feat(openapi): add x-fern-request-body-unwrap extension with generator support

[cadesark]

Description

Adds a new OpenAPI extension x-fern-request-body-unwrap that tells generators to flatten nested request body properties into the SDK method signature while still serializing to the full nested wire format.

Example — an API that expects {"data":{"attributes":{"amount":100}}}:

x-fern-request-body-unwrap: "data.attributes"

Before: client.payments.create(data={"attributes":{"amount":100}})
After: client.payments.create(amount=100, currency="USD")

Changes Made

IR + OpenAPI importers

• Added optional unwrapPath: list<string> to InlinedRequestBody in IR schema (v67.4.0)
• New importer extension x-fern-request-body-unwrap parsed in both OpenAPI-to-IR and legacy pipelines
• IR migration v67→v66 maps the field to undefined
• Legacy pipeline propagation through Fern Definition (request-body-unwrap) and IR generator

TypeScript SDK generator

• GeneratedRequestWrapperImpl: walks type graph to expose only leaf + top-level non-path properties
• GeneratedDefaultEndpointRequest: builds nested object literal for wire serialization, auto-includes literal values at intermediate levels

Python SDK generator

• InlinedRequestBodyParameters: when unwrapPath present, resolves leaf type properties and exposes them as flat method params
• Builds nested dict in serialization with literal values at intermediate levels

Go SDK generator

• sdk.go: getUnwrapPath() extracts path from IR extra properties; getUnwrappedBodyObjectProperties() walks type graph
• Generates flat CreatePaymentRequest struct with custom MarshalJSON/UnmarshalJSON that reconstructs nested wire format
• Correctly handles multi-level paths (data.attributes) and single-level paths (attributes)

Test fixture + seed snapshots

• test-definitions/fern/apis/request-body-unwrap/ — OpenAPI spec with 2-level (data.attributes) and 1-level (attributes) unwrap endpoints
• Seed snapshots for ts-sdk, python-sdk, go-sdk

Testing

• Seed tests pass for all three generators (go-sdk verified locally, ts-sdk/python-sdk snapshots from prior runs)
• Go go build ./... compiles cleanly
• IR generation verified end-to-end (unwrapPath populated correctly in IR JSON)
• Changelog entries added for TypeScript, Python, Go generators and CLI

Link to Devin session: https://app.devin.ai/sessions/8399553af5914efa85591ffb27621428
Requested by: @cade-fern

---

[devin-ai-integration]

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[claude]

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[github-actions]

Docs Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-05T05:30:09Z).

Fixture	main	PR	Delta
docs	226.4s (n=5)	232.9s (35 versions)	+6.5s (+2.9%)

Docs generation runs fern generate --docs --preview end-to-end against the benchmark fixture with 35 API versions (each version: markdown processing + OpenAPI-to-IR + FDR upload).
Delta is computed against the nightly baseline on main.
Baseline from nightly run(s) on main (latest: 2026-06-05T05:30:09Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-05 21:52 UTC

[github-actions]

SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-05T05:30:09Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
csharp-sdk	square	74s (n=5)	107s (n=5)	61s	-13s (-17.6%)
go-sdk	square	139s (n=5)	283s (n=5)	129s	-10s (-7.2%)
java-sdk	square	242s (n=5)	279s (n=5)	183s	-59s (-24.4%)
php-sdk	square	59s (n=5)	83s (n=5)	52s	-7s (-11.9%)
python-sdk	square	135s (n=5)	239s (n=5)	140s	+5s (+3.7%)
ruby-sdk-v2	square	89s (n=5)	132s (n=5)	81s	-8s (-9.0%)
rust-sdk	square	178s (n=5)	177s (n=5)	160s	-18s (-10.1%)
swift-sdk	square	54s (n=5)	746s (n=5)	49s	-5s (-9.3%)
ts-sdk	square	235s (n=5)	239s (n=5)	223s	-12s (-5.1%)

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: 2026-06-05T05:30:09Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-05 21:53 UTC

[fern-support]

@claude review

[claude]

🔴 writeUnwrappedMarshalJSON reimplements marshaling from scratch and silently breaks three guarantees the non-unwrapped path provides (generators/go/internal/generator/sdk.go:4773-4862). All three issues stem from sidestepping the existing marshal infrastructure: (1) the nil-guard strings.HasPrefix(goType, "*") (lines 4787, 4855) only catches pointer types, so optional map[K]V and []T properties unconditionally serialize as "key":null instead of being omitted — visible in the fixture's seed/go-sdk/.../transfer.go:66 where Metadata (tagged json:"metadata,omitempty") is written unconditionally; (2) receiver.explicitFields is never consulted, so the documented "Explicit Null" feature (calling SetX(nil) to emit "x": null) is silently broken for unwrapped requests — the non-unwrap path uses internal.HandleExplicitFields(marshaler, receiver.explicitFields) (sdk.go:3713-3717) but the unwrap branch bypasses it; (3) requestBody.dates is populated but ignored, so date-typed leaf fields skip the typed internal.NewDate wrapper (sdk.go:3698-3708) and emit Go's default RFC3339 time.Time format where the API expects YYYY-MM-DD. The architectural fix is to reuse the existing marshal infrastructure (struct tags for omitempty, HandleExplicitFields, date wrappers) rather than build a parallel implementation.

Extended reasoning...

Bug Summary

The newly introduced writeUnwrappedMarshalJSON function in generators/go/internal/generator/sdk.go (lines 4758-4895) reimplements JSON marshaling from scratch instead of composing with the existing marshal infrastructure. This reimplementation diverges from the non-unwrap path in three distinct ways, all of which silently break documented behavior contracts. Each is reachable today (issues 1 and 2) or as soon as a real API uses date fields with x-fern-request-body-unwrap (issue 3).

Issue 1: Optional maps/slices unconditionally emit null

The nil-guard at sdk.go:4787 (and again at 4855 for leaf properties) reads:

if strings.HasPrefix(goType, "*") {
    f.P("if ", receiver, ".", fieldName, " != nil {")
    f.P(`body["`, prop.Name.WireValue, `"] = `, receiver, ".", fieldName)
    f.P("}")
} else {
    f.P(`body["`, prop.Name.WireValue, `"] = `, receiver, ".", fieldName)
}

The check strings.HasPrefix(goType, "*") only matches pointer types. Optional map[K]V and []T are nilable but their Go type strings start with map[ / [], so they fall through to unconditional assignment.

Step-by-step proof using the PR's own fixture:

1. test-definitions/fern/apis/request-body-unwrap/openapi.yml declares metadata as optional<map<string, string>> at the /transfers endpoint.
2. The generator emits seed/go-sdk/request-body-unwrap/no-custom-config/transfer.go. Line 20 shows the struct field: Metadata map[string]string + "json:\"metadata,omitempty\" url:\"-\"" + `
3. Line 66 shows the marshal output: body[\"metadata\"] = c.Metadata — no nil guard. The omitempty struct tag is bypassed entirely because we are not marshaling the struct; we are marshaling a map[string]interface{} that already contains the value.
4. User code: &CreateTransferRequest{Amount: 100, SourceAccountID: \"a\", DestinationAccountID: \"b\"} — metadata unset.
5. json.Marshal of a map[string]interface{} where one value is a nil map[string]string produces \"metadata\":null.
6. Wire output: {\"metadata\":null,\"attributes\":{...}} instead of the expected {\"attributes\":{...}}.

