## Problem
Fixes#21695.
An AI chat thread became **permanently unusable** — every subsequent
message failed with `AI_APICallError: Internal server error` from
Anthropic — when the thread history contained a tool part in
`output-error` state with a **null input** (e.g. a tool call that failed
input validation before execution, so neither `toolInput` nor
`toolOutput` was ever captured).
## Validation of the reported findings
I reproduced and confirmed the root cause empirically against the pinned
`ai@6.0.97` SDK before writing the fix.
**Root cause (confirmed from SDK source).** `convertToModelMessages`
serializes every non-`input-streaming` tool part into a provider
`tool_use` block, and for errored parts it uses:
```ts
input: part.state === 'output-error'
? (part.input ?? ('rawInput' in part ? part.rawInput : undefined))
: part.input,
```
When both `input` and `rawInput` are nullish, the block is built with
`input: undefined`, which `JSON.stringify` drops — so the HTTP payload
carries a `tool_use` with **no `input` field**. This matches the
reporter's minimal repro exactly (no `input` → `400 Field required`;
`input: {}` → `200`). Inside a large streamed conversation the same
malformed block surfaces as the generic `500`, and because the bad part
is replayed on every turn the thread stays bricked.
**Why #21276 didn't catch it.** `finalizeDanglingToolParts` only rewrote
`input-available` parts; a part that arrives already in `output-error`
with a null input was passed through untouched.
**Note on current `main`.** A read-path default added recently
(`mapDBPartToUIMessagePart`: `input: part.toolInput ?? {}`) already
masks the live 500 on the standard reload path. However the gap is real
and worth closing: the persist path still writes `toolInput = NULL` (the
exact malformed rows the reporter found in `core."agentMessagePart"`),
`finalizeDanglingToolParts` still doesn't normalize this case, and the
protection rested on a single implicit default with no regression
coverage. A small repro harness confirmed all of this: persisted
`toolInput` was `undefined`, and a raw (non-defaulted) `output-error`
part produced a `tool-call` whose `input` value was `undefined`.
## Fix
Defense-in-depth so the invariant *"a tool part always carries a defined
input"* holds at both the finalize and storage boundaries:
- **`finalizeDanglingToolParts`** now backfills `input: {}` for
`output-error` parts whose input is null, while preserving the original
error message. This is the natural chokepoint (it already runs
immediately before every persist).
- **`mapUIMessagePartsToDBParts`** defaults a nullish tool input to `{}`
so malformed rows are never persisted, independent of the caller.
The existing read-path `?? {}` default is kept as a third safety net.
## Tests
- Unit tests for `finalizeDanglingToolParts`: backfills `{}` for an
`output-error` part missing its input, and preserves the existing
validation error message.
- Persistence test: `mapUIMessagePartsToDBParts` stores `{}` (never
`null`) for a missing input.
- End-to-end round-trip test: after finalize → persist → reload,
`convertToModelMessages` produces a `tool-call` with a defined input and
the errored call stays resolved.
All three new core assertions were verified to **fail without the fix**
and pass with it. Full AI module suite (97 tests) passes; `oxlint
--type-aware`, `oxfmt`, and `tsgo` typecheck are clean.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
https://claude.ai/code/session_01SpuX6Pp2yTevk1zKTRiB9G
---
_Generated by [Claude
Code](https://claude.ai/code/session_01SpuX6Pp2yTevk1zKTRiB9G)_
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---------
Co-authored-by: Claude <noreply@anthropic.com>
Félix Malfait
f96e36d3e6
