diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index d1b4c6e4e4..9a1f83e7d7 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -39,7 +39,7 @@ from agent.gemini_native_adapter import is_native_gemini_base_url
 # misidentify and, without an api_key, return 401 on every leg (issue #89863).
 from agent.model_metadata import is_local_endpoint
 from agent.message_content import flatten_message_text
-from agent.message_metadata import append_message, stamp_message_timestamp
+from agent.message_metadata import PERSISTENCE_ONLY_MESSAGE_FIELDS, append_message, stamp_message_timestamp
 from agent.message_sanitization import (
     _sanitize_surrogates, _repair_tool_call_arguments, normalize_finish_reason as _normalize_finish_reason,
     sanitize_outbound_kwargs, strip_images_for_rejecting_model,
@@ -2129,8 +2129,8 @@ def try_activate_fallback(agent, reason: "FailoverReason | None" = None, reset_a
 # Keys outside the Chat Completions schema that strict gateways (Fireworks-backed OpenCode
 # Go, Mistral, Moonshot/Kimi) reject with 422. The transport's convert_messages() drops them
 # in the main loop; the summary path calls chat.completions.create() directly, so mirror it.
-_SUMMARY_FOREIGN_MESSAGE_KEYS = ("reasoning", "finish_reason", "tool_name", "codex_reasoning_items",
-    "codex_message_items", "timestamp", "platform_message_id")
+_SUMMARY_FOREIGN_MESSAGE_KEYS = PERSISTENCE_ONLY_MESSAGE_FIELDS | {"reasoning", "finish_reason", "tool_name",
+    "codex_reasoning_items", "codex_message_items", "platform_message_id"}
 _EMPTY_SUMMARY_RESPONSE = "I reached the iteration limit and couldn't generate a summary."
 
 
diff --git a/agent/message_metadata.py b/agent/message_metadata.py
index e13acb663f..687942156a 100644
--- a/agent/message_metadata.py
+++ b/agent/message_metadata.py
@@ -6,9 +6,12 @@ from time import time as wall_time
 from typing import Any, MutableMapping, Optional, TypeVar
 
 
-# These fields describe Hermes' durable record, not provider-visible message
-# content. They must not influence context-pressure decisions.
-PERSISTENCE_ONLY_MESSAGE_FIELDS = frozenset({"timestamp"})
+# These fields describe Hermes' durable record and timeline display, not
+# provider-visible message content. The request builder strips them from every
+# outgoing copy and the token estimator ignores them: one set, so an estimate
+# never prices bytes the provider never receives (an edit's inline_diff in
+# display_metadata is ~9KB and would trigger premature compaction).
+PERSISTENCE_ONLY_MESSAGE_FIELDS = frozenset({"timestamp", "display_kind", "display_metadata", "_row_id"})
 
 _Message = TypeVar("_Message", bound=MutableMapping[str, Any])
 
diff --git a/agent/turn_context.py b/agent/turn_context.py
index 43f8ae03b4..113527c540 100644
--- a/agent/turn_context.py
+++ b/agent/turn_context.py
@@ -22,7 +22,7 @@ from agent.iteration_budget import IterationBudget
 from agent.memory_manager import build_memory_context_block
 from agent.memory_provider import is_trivial_prompt
 from agent.message_content import flatten_message_text
-from agent.message_metadata import append_message, stamp_message_timestamp
+from agent.message_metadata import PERSISTENCE_ONLY_MESSAGE_FIELDS, append_message, stamp_message_timestamp
 from agent.model_metadata import estimate_messages_tokens_rough, estimate_request_tokens_rough
 from agent.image_token_cost import bind_image_token_cost
 from agent.usage_anchor import anchored_context_tokens, restore_usage_anchor
@@ -1220,11 +1220,12 @@ def build_api_messages(
         # persisted history via nested containers; see _clone_message_for_send.
         api_msg = _clone_message_for_send(msg)
         # api_content is bookkeeping (exact bytes sent), never a provider field — pop
-        # it from EVERY outgoing copy. display_* is display-only timeline metadata
-        # (strict OpenAI backends reject unknown keys); _row_id is the durable row id
-        # from _rows_to_conversation and only chat-completions strips underscore keys.
+        # it from EVERY outgoing copy. Persistence/display fields (display_*, _row_id,
+        # timestamp) are local bookkeeping: strict OpenAI backends reject unknown keys
+        # and only chat-completions strips underscore keys. The token estimator drops
+        # the same set, so it never prices bytes the provider never receives.
         _api_content = api_msg.pop("api_content", None)
-        for key in ("display_kind", "display_metadata", "_row_id"):
+        for key in PERSISTENCE_ONLY_MESSAGE_FIELDS:
             api_msg.pop(key, None)
 
         # Inject ephemeral context (memory prefetch + pre_llm_call user hooks)
diff --git a/tests/agent/test_model_metadata.py b/tests/agent/test_model_metadata.py
index 3d8736f3c7..76088df438 100644
--- a/tests/agent/test_model_metadata.py
+++ b/tests/agent/test_model_metadata.py
@@ -23,6 +23,7 @@ from agent.model_metadata import (
     _strip_provider_prefix,
     estimate_tokens_rough,
     estimate_messages_tokens_rough,
+    estimate_request_tokens_rough,
     get_model_context_length,
     get_next_probe_tier,
     get_cached_context_length,
@@ -70,6 +71,17 @@ class TestEstimateMessagesTokensRough:
             estimate_messages_tokens_rough([msg])
         )
 
+    def test_display_only_fields_do_not_change_estimate(self):
+        """An edit row's inline_diff rides display_metadata, which the request builder strips;
+        pricing it would compact early and break the prompt cache."""
+        wire = {"role": "tool", "tool_call_id": "call-1", "name": "write_file",
+                "content": '{"bytes_written": 18000}'}
+        rich = {**wire, "_row_id": 42, "display_kind": "tool_result",
+                "display_metadata": {"tool_result_metadata": {"inline_diff": "\x1b[32m+ line\x1b[0m\n" * 600}}}
+
+        assert estimate_messages_tokens_rough([rich]) == estimate_messages_tokens_rough([wire])
+        assert estimate_request_tokens_rough([rich]) == estimate_request_tokens_rough([wire])
+
     def test_message_with_list_content(self):
         """Vision messages with multimodal content arrays.