diff --git a/tests/features/processings/log-flood.api.spec.ts b/tests/features/processings/log-flood.api.spec.ts
new file mode 100644
index 00000000..665b2a14
--- /dev/null
+++ b/tests/features/processings/log-flood.api.spec.ts
@@ -0,0 +1,59 @@
+import { test, expect } from '@playwright/test'
+import { execSync } from 'node:child_process'
+import path from 'node:path'
+import os from 'node:os'
+import fs from 'node:fs'
+import { axiosAuth, clean, waitForRunStatus } from '../../support/axios.ts'
+import { publishFixturePlugin } from '../../support/registry.ts'
+
+const buildLogFloodTarball = (): string => {
+  const fixtureDir = path.resolve(import.meta.dirname, '../../fixtures/processing-log-flood')
+  const outDir = fs.mkdtempSync(path.join(os.tmpdir(), 'log-flood-pack-'))
+  execSync('npm pack --pack-destination ' + outDir, { cwd: fixtureDir, stdio: 'pipe' })
+  const tarball = fs.readdirSync(outDir).find(f => f.endsWith('.tgz'))
+  if (!tarball) throw new Error('npm pack did not produce a tarball')
+  return path.join(outDir, tarball)
+}
+
+// Worker default (worker/config/default.mjs). The fixture writes 20 000 chars
+// per entry, so every entry is truncated to maxLogEntryLength.
+const maxLogEntryLength = 10000
+const mongoDocumentLimit = 16 * 1024 * 1024
+
+test.describe('run log size guard', () => {
+  test.beforeEach(clean)
+  test.afterAll(clean)
+
+  test('a task flooding its log is failed once the run document is full', async () => {
+    test.setTimeout(300_000)
+    const superadmin = await axiosAuth('test_superadmin@test.com')
+
+    const plugin = await publishFixturePlugin({
+      name: '@data-fair-tests/processing-log-flood',
+      version: '1.0.0',
+      tarballPath: buildLogFloodTarball()
+    })
+    const processing = (await superadmin.post('/api/v1/processings', {
+      title: 'Log flood test',
+      plugin: plugin.pluginId,
+      owner: { type: 'user', id: 'test_superadmin', name: 'Test Super Admin' },
+      active: true,
+      config: {}
+    })).data
+    const triggered = (await superadmin.post(`/api/v1/processings/${processing._id}/_trigger`)).data
+    const finalRun = await waitForRunStatus(triggered._id, ['finished', 'error'], 270_000)
+    expect(finalRun.status).toBe('error')
+
+    const log = finalRun.log as Array<{ type: string, msg: string }>
+    // the loop of the fixture never completes
+    expect(log.map(l => l.msg)).not.toContain('log-flood fixture finished, the guard did not trigger')
+    // every flooding entry was truncated
+    const flooding = log.filter(l => l.msg.startsWith('xxxx'))
+    expect(flooding.length).toBeGreaterThan(0)
+    for (const entry of flooding) expect(entry.msg).toBe('x'.repeat(maxLogEntryLength) + '...')
+    // the document filled up, the log was truncated to make room for the guard message and finish()
+    expect(log.some(l => l.type === 'error' && /taille maximale/.test(l.msg))).toBe(true)
+    expect(Buffer.byteLength(JSON.stringify(log))).toBeLessThan(mongoDocumentLimit / 4)
+    expect(log[log.length - 1].type).toBe('debug') // finish() could still add its entry
+  })
+})
diff --git a/tests/features/worker-utils/runs-operations.unit.spec.ts b/tests/features/worker-utils/runs-operations.unit.spec.ts
index ac866905..90b00633 100644
--- a/tests/features/worker-utils/runs-operations.unit.spec.ts
+++ b/tests/features/worker-utils/runs-operations.unit.spec.ts
@@ -1,5 +1,5 @@
 import { test, expect } from '@playwright/test'
-import { shouldDisableForFailures, buildFinishStatusPatch } from '../../../worker/src/utils/runs-operations.ts'
+import { shouldDisableForFailures, buildFinishStatusPatch, isDocumentTooLargeError, truncateLogValue } from '../../../worker/src/utils/runs-operations.ts'
 
 test.describe('shouldDisableForFailures', () => {
   const maxFailures = 3
@@ -59,3 +59,33 @@ test.describe('buildFinishStatusPatch', () => {
     expect(buildFinishStatusPatch('running', undefined, finishedAt)).toEqual({ status: 'finished', finishedAt })
   })
 })
+
+test.describe('isDocumentTooLargeError', () => {
+  test('matches the mongo error of an update overflowing the document limit', () => {
+    expect(isDocumentTooLargeError(new Error('Plan executor error during findAndModify :: caused by :: Resulting document after update is larger than 16777216'))).toBe(true)
+    expect(isDocumentTooLargeError(Object.assign(new Error('BSONObjectTooLarge'), { code: 17419 }))).toBe(true)
+  })
+
+  test('ignores other errors', () => {
+    expect(isDocumentTooLargeError(new Error('Run not found'))).toBe(false)
+    expect(isDocumentTooLargeError(undefined)).toBe(false)
+  })
+})
+
+test.describe('truncateLogValue', () => {
+  test('leaves short values untouched', () => {
+    expect(truncateLogValue('hello', 100)).toBe('hello')
+    expect(truncateLogValue({ a: 1 }, 100)).toEqual({ a: 1 })
+    expect(truncateLogValue(undefined, 100)).toBeUndefined()
+    expect(truncateLogValue('', 100)).toBe('')
+  })
+
+  test('truncates a long string', () => {
+    expect(truncateLogValue('a'.repeat(50), 10)).toBe('a'.repeat(10) + '...')
