From 2037023d8eed3024a61751e324e96a02a5679501 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Fri, 3 Oct 2025 17:15:17 +0100
Subject: [PATCH 1/7] feat: support custom log function

---
 packages/build/src/error/monitor/start.ts     |   4 +-
 packages/build/src/log/logger.ts              |  46 ++++++++++++------
 packages/build/src/log/messages/config.js     |   2 +-
 packages/build/src/log/messages/core.ts       |   4 +-
 .../build/tests/log/snapshots/tests.js.md     |  32 +++++++++++-
 .../build/tests/log/snapshots/tests.js.snap   | Bin 991 -> 1096 bytes
 packages/build/tests/log/tests.js             |  10 ++++
 7 files changed, 76 insertions(+), 22 deletions(-)

diff --git a/packages/build/src/error/monitor/start.ts b/packages/build/src/error/monitor/start.ts
index 50094d3e13..d07722b224 100644
--- a/packages/build/src/error/monitor/start.ts
+++ b/packages/build/src/error/monitor/start.ts
@@ -4,13 +4,13 @@ import Bugsnag from '@bugsnag/js'
 import memoizeOne from 'memoize-one'
 
 import type { ResolvedFlags } from '../../core/normalize_flags.js'
-import { BufferedLogs, log } from '../../log/logger.js'
+import { Logs, log } from '../../log/logger.js'
 import { ROOT_PACKAGE_JSON } from '../../utils/json.js'
 
 const projectRoot = fileURLToPath(new URL('../../..', import.meta.url))
 
 // Start a client to monitor errors
-export const startErrorMonitor = function (config: { flags: ResolvedFlags; logs?: BufferedLogs; bugsnagKey?: string }) {
+export const startErrorMonitor = function (config: { flags: ResolvedFlags; logs?: Logs; bugsnagKey?: string }) {
   const {
     flags: { mode },
     logs,
diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index 4e82b61497..cfc3a26dca 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -12,7 +12,7 @@ import { THEME } from './theme.js'
 
 export type Logs = BufferedLogs | StreamedLogs
 export type BufferedLogs = { stdout: string[]; stderr: string[]; outputFlusher?: OutputFlusher }
-export type StreamedLogs = { outputFlusher?: OutputFlusher }
+export type StreamedLogs = { outputFlusher?: OutputFlusher; logFunction?: (message: string) => void }
 
 export const logsAreBuffered = (logs: Logs | undefined): logs is BufferedLogs => {
   return logs !== undefined && 'stdout' in logs
@@ -31,8 +31,16 @@ const EMPTY_LINE = '\u{200B}'
  * When the `buffer` option is true, we return logs instead of printing them
  * on the console. The logs are accumulated in a `logs` array variable.
  */
-export const getBufferLogs = (config: { buffer?: boolean }): BufferedLogs | undefined => {
-  const { buffer = false } = config
+export const getBufferLogs = (config: {
+  buffer?: boolean
+  logs?: { logFunction?: (message: string) => void }
+}): Logs | undefined => {
+  const { buffer = false, logs } = config
+
+  if (logs?.logFunction) {
+    return { logFunction: logs.logFunction }
+  }
+
   if (!buffer) {
     return
   }
@@ -64,6 +72,12 @@ export const log = function (
     return
   }
 
+  if (typeof logs?.logFunction === 'function') {
+    logs.logFunction(stringC)
+
+    return
+  }
+
   console.log(stringC)
 }
 
@@ -75,61 +89,61 @@ const serializeIndentedItem = function (item) {
   return indentString(item, INDENT_SIZE + 1).trimStart()
 }
 
-export const logError = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logError = function (logs: Logs | undefined, string: string, opts = {}) {
   log(logs, string, { color: THEME.errorLine, ...opts })
 }
 
-export const logWarning = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logWarning = function (logs: Logs | undefined, string: string, opts = {}) {
   log(logs, string, { color: THEME.warningLine, ...opts })
 }
 
 // Print a message that is under a header/subheader, i.e. indented
-export const logMessage = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logMessage = function (logs: Logs | undefined, string: string, opts = {}) {
   log(logs, string, { indent: true, ...opts })
 }
 
 // Print an object
-export const logObject = function (logs: BufferedLogs | undefined, object, opts) {
+export const logObject = function (logs: Logs | undefined, object, opts) {
   logMessage(logs, serializeObject(object), opts)
 }
 