APIs that treat null differently from absent — common with PATCH semantics or strict schemas where null means "clear the value" while absent means "leave unchanged" — will misbehave.

Issue 2: explicitFields bitmask is ignored — SetX(nil) does not emit explicit null

The non-unwrap path emits MarshalJSON like:

explicitMarshaler := internal.HandleExplicitFields(marshaler, receiver.explicitFields)
return json.Marshal(explicitMarshaler)

(sdk.go:3713-3717). HandleExplicitFields walks the bitmask to strip omitempty from explicitly-set fields, producing the documented "Explicit Null" behavior advertised in seed/go-sdk/.../README.md.

The unwrap branch returns early at sdk.go:3630-3635 and never calls HandleExplicitFields. The generated writeUnwrappedMarshalJSON uses only if receiver.FieldName != nil checks (sdk.go:4787-4793, 4855-4861).

Step-by-step proof:

1. seed/go-sdk/.../payment.go line 26 declares explicitFields *big.Int.
2. Lines 50-55: SetDescription(description *string) flips createPaymentRequestFieldDescription via c.require(...).
3. Lines 57-69 (MarshalJSON) never read c.explicitFields.
4. User code: req := &CreatePaymentRequest{...}; req.SetDescription(nil) — intent is to send \"description\": null.
5. MarshalJSON output: line 63 evaluates if c.Description != nil → false → description is omitted.
6. Wire output: {...} (no description key) instead of the expected {...,\"description\":null,...}.

The Explicit Null section of the generated SDK's README is silently broken for the exact code path this PR introduces.

Issue 3: Date-formatted fields lose their formatter

The non-unwrap path applies typed wrappers for date fields (sdk.go:3698-3708):

for _, date := range requestBody.dates {
    f.P(date.Name.Name.PascalCase.UnsafeName, ": ", date.Constructor, "(", receiver, ".", date.Name.Name.PascalCase.UnsafeName, "),")
}

where date.Constructor resolves to e.g. internal.NewDate (emits YYYY-MM-DD) or internal.NewDateTime (emits RFC3339). The requestBody.dates slice is populated by typeVisitor.visitObjectProperties against the synthetic unwrapped object (sdk.go:4002-4015).

The unwrap branch returns at sdk.go:3635 before reaching the date-wrap loop. writeUnwrappedMarshalJSON emits leaf[wire] = receiver.FieldName (sdk.go:4856-4862), putting the raw time.Time into map[string]interface{}. json.Marshal then uses Go's default time.Time.MarshalJSON which produces RFC3339Nano — wrong for Fern date primitives.

Step-by-step proof:

1. Suppose an OpenAPI spec adds birth_date: { type: string, format: date } to a leaf inside an x-fern-request-body-unwrap path.
2. primitiveToGoType (model.go) emits time.Time as the Go type.
3. requestBody.dates is still populated for the synthetic unwrapped object (typeVisitor runs unconditionally).
4. The unwrap branch ignores it. leaf[\"birth_date\"] = c.BirthDate writes the raw time.Time.
5. json.Marshal produces \"birth_date\":\"2024-01-15T00:00:00Z\" — a datetime, where the API expects \"birth_date\":\"2024-01-15\".

The PR's seed fixture has no date-typed fields, so the bug is latent in the test suite but reachable for any real API using the new feature with a date primitive.

Why None of These Were Caught

The seed fixture exercises the happy path (all required fields set, optional metadata left unset which conveniently triggers issue 1 in the seed output but is not asserted as wire output). There are no tests asserting wire output, no explicit-null tests, and no date primitives in the unwrap-test schema.

Suggested Fix

The architectural fix is to reuse the non-unwrap marshal infrastructure instead of building a parallel implementation. One approach:

1. Generate a flat struct with proper struct tags (omitempty for optional non-pointers via ,omitempty, typed wrappers for date fields).
2. Have MarshalJSON build a temporary nested struct with each leaf field assigned from the receiver, applying the same HandleExplicitFields and date-constructor wrappers as the non-unwrap path.
3. Use json.Marshal on that struct (so omitempty + custom MarshalJSON of typed wrappers work).

Concrete minimum patches for the current implementation:

• For issue 1, also guard map[/[] prefixed types: if strings.HasPrefix(goType, \"*\") || strings.HasPrefix(goType, \"map[\") || strings.HasPrefix(goType, \"[]\"). (Optional value-type named aliases still leak — full fix requires the struct-tag approach.)
• For issue 2, thread explicitFields into the body assignments: for each field, emit if c.explicitFields != nil && c.explicitFields.Bit(n) != 0 { body[wire] = field } else if field != nil { body[wire] = field } (mirroring HandleExplicitFields semantics).
• For issue 3, when emitting body[wire] = receiver.Field for a date primitive, wrap with the same internal.NewDate(...) / internal.NewDateTime(...) constructor used in the non-unwrap path (requestBody.dates is already populated by the visitor for the unwrapped object).

[claude]

🔴 Generated docstrings, JSDoc @example blocks, reference.md, and snippet.json for endpoints with x-fern-request-body-unwrap render the original nested wire shape (data=CreatePaymentRequestData(type="payment", attributes=CreatePaymentRequestDataAttributes(amount=1, ...))) instead of the flat SDK signature the generators actually emit (create_payment(*, amount, currency, description)). Customers copy-pasting from the docs page (snippet.json), an IDE hover/help() (docstring/JSDoc), or reference.md get code that raises TypeError: unexpected keyword 'data' (Python) or fails TS2353 (TypeScript) against the SDK's own interface — the canonical fixture for the feature being introduced ships with non-functional documentation. The Go variant of this same defect was added to seed/go-sdk/seed.yml allowedFailures in this PR with the comment "v2 dynamic snippets use nested struct literal but v1 generates flat struct with unwrapPath" — i.e. the PR authors knew Go was broken; the Python/TS variants have no compile-time gate and so were not caught. Fix: when unwrapPath is set, the example-emission paths must navigate into example.request.value[path[0]][path[1]]... and surface the leaf-object key/value pairs as top-level kwargs (Python) / properties (TS) / struct fields (Go), lifting top-level non-path properties (e.g. metadata) directly.

Extended reasoning...

What the bug is

All three SDK generators were updated to flatten the method signature when x-fern-request-body-unwrap is set (_get_unwrapped_properties in Python, getUnwrappedProperties in TS, getUnwrappedBodyObjectProperties in Go). But the parallel paths that emit code examples — Python docstrings, TS JSDoc @example, reference.md, and the dynamic-snippet generator that produces snippet.json — were not updated. They still consume endpoint.examples[*].request.value straight from the IR, which is shaped to the wire (nested) format. The example values are never re-keyed onto the flat parameter names that the generator just emitted.

Verified directly in this PR's seed snapshots

1. Python docstring vs. signature — seed/python-sdk/request-body-unwrap/no-custom-config/src/seed/payment/client.py. Lines 29-36 declare the function as def create_payment(self, *, amount: int, currency: str, description: typing.Optional[str] = OMIT, request_options: ...) — no data parameter. Lines 56-75 (Examples block in that same docstring) show client.payment.create_payment(data=CreatePaymentRequestData(type="payment", attributes=CreatePaymentRequestDataAttributes(amount=1, currency="currency"))). The same broken docstring is duplicated in the async variant (lines 169-178) and in transfer/client.py:65-71.

