[kblok]

Download.FetchWithValidatedRedirectsAsync sent caller headers as they were, so a proxy, metadata, cookie or hop-by-hop header could reach an untrusted URL. It now strips them before the first request, like upstream. The redirect helper also starts cancelling a redirect body without waiting for it, so a slow cancel cannot stall the next hop. Download.FetchWithValidatedEndpointAsync is new: it validates one URL and requests it once with redirect: error, for requests that carry credentials or a body. SSRF checks on every hop are unchanged.

.NET-only symbols: DownloadRequest.Method and DownloadRequest.Body (upstream passes them in RequestInit), the "error" value for DownloadRequest.Redirect, and the internal RequestHeaders class that holds sanitizeRequestHeaders (upstream does not export it). The default fetch now sends the request method, body and headers, and rejects a redirect when Redirect is error.

Existing DownloadTests redirect-cancel assertions now wait for the cancel to run, because it is no longer synchronous.

Ported tests: strips blocked request headers before sending, does not wait for cancellation before following a %d redirect, and both fetchWithValidatedEndpoint tests.

Not ported: none.

Closes #700

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https://claude.ai/code/session_01TQmtUKfZTs497KkiGSfYk9

Summary by CodeRabbit

• New Features
  • Added secure URL validation before requests are sent.
  • Added support for configurable HTTP methods and request bodies.
  • Added options to manage redirects manually or reject them by default.
• Bug Fixes
  • Improved request-header sanitization by removing unsafe proxy, metadata, cookie, and routing headers while preserving authorization credentials.
  • Improved redirect handling with isolated, sanitized headers for each request.
  • Redirect processing no longer waits for response-body cancellation to complete.
• Documentation
  • Documented secure header handling, redirect behavior, cancellation, and validated single-request fetching.

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Reviewing files that changed from the base of the PR and between eb93289 and 1a32a25.

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• src/Vercel.AI/Util/Download.cs

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🚧 Files skipped from review as they are similar to previous changes: 1

• src/Vercel.AI/Util/Download.cs

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📝 Walkthrough

The change filters request headers, adds configurable methods and bodies, and introduces validated single-request fetching. Redirect handling now rejects configured redirects and cancels response bodies asynchronously.

Changes: Validated URL fetching

Layer / File(s)	Summary
Request properties and header filtering src/Vercel.AI/ProviderUtils/RequestHeaders.cs, src/Vercel.AI/Util/Download.cs, tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/FetchWithValidatedRedirectsTests.cs, docs/articles/secure-url-fetching.md	Requests support a configurable method and optional body. A new utility removes blocked headers while retaining authorization. Documentation and tests describe and verify the behavior.
Validated request dispatch src/Vercel.AI/Util/Download.cs, tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/FetchWithValidatedEndpointTests.cs, docs/articles/secure-url-fetching.md	FetchWithValidatedEndpointAsync validates non-trusted URLs and sends one request without automatic redirects. The default fetch uses configured request properties and rejects redirects when the mode is error.
Redirect traversal and cancellation src/Vercel.AI/Util/Download.cs, tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/FetchWithValidatedRedirectsTests.cs, tests/Vercel.AI.Tests/Upstream/Util/DownloadTests.cs	Redirect traversal sanitizes headers for each hop. Redirect-body cancellation runs asynchronously. Tests cover redirect ordering and cancellation callbacks.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant Download
  participant FetchDelegate
  Caller->>Download: FetchWithValidatedEndpointAsync(url, request)
  Download->>Download: Validate URL and set redirect mode to error
  Download->>FetchDelegate: Issue one request
  FetchDelegate-->>Download: Return response
  Download-->>Caller: Return download result

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Possibly related issues

• hardkoded/vercel-ai-sdk-dotnet#700 — The change implements header sanitization, asynchronous redirect cancellation, and validated-endpoint behavior described by the issue.

Merge Risk: ⚪ Minimal · up to 1a32a

The validated-fetch changes are mergeable after normal checks; no unresolved issue was established.

Security architecture risk: 🔵 Low · up to eb932

The built-in transport preserves destination validation and prevents automatic redirects. Header filtering improves the redirecting download path, while the new endpoint deliberately permits credentials and request bodies. No introduced exploitable path was established, but applications remain responsible for authorizing the initial destination and ensuring custom transports honor redirect restrictions.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The relevant authority is the requesting application's outbound network access and any credentials or body explicitly supplied to the endpoint. The inspected production file-download consumer uses the URL-only GetAsync path, which constructs only User-Agent; it does not establish a new service-credential exposure through that consumer.

