feat(csharp): add opt-in User-Agent fallback flag

[fern-support]

Description

Linear ticket: Refs

Bring the C# SDK generator to optional parity with the TypeScript generator's User-Agent fallback. When sdkConfig.platformHeaders.userAgent is unset in the IR (which happens for SDKs imported from OpenAPI when version/packageName aren't both passed to generateIntermediateRepresentation), the C# generator emits no User-Agent header at all. The TS generator already falls back to <npm-package-name>/<version> in the analogous branch — see <ref_snippet file="/home/ubuntu/repos/fern/generators/typescript/sdk/client-class-generator/src/BaseClientTypeGenerator.ts" lines="171-181" />.

This PR adds an opt-in user-agent-name-from-package boolean custom config (default false) that makes C# emit $"<NuGetPackageId>/{Version.Current}" in that fallback branch, where:

• <NuGetPackageId> resolves via this.generation.names.project.packageId. The cascade is package-id → namespace → PascalCase(<organization>_<api-name>), so the prefix always matches the name the SDK ships under on NuGet (e.g. DocuSign.eSign, or FernPlantstore if neither package-id nor namespace is set).
• Version.Current is the same compile-time constant already used for the X-Fern-SDK-Version header.

Defaulting to false preserves the historical "no User-Agent header for OpenAPI-imported SDKs" behavior; turning it on is now a one-line generator config change.

The companion docs PR is fern-api/docs#5439, which adds the new flag to the C# configuration reference.

Changes Made

• generators/csharp/codegen/src/custom-config/CsharpConfigSchema.ts: new optional boolean user-agent-name-from-package.
• generators/csharp/codegen/src/context/generation-info.ts: new userAgentNameFromPackage setting wired to the new schema key, defaulting to false.
• generators/csharp/sdk/src/root-client/buildUserAgentHeaderEntry.ts: new pure helper that returns the platform-headers Dictionary.MapEntry for User-Agent, branching on whether userAgent is set on the IR and whether the flag is on.
• generators/csharp/sdk/src/root-client/RootClientGenerator.ts: the existing if (userAgent != null) block is replaced with a call to the new helper, gated on this.settings.userAgentNameFromPackage.
• generators/csharp/sdk/src/root-client/__test__/buildUserAgentHeaderEntry.test.ts: 5 unit tests covering both branches (explicit IR header, fallback) plus regression guards for dotted NuGet ids, the version-expression interpolation, and — crucially — the names.project.packageId cascade when no package-id/namespace is configured.
• generators/csharp/sdk/changes/unreleased/user-agent-fallback.yml: feat changelog entry.
• Updated README.md generator (if applicable) — N/A

Testing

• Unit tests added/updated — 5 cases in buildUserAgentHeaderEntry.test.ts; all pass via pnpm --filter=@fern-api/fern-csharp-sdk run test (41 passed, 41 total).

• pnpm run check (biome) clean.

• pnpm turbo run compile --filter=@fern-api/fern-csharp-sdk clean.

• All required CI checks green (49 pass, 0 fail), including every csharp-sdk seed-fixture matrix job and seed-test-results (csharp-sdk).

• Manual end-to-end against an IR with platformHeaders.userAgent unset (fern-unions test fixture, run through the locally-built cli.cjs).

  Flag off (default), package-id: "Plantstore.eSign" — generated FernPlantstoreClient.cs has no User-Agent entry (matches pre-PR behavior):

  new Dictionary<string, string>()
  {
      { "X-Fern-Language", "C#" },
      { "X-Fern-SDK-Name", "FernUnions" },
      { "X-Fern-SDK-Version", Version.Current },
  }

  Flag on, package-id: "Plantstore.eSign" — fallback entry is emitted:

  { "User-Agent", $"Plantstore.eSign/{Version.Current}" },

  Flag on, no package-id, no namespace — fallback degrades to the cascade, producing $"FernUnions/{Version.Current}" (the same string the SDK ships under as the NuGet project), confirming the User-Agent prefix is never empty.

  The Fern-Definition path is byte-identical: seed/csharp-sdk/simple-api/no-custom-config/.../SeedSimpleApiClient.cs continues to emit { "User-Agent", "Fernsimple-api/0.0.1" } regardless of the flag.

Things worth a careful look

• The fallback emits a single interpolated string by combining a literal prefix with writer.writeNode(this.context.getCurrentVersionValueAccess()). Confirmed via the unit tests and manual e2e that the rendered output is exactly $"<PackageId>/{Version.Current}" and that the Version using directive is wired up the same way as the existing X-Fern-SDK-Version entry.
• packageName, userAgent.header, and userAgent.value are interpolated unescaped into $"..." / "...". NuGet package ids in practice only use letters/digits/./-/_, so this matches the existing convention used by the X-Fern-SDK-Name entry; the IR-supplied header/value path is identical to the pre-PR behavior. Worth a sanity check that we're comfortable with that assumption.
• The cascade test uses real Generation rather than the {} as mock used elsewhere in this file, so it actually exercises names.project.packageId end-to-end. If anyone changes that cascade in generation-info.ts, this test should fail.
• Renaming the flag from my initial user-agent-from-package to user-agent-name-from-package happened mid-review per @fern-support's request — sanity-check that no stragglers from the old name remain (e.g. in changelog filenames). The changelog file is still named user-agent-fallback.yml to avoid bake-in of either flag spelling.

Link to Devin session: https://app.devin.ai/sessions/02d3233caaac4616b6f38488ff08ef1a

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-05-08T04:59:46Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
csharp-sdk	square	58s (n=5)	89s (n=5)	68s	+10s (+17.2%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-05-08T04:59:46Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-05-08 05:47 UTC