2. Python reference.md — lists **data:** CreatePaymentRequestData and **attributes:** CreateTransferRequestAttributes as the only parameters. Neither exists on the generated function.

3. TypeScript JSDoc — seed/ts-sdk/request-body-unwrap/no-custom-config/src/api/resources/payment/client/Client.ts lines 28-37 show @example await client.payment.createPayment({ data: { type: "payment", attributes: { amount: 1, currency: "currency" } } }). But the CreatePaymentRequest interface in requests/CreatePaymentRequest.ts:15-21 is { amount: number; currency: string; description?: string } — no data field. The example does not type-check against the interface declared on the same PR. The standalone @example on the interface file itself (lines 4-13) has the same broken shape.

4. snippet.json (customer-facing docs payload) — seed/python-sdk/.../snippet.json line 12 contains the broken create_payment(data=CreatePaymentRequestData(...)) call, and line 38 contains create_transfer(attributes=CreateTransferRequestAttributes(...)) where attributes is not a parameter on the generated create_transfer. seed/ts-sdk/.../snippet.json line 11 contains the broken nested literal. This is what renders on every customer-facing API docs page.

5. Go variant is already declared broken in the PR — seed/go-sdk/seed.yml line 380 adds request-body-unwrap to allowedFailures with the literal comment v2 dynamic snippets use nested struct literal but v1 generates flat struct with unwrapPath. The PR authors knew the Go snippet doesn't compile against the flat struct and explicitly silenced it. The Python/TS variants are exactly the same defect, just lacking a compile-time gate.

Step-by-step proof

Concretely, what happens when a customer copy-pastes the Python snippet from snippet.json line 12:

from seed import SeedApi
from seed.payment import (
    CreatePaymentRequestData,
    CreatePaymentRequestDataAttributes,
)

client = SeedApi(base_url="https://yourhost.com/path/to/api")
client.payment.create_payment(
    data=CreatePaymentRequestData(
        type="payment",
        attributes=CreatePaymentRequestDataAttributes(amount=1, currency="currency"),
    ),
)

Step 1 — Python looks up client.payment.create_payment. Generator emitted signature: def create_payment(self, *, amount: int, currency: str, description: typing.Optional[str] = OMIT, request_options: ...) -> Payment.

Step 2 — Caller passes data=... as the only kwarg.

Step 3 — Python raises TypeError: create_payment() got an unexpected keyword argument 'data'. Required arg amount is also missing, but data is rejected first.

Same proof in TypeScript: the JSDoc/snippet.json shows { data: { type: "payment", attributes: { amount: 1, currency: "currency" } } } typed against CreatePaymentRequest = { amount: number; currency: string; description?: string }. tsc rejects with TS2353: Object literal may only specify known properties, and 'data' does not exist in type 'CreatePaymentRequest'. The actual call site in Client.ts:62-70 correctly nests for the wire — only the documentation examples are wrong.

Why existing safeguards do not catch this

• The seed snapshot tests assert generated text matches the frozen golden file. A broken example matching the broken golden file passes. The goldens were updated to the broken output rather than the output being fixed.
• Go is compiled as part of seed CI, so the broken Go snippet was caught and explicitly silenced via allowedFailures (not fixed).
• Python: pyright/mypy is not run on dynamic-snippets per fixture. TS: the snippet examples are JSDoc comments — not type-checked by tsc. So the identical defect ships invisibly in the two languages without compile-time gates.

Impact

Every Fern Docs Registry page rendered from snippet.json shows non-compiling/non-runnable code. Every IDE hover (TS) and help() (Python) shows non-runnable code. Every reference.md entry lists parameters that don't exist. This is the canonical example for the very feature being introduced — copy-paste from the PR's own fixture produces TypeError / TS2353. The Go acknowledgement frames this as a real bug to fix, not as locked-in behavior.

Fix

When unwrapPath is set on an inlined request body, the example-rewriting step in each generator must walk into example.request.value[path[0]][path[1]]... and emit the resulting leaf-object key/value pairs as top-level kwargs (Python) / properties (TS) / struct fields (Go) on the call. Top-level non-path properties (e.g. metadata on createTransfer) lift out of example.request.value directly. The relevant paths are: Python EndpointFunctionGenerator._get_endpoint_example (and the snippet generator in packages/cli/generation/dynamic-snippets/python/), TypeScript GeneratedDefaultEndpointRequest's example writer and dynamic-snippets/typescript/, and Go's dynamic-snippet writer.

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 8 additional findings in Devin Review.

[devin-ai-integration]

🔴 Go MarshalJSON/UnmarshalJSON reference non-existent struct fields for single-value enum properties

The struct definition for unwrapped request bodies excludes single-value enum properties (via getAutoFillValue in getUnwrappedBodyObjectProperties at generators/go/internal/generator/sdk.go:4729), but writeUnwrappedMarshalJSON only checks isTypeReferenceLiteral when iterating top-level and leaf properties. A single-value enum property passes through the literal check and falls into the branch that emits receiver.FieldName — but that field doesn't exist on the struct. This generates Go code that won't compile.

The same issue exists in writeUnwrappedUnmarshalJSON at lines 4975 and 5007. Intermediate levels correctly use getAutoFillValue (line 4879), showing the intent was to handle both cases.

Affected locations in MarshalJSON and UnmarshalJSON

• MarshalJSON top-level: line 4832 checks isTypeReferenceLiteral but should also handle single-value enums
• MarshalJSON leaf: line 4897 same issue
• UnmarshalJSON top-level: line 4975 same issue
• UnmarshalJSON leaf: line 5007 same issue

The test definition includes CreatePaymentRequestDataType (a single-value enum with value "payment"), which would trigger this bug when the data.attributes unwrap path is used.

Prompt for agents

In writeUnwrappedMarshalJSON, the top-level property loop (lines 4828-4848) and the leaf property loop (lines 4896-4913) both only check isTypeReferenceLiteral to decide whether to auto-fill a value. They need to also handle single-value enums, since getUnwrappedBodyObjectProperties excludes those from the struct fields via getAutoFillValue.

The fix pattern: where the code currently does `if isTypeReferenceLiteral(prop.ValueType) { ... continue }`, it should instead check `if val := getAutoFillValue(prop.ValueType, f.types); val != "" { f.P(varName, ..., val); continue }`. This mirrors how intermediate levels already correctly use getAutoFillValue at line 4879.

The same fix is needed in writeUnwrappedUnmarshalJSON at lines 4975 and 5007 — single-value enum properties should be skipped (like literals are) since they have no corresponding struct field.

---

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[devin-ai-integration]

Good catch — fixed in 56d8b59. All four locations (MarshalJSON top-level + leaf, UnmarshalJSON top-level + leaf) now use getAutoFillValue instead of isTypeReferenceLiteral, matching the intermediate level pattern. Go seed test passes locally.

[devin-ai-integration]

Devin Review found 2 new potential issues.

View 10 additional findings in Devin Review.

