Bedrock models resolved their context window from a hardcoded table
(BEDROCK_CONTEXT_LENGTHS) keyed by longest-substring match. AWS ships
new model versions faster than the table tracks, so a new model like
claude-opus-4-8 (1M-token window) silently matched the older
"anthropic.claude-opus-4" entry and got capped at 200K — wasting 80%
of the available context.
Bedrock exposes the real window nowhere in metadata: get-foundation-model
omits it, Converse usage metrics omit it, CountTokens is unsupported on
several models. The only authoritative source is the ValidationException
raised when a prompt exceeds the window:
"prompt is too long: 1300032 tokens > 1000000 maximum"
Length validation runs before inference, so an oversized request is
rejected immediately and cheaply (no tokens generated, no input
processed). This adds probe_bedrock_context_length(): pad a request just
past a tier, parse the reported maximum, return it. get_bedrock_context_length()
now probes first and falls back to the static table only when the probe
can't run (missing creds, network error, unparseable error). The static
table stays as a safety net.
get_model_context_length() caches the probe result per model+region, so
the network cost is paid once, not every turn. probe=False / empty region
disables probing for offline/display paths — backward compatible with the
single-arg callers.
Verified E2E against live Bedrock (eu-central-1): claude-opus-4-8 resolves
to 1000000. Unit tests cover error parsing, unparseable errors, missing
client, probe-beats-table, and table fallback.
kubolko
6be4944bc0