Enum ChatMessage 

pub enum ChatMessage {
    User {
        content: String,
        attachments: Vec<UserAttachment>,
    },
    Assistant {
        text: Option<String>,
        tool_calls: Vec<ToolInvocation>,
        thinking: Option<AssistantThinking>,
        usage: Option<HarnessUsage>,
    },
    Tool {
        tool_call_id: String,
        content: String,
        is_error: bool,
        attachments: Vec<UserAttachment>,
    },
}

One conversation entry as seen by the model. Mirrors the system / user / assistant / tool set OpenAI chat/completions expects. Anthropic’s Messages API uses a different shape (tool_use / tool_result content blocks instead of separate tool role) but consumes the same ChatMessage history — the projection lives in the per-provider model client, keeping the harness loop provider-agnostic.

Variants

User

Fields

content: String

attachments: Vec<UserAttachment>

Non-text attachments (images today; file_id / documents later). Renders as additional content blocks alongside the content text on the wire — projection per provider in §8 / §9.

Assistant

Assistant turn. May carry any combination of: a thinking block (Anthropic extended thinking), final text, and one+ tool calls. All three render into the assistant message’s content array on the Anthropic wire; OpenAI uses tool_calls for tool calls and ignores thinking entirely.

Fields

text: Option<String>

tool_calls: Vec<ToolInvocation>

thinking: Option<AssistantThinking>

Thinking block to round-trip verbatim. None for OpenAI / Anthropic-without-extended-thinking turns.

usage: Option<HarnessUsage>

Provider-reported token tally for this assistant turn, when the provider supplied one. Records the exact context size up to and including this message, so context-size estimation can anchor on a measured value and estimate only the messages appended after it. None when the turn carried no reported usage.

Tool

Tool response paired by tool_call_id. content is the serialized tool output (model side sees a string regardless of the underlying JSON shape). attachments carries structured non-text content the tool produced — e.g. a screenshot MCP tool returning an image. Only providers with a tool-content array (Anthropic) surface these on the wire; OpenAI tool role is strictly string-typed, so attachments degrade to a placeholder in the content string there.

Fields

tool_call_id: String

content: String

is_error: bool

attachments: Vec<UserAttachment>

Trait Implementations

impl Clone for ChatMessage

fn clone(&self) -> ChatMessage

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ChatMessage

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for ChatMessage

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for ChatMessage

fn eq(&self, other: &ChatMessage) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl Serialize for ChatMessage

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for ChatMessage