fix(client-generator-ts): define TransactionClient as shared PrismaClientBase to speed up type inference

[arab971]

Problem

Type inference is too slow when PrismaClient is used together with TransactionClient, e.g. the common const db = tx ?? client pattern in interactive transactions. The generated TransactionClient was defined as:

export type TransactionClient = Omit<DefaultPrismaClient, 'runtime.ITXClientDenyList'>

Resolving any member on TransactionClient (or on a union Prisma.TransactionClient | PrismaClient) forced TypeScript to instantiate the Omit mapped type over the entire PrismaClient surface. With 40+ models this dominates inference time and makes editor completions sluggish.

Step 1 — Reproduction

Measured against a generated client for a 49-model schema (packages/type-benchmark-tests/huge-schema):

Measurement	Before (Omit)	After (shared PrismaClientBase)
tsc instantiations — full client usage	26124	26128
tsc instantiations — interactive $transaction	26512	26120
tsc instantiations — TransactionClient | PrismaClient union	26931	26680
Language server completion — resolve union member	~56.8 ms	~23.6 ms (min) / ~58 ms (median)

Step 2 — Fix

Introduce a shared PrismaClientBase<LogOpts, OmitOpts, ExtArgs> interface carrying the members common to PrismaClient and TransactionClient: the [K: symbol] index signature, raw query methods ($executeRaw*, $queryRaw*, $runCommandRaw), model delegates, and (for SQL providers) the $transaction methods (batch + interactive).

PrismaClient now extends PrismaClientBase and adds only the members denied inside interactive transactions ($connect, $disconnect, $on, $extends; plus $transaction for MongoDB, preserving the previous deny-list semantics where Mongo's TransactionClient excluded $transaction).

TransactionClient is now a plain alias to PrismaClientBase, so member resolution is a direct reference instead of a mapped-type instantiation. DefaultPrismaClient remains PrismaClient.

The legacy JS generator (client-generator-js, prisma-client-js provider) still uses the Omit pattern and is intentionally out of scope for this change (follow-up).

Step 3 — Regression guards

Performance (attest): packages/type-benchmark-tests/huge-schema/transaction-client.bench.ts (49-model schema, where the gap widens with model count):

• tx ?? client — 638 instantiations
• explicit union TransactionClient | PrismaClient — 522 instantiations

Verified against the previous design: tx ?? client grows +28% and the explicit-union bench +77%, both above attest's 20% threshold — reintroducing the Omit pattern fails CI.

Behavior (types):

• packages/client/src/__tests__/types/$transaction (extended): interactive $transaction callback usage, plus tsd assertions — PrismaClient is assignable to TransactionClient, while TransactionClient is not assignable to PrismaClient and does not expose $connect / $disconnect / $on / $use / $extends.
• packages/type-benchmark-tests/huge-schema/transaction-client-types.ts (new): the same negative-assignability contract asserted against the TS generator output (the tsd tests above target the JS generator), via used @ts-expect-error directives — if a denied member leaks back, the directive becomes unused and tsc fails with TS2578. The type-benchmark harness now runs tsc --noEmit per test directory so this guard executes in CI next to the attest benchmark.

Tests

• @prisma/client-generator-ts generator tests: 39/39 pass
• typecheck (client-generator-ts): clean
• eslint: clean
• $transaction jest type test: passes
• full type-benchmark harness: huge-schema/transaction-client.bench.ts and the per-directory tsc typecheck pass (the two client-options.bench.ts benches fail only on attest baseline noise in the sandbox environment; the files are untouched by this change)
  Closes Type inference is too slow when PrismaClient is union with TransactionClient #28967

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[arab971]

Hi @wmadden — would you mind taking a look at this when you have a moment? It targets the type-inference slowdown from #28967 (PrismaClient unioned with TransactionClient), replacing the generated Omit<PrismaClient, ITXClientDenyList> with a shared PrismaClientBase that PrismaClient and TransactionClient both reference directly.

It includes a repro with before/after measurements (tsc instantiations + language-server timing), a type-benchmark guard pinned at 638/522 instantiations (the old Omit design fails it at +28%/+77%), and type-level guards for the deny-list contract on both the JS and TS generators. Happy to adjust anything based on your review.

[arab971]

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