diff --git a/src/app/runtime/stream/turn.rs b/src/app/runtime/stream/turn.rs
index bc7511e..47994ee 100644
--- a/src/app/runtime/stream/turn.rs
+++ b/src/app/runtime/stream/turn.rs
@@ -157,6 +157,28 @@ impl StreamedTurn {
         msg
     }
 
+    /// Find the first named tool call whose accumulated `arguments` do not
+    /// parse as valid JSON.
+    ///
+    /// Why: a connection that closes mid-stream (no `[DONE]` event) still
+    /// leaves partial argument text in the accumulator — e.g. a `write`
+    /// tool call cut off mid-string. Parsing that fragment always fails,
+    /// so a parse failure at end-of-stream is a reliable signal that the
+    /// response was truncated, not that the model legitimately finished
+    /// without sending `[DONE]`.
+    ///
+    /// Return: `Some((name, parse_error))` for the first bad tool call, or
+    /// `None` if every tool call's arguments are complete, parsable JSON.
+    pub fn incomplete_tool_call(&self) -> Option<(&str, String)> {
+        self.tool_calls.iter()
+            .filter(|tc| !tc.name.is_empty())
+            .find_map(|tc| {
+                serde_json::from_str::<Value>(&tc.arguments)
+                    .err()
+                    .map(|e| (tc.name.as_str(), e.to_string()))
+            })
+    }
+
     /// Reserved accessor for callers that want to branch mid-stream before the turn
     /// completes; the current wiring only inspects the final `build_assistant_message()`.
     #[allow(dead_code)]
@@ -176,3 +198,41 @@ impl Default for StreamedTurn {
         Self::new()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn tool_call(name: &str, arguments: &str) -> ParsedToolCall {
+        ParsedToolCall {
+            id: "call_1".to_string(),
+            name: name.to_string(),
+            arguments: arguments.to_string(),
+            is_complete: false,
+        }
+    }
+
+    #[test]
+    fn incomplete_tool_call_flags_truncated_json() {
+        let mut turn = StreamedTurn::new();
+        turn.tool_calls.push(tool_call("write", "{\"path\": \"a.txt\", \"content\": \"unterm"));
+        let bad = turn.incomplete_tool_call();
+        assert_eq!(bad.map(|(name, _)| name), Some("write"));
+    }
+
+    #[test]
+    fn incomplete_tool_call_accepts_complete_json() {
+        let mut turn = StreamedTurn::new();
+        turn.tool_calls.push(tool_call("write", "{\"path\": \"a.txt\", \"content\": \"done\"}"));
+        assert!(turn.incomplete_tool_call().is_none());
+    }
+
+    #[test]
+    fn incomplete_tool_call_ignores_calls_without_a_name() {
+        // A slot reserved by `apply_event` (via index padding) but never
+        // filled with a name shouldn't be mistaken for a truncated call.
+        let mut turn = StreamedTurn::new();
+        turn.tool_calls.push(tool_call("", "not json at all"));
+        assert!(turn.incomplete_tool_call().is_none());
+    }
+}
diff --git a/src/app/subagent/engine.rs b/src/app/subagent/engine.rs
index 2587699..3aa2ac3 100644
--- a/src/app/subagent/engine.rs
+++ b/src/app/subagent/engine.rs
@@ -54,8 +54,10 @@ fn build_subagent_tools(allowed_tools: &[String]) -> (Vec<Box<dyn crate::tool::T
 /// Why: matches the main agent's credential resolution exactly, so
 /// subagents automatically inherit the same provider settings.
 ///
-/// Return: `(api_key, model, optional_base_url)`.
-fn resolve_provider_config() -> (String, String, Option<String>) {
+/// Return: `(api_key, model, optional_base_url, provider_name)`. `api_key`
+/// is empty when every resolution path was exhausted — callers must check
+/// for this before issuing requests (see `run_subagent`).
+fn resolve_provider_config() -> (String, String, Option<String>, String) {
     let settings = crate::model::settings::Settings::load();
     let app_config = crate::model::app_config::AppConfig::load();
 
@@ -79,7 +81,21 @@ fn resolve_provider_config() -> (String, String, Option<String>) {
         }
     }
 
-    (api_key, model, base_url)
+    (api_key, model, base_url, settings.provider)
+}
+
+/// Reject an empty API key with an actionable error instead of letting the
+/// caller send a request that is guaranteed to fail once it reaches the network.
+///
+/// Return: `Ok(())` if `api_key` is non-empty, `Err` with a message naming
+/// `provider` and where to fix it otherwise.
+fn require_api_key(api_key: &str, provider: &str) -> anyhow::Result<()> {
+    if api_key.is_empty() {
+        anyhow::bail!(
+            "no API key configured for provider '{provider}' — set one in Settings or ~/.claude/settings.json"
+        );
+    }
+    Ok(())
 }
 
 // ─── Subagent-level tool gating (mirrors Harness checks) ───
@@ -325,7 +341,20 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
     // Cache provider config once before the loop instead of re-resolving
     // from disk on every step (Settings::load + AppConfig::load each parse
     // JSON files, and the config cannot change between steps).
-    let (api_key, model, base_url) = resolve_provider_config();
+    let (api_key, model, base_url, provider) = resolve_provider_config();
+
+    // Fail fast on a missing key instead of sending a doomed request: an
+    // empty api_key still reaches the network (base_url falls back to a
+    // default endpoint), so without this check every step burns a full
+    // 10-retry timeout/backoff cycle against a server that was never going
+    // to authenticate, and the real cause (no key configured) never
+    // surfaces past a buried WARN log.
+    if let Err(error) = require_api_key(&api_key, &provider) {
+        let error = error.to_string();
+        let _ = tx.blocking_send(SubagentEvent::StepFailed { step: 0, error: error.clone() });
+        anyhow::bail!(error);
+    }
+
     let client = crate::service::provider::LlmClient::new(api_key, model, base_url);
 
     for step in 0..ctx.max_steps {
@@ -559,3 +588,19 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
     let _ = tx.blocking_send(SubagentEvent::Completed { output: output.clone() });
     Ok(output)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn require_api_key_rejects_empty_key_with_provider_named_in_message() {
+        let err = require_api_key("", "claude").unwrap_err();
+        assert!(err.to_string().contains("claude"));
+    }
+
+    #[test]
+    fn require_api_key_accepts_non_empty_key() {
+        assert!(require_api_key("sk-live-abc123", "claude").is_ok());
+    }
+}
diff --git a/src/service/provider.rs b/src/service/provider.rs
index 6d93f58..905893b 100644
--- a/src/service/provider.rs
+++ b/src/service/provider.rs
@@ -318,6 +318,19 @@ impl LlmClient {
             }
         }
 
+        // The connection closed without an explicit `[DONE]` event. Some
+        // providers legitimately omit it, so EOF alone isn't an error —
+        // but if it leaves a tool call's arguments as unparsable JSON, the
+        // response was truncated mid-generation, not finished. Report that
+        // honestly instead of silently double-stringifying the fragment
+        // into a tool call that will misbehave (e.g. a `write` call with a
+        // half-written file body).
+        if let Some((name, err)) = turn.incomplete_tool_call() {
+            anyhow::bail!(
+                "stream ended before tool call '{name}' arguments were complete: {err}"
+            );
+        }
+
         turn.is_complete = true;
         Ok((turn.build_assistant_message(), usage))
     }