Claude Fable 5 on Bedrock returns raw 400 (data retention mode 'default' not available)

Summary

Selecting Claude Fable 5 on the Amazon Bedrock provider and sending any message returns HTTP 400 before generation:

data retention mode 'default' is not available for this model

Claude Fable 5 (and Claude Mythos 5) require 30-day data retention per Anthropic policy and cannot run under zero/"default" data retention. On Bedrock this requirement is enforced by the platform. We surface the raw 400 rather than handling it.

Environment (reproduced)

• Posit Assistant 0.8.0, Positron 2026.08.0 (build 124), Darwin arm64
• Provider: Amazon Bedrock
• Model: claude-fable-5 (very likely claude-mythos-5 too — same 30-day retention requirement)

Steps to reproduce

1. Configure a Bedrock provider and select Claude Fable 5.
2. Send any message.
3. Request fails with HTTP 400: data retention mode 'default' is not available for this model.

Notes / background

• This is not the eager_input_streaming issue (#33 / PR #34) — different root cause and fix location, filed separately.
• We already know Fable 5 / Mythos 5 are the 30-day-retention exception: see the model descriptions in packages/core/src/utils/model-helpers.ts (~L294–299), which note that, unlike other Posit AI models (zero data retention), Fable 5 / Mythos 5 retain data for 30 days.
• Our Bedrock path (packages/ai-provider-bridge/src/model-clients/BedrockClient.ts) does not set any data-retention parameter on the request, so the "default" mode is coming from the account/SDK default.

Root cause not yet confirmed

Unclear whether this is:

1. AWS Bedrock account / inference-profile configuration — the account's data-retention mode doesn't meet Fable 5's 30-day requirement (would be resolved on the AWS side, not in our code), or
2. Something the client should send or handle — e.g. a retention-mode parameter, or model availability that we should detect.

Suggested directions

• Confirm whether configuring appropriate data retention on the Bedrock account resolves it (rules in/out the account-config hypothesis).
• If it's account config: document the requirement, and consider surfacing a clear, actionable message instead of the raw 400, and/or gating/annotating Fable 5 + Mythos 5 on the Bedrock provider.
• If it's a request parameter: determine whether/how to pass a data-retention mode on the Bedrock request for these models.

Also affects

Likely claude-mythos-5 on Bedrock (same retention requirement) — worth confirming during investigation.

Start with packages/ai-provider-bridge/src/model-clients/BedrockClient.ts and the Fable 5/Mythos 5 descriptions in packages/core/src/utils/model-helpers.ts. Confirm whether Bedrock account retention configuration resolves the 400, then determine whether the client needs a request parameter or clearer handling for these models. Done means the retention requirement is correctly handled or surfaced as an actionable message, with Mythos 5 checked as well.