feat(go): add clientDefault support to Go SDK generator

[Swimburger]

Description

Adds clientDefault support (from the x-fern-default OpenAPI extension) to the Go SDK generator. When a header, query parameter, or path parameter has a clientDefault value, the generated SDK uses that value as a fallback when the caller does not provide one.

Both the v1 and v2 Go generators are updated.

Changes Made

Go v1 generator (generators/go/internal/generator/sdk.go)

• IR types (http.go): Added ClientDefault *Literal field to HttpHeader, QueryParameter, and PathParameter structs with getters, setters, and bitmask serialization control.
• Request-options headers (sdk.go:WriteRequestOptionsDefinition): Headers with clientDefault initialize a local var with the default, optionally fall back to an env var, then use the caller-provided value if present.
• Client constructor headers (sdk.go:WriteClient): After the existing env-var fallback, a second fallback to clientDefault is applied for string-typed headers. Note: the prior continue after the env block was removed so execution falls through to the new clientDefault check.
• Endpoint headers: Endpoint-scoped headers with clientDefault use a local variable initialized with the default, overridden by the request field if provided.
• Query parameters: After normal serialization, a clientDefault is added only if the key is not already present (if _, ok := queryParams[...]; !ok).
• Path parameters: For both wrapped-request and positional styles, a local variable is initialized from the request field (or param), falling back to the clientDefault when the value is the zero string.
• Helpers: Added isStringType() which guards clientDefault generation to non-optional string primitives, and pathParameterDefault struct for path param default tracking.

Go v2 generator (generators/go-v2/sdk/src/)

• authUtils.ts: Added isPlainStringType() helper — returns true only for non-optional, non-nullable string primitives. Guards clientDefault generation for path params and headers since the == "" zero-value check only works for Go string (not *string).
• ClientGenerator.ts: Modified writeHeaderEnvironmentVariables() to apply clientDefault fallback for global headers after the env-var fallback. Added writeClientDefaultConditional() method using safe getLiteralValue (AST-based, properly escaped).
• HttpEndpointGenerator.ts:
  • Path parameters: Before URL encoding, applies clientDefault fallback using local variables (avoids mutating caller's request struct). Uses getFieldName() for safe Go identifiers. endpointURL := assignment moved after local var declarations.
  • Query parameters: After type-level defaults, adds clientDefault entries for query params that don't already have a type-level default. No type guard — works for both string and boolean literals (matches v1 behavior).
  • Endpoint headers: After existing header handling, applies clientDefault fallback using local variables (avoids request struct mutation).

Other

• Unreleased changelog: Added generators/go/sdk/changes/unreleased/add-client-default-support.yml (uses the unreleased version mechanism instead of editing versions.yml directly).
• Seed snapshots: Updated seed/go-sdk/x-fern-default/ (clientDefault output) and seed/go-sdk/exhaustive/ (unrelated UUID→Uuid naming regeneration from main).

Issues found during review & fixed

• Security: unescaped string interpolation — Replaced all 3 getLiteralAsString callsites (which do no escaping) with safe getLiteralValue (AST-based go.TypeInstantiation.string() → escapeGoString). Affected: ClientGenerator.ts:writeClientDefaultConditional, HttpEndpointGenerator.ts:buildEndpointUrl, HttpEndpointGenerator.ts:buildHeaders.
• Request struct mutation — v2 endpoint headers and path parameters now use local variables (_FieldName := request.FieldName) instead of mutating the caller's *Request struct through the pointer.
• Invalid Go syntax — Moved writer.write("endpointURL := ") from before clientDefault local var declarations to after them, preventing endpointURL := _region := request.Region (double :=).
• Unsafe Go identifiers — Path param local variable names now use getFieldName() instead of getOriginalName() to handle names with hyphens/special characters.
• Silently dropped boolean defaults — Removed isPlainStringType guard for query param clientDefault in v2 (query params use a defaults map, so any literal type works — matches v1 behavior).

Human Review Checklist

Items worth a reviewer's attention — not all are bugs, but they carry risk.

