fix(wac): return the checkpointed value from step(), not the live object (#10367)

* fix(wac): return the checkpointed value from step(), not the live object

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* docs(wac): regenerate system prompts and narrow the round-trip claim

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* style(wac): condense the round-trip comments and fix the fallback note

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* feat(sdk): type step() as the JSON round trip of its body's result

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* feat(sdk): apply the JSON round trip to task() and the standalone paths

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(sdk): encode bigint, keep unknown as unknown, align dropped-key results

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(wac): null out results whose key JSON.stringify would drop

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(wac): normalize only the top-level result, keeping nested keys as they were

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(wac): normalize a child task's result so a deployed job cannot fail to parse

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* docs(sdk): pin non-finite number behavior in Jsonified and its tests

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(sdk): admit undefined for keys whose value JSON.stringify may omit

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(sdk): make a key JSON.stringify may omit optional, not just nullable

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* fix(sdk): treat a class-valued property as dropped, like any other function

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

system_prompts/auto-generated/skills/write-script-python3/SKILL.md

@@ -816,6 +816,10 @@ def parse_sql_client_name(name: str) -> tuple[str, Optional[str]]
 # - **v1 (WM_JOB_ID set, no @workflow)**: dispatches via HTTP API.
 # - **Standalone**: executes the function body directly.
 # 
+# A task runs as its own job, so its result is always encoded as JSON and
+# decoded back before the caller sees it: a ``datetime`` comes back as a
+# string, a tuple as a list.
+# 
 # Usage::
 # 
 #     @task
@@ -861,6 +865,10 @@ def workflow(func)
 # On replay the cached value is returned without re-executing ``fn``.
 # Use for lightweight deterministic operations (timestamps, random IDs,
 # config reads) that should not incur the overhead of a child job.
+# 
+# ``fn``'s result is encoded as JSON and decoded back before it is returned,
+# so the round that runs the body sees the same types every replay sees:
+# a ``datetime`` comes back as a string, a tuple as a list.
 async def step(name: str, fn)
 
 # Server-side sleep — suspend the workflow for the given duration without holding a worker.