diff --git a/tests/antigravity-statusline.test.ts b/tests/antigravity-statusline.test.ts
index 1eca3104..8a41494b 100644
--- a/tests/antigravity-statusline.test.ts
+++ b/tests/antigravity-statusline.test.ts
@@ -16,9 +16,6 @@ describe('Antigravity CLI statusLine hook installer', () => {
     const settingsPath = join(dir, 'settings.json')
     const binDir = join(dir, 'bin')
     const codeburnPath = join(binDir, process.platform === 'win32' ? 'codeburn.cmd' : 'codeburn')
-    const oldSettingsPath = process.env['CODEBURN_ANTIGRAVITY_SETTINGS_PATH']
-    const oldCacheDir = process.env['CODEBURN_CACHE_DIR']
-    const oldPath = process.env.PATH
     await mkdir(binDir, { recursive: true })
     await writeFile(codeburnPath, process.platform === 'win32' ? '@echo off\r\n' : '#!/bin/sh\n')
     await chmod(codeburnPath, 0o755)
@@ -29,12 +26,6 @@ describe('Antigravity CLI statusLine hook installer', () => {
     try {
       await run(dir, settingsPath)
     } finally {
-      if (oldSettingsPath === undefined) delete process.env['CODEBURN_ANTIGRAVITY_SETTINGS_PATH']
-      else process.env['CODEBURN_ANTIGRAVITY_SETTINGS_PATH'] = oldSettingsPath
-      if (oldCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else process.env['CODEBURN_CACHE_DIR'] = oldCacheDir
-      if (oldPath === undefined) delete process.env.PATH
-      else process.env.PATH = oldPath
       await rm(dir, { recursive: true, force: true })
     }
   }
diff --git a/tests/daily-cache.test.ts b/tests/daily-cache.test.ts
index 46d774d5..98923192 100644
--- a/tests/daily-cache.test.ts
+++ b/tests/daily-cache.test.ts
@@ -45,7 +45,6 @@ beforeEach(() => {
 
 afterEach(async () => {
   vi.useRealTimers()
-  delete process.env['CODEBURN_CACHE_DIR']
   if (existsSync(TMP_CACHE_ROOT)) {
     await rm(TMP_CACHE_ROOT, { recursive: true, force: true })
   }
diff --git a/tests/models.test.ts b/tests/models.test.ts
index 40981c0a..2943f2e3 100644
--- a/tests/models.test.ts
+++ b/tests/models.test.ts
@@ -429,7 +429,6 @@ describe('DeepSeek v4 models resolve to pricing', () => {
   })
 
   it('keeps bundled DeepSeek v4 fallback entries when runtime pricing cache is stale', async () => {
-    const previousCacheDir = process.env['CODEBURN_CACHE_DIR']
     const cacheRoot = await mkdtemp(join(tmpdir(), 'codeburn-pricing-cache-'))
 
     try {
@@ -455,11 +454,6 @@ describe('DeepSeek v4 models resolve to pricing', () => {
       expect(getModelCosts('deepseek-v4-pro')!.inputCostPerToken).toBe(4.35e-7)
       expect(getModelCosts('deepseek-v4-flash')!.inputCostPerToken).toBe(1.4e-7)
     } finally {
-      if (previousCacheDir === undefined) {
-        delete process.env['CODEBURN_CACHE_DIR']
-      } else {
-        process.env['CODEBURN_CACHE_DIR'] = previousCacheDir
-      }
       await rm(cacheRoot, { recursive: true, force: true })
       await loadPricing()
     }
diff --git a/tests/optimize-fs.test.ts b/tests/optimize-fs.test.ts
index 29d583e1..2364e08a 100644
--- a/tests/optimize-fs.test.ts
+++ b/tests/optimize-fs.test.ts
@@ -19,8 +19,6 @@ import {
   detectBloatedClaudeMd,
   detectUnusedMcp,
   detectBashBloat,
-  detectGhostAgents,
-  detectGhostSkills,
   detectGhostCommands,
   loadMcpConfigs,
   scanJsonlFile,
@@ -217,16 +215,6 @@ describe('detectUnusedMcp', () => {
 // ============================================================================
 
