fix(agent): discard truncated tool calls when finish_reason == Length (#1631) (#1632)

src/agent/agentic_loop.rs

@@ -10,7 +10,7 @@ use std::borrow::Cow;
 
 use crate::agent::session::PendingApproval;
 use crate::error::Error;
-use crate::llm::{ChatMessage, Reasoning, ReasoningContext, RespondResult};
+use crate::llm::{ChatMessage, FinishReason, Reasoning, ReasoningContext, RespondResult};
 
 /// Signal from the delegate indicating how the loop should proceed.
 pub enum LoopSignal {
@@ -134,6 +134,9 @@ pub async fn run_agentic_loop(
     config: &AgenticLoopConfig,
 ) -> Result<LoopOutcome, Error> {
     let mut consecutive_tool_intent_nudges: u32 = 0;
+    // Accumulates across all iterations (not reset by text responses) so
+    // non-consecutive truncations still escalate to force_text.
+    let mut truncation_count: u32 = 0;
 
     for iteration in 1..=config.max_iterations {
         // Check for external signals (stop, cancellation, user messages)
@@ -215,7 +218,35 @@ pub async fn run_agentic_loop(
                 tool_calls,
                 content,
             } => {
+                // If the response was truncated, tool call parameters are likely
+                // incomplete. Discard them and tell the LLM to try a different
+                // approach rather than executing malformed tool calls.
+                if output.finish_reason == FinishReason::Length {
+                    truncation_count += 1;
+                    let names: Vec<&str> = tool_calls.iter().map(|tc| tc.name.as_str()).collect();
+                    tracing::warn!(
+                        iteration,
+                        tools = ?names,
+                        truncation_count,
+                        "Discarding truncated tool calls (finish_reason=Length)"
+                    );
+                    if let Some(ref text) = content {
+                        reason_ctx.messages.push(ChatMessage::assistant(text));
+                    }
+                    reason_ctx
+                        .messages
+                        .push(ChatMessage::user(crate::llm::TRUNCATED_TOOL_CALL_NOTICE));
+                    // After repeated truncations, force text-only mode so the LLM
+                    // stops attempting tool calls it can't fit in the output budget.
+                    if truncation_count >= 3 {
+                        reason_ctx.force_text = true;
+                    }
+                    delegate.after_iteration(iteration).await;
+                    continue;
+                }
+
                 consecutive_tool_intent_nudges = 0;
+                truncation_count = 0;
 
                 if let Some(outcome) = delegate
                     .execute_tool_calls(tool_calls, content, reason_ctx)
@@ -271,6 +302,7 @@ mod tests {
         RespondOutput {
             result: RespondResult::Text(text.to_string()),
             usage: zero_usage(),
+            finish_reason: FinishReason::Stop,
         }
     }
 
@@ -281,6 +313,7 @@ mod tests {
                 content: None,
             },
             usage: zero_usage(),
+            finish_reason: FinishReason::ToolUse,
         }
     }
 
@@ -622,4 +655,95 @@ mod tests {
         let result = truncate_for_preview("café", 4);
         assert_eq!(result, "caf...");
     }
+
+    #[tokio::test]
+    async fn test_truncated_tool_calls_discarded_on_length() {
+        let truncated_tool_call = ToolCall {
+            id: "call_1".to_string(),
+            name: "memory_write".to_string(),
+            arguments: serde_json::json!({}), // empty — truncated
+            reasoning: None,
+        };
+        let truncated_output = RespondOutput {
+            result: RespondResult::ToolCalls {
+                tool_calls: vec![truncated_tool_call],
+                content: Some("I'll write the report.".to_string()),
+            },
+            usage: zero_usage(),
+            finish_reason: FinishReason::Length, // response was truncated
+        };
+        let delegate = MockDelegate::new(vec![truncated_output, text_output("Summarized it.")]);
+        let reasoning = stub_reasoning();
+        let mut ctx = ReasoningContext::new();
+        let config = AgenticLoopConfig {
+            max_iterations: 5,
+            ..Default::default()
+        };
+
+        let outcome = run_agentic_loop(&delegate, &reasoning, &mut ctx, &config)
+            .await
+            .unwrap();
+
+        // Tool calls should NOT have been executed
+        assert_eq!(delegate.tool_exec_count.load(Ordering::SeqCst), 0);
+        // The loop should have continued and returned the text response
+        assert!(matches!(outcome, LoopOutcome::Response(ref t) if t == "Summarized it."));
+        // A truncation notice should have been injected into context
+        assert!(
+            ctx.messages
+                .iter()
+                .any(|m| m.role == crate::llm::Role::User && m.content.contains("truncated")),
+            "Should inject truncation notice into context"
+        );
+        // The partial assistant content should have been preserved
+        assert!(
+            ctx.messages
+                .iter()
+                .any(|m| m.role == crate::llm::Role::Assistant
+                    && m.content.contains("write the report")),
+            "Should preserve partial assistant content"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_repeated_truncations_force_text_mode() {
+        let make_truncated = || RespondOutput {
+            result: RespondResult::ToolCalls {
+                tool_calls: vec![ToolCall {
+                    id: "call_1".to_string(),
+                    name: "memory_write".to_string(),
+                    arguments: serde_json::json!({}),
+                    reasoning: None,
+                }],
+                content: None,
+            },
+            usage: zero_usage(),
+            finish_reason: FinishReason::Length,
+        };
+        // Three truncated responses, then a text response
+        let delegate = MockDelegate::new(vec![
+            make_truncated(),
+            make_truncated(),
+            make_truncated(),
+            text_output("Gave up on tool calls."),
+        ]);
+        let reasoning = stub_reasoning();
+        let mut ctx = ReasoningContext::new();
+        let config = AgenticLoopConfig {
+            max_iterations: 5,
+            ..Default::default()
+        };
+
+        let outcome = run_agentic_loop(&delegate, &reasoning, &mut ctx, &config)
+            .await
+            .unwrap();
+
+        assert!(matches!(outcome, LoopOutcome::Response(_)));
+        assert_eq!(delegate.tool_exec_count.load(Ordering::SeqCst), 0);
+        // After 3 truncations, force_text should be set
+        assert!(
+            ctx.force_text,
+            "Should escalate to force_text after repeated truncations"
+        );
+    }
 }

src/agent/dispatcher.rs

@@ -306,6 +306,8 @@ impl<'a> LoopDelegate for ChatDelegate<'a> {
 
         // Update context for this iteration
         reason_ctx.available_tools = tool_defs;
+        // Preserve force_text if already set (e.g. by truncation escalation).
+        let force_text = force_text || reason_ctx.force_text;
         reason_ctx.system_prompt = Some(if force_text {
             self.cached_prompt_no_tools.clone()
         } else {