fix(agent): treat tool_use/tool_result as atomic groups in history pruning

src/agent/history.rs

@@ -76,6 +76,8 @@ pub(crate) fn fast_trim_tool_results(
 }
 
 /// Emergency: drop oldest non-system, non-recent messages from history.
+/// Tool groups (assistant + consecutive tool messages) are dropped
+/// atomically to preserve tool_use/tool_result pairing. See #4810.
 /// Returns number of messages dropped.
 pub(crate) fn emergency_history_trim(
     history: &mut Vec<crate::providers::ChatMessage>,
@@ -87,6 +89,18 @@ pub(crate) fn emergency_history_trim(
     while dropped < target_drop && i < history.len().saturating_sub(keep_recent) {
         if history[i].role == "system" {
             i += 1;
+        } else if history[i].role == "assistant" {
+            // Count following tool messages — drop as atomic group
+            let mut tool_count = 0;
+            while i + 1 + tool_count < history.len().saturating_sub(keep_recent)
+                && history[i + 1 + tool_count].role == "tool"
+            {
+                tool_count += 1;
+            }
+            for _ in 0..=tool_count {
+                history.remove(i);
+                dropped += 1;
+            }
         } else {
             history.remove(i);
             dropped += 1;

src/agent/history_pruner.rs

@@ -101,44 +101,79 @@ pub fn prune_history(messages: &mut Vec<ChatMessage>, config: &HistoryPrunerConf
 
     let mut collapsed_pairs: usize = 0;
 
-    // Phase 1 – collapse assistant+tool pairs
+    // Phase 1 – collapse assistant+tool groups atomically.
+    // An assistant message followed by one or more consecutive tool messages
+    // forms an atomic group (tool_use + tool_result pairing). Collapsing only
+    // part of the group would orphan tool_use blocks, causing API 400 errors
+    // from providers that enforce pairing (e.g., Anthropic). See #4810.
     if config.collapse_tool_results {
         let mut i = 0;
-        while i + 1 < messages.len() {
+        while i < messages.len() {
             let protected = protected_indices(messages, config.keep_recent);
-            if messages[i].role == "assistant"
-                && messages[i + 1].role == "tool"
-                && !protected[i]
-                && !protected[i + 1]
-            {
-                let tool_content = &messages[i + 1].content;
-                let truncated: String = tool_content.chars().take(100).collect();
-                let summary = format!("[Tool result: {truncated}...]");
-                messages[i] = ChatMessage {
-                    role: "assistant".to_string(),
-                    content: summary,
-                };
-                messages.remove(i + 1);
-                collapsed_pairs += 1;
-            } else {
-                i += 1;
+            if messages[i].role == "assistant" && !protected[i] {
+                // Count consecutive tool messages following this assistant
+                let mut tool_count = 0;
+                while i + 1 + tool_count < messages.len()
+                    && messages[i + 1 + tool_count].role == "tool"
+                    && !protected[i + 1 + tool_count]
+                {
+                    tool_count += 1;
+                }
+                if tool_count > 0 {
+                    let summary =
+                        format!("[Tool exchange: {tool_count} tool call(s) — results collapsed]");
+                    messages[i] = ChatMessage {
+                        role: "assistant".to_string(),
+                        content: summary,
+                    };
+                    for _ in 0..tool_count {
+                        messages.remove(i + 1);
+                    }
+                    collapsed_pairs += tool_count;
+                    continue;
+                }
             }
+            i += 1;
         }
     }
 
-    // Phase 2 – budget enforcement
+    // Phase 2 – budget enforcement: drop messages to fit token budget.
+    // Tool groups (assistant + consecutive tool messages) are dropped
+    // atomically to preserve tool_use/tool_result pairing. See #4810.
     let mut dropped_messages: usize = 0;
     while estimate_tokens(messages) > config.max_tokens {
         let protected = protected_indices(messages, config.keep_recent);
-        if let Some(idx) = protected
-            .iter()
-            .enumerate()
-            .find(|&(_, &p)| !p)
-            .map(|(i, _)| i)
-        {
-            messages.remove(idx);
+        let mut dropped_any = false;
+        let mut i = 0;
+        while i < messages.len() {
+            if protected[i] {
+                i += 1;
+                continue;
+            }
+            if messages[i].role == "assistant" {
+                // Count following tool messages — drop as atomic group
+                let mut tool_count = 0;
+                while i + 1 + tool_count < messages.len()
+                    && messages[i + 1 + tool_count].role == "tool"
+                {
+                    tool_count += 1;
+                }
+                if tool_count > 0 {
+                    for _ in 0..=tool_count {
+                        messages.remove(i);
+                    }
+                    dropped_messages += 1 + tool_count;
+                    dropped_any = true;
+                    break;
+                }
+            }
+            // Non-tool-group message — safe to drop individually
+            messages.remove(i);
             dropped_messages += 1;
-        } else {
+            dropped_any = true;
+            break;
+        }
+        if !dropped_any {
             break;
         }
     }
@@ -220,7 +255,7 @@ mod tests {
         assert_eq!(stats.collapsed_pairs, 1);
         assert_eq!(messages.len(), 4);
         assert_eq!(messages[1].role, "assistant");
-        assert!(messages[1].content.starts_with("[Tool result: "));
+        assert!(messages[1].content.contains("1 tool call(s)"));
     }
 