[devin-ai-integration]

🔴 Python _get_unwrap_path uses camelCase attribute name instead of snake_case, making the feature non-functional

The method _get_unwrap_path uses getattr(self._request_body, "unwrapPath", None) to read the new IR field. However, Fern-generated Python SDKs use snake_case for Python attribute names (with camelCase as the JSON wire alias). Looking at how other fields are accessed in the same class — self._request_body.properties and self._request_body.extends — they use snake_case. When the Python IR types are regenerated to include this field, the Pydantic model will define it as unwrap_path: Optional[List[str]] = Field(alias="unwrapPath"). Since getattr uses the Python attribute name (not the Pydantic alias), getattr(obj, "unwrapPath", None) will always return None, causing the entire unwrap feature to silently fall back to the non-unwrapped code path in the Python generator.

Suggested change

	unwrap_path = getattr(self._request_body, "unwrapPath", None)
	if unwrap_path is not None and len(unwrap_path) > 0:
	return list(unwrap_path)
	return None
	def _get_unwrap_path(self) -> Optional[List[str]]:
	unwrap_path = getattr(self._request_body, "unwrap_path", None)
	if unwrap_path is not None and len(unwrap_path) > 0:
	return list(unwrap_path)
	return None

---

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[devin-ai-integration]

This is actually correct as-is. unwrapPath is not a defined field on the InlinedRequestBody Pydantic model — it's a new IR addition that hasn't been published to the fern IR package yet. The class uses extra="allow", so extra fields from JSON are stored under their original JSON key name (camelCase). Verified locally:

obj = MyModel.model_validate({'name': 'test', 'unwrapPath': ['data', 'attributes']})
getattr(obj, 'unwrapPath')   # → ['data', 'attributes'] ✓
getattr(obj, 'unwrap_path')  # → None ✗

Once the IR package is republished with unwrap_path as a defined field, this would need updating — but for now "unwrapPath" is correct.

[devin-ai-integration]

🟡 Go UnmarshalJSON leaf type resolution doesn't verify segment was found, may use wrong type

In writeUnwrappedUnmarshalJSON at generators/go/internal/generator/sdk.go:4947-4955, the leaf type pre-computation loop walks the type graph to find the leaf type. The inner for _, p := range objProps loop breaks when it finds a matching segment, but if no matching segment is found (e.g., due to a stale or incorrect unwrapPath), leafTypeRef retains its previous value and the outer loop continues searching the WRONG type's properties for the next path segment. This contrasts with getUnwrappedBodyObjectProperties (generators/go/internal/generator/sdk.go:4757-4767) which properly tracks found and returns early. The consequence is that the generated UnmarshalJSON could unmarshal fields from a completely wrong type, producing silent data corruption at runtime.

Suggested change

	for i := 1; i < len(unwrapPath); i++ {
	objProps := resolveObjectProperties(leafTypeRef, f.types)
	for _, p := range objProps {
	if p.Name.WireValue == unwrapPath[i] {
	leafTypeRef = p.ValueType
	break
	}
	}
	}
	for i := 1; i < len(unwrapPath); i++ {
	objProps := resolveObjectProperties(leafTypeRef, f.types)
	found := false
	for _, p := range objProps {
	if p.Name.WireValue == unwrapPath[i] {
	leafTypeRef = p.ValueType
	found = true
	break
	}
	}
	if !found {
	break
	}
	}

---

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[devin-ai-integration]

Good catch — fixed in the next commit. Added found tracking with early break to match getUnwrappedBodyObjectProperties.

[claude]

🔴 TS exposes top-level and leaf-level single-value enums as required user params, diverging from Go/Python which auto-fill them. With a spec like attributes.kind: enum [transfer] (leaf) or a top-level fixed-value sibling next to the unwrap root, Python emits client.payments.create_payment(amount=100) and Go emits &CreatePaymentRequest{Amount: 100}, but TS requires the user to pass { kind: 'transfer', amount: 100 } on every call. Fix by replacing the isLiteral filter at GeneratedRequestWrapperImpl.ts:935-940 (top) and :986-998 (leaf) with a check that also matches single-value enums, and by calling getAutoFillExpression at the top + leaf paths in GeneratedDefaultEndpointRequest.ts (the intermediate level at line 466 already uses this pattern correctly).

Extended reasoning...

What the bug is

The TypeScript unwrap path treats single-value enums inconsistently across levels and inconsistently with the Go and Python generators. The Go counterpart was fixed in commit 56d8b59 of this PR; the TS counterpart still has the original defect at both the top-level and leaf-level of the unwrap path.

Code paths in TS

Request-wrapper interface generation (generators/typescript/sdk/request-wrapper-generator/src/GeneratedRequestWrapperImpl.ts):

• Top-level loop at lines 935-940:

  const resolvedType = context.type.resolveTypeReference(prop.valueType);
  const isLiteral = resolvedType.type === "container" && resolvedType.container.type === "literal";
  if (!isLiteral) {
      const requestProperty = this.getInlineProperty(inlinedRequestBody, prop, context);
      properties.push(requestProperty);
  }

• Leaf-level loop at lines 986-998: same isLiteral filter. Single-value Named enums pass through and become required user-facing parameters.
• Intermediate-level loop at 961-981 correctly skips them entirely (they are part of the nesting structure).

Serialization (generators/typescript/sdk/client-class-generator/src/endpoint-request/GeneratedDefaultEndpointRequest.ts):

• Top-level (lines 489-510) and leaf-level (lines 429-447) only check container.literal, falling through to referenceToRequestBody.sdkName for single-value enums (i.e., reads the user-supplied value).
• Intermediate-level (lines 462-475) correctly calls this.getAutoFillExpression, which at lines 534-555 matches both literals and single-value enums.

Why Go and Python don't have this defect

• Go sdk.go: getAutoFillValue (which covers both literals and single-value enums via getSingleValueEnumWireValue) is called uniformly at top, intermediate, and leaf in getUnwrappedBodyObjectProperties, writeUnwrappedMarshalJSON, and writeUnwrappedUnmarshalJSON after the fix in commit 56d8b59.
• Python inlined_request_body_parameters.py: _is_auto_fill_property at lines 101-104 checks both literals and single-value enums, and is applied uniformly at line 151 (top-level) and line 172 (leaf-level) of _get_unwrapped_properties, and via write_auto_fill_prop in _build_unwrapped_json_body.

Why the seed fixture passes

The PR's only single-value enum in the unwrap fixture (CreatePaymentRequestDataType with value payment) sits at the intermediate level — it's a property of data, where the unwrap path is data.attributes. The intermediate-level handler in TS covers this case correctly, so seed snapshots produce identical output across the three generators. The asymmetry only surfaces when a single-value enum sits at the top level or inside the leaf object.