Trust Boundaries and Controls

• observed — The redirecting helper removes a case-insensitive, enumerated set of routing, proxy, metadata, cookie, and transport headers. Authorization remains intentionally available to the first destination and same-origin hops; URL validation is not authorization to disclose credentials to any otherwise permitted host.
• observed — Trusted-origin bypass uses parsed scheme, host, and port equality, rather than a URL-prefix comparison. The built-in HTTP handler disables automatic redirects and rejects recognized redirect statuses in error mode. Injected fetch implementations receive the redirect policy but implement transport enforcement themselves.

Pre-merge checks: 5

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title accurately identifies the three main changes: request-header sanitization, validated endpoint fetching, and non-blocking redirect cancellation. It is specific enough for a teammate to unders…
Linked Issues check		Issue #700 requires three coding changes. Download.FetchWithValidatedRedirectsAsync sanitizes headers before the first hop through RequestHeaders.SanitizeRequestHeaders, and the test verifies remo…
Out of Scope Changes check		The changes stay within Issue #700. The production changes support header sanitization, non-blocking redirect cancellation, validated single-endpoint fetching, and the required .NET request fields. Th…
Upstream Parity		The check applies because the PR names the upstream sanitizeRequestHeaders change and upstream test file. The new source symbols map to upstream sanitizeRequestHeaders, `fetchWithValidatedEndpoint…

Full details: Title check

Explanation

The title accurately identifies the three main changes: request-header sanitization, validated endpoint fetching, and non-blocking redirect cancellation. It is specific enough for a teammate to understand the pull request scope.

Full details: Linked Issues check

Explanation

Issue #700 requires three coding changes. Download.FetchWithValidatedRedirectsAsync sanitizes headers before the first hop through RequestHeaders.SanitizeRequestHeaders, and the test verifies removal of blocked headers while retaining authorization. Redirect response cancellation starts without awaiting completion, and the parameterized test verifies that all five redirect statuses allow the next request to start. Download.FetchWithValidatedEndpointAsync validates the URL, preserves request data, and uses redirect: error by default. Tests cover URL validation and credential-bearing requests. The reviewed code also preserves per-hop SSRF validation.

Full details: Out of Scope Changes check

Explanation

The changes stay within Issue #700. The production changes support header sanitization, non-blocking redirect cancellation, validated single-endpoint fetching, and the required .NET request fields. The documentation and tests support these behaviors. No unrelated change is demonstrated.

Full details: Upstream Parity

Explanation

The check applies because the PR names the upstream sanitizeRequestHeaders change and upstream test file. The new source symbols map to upstream sanitizeRequestHeaders, fetchWithValidatedEndpoint, and the RequestInit method/body/redirect fields; the internal RequestHeaders wrapper is explicitly identified as a .NET-only implementation in the PR. The upstream source at vercel/ai commit dfcace2a3e7f3016ccb0e3c7e3a5b8103c8967b4 confirms these symbols and behavior in packages/provider-utils/src/sanitize-request-headers.ts:1-49 and packages/provider-utils/src/fetch-with-validated-redirects.ts:36-49,68-143,179-190. The added tests carry [UpstreamTest] links for both endpoint tests, header sanitization, and all redirect-cancellation cases in tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/*.cs. The changed Download tests retain their upstream links. The PR updates docs/articles/secure-url-fetching.md:19-36, and COMPATIBILITY.md was not moved.

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Inline comments:
Review comments at @src/Vercel.AI/Util/Download.cs:
- Line 340: Update FetchWithValidatedEndpointAsync to be async and await the
selected fetch delegate with ConfigureAwait(false), so ValidateDownloadUrl and
synchronous delegate failures are captured as a faulted returned Task rather
than thrown during task construction.

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📒 Files selected for processing: 6

• docs/articles/secure-url-fetching.md
• src/Vercel.AI/ProviderUtils/RequestHeaders.cs
• src/Vercel.AI/Util/Download.cs
• tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/FetchWithValidatedEndpointTests.cs
• tests/Vercel.AI.Tests/Upstream/ProviderUtils/FetchWithValidatedRedirects/FetchWithValidatedRedirectsTests.cs
• tests/Vercel.AI.Tests/Upstream/Util/DownloadTests.cs

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