 // Print an array
-export const logArray = function (logs: BufferedLogs | undefined, array, opts = {}) {
+export const logArray = function (logs: Logs | undefined, array, opts = {}) {
   logMessage(logs, serializeIndentedArray(array), { color: THEME.none, ...opts })
 }
 
 // Print an array of errors
-export const logErrorArray = function (logs: BufferedLogs | undefined, array, opts = {}) {
+export const logErrorArray = function (logs: Logs | undefined, array, opts = {}) {
   logMessage(logs, serializeIndentedArray(array), { color: THEME.errorLine, ...opts })
 }
 
 // Print an array of warnings
-export const logWarningArray = function (logs: BufferedLogs | undefined, array, opts = {}) {
+export const logWarningArray = function (logs: Logs | undefined, array, opts = {}) {
   logMessage(logs, serializeIndentedArray(array), { color: THEME.warningLine, ...opts })
 }
 
 // Print a main section header
-export const logHeader = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logHeader = function (logs: Logs | undefined, string: string, opts = {}) {
   log(logs, `\n${getHeader(string)}`, { color: THEME.header, ...opts })
 }
 
 // Print a main section header, when an error happened
-export const logErrorHeader = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logErrorHeader = function (logs: Logs | undefined, string: string, opts = {}) {
   logHeader(logs, string, { color: THEME.errorHeader, ...opts })
 }
 
 // Print a sub-section header
-export const logSubHeader = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logSubHeader = function (logs: Logs | undefined, string: string, opts = {}) {
   log(logs, `\n${figures.pointer} ${string}`, { color: THEME.subHeader, ...opts })
 }
 
 // Print a sub-section header, when an error happened
-export const logErrorSubHeader = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logErrorSubHeader = function (logs: Logs | undefined, string: string, opts = {}) {
   logSubHeader(logs, string, { color: THEME.errorSubHeader, ...opts })
 }
 
 // Print a sub-section header, when a warning happened
-export const logWarningSubHeader = function (logs: BufferedLogs | undefined, string: string, opts = {}) {
+export const logWarningSubHeader = function (logs: Logs | undefined, string: string, opts = {}) {
   logSubHeader(logs, string, { color: THEME.warningSubHeader, ...opts })
 }
 
@@ -162,7 +176,7 @@ export const reduceLogLines = function (lines) {
  * the user-facing build logs)
  */
 export const getSystemLogger = function (
-  logs: BufferedLogs | undefined,
+  logs: Logs | undefined,
   debug: boolean,
   /** A system log file descriptor, if non is provided it will be a noop logger */
   systemLogFile?: number,
diff --git a/packages/build/src/log/messages/config.js b/packages/build/src/log/messages/config.js
index 32342369b5..a68c4458b8 100644
--- a/packages/build/src/log/messages/config.js
+++ b/packages/build/src/log/messages/config.js
@@ -61,7 +61,7 @@ const INTERNAL_FLAGS = [
   'eventHandlers',
 ]
 const HIDDEN_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, ...INTERNAL_FLAGS]
-const HIDDEN_DEBUG_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, 'eventHandlers']
+const HIDDEN_DEBUG_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, 'eventHandlers', 'logs']
 
 export const logBuildDir = function (logs, buildDir) {
   logSubHeader(logs, 'Current directory')
diff --git a/packages/build/src/log/messages/core.ts b/packages/build/src/log/messages/core.ts
index e856a3f8c2..f3c4b08233 100644
--- a/packages/build/src/log/messages/core.ts
+++ b/packages/build/src/log/messages/core.ts
@@ -6,13 +6,13 @@ import { serializeLogError } from '../../error/parse/serialize_log.js'
 import { roundTimerToMillisecs } from '../../time/measure.js'
 import { ROOT_PACKAGE_JSON } from '../../utils/json.js'
 import { getLogHeaderFunc } from '../header_func.js'
-import { log, logMessage, logWarning, logHeader, logSubHeader, logWarningArray, BufferedLogs } from '../logger.js'
+import { log, logMessage, logWarning, logHeader, logSubHeader, logWarningArray, Logs } from '../logger.js'
 import { OutputFlusher } from '../output_flusher.js'
 import { THEME } from '../theme.js'
 
 import { logConfigOnError } from './config.js'
 
