src/message.rs

Everything your model ever sees is built from these types: Message, Role, MessagePart, ImageSource, ToolContent, and ToolContentPart.

Key items

Message { role, parts } gives you the constructors user, assistant, and new, plus the readers text_content() and tool_call_parts().

Role is User, Assistant, or System, serialized in snake_case.

MessagePart covers Text, Image, and ToolCall { id, name, input }.

Thinking { text, signature, redacted } arrived in 0.4.0, where provider signatures latch and replay, and redacted blocks carry their opaque payload.

ToolResult { call_id, name, output, is_error, retention } carries the compaction and prune class since 0.4.0, set with tool_result(...).with_retention(...).

Constructors are text, tool_call, and tool_result, and predicates include is_text etc., plus as_text().

ImageSource::new_base64(media_type, data) is the only way to build one, and your data must be raw base64 with no data-URI prefix.

ToolContent is Text(String) or Multipart(Vec<ToolContentPart>), with From<String> and From<&str>. Its Display joins text parts and skips images.

Behavior notes

Tool results ride Role::User messages, which is provider convention and handled everywhere.

Pairing ToolCall.id with ToolResult.call_id is the load-bearing link, and your engine stamps ids authoritatively.

Everything serializes with serde, and that is what makes machine checkpointing work for you. signature, redacted, and retention ride skip_serializing_if, so old saved conversations load unchanged.

Thinking rides the conversation verbatim since 0.4.0, with no turn-boundary strip, and a compaction pass ages it out.

Your prefix stays byte-stable so provider prompt caches keep hitting.

Adjacent assistant runs fold since 0.4.1 through coalesce_adjacent_assistants, which serves both the compaction seam and the Anthropic request build. Runs of assistant messages merge into one carrying the parts of every member in order, with a \n\n separator when two texts would glue.

Some providers, Z.ai’s GLM among them, reject the adjacent shape with HTTP 400 [1214].

ToolResult.name defaults to "" when you deserialize old data, because the field came later.

Deep dive: Messages.