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fix(caching): prevent whitespace-only text blocks in prompt cache prefix splits

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This commit is contained in:
ericmaddox
2026-08-28 22:43:08 -04:00
committed by kshitij
parent 9c137e6163
commit f0d5f1298b
3 changed files with 26 additions and 21 deletions
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agent/prompt_caching.py
+18 -16
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@@ -63,20 +63,22 @@ def _apply_cache_marker(msg: dict, cache_marker: dict, native_anthropic: bool =
if role == "user":
stable_prefix = find_stable_prefix(content)
if stable_prefix is not None:
# Builder-declared boundary (#81867): the scaffold carries the
# breakpoint, the volatile invocation tail rides unmarked so a
# changed ticket ID or timestamp no longer invalidates the
# whole skill body. Request-local only — the canonical session
# message stays a plain string.
msg["content"] = [
{
"type": "text",
"text": stable_prefix,
"cache_control": cache_marker,
},
{"type": "text", "text": content[len(stable_prefix):]},
]
return
suffix = content[len(stable_prefix):]
if suffix.strip():
# Builder-declared boundary (#81867): the scaffold carries the
# breakpoint, the volatile invocation tail rides unmarked so a
# changed ticket ID or timestamp no longer invalidates the
# whole skill body. Request-local only — the canonical session
# message stays a plain string.
msg["content"] = [
{
"type": "text",
"text": stable_prefix,
"cache_control": cache_marker,
},
{"type": "text", "text": suffix},
]
return
msg["content"] = [
{"type": "text", "text": content, "cache_control": cache_marker}
]
@@ -204,7 +206,7 @@ def _apply_system_cache_markers(
and content.startswith(static_system_prefix)
):
suffix = content[len(static_system_prefix):]
if suffix:
if suffix.strip():
suffix_part: dict = {"type": "text", "text": suffix}
if mark_suffix:
suffix_part["cache_control"] = cache_marker
@@ -217,7 +219,7 @@ def _apply_system_cache_markers(
suffix_part,
]
return 2 if mark_suffix else 1
# Empty suffix: the stored prompt IS the static prefix. Mark it as
# Empty/whitespace-only suffix: the stored prompt IS the static prefix. Mark it as
# one whole block — a [marked-prefix, ""] split would put an empty
# text block on the wire (HTTP 400 on native Anthropic).
_apply_cache_marker(message, cache_marker, native_anthropic=native_anthropic)