 describe('detectBashBloat', () => {
-  const originalEnv = process.env['BASH_MAX_OUTPUT_LENGTH']
-
-  beforeEach(() => {
-    delete process.env['BASH_MAX_OUTPUT_LENGTH']
-  })
-
-  afterAll(() => {
-    if (originalEnv !== undefined) process.env['BASH_MAX_OUTPUT_LENGTH'] = originalEnv
-  })
-
   it('flags when env var is unset (uses default 30K)', () => {
     const finding = detectBashBloat()
     expect(finding).not.toBeNull()
diff --git a/tests/otel-cache-aggregation.test.ts b/tests/otel-cache-aggregation.test.ts
index 78f6f50b..912263d0 100644
--- a/tests/otel-cache-aggregation.test.ts
+++ b/tests/otel-cache-aggregation.test.ts
@@ -92,16 +92,12 @@ describe.skipIf(!isSqliteAvailable())(
     let tmpHome: string
     let tmpCache: string
     let dbPath: string
-    let prevHome: string | undefined
-    let prevCache: string | undefined
 
     beforeEach(async () => {
       tmpHome  = await mkdtemp(join(tmpdir(), 'cb-otel-agg-home-'))
       tmpCache = await mkdtemp(join(tmpdir(), 'cb-otel-agg-cache-'))
       dbPath   = join(tmpHome, 'agent-traces.db')
 
-      prevHome  = process.env['HOME']
-      prevCache = process.env['CODEBURN_CACHE_DIR']
       process.env['HOME']              = tmpHome
       process.env['CODEBURN_CACHE_DIR'] = tmpCache
 
@@ -116,10 +112,6 @@ describe.skipIf(!isSqliteAvailable())(
     afterEach(async () => {
       clearSessionCache()
       vi.unstubAllEnvs()
-      if (prevHome  === undefined) delete process.env['HOME']
-      else                          process.env['HOME'] = prevHome
-      if (prevCache === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else                          process.env['CODEBURN_CACHE_DIR'] = prevCache
       await rm(tmpHome,  { recursive: true, force: true })
       await rm(tmpCache, { recursive: true, force: true })
     })
diff --git a/tests/parser-claude-cwd.test.ts b/tests/parser-claude-cwd.test.ts
index 9206e6f4..57ab04b1 100644
--- a/tests/parser-claude-cwd.test.ts
+++ b/tests/parser-claude-cwd.test.ts
@@ -7,13 +7,9 @@ import { parseAllSessions } from '../src/parser.js'
 import type { DateRange } from '../src/types.js'
 
 let tmpDir: string
-let originalConfigDir: string | undefined
-let originalDesktopSessionsDir: string | undefined
 
 beforeEach(async () => {
   tmpDir = await mkdtemp(join(tmpdir(), 'claude-cwd-test-'))
-  originalConfigDir = process.env['CLAUDE_CONFIG_DIR']
-  originalDesktopSessionsDir = process.env['CODEBURN_DESKTOP_SESSIONS_DIR']
   process.env['CLAUDE_CONFIG_DIR'] = tmpDir
   // Point desktop sessions at an empty subdir by default so real sessions
   // on the developer's machine do not bleed into the unit tests.
@@ -21,16 +17,6 @@ beforeEach(async () => {
 })
 
 afterEach(async () => {
-  if (originalConfigDir === undefined) {
-    delete process.env['CLAUDE_CONFIG_DIR']
-  } else {
-    process.env['CLAUDE_CONFIG_DIR'] = originalConfigDir
-  }
-  if (originalDesktopSessionsDir === undefined) {
-    delete process.env['CODEBURN_DESKTOP_SESSIONS_DIR']
-  } else {
-    process.env['CODEBURN_DESKTOP_SESSIONS_DIR'] = originalDesktopSessionsDir
-  }
   await rm(tmpDir, { recursive: true, force: true })
 })
 
diff --git a/tests/parser-gemini-cache.test.ts b/tests/parser-gemini-cache.test.ts
index 240e2afc..c2542b69 100644
--- a/tests/parser-gemini-cache.test.ts
+++ b/tests/parser-gemini-cache.test.ts
@@ -10,24 +10,16 @@ import type { DateRange } from '../src/types.js'
 
 let home: string
 let cacheDir: string
-let previousHome: string | undefined
-let previousCacheDir: string | undefined
 
 beforeEach(async () => {
   home = await mkdtemp(join(tmpdir(), 'codeburn-gemini-home-'))
   cacheDir = await mkdtemp(join(tmpdir(), 'codeburn-gemini-cache-'))
-  previousHome = process.env['HOME']
-  previousCacheDir = process.env['CODEBURN_CACHE_DIR']
   process.env['HOME'] = home
   process.env['CODEBURN_CACHE_DIR'] = cacheDir
 })
 
 afterEach(async () => {
   clearSessionCache()
-  if (previousHome === undefined) delete process.env['HOME']
-  else process.env['HOME'] = previousHome
-  if (previousCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-  else process.env['CODEBURN_CACHE_DIR'] = previousCacheDir
   await rm(home, { recursive: true, force: true })
   await rm(cacheDir, { recursive: true, force: true })
 })
diff --git a/tests/parser-large-session.test.ts b/tests/parser-large-session.test.ts
index 10eb7775..9ef1b8bc 100644
--- a/tests/parser-large-session.test.ts
+++ b/tests/parser-large-session.test.ts
@@ -16,7 +16,6 @@ beforeEach(async () => {
 
 afterEach(async () => {
   clearSessionCache()
-  delete process.env['CLAUDE_CONFIG_DIR']
   await rm(home, { recursive: true, force: true })
 })
 
diff --git a/tests/parser-local-savings.test.ts b/tests/parser-local-savings.test.ts
index 166e8f07..ae5dd258 100644
--- a/tests/parser-local-savings.test.ts
+++ b/tests/parser-local-savings.test.ts
@@ -20,25 +20,18 @@ function makeRange(): DateRange {
 }
 
 let tmpDirs: string[] = []
-let originalConfigDir: string | undefined
 
 beforeAll(async () => {
   await loadPricing()
 })
 
 beforeEach(() => {
-  originalConfigDir = process.env['CLAUDE_CONFIG_DIR']
   setLocalModelSavings({})
   setModelAliases({})
 })
 
 afterEach(async () => {
   delete (Object.prototype as Record<string, unknown>).calls
-  if (originalConfigDir === undefined) {
-    delete process.env['CLAUDE_CONFIG_DIR']
-  } else {
-    process.env['CLAUDE_CONFIG_DIR'] = originalConfigDir
-  }
   clearSessionCache()
   while (tmpDirs.length > 0) {
     const d = tmpDirs.pop()
diff --git a/tests/parser-proxy-codex-only.test.ts b/tests/parser-proxy-codex-only.test.ts
index 9c59949a..1bb92648 100644
--- a/tests/parser-proxy-codex-only.test.ts
+++ b/tests/parser-proxy-codex-only.test.ts
@@ -18,8 +18,6 @@ const CWD = '/Users/test/codexonlyproxied'
 let tmpDirs: string[] = []
 
 afterEach(async () => {
-  delete process.env['CODEX_HOME']
-  delete process.env['CLAUDE_CONFIG_DIR']
   while (tmpDirs.length > 0) {
     const d = tmpDirs.pop()
     if (d) await rm(d, { recursive: true, force: true })
diff --git a/tests/parser-proxy-merge.test.ts b/tests/parser-proxy-merge.test.ts
index 19dd3c85..a0d8dc95 100644
--- a/tests/parser-proxy-merge.test.ts
+++ b/tests/parser-proxy-merge.test.ts
@@ -19,8 +19,6 @@ const MERGE_CWD = '/Users/test/proxiedmerge'
 let tmpDirs: string[] = []
 