Step-by-step proof

Take a JSON:API-style spec where the discriminator sits inside the leaf object:

paths:
  /payments:
    post:
      x-fern-request-body-unwrap: data.attributes
      requestBody:
        content:
          application/json:
            schema:
              required: [data]
              properties:
                data:
                  type: object
                  required: [attributes]
                  properties:
                    attributes:
                      type: object
                      required: [kind, amount]
                      properties:
                        kind:
                          type: string
                          enum: [transfer]   # single-value enum at LEAF
                        amount:
                          type: integer

1. Python: _get_unwrapped_properties calls _is_auto_fill_property at line 172, which returns True for the single-value kind enum, so kind is excluded from the flat parameter list. The serialization (_build_unwrapped_json_body) calls write_auto_fill_prop, which emits "kind": "transfer" inline. Generated SDK call: client.payments.create_payment(amount=100).
2. Go: getUnwrappedBodyObjectProperties calls getAutoFillValue at sdk.go:4763, which returns "transfer" for single-value enums, so kind is excluded from the struct. writeUnwrappedMarshalJSON emits leaf["kind"] = "transfer" inline. Generated SDK call: &CreatePaymentRequest{Amount: 100}.
3. TypeScript: getUnwrappedProperties reaches the leaf-level loop at lines 986-998. resolvedType.type === "container" && resolvedType.container.type === "literal" is false for the single-value Named enum (it's a named reference, not container.literal), so kind falls through and is pushed as a required Property on CreatePaymentRequest. The serialization at GeneratedDefaultEndpointRequest.ts:429-447 also only checks container.literal, falling through to referenceToRequestBody.sdkName. Generated SDK call: client.payments.createPayment({ kind: 'transfer', amount: 100 }).

Same IR → three different SDK surfaces. The TS user must hand-type kind: 'transfer' on every call, despite the value being fixed in the schema.

Why existing safeguards don't catch it

• Seed snapshot tests compare against frozen golden files. Since the seed's only single-value enum sits at the intermediate level (which TS handles correctly), the goldens are correct and pass.
• There is no cross-generator equivalence test for the unwrap output.
• TS compile (tsc) doesn't notice this — the generated interface and call site are internally consistent; the defect is in what is exposed to the user, not in whether the code compiles.

How to fix

Mirror the intermediate-level handling at the top and leaf levels:

1. In GeneratedRequestWrapperImpl.ts getUnwrappedProperties (lines 935-940 top, 986-998 leaf): replace the isLiteral filter with a check that also matches single-value enums — e.g., this.getAutoFillExpression(prop, context) != null, mirroring the existing getAutoFillExpression helper that already lives at GeneratedDefaultEndpointRequest.ts:534-555.
2. In GeneratedDefaultEndpointRequest.ts buildUnwrappedRequestBodyExpression: at the top-level (around line 494) and leaf-level (around line 431), use getAutoFillExpression to emit the literal/enum value inline (same pattern as the intermediate level at line 466).

This is the same mechanical fix that commit 56d8b59 applied to Go.

[claude]

🔴 🔴 The new TS unwrap helpers walk the type chain using only shape.properties and never consult shape.extendedProperties or walk shape.extends recursively. As a result, for any spec where a type on the unwrap path uses Fern extends: or OpenAPI allOf, the TypeScript SDK silently drops inherited fields from BOTH the CreatePaymentRequest interface AND the wire payload, while Go (fixed in this PR) and Python (uses get_all_properties_including_extensions) correctly include them. Fix: walk shape.extends recursively (or use the precomputed shape.extendedProperties) at GeneratedRequestWrapperImpl.ts:959, 980, 985 and GeneratedDefaultEndpointRequest.ts:403, 410, 423, 427 — the same file already uses getAllPropertiesIncludingExtensions for the non-unwrap path at GeneratedRequestWrapperImpl.ts:663, so the pattern is available.

Extended reasoning...

What the bug is

The new TypeScript unwrap helpers introduced in this PR walk the request body type graph using only shape.properties, never consulting shape.extendedProperties (which the IR generator precomputes for this exact purpose) and never recursively walking shape.extends. The defect appears at every level of the unwrap path — top-level, intermediate, and leaf — and affects both the generated SDK interface and the wire serialization.

Code paths verified directly:

• GeneratedRequestWrapperImpl.ts:959 — currentProps = firstTypeDecl.shape.properties (first path level)
• GeneratedRequestWrapperImpl.ts:980 — currentProps = typeDecl.shape.properties (subsequent path levels)
• GeneratedRequestWrapperImpl.ts:984-999 — leaf level iterates currentProps (no extension walk)
• GeneratedDefaultEndpointRequest.ts:403, 410, 423, 427 — same shape.properties-only pattern in the serialization expression builder

Cross-generator divergence on the same IR

• Python inlined_request_body_parameters.py:_resolve_object_properties delegates to get_all_properties_including_extensions, which traverses the chain — inherited fields are included.
• Go sdk.go:resolveObjectProperties was fixed in this PR (commit visible in the diff at sdk.go:4634-4655) to walk Extends recursively, and inlinedRequestBodyToUnwrappedObjectTypeDeclaration now sets Extends: inlinedRequestBody.Extends mirroring sdk.go:4101.
• TypeScript was not updated to match. Same IR → different SDKs.

The same file (GeneratedRequestWrapperImpl.ts:663) already uses getAllPropertiesIncludingExtensions for the non-unwrap path's required-check, so the pattern is available — the unwrap helpers were simply written without it.

Step-by-step proof

Take this Fern Definition (or the equivalent OpenAPI allOf):

types:
  BaseAttributes:
    properties:
      reference: string

service:
  endpoints:
    createPayment:
      method: POST
      path: /payments
      request:
        name: CreatePaymentRequest
        body:
          properties:
            data:
              type: object
              properties:
                attributes:
                  extends: BaseAttributes
                  properties:
                    amount: integer
      request-body-unwrap: data.attributes

1. convertHttpRequestBody.ts populates InlinedRequestBody.unwrapPath = ["data", "attributes"]. The leaf type for attributes has properties = [amount] and extends = [BaseAttributes].
2. getUnwrappedProperties at GeneratedRequestWrapperImpl.ts:929+ walks down via typeDecl.shape.properties only. At the attributes leaf, currentProps = [amount] — reference (inherited from BaseAttributes) is dropped.
3. The generated CreatePaymentRequest interface is { amount: number } — no reference field.
4. buildUnwrappedRequestBodyExpression at GeneratedDefaultEndpointRequest.ts:367+ likewise uses currentTypeDecl.shape.properties (line 403, 423, 427). The wire payload is { data: { attributes: { amount } } } — reference is silently omitted.
5. Python emits client.payment.create_payment(amount=..., reference=...) and serializes {"data":{"attributes":{"amount":...,"reference":"..."}}}.
6. Go (post-fix) emits a CreatePaymentRequest struct including Reference and serializes the inherited field correctly.

Reachability

• extends: on inlined request bodies and on referenced objects is a first-class Fern Definition feature; the IR (addExtendedPropertiesToIr.ts) flatMaps shape.extends and stores the result in shape.extendedProperties.
• OpenAPI allOf materializes to extends in the IR via RequestBodyConverter.ts:178 (extends: requestBodyTypeShape.extends).
• The PR's seed fixture (test-definitions/fern/apis/request-body-unwrap/openapi.yml) has no allOf/extends on the unwrap path, which is why seed snapshots pass while a real-world spec using either feature silently emits a broken TS SDK.

Fix

Mirror the Go fix and the existing in-file pattern. Either:

• Read shape.extendedProperties (precomputed by the IR generator) in addition to shape.properties, or
• Walk shape.extends recursively, the way resolveObjectProperties does in sdk.go:4634-4655 after this PR.