-export const logBuildStart = function (logs?: BufferedLogs) {
+export const logBuildStart = function (logs?: Logs) {
   logHeader(logs, 'Netlify Build')
   logSubHeader(logs, 'Version')
   logMessage(logs, `${ROOT_PACKAGE_JSON.name} ${ROOT_PACKAGE_JSON.version}`)
diff --git a/packages/build/tests/log/snapshots/tests.js.md b/packages/build/tests/log/snapshots/tests.js.md
index 2513e7c1d9..736aa4434e 100644
--- a/packages/build/tests/log/snapshots/tests.js.md
+++ b/packages/build/tests/log/snapshots/tests.js.md
@@ -1,4 +1,4 @@
-# Snapshot report for `tests/log/tests.js`
+# Snapshot report for `packages/build/tests/log/tests.js`
 
 The actual snapshot is saved in `tests.js.snap`.
 
@@ -204,3 +204,33 @@ Generated by [AVA](https://avajs.dev).
         - inputs: {}␊
           origin: config␊
           package: ./plugin`
+
+## Accepts a custom log function
+
+> Snapshot 1
+
+    `␊
+    Netlify Build                                                 ␊
+    ────────────────────────────────────────────────────────────────␊
+    > Version  @netlify/build 1.0.0␊
+    > Flags  debug: true␊
+      repositoryRoot: packages/build/tests/log/fixtures/verbose␊
+      testOpts:␊
+        pluginsListUrl: test␊
+        silentLingeringProcesses: true␊
+      verbose: true␊
+    > Current directory  packages/build/tests/log/fixtures/verbose␊
+    > Config file  packages/build/tests/log/fixtures/verbose/netlify.toml␊
+    > Resolved config  build:␊
+        publish: packages/build/tests/log/fixtures/verbose␊
+        publishOrigin: default␊
+      plugins:␊
+        - inputs: {}␊
+          origin: config␊
+          package: ./plugin␊
+    > Context  production␊
+    > Loading plugins   - ./plugin@1.0.0 from netlify.toml␊
+    ./plugin (onPreBuild event)                                   ␊
+    ────────────────────────────────────────────────────────────────Step completed.(./plugin onPreBuild completed in 1ms)Build step duration: ./plugin onPreBuild completed in 1msStep starting.Step started.Step ended.␊
+    Netlify Build Complete                                        ␊
+    ────────────────────────────────────────────────────────────────(Netlify Build completed in 1ms)Build step duration: Netlify Build completed in 1ms`
diff --git a/packages/build/tests/log/snapshots/tests.js.snap b/packages/build/tests/log/snapshots/tests.js.snap
index d107a909d4ddf98a27d21896e3ebdbc659de1baf..845e53837959276f8a128120318597ccb6a67432 100644
GIT binary patch
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diff --git a/packages/build/tests/log/tests.js b/packages/build/tests/log/tests.js
index 601befc90f..7a0df73520 100644
--- a/packages/build/tests/log/tests.js
+++ b/packages/build/tests/log/tests.js
@@ -58,3 +58,13 @@ test('Does not truncate long redirects in logs', async (t) => {
   const output = await new Fixture('./fixtures/truncate_redirects').runWithBuild()
   t.false(output.includes('999'))
 })
+
+test('Accepts a custom log function', async (t) => {
+  const logs = []
+  const logFunction = (message) => {
+    logs.push(message)
+  }
+  await new Fixture('./fixtures/verbose').withFlags({ logs: { logFunction }, verbose: true }).runBuildProgrammatic()
+
+  t.snapshot(normalizeOutput(logs.join('')))
+})

From fa000ff896b59474091d86f74eeae9c8879b4572 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Fri, 3 Oct 2025 17:18:47 +0100
Subject: [PATCH 2/7] refactor: simplify logic

---
 packages/build/src/log/logger.ts | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index cfc3a26dca..ece0201f85 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -37,15 +37,13 @@ export const getBufferLogs = (config: {
 }): Logs | undefined => {
   const { buffer = false, logs } = config
 
-  if (logs?.logFunction) {
-    return { logFunction: logs.logFunction }
+  if (buffer) {
+    return { stdout: [], stderr: [] }
   }
 
-  if (!buffer) {
-    return
+  if (logs?.logFunction) {
+    return { logFunction: logs.logFunction }
   }
-
-  return { stdout: [], stderr: [] }
 }
 
 // Core logging utility, used by the other methods.

