aaif-goose/goose · error · anyhow::Error

ACP agent rejected session/set_config_option for '{}': {err}

Error message

ACP agent rejected session/set_config_option for '{}': {err}

What it means

After a session is created or loaded, goose replays config.session_config_options via session/set_config_option RPCs. This error means the agent answered that RPC with an error for the named config id — the option is unknown, the value is rejected, or the agent considers the state invalid. It fires per option, naming the exact config_id that was refused.

Source

Thrown at crates/goose/src/acp/provider.rs:1436

    Ok(())
}

async fn apply_session_config_options(
    config: &AcpProviderConfig,
    cx: &ConnectionTo<Agent>,
    session_id: SessionId,
) -> Result<()> {
    for (config_id, value) in &config.session_config_options {
        let value_id = agent_client_protocol::schema::v1::SessionConfigValueId::new(value.clone());
        cx.send_request(SetSessionConfigOptionRequest::new(
            session_id.clone(),
            config_id.clone(),
            value_id,
        ))
        .block_task()
        .await
        .map_err(|err| {
            anyhow::anyhow!(
                "ACP agent rejected {} for '{}': {err}",
                AGENT_METHOD_NAMES.session_set_config_option,
                config_id
            )
        })?;
    }
    Ok(())
}

async fn apply_session_mode(
    config: &AcpProviderConfig,
    goose_mode: &Arc<Mutex<GooseMode>>,
    cx: &ConnectionTo<Agent>,
    session: NewSessionResponse,
) -> Result<NewSessionResponse> {
    let current_mode = goose_mode.lock().ok().map(|mode| *mode);
    let candidates = initial_mode_candidates(config, current_mode);

View on GitHub (pinned to 3810898a74)

Solutions

  1. Read the agent's config_options from its session/new response (or docs) and delete options it does not advertise
  2. Correct the value for the named option to one of the agent's allowed choices
  3. After upgrading an agent, reconcile your session_config_options with the new capability set

Example fix

# before
session_config_options:
  legacyMode: true      # agent no longer knows this option

# after
session_config_options:
  nestedMode: auto      # an option the agent advertises
Defensive patterns

Strategy: validation

Validate before calling

let advertised: HashSet<_> = session
    .config_options
    .iter()
    .flat_map(|o| o.options.iter().map(|o| o.property_id.0.clone()))
    .collect();
let accepted: Vec<_> = config.session_config_options
    .iter()
    .filter(|(id, _)| advertised.contains(id))
    .cloned()
    .collect();

Try / catch

match apply_session_config_options(&config, &cx, sid).await {
    Err(e) if e.to_string().contains("set_config_option") => {
        tracing::warn!(%e, "agent rejected a config option; trim session_config_options to advertised ids");
        // continue with defaults rather than aborting the session
    }
    r => r?,
}

Prevention

When it happens

Trigger: config.session_config_options contains an entry like ('nestedMode', 'auto') but the agent's NewSessionResponse.config_options does not list that option id, or lists it with different allowed values; sending a value the agent validates and refuses.

Common situations: Hand-writing session_config_options copied from a different agent's docs; an agent upgrade renames or removes a config option while goose config still sets the old id.

Related errors

AI-assisted analysis of aaif-goose/goose@3810898a74 (2026-08-16). Data as JSON: /api/errors/afc310b7d2c59a87. Report an issue: GitHub.