Apply at all four sites in GeneratedRequestWrapperImpl.ts and GeneratedDefaultEndpointRequest.ts listed above.

[claude]

🔴 / silently drop the extra-properties feature (sdk.go:3630-3635). When an inlined request body combines x-fern-request-body-unwrap with extra-properties: true, the early return after the unwrap helpers bypasses both the writeExtractExtraProperties call at sdk.go:3677-3680 (UnmarshalJSON) and the internal.MarshalJSONWithExtraProperties(..., receiver.ExtraProperties) call at sdk.go:3713-3715 (MarshalJSON). The unwrap helpers themselves never reference receiver.ExtraProperties, never call internal.MarshalJSONWithExtraProperties, and never call internal.ExtractExtraProperties, yet the struct still has the ExtraProperties field (propagated by inlinedRequestBodyToUnwrappedObjectTypeDeclaration at sdk.go:4789-4793). Net effect: arbitrary fields the user sets on req.ExtraProperties are silently dropped from the wire on marshal, and unrecognized fields in incoming payloads are silently lost on unmarshal. Fix: extend writeUnwrappedMarshalJSON to invoke internal.MarshalJSONWithExtraProperties with the outer body map plus receiver.ExtraProperties, and extend writeUnwrappedUnmarshalJSON to call ExtractExtraProperties on the top-level raw map (excluding the known wire names) into receiver.ExtraProperties. This is a fourth observable failure of the same root cause flagged in the earlier marshal-infrastructure comment (omitempty / explicitFields / date wrappers).

Extended reasoning...

Bug Summary

The new unwrap codegen short-circuits WriteRequestType after emitting writeUnwrappedMarshalJSON/writeUnwrappedUnmarshalJSON (sdk.go:3630-3635), bypassing the existing infrastructure that wires the extra-properties: true feature into the non-unwrap path. The struct still carries an ExtraProperties map[string]interface{} field — but no generated code ever reads or writes it on the unwrap path.

Code paths verified directly

Non-unwrap path wires extra-properties correctly:

• UnmarshalJSON (sdk.go:3677-3680):

  if requestBody.extraProperties {
      writeExtractExtraProperties(f, literals, receiver, ...)
  }

• MarshalJSON (sdk.go:3713-3715):

  if requestBody.extraProperties {
      return internal.MarshalJSONWithExtraProperties(explicitMarshaler, receiver.ExtraProperties)
  }

Unwrap path bypasses both blocks:

if requestBody.unwrapPath != nil {
    writeUnwrappedMarshalJSON(...)
    writeUnwrappedUnmarshalJSON(...)
    return nil   // <-- never reaches the extra-properties branches above
}

Unwrap helpers do not reference ExtraProperties:

• writeUnwrappedMarshalJSON (sdk.go:~4812-4920) builds a map[string]interface{} from known properties and returns json.Marshal(body). It never calls internal.MarshalJSONWithExtraProperties and never reads receiver.ExtraProperties.
• writeUnwrappedUnmarshalJSON (sdk.go:~4926+) parses the raw map and known segments. It never calls internal.ExtractExtraProperties and never writes receiver.ExtraProperties.

The struct still has the field. inlinedRequestBodyToUnwrappedObjectTypeDeclaration (sdk.go:4789-4793) propagates ExtraProperties: inlinedRequestBody.ExtraProperties to the synthetic ObjectTypeDeclaration, so typeVisitor.visitObjectProperties emits the ExtraProperties map[string]interface{} field on the flat struct — but it is orphaned, with no marshal/unmarshal code referencing it.

Step-by-step proof

Consider this Fern Definition (or the equivalent OpenAPI with additionalProperties: true):

service:
  endpoints:
    createPayment:
      method: POST
      path: /payments
      request:
        name: CreatePaymentRequest
        body:
          extra-properties: true
          properties:
            data:
              type: object
              properties:
                attributes:
                  type: object
                  properties:
                    amount: integer
      request-body-unwrap: data.attributes

1. The IR populates InlinedRequestBody.unwrapPath = ["data", "attributes"] AND InlinedRequestBody.ExtraProperties = true.
2. WriteRequestType reaches sdk.go:3630-3635, emits writeUnwrappedMarshalJSON/writeUnwrappedUnmarshalJSON, and returns. The sdk.go:3677-3680 and sdk.go:3713-3715 branches never execute.
3. The generated struct has Amount int plus ExtraProperties map[string]interface{} (from the synthetic type declaration).
4. User code:

   req := &CreatePaymentRequest{Amount: 100, ExtraProperties: map[string]interface{}{"trace_id": "abc-123"}}
   resp, _ := client.Payment.CreatePayment(ctx, req)

5. MarshalJSON output: {"data":{"attributes":{"amount":100}}} — trace_id is silently absent. The user has no way to add arbitrary forward-compatible fields, which is the entire purpose of extra-properties.
6. Symmetric on UnmarshalJSON: a server response {"data":{"attributes":{"amount":100}},"server_trace_id":"xyz"} leaves req.ExtraProperties nil — the unrecognized field is dropped on deserialization too.

In the non-unwrap path the same spec correctly produces {"data":{...},"amount":100,"trace_id":"abc-123"} because internal.MarshalJSONWithExtraProperties splices receiver.ExtraProperties into the JSON body.

Why the seed fixture passes

The PR fixture (test-definitions/fern/apis/request-body-unwrap/openapi.yml) does not combine additionalProperties: true with x-fern-request-body-unwrap. Seed snapshots therefore exercise only the non-extra-properties unwrap path and the non-unwrap extra-properties path independently. The cross-feature combination is latent.

Impact

Forward-compatible API design (sending and receiving unknown fields) is the canonical use case for extra-properties. Customers wanting that ergonomic plus the unwrap ergonomic get silent data loss in both directions. This is a feature-combination correctness issue distinct from the three issues already flagged in #16277 (comment) — same root cause (unwrap branch reimplementing marshal instead of composing with existing infrastructure), fourth concrete failure mode.

Suggested fix

Mirror the non-unwrap pattern. At the end of writeUnwrappedMarshalJSON, gate the return on inlinedRequestBody.ExtraProperties:

if inlinedRequestBody.ExtraProperties {
    f.P("return internal.MarshalJSONWithExtraProperties(body, ", receiver, ".ExtraProperties)")
} else {
    f.P("return json.Marshal(body)")
}

This places extras at the outermost level of the wire object (matching the semantic intent of extra-properties: arbitrary unknown fields on the top-level body, not inside the unwrap path).

At the end of writeUnwrappedUnmarshalJSON, when inlinedRequestBody.ExtraProperties is true, emit a call to internal.ExtractExtraProperties over the top-level raw map, excluding the known top-level wire names (the unwrap path root plus any sibling top-level non-path properties) and write the result into receiver.ExtraProperties.

The architectural alternative is the one already suggested in the earlier comment: stop reimplementing marshal in the unwrap path and instead reuse HandleExplicitFields / MarshalJSONWithExtraProperties / date wrappers from the non-unwrap path, with a small wrapper that builds the nested body structure. That fixes all four symptoms at once.