+  })
+
+  test('serializes then truncates a long non-string value', () => {
+    const value = { items: new Array(100).fill('x') }
+    expect(truncateLogValue(value, 10)).toBe(JSON.stringify(value).slice(0, 10) + '...')
+  })
+})
diff --git a/tests/fixtures/processing-log-flood/index.js b/tests/fixtures/processing-log-flood/index.js
new file mode 100644
index 00000000..b97fd9d7
--- /dev/null
+++ b/tests/fixtures/processing-log-flood/index.js
@@ -0,0 +1,14 @@
+// Test fixture: push far more log data than fits in the run document (mongo's
+// 16MB limit). Each message is longer than worker.task.maxLogEntryLength so
+// the per-entry truncation is exercised too, and so the document fills up in
+// a couple thousand entries instead of hundreds of thousands. The worker is
+// expected to fail the run before the loop ends.
+export const run = async (context) => {
+  const { log } = context
+  await log.step('starting log-flood fixture')
+  const message = 'x'.repeat(20_000)
+  for (let i = 0; i < 5000; i++) {
+    await log.info(message)
+  }
+  await log.info('log-flood fixture finished, the guard did not trigger')
+}
diff --git a/tests/fixtures/processing-log-flood/package.json b/tests/fixtures/processing-log-flood/package.json
new file mode 100644
index 00000000..d80cee42
--- /dev/null
+++ b/tests/fixtures/processing-log-flood/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "@data-fair-tests/processing-log-flood",
+  "version": "1.0.0",
+  "description": "Test plugin that floods the run log to trigger the worker log size guard.",
+  "main": "index.js",
+  "type": "module",
+  "files": ["index.js", "processing-config-schema.json"]
+}
diff --git a/tests/fixtures/processing-log-flood/processing-config-schema.json b/tests/fixtures/processing-log-flood/processing-config-schema.json
new file mode 100644
index 00000000..83b5c3a8
--- /dev/null
+++ b/tests/fixtures/processing-log-flood/processing-config-schema.json
@@ -0,0 +1,5 @@
+{
+  "type": "object",
+  "additionalProperties": true,
+  "properties": {}
+}
diff --git a/worker/config/custom-environment-variables.mjs b/worker/config/custom-environment-variables.mjs
index ef7e215d..0100abbf 100644
--- a/worker/config/custom-environment-variables.mjs
+++ b/worker/config/custom-environment-variables.mjs
@@ -46,7 +46,8 @@ export default {
       externalSamplerEnabled: {
         __name: 'WORKER_TASK_EXTERNAL_SAMPLER_ENABLED',
         __format: 'json'
-      }
+      },
+      maxLogEntryLength: 'WORKER_TASK_MAX_LOG_ENTRY_LENGTH'
     }
   }
 }
diff --git a/worker/config/default.mjs b/worker/config/default.mjs
index 55262b3d..2a9b969f 100644
--- a/worker/config/default.mjs
+++ b/worker/config/default.mjs
@@ -89,7 +89,10 @@ export default {
       // for each task child. Becomes the authoritative writer for the per-slot
       // RSS gauge (the in-process df-mem RSS write is suppressed). Auto-disabled
       // at boot on non-Linux platforms.
-      externalSamplerEnabled: true
+      externalSamplerEnabled: true,
+      // Max length (chars) of the msg / extra of a single run log entry,
+      // longer values are truncated.
