docs: add v1 to v2 migration guide

Add docs/migration-1.x-to-2.x.md covering the 2.x breaking changes: typed
per-operation error hierarchy, first-run serialize/deserialize round trip,
removed config types and fields, construction-time config validation, and
wait_for_condition changes; plus the optional should_complete feature.
Also document the initial_state serdes requirement on WaitForConditionConfig.

Author: Ayushi Ahjolia

docs/migration-1.x-to-2.x.md

@@ -0,0 +1,152 @@
+# Migrating from 1.x to 2.x
+
+`2.x` is a breaking major release. Every change is a bug fix or brings Python to
+parity with the JavaScript and Java SDKs. The two changes most likely to touch
+your code are the typed, per-operation **error hierarchy** and the
+**serialize/deserialize round trip on the first run**.
+
+There is no compatibility shim: removed names (for example `CallableRuntimeError`)
+are gone with no alias. If you are not ready to migrate, stay on `1.x`.
+
+## What Changed and What to Do
+
+| Change | What you must do |
+| --- | --- |
+| `CallableRuntimeError`, `UserlandError`, `CallableRuntimeErrorSerializableDetails` removed; typed per-operation errors added | Catch `StepError`, `InvokeError`, `ChildContextError`, or `WaitForConditionError` (or the base `DurableOperationError`) instead of `CallableRuntimeError`. |
+| `CallbackError` moved out of the termination tree; graded subtypes added | Remove any `termination_reason == TerminationReason.CALLBACK_ERROR` check (the enum member is gone). Optionally catch `CallbackTimeoutError` / `CallbackExternalError` / `CallbackSubmitterError`. |
+| `BatchResult.throw_if_error()` now raises a typed error | Catch `ChildContextError` instead of `CallableRuntimeError`. |
+| First-run serialize/deserialize round trip for `step`, child contexts, `map`/`parallel`, and `wait_for_condition` | Make custom `SerDes` round-trip safe: `deserialize(serialize(x)) == x`. Ensure `wait_for_condition` `initial_state` is serializable by the configured serdes. For a transient serdes failure, raise the new `RetryableSerDesError` (retries) instead of `SerDesError` (permanent). |
+| `InvokeConfig.timeout` and `InvokeConfig.timeout_seconds` removed | Remove them. Enforce any timeout inside the invoked function or as a separate timer. |
+| Removed `ItemBatcher`, `ItemsPerBatchUnit`, `BatchedInput`, `TerminationMode`, `StepFuture`, `MapConfig.item_batcher`, `ChildConfig.item_serdes` | Remove all uses. Replace `ChildConfig.item_serdes` with `ChildConfig.serdes`. |
+| `MapConfig` / `ParallelConfig` / `CompletionConfig` now validate at construction | Wrap construction in `try/except ValidationError` if you build configs from external input. |
+| `CompletionConfig.all_completed()` now actually tolerates all failures | If you hand-built the old all-`None` config, use the factory instead. |
+| `WaitDecision` removed; `WaitStrategyConfig.timeout` / `timeout_seconds` removed | Use `WaitForConditionDecision` (`stop_polling()` / `continue_waiting(delay)`). |
+| `wait_for_condition` raises `WaitForConditionError` when it exhausts `max_attempts` | Catch `WaitForConditionError` instead of inspecting the returned state. |
+
+Find affected code before upgrading:
+
+```bash
+rg -n "CallableRuntimeError|UserlandError|CallableRuntimeErrorSerializableDetails" .
+rg -n "CallbackError|CALLBACK_ERROR" .
+rg -n "InvokeConfig\(|\.timeout_seconds" .
+rg -n "WaitDecision|WaitStrategyConfig\(|item_batcher|ItemBatcher|ItemsPerBatchUnit" .
+rg -n "TerminationMode|BatchedInput|StepFuture|ChildConfig\(" .
+```
+
+## Error Handling (the biggest change)
+
+In `1.x` nearly every user-land failure surfaced as one `CallableRuntimeError`,
+so a failed step was indistinguishable from a failed invoke or child branch. `2.x`
+raises a specific type per operation, all under a new base `DurableOperationError`,
+and preserves the original error as `__cause__` (on replay, `__cause__` is
+reconstructed from the checkpointed wire fields `error_type`/`message`/`data`/`stack_trace`).
+
+```python
+# 1.x
+from aws_durable_execution_sdk_python.exceptions import CallableRuntimeError
+try:
+    result = context.step(charge_card, name="charge")
+except CallableRuntimeError as e:
+    log.error("something failed: %s", e.message)
+
+# 2.x
+from aws_durable_execution_sdk_python import StepError, DurableOperationError
+try:
+    result = context.step(charge_card, name="charge")
+except StepError as e:               # or `except DurableOperationError` to catch any operation
+    log.error("charge step failed: %s", e.message)
+```
+
+New types, all exported from the package root: `DurableOperationError` (base),
+`StepError`, `InvokeError`, `ChildContextError`, `WaitForConditionError`,
+`CallbackError` (+ `CallbackExternalError`, `CallbackTimeoutError`,
+`CallbackSubmitterError`), plus `SerDesError` (now exported) and
+`RetryableSerDesError`. `SerDesError` stays a direct child of
+`DurableExecutionsError`; `RetryableSerDesError` is a retryable `InvocationError`.
