diff --git a/src/amplifier_agent_http/_wire.py b/src/amplifier_agent_http/_wire.py
index b801e90c..4e032c96 100644
--- a/src/amplifier_agent_http/_wire.py
+++ b/src/amplifier_agent_http/_wire.py
@@ -180,6 +180,7 @@ def stop_chunk(
     completion_tokens: int = 0,
     cached_tokens: int = 0,
     cost_usd: str | None = None,
+    total_nano_aiu: int | None = None,
     include_usage: bool = True,
 ) -> dict[str, Any]:
     """Final chunk -- empty delta, finish_reason: stop, optional usage block.
@@ -206,6 +207,8 @@ def stop_chunk(
             cached_tokens=cached_tokens,
             cost_usd=cost_usd,
         )
+        if total_nano_aiu is not None:
+            chunk["copilot_usage"] = {"total_nano_aiu": total_nano_aiu}
     return chunk
 
 
@@ -262,6 +265,7 @@ def tool_calls_stop_chunk(
     completion_tokens: int = 0,
     cached_tokens: int = 0,
     cost_usd: str | None = None,
+    total_nano_aiu: int | None = None,
     include_usage: bool = True,
 ) -> dict[str, Any]:
     """Terminal chunk for a turn that ends with host-delegated tool calls.
@@ -287,6 +291,8 @@ def tool_calls_stop_chunk(
             cached_tokens=cached_tokens,
             cost_usd=cost_usd,
         )
+        if total_nano_aiu is not None:
+            chunk["copilot_usage"] = {"total_nano_aiu": total_nano_aiu}
     return chunk
 
 
diff --git a/src/amplifier_agent_http/routes/chat_completions.py b/src/amplifier_agent_http/routes/chat_completions.py
index 8ef1d2c3..a5c18b0e 100644
--- a/src/amplifier_agent_http/routes/chat_completions.py
+++ b/src/amplifier_agent_http/routes/chat_completions.py
@@ -26,7 +26,7 @@
 import logging
 import time
 from collections.abc import AsyncGenerator
-from decimal import Decimal, InvalidOperation
+from decimal import ROUND_HALF_UP, Decimal, InvalidOperation
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException, Request, status
@@ -427,6 +427,14 @@ async def _stream_chat_completion(
         #   runs the tool host-side, then re-POSTs.
         # - "stop" for the normal end-of-turn path (with or without text).
         cost_str_final: str | None = str(usage_cost) if usage_cost is not None else None
+        # opencode reads a top-level copilot_usage.total_nano_aiu and overrides its
+        # own token x rate cost estimate with total_nano_aiu / 1e11, so emitting
+        # round(cost_usd * 1e11) makes its displayed cost exactly cost_usd.
+        total_nano_aiu: int | None = (
+            int((usage_cost * Decimal(10**11)).to_integral_value(rounding=ROUND_HALF_UP))
+            if usage_cost is not None
+            else None
+        )
         if finish_reason_tool_calls:
             yield sse_data(
                 tool_calls_stop_chunk(
@@ -436,6 +444,7 @@ async def _stream_chat_completion(
                     completion_tokens=usage_completion,
                     cached_tokens=usage_cached,
                     cost_usd=cost_str_final,
+                    total_nano_aiu=total_nano_aiu,
                     include_usage=True,
                 )
             )
@@ -448,6 +457,7 @@ async def _stream_chat_completion(
                     completion_tokens=usage_completion,
                     cached_tokens=usage_cached,
                     cost_usd=cost_str_final,
+                    total_nano_aiu=total_nano_aiu,
                     include_usage=True,
                 )
             )