+      maxLogEntryLength: 10000
     }
   },
   upgradeRoot: '/app/'
diff --git a/worker/config/type/schema.json b/worker/config/type/schema.json
index 4dceb839..53b4aa2f 100644
--- a/worker/config/type/schema.json
+++ b/worker/config/type/schema.json
@@ -146,12 +146,13 @@
         "gracePeriod": { "type": "number" },
         "task": {
           "type": "object",
-          "required": ["maxHeapMB", "memorySampleIntervalMs", "memoryHeadroomWarnPct", "externalSamplerEnabled"],
+          "required": ["maxHeapMB", "memorySampleIntervalMs", "memoryHeadroomWarnPct", "externalSamplerEnabled", "maxLogEntryLength"],
           "properties": {
             "maxHeapMB": { "type": "number", "minimum": 64, "description": "Per-task V8 old-space heap cap in MB, passed as --max-old-space-size to the child. Defaults to V8's own heap_size_limit for the host." },
             "memorySampleIntervalMs": { "type": "number", "minimum": 0 },
             "memoryHeadroomWarnPct": { "type": "number", "minimum": 0, "maximum": 100 },
-            "externalSamplerEnabled": { "type": "boolean", "description": "Enable parent-side /proc-based RSS/CPU sampler for each task child. Auto-disabled at boot on non-Linux platforms." }
+            "externalSamplerEnabled": { "type": "boolean", "description": "Enable parent-side /proc-based RSS/CPU sampler for each task child. Auto-disabled at boot on non-Linux platforms." },
+            "maxLogEntryLength": { "type": "number", "minimum": 100, "description": "Max length in chars of the msg/extra of a single run log entry, longer values are truncated." }
           }
         }
       }
diff --git a/worker/src/task/task.ts b/worker/src/task/task.ts
index 7020ed52..085caec4 100644
--- a/worker/src/task/task.ts
+++ b/worker/src/task/task.ts
@@ -12,7 +12,8 @@ import * as wsEmitter from '@data-fair/lib-node/ws-emitter.js'
 import { ensureArtefact } from '@data-fair/lib-node-registry'
 import { decipher } from '@data-fair/processings-shared/cipher.ts'
 import { importPluginModule } from '@data-fair/processings-shared/plugin-load.ts'
-import { running } from '../utils/runs.ts'
+import { running, truncateLog } from '../utils/runs.ts'
+import { isDocumentTooLargeError, truncateLogValue } from '../utils/runs-operations.ts'
 import config, { registryCacheDir } from '#config'
 import mongo from '#mongo'
 import { getAxiosInstance, getHttpErrorMessage, prepareAxiosError } from './axios.ts'
@@ -44,9 +45,29 @@ const wsInstance = (log: LogFunctions, owner: Account): DataFairWsClient => {
  * Prepare log functions.
  */
 const prepareLog = (processing: Processing, run: Run): LogFunctions => {
+  const { maxLogEntryLength } = config.worker.task
+  let overflowed = false
   const pushLog = async (log: any) => {
+    // the task is failing because of its log, drop what it writes on the way out
+    if (overflowed) return
+    log.msg = truncateLogValue(log.msg, maxLogEntryLength)
+    log.extra = truncateLogValue(log.extra, maxLogEntryLength)
     log.date = new Date().toISOString()
-    await mongo.runs.updateOne({ _id: run._id }, { $push: { log } })
+    try {
+      await mongo.runs.updateOne({ _id: run._id }, { $push: { log } })
+    } catch (err) {
+      if (!isDocumentTooLargeError(err)) throw err
+      // The whole log lives in the run document and it reached mongo's 16MB
+      // limit. Make room so the run can still be finished, leave an explicit
+      // error and fail the task.
+      overflowed = true
+      const msg = 'Le journal d\'exécution a atteint la taille maximale autorisée, l\'exécution est interrompue. Le traitement produit trop de messages, réduisez sa verbosité.'
+      await truncateLog(run._id)
+      const errorLog: any = { type: 'error', msg, extra: '', date: log.date }
+      await mongo.runs.updateOne({ _id: run._id }, { $push: { log: errorLog } })
+      await wsEmitter.emit(`processings/${processing._id}/run-log`, { _id: run._id, log: errorLog })
+      throw new Error(msg)
+    }
     await wsEmitter.emit(`processings/${processing._id}/run-log`, { _id: run._id, log })
   }
 
@@ -185,13 +206,9 @@ export const run = async (mailTransport: any) => {
     const httpMessage = getHttpErrorMessage(err)
 
     if (httpMessage) {
-      let errStr = util.inspect(err, { depth: 5 })
-      if (errStr.length > 10000) {
-        errStr = errStr.slice(0, 10000) + '...'
