An agent set up by its program and a model runs that agent with that model (#46)

An agent CLI's `model` names the agent (opencode) and the model it should use goes in cli_model,
which nothing in the app shows. People write the program in `command` and the model in `model`
instead (command = "opencode", model = "deepseek/deepseek-flash"), which ran the plain adapter
with the agent's own default model until its free quota ran out. Settings read that way now run the
agent the command names, asked for that model (OpenCode gets --model). A model that names an
agent, a cli_model already set, and unknown programs are left as they are.

Fixes #34

crates/ai/src/config.rs

@@ -70,6 +70,29 @@ pub struct BackendEntry {
     pub options: BackendOptions,
 }
 
+impl BackendEntry {
+    /// An agent CLI's `model` names the agent (`opencode`), and the model it should use goes in
+    /// `cli_model`. People also write the agent's program in `command` and the model in `model`
+    /// (`command = "opencode"`, `model = "deepseek/deepseek-flash"`); that ran the plain adapter
+    /// with the agent's own default model (#34). Read that way, it's the agent the command names,
+    /// asked for that model.
+    fn model_from_command(&mut self) {
+        let known = crate::cli_agents::KNOWN;
+        let model = self.config.model.trim();
+        let names_an_agent = known.iter().any(|(id, prog, display)| model == *id || model == *prog || model.eq_ignore_ascii_case(display));
+        if self.config.kind != BackendKind::AgentCli || model.is_empty() || names_an_agent {
+            return;
+        }
+        // The file name without `.exe`, whichever system's separators the path uses.
+        let program = self.options.command.as_deref().and_then(|c| c.trim().rsplit(['/', '\\']).next()).map(|f| f.strip_suffix(".exe").unwrap_or(f));
+        let Some((id, _, _)) = program.and_then(|p| known.iter().find(|(_, prog, _)| prog.eq_ignore_ascii_case(p))) else { return };
+        if self.options.cli_model.is_none() {
+            self.options.cli_model = Some(model.to_string());
+        }
+        self.config.model = id.to_string();
+    }
+}
+
 /// USD per million tokens.
 #[derive(Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
 pub struct Price {
@@ -109,10 +132,12 @@ pub fn role_key(r: Role) -> &'static str {
 impl AiConfig {
     /// Parse `settings.ai`. A malformed table gives an error the UI can show (never a panic).
     pub fn from_table(t: &toml::Table) -> Result<AiConfig, ToolError> {
-        toml::Value::Table(t.clone()).try_into().map_err(|e: toml::de::Error| {
+        let mut c: AiConfig = toml::Value::Table(t.clone()).try_into().map_err(|e: toml::de::Error| {
             ToolError::new(ErrorCode::BadSettings, format!("The [ai] settings couldn't be read: {e}"))
                 .with_remediation("Fix or remove the [ai] section in config.toml.")
-        })
+        })?;
+        c.backends.iter_mut().for_each(BackendEntry::model_from_command);
+        Ok(c)
     }
 
     /// Parse from settings; unreadable config is logged and treated as empty.
@@ -263,4 +288,23 @@ mod tests {
         let s = Settings { ai: t, ..Default::default() };
         assert!(AiConfig::from_settings(&s).backends.is_empty());
     }
+
+    /// #34: the agent's program in `command` and the model in `model` runs that agent with it.
+    #[test]
+    fn an_agent_named_by_its_command_gets_the_model() {
+        let one = |toml: &str| {
+            let t: toml::Table = toml::from_str(&format!("[[backends]]\nname = \"A\"\nkind = \"agent_cli\"\n{toml}")).unwrap();
+            let b = AiConfig::from_table(&t).unwrap().backends.remove(0);
+            (b.config.model, b.options.cli_model)
+        };
+        let reported = "model = \"deepseek/deepseek-flash\"\ncommand = \"opencode\"\nargs = [\"run\", \"--auto\"]";
+        assert_eq!(one(reported), ("opencode".into(), Some("deepseek/deepseek-flash".into())));
+        assert_eq!(one("model = \"sonnet\"\ncommand = 'C:\\Users\\u\\.bun\\bin\\opencode.exe'").0, "opencode", "a full path names it too");
+        // Left as they are: a model that names the agent, a cli_model already set, an unknown program.
+        assert_eq!(one("model = \"opencode\"\ncommand = \"opencode\""), ("opencode".into(), None));
+        assert_eq!(one("model = \"gemini\"\ncommand = \"gemini\""), ("gemini".into(), None));
+        assert_eq!(one("model = \"x/y\"\ncommand = \"opencode\"\ncli_model = \"z\""), ("opencode".into(), Some("z".into())));
+        assert_eq!(one("model = \"my-agent\"\ncommand = \"my-agent-cli\""), ("my-agent".into(), None));
+        assert_eq!(one("model = \"claude-code\""), ("claude-code".into(), None));
+    }
 }