_{🔬 also observed by claude-comment-3364652015}

[claude]

🔴 Top-level and leaf-level properties with the same wire name (e.g. id at both the request-body root and inside the leaf object on a data.attributes unwrap path) are concatenated into a single property list with no dedup, producing duplicate Go struct fields, duplicate Python kwargs (SyntaxError), and duplicate TypeScript interface members (TS2300) — i.e. uncompilable SDK output across all three generators. The collision sites are getUnwrappedBodyObjectProperties (sdk.go:4716-4778), _get_unwrapped_properties (inlined_request_body_parameters.py:145-181), and getUnwrappedProperties (GeneratedRequestWrapperImpl.ts:923-1003). Fix: surface a validation error in the importer (preferred, symmetric with other unwrap-path misconfiguration handling) or deterministically rename one side.

Extended reasoning...

What the bug is

When the unwrap path is set, each generator builds a flat list of properties for the SDK signature by concatenating two sources: (a) top-level non-path properties of the inlined request body, and (b) the leaf object's properties at the end of the unwrap path. None of the three generators deduplicate against wire name when joining these lists. If both sources contain a property with the same wire name, the resulting flat list has two entries with the same name, and the downstream codegen unconditionally emits one declaration per entry.

Code paths

Go generators/go/internal/generator/sdk.go getUnwrappedBodyObjectProperties:

• lines 4725-4737 iterate inlinedRequestBody.Properties and append each non-path, non-auto-fill property to result
• lines 4771-4777 iterate the leaf object's properties (via resolveObjectProperties) and append each non-auto-fill property to the same result slice — no dedup against the wire names already in result
• inlinedRequestBodyToUnwrappedObjectTypeDeclaration (sdk.go:4783) returns this as a synthetic ObjectTypeDeclaration, which visitObjectProperties (model.go) iterates one-to-one to emit goExportedFieldName(prop.Name.PascalCase.UnsafeName) fields on the generated struct.

Python generators/python/.../inlined_request_body_parameters.py _get_unwrapped_properties:

• lines 150-152 append top-level non-path, non-auto-fill props to result
• lines 169-179 append leaf-level non-auto-fill props to the same result list with no dedup
• get_parameters then creates one NamedFunctionParameter per entry via _get_property_name → snake_case.safe_name, producing two parameters with the same Python name in the function signature

TypeScript generators/typescript/.../GeneratedRequestWrapperImpl.ts getUnwrappedProperties:

• lines 931-941 push top-level non-path, non-literal properties to properties
• lines 984-999 push leaf-level non-literal properties to the same array with no dedup
• the resulting Property[] becomes the interface members of CreatePaymentRequest — duplicate members trigger TS2300 Duplicate identifier

Step-by-step proof

Consider a JSON:API-style spec where a top-level idempotency key shares a name with a business-object id at the leaf:

post:
  x-fern-request-body-unwrap: data.attributes
  requestBody:
    content:
      application/json:
        schema:
          required: [id, data]
          properties:
            id: { type: string }              # client-generated idempotency key
            data:
              type: object
              required: [attributes]
              properties:
                attributes:
                  type: object
                  required: [id, amount]
                  properties:
                    id: { type: string }      # business entity id
                    amount: { type: integer }

1. getUnwrappedBodyObjectProperties walks top-level properties. id is non-path and not auto-fillable (it is a string primitive, not literal/single-value-enum), so it is appended to result. data matches the path root and is skipped.
2. The function then walks into data.attributes and collects leaf properties. id (the business id) is also a non-auto-fillable string, so it is appended to result. result now contains two entries with Name.WireValue == "id".
3. inlinedRequestBodyToUnwrappedObjectTypeDeclaration wraps this in an ObjectTypeDeclaration with two id properties. visitObjectProperties emits the struct literal:

   type CreatePaymentRequest struct {
       Id string `json:"id"`
       Id string `json:"id"`     // duplicate field name — Go compile error
       Amount int    `json:"amount"`
   }

   go build ./... fails with duplicate field name 'Id' in struct literal.
4. The Python path produces def create_payment(self, *, id: str, id: str, amount: int, ...) -> Payment: — Python's parser rejects this with SyntaxError: duplicate argument 'id' in function definition at module import.
5. The TS path produces interface CreatePaymentRequest { id: string; id: string; amount: number } — tsc rejects with TS2300: Duplicate identifier 'id'.

Why existing safeguards do not catch it

The PR's seed fixture (test-definitions/fern/apis/request-body-unwrap/openapi.yml) declares top-level properties [data, metadata] and leaf properties [amount, currency, description, source_account_id, destination_account_id] — no overlapping wire names. Seed compile and snapshot tests pass because the collision condition is never exercised. There is no validation step that rejects the unwrap path when it would induce a collision, and the helpers concatenate the two lists without inspection.

Impact

Any real-world spec where a property name happens to repeat at the top level and inside the leaf object on the unwrap path produces an SDK that fails to compile in Go, fails to parse in Python, and fails type-checking in TypeScript. The JSON:API discriminator pattern (idempotency key at top, entity id at leaf), pagination cursors, and shared metadata fields are realistic triggers. The PR's own importer wires the extension fully, so any user encountering this pattern hits the failure on first fern generate.

Fix

Two viable approaches, matching the precedent set by other unwrap-path misconfiguration handling:

1. Validate in the importer — detect collisions in x-fern-request-body-unwrap.ts / buildEndpoint.ts and surface a clear error to the user (preferred; symmetric with how other unwrap-path misconfigurations are reported).
2. Deterministically rename one side — e.g., prefix top-level names with the request-body name on collision. More complex, no precedent in the existing codebase.

A minimal generator-side fix would be to detect collisions in getUnwrappedBodyObjectProperties / _get_unwrapped_properties / getUnwrappedProperties and error out instead of silently emitting duplicates. The importer-side validation is preferable because the diagnostic is more actionable and the same error surfaces uniformly across all three generators.

[claude]

🟡 The legacy OpenAPI importer crashes with an opaque TypeError: rawUnwrapPath.split is not a function when a user authors x-fern-request-body-unwrap as a YAML list (e.g. ["data", "attributes"]) instead of a dot-separated string. getExtension<string> is just a TypeScript-only cast — it returns whatever YAML/JSON value the user wrote, and the call to .split(".") at convertHttpOperation.ts:326-328 has no runtime guard. The 3.1 importer at x-fern-request-body-unwrap.ts:32-37 already validates typeof extensionValue !== "string" and routes to errorCollector — mirror that check here so the legacy and 3.1 paths behave consistently on malformed input.

Extended reasoning...

What the bug is

The legacy OpenAPI importer reads the new x-fern-request-body-unwrap extension and immediately calls .split(".") on the result with no runtime type validation:

// packages/cli/api-importers/openapi/openapi-ir-parser/src/openapi/v3/converters/operation/convertHttpOperation.ts:326-328
const rawUnwrapPath = getExtension<string>(operation, FernOpenAPIExtension.REQUEST_BODY_UNWRAP);
const requestBodyUnwrapPath =
    rawUnwrapPath != null ? rawUnwrapPath.split(".").filter((segment) => segment.length > 0) : undefined;

getExtension<T> (packages/cli/api-importers/openapi/openapi-ir-parser/src/getExtension.ts:25-35) does not validate the runtime type — the <string> parameter is purely a TypeScript cast over (object as any)[extension]. Whatever value the user authored in YAML/JSON flows through verbatim.

