§12 Cross-Cutting Concerns

§12.1 Async Contract

All pipeline functions MUST have async variants. Pipeline functions SHOULD also have sync variants where the language supports synchronous execution.

The following functions MUST exist in both sync and async forms:

Sync	Async
load	load_async
render	render_async
parse	parse_async
prepare	prepare_async
run	run_async
invoke	invoke_async
process	process_async
invoke_agent	invoke_agent_async

Naming convention: function_name for sync, function_name_async for async. Language-idiomatic alternatives are acceptable (e.g., TypeScript MAY provide only async functions since its ecosystem is async-first).

Registry operations (register_connection, get_connection, register_tool, get_tool, clear_cache, etc.) are sync-only. They perform in-memory lookups and MUST NOT perform I/O.

§12.2 Input Validation

The validate_inputs function MUST be called as part of prepare(), before rendering:

function validate_inputs(agent, inputs) → dict:
  validated = shallow_copy(inputs)

  for prop in agent.inputs:
    if prop.name not in validated:
      if prop.default is not null:
        validated[prop.name] = prop.default
      elif prop.required:
        raise ValueError("Missing required input: " + prop.name)
      // else: optional with no default — omit

  return validated

Behaviour requirements:

• MUST fill missing inputs from default when available.
• MUST raise ValueError for missing inputs that are required and have no default.
• MUST NOT raise for optional inputs with no value and no default — they are simply omitted from the validated dict.
• MUST NOT use example as a runtime value under any circumstances — it exists solely for documentation and tooling. Using example as a fallback is the fastest path to unexpected behavior.
• MUST NOT type-check input values (the template engine handles coercion).
• MUST NOT reject extra inputs not declared in inputs (they are passed through to the template).

§12.3 Dotenv Loading

Loading .env files is an application-level concern. Runtime libraries MUST NOT automatically load .env files as a side effect of loading a .prompty file.

Guidance for application authors:

• Load .env files at application startup, before calling load().
• Look for .env in the .prompty file’s parent directory or the project root.
• Format: standard dotenv (KEY=VALUE, one per line, # comments, optional quoting).
• .env files MUST NOT override environment variables that are already set in the process environment.

§12.4 Error Conditions

Implementations MUST raise appropriate errors for the conditions listed below. This specification defines what error to raise and when; the specific exception class SHOULD follow language idioms.

Stage	Condition	Error Type	Message Pattern
Load	File not found	FileNotFoundError	"Prompty file not found: {path}"
Load	Invalid YAML frontmatter	ValueError	"Invalid frontmatter YAML: {details}"
Load	Missing env var (no default)	ValueError	"Environment variable '{name}' not set"
Load	Referenced file not found (${file:})	FileNotFoundError	"Referenced file not found: {path}"
Render	Undefined template variable	ValueError	"Undefined template variable: {name}"
Render	Template syntax error	ValueError	"Template syntax error: {details}"
Parse	Role marker nonce mismatch (strict mode)	ValueError	"Role marker nonce mismatch (possible injection)"
Execute	Unsupported apiType	ValueError	"Unsupported API type: {type}"
Execute	Connection failure	ConnectionError	Provider-specific message
Process	Unexpected response format	ValueError	"Unexpected response format"
Agent Loop	Tool not registered	ValueError	"Tool not registered: {name}"
Agent Loop	Max iterations exceeded	RuntimeError	"Agent loop exceeded {n} iterations"
Agent Loop	Model refusal	ValueError	"Model refused: {message}"
Discovery	No implementation for key	InvokerError	"No {component} registered for key: {key}"

§12.6 Rich Kinds

RICH_KINDS = { thread, image, file, audio }

These are input property kinds that contain structured data requiring special handling during rendering and parsing. They MUST NOT be passed directly to the template engine as raw values.

Kind	Contains	Rendering	Parsing	Wire Format
thread	Message[]	Nonce replacement	Expanded back to Message[]	N/A (already messages)
image	Image data or URL	Nonce replacement	Preserved as nonce	Resolved to ImagePart
file	File data or URL	Nonce replacement	Preserved as nonce	Resolved to FilePart
audio	Audio data or URL	Nonce replacement	Preserved as nonce	Resolved to AudioPart

Nonce format: All rich kinds use the same nonce format:

__PROMPTY_THREAD_<hex8>_<name>__

Where <hex8> is 8 random hexadecimal characters and <name> is the input property name. The nonce MUST be unique per render invocation.

Purpose: The nonce prevents template engines from interpreting or mangling structured data. During rendering, the structured value is replaced with a nonce string. After parsing, the nonce is resolved back to the appropriate content:

• Thread nonces are expanded during the parse/prepare phase into the actual Message[] sequence, spliced into the message list at the nonce’s position.
• Image, file, and audio nonces are resolved during wire-format conversion (§7 Wire Format) when the actual ImagePart, FilePart, or AudioPart content types are constructed.