github · TheovanKraay · Sep 1, 2026 · Sep 1, 2026 · Sep 1, 2026 · Sep 2, 2026
@@ -205,6 +205,19 @@ specify preset add team-workflow --priority 10
 
 For any file that both provide, `compliance` wins (priority 5 < 10). For files only one provides, that one is used. For files neither provides, the core default is used.
 
+## Always-on instructions
+
+A preset can also contribute an always-on instruction block through `provides.instructions`. Unlike templates, commands, and scripts (which are resolved by the priority stack when Spec Kit needs them), an instruction block is composed into the coding agent's always-on context file so it reaches the agent's work generally, including outside a Spec Kit workflow.
+
+```yaml
+provides:
+  instructions:
+    - file: "instructions/best-practices.md"
+      description: "Always-on engineering rules"
+```
+
+This is opt-in and owned by the `agent-context` extension: nothing is written unless `agent-context` is installed and the preset is enabled. When both hold, `agent-context` composes each enabled preset's block into the routed context file (for example `.github/copilot-instructions.md`) inside a namespaced `<!-- SPECKIT PRESET:<id> START/END -->` block, and drops it again on `preset disable`/`remove` at the next refresh. Enabling the preset is the explicit opt-in; installing an extension does not by itself change the agent's context.
+
 ## FAQ
 
 ### Can I use multiple presets at the same time?

@@ -345,6 +345,21 @@ PY
 fi
 
 # Build the managed section
+_SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# Always-on instruction blocks contributed by enabled presets (#4200).
+# Delegated to the python twin's --emit-preset-blocks so all three twins emit
+# byte-identical block text from a single implementation. Capture only stdout so
+# the composer's warnings (oversized, marker-colliding, or skipped entries) still
+# reach stderr. The assignment is the condition of an `if` so `set -e` does not
+# abort on a nonzero composer status before this diagnostic runs; on failure we
+# abort here so a composer failure never rewrites the section with previously
+# composed preset blocks silently dropped.
+if ! _PRESET_BLOCKS="$("$_python" "$_SCRIPT_DIR/../python/update_agent_context.py" --emit-preset-blocks --marker-start "$MARKER_START" --marker-end "$MARKER_END")"; then
+  echo "agent-context: preset instruction composer failed; aborting so the managed section is not rewritten with preset blocks dropped." >&2
+  exit 1
+fi
+
 TMP_SECTION="$(mktemp)"
 trap 'rm -f "$TMP_SECTION"' EXIT
 {
@@ -354,6 +369,9 @@ trap 'rm -f "$TMP_SECTION"' EXIT
   if [[ -n "$PLAN_PATH" ]]; then
     echo "at $PLAN_PATH"
   fi
+  if [[ -n "$_PRESET_BLOCKS" ]]; then
+    printf '%s\n' "$_PRESET_BLOCKS"
+  fi
   echo "$MARKER_END"
 } > "$TMP_SECTION"
 

