subagent data types

The module has exactly two data types: the call’s input and its output. Both derive Serialize + Deserialize, so they round-trip through the tool-call JSON directly.

SubAgentCallInput — the subagent_call arguments

pub struct SubAgentCallInput {
    /// The agent CLI to call (e.g. "opencode"). Must be one of the
    /// supported agents listed in the tool description.
    ///
    /// Optional: if omitted (or empty), an internal agent runs the task
    /// instead — a fresh nested harness with an empty context, in
    /// auto-approve mode, that persists nothing and returns only its final
    /// report.
    pub agent: Option<String>,

    /// The message to send to the sub-agent.
    ///
    /// Optional: if omitted (or empty), the last message sent to a
    /// sub-agent in this session is reused automatically. If no sub-agent
    /// has been called yet, an error is returned telling the caller to
    /// provide one.
    pub input: Option<String>,
}

Both fields are optional — deliberately. The combination of what is present and what is missing selects the behavior:

The agent value must be one of the names in the agent registry (AGENTS); anything else is rejected by validate_agent with a "Unsupported agent …" error.

SubAgentCallOutput — the result

pub struct SubAgentCallOutput {
    /// Accumulated output from the sub-agent.
    pub output: String,
    /// Exit code of the process.
    pub exit_code: i32,
}

The harness returns this as JSON after the sub-agent finishes. The two fields tell the caller everything needed to judge the outcome:

• output — the full accumulated stdout/stderr of the external CLI, or the internal agent’s final report.
• exit_code — 0 on success; a non-zero process exit code on failure; or -1 when the process was killed by a timeout (partial output may still be present, and the harness logs a warning in that case).

For the external path, a hard failure (spawn error, or a timeout with no output at all) is returned as an Err string instead of an output struct — see the call page for the exact cases.