§7 Wire Format

This section defines how the internal Message[] representation (produced by the parser in §6 Parsing) is converted to the wire format expected by each LLM provider’s API. Implementations MUST support at least one provider; OpenAI Chat Completions is the reference format.

§7.1 OpenAI Chat Completions

§7.1.1 Message Conversion

Each internal Message MUST be converted to the OpenAI wire format before submission. The algorithm is:

function message_to_wire(message) → dict:
  wire = { role: message.role }

  // Metadata pass-through (tool_call_id, name, tool_calls, etc.)
  if message.metadata exists and is non-empty:
    merge metadata keys into wire

  // Content: single TextPart → string; otherwise → array of content parts
  if message.parts has exactly 1 element AND that element is a TextPart:
    wire.content = message.parts[0].value        // plain string
  else:
    wire.content = [part_to_wire(part) for part in message.parts]

  return wire

Implementations MUST preserve the single-string optimisation for messages containing exactly one TextPart. Multi-part messages MUST use the array form.

§7.1.2 Part Conversion

Each ContentPart MUST be mapped to the corresponding OpenAI content-block type:

function part_to_wire(part) → dict:
  match part.kind:
    "text"  → { type: "text", text: part.value }
    "image" → { type: "image_url",
                image_url: { url: part.value,
                             detail: part.detail if present } }
    "audio" → { type: "input_audio",
                input_audio: { data: part.value,
                               format: map_audio_format(part.mediaType) } }
    "file"  → { type: "file", file: { url: part.value } }

Audio format mapping. The mediaType field MUST be mapped as follows:

mediaType value	API format value
audio/wav, audio/x-wav	wav
audio/mp3, audio/mpeg	mp3
audio/flac	flac
audio/ogg	ogg
any other audio/*	strip audio/ prefix

Implementations MUST NOT send an empty detail field; it SHOULD be omitted when no detail level is specified on the ImagePart.

§7.1.3 Tool Conversion

All tool kinds MUST be projected as OpenAI function definitions in the wire tools array so the LLM can discover and invoke them. Each tool kind requires a different projection strategy, but the wire format is always the same: { type: "function", function: { name, description, parameters } }.

Projection by tool kind:

Tool Kind	Projection Strategy
function	Already function-shaped — convert parameters (list[Property]) to JSON Schema directly
mcp	Resolve MCP server connection, discover its tools, project each as a function definition
openapi	Parse OpenAPI specification, project each operation as a function definition
custom (*)	Look up in tool registry, use registered function signature

The agent loop (§9 Agent Loop) intercepts tool calls from the LLM response and routes them to the appropriate handler based on the original tool kind.

function tools_to_wire(tools, inputs) → list | null:
  wire_tools = []

  for tool in tools:
    func_defs = project_tool(tool)
    for func_def in func_defs:
      // Strip bound parameters — these are injected at call time, not
      // exposed to the LLM.
      if tool.bindings:
        params = func_def.function.parameters
        for bound_param in tool.bindings:
          remove bound_param from params.properties
          remove bound_param from params.required (if present)

      // Strict mode: flag lives on function definition (NOT inside schema)
      if tool.strict:
        func_def.function.strict = true
        func_def.function.parameters.additionalProperties = false

      wire_tools.append(func_def)

  return wire_tools if non-empty else null


function project_tool(tool) → list of func_defs:
  match tool.kind:
    "function":
      return [{
        type: "function",
        function: {
          name: tool.name,
          description: tool.description,
          parameters: schema_to_wire(tool.parameters)
        }
      }]

    "mcp":
      // Resolve MCP server → returns list of tool definitions
      mcp_tools = resolve_mcp_server(tool.connection, tool.serverName)
      if tool.allowedTools:
        mcp_tools = filter(mcp_tools, name in tool.allowedTools)
      return [mcp_tool_to_func_def(t) for t in mcp_tools]

    "openapi":
      // Parse OpenAPI spec → returns list of operation definitions
      operations = parse_openapi_spec(tool.specification, tool.connection)
      return [openapi_op_to_func_def(op) for op in operations]

    default:  // CustomTool (wildcard *)
      // Look up in tool registry for function signature
      handler = get_tool(tool.name)
      return [{
        type: "function",
        function: {
          name: tool.name,
          description: tool.description,
          parameters: handler.parameters_schema
        }
      }]

Note

McpTool and OpenApiTool can project to multiple function definitions (one per MCP tool or API operation). All other kinds project to exactly one. The agent loop uses the tool name to route calls back to the correct handler.

When tools_to_wire returns null, the tools key MUST be omitted from the request entirely (not sent as an empty array).

§7.1.4 Schema Conversion

A list[Property] (used for inputs, outputs, and FunctionTool.parameters) MUST be converted to a JSON Schema object for wire transmission:

function schema_to_wire(properties: list[Property], strict: bool = false) → dict:
  schema = { type: "object", properties: {}, required: [] }

  for prop in properties:
    optional = strict and not prop.required
    prop_schema = property_to_json_schema(prop, strict, optional)
    if prop.description:
      prop_schema.description = prop.description
    if prop.enumValues:
      prop_schema.enum = prop.enumValues
    schema.properties[prop.name] = prop_schema
    if strict or prop.required:
      schema.required.append(prop.name)

  if schema.required is empty:
    delete schema.required

  return schema

property_to_json_schema MUST recurse through array items and object properties. Outside provider strict mode, each object lists only explicitly required children in required. In OpenAI strict mode, every property of every object MUST appear in that object’s required array. A property that is optional in the Prompty model MUST instead be represented as nullable in the strict wire schema.

Kind → JSON Schema type mapping. Implementations MUST use this table:

Property kind	JSON Schema type
string	string
integer	integer
float	number
boolean	boolean
array	array
object	object

§7.1.5 Options Mapping

ModelOptions fields MUST be mapped to OpenAI request parameters:

function build_options(model_options) → dict:
  opts = {}
  if model_options is null:
    return opts

  mapping = {
    temperature      → temperature,
    maxOutputTokens  → max_completion_tokens,   // NOT max_tokens (deprecated)
    topP             → top_p,
    frequencyPenalty → frequency_penalty,
    presencePenalty  → presence_penalty,
    stopSequences    → stop,
    seed             → seed
  }

  for each field in model_options:
    if field.name in mapping:
      opts[mapping[field.name]] = field.value

  // Pass through additionalProperties unmapped
  if model_options.additionalProperties:
    for key, value in model_options.additionalProperties:
      if key not in opts:
        opts[key] = value

  return opts

Important

Implementations MUST map maxOutputTokens to max_completion_tokens, not max_tokens. The max_tokens parameter is deprecated by OpenAI and is incompatible with o-series reasoning models.

§7.1.6 Structured Output

When agent.outputs is non-empty, the executor MUST convert it to an OpenAI response_format parameter:

function output_schema_to_wire(outputs: Properties) → dict | null:
  if outputs is empty:
    return null

  json_schema = schema_to_wire(outputs)
  json_schema.additionalProperties = false

  return {
    type: "json_schema",
    json_schema: {
      name: "structured_output",
      strict: true,
      schema: json_schema
    }
  }

The processor MUST JSON-parse the response content when outputs is present (see §8 Processing).

§7.1.7 Full Chat Request Building

function build_chat_args(agent, messages) → dict:
  args = {
    model: agent.model.id,
    messages: [message_to_wire(m) for m in messages],
    **build_options(agent.model.options)
  }

  tools = tools_to_wire(agent.tools, null)
  if tools:
    args.tools = tools

  response_format = output_schema_to_wire(agent.outputs)
  if response_format:
    args.response_format = response_format

  return args

§7.1.8 Tracing Requirements

The executor MUST emit an execute trace span with the following OpenTelemetry Semantic Conventions for GenAI attributes:

Attribute	Value
gen_ai.operation.name	"chat"
gen_ai.provider.name	agent.model.provider
gen_ai.request.model	agent.model.id
All request options	gen_ai.request.*
gen_ai.usage.input_tokens	From response usage
gen_ai.usage.output_tokens	From response usage
gen_ai.response.finish_reasons	From response choices
gen_ai.response.id	From response id

§7.2 OpenAI Embeddings

When agent.model.apiType is "embedding", the executor MUST build an embeddings request:

function build_embedding_args(agent, messages) → dict:
  // Extract text content from all messages
  texts = []
  for msg in messages:
    for part in msg.parts:
      if part.kind == "text":
        texts.append(part.value)

  input = texts[0] if len(texts) == 1 else texts

  return {
    model: agent.model.id,
    input: input
  }

Implementations MUST use a single string when there is exactly one text input and an array of strings when there are multiple.

Tracing: The span MUST set gen_ai.operation.name = "embeddings".

§7.3 OpenAI Images

When agent.model.apiType is "image", the executor MUST build an image generation request:

function build_image_args(agent, messages) → dict:
  // Extract prompt from last user message
  prompt = ""
  for msg in reversed(messages):
    if msg.role == "user":
      for part in msg.parts:
        if part.kind == "text":
          prompt = part.value
          break
      break

  args = { model: agent.model.id, prompt: prompt }

  // Pass through model options (size, quality, n, etc.)
  opts = build_options(agent.model.options)
  args.update(opts)

  return args

The prompt MUST be extracted from the last user-role message. If no user message exists, the prompt MUST be the empty string.

§7.4 OpenAI Responses API

The Responses API uses a different request/response model from Chat Completions. When agent.model.apiType is "responses", the executor MUST use this wire format instead of the Chat Completions format.