@@ -457,6 +457,73 @@ $lines = @($MarkerStart,
 if ($PlanPath) {
     $lines += "at $PlanPath"
 }
+# Always-on instruction blocks contributed by enabled presets (#4200): delegate
+# to the python twin's --emit-preset-blocks so all three twins emit byte-identical
+# block text from a single implementation.
+$pyTwin = Join-Path (Join-Path (Join-Path $PSScriptRoot '..') 'python') 'update_agent_context.py'
+$pyForBlocks = $null
+foreach ($candidate in @($env:SPECKIT_PYTHON, 'python3', 'python')) {
+    if (-not $candidate) { continue }
+    if (-not (Get-Command $candidate -ErrorAction SilentlyContinue)) { continue }
+    # Require a real Python 3 that can import PyYAML (the composer imports yaml),
+    # skipping the Windows Store 'python3' alias stub.
+    try {
+        & $candidate -c "import sys, yaml; sys.exit(0 if sys.version_info[0] == 3 else 1)" 2>$null | Out-Null
+        if ($LASTEXITCODE -eq 0) { $pyForBlocks = $candidate; break }
+    } catch { }
+}
+if (-not $pyForBlocks) {
+    # The base section is written natively, but preset instruction blocks are
+    # composed by the Python twin only. If no Python 3 + PyYAML is available and
+    # presets are installed, warn instead of silently dropping their rules.
+    $presetReg = Join-Path $ProjectRoot '.specify/presets/.registry'
+    if (Test-Path -LiteralPath $presetReg) {
+        try {
+            $preg = Get-Content -LiteralPath $presetReg -Raw -Encoding UTF8 | ConvertFrom-Json
+            $enabled = @($preg.presets.PSObject.Properties | Where-Object { $_.Value.enabled -ne $false })
+            if ($enabled.Count -gt 0) {
+                [Console]::Error.WriteLine("agent-context: Python 3 with PyYAML not found; preset always-on instruction blocks (provides.instructions) were NOT composed. Base context section written.")
+            }
+        } catch { }
+    }
+}
+if ($pyForBlocks) {
+    if (-not (Test-Path -LiteralPath $pyTwin)) {
+        # Python is available but the sibling composer is gone: a partial or
+        # corrupt install. Treat it as a hard failure (like a nonzero composer
+        # exit) so the managed section is not rewritten with existing preset
+        # blocks silently dropped.
+        [Console]::Error.WriteLine("agent-context: preset instruction composer '$pyTwin' is missing (corrupt or partial install); aborting so the managed section is not rewritten with preset blocks dropped.")
+        exit 1
+    }
+    # Windows PowerShell decodes native-command stdout using the console code
+    # page; force UTF-8 so non-ASCII rule text (e.g. em-dashes) survives capture.
+    # Keep stderr (the composer's oversized/marker-colliding/skipped warnings) out
+    # of the captured stdout by routing it to a temp file, then surface it, and
+    # abort on a nonzero exit so a composer failure never rewrites the section
+    # with previously composed preset blocks silently dropped.
+    $errFile = [System.IO.Path]::GetTempFileName()
+    $prevOutEnc = [Console]::OutputEncoding
+    try {
+        [Console]::OutputEncoding = [System.Text.Encoding]::UTF8
+        $emitted = (& $pyForBlocks $pyTwin --emit-preset-blocks --marker-start $MarkerStart --marker-end $MarkerEnd 2>$errFile | Out-String)
+    } finally {
+        [Console]::OutputEncoding = $prevOutEnc
+    }
+    $emitRc = $LASTEXITCODE
+    $errText = if (Test-Path -LiteralPath $errFile) { Get-Content -LiteralPath $errFile -Raw } else { '' }
+    Remove-Item -LiteralPath $errFile -ErrorAction SilentlyContinue
+    if ($errText) { [Console]::Error.Write($errText) }
+    if ($emitRc -ne 0) {
+        [Console]::Error.WriteLine("agent-context: preset instruction composer failed (exit $emitRc); aborting so the managed section is not rewritten with preset blocks dropped.")
+        exit 1
+    }
+    if ($emitted) {
+        $emitted = ($emitted -replace "`r`n", "`n") -replace "`r", "`n"
+        $emitted = $emitted.TrimEnd("`n")
+        foreach ($bl in ($emitted -split "`n")) { $lines += $bl }
+    }
+}
 $lines += $MarkerEnd
 $Section = ($lines -join "`n") + "`n"
 

@@ -27,6 +27,21 @@
 DEFAULT_START = "<!-- SPECKIT START -->"
 DEFAULT_END = "<!-- SPECKIT END -->"
 
+# Any SPECKIT marker comment (the outer managed-section markers or the
+# per-preset ``PRESET:<id> START/END`` sub-markers). Instruction payloads that
+# embed one would collide with the find/replace in _upsert_section, so they are
+# rejected.
+_SPECKIT_MARKER_RE = re.compile(r"<!--\s*SPECKIT\b")
+
+# Deliberately small budget for always-on instruction payloads. The composed
+# managed section is re-sent as agent context on every request, so an oversized
+# preset file (a bundled archive member or an unbounded ``--dev`` source) must
+# not be allowed to bloat it. A single file over the per-file cap is skipped
+# with a warning; once the aggregate cap across all presets is reached, the
+# remaining entries are skipped too.
+_MAX_INSTRUCTION_FILE_BYTES = 32 * 1024
+_MAX_INSTRUCTION_TOTAL_BYTES = 64 * 1024
+
 
 def _err(message: str) -> None:
     print(message, file=sys.stderr)
@@ -201,18 +216,192 @@ def _resolved_rel(p: Path) -> Path | None:
     return plan_path
 