• continue removal in v1 client constructor (~line 1330): The continue after the env-var block was removed so execution falls through to the clientDefault check. When env != nil but clientDefault == nil, the code enters the env block then skips the clientDefault block (guarded by clientDefault != nil). Behavior should be equivalent, but worth verifying.
• Space in options. field access (v1 lines ~1334, 1336): f.P("options. ", ...) has a space — this is pre-existing in the codebase. Go's f.P concatenates args, so it generates options. HeaderName. Confirm this is intentional.
• isPlainStringType / isStringType don't resolve aliases: If a type is an alias of string, clientDefault will be silently skipped. Same limitation in both v1 and v2.
• Pre-existing getLiteralAsString calls for type-level literals: Lines 832 and 886 of HttpEndpointGenerator.ts still use getLiteralAsString for type-level literal values (not clientDefault). These have the same escaping gap but are pre-existing and out of scope for this PR.
• _FieldName local variable prefix convention: Path param and header clientDefault logic creates local variables prefixed with _ (e.g., _Region). Verify this doesn't collide with any Go reserved identifiers or existing generated variable names.
• Query param clientDefault extracts raw string from Literal: queryParameter.clientDefault.string is passed to go.TypeInstantiation.string() which handles escaping. Confirm this is safe for all string contents (quotes, backslashes, newlines).
• Exhaustive snapshot churn: The seed/go-sdk/exhaustive/ changes are unrelated to clientDefault — they're from main's UUID→Uuid naming convention update and metadata cleanup that got regenerated when running seed tests.

Testing

• v2 TypeScript compilation (tsc) passed via seed test build
• Self-review of full diff completed
• All Devin Review issues resolved (6 findings addressed across 6 commits)
• hex-security-app finding addressed (unescaped getLiteralAsString → safe getLiteralValue)
• Docker seed tests: x-fern-default fixture — generated Go code compiles (go build) and passes tests (go test) ✓
• Docker seed tests: exhaustive fixture (both variants) — compiles and passes tests ✓
• Seed snapshots committed (x-fern-default + exhaustive regeneration)
• CI fully green: 53/53 checks passing (compile, biome, depcheck, lint, test, test-ete, all 16 go-sdk seed jobs)
• Unit tests added/updated

Link to Devin session: https://app.devin.ai/sessions/dae61f87717a46d781e579e47b4758e5
Requested by: @Swimburger

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[claude]

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[devin-ai-integration]

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[github-actions]

SDK Generation Benchmark Results

Comparing PR branch against latest nightly baseline on main (2026-04-23T04:59:11Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
go-sdk	square	129s	166s	119s	-10s (-7.8%)

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: 2026-04-23T04:59:11Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-04-23 16:53 UTC

[iamnamananand996]

Can update this with the new unreleased mechanism.

[devin-ai-integration]

Thanks for the suggestion! I'm not familiar with the "unreleased mechanism" — I don't see it used in any other versions.yml files on main currently. Could you point me to an example or docs? Happy to update if there's a specific format to follow.

[Swimburger]

Instead of editing versions.yml, use the unreleased version mechanism. It is documented in CLAUDE.md and other places.

[devin-ai-integration]

Done — reverted the versions.yml edit and created generators/go/sdk/changes/unreleased/add-client-default-support.yml instead (commit d584b11a925).

[iamnamananand996]

Looks good

[devin-ai-integration]

Thanks! 👍

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 19 additional findings in Devin Review.

[devin-ai-integration]

🟡 Boolean clientDefault for query parameter produces wrong Go type in defaults map

When a query parameter has a boolean clientDefault, the v2 code at lines 788-794 converts it to a Go string literal (e.g., "true") instead of a Go boolean literal (true). This is inconsistent with how type-level boolean defaults are stored — those use go.TypeInstantiation.bool(value). While the URL-encoded output is the same (both stringify to "true"), the actual bug manifests when QueryValuesWithDefaults (generators/go/internal/generator/sdk/internal/query.go:95) checks field.IsZero() on the struct field. For a boolean field set to false, IsZero() returns true, so the default map is consulted. The default should be a Go bool value (true/false), not a Go string value ("true"/"false"). A string default in a map[string]interface{} would be passed to valueString(reflect.ValueOf("true")), which produces the correct URL string, so the end result is functionally equivalent. However, the comment on line 782-783 claims "any literal type works here" yet always forces the value to a string type, making the comment misleading and the implementation inconsistent with the type-level defaults mechanism.

Suggested change

	const literalString =
	queryParameter.clientDefault.type === "string"
	? queryParameter.clientDefault.string
	: String(queryParameter.clientDefault.boolean);
	defaults.push({
	key: go.TypeInstantiation.string(wireValue),
	value: go.TypeInstantiation.string(literalString)
	});
	const literalValue =
	queryParameter.clientDefault.type === "string"
	? go.TypeInstantiation.string(queryParameter.clientDefault.string)
	: go.TypeInstantiation.bool(queryParameter.clientDefault.boolean);
	defaults.push({
	key: go.TypeInstantiation.string(wireValue),
	value: literalValue
	});

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