`apply_anthropic_cache_control` runs once per call block, before the retry
loop, and splits the system prompt into `[static prefix, volatile tail]` text
blocks carrying the cache_control breakpoints.
A failover fires *inside* that retry loop, and `_sync_failover_system_message`
assigns a bare string over `api_messages[0]["content"]` to refresh the
`Model:`/`Provider:` identity lines. That drops the block list and both
breakpoints. `convert_messages_to_anthropic` only emits system cache blocks for
list content (`isinstance(content, list)`), so the retried request ships
`system` as one plain string: zero breakpoints, nothing written to cache, and
the whole system prompt re-billed at full write price. The next call misses
too, so a failover costs two full-price prompts instead of one write + one
read.
Measured with the real functions, same history, native Anthropic layout:
normal turn system=BLOCK LIST(2) system breakpoints=2
after failover system=plain string system breakpoints=0
`_sync_failover_system_message`'s own docstring notes this fires on "every
gateway turn, since fallback re-activates per message while the primary is
down" -- so a gateway running on a degraded primary pays it on every message.
Fix: rewrite the decorated blocks in place instead of flattening them.
`rewrite_prompt_model_identity` only touches the LAST `Model:`/`Provider:`
lines, and those live in the volatile tail, so the static prefix stays
byte-identical and its cache entry keeps matching -- the failover retry keeps
both breakpoints AND the warm prefix. Shapes we cannot safely patch fall back
to the existing plain-string assignment.
This is the same class the retry loop already guards against eight lines below
the gap, for reasoning fields:
# api_messages is built once, before this retry loop, while the primary
# provider is active. [...] so the fallback request isn't sent with stale,
# primary-shaped reasoning fields.
agent._reapply_reasoning_echo_for_provider(api_messages)
There was no equivalent for cache decoration.
The session-stable system prompt embeds Model:/Provider: identity lines,
but mid-turn failover (try_activate_fallback) swaps the runtime without
touching them, so a fallback model misreports itself as the primary when
asked "what model are you?".
rewrite_prompt_model_identity() rewrites the last occurrence of each line
on _cached_system_prompt when a fallback activates (and back on restore,
byte-identical so the primary's prefix cache still hits). The rewrite is
never persisted to the session DB. _sync_failover_system_message() patches
the in-flight api_messages[0] at all 8 failover sites so the current turn
ships the corrected identity. Cache-safe: the fallback's prefix cache is
cold on a model switch anyway.
Co-authored-by: Hermes Agent <noreply@nousresearch.com>
