Model/provider not updated when switching agents

[dpshde]

Bug

When switching agents via /forge, /muse, /sage, /agent, or /agent-{id}, the model and provider are not updated to reflect the new agent's configured model/provider. This affects analytics tracking and user feedback, even though the prompt display and actual LLM calls work correctly.

Root Cause

on_agent_change() at crates/forge_main/src/ui.rs:186-209 only calls set_active_agent() (which stores the agent ID). It does not:

1. Resolve the new agent's model via get_agent_model()
2. Call update_model() to update the analytics tracker
3. Display the model/provider change to the user

Current code

async fn on_agent_change(&mut self, agent_id: AgentId) -> Result<()> {
    let agent = self.api.get_agents().await?
        .iter().find(|agent| agent.id == agent_id)
        .cloned()
        .ok_or(anyhow::anyhow!("Undefined agent: {agent_id}"))?;

    // Only sets the active agent ID — does NOT update model/provider
    self.api.set_active_agent(agent.id.clone()).await?;

    let name = agent.id.as_str().to_case(Case::UpperSnake).bold();
    let title = format!("∙ {}", agent.title.as_deref().unwrap_or(MISSING_AGENT_TITLE)).dimmed();
    self.writeln_title(TitleFormat::action(format!("{name} {title}")))?;
    Ok(())
}

Comparison: How model IS updated elsewhere

When the user runs /model (crates/forge_main/src/ui.rs:2749-2753):

self.api.set_default_model(model.clone()).await?;
self.update_model(Some(model.clone()));
self.writeln_title(TitleFormat::action(format!("Switched to model: {model}")))?;

When init_state() runs (crates/forge_main/src/ui.rs:2957-3008):

let mut operating_model = self.get_agent_model(active_agent.clone()).await;
self.update_model(operating_model);

Proposed Solution

Update on_agent_change() to resolve and propagate the agent's model after switching:

async fn on_agent_change(&mut self, agent_id: AgentId) -> Result<()> {
    let agent = self
        .api
        .get_agents()
        .await?
        .iter()
        .find(|agent| agent.id == agent_id)
        .cloned()
        .ok_or(anyhow::anyhow!("Undefined agent: {agent_id}"))?;

    self.api.set_active_agent(agent.id.clone()).await?;

    // Update model tracking to reflect the new agent's model
    let model = self.get_agent_model(Some(agent.id.clone())).await;
    self.update_model(model.clone());

    let name = agent.id.as_str().to_case(Case::UpperSnake).bold();
    let title = format!(
        "∙ {}",
        agent.title.as_deref().unwrap_or(MISSING_AGENT_TITLE)
    )
    .dimmed();

    // Show model info if agent uses a specific model
    let model_info = model
        .map(|m| format!(" ∙ model: {m}").dimmed().to_string())
        .unwrap_or_default();

    self.writeln_title(TitleFormat::action(format!("{name} {title}{model_info}")))?;
    Ok(())
}

Affected Call Sites

All callers of on_agent_change() are affected:

Command	File	Line
/forge	crates/forge_main/src/ui.rs	1874
/muse	crates/forge_main/src/ui.rs	1877
/sage	crates/forge_main/src/ui.rs	1880
/agent (interactive selection)	crates/forge_main/src/ui.rs	1994
/agent-{id} (direct switch)	crates/forge_main/src/ui.rs	2017

Impact

• Analytics/tracking: The model sent to the tracker is stale after agent switch
• User feedback: No model/provider info shown when switching to an agent with a different model
• Chat flow: NOT affected — ForgeApp::chat() independently resolves the agent's model/provider via AgentProviderResolver, so LLM calls use the correct model

Why This Was Missed

The built-in agents (forge, muse, sage) don't specify their own model or provider — they inherit session defaults. The bug only manifests with custom agents (in ~/.forge/agents/*.md or .forge/agents/*.md) that define their own model/provider. The AgentDefinition schema supports optional model and provider fields (crates/forge_repo/src/agent_definition.rs:34-38), but no built-in agents use them.

Additional Notes

• The prompt display is correct — it independently resolves the model per-agent on every call via get_agent_model(). This makes the bug harder to notice visually.
• Consider also checking if /config reflects the agent's model correctly — on_show_config() at crates/forge_main/src/ui.rs:1317-1384 uses get_agent_model(None) which returns the default model, not the agent-specific one.