-      }
       console.error(httpMessage)
       await log.error(httpMessage)
-      await log.debug('axios error', errStr)
+      await log.debug('axios error', util.inspect(err, { depth: 5 }))
     } else {
       console.error(err.message || err)
       await log.error(err.message)
diff --git a/worker/src/utils/runs-operations.ts b/worker/src/utils/runs-operations.ts
index ba1649a2..48b3b51b 100644
--- a/worker/src/utils/runs-operations.ts
+++ b/worker/src/utils/runs-operations.ts
@@ -36,3 +36,20 @@ export const buildFinishStatusPatch = (
   if (errorMessage) return { status: 'error', finishedAt }
   return { status: 'finished', finishedAt }
 }
+
+/**
+ * Whether a mongo error means the updated document would exceed the 16MB limit
+ * ("Resulting document after update is larger than 16777216").
+ */
+export const isDocumentTooLargeError = (err: any): boolean =>
+  err?.code === 17419 || /larger than \d+/.test(err?.message ?? '')
+
+/**
+ * Cap a log msg/extra at `maxLength` chars. A non-string value that is too
+ * long once serialized is replaced by its truncated serialization.
+ */
+export const truncateLogValue = (value: any, maxLength: number): any => {
+  const str = typeof value === 'string' ? value : JSON.stringify(value)
+  if (!str || str.length <= maxLength) return value
+  return str.slice(0, maxLength) + '...'
+}
diff --git a/worker/src/utils/runs.ts b/worker/src/utils/runs.ts
index 966eb286..af40a973 100644
--- a/worker/src/utils/runs.ts
+++ b/worker/src/utils/runs.ts
@@ -7,7 +7,7 @@ import eventsQueue from '@data-fair/lib-node/events-queue.js'
 import config from '#config'
 import mongo from '#mongo'
 import { internalError } from '@data-fair/lib-node/observer.js'
-import { shouldDisableForFailures } from './runs-operations.ts'
+import { isDocumentTooLargeError, shouldDisableForFailures } from './runs-operations.ts'
 
 const sendProcessingEvent = (
   run: Run,
@@ -54,6 +54,15 @@ export const running = async (run: Run) => {
     .updateOne({ _id: run.processing._id }, { $set: { lastRun }, $unset: { nextRun: '' } })
 }
 
+/**
+ * Keep only the tail of a run log. Used when the run document reached mongo's
+ * 16MB limit and can not be updated anymore.
+ */
+export const truncateLog = async (runId: string, keep = 100) => {
+  const truncate: Record<string, any> = { $push: { log: { $each: [], $slice: -keep } } }
+  await mongo.runs.updateOne({ _id: runId }, truncate)
+}
+
 /**
  * Update the database when a run is finished (edit status, log, duration, etc.)
  */
@@ -75,11 +84,25 @@ export const finish = async (run: Run, errorMessage: string | undefined = undefi
     if (metricsMessage) logs.push({ type: 'debug', msg: metricsMessage, date: new Date(now + 1).toISOString() })
     query.$push = { log: { $each: logs } }
   }
-  let lastRun = (await mongo.runs.findOneAndUpdate(
-    { _id: run._id },
-    query,
-    { returnDocument: 'after', projection: { processing: 0, owner: 0 } }
-  ))
+  let lastRun
+  try {
+    lastRun = await mongo.runs.findOneAndUpdate(
+      { _id: run._id },
+      query,
+      { returnDocument: 'after', projection: { processing: 0, owner: 0 } }
+    )
+  } catch (err) {
+    if (!isDocumentTooLargeError(err)) throw err
+    // A run whose log filled the document would otherwise be impossible to
+    // finish or kill, and the kill loop would retry it forever.
+    console.warn('run document too large to be finished, truncating its log', run._id)
+    await truncateLog(run._id)
+    lastRun = await mongo.runs.findOneAndUpdate(
+      { _id: run._id },
+      query,
+      { returnDocument: 'after', projection: { processing: 0, owner: 0 } }
+    )
+  }
   if (!lastRun) return internalError('processing-worker', 'Last run not found after finish update')
   if (!lastRun.startedAt) {
     lastRun = (await mongo.runs.findOneAndUpdate(