typeDiagram 0.11: Native DateTime, Uuid and Decimal Scalars — Codegen That Refuses to Lie

If you generate types from one schema across several languages, the failure you fear most is schema drift — the moment your generated code and your hand-written code quietly disagree. Drift detection is one of the defining backend concerns of 2026: API teams are building whole spec-conformance pipelines just to catch the day a service's real responses stop matching its documented schema.

typeDiagram exists to remove that whole class of bug — one .td file generates TypeScript, Python, Rust, Go, C#, F#, Dart, PHP, and Protobuf types that are in sync by construction. But until now it had a gap exactly where drift hurts most: timestamps and ids.

typeDiagram 0.11 closes that gap. Two changes, both aimed at making the generated code the honest single source of truth.

1. Native semantic scalars: DateTime, Uuid, Decimal

Before 0.11, the only primitives were Bool, Int, Float, String, Bytes, and Unit. A created_at timestamp or a conversation_id could only be modelled as String. So the generated DTOs typed them as str / string — and every consumer that actually wanted a datetime or a UUID had to hand-patch the output. That hand-patched copy is precisely what drifts.

0.11 makes DateTime, Uuid, and Decimal built-in scalars. They render like any other primitive, but code generation maps each one to the native type of the target language:

The emitters also pull in the imports each type needs — import datetime, import "time", import "google/protobuf/timestamp.proto" — so the output compiles as-is.

So this schema:

type AuditEvent {
  id: Uuid
  createdAt: DateTime
  amount: Decimal
}
 

now generates this Python, with the real types:

from __future__ import annotations
import datetime
import uuid
import decimal
from dataclasses import dataclass

@dataclass
class AuditEvent:
    id: uuid.UUID
    createdAt: datetime.datetime
    amount: decimal.Decimal

The conversion is bidirectional: point --from at existing code and a datetime.datetime, Guid, or chrono::DateTime maps straight back to the scalar.

Already using a type called Uuid? Nothing breaks. A declaration with the same name as a built-in shadows it — alias Uuid = String still means String. Declared names always win.

2. Unknown type names now fail generation — loudly

The more dangerous bug was silent. Given a typo or an unsupported type:

type Probe {
  a: Timestamp   # not a real typeDiagram type
}
 

the old CLI emitted a: Timestamp into your Python with exit code 0. The reference is to a symbol that doesn't exist — a NameError at import time — and nothing warned you. A bad type name was indistinguishable from a good one until the consuming project failed to build.

In 0.11, code generation is strict. Every referenced name must resolve to a primitive, a generic built-in (List, Map, Option, Any), a declared type, or a generic parameter. Anything else fails the generation:

$ typediagram --to python schema.td
  0:0  error   unknown type 'Timestamp': not a primitive, builtin, or declared type — declare it or use a builtin scalar
$ echo $?
1

The error stops at the generator, where you can fix it, instead of leaking into a downstream build. Rendering a diagram is unchanged — unknown names still draw as opaque external references, because a picture of an external type is useful while a compile of one is not.

Why this matters for drift

Schema-driven development only pays off if every consumer can build directly from the generated output with zero hand-edits. The two gaps above each forced a hand-edit — patch the str back to a UUID, or notice the bad type before it shipped — and every hand-edit is a place drift creeps in. Closing them keeps the schema as the one source of truth, all the way down to the timestamp.

Get it

• CLI / library: npm install -g typediagram (or npm i typediagram-core)
• VS Code extension: Install from the Marketplace
• Try it now: the playground and converter run entirely in your browser.

Full type-mapping details are in the Language Reference and Converters docs.