fix(copilot): ensure tool_call/tool_response pairs stay intact during context compaction (#11863)

## Summary

Fixes context compaction breaking tool_call/tool_response pairs, causing
API validation errors.

## Problem

When context compaction slices messages with `messages[-KEEP_RECENT:]`,
a naive slice can separate an assistant message containing `tool_calls`
from its corresponding tool response messages. This causes API
validation errors like:

```
messages.0.content.1: unexpected 'tool_use_id' found in 'tool_result' blocks: orphan_12345.
Each 'tool_result' block must have a corresponding 'tool_use' block in the previous message.
```

## Solution

Added `_ensure_tool_pairs_intact()` helper function that:
1. Detects orphan tool responses in a slice (tool messages whose
`tool_call_id` has no matching assistant message)
2. Extends the slice backwards to include the missing assistant messages
3. Falls back to removing orphan tool responses if the assistant cannot
be found (edge case)

Applied this safeguard to:
- The initial `KEEP_RECENT` slice (line ~990)
- The progressive fallback slices when still over token limit (line
~1079)

## Testing

- Syntax validated with `python -m py_compile`
- Logic reviewed for correctness

## Linear

Fixes SECRT-1839

---
*Debugged by Toran & Orion in #agpt Discord*

autogpt_platform/backend/backend/api/features/chat/service.py

@@ -800,6 +800,114 @@ async def _summarize_messages(
     return summary or "No summary available."
 
 
+def _ensure_tool_pairs_intact(
+    recent_messages: list[dict],
+    all_messages: list[dict],
+    start_index: int,
+) -> list[dict]:
+    """
+    Ensure tool_call/tool_response pairs stay together after slicing.
+
+    When slicing messages for context compaction, a naive slice can separate
+    an assistant message containing tool_calls from its corresponding tool
+    response messages. This causes API validation errors (e.g., Anthropic's
+    "unexpected tool_use_id found in tool_result blocks").
+
+    This function checks for orphan tool responses in the slice and extends
+    backwards to include their corresponding assistant messages.
+
+    Args:
+        recent_messages: The sliced messages to validate
+        all_messages: The complete message list (for looking up missing assistants)
+        start_index: The index in all_messages where recent_messages begins
+
+    Returns:
+        A potentially extended list of messages with tool pairs intact
+    """
+    if not recent_messages:
+        return recent_messages
+
+    # Collect all tool_call_ids from assistant messages in the slice
+    available_tool_call_ids: set[str] = set()
+    for msg in recent_messages:
+        if msg.get("role") == "assistant" and msg.get("tool_calls"):
+            for tc in msg["tool_calls"]:
+                tc_id = tc.get("id")
+                if tc_id:
+                    available_tool_call_ids.add(tc_id)
+
+    # Find orphan tool responses (tool messages whose tool_call_id is missing)
+    orphan_tool_call_ids: set[str] = set()
+    for msg in recent_messages:
+        if msg.get("role") == "tool":
+            tc_id = msg.get("tool_call_id")
+            if tc_id and tc_id not in available_tool_call_ids:
+                orphan_tool_call_ids.add(tc_id)
+
+    if not orphan_tool_call_ids:
+        # No orphans, slice is valid
+        return recent_messages
+
+    # Find the assistant messages that contain the orphan tool_call_ids
+    # Search backwards from start_index in all_messages
+    messages_to_prepend: list[dict] = []
+    for i in range(start_index - 1, -1, -1):
+        msg = all_messages[i]
+        if msg.get("role") == "assistant" and msg.get("tool_calls"):
+            msg_tool_ids = {tc.get("id") for tc in msg["tool_calls"] if tc.get("id")}
+            if msg_tool_ids & orphan_tool_call_ids:
+                # This assistant message has tool_calls we need
+                # Also collect its contiguous tool responses that follow it
+                assistant_and_responses: list[dict] = [msg]
+
+                # Scan forward from this assistant to collect tool responses
+                for j in range(i + 1, start_index):
+                    following_msg = all_messages[j]
+                    if following_msg.get("role") == "tool":
+                        tool_id = following_msg.get("tool_call_id")
+                        if tool_id and tool_id in msg_tool_ids:
+                            assistant_and_responses.append(following_msg)
+                    else:
+                        # Stop at first non-tool message
+                        break
+
+                # Prepend the assistant and its tool responses (maintain order)
+                messages_to_prepend = assistant_and_responses + messages_to_prepend
+                # Mark these as found
+                orphan_tool_call_ids -= msg_tool_ids
+                # Also add this assistant's tool_call_ids to available set
+                available_tool_call_ids |= msg_tool_ids
+
+        if not orphan_tool_call_ids:
+            # Found all missing assistants
+            break
+
+    if orphan_tool_call_ids:
+        # Some tool_call_ids couldn't be resolved - remove those tool responses
+        # This shouldn't happen in normal operation but handles edge cases
+        logger.warning(
+            f"Could not find assistant messages for tool_call_ids: {orphan_tool_call_ids}. "
+            "Removing orphan tool responses."
+        )
+        recent_messages = [
+            msg
+            for msg in recent_messages
+            if not (
+                msg.get("role") == "tool"
+                and msg.get("tool_call_id") in orphan_tool_call_ids
+            )
+        ]
+
+    if messages_to_prepend:
+        logger.info(
+            f"Extended recent messages by {len(messages_to_prepend)} to preserve "
+            f"tool_call/tool_response pairs"
+        )
+        return messages_to_prepend + recent_messages
+
+    return recent_messages
+
+
 async def _stream_chat_chunks(
     session: ChatSession,
     tools: list[ChatCompletionToolParam],
@@ -891,7 +999,15 @@ async def _stream_chat_chunks(
             # Always attempt mitigation when over limit, even with few messages
             if messages:
                 # Split messages based on whether system prompt exists
-                recent_messages = messages[-KEEP_RECENT:]
+                # Calculate start index for the slice
+                slice_start = max(0, len(messages_dict) - KEEP_RECENT)
+                recent_messages = messages_dict[-KEEP_RECENT:]
+
+                # Ensure tool_call/tool_response pairs stay together
+                # This prevents API errors from orphan tool responses
+                recent_messages = _ensure_tool_pairs_intact(
+                    recent_messages, messages_dict, slice_start
+                )
 