§7.4.1 Request Format

function build_responses_args(agent, messages) → dict:
  input_items = []

  for msg in messages:
    item = { role: msg.role }

    // Function-call metadata from a previous agent-loop iteration
    if msg.metadata and "responses_function_call" in msg.metadata:
      input_items.append(msg.metadata["responses_function_call"])
      input_items.append({
        type: "function_call_output",
        call_id: msg.metadata.get("tool_call_id"),
        output: msg.parts[0].value if msg.parts else ""
      })
      continue

    // Normal message
    if len(msg.parts) == 1 and msg.parts[0].kind == "text":
      item.content = msg.parts[0].value
    else:
      item.content = [part_to_wire(part) for part in msg.parts]

    input_items.append(item)

  args = { model: agent.model.id, input: input_items }

  // Tools
  tools = tools_to_wire(agent.tools, null)
  if tools:
    args.tools = tools

  // Structured output
  if agent.outputs:
    schema = schema_to_wire(agent.outputs)
    schema.additionalProperties = false
    args.text = {
      format: {
        type: "json_schema",
        name: "structured_output",
        strict: true,
        schema: schema
      }
    }

  return args

Response processing for the Responses API is defined in §8.4.

§7.5 Anthropic Messages API

Anthropic’s Messages API differs from OpenAI in several key ways. Implementations MAY support Anthropic as a provider.

function build_anthropic_args(agent, messages) → dict:
  // Anthropic REQUIRES the system message as a separate top-level field
  system_text = null
  non_system = []

  for msg in messages:
    if msg.role == "system":
      system_text = extract_text(msg)
    else:
      non_system.append(anthropic_message(msg))

  args = {
    model: agent.model.id,
    messages: non_system,
    max_tokens: agent.model.options.maxOutputTokens or 4096
  }

  if system_text:
    args.system = system_text

  // Options mapping (Anthropic-specific)
  if agent.model.options:
    if agent.model.options.temperature is not null:
      args.temperature = agent.model.options.temperature
    if agent.model.options.topP is not null:
      args.top_p = agent.model.options.topP
    if agent.model.options.topK is not null:
      args.top_k = agent.model.options.topK
    if agent.model.options.stopSequences is not null:
      args.stop_sequences = agent.model.options.stopSequences

  // Tools
  if agent.tools:
    tools = [anthropic_tool(t) for t in agent.tools if t.kind == "function"]
    if tools:
      args.tools = tools

  return args

Anthropic tool format:

function anthropic_tool(tool) → dict:
  return {
    name: tool.name,
    description: tool.description,
    input_schema: schema_to_wire(tool.parameters)
  }

Anthropic message format:

function anthropic_message(msg) → dict:
  // Anthropic always uses an array of typed content blocks
  blocks = []
  for part in msg.parts:
    match part.kind:
      "text"  → blocks.append({ type: "text", text: part.value })
      "image" → blocks.append({
                  type: "image",
                  source: {
                    type: "base64",
                    media_type: part.mediaType,
                    data: part.value
                  }
                })

  return { role: msg.role, content: blocks }

Implementations MUST always use the array-of-blocks form for Anthropic messages, even when there is only one text block.

§7.6 Options Mapping Table

The following table defines the canonical mapping from Prompty ModelOptions fields to provider-specific parameter names. Implementations MUST respect these mappings for each supported provider.

Prompty (ModelOptions)	OpenAI Chat	Anthropic Messages	Notes
temperature	temperature	temperature
maxOutputTokens	max_completion_tokens	max_tokens	OpenAI deprecated max_tokens
topP	top_p	top_p
topK	—	top_k	OpenAI does not support
frequencyPenalty	frequency_penalty	—	Anthropic does not support
presencePenalty	presence_penalty	—	Anthropic does not support
stopSequences	stop	stop_sequences	Different parameter name
seed	seed	—	Anthropic does not support

When a provider does not support an option, the implementation MUST silently ignore it (MUST NOT raise an error).

§7.7 Structured Output

When agent.outputs is non-empty, the following two-phase process applies:

1. Request phase: The executor MUST convert the output schema to the provider’s structured-output mechanism (e.g., response_format for OpenAI, or constrained decoding where supported).
2. Response phase: The processor MUST JSON-parse the content string from the response and return the parsed object instead of a raw string. If parsing fails, the processor SHOULD return the raw string as a fallback.

See §7.1.6 for the OpenAI wire format and §8 Processing for processing details.

§7.8 Adding a New Provider

To add support for a new LLM provider, an implementation MUST:

1. Implement the executor interface (execute / execute_async) as defined in §11.3.
2. Implement the processor interface (process / process_async) as defined in §11.3.
3. Register both via the invoker discovery mechanism (§11.3) under the provider key (e.g., "anthropic").
4. Document the wire-format mappings (message conversion, tool format, options mapping) in a provider-specific subsection.

User-Agent headers. Implementations SHOULD send User-Agent: prompty/<version> on all API requests to aid provider-side diagnostics.