From b9033cf62be0caeeeed196b548215d14131f324c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Fri, 3 Oct 2025 17:49:33 +0100
Subject: [PATCH 3/7] refactor: rename property

---
 packages/build/src/core/main.ts                |  4 ++--
 packages/build/src/core/types.ts               |  2 ++
 packages/build/src/log/logger.ts               | 12 ++++++------
 packages/build/src/log/messages/config.js      |  3 ++-
 packages/build/tests/log/snapshots/tests.js.md |  2 +-
 packages/build/tests/log/tests.js              | 10 +++++++---
 packages/testing/src/fixture.ts                |  7 ++++---
 7 files changed, 24 insertions(+), 16 deletions(-)

diff --git a/packages/build/src/core/main.ts b/packages/build/src/core/main.ts
index 7c7c4e5b3a..2fcdb43b8b 100644
--- a/packages/build/src/core/main.ts
+++ b/packages/build/src/core/main.ts
@@ -4,7 +4,7 @@ import { trace, context } from '@opentelemetry/api'
 import { handleBuildError } from '../error/handle.js'
 import { reportError } from '../error/report.js'
 import { getSystemLogger } from '../log/logger.js'
-import type { BufferedLogs } from '../log/logger.js'
+import type { Logs } from '../log/logger.js'
 import { logTimer, logBuildSuccess } from '../log/messages/core.js'
 import { getGeneratedFunctions } from '../steps/return_values.js'
 import { trackBuildComplete } from '../telemetry/main.js'
@@ -27,7 +27,7 @@ const tracer = trace.getTracer('core')
 export async function buildSite(flags: Partial<BuildFlags> = {}): Promise<{
   success: boolean
   severityCode: number
-  logs: BufferedLogs | undefined
+  logs: Logs | undefined
   netlifyConfig?: any
   configMutations?: any
 }> {
diff --git a/packages/build/src/core/types.ts b/packages/build/src/core/types.ts
index d746c361df..39336b5066 100644
--- a/packages/build/src/core/types.ts
+++ b/packages/build/src/core/types.ts
@@ -44,6 +44,8 @@ export type BuildCLIFlags = {
 export type BuildFlags = BuildCLIFlags & {
   env?: Record<string, unknown>
   eventHandlers?: EventHandlers
+  /** Custom logger function to capture build output */
+  logger?: (message: string) => void
 }
 
 type EventHandlers = {
diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index ece0201f85..d8b8041110 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -33,16 +33,16 @@ const EMPTY_LINE = '\u{200B}'
  */
 export const getBufferLogs = (config: {
   buffer?: boolean
-  logs?: { logFunction?: (message: string) => void }
+  logger?: (message: string) => void
 }): Logs | undefined => {
-  const { buffer = false, logs } = config
+  const { buffer = false, logger } = config
 
-  if (buffer) {
-    return { stdout: [], stderr: [] }
+  if (logger) {
+    return { logFunction: logger }
   }
 
-  if (logs?.logFunction) {
-    return { logFunction: logs.logFunction }
+  if (buffer) {
+    return { stdout: [], stderr: [] }
   }
 }
 
diff --git a/packages/build/src/log/messages/config.js b/packages/build/src/log/messages/config.js
index a68c4458b8..9aadf3606a 100644
--- a/packages/build/src/log/messages/config.js
+++ b/packages/build/src/log/messages/config.js
@@ -59,9 +59,10 @@ const INTERNAL_FLAGS = [
   'enhancedSecretScan',
   'edgeFunctionsBootstrapURL',
   'eventHandlers',
+  'logger',
 ]
 const HIDDEN_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, ...INTERNAL_FLAGS]
-const HIDDEN_DEBUG_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, 'eventHandlers', 'logs']
+const HIDDEN_DEBUG_FLAGS = [...SECURE_FLAGS, ...TEST_FLAGS, 'eventHandlers', 'logger']
 
 export const logBuildDir = function (logs, buildDir) {
   logSubHeader(logs, 'Current directory')
diff --git a/packages/build/tests/log/snapshots/tests.js.md b/packages/build/tests/log/snapshots/tests.js.md
index 736aa4434e..0782f03e14 100644
--- a/packages/build/tests/log/snapshots/tests.js.md
+++ b/packages/build/tests/log/snapshots/tests.js.md
@@ -1,4 +1,4 @@
-# Snapshot report for `packages/build/tests/log/tests.js`
+# Snapshot report for `tests/log/tests.js`
 
 The actual snapshot is saved in `tests.js.snap`.
 
diff --git a/packages/build/tests/log/tests.js b/packages/build/tests/log/tests.js
index 7a0df73520..dac64bf956 100644
--- a/packages/build/tests/log/tests.js
+++ b/packages/build/tests/log/tests.js
@@ -61,10 +61,14 @@ test('Does not truncate long redirects in logs', async (t) => {
 
 test('Accepts a custom log function', async (t) => {
   const logs = []
-  const logFunction = (message) => {
+  const logger = (message) => {
     logs.push(message)
   }
-  await new Fixture('./fixtures/verbose').withFlags({ logs: { logFunction }, verbose: true }).runBuildProgrammatic()
+  await new Fixture('./fixtures/verbose')
+    .withFlags({ logger, verbose: true })
+    .runBuildProgrammatic()
 
-  t.snapshot(normalizeOutput(logs.join('')))
+  t.true(logs.length > 0, 'logger should have been called with messages')
+  t.true(logs.some(log => log.includes('Netlify Build')), 'logs should contain build header')
+  t.true(logs.some(log => log.includes('onPreBuild')), 'logs should contain plugin event')
 })
diff --git a/packages/testing/src/fixture.ts b/packages/testing/src/fixture.ts
index d096124684..f227f37761 100644
--- a/packages/testing/src/fixture.ts
+++ b/packages/testing/src/fixture.ts
@@ -205,9 +205,10 @@ export class Fixture {
 
   async runWithBuildAndIntrospect(): Promise<Awaited<ReturnType<typeof build>> & { output: string }> {
     const buildResult = await build(this.getBuildFlags())
-    const output = [buildResult.logs?.stdout.join('\n'), buildResult.logs?.stderr.join('\n')]
-      .filter(Boolean)
-      .join('\n\n')
+    const output =
+      buildResult.logs && 'stdout' in buildResult.logs
+        ? [buildResult.logs.stdout.join('\n'), buildResult.logs.stderr.join('\n')].filter(Boolean).join('\n\n')
+        : ''
 
     return {
       ...buildResult,

From b13fa68951406409315d8cedb8bdd9bf57cd16a0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Fri, 3 Oct 2025 17:59:05 +0100
Subject: [PATCH 4/7] refactor: improve output

---
 packages/build/src/core/main.ts               |   4 +--
 packages/build/src/index.ts                   |   1 +
 packages/build/src/log/logger.ts              |  16 +++++++---
 .../build/tests/log/snapshots/tests.js.md     |  30 ------------------
 .../build/tests/log/snapshots/tests.js.snap   | Bin 1096 -> 991 bytes
 packages/build/tests/log/tests.js             |  16 +++++++---
 6 files changed, 26 insertions(+), 41 deletions(-)

diff --git a/packages/build/src/core/main.ts b/packages/build/src/core/main.ts
index 2fcdb43b8b..fae361d1d4 100644
--- a/packages/build/src/core/main.ts
+++ b/packages/build/src/core/main.ts
@@ -3,7 +3,7 @@ import { trace, context } from '@opentelemetry/api'
 
 import { handleBuildError } from '../error/handle.js'
 import { reportError } from '../error/report.js'
-import { getSystemLogger } from '../log/logger.js'
+import { getLogsOutput, getSystemLogger } from '../log/logger.js'
 import type { Logs } from '../log/logger.js'
 import { logTimer, logBuildSuccess } from '../log/messages/core.js'
 import { getGeneratedFunctions } from '../steps/return_values.js'
@@ -123,7 +123,7 @@ export async function buildSite(flags: Partial<BuildFlags> = {}): Promise<{
         success,
         severityCode,
         netlifyConfig: netlifyConfigA,
-        logs,
+        logs: getLogsOutput(logs),
         configMutations,
         generatedFunctions: getGeneratedFunctions(returnValues),
       }
diff --git a/packages/build/src/index.ts b/packages/build/src/index.ts
index 888e21ed1b..318469080d 100644
--- a/packages/build/src/index.ts
+++ b/packages/build/src/index.ts
@@ -1,6 +1,7 @@
 import { buildSite } from './core/main.js'
 export { NetlifyPluginConstants } from './core/constants.js'
 
+export type { BufferedLogs as Logs } from './log/logger.js'
 export type { GeneratedFunction } from './steps/return_values.js'
 // export the legacy types
 export type { NetlifyPlugin } from './types/netlify_plugin.js'
diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index d8b8041110..10840ffa43 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -31,10 +31,7 @@ const EMPTY_LINE = '\u{200B}'
  * When the `buffer` option is true, we return logs instead of printing them
  * on the console. The logs are accumulated in a `logs` array variable.
  */
-export const getBufferLogs = (config: {
-  buffer?: boolean
-  logger?: (message: string) => void
-}): Logs | undefined => {
+export const getBufferLogs = (config: { buffer?: boolean; logger?: (message: string) => void }): Logs | undefined => {
   const { buffer = false, logger } = config
 
   if (logger) {
@@ -79,6 +76,17 @@ export const log = function (
   console.log(stringC)
 }
 
+// Returns a `logs` object to be returned in the public interface,
+// always containing a `stderr` and `stdout` arrays, regardless of
+// whether the `buffer` input property was used.
+export const getLogsOutput = (logs: Logs | undefined): BufferedLogs | undefined => {
+  if (!logs || logsAreBuffered(logs)) {
+    return logs
+  }
+
+  return { stdout: [], stderr: [] }
+}
+
 const serializeIndentedArray = function (array) {
   return serializeArray(array.map(serializeIndentedItem))
 }
diff --git a/packages/build/tests/log/snapshots/tests.js.md b/packages/build/tests/log/snapshots/tests.js.md
index 0782f03e14..2513e7c1d9 100644
--- a/packages/build/tests/log/snapshots/tests.js.md
+++ b/packages/build/tests/log/snapshots/tests.js.md
@@ -204,33 +204,3 @@ Generated by [AVA](https://avajs.dev).
         - inputs: {}␊
           origin: config␊
           package: ./plugin`
-
-## Accepts a custom log function
-
-> Snapshot 1
-
-    `␊
-    Netlify Build                                                 ␊
-    ────────────────────────────────────────────────────────────────␊
-    > Version  @netlify/build 1.0.0␊
-    > Flags  debug: true␊
-      repositoryRoot: packages/build/tests/log/fixtures/verbose␊
-      testOpts:␊
-        pluginsListUrl: test␊
-        silentLingeringProcesses: true␊
-      verbose: true␊
-    > Current directory  packages/build/tests/log/fixtures/verbose␊
-    > Config file  packages/build/tests/log/fixtures/verbose/netlify.toml␊
-    > Resolved config  build:␊
-        publish: packages/build/tests/log/fixtures/verbose␊
-        publishOrigin: default␊
-      plugins:␊
-        - inputs: {}␊
-          origin: config␊
-          package: ./plugin␊
-    > Context  production␊
-    > Loading plugins   - ./plugin@1.0.0 from netlify.toml␊
-    ./plugin (onPreBuild event)                                   ␊
-    ────────────────────────────────────────────────────────────────Step completed.(./plugin onPreBuild completed in 1ms)Build step duration: ./plugin onPreBuild completed in 1msStep starting.Step started.Step ended.␊
-    Netlify Build Complete                                        ␊
-    ────────────────────────────────────────────────────────────────(Netlify Build completed in 1ms)Build step duration: Netlify Build completed in 1ms`
diff --git a/packages/build/tests/log/snapshots/tests.js.snap b/packages/build/tests/log/snapshots/tests.js.snap
index 845e53837959276f8a128120318597ccb6a67432..d107a909d4ddf98a27d21896e3ebdbc659de1baf 100644
GIT binary patch
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diff --git a/packages/build/tests/log/tests.js b/packages/build/tests/log/tests.js
index dac64bf956..642442b14f 100644
--- a/packages/build/tests/log/tests.js
+++ b/packages/build/tests/log/tests.js
@@ -64,11 +64,17 @@ test('Accepts a custom log function', async (t) => {
   const logger = (message) => {
     logs.push(message)
   }
-  await new Fixture('./fixtures/verbose')
-    .withFlags({ logger, verbose: true })
-    .runBuildProgrammatic()
+  const result = await new Fixture('./fixtures/verbose').withFlags({ logger, verbose: true }).runBuildProgrammatic()
 
