Session still crashes with "JSON.stringify cannot serialize BigInt" after parallel scratch queries #1486

Problem

A session died with UnknownError: JSON.stringify cannot serialize BigInt. after four parallel scratch sql_execute calls returned BIGINT counts. The run exited with status 1 and no answer, even though the answer had already been computed.

Mechanism

normalizeBigInts(input.messages) already guards outgoing model messages (packages/opencode/src/session/llm.ts:112, packages/opencode/src/altimate/util/json.ts:7), and an earlier fix landed for the same error. A different serialization path (tool-result storage, session persistence or tracing; the exact call site is not yet identified) is still unguarded when DuckDB returns BIGINT values from several concurrent tool calls.

Evidence

• 1 session out of about 1,255 scanned; it had the final table in hand at 47 s and then crashed on the next turn. The build already contained the earlier normalizeBigInts fix.
• Not seen in sessions with Claude Opus.

Impact

Answer-losing but rare. Fatal when it happens.

Proposed fix

Normalise tool results to JSON-safe values at the single point where a tool result is stored, not only on outgoing model messages (BigInt to number when safe, string otherwise). Find the remaining throw site by reproducing with four parallel BIGINT-returning scratch queries.

Acceptance check

Session-level test: four parallel scratch sql_execute calls returning BIGINT columns, followed by a second model turn, completes without error and the stored/traced results are JSON-serializable.