule inline (the hook/helper is the single source of truth).

---

## §2. ⛔ Two non-negotiable rules

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

### Rule 1 — Fill `content` from the file (CLI preferred, bash-wrapper fallback); 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** fill the `content` field from the local file's bytes, never from a hand-typed body. 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. Use whichever transport is available, preferred first:

- **Primary — the `chorus` CLI:** `chorus mcp call <tool_name> '<json-without-content>' --arg-file content=<file>`. `--arg-file content=<path>` reads the file's raw bytes and injects them as the JSON `content` string, byte-exact — the CLI's built-in replacement for `json_encode_file`, so no helper is needed. See §3.6. **Requires chorus >= 0.17.0** (the `chorus mcp` subcommand was added then; an older CLI errors with "unknown command"); on any version or unknown-command failure, upgrade with `npm install -g @chorus-aidlc/chorus`.
- **Fallback — the `chorus-api.sh` wrapper, when `chorus` is not on `PATH`:** build `$PAYLOAD` with the `json_encode_file` helper and call `chorus-api.sh mcp-tool <tool_name> "$PAYLOAD"` (`chorus-api.sh` is on `PATH` — call it by name). Defined in the §3.6 fallback block.

> **New to the `chorus` CLI?** See the **`chorus-cli`** skill for install, configuring agents (`chorus agents add|remove|list`), the connection env vars, and `chorus mcp` basics.

> **Acting identity — which agent the call acts as.** `chorus mcp call` resolves the agent from, in order: `CHORUS_AGENT_PROFILE` (a name or UUID) → `CHORUS_URL` + `CHORUS_API_KEY` in the environment → the single agent configured in `~/.chorus/daemon.json`. A daemon-woken session already has `CHORUS_AGENT_PROFILE` set. If a mirror call fails with `Multiple agents … specify --agent` (several agents configured and no profile/creds in the env), pass your own identity explicitly: `chorus mcp call <tool> … --agent <your-agentUuid>` — your UUID is in your `chorus_checkin` result, and `chorus agents` lists every configured name/UUID.

Reasons (they apply to both paths):

1. **Token cost.** Re-typing a multi-thousand-line markdown body through the LLM burns input + output tokens for every draft. Both the CLI's `--arg-file` and the fallback's `json_encode_file` stream the file's bytes into the JSON string — content never enters LLM context. A typical 3-doc proposal mirror costs roughly zero content-tokens this way; via direct MCP with a re-typed body it routinely costs 20k+.
2. **Byte-equality.** A file-fill path (CLI `--arg-file`, or the fallback's `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 a file-fill path, never on LLM re-emission.
3. **Single source of truth.** With a file-fill mirror, 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 Filling the `content` field byte-exact

The document `content` must be inserted **byte-for-byte** from the local file — never re-typed by the LLM. Two mechanisms, preferred first:

- **Primary — `chorus mcp call … --arg-file content=<path>`** (§3.6). The CLI reads the file's raw bytes and injects them as the JSON `content` string. This is the byte-faithful replacement for `json_encode_file`, so on the CLI path **no helper is needed** — pass the base JSON without a `content` field and let `--arg-file` fill it.
- **Fallback — `json_encode_file`** (defined in the §3.6 fallback block, used only when `chorus` is not on `PATH`). With `jq` available it streams the file through `jq -Rs '.'`; the pure-shell branch matches `chorus-api.sh`'s own escaping when `jq` is missing.

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 (CLI primary, wrapper fallback)

> **Rule 1 reminder:** `content` comes from the file's bytes, never a hand-typed body. The agent must not retype the document body.

Define the halt-on-error helper from §6 once at the top. **Primary path — the `chorus` CLI:** pass the base JSON *without* a `content` field and let `--arg-file content=<file>` fill it byte-exact. One call per file:

```bash
# PRD draft — --arg-file fills content byte-exact from the file; no json_encode_file needed.
RESULT=$(chorus mcp call chorus_pm_add_document_draft \
  "{\"proposalUuid\":\"$PROPOSAL_UUID\",\"type\":\"prd\",\"title\":\"PRD: $HUMAN_TITLE\"}" \
  --arg-file content="openspec/changes/$SLUG/proposal.md")
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.

#### Fallback — when the `chorus` CLI is not on `PATH`

If `command -v chorus` fails, mirror through the bundled `chorus-api.sh` wrapper instead. Define `json_encode_file` here (it is used **only** on this fallback path), then build `$PAYLOAD` with an embedded `content` and call the wrapper. The `chorus_check_response` halt-on-error check applies exactly as on the primary path.

