python: decode boolean-discriminated unions

[examon]

Fixes #2122

The problem

scripts/codegen/python.ts captured a union discriminator's JSON Schema const with String(...), so a boolean const became the JavaScript string "true" / "false" before any emitter ran. The dispatch table was typed Array<{ value: string; typeName: string }>, so the type could not survive even if it had been captured.

The generated Python therefore matched strings:

def _load_SessionListEntry(obj: Any) -> "SessionListEntry":
    kind = obj.get("isRemote")
    match kind:
        case "false": return LocalSessionMetadataValue.from_dict(obj)
        case "true": return RemoteSessionMetadataValue.from_dict(obj)
        case _: raise ValueError(f"Unknown SessionListEntry isRemote: {kind!r}")

A JSON boolean decodes to Python True, which never equals "true", so both boolean-discriminated unions in the schema fell through to raise ValueError:

• sessions.list() raised ValueError: Unknown SessionListEntry isRemote: False for any non-empty session list.
• QueuedCommandResult failed to decode, and QueuedCommandHandled.to_dict() emitted {"handled": "true"} where the schema declares {"type": "boolean", "const": true} with additionalProperties: false.

The fix

Keep the const's JSON type through codegen and render it as a Python literal.

• PyDiscriminatorValue = string | boolean, with the capture keeping booleans as booleans and stringifying everything else exactly as before.
• A literal renderer emits True / False for booleans. JSON.stringify cannot be reused here: it yields lowercase true, which Python parses as a capture pattern rather than a literal, and in a multi-arm match that is a hard SyntaxError.
• The discriminator ClassVar is annotated bool when the const is boolean, so the encode direction puts a real JSON boolean on the wire.

This mirrors scripts/codegen/go.ts, which already models discriminator values as string | boolean and is unaffected by the bug.

Regenerating changes six lines of python/copilot/generated/rpc.py:

-    handled: ClassVar[str] = "true"
+    handled: ClassVar[bool] = True
-    handled: ClassVar[str] = "false"
+    handled: ClassVar[bool] = False

 def _load_QueuedCommandResult(obj: Any) -> "QueuedCommandResult":
-        case "true": return QueuedCommandHandled.from_dict(obj)
-        case "false": return QueuedCommandNotHandled.from_dict(obj)
+        case True: return QueuedCommandHandled.from_dict(obj)
+        case False: return QueuedCommandNotHandled.from_dict(obj)

 def _load_SessionListEntry(obj: Any) -> "SessionListEntry":
-        case "false": return LocalSessionMetadataValue.from_dict(obj)
-        case "true": return RemoteSessionMetadataValue.from_dict(obj)
+        case False: return LocalSessionMetadataValue.from_dict(obj)
+        case True: return RemoteSessionMetadataValue.from_dict(obj)

No other generated file changes, in any language.

Scope

Deliberately left alone:

• findPyDiscriminator's mapping.set(String(...)). Those keys feed only the variant-count validity check and the flat-union path, which both boolean unions bypass because they are $ref-based. No union in the schema reaches it today.
• The ClassVar collapse pass's field lookup, which builds its regex from the raw schema property name (isRemote) and so never matches the snake_cased is_remote. That is a separate defect, and it is currently load-bearing: it is why LocalSessionMetadataValue keeps a real is_remote: bool field that already encodes correctly. Changing it would drop a required constructor parameter.

Verification

Against the published 1.0.8 wheel, the reproducer raises ValueError: Unknown SessionListEntry isRemote: False and exits 1. Against this branch it exits 0:

SessionList(sessions=[LocalSessionMetadataValue(is_remote=False, modified_time='2026-07-26T10:05:00.000Z', session_id='example-local', start_time='2026-07-26T10:00:00.000Z', client_name=None, context=None, is_detached=None, mc_task_id=None, name=None, summary=None)])

Also verified in a scratch environment against the built branch:

probe	before	after
one local session	ValueError	LocalSessionMetadataValue
one remote session	ValueError	RemoteSessionMetadataValue
mixed local + remote list	ValueError	both variants, round-trips to "isRemote": false/true
_load_QueuedCommandResult on true / false	ValueError	QueuedCommandHandled / QueuedCommandNotHandled
json.dumps(QueuedCommandHandled().to_dict())	{"handled": "true"}	{"handled": true}
handled: "true" (a string)	silently dispatched to QueuedCommandHandled	ValueError
isRemote missing / None / "yes" / 1 / 0	ValueError (or AssertionError)	ValueError

case True: compiles to an identity comparison, so 1 and 0 do not match it despite True == 1. The error path stays intact.

Tests

python/test_rpc_generated.py gains coverage for both unions, routed through the real dispatchers rather than the variant classes (a variant's from_dict ignores the discriminator, so calling it directly would pass even with the bug present):

• SessionList.from_dict decodes a local and a remote entry in one payload to the right variants.
• CommandsRespondToQueuedCommandRequest.from_dict decodes both handled values.
• The encode direction asserts is True / is False identity and that json.dumps produces {"handled": true} / {"handled": false}.
• A string-discriminated union case guards against a regression on that path.

The new tests fail against the pre-fix generated file and pass after it.

Checks run

• npm run generate for all five languages plus the pinned nightly cargo fmt step, leaving the tree byte-clean apart from the six intended Python lines.
• uv run ruff format --check ., uv run ruff check, uv run ty check copilot (two pre-existing warnings in unrelated hand-written files, exit 0).
• uv run pytest test_rpc_generated.py -v: 7 passed, repeated across runs and on Python 3.11.
• The offline Python test set: 291 passed.
• Docs validation: 56 files passed.

[Copilot]

Pull request overview

Fixes Python decoding and serialization for boolean-discriminated RPC unions.

Changes:

• Preserves boolean discriminator values during Python code generation.
• Regenerates affected RPC models and dispatchers.
• Adds decode, encode, round-trip, and string-discriminator regression tests.

Show a summary per file

File	Description
scripts/codegen/python.ts	Emits correctly typed Python discriminator literals.
python/copilot/generated/rpc.py	Uses boolean constants and match arms.
python/test_rpc_generated.py	Tests boolean union serialization and dispatch.

Review details

• Files reviewed: 2/3 changed files
• Comments generated: 0
• Review effort level: Medium

[github-actions]

Cross-SDK Consistency Review ✅

This PR fixes a Python-specific codegen bug where boolean JSON Schema const discriminators were coerced to strings ("true"/"false") before code was emitted, causing deserialization failures at runtime.

Consistency check result: No issues found.

• The fix is scoped to scripts/codegen/python.ts — the codegen layer, not the SDK's public API surface.
• The PR description notes that go.ts already used string | boolean for discriminator values (GoDiscriminatorValue) — this fix mirrors that existing correct behavior in Go.
• The PR author ran npm run generate for all five languages and confirmed the tree was byte-clean except for the intended six Python lines — meaning no other language SDK was affected by this bug or requires a corresponding fix.
• No public API signatures changed; this is a wire-format correctness fix (booleans now serialize as JSON true/false instead of strings).

The changes maintain full cross-SDK consistency.

Generated by SDK Consistency Review Agent for #2123 · sonnet46 18.4 AIC · ⌖ 5.38 AIC · ⊞ 6.6K · ◷