 
-def _build_section(marker_start: str, marker_end: str, plan_path: str) -> str:
+def _build_section(
+    marker_start: str,
+    marker_end: str,
+    plan_path: str,
+    preset_blocks: list[str] | None = None,
+) -> str:
     lines = [
         marker_start,
         "For additional context about technologies to be used, project structure,",
         "shell commands, and other important information, read the current plan",
     ]
     if plan_path:
         lines.append(f"at {plan_path}")
+    # Always-on instruction blocks contributed by explicitly-enabled presets,
+    # each in its own namespaced sub-block so multiple presets coexist and each
+    # can be regenerated or dropped independently on the next update.
+    lines.extend(preset_blocks or [])
     lines.append(marker_end)
     return "\n".join(lines) + "\n"
 
 
+def _collect_preset_instruction_blocks(
+    project_root: str,
+    marker_start: str = DEFAULT_START,
+    marker_end: str = DEFAULT_END,
+) -> list[tuple[str, str]]:
+    """Collect always-on instruction blocks from installed + enabled presets.
+
+    A preset the user explicitly added (``specify preset add``) that declares
+    ``provides.instructions`` gets its rule block composed into the managed
+    section. Reads ``.specify/presets/.registry`` and each preset's
+    ``preset.yml`` directly, with no dependency on the Specify CLI (mirrors this
+    extension's by-design independence). Returns ``(preset_id, content)`` in
+    deterministic id order. Each referenced file must resolve inside its own
+    preset directory; path-unsafe, unreadable, non-UTF-8, oversized (per-file or
+    aggregate budget), or marker-colliding entries are skipped (fail closed).
+    Fails closed on an unreadable registry.
+    """
+    presets_dir = Path(project_root) / ".specify" / "presets"
+    registry = presets_dir / ".registry"
+    if not registry.is_file():
+        return []
+    try:
+        import yaml
+    except ImportError:
+        return []
+    try:
+        with open(registry, "r", encoding="utf-8") as fh:
+            reg = json.load(fh)
+    except (json.JSONDecodeError, OSError, UnicodeDecodeError):
+        return []
+    if not isinstance(reg, dict) or not isinstance(reg.get("presets"), dict):
+        return []
+
+    presets_root = presets_dir.resolve()
+    blocks: list[tuple[str, str]] = []
+    total_bytes = 0
+    for preset_id in sorted(reg["presets"]):
+        # The registry lives on disk and is untrusted. Reject ids that are not
+        # simple names (no path separators, '..' traversal, or absolute/drive
+        # forms), then confirm the resolved directory stays inside
+        # .specify/presets, so a crafted key or a symlink cannot read a manifest
+        # or payload outside it.
+        if not isinstance(preset_id, str) or not re.match(r"^[a-z0-9][a-z0-9._-]*$", preset_id):
+            continue
+        preset_root = (presets_dir / preset_id).resolve()
+        try:
+            preset_root.relative_to(presets_root)
+        except ValueError:
+            continue
+        meta = reg["presets"][preset_id]
+        if not isinstance(meta, dict) or not meta.get("enabled", True):
+            continue
+        manifest = preset_root / "preset.yml"
+        if not manifest.is_file():
+            continue
+        try:
+            with open(manifest, "r", encoding="utf-8") as fh:
+                pdata = yaml.safe_load(fh)
+        except Exception:
+            continue
+        provides = pdata.get("provides") if isinstance(pdata, dict) else None
+        instructions = provides.get("instructions") if isinstance(provides, dict) else None
+        if not isinstance(instructions, list):
+            continue
+        parts: list[str] = []
+        for entry in instructions:
+            if not isinstance(entry, dict):
+                continue
+            rel = entry.get("file")
+            if not isinstance(rel, str) or not rel.strip():
+                continue
+            # Path-unsafe entries (absolute, backslash, parent traversal, or a
+            # target escaping the preset directory) are skipped silently: this is
+            # a security fail-closed decision, so no diagnostic is emitted.
+            if rel.startswith("/") or "\\" in rel or ".." in rel.split("/"):
+                continue
+            target = (preset_root / rel).resolve()
+            try:
+                target.relative_to(preset_root)
+            except ValueError:
+                continue
+            if not target.is_file():
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    f"file '{rel}' not found."
+                )
+                continue
+            # Reject an oversized file by its on-disk size before reading it, so
+            # a huge member never gets allocated into memory.
+            try:
+                size = target.stat().st_size
+            except OSError:
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    f"file '{rel}' is not readable."
+                )
+                continue
+            if size > _MAX_INSTRUCTION_FILE_BYTES:
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    f"file '{rel}' is {size} bytes (per-file limit "
+                    f"{_MAX_INSTRUCTION_FILE_BYTES})."
+                )
+                continue
+            try:
+                text = target.read_text(encoding="utf-8").strip()
+            except (OSError, UnicodeDecodeError):
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    f"file '{rel}' is not readable UTF-8 text."
+                )
+                continue
+            if marker_start in text or marker_end in text or _SPECKIT_MARKER_RE.search(text):
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    "content contains a managed section marker."
+                )
+                continue
+            # Count the bytes this entry actually adds to the rendered section,
+            # not just its raw payload: the first entry of a preset materializes
+            # the surrounding marker block, and every later entry adds a blank-line
+            # separator. Without this, a flood of tiny entries would slip past the
+            # aggregate cap even though the composed section is far larger.
+            entry_bytes = len(text.encode("utf-8"))
+            if parts:
+                overhead = 2  # the "\n\n" joining this entry to the previous one
+            else:
+                overhead = (
+                    len(f"<!-- SPECKIT PRESET:{preset_id} START -->")
+                    + len(f"<!-- SPECKIT PRESET:{preset_id} END -->")
+                    + 4  # surrounding newlines and the leading blank line
+                )
+            if total_bytes + entry_bytes + overhead > _MAX_INSTRUCTION_TOTAL_BYTES:
+                _err(
+                    f"agent-context: skipping instructions from preset '{preset_id}': "
+                    f"aggregate instruction budget ({_MAX_INSTRUCTION_TOTAL_BYTES} "
+                    "bytes) exceeded."
+                )
+                continue
+            total_bytes += entry_bytes + overhead
+            parts.append(text)
+        if parts:
+            blocks.append((preset_id, "\n\n".join(parts)))
+    return blocks
+
+
+def _render_preset_block_lines(
+    project_root: str,
+    marker_start: str = DEFAULT_START,
+    marker_end: str = DEFAULT_END,
+) -> list[str]:
+    """Render the namespaced sub-block lines for all enabled presets' instruction
+    blocks, to be embedded inside the managed section.
+    """
+    lines: list[str] = []
+    for preset_id, content in _collect_preset_instruction_blocks(
+        project_root, marker_start, marker_end
+    ):
+        lines.append("")
+        lines.append(f"<!-- SPECKIT PRESET:{preset_id} START -->")
+        lines.append(content)
+        lines.append(f"<!-- SPECKIT PRESET:{preset_id} END -->")
+    return lines
+
+
 def ensure_mdc_frontmatter(content: str) -> str:
     """Ensure ``.mdc`` content has YAML frontmatter with ``alwaysApply: true``.
 