```bash
# Define once, fallback-only: byte-faithful file → JSON string.
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
}

# 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/')
```

### 3.7 Editing a draft after the first mirror

Local file changes propagate via `chorus_pm_update_document_draft` — same primary/fallback split as §3.6, same halt check. Primary (CLI):

```bash
RESULT=$(chorus mcp call chorus_pm_update_document_draft \
  "{\"proposalUuid\":\"$PROPOSAL_UUID\",\"draftUuid\":\"$PRD_DRAFT_UUID\"}" \
  --arg-file content="openspec/changes/$SLUG/proposal.md")
RC=$?
chorus_check_response "chorus_pm_update_document_draft" "$RC" "$RESULT"
```

Fallback (no `chorus` on `PATH`) — `json_encode_file` from the §3.6 fallback block:

```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`. Primary (CLI):

```bash
RESULT=$(chorus mcp call chorus_pm_update_document \
  "{\"documentUuid\":\"$SPEC_DOCUMENT_UUID\"}" \
  --arg-file content="openspec/changes/$SLUG/specs/<capability>/spec.md")
RC=$?
chorus_check_response "chorus_pm_update_document" "$RC" "$RESULT"
```

Fallback (no `chorus` on `PATH`) — `json_encode_file` from the §3.6 fallback block:

```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.)

2. **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.

3. **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.)

4. **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. When OpenSpec is not the resolved mode

When `CHORUS_OPENSPEC_ACTIVE` is not `1`, this skill is a **no-op** — return to the calling skill, which follows whatever the resolved `SPEC_MODE` is: **spec-lite** (the default when OpenSpec isn't usable) or **free-form** (`CHORUS_SPEC_MODE=off`). Either way, from this skill's side:

- No `openspec/changes/` folder is created or referenced.
- No `OpenSpec change slug: …` line is added to the proposal description.
- Document authoring is whatever the resolved mode dictates (spec-lite: mirror `.chorus/specs/<slug>/<YYYY-MM-DD>-<change-slug>/<type>.md`; free-form: inline `chorus_pm_add_document_draft`) — not this skill's concern.
- Rule 1 (wrapper-only mirror) does not apply to OpenSpec here — there is no `openspec/` local file for this skill to mirror.
- 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

This helper guards **both** the primary CLI path and the fallback wrapper path. On the fallback path 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. (`chorus mcp call` exits non-zero on tool/transport errors, so `RC` is reliable on the primary path — but run the same three-signal check on both, as defense in depth.)

Define this helper **once** at the top of the authoring session and use it after every mirror call (CLI or wrapper):

```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 (both paths):**

```bash
# Primary — chorus CLI:
RESULT=$(chorus mcp call <tool_name> '<json-without-content>' --arg-file content=<file>)
RC=$?
chorus_check_response "<tool_name>" "$RC" "$RESULT"

# Fallback — chorus-api.sh wrapper (chorus not on PATH):
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 `## Spec 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` is not `1` → no-op; return to the caller, which follows the resolved `SPEC_MODE` (spec-lite or free-form) — see §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: $SLUG` line in description (§3.5).
   f. Define the `chorus_check_response` helper. Prefer `chorus mcp call … --arg-file content=<file>` for mirrors (§3.6) — no `json_encode_file` needed on that path; define `json_encode_file` only when falling back to the wrapper because `chorus` is not on `PATH`. (`chorus-api.sh` is on PATH — no `$API` variable needed.)
   g. For each row in §5 with "yes" — mirror via `chorus mcp call chorus_pm_add_document_draft … --arg-file content=<file>` (§3.6; fallback = `chorus-api.sh mcp-tool`). Record each `$DRAFT_UUID`.
   h. On any failed `chorus_check_response` — halt, surface the error, do NOT proceed.
4. Edits before approval → §3.7. Edits after approval → §3.8.
5. Last task verified → hook fires → run §3.9 archive flow.