     #[test]
@@ -280,4 +315,164 @@ mod tests {
         assert_eq!(stats.messages_before, 0);
         assert_eq!(stats.messages_after, 0);
     }
+
+    #[test]
+    fn prune_collapses_multi_tool_group() {
+        let mut messages = vec![
+            msg("system", "sys"),
+            msg(
+                "assistant",
+                r#"{"content":null,"tool_calls":[{"id":"t1","name":"shell","arguments":"{}"},{"id":"t2","name":"web","arguments":"{}"}]}"#,
+            ),
+            msg("tool", r#"{"tool_call_id":"t1","content":"result1"}"#),
+            msg("tool", r#"{"tool_call_id":"t2","content":"result2"}"#),
+            msg("user", "thanks"),
+            msg("assistant", "done"),
+        ];
+        let config = HistoryPrunerConfig {
+            enabled: true,
+            max_tokens: 100_000,
+            keep_recent: 2,
+            collapse_tool_results: true,
+        };
+        let stats = prune_history(&mut messages, &config);
+        assert_eq!(stats.collapsed_pairs, 2);
+        // assistant(tool_calls) + 2 tool messages → 1 summary assistant
+        assert_eq!(messages.len(), 4); // sys, summary, user, assistant
+        assert!(messages[1].content.contains("2 tool call(s)"));
+        // No tool messages remain
+        assert!(!messages.iter().any(|m| m.role == "tool"));
+    }
+
+    #[test]
+    fn prune_drops_tool_group_atomically() {
+        let big = "x".repeat(2000);
+        let mut messages = vec![
+            msg("system", "sys"),
+            msg("assistant", &big),
+            msg("tool", &big),
+            msg("tool", &big),
+            msg("user", "recent"),
+            msg("assistant", "recent reply"),
+        ];
+        let config = HistoryPrunerConfig {
+            enabled: true,
+            max_tokens: 50, // very low — forces drops
+            keep_recent: 2,
+            collapse_tool_results: false, // skip collapse, go straight to drop
+        };
+        let stats = prune_history(&mut messages, &config);
+        assert!(stats.dropped_messages >= 3); // assistant + 2 tools dropped together
+        // No orphaned tool messages
+        for (i, m) in messages.iter().enumerate() {
+            if m.role == "tool" {
+                assert!(
+                    i > 0 && messages[i - 1].role == "assistant",
+                    "tool message at index {i} has no preceding assistant"
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn prune_never_orphans_tool_use() {
+        // Simulate a conversation with multiple tool groups
+        let filler = "y".repeat(500);
+        let mut messages = vec![
+            msg("system", "sys"),
+            msg("user", "q1"),
+            msg("assistant", &filler), // tool group 1
+            msg("tool", &filler),
+            msg("user", "q2"),
+            msg("assistant", &filler), // tool group 2
+            msg("tool", &filler),
+            msg("tool", &filler),
+            msg("user", "recent"),
+            msg("assistant", "recent reply"),
+        ];
+        let config = HistoryPrunerConfig {
+            enabled: true,
+            max_tokens: 100,
+            keep_recent: 2,
+            collapse_tool_results: true,
+        };
+        prune_history(&mut messages, &config);
+        // Verify invariant: no tool message without a preceding assistant
+        for (i, m) in messages.iter().enumerate() {
+            if m.role == "tool" {
+                assert!(
+                    i > 0 && messages[i - 1].role == "assistant",
+                    "orphaned tool message at index {i}: {:?}",
+                    messages.iter().map(|m| &m.role).collect::<Vec<_>>()
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn prune_protects_recent_tool_groups() {
+        let mut messages = vec![
+            msg("system", "sys"),
+            msg("user", "old"),
+            msg("assistant", "old reply"),
+            msg("assistant", "tool call"),
+            msg("tool", "tool result"),
+            msg("user", "recent"),
+        ];
+        let config = HistoryPrunerConfig {
+            enabled: true,
+            max_tokens: 100_000,
+            keep_recent: 3, // protects last 3: tool call, tool result, recent
+            collapse_tool_results: true,
+        };
+        let stats = prune_history(&mut messages, &config);
+        // Protected tool group should not be collapsed
+        assert!(messages.iter().any(|m| m.role == "tool"));
+        assert_eq!(stats.collapsed_pairs, 0);
+    }
+
+    #[test]
+    fn prune_under_realistic_token_pressure_preserves_tool_pairing() {
+        // Simulate 15 tool iterations with realistic content sizes
+        let mut messages = vec![msg("system", "You are helpful.")];
+        messages.push(msg("user", "Research this topic thoroughly"));
+
+        // 15 tool iterations — each adds assistant(tool_calls) + tool(result)
+        for i in 0..15 {
+            let tool_json = format!(
+                r#"{{"content":"iteration {i}","tool_calls":[{{"id":"t{i}","name":"web_search","arguments":"{{}}"}}]}}"#
+            );
+            messages.push(msg("assistant", &tool_json));
+            // Realistic tool result size (~2K chars each)
+            let result = format!(
+                r#"{{"tool_call_id":"t{i}","content":"{}"}}"#,
+                "x".repeat(2000)
+            );
+            messages.push(msg("tool", &result));
+        }
+        messages.push(msg("assistant", "Here's what I found..."));
+
+        // 33 messages total: system + user + 15*(assistant+tool) + final assistant
+        assert_eq!(messages.len(), 33);
+
+        let config = HistoryPrunerConfig {
+            enabled: true,
+            max_tokens: 2000, // Forces pruning of older iterations
+            keep_recent: 4,
+            collapse_tool_results: true,
+        };
+
+        prune_history(&mut messages, &config);
+
+        // Invariant: no orphaned tool messages after pruning
+        for (i, m) in messages.iter().enumerate() {
+            if m.role == "tool" {
+                assert!(
+                    i > 0 && messages[i - 1].role == "assistant",
+                    "orphaned tool at index {i}: roles = {:?}",
+                    messages.iter().map(|m| &m.role).collect::<Vec<_>>()
+                );
+            }
+        }
+    }
 }