Update DTS large payload doc: 256 KiB threshold and THRESHOLD_BYTES env var

[torosent]

Summary

Updates the "Large payload support with Durable Task Scheduler (Preview)" article so the .NET and Python examples match the shipped SDK defaults and current samples.

• .NET SDK default threshold → 262,144 bytes (256 KiB) — Set default large payload offloading threshold to 256 KiB microsoft/durabletask-dotnet#755.
• .NET samples renamed the env var EXTERNALIZE_THRESHOLD_BYTES → THRESHOLD_BYTES and use 262144; Durable Functions host.json keys fixed — Align large-payload samples to 256 KiB and fix Durable Functions host.json keys Azure-Samples/Durable-Task-Scheduler#343.
• Python SDK default → 262,144 (Change default large-payload threshold to 256 KiB (mirror .NET) microsoft/durabletask-python#166) and the Python sample updated (Set Python large payload sample threshold to 256 KiB Azure-Samples/Durable-Task-Scheduler#344).

Changes

1. SDK env-var table (C#): EXTERNALIZE_THRESHOLD_BYTES → THRESHOLD_BYTES, 900000 → 262144. The old variable no longer exists in the sample.
2. SDK C# prose: "The sample uses 900,000 bytes" → "262,144 bytes (256 KiB), which is also the SDK default".
3. SDK C# code: options.ThresholdBytes = 900_000 → 262_144.
4. Durable Functions host.json: payloadStorageThresholdBytes: 900000 → 262144, matching both Durable Functions samples.
5. Python (C# tab): code threshold_bytes = 900_000 → 262_144, and prose updated to "262,144 bytes (256 KiB), which is also the SDK default". (The block was previously inconsistent — the code showed 900_000 while the prose said 1,024.)

PAYLOAD_SIZE_BYTES (the 1.5 MiB demo payload size) is intentionally left unchanged — it must stay larger than both the threshold and the 1 MiB scheduler message limit for the samples to demonstrate offload.

Note: this changes the .NET and Python portable-SDK default guidance and sample values. The Durable Functions extension's own payloadStorageThresholdBytes default is unchanged; the host.json value here is a sample-consistent example.

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[learn-build-service-prod]

Learn Build status updates of commit e0a5156:

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[learn-build-service-prod]

Learn Build status updates of commit 9c2a0f0:

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[hhunter-ms]

#sign-off

[Copilot]

Pull request overview

Note

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Updates the “Large payload support with Durable Task Scheduler (Preview)” documentation to align .NET and Python examples with current SDK defaults and sample configuration, especially around the 256 KiB (262,144 bytes) offload threshold and the renamed threshold environment variable.

Changes:

• Updated Durable Functions host.json example threshold from 900000 to 262144.
• Updated .NET and Python SDK sample code/prose to use 262_144 / 262,144 bytes (256 KiB) and note it’s the SDK default.
• Updated the environment variable table to replace EXTERNALIZE_THRESHOLD_BYTES with THRESHOLD_BYTES and updated the default value.

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