0019. Tool-call continuation chunks append to the open call

• Status: Trial
• Date: 2026-11-03

Context

delta.tool_calls entries in a chat-completions stream are assembled by exact key: stream index, then call id. Some model families routed through the chat-completions endpoint emit continuation chunks — argument-only deltas that carry neither id nor index nor function.name. With exact-key matching alone, such a chunk matches no existing block and falls through to creating a new tool call with an empty name: a phantom call whose arguments are split across two blocks, and whose toolcall_start fires twice for one logical call. The same hole exists in both assembly paths of this provider: the client tool-call block path and the server-tools pending-candidate path.

Decision

A delta with no function.name and no id is a continuation: when no exact index/id match exists, it appends to the most recently opened tool-call block (client path) or the most recently opened, not-yet-folded pending candidate (server-tools path). A continuation with no open block/candidate is dropped — there is nothing valid to append to, and creating a block would be the phantom-call bug. Exact-key matching stays primary; the continuation fallback only runs when no exact match exists. The !toolCall.id term in the discriminator is load-bearing: a nameless delta with an id is a legitimate call header whose name is backfilled by a later delta, and must never be treated as a continuation.

Consequences

• Argument fragments split across id-less/index-less/name-less chunks reassemble into the single real call; toolcall_start fires once per real call and toolcall_delta carries the appended arguments on the same block.
• The stream-event contract is unchanged; only the block-resolution fallback is new.
• A continuation arriving with no open call is silently dropped (same choice as the reference behavior upstream), which is strictly safer than inventing an empty-name call.
• Both assembly helpers can now return undefined (dropped continuation), so their callers guard with a continue.