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Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

When @fastify/compress is registered on a NestFastifyApplication, Swagger UI renders blank because GET /docs-json, GET /docs-yaml, and GET /docs/swagger-ui-init.js all return HTTP 200 with an empty body as soon as the client sends an Accept-Encoding: br|gzip header (which every modern browser does by default).

In 11.3.0, the JSON, YAML and swagger-ui-init.js route handlers in lib/swagger-module.ts were converted from sync to async to support an async patchDocumentOnRequest hook, but the bare res.send(...) calls were left without a return. The async function therefore resolves to undefined. When @fastify/compress attaches its onSend hook to compress the response, it serializes undefined and emits an empty compressed body — even though reply.send() was called manually inside the handler.

The HTML page and static assets (served through @fastify/static) are unaffected, which is why the page loads but the UI never initializes.

Plain curl requests without Accept-Encoding look fine, which made the regression invisible to the existing test suite.

Closes #3908

What is the new behavior?

The 4 affected async handlers now return res.send(...), mirroring the pattern already used in the patchDocumentOnRequest branches. The async function resolves to the reply, the onSend hook receives the actual payload, and compression works correctly.

Affected handlers (line numbers from this PR):

• swagger-module.ts:212 — swagger-ui-init.js (primary route)
• swagger-module.ts:244 — swagger-ui-init.js (trailing-slash route)
• swagger-module.ts:326 — /docs-json
• swagger-module.ts:353 — /docs-yaml

The sync HTML handler (swaggerUiHtml, line 276) was left unchanged because it is not affected.

Tests

Added a new describe('with @fastify/compress registered', ...) block in e2e/fastify.e2e-spec.ts covering Accept-Encoding: br and gzip against /apidoc-json, /apidoc-yaml, and /apidoc/swagger-ui-init.js. Verified that all 4 new tests fail on master (each asserts body length > 0 — currently 0) and pass with this fix.

Added @fastify/compress as a devDependency (already a peer of @nestjs/platform-fastify consumers).

Full test suites green: 122 e2e passed, 368 unit passed, tsc -p tsconfig.build.json clean.

Does this PR introduce a breaking change?

• Yes
• No

The change preserves the existing behavior for callers not using @fastify/compress (since reply.send() already terminates the response) and is identical to what the patchDocumentOnRequest branches do today.

Other information

The fix was validated end-to-end in a real project (NestJS 11.1.20 + @nestjs/platform-fastify 11.1.20 + @fastify/compress 8.3.1 + fastify 5.8.5) by patching node_modules locally and confirming that:

• curl -s -H 'Accept-Encoding: br' /api/docs-json | wc -c returns ~12 KB of brotli payload (was 0 before)
• the decompressed body matches the full 198 KB OpenAPI document
• Swagger UI renders normally in Safari, Chrome and Firefox