+  t.deepEqual(result.logs.stdout, [])
+  t.deepEqual(result.logs.stderr, [])
   t.true(logs.length > 0, 'logger should have been called with messages')
-  t.true(logs.some(log => log.includes('Netlify Build')), 'logs should contain build header')
-  t.true(logs.some(log => log.includes('onPreBuild')), 'logs should contain plugin event')
+  t.true(
+    logs.some((log) => log.includes('Netlify Build')),
+    'logs should contain build header',
+  )
+  t.true(
+    logs.some((log) => log.includes('onPreBuild')),
+    'logs should contain plugin event',
+  )
 })

From 8e6584f56c6888ab4b0fb772a107177d9e503046 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Wed, 15 Oct 2025 12:27:23 +0100
Subject: [PATCH 5/7] fix: pass `logFunction` to plugins and core steps

---
 packages/build/src/log/logger.ts              | 16 +++--------
 .../with_plugin_and_functions/manifest.yml    |  2 ++
 .../with_plugin_and_functions/netlify.toml    |  2 ++
 .../netlify/functions/hello.mjs               |  5 ++++
 .../with_plugin_and_functions/plugin.js       |  3 ++
 packages/build/tests/log/tests.js             | 28 +++++++++++--------
 6 files changed, 33 insertions(+), 23 deletions(-)
 create mode 100644 packages/build/tests/log/fixtures/with_plugin_and_functions/manifest.yml
 create mode 100644 packages/build/tests/log/fixtures/with_plugin_and_functions/netlify.toml
 create mode 100644 packages/build/tests/log/fixtures/with_plugin_and_functions/netlify/functions/hello.mjs
 create mode 100644 packages/build/tests/log/fixtures/with_plugin_and_functions/plugin.js

diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index 10840ffa43..bf1bef75af 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -212,15 +212,7 @@ export const getSystemLogger = function (
   return (...args) => fileDescriptor.write(`${reduceLogLines(args)}\n`)
 }
 
-export const addOutputFlusher = (logs: Logs, outputFlusher: OutputFlusher): Logs => {
-  if (logsAreBuffered(logs)) {
-    return {
-      ...logs,
-      outputFlusher,
-    }
-  }
-
-  return {
-    outputFlusher,
-  }
-}
+export const addOutputFlusher = (logs: Logs, outputFlusher: OutputFlusher): Logs => ({
+  ...logs,
+  outputFlusher,
+})
diff --git a/packages/build/tests/log/fixtures/with_plugin_and_functions/manifest.yml b/packages/build/tests/log/fixtures/with_plugin_and_functions/manifest.yml
new file mode 100644
index 0000000000..a3512f0259
--- /dev/null
+++ b/packages/build/tests/log/fixtures/with_plugin_and_functions/manifest.yml
@@ -0,0 +1,2 @@
+name: test
+inputs: []
diff --git a/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify.toml b/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify.toml
new file mode 100644
index 0000000000..81b0ce8bb1
--- /dev/null
+++ b/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify.toml
@@ -0,0 +1,2 @@
+[[plugins]]
+package = "./plugin"
diff --git a/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify/functions/hello.mjs b/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify/functions/hello.mjs
new file mode 100644
index 0000000000..f45d45d8d1
--- /dev/null
+++ b/packages/build/tests/log/fixtures/with_plugin_and_functions/netlify/functions/hello.mjs
@@ -0,0 +1,5 @@
+export default async () => new Response("Hello")
+
+export const config = {
+  path: "/hello"
+}
\ No newline at end of file
diff --git a/packages/build/tests/log/fixtures/with_plugin_and_functions/plugin.js b/packages/build/tests/log/fixtures/with_plugin_and_functions/plugin.js
new file mode 100644
index 0000000000..f7e96fc421
--- /dev/null
+++ b/packages/build/tests/log/fixtures/with_plugin_and_functions/plugin.js
@@ -0,0 +1,3 @@
+export const onPreBuild = function () {
+  console.log('test')
+}
diff --git a/packages/build/tests/log/tests.js b/packages/build/tests/log/tests.js
index 642442b14f..ebe8f798b7 100644
--- a/packages/build/tests/log/tests.js
+++ b/packages/build/tests/log/tests.js
@@ -59,22 +59,28 @@ test('Does not truncate long redirects in logs', async (t) => {
   t.false(output.includes('999'))
 })
 