+
+### Callbacks
+
+`context.wait_for_callback(...)` returns the payload directly and raises the
+callback error from the call itself (there is no `callback.result()`):
+
+```python
+from aws_durable_execution_sdk_python import (
+    CallbackError, CallbackTimeoutError, CallbackSubmitterError,
+)
+try:
+    payload = context.wait_for_callback(submit_approval, name="approval")
+except CallbackTimeoutError:
+    ...                               # timeout / heartbeat expiry
+except CallbackSubmitterError:
+    ...                               # the submitter step failed
+except CallbackError as e:            # external + internal; `callback_id` still available
+    log.error("callback %s failed", e.callback_id)
+```
+
+### map / parallel
+
+```python
+result = context.map(items, process_item)
+try:
+    result.throw_if_error()           # raises ChildContextError for the first failure
+except ChildContextError:
+    for err in result.get_errors():   # every failed item's ErrorObject
+        log.error("%s: %s", err.type, err.message)
+```
+
+## Serialize/Deserialize Round Trip
+
+`1.x` returned the raw in-memory result on the first run but the deserialized
+result on replay, so a non-identity custom `SerDes` produced different values.
+`2.x` round-trips (`serialize` then `deserialize`) on the first run for `step`,
+child contexts, `map`/`parallel`, and `wait_for_condition` (which also feeds the
+deserialized state to the wait strategy). No API change, but a `SerDes` that is
+not round-trip safe now surfaces the discrepancy (and any serialization bug) on
+the first run. Fix it so `deserialize(serialize(x)) == x`. Async operations
+(`invoke`, `wait_for_callback`, `wait`) are unaffected.
+
+`wait_for_condition` also round-trips `initial_state` through the serdes before
+the first check, so `initial_state` must now be serializable by the configured
+serdes.
+
+## New in 2.x: Custom Completion Predicate (Optional)
+
+`2.x` adds a `should_complete` predicate to `CompletionConfig`, giving `map` and
+`parallel` full control over when a batch completes early. This is a new feature,
+not a breaking change - no action is required unless you adopt it.
+
+```python
+from aws_durable_execution_sdk_python import complete_batch, continue_batch
+
+config = CompletionConfig(
+    should_complete=lambda status: (
+        complete_batch() if status.success_count >= 2 else continue_batch()
+    )
+)
+```
+
+The predicate receives a `CompletionStatus` snapshot (counts plus per-item
+statuses) and returns a `CompletionDecision` - `continue_batch()` or
+`complete_batch(outcome)`. The outcome reports `CUSTOM_COMPLETION_SUCCEEDED` or
+`CUSTOM_COMPLETION_FAILED`; a failed custom completion surfaces through
+`throw_if_error()` as a `ChildContextError`, so there is still no separate
+batch-completion error type to catch. Notes:
+
+- It cannot be combined with `min_successful` or the `tolerated_failure_*`
+  fields; doing so raises `ValidationError` at construction.
+- The predicate must be deterministic and side-effect-free. Replay uses the
+  checkpointed decision and never re-invokes it.
+- New exports: `complete_batch`, `continue_batch`, `CompletionStatus`,
+  `CompletionDecision`, `CompletionOutcome`, `CompletionItemStatus`,
+  `BatchItemStatus`.
+
+## Recommended Validation After Upgrading
+
+1. Build and run your test suite against `2.x`, and grep for the removed names above.
+2. Trigger a failure in a `step`, an `invoke`, and a `map`/`parallel` branch;
+   confirm you catch `StepError`, `InvokeError`, and `ChildContextError`.
+3. Exercise a `wait_for_callback` timeout and a submitter-step failure
+   (`CallbackTimeoutError`, `CallbackSubmitterError`).
+4. Exercise a `wait_for_condition` that exhausts its attempts (`WaitForConditionError`).
+5. If you use a custom `SerDes`, run a workflow that checkpoints both a result and
+   an error payload and confirm first-run output equals replay output.

packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/waits.py

@@ -99,7 +99,17 @@ def wait_strategy(result: T, attempts_made: int) -> WaitForConditionDecision:
 
 @dataclass(frozen=True)
 class WaitForConditionConfig(Generic[T]):
-    """Configuration for wait_for_condition."""
+    """Configuration for wait_for_condition.
+
+    Attributes:
+        wait_strategy: Called after each poll with (state, attempts_made) and
+            returns a WaitForConditionDecision (continue_waiting or stop_polling).
+        initial_state: State passed to the first poll. It is round-tripped
+            through serdes (serialize then deserialize) before the first check,
+            so it must be serializable by the configured serdes.
+        serdes: SerDes used to serialize and deserialize the polled state at
+            each checkpoint. Defaults to the SDK's default JSON serdes when None.
+    """
 
     wait_strategy: Callable[[T, int], WaitForConditionDecision]
     initial_state: T