fix: Claude code system prompt injected into claude requests to make them valid once again

src/client/claude.rs

@@ -11,6 +11,7 @@ use serde::Deserialize;
 use serde_json::{Value, json};
 
 const API_BASE: &str = "https://api.anthropic.com/v1";
+const CLAUDE_CODE_PREFIX: &str = "You are Claude Code, Anthropic's official CLI for Claude.";
 
 #[derive(Debug, Clone, Deserialize)]
 pub struct ClaudeConfig {
@@ -94,6 +95,7 @@ async fn prepare_chat_completions(
         for (key, value) in provider.extra_request_headers() {
             request_data.header(key, value);
         }
+        inject_oauth_system_prompt(&mut request_data.body);
     } else if let Ok(api_key) = self_.get_api_key() {
         request_data.header("x-api-key", api_key);
     } else {
@@ -107,6 +109,43 @@ async fn prepare_chat_completions(
     Ok(request_data)
 }
 
+/// Anthropic requires OAuth-authenticated requests to include a Claude Code
+/// system prompt prefix in order to consider a request body as "valid".
+///
+/// This behavior was discovered 2026-03-17.
+///
+/// So this function injects the Claude Code system prompt into the request
+/// body to make it a valid request.
+fn inject_oauth_system_prompt(body: &mut Value) {
+    let prefix_block = json!({
+        "type": "text",
+        "text": CLAUDE_CODE_PREFIX,
+    });
+
+    match body.get("system") {
+        Some(Value::String(existing)) => {
+            let existing_block = json!({
+                "type": "text",
+                "text": existing,
+            });
+            body["system"] = json!([prefix_block, existing_block]);
+        }
+        Some(Value::Array(_)) => {
+            if let Some(arr) = body["system"].as_array_mut() {
+                let already_injected = arr
+                    .iter()
+                    .any(|block| block["text"].as_str() == Some(CLAUDE_CODE_PREFIX));
+                if !already_injected {
+                    arr.insert(0, prefix_block);
+                }
+            }
+        }
+        _ => {
+            body["system"] = json!([prefix_block]);
+        }
+    }
+}
+
 pub async fn claude_chat_completions(
     builder: RequestBuilder,
     _model: &Model,

src/client/model.rs

@@ -177,6 +177,10 @@ impl Model {
         self.data.max_output_tokens
     }
 
+    pub fn supports_function_calling(&self) -> bool {
+        self.data.supports_function_calling
+    }
+
     pub fn no_stream(&self) -> bool {
         self.data.no_stream
     }

src/config/input.rs

@@ -239,12 +239,17 @@ impl Input {
         patch_messages(&mut messages, model);
         model.guard_max_input_tokens(&messages)?;
         let (temperature, top_p) = (self.role().temperature(), self.role().top_p());
-        let functions = self.config.read().select_functions(self.role());
-        if let Some(vec) = &functions {
-            for def in vec {
-                debug!("Function definition: {:?}", def.name);
+        let functions = if model.supports_function_calling() {
+            let fns = self.config.read().select_functions(self.role());
+            if let Some(vec) = &fns {
+                for def in vec {
+                    debug!("Function definition: {:?}", def.name);
+                }
             }
-        }
+            fns
+        } else {
+            None
+        };
         Ok(ChatCompletionsData {
             messages,
             temperature,

src/config/mod.rs

@@ -1842,6 +1842,12 @@ impl Config {
             bail!("Already in an agent, please run '.exit agent' first to exit the current agent.");
         }
         let agent = Agent::init(config, agent_name, abort_signal.clone()).await?;
+        if !agent.model().supports_function_calling() {
+            eprintln!(
+                "Warning: The model '{}' does not support function calling. Agent tools (including todo, spawning, and user interaction) will not be available.",
+                agent.model().id()
+            );
+        }
         let session = session_name.map(|v| v.to_string()).or_else(|| {
             if config.read().macro_flag {
                 None