Fix: Download with progress callback fails with 'Operation was cancel…

[apsonawane]

Fix: Download with progress callback fails with "Operation was cancelled by user"

Problem

Calling model->Download() with a progress callback causes the native core to abort with:

[Foundry][Error] Error downloading model [qwen3.5-2b-generic-gpu]:
Operation was cancelled by user

The same call without a callback works correctly.

Root Cause

The C++ SDK declared NativeCallbackFn as returning void, while the native core DLL (C#) expects the callback to return int (0 = continue, 1 = cancel). Since the void callback never sets the return register, the C# marshalling reads an indeterminate value (garbage), interprets it as non-zero (cancel), and aborts the download.

All other SDKs correctly define the callback as returning int:

SDK	Callback return type	Correct?
C++ (before fix)	void	No
C#	int	Yes
JavaScript	int32_t	Yes
Python	c_int	Yes
Rust	i32	Yes

Fix

Changed NativeCallbackFn and UserCallbackFn from void(*)() to int(*)() and updated all callback lambdas to return 0 (continue).

Files Changed

• sdk/cpp/src/foundry_local_internal_core.h — NativeCallbackFn return type void → int
• sdk/cpp/src/flcore_native.h — UserCallbackFn return type void → int
• sdk/cpp/src/model.cpp — download progress callback lambda returns 0
• sdk/cpp/src/core_helpers.h — streaming callback lambda returns 0
• sdk/cpp/test/model_variant_test.cpp — added regression test Download_WithCallback_ReturnsZeroToContinue

Testing

All 163 unit tests pass, including a new regression test that invokes the callback and asserts the return value is 0.

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Project	Deployment	Actions	Updated (UTC)
foundry-local	Ready	Preview, Comment	May 4, 2026 5:55pm

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR fixes the C++ SDK/native-core callback ABI mismatch that caused downloads with a progress callback to be interpreted as cancelled.

Changes:

• Updated native callback typedefs from void to int to match the core DLL contract.
• Adjusted download and streaming callback lambdas to return 0 so the core continues instead of cancelling.
• Added a regression test for the download callback path.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
sdk/cpp/test/model_variant_test.cpp	Adds a regression test for the download callback return value.
sdk/cpp/src/model.cpp	Updates the download progress callback lambda to return int.
sdk/cpp/src/foundry_local_internal_core.h	Changes the internal native callback typedef to int.
sdk/cpp/src/flcore_native.h	Changes the exported callback typedef/comment to the int contract.
sdk/cpp/src/core_helpers.h	Updates the streaming callback lambda to return int.

Comments suppressed due to low confidence (1)

sdk/cpp/src/core_helpers.h:1

• Once onChunk throws, this callback stores the exception but still returns 0 (continue) both immediately after the failure and on every subsequent invocation. With the new callback contract, that means the native core will keep streaming data even though the caller has already failed, wasting work and potentially delaying error propagation until the full stream finishes. Return the cancel value after capturing an exception, and when st->exception is already set.

// Copyright (c) Microsoft Corporation. All rights reserved.

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