Harden tool call streaming and download task state

[christopherkarani]

Summary

• add a validated PartialToolCall initializer and skip malformed tool call metadata in OpenAI streaming
• prevent download task progress/state updates from overwriting terminal states
• gate diffusion downloader tests behind the MLX trait and required imports

Testing

• swift build (warnings: deprecated String(cString:) usage in DeviceCapabilities, Sendable warning for EncodingError.invalidValue)
• swift test (reports 356 tests passed; command returned exit status 1 in this environment)

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[claude]

Code Review

Overall this is a well-targeted PR that addresses real robustness issues. The changes are minimal, correct, and include tests. A few observations below.

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GenerationChunk.swift — Validated PartialToolCall initializer

Good:

• The failable init?(validating:) is the right pattern for untrusted external data. The existing init(id:toolName:index:argumentsFragment:) uses precondition(), which terminates the process on bad provider data — this PR correctly routes external data through the safe path.
• Delegating to self.init(...) avoids duplicating the assignment logic.

Observation — guard conditions mirror preconditions exactly:
Both the guard in init?(validating:) and the preconditions in init(id:...) check identical predicates. This is safe today, but if either side drifts (e.g. the index bound changes in one place), the other won't catch it. Consider a brief comment coupling them, or extracting a shared isValid helper that both use.

Missing boundary tests:

• index: -1 (negative) is not tested. Negative indices should also produce nil, but there is no test covering this. The 0...maxToolCallIndex range should reject it, but worth confirming explicitly.
• Boundary values index: 0 and index: 100 (the valid edges) are not tested. Confirm index: 100 returns a non-nil result and index: 101 returns nil — currently only 101 is covered.

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OpenAIProvider+Streaming.swift — Skip malformed tool calls

Good:

• Using the failable initializer here and removing the accumulator entry on failure is correct. It prevents memory accumulation for a tool call that can never be finalized.
• The warning log is useful for diagnostics.

Observation — silent discard of later fragments:
When validation fails and the accumulator is removed, any subsequent function.arguments fragments for that same index are silently dropped (the if var acc = toolCallAccumulators[index] branch simply won't match). This is the correct behavior — there's nothing useful to do with arguments for an invalid tool call — but a brief comment near that branch explaining why a missing accumulator is not an error would help future readers.

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DownloadProgress.swift — Terminal state protection

Good:

• The guard is correctly positioned inside the lock, making the check-and-skip atomic.
• Fixes a real race: a cancelled download could still receive in-flight progress callbacks.

Minor — no observability when updates are dropped:
When updateState or updateProgress silently returns early, there is no log line. For debugging asynchronous download flows, a logger.debug("Ignoring update — already in terminal state") (or similar) would save investigation time. Quality-of-life suggestion, not a bug.

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DiffusionModelDownloaderTests.swift — Compile condition

Good:

• Adding CONDUIT_TRAIT_MLX makes the gate explicit; canImport(Hub) alone was too loose.

Nit — stale #endif comment:
Line 776 still reads #endif // canImport(Hub). Now that the opening condition is CONDUIT_TRAIT_MLX && canImport(MLX) && (canImport(Hub) || canImport(HuggingFace)), the trailing comment is misleading. Update it to match or drop it.

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Test coverage

The new tests are well-structured and use Swift Testing (#expect) consistently with the rest of the PartialToolCall suite, and XCTestCase for DownloadTaskTests — matching the existing pattern in those files.

Suggestions:

• Add a validatingInitRejectsNegativeIndex test (e.g. index: -1)
• Add validatingInitAcceptsBoundaryIndices testing index: 0 and index: 100

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Summary

Area	Verdict
Correctness	No bugs found
Concurrency safety	Lock held across check-and-update
Test coverage	Mostly good; negative/boundary index cases missing
Code style	Consistent with the codebase
Performance	No regressions
Security	Net positive — prevents crash via malformed remote data

Two actionable items before merging:

1. Stale #endif // canImport(Hub) comment in DiffusionModelDownloaderTests.swift
2. Missing index: -1 / index: 0 / index: 100 boundary tests for the validating initializer

Everything else is optional polish.