feat(csharp): read idempotency-key generation config from the IR

[devin-ai-integration]

Description

Switches the C# SDK generator from its own auto-generate-idempotency-key config flag to reading the centralized sdkConfig.idempotencyKeyGeneration block from the IR (added + threaded by the CLI in #17018, published as @fern-fern/ir-sdk@67.11.0).

The behavior is unchanged from a caller's perspective: when idempotency-key generation is enabled, the generated SDK attaches a UUIDv4 idempotency-key header on eligible HTTP methods, unless the caller supplies one. The difference is where the config lives: enablement, header name, and the eligible-method set now come from the IR instead of being defined per-generator and hardcoded to POST/PUT. This is the C# generator half of the IR centralization (PR2); #17018 was PR1, and #17007 is the TypeScript reference.

// before: per-generator flag + hardcoded methods
settings.autoGenerateIdempotencyKey && (method === "POST" || method === "PUT")

// after: read from IR
const gen = this.ir.sdkConfig.idempotencyKeyGeneration;
const headerName = gen?.headerName ?? "Idempotency-Key";
const enabled = gen != null && gen.methods.includes(endpoint.method);

Changes Made

• GeneratorContext.ts: added shouldAutoGenerateIdempotencyKey(endpoint) (true iff ir.sdkConfig.idempotencyKeyGeneration != null and .methods includes the endpoint's method) and getIdempotencyKeyGenerationHeaderName() (from .headerName, defaulting to Idempotency-Key). Enablement + eligible methods + header name all come from the IR.
• Header injection unchanged in shape across all four header-builder paths (HttpEndpointGenerator, WrappedEndpointRequest, ReferencedEndpointRequest, BytesOnlyEndpointRequest): a fallback .Add(<headerName>, global::System.Guid.NewGuid().ToString()) emitted before the declared idempotency headers and request-option headers so a caller-supplied value wins.
• Removed the per-generator auto-generate-idempotency-key config plumbing (custom-config schema key + autoGenerateIdempotencyKey threading through generation-info). The CLI now owns the auto-generate-idempotency-key config key and threads it into the IR.
• Bumped @fern-fern/ir-sdk to 67.11.0 (contains idempotencyKeyGeneration with methods) and reconciled pnpm-lock.yaml.
• fix(seed): made the CLI-only-key threader (getIdempotencyKeyGenerationFromGeneratorConfig) fall back to the resolved generatorInvocation.config when raw is absent, so the seed harness's synthetic invocations thread the IR field. Production (where raw is always populated) is unchanged. Added unit tests. (Brought in verbatim from the TS reference branch.)
• Added an unreleased feat changelog entry, reworded to describe the IR-driven approach.
• Updated README.md generator (if applicable) — N/A

Seed fixture proof

• idempotency-headers fixture (no-custom-config baseline + auto-generate-idempotency-key) drives the feature via the CLI key (auto-generate-idempotency-key: true); regenerated flag-on output injects the fallback UUID header on the declared POST endpoint, flag-off has none.
• exhaustive/auto-generate-idempotency-key variant proves the standalone path across the full API.
• Committed fixture .fern/metadata.json normalized to canonical field order with invokedBy: ci / ciProvider: github to match sibling committed fixtures.

Injection confirmation:

seed/csharp-sdk/idempotency-headers/auto-generate-idempotency-key/src/SeedIdempotencyHeaders/Payment/PaymentClient.cs:27:            .Add("Idempotency-Key", global::System.Guid.NewGuid().ToString())

Testing

• Unit tests added/updated — csharp-base, csharp-codegen, csharp-sdk vitest suites pass; commons idempotency-threading tests pass.
• Manual testing completed — pnpm seed test --generator csharp-sdk --fixture idempotency-headers --local --skip-scripts → 2/2 pass; --fixture exhaustive → 8/8 pass. biome format clean.

Held for Twilio design-doc sign-off before merge.

[devin-ai-integration]

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-07-13T05:08:13Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
csharp-sdk	square	76s (n=5)	115s (n=5)	66s	-10s (-13.2%)

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: 2026-07-13T05:08:13Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-07-13 22:36 UTC

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[Swimburger]

Can we generate a C# extension method that is called AddIdempotencyHeader(), so the magic string "Idempotency-Key" is in a single location, and the new guid code as well?
This could be a separate file that is generated dynamically.

[devin-ai-integration]

Done — generated a dedicated IdempotencyHeaderExtensions core file (emitted only when the IR enables auto-generation) so the "Idempotency-Key" magic string and the new-guid code live in a single generated location:

internal static class IdempotencyHeaderExtensions
{
    private const string IdempotencyKeyHeaderName = "Idempotency-Key";

    internal static HeadersBuilder.Builder AddIdempotencyHeader(this HeadersBuilder.Builder builder)
    {
        return builder.Add(IdempotencyKeyHeaderName, global::System.Guid.NewGuid().ToString());
    }
}

All four request-builder paths (HttpEndpointGenerator, WrappedEndpointRequest, ReferencedEndpointRequest, BytesOnlyEndpointRequest) now call .AddIdempotencyHeader() instead of repeating .Add("Idempotency-Key", global::System.Guid.NewGuid().ToString()). The configured header name (custom names from the IR) is baked into IdempotencyKeyHeaderName. Merge order is preserved (.AddIdempotencyHeader() before the caller/declared headers), so a caller-supplied value still wins.