The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate/Related PRs Found:

1. PR fix(provider): skip empty-text filtering for assistant messages in normalizeMessages (#16748) #16750 - fix(provider): skip empty-text filtering for assistant messages in normalizeMessages

   • Directly related to the empty content filtering issue addressed in the current PR

2. PR fix(provider): preserve assistant message content when reasoning blocks present #21370 - fix(provider): preserve assistant message content when reasoning blocks present

   • Addresses similar issue of preserving assistant messages when reasoning blocks are present

3. PR fix(provider): preserve redacted_thinking blocks and fix signature validation #12131 - fix(provider): preserve redacted_thinking blocks and fix signature validation

   • Related to preserving redacted_thinking block integrity

4. PR fix: replace empty text in reasoning messages to preserve thinking block positions #21860 - fix: replace empty text in reasoning messages to preserve thinking block positions

   • Similar fix for preserving thinking block positions with empty text handling

These PRs appear to be addressing overlapping issues with preserving thinking/reasoning blocks in Anthropic message transforms. The current PR (23755) consolidates fixes for three specific code paths (empty content filtering, tool-use reordering, and cache control hints) that were inadvertently modifying these blocks.

@rekram1-node Any chance you could have a look? ~20 LOC of actual changes, rest is tests

Confirmed this fixes the three transform.ts paths cleanly — verified against messages.X.content.Y: 'thinking' or 'redacted_thinking' blocks in the latest assistant message cannot be modified on claude-opus-4-x (extended thinking, tool-heavy multi-turn).

One gap worth folding in: message-v2.ts strips the thinking signature before transform.ts ever runs, on a path this PR doesn't touch. In toModelMessagesEffect, the reasoning branch is roughly:

if (part.type === "reasoning") {
  if (differentModel) { /* demote to text, drop providerMetadata */ }
  // providerMetadata only kept when !differentModel
}

with differentModel = ${model.providerID}/${model.id} !== ${msg.info.providerID}/${msg.info.modelID}. For Anthropic thinking blocks the providerMetadata carries the cryptographic signature, so dropping it on differentModel re-triggers the exact same cannot be modified 400 on the next turn — even with this PR's transform.ts fix applied.

It reliably reproduces whenever provider/model identity shifts between turns: fallback-model switches, variant changes, and proxy setups where the stored providerID differs from the live one (we hit it through a proxy front where requests resolve under a different provider id than what's persisted in the session). Stripping text/tool metadata there is correct; stripping thinking metadata is not.

Minimal fix that pairs with this PR:

if (part.type === "reasoning") {
  const isAnthropicReasoning =
    model.providerID === "anthropic" ||
    model.providerID.includes("anthropic") ||
    model.api?.npm === "@ai-sdk/anthropic"
  // keep the signature for Anthropic even across differentModel
  ...(differentModel && !isAnthropicReasoning ? {} : { providerMetadata: part.metadata })
}

Happy to open a small follow-up PR (rebased on this) if useful.

Disclosure: investigation + draft assisted by an AI agent; reproduction and the message-v2.ts gap were validated against a live multi-provider (CLIProxyAPI) deployment.

Heads-up on the empty-reasoning filter in this PR — the providerOptions != null check is too loose and still lets the 400 through in one case:

if (part.type === "reasoning") {
  return part.text !== "" || part.providerOptions != null
}

With Anthropic interleaved thinking across several tool-call rounds, a turn that is cut off (e.g. output-length / max-tokens) can leave trailing reasoning parts that have empty text and providerOptions: {} (an empty object — still != null). Those get kept and re-sent as malformed thinking blocks, and Anthropic rejects the next turn with the very error this PR targets:

messages.N.content.M: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified.

We hit this in production after applying an equivalent of this filter: the error recurred on a long Opus extended-thinking + multi-tool session. Tightening the predicate to require an actual signature/redactedData fixed it:

if (part.type === "reasoning") {
  if (part.text !== "") return true
  const po = part.providerOptions ?? {}
  // keep ONLY genuine redacted_thinking / signature-only blocks
  return Object.values(po).some((v) => v != null && (v.signature != null || v.redactedData != null))
}

Empty reasoning with {} / no signature is a truncated remnant and should be dropped, not replayed. Might be worth folding into this PR. (Related: #30046 fixes the upstream message-v2.ts strip that drops the signature on differentModel.)