-test('Accepts a custom log function', async (t) => {
+test.only('Accepts a custom log function', async (t) => {
   const logs = []
   const logger = (message) => {
     logs.push(message)
   }
-  const result = await new Fixture('./fixtures/verbose').withFlags({ logger, verbose: true }).runBuildProgrammatic()
+  const result = await new Fixture('./fixtures/with_plugin_and_functions')
+    .withFlags({ logger, verbose: true })
+    .runBuildProgrammatic()
 
   t.deepEqual(result.logs.stdout, [])
   t.deepEqual(result.logs.stderr, [])
-  t.true(logs.length > 0, 'logger should have been called with messages')
-  t.true(
-    logs.some((log) => log.includes('Netlify Build')),
-    'logs should contain build header',
-  )
-  t.true(
-    logs.some((log) => log.includes('onPreBuild')),
-    'logs should contain plugin event',
-  )
+
+  t.true(logs.length > 0)
+
+  // From main logic.
+  t.true(logs.some((log) => log.includes('Netlify Build')))
+  t.true(logs.some((log) => log.includes('onPreBuild')))
+
+  // From core step.
+  t.true(logs.some((log) => log.includes('Packaging Functions from ')))
+
+  // From plugin.
+  t.true(logs.some((log) => log.includes('Step started.')))
+  t.true(logs.some((log) => log.includes('Step ended.')))
 })

From 63a6a262775b8f3801cd77a1cd4ccb04e1389720 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Wed, 15 Oct 2025 12:30:27 +0100
Subject: [PATCH 6/7] fix: simplify types

---
 packages/build/src/index.ts      |  2 +-
 packages/build/src/log/logger.ts | 10 ++++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/packages/build/src/index.ts b/packages/build/src/index.ts
index 318469080d..4e7b97b525 100644
--- a/packages/build/src/index.ts
+++ b/packages/build/src/index.ts
@@ -1,7 +1,7 @@
 import { buildSite } from './core/main.js'
 export { NetlifyPluginConstants } from './core/constants.js'
 
-export type { BufferedLogs as Logs } from './log/logger.js'
+export type { LogOutput as Logs } from './log/logger.js'
 export type { GeneratedFunction } from './steps/return_values.js'
 // export the legacy types
 export type { NetlifyPlugin } from './types/netlify_plugin.js'
diff --git a/packages/build/src/log/logger.ts b/packages/build/src/log/logger.ts
index bf1bef75af..b6199e1eb7 100644
--- a/packages/build/src/log/logger.ts
+++ b/packages/build/src/log/logger.ts
@@ -76,15 +76,17 @@ export const log = function (
   console.log(stringC)
 }
 
+export type LogOutput = Pick<BufferedLogs, 'stderr' | 'stdout'>
+
 // Returns a `logs` object to be returned in the public interface,
 // always containing a `stderr` and `stdout` arrays, regardless of
 // whether the `buffer` input property was used.
-export const getLogsOutput = (logs: Logs | undefined): BufferedLogs | undefined => {
-  if (!logs || logsAreBuffered(logs)) {
-    return logs
+export const getLogsOutput = (logs: Logs | undefined): LogOutput => {
+  if (!logs || !logsAreBuffered(logs)) {
+    return { stdout: [], stderr: [] }
   }
 
-  return { stdout: [], stderr: [] }
+  return { stdout: logs.stdout, stderr: logs.stderr }
 }
 
 const serializeIndentedArray = function (array) {

From 1665e1ccddf8eefc85c7ca8d6bbbba6d8b002587 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Bou=C3=A7as?= <mail@eduardoboucas.com>
Date: Wed, 15 Oct 2025 12:37:16 +0100
Subject: [PATCH 7/7] chore: ooops

---
 packages/build/tests/log/tests.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/build/tests/log/tests.js b/packages/build/tests/log/tests.js
index ebe8f798b7..1de8a698d9 100644
--- a/packages/build/tests/log/tests.js
+++ b/packages/build/tests/log/tests.js
@@ -59,7 +59,7 @@ test('Does not truncate long redirects in logs', async (t) => {
   t.false(output.includes('999'))
 })
 
-test.only('Accepts a custom log function', async (t) => {
+test('Accepts a custom log function', async (t) => {
   const logs = []
   const logger = (message) => {
     logs.push(message)