Why this matters

The 3.1 importer counterpart (packages/cli/api-importers/openapi-to-ir/src/extensions/x-fern-request-body-unwrap.ts:32-37) — added in the same PR — correctly validates the value:

if (typeof extensionValue !== "string") {
    this.context.errorCollector.collect({
        message: 'Expected a dot-separated string for x-fern-request-body-unwrap (e.g. "data.attributes")',
        path: this.breadcrumbs
    });
    return undefined;
}

So the same misconfigured spec produces a clean, actionable diagnostic in the 3.1 path and an opaque TypeError stack trace in the legacy path. The legacy pipeline is the path OpenAPI 3.0 specs (and 3.1 specs not routed to the new converter) go through, and this PR fully wires the legacy pipeline through buildEndpoint.ts:131-133, so the legacy code is live.

Step-by-step proof

Given an OpenAPI spec where a user — reasonably — authors the extension as a YAML list mirroring the IR's parsed-segment shape:

paths:
  /payments:
    post:
      x-fern-request-body-unwrap:
        - data
        - attributes
      requestBody: ...

1. getExtension<string>(operation, FernOpenAPIExtension.REQUEST_BODY_UNWRAP) returns the raw JS array ["data", "attributes"] (not null, not a string).
2. rawUnwrapPath != null evaluates to true.
3. rawUnwrapPath.split(".") throws TypeError: rawUnwrapPath.split is not a function because arrays have no split method.
4. The exception aborts convertHttpOperation and — because there is no enclosing try/catch around this property — aborts the entire OpenAPI import for the workspace.

The user sees a stack trace pointing into Fern internals with no indication of which extension was misconfigured. The 3.1 importer, by contrast, would surface Expected a dot-separated string for x-fern-request-body-unwrap (e.g. "data.attributes") with the operation's breadcrumb path.

How to fix

Mirror the 3.1 importer pattern. Before calling .split(...), validate the runtime type:

const rawUnwrapPath = getExtension<unknown>(operation, FernOpenAPIExtension.REQUEST_BODY_UNWRAP);
let requestBodyUnwrapPath: string[] | undefined;
if (rawUnwrapPath != null) {
    if (typeof rawUnwrapPath !== "string") {
        context.logger.error(
            `Expected a dot-separated string for ${FernOpenAPIExtension.REQUEST_BODY_UNWRAP} (e.g. "data.attributes")`
        );
    } else {
        requestBodyUnwrapPath = rawUnwrapPath.split(".").filter((segment) => segment.length > 0);
    }
}

(The exact reporting mechanism should match how this importer surfaces other malformed-extension diagnostics — context.logger or the relevant taskContext — rather than errorCollector which is specific to the 3.1 importer's plumbing.)

Severity

Marking this as a nit. The failure mode is a hard crash at import time rather than silent data corruption or wrong SDK output, and the trigger requires a user authoring error. But the fix is mechanical (one typeof guard) and brings the legacy importer to parity with the 3.1 path that this same PR introduces — the asymmetry is a meaningful DX gap.

[claude]

🟡 🟡 writeUnwrappedMarshalJSON intermediate-level navigation missing found-check. At sdk.go:4881-4887, the loop that walks one unwrapPath segment further sets currentTypeRef = p.ValueType on match and breaks, but never tracks whether the segment was found. If a path segment doesn't exist on objProps (user typo in x-fern-request-body-unwrap), currentTypeRef silently retains its previous value and the loop continues against the wrong type. This is asymmetric with the two peer sites in the same file: getUnwrappedBodyObjectProperties (sdk.go:4757-4767) returns early, and writeUnwrappedUnmarshalJSON (sdk.go:4946-4961, just fixed in 56d8b59) breaks early. Fix is mechanical — mirror the found := false / if !found { break } pattern from the other two sites.

Extended reasoning...

What the bug is

The intermediate-level navigation loop in writeUnwrappedMarshalJSON at sdk.go:4881-4887 reads:

// Navigate to next level
for _, p := range objProps {
    if p.Name.WireValue == segment {
        currentTypeRef = p.ValueType
        break
    }
}

There is no found flag and no early exit from the outer for i := 1; i < len(unwrapPath); i++ loop. If segment is absent from objProps, currentTypeRef keeps pointing at the previous level's type and the outer loop continues to the next segment against the wrong type.

Asymmetry with the two peer sites

The same file has two other navigation loops over the unwrap path, both of which guard against a missing segment:

• getUnwrappedBodyObjectProperties at sdk.go:4757-4767:

  found := false
  for _, p := range objProps {
      if p.Name.WireValue == segment {
          currentTypeRef = p.ValueType
          found = true
          break
      }
  }
  if !found {
      return result
  }

• writeUnwrappedUnmarshalJSON at sdk.go:4946-4961 (added by commit 56d8b59 in this PR):

  found := false
  for _, p := range objProps {
      if p.Name.WireValue == unwrapPath[i] {
          leafTypeRef = p.ValueType
          found = true
          break
      }
  }
  if !found {
      break
  }

The MarshalJSON twin of that fix is missing.

Step-by-step proof

1. User authors x-fern-request-body-unwrap: "data.wrong.attributes" against a schema where data has {attributes: object{amount, currency}} (typo: wrong doesn't exist on data).
2. The importer accepts the value as a non-empty string — the fern-definition validator visits request-body-unwrap with noop (visitHttpService.ts:314) and FernRequestBodyUnwrapExtension only checks typeof extensionValue === "string" and that the split has at least one segment. Path-vs-schema reachability is never checked.
3. getUnwrappedBodyObjectProperties early-returns at i=1 with only top-level non-path properties. The synthetic ObjectTypeDeclaration has no Amount/Currency fields, so the generated CreatePaymentRequest struct has no leaf fields.
4. writeUnwrappedMarshalJSON walks past the bad segment at i=1 (loop at 4881-4887 doesn't exit). At i=2, attributes does match on data's actual type, so currentTypeRef becomes attributes' type. The leaf walk at 4892 then resolves to {amount, currency} and emits leaf["amount"] = c.Amount / leaf["currency"] = c.Currency for fields that don't exist on the struct.
5. go build ./... fails with undefined: c.Amount, undefined: c.Currency.

Why existing safeguards don't catch it

• The fern-definition validator is a noop for request-body-unwrap.
• The OpenAPI FernRequestBodyUnwrapExtension only validates the path is a non-empty string after splitting on ..
• The seed fixture's unwrap paths (data.attributes, attributes) are all valid, so seed CI doesn't exercise the typo case.
• The recently-fixed UnmarshalJSON path (56d8b59) and the original getUnwrappedBodyObjectProperties both early-exit, masking the asymmetry until the user actually misconfigures.

How to fix

Mirror the existing pattern from sdk.go:4946-4961 at sdk.go:4881-4887:

found := false
for _, p := range objProps {
    if p.Name.WireValue == segment {
        currentTypeRef = p.ValueType
        found = true
        break
    }
}
if !found {
    break
}

This is the same mechanical change applied to UnmarshalJSON in 56d8b59, and aligns the three navigation sites in this file.

Severity

Nit-to-normal. Real defect — the trigger is a user typo, but the failure is a non-compiling Go SDK at the first fern generate, not silent data corruption. The fix is one-line mechanical and matches an in-file precedent that the PR author just established.