                 if has_system_prompt:
                     # Keep system prompt separate, summarize everything between system and recent
@@ -978,6 +1094,13 @@ async def _stream_chat_chunks(
                                     if len(recent_messages) >= keep_count
                                     else recent_messages
                                 )
+                                # Ensure tool pairs stay intact in the reduced slice
+                                reduced_slice_start = max(
+                                    0, len(recent_messages) - keep_count
+                                )
+                                reduced_recent = _ensure_tool_pairs_intact(
+                                    reduced_recent, recent_messages, reduced_slice_start
+                                )
                                 if has_system_prompt:
                                     messages = [
                                         system_msg,
@@ -1036,7 +1159,10 @@ async def _stream_chat_chunks(
 
                     # Create a base list excluding system prompt to avoid duplication
                     # This is the pool of messages we'll slice from in the loop
-                    base_msgs = messages[1:] if has_system_prompt else messages
+                    # Use messages_dict for type consistency with _ensure_tool_pairs_intact
+                    base_msgs = (
+                        messages_dict[1:] if has_system_prompt else messages_dict
+                    )
 
                     # Try progressively smaller keep counts
                     new_token_count = token_count  # Initialize with current count
@@ -1059,6 +1185,12 @@ async def _stream_chat_chunks(
                             # Slice from base_msgs to get recent messages (without system prompt)
                             recent_messages = base_msgs[-keep_count:]
 
+                            # Ensure tool pairs stay intact in the reduced slice
+                            reduced_slice_start = max(0, len(base_msgs) - keep_count)
+                            recent_messages = _ensure_tool_pairs_intact(
+                                recent_messages, base_msgs, reduced_slice_start
+                            )
+
                             if has_system_prompt:
                                 messages = [system_msg] + recent_messages
                             else: