Module codec 

Bijection codec between RunItem and raw OpenAI chat messages

What this module provides (spec)

• Deterministic, lossless conversions used by recording/replay and persistence layers
• Pure utility functions decoupled from services

Exports

• Models (reuse from items.rs if desired)
  • RunItem::{Message, ToolCall, ToolOutput, Handoff}
• Utils (pure)
  • messages_to_items(messages: &[RawChatMessage]) -> Vec<RunItem>
  • items_to_messages(items: &[RunItem]) -> Vec<RawChatMessage>
  • CodecError for invalid sequences (e.g., tool output without prior tool_call)

Implementation strategy

• One-pass state machine for messages_to_items:
  • Accumulate assistant tool_calls to attach to the preceding assistant message
  • Immediately emit ToolOutput for tool role messages with resolved tool_call_id
• The inverse reconstructs messages, ensuring assistant tool_calls coalesce correctly
• Avoid allocations by pre-sizing vectors and reusing buffers

Composition

• Recorder and session layers call these functions at the edge
• Does not depend on Tower; purely functional

Testing strategy

• Property tests (e.g., with proptest) asserting round-trip identity
• Golden-case unit tests for:
  • Multiple tool_calls in a single assistant message
  • Interleaved ToolCall/ToolOutput
  • Empty content and error outputs
• Fuzz invalid sequences to ensure CodecError surfaces clearly

Enums

CodecError

Functions

items_to_messages

Convert RunItems back to raw OpenAI request messages, preserving tool_calls semantics.

messages_to_items

Convert raw OpenAI request messages to a sequence of RunItems.