@@ -298,6 +487,25 @@ def _upsert_section(
 def main(argv: list[str] | None = None) -> int:
     args = sys.argv[1:] if argv is None else argv
     project_root = os.getcwd()
+
+    # --emit-preset-blocks: print only the composed preset instruction sub-block
+    # lines and exit. Used by the bash/PowerShell twins so all three produce
+    # identical output from this single implementation. Does not require the
+    # agent-context config (the twin already validated it before calling).
+    if "--emit-preset-blocks" in args:
+        def _opt(name: str, default: str) -> str:
+            if name in args:
+                i = args.index(name)
+                if i + 1 < len(args):
+                    return args[i + 1]
+            return default
+        marker_start = _opt("--marker-start", DEFAULT_START)
+        marker_end = _opt("--marker-end", DEFAULT_END)
+        block_lines = _render_preset_block_lines(project_root, marker_start, marker_end)
+        if block_lines:
+            sys.stdout.buffer.write("\n".join(block_lines).encode("utf-8"))
+        return 0
+
     ext_config = (
         f"{project_root}/.specify/extensions/agent-context/agent-context-config.yml"
     )
@@ -353,7 +561,8 @@ def main(argv: list[str] | None = None) -> int:
     if not plan_path:
         plan_path = _resolve_plan_path(project_root)
 
-    section = _build_section(marker_start, marker_end, plan_path)
+    preset_blocks = _render_preset_block_lines(project_root, marker_start, marker_end)
+    section = _build_section(marker_start, marker_end, plan_path, preset_blocks)
 
     for context_file in context_files:
         ctx_path = os.path.join(project_root, context_file)