 afterEach(async () => {
-  delete process.env['CLAUDE_CONFIG_DIR']
-  delete process.env['CODEX_HOME']
   while (tmpDirs.length > 0) {
     const d = tmpDirs.pop()
     if (d) await rm(d, { recursive: true, force: true })
diff --git a/tests/parser-proxy-pricing.test.ts b/tests/parser-proxy-pricing.test.ts
index 10cf768e..26df51b1 100644
--- a/tests/parser-proxy-pricing.test.ts
+++ b/tests/parser-proxy-pricing.test.ts
@@ -127,14 +127,12 @@ const makeRange = (): DateRange => ({ start: RANGE_START, end: RANGE_END })
 const FIXTURE_CWD = '/private/var/eywa-proxy-fixture/acme'
 
 let tmpDirs: string[] = []
-let originalConfigDir: string | undefined
 
 beforeAll(async () => {
   await loadPricing()
 })
 
 beforeEach(() => {
-  originalConfigDir = process.env['CLAUDE_CONFIG_DIR']
   setProxyPaths([])
   setLocalModelSavings({})
   setModelAliases({})
@@ -143,8 +141,6 @@ beforeEach(() => {
 
 afterEach(async () => {
   setProxyPaths([])
-  if (originalConfigDir === undefined) delete process.env['CLAUDE_CONFIG_DIR']
-  else process.env['CLAUDE_CONFIG_DIR'] = originalConfigDir
   clearSessionCache()
   while (tmpDirs.length > 0) {
     const d = tmpDirs.pop()
diff --git a/tests/parser.test.ts b/tests/parser.test.ts
index 46edeb42..1a2a4ece 100644
--- a/tests/parser.test.ts
+++ b/tests/parser.test.ts
@@ -172,15 +172,11 @@ function totalOutput(projects: Awaited<ReturnType<typeof parseAllSessions>>): nu
 // ── Common env setup ──────────────────────────────────────────────────────
 let tmpHome: string
 let tmpCache: string
-let prevHome: string | undefined
-let prevCache: string | undefined
 
 beforeEach(async () => {
   tmpHome  = await mkdtemp(join(tmpdir(), 'cb-parser-test-home-'))
   tmpCache = await mkdtemp(join(tmpdir(), 'cb-parser-test-cache-'))
 
-  prevHome  = process.env['HOME']
-  prevCache = process.env['CODEBURN_CACHE_DIR']
   process.env['HOME']               = tmpHome
   process.env['CODEBURN_CACHE_DIR'] = tmpCache
 
@@ -194,11 +190,6 @@ afterEach(async () => {
   clearSessionCache()
   vi.unstubAllEnvs()
 
-  if (prevHome  === undefined) delete process.env['HOME']
-  else                          process.env['HOME'] = prevHome
-  if (prevCache === undefined) delete process.env['CODEBURN_CACHE_DIR']
-  else                          process.env['CODEBURN_CACHE_DIR'] = prevCache
-
   _synthSources = []
 
   await rm(tmpHome,  { recursive: true, force: true })
diff --git a/tests/plan-usage.test.ts b/tests/plan-usage.test.ts
index b8a81c61..eabfe5a5 100644
--- a/tests/plan-usage.test.ts
+++ b/tests/plan-usage.test.ts
@@ -185,7 +185,6 @@ describe('getPlanUsage', () => {
 
   it('keeps the provider-specific parser filter for one active plan', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-usage-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -217,18 +216,12 @@ describe('getPlanUsage', () => {
       expect(usages).toHaveLength(1)
       expect(usages[0]?.spentApiEquivalentUsd).toBe(80)
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('computes multiple active plan usages from one all-provider parse', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-usage-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -344,11 +337,6 @@ describe('getPlanUsage', () => {
       expect(usages.map(usage => usage.plan.provider)).toEqual(['claude', 'codex'])
       expect(usages.map(usage => usage.spentApiEquivalentUsd)).toEqual([100, 50])
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
diff --git a/tests/plans.test.ts b/tests/plans.test.ts
index 4ed7cc86..ca4553b3 100644
--- a/tests/plans.test.ts
+++ b/tests/plans.test.ts
@@ -29,7 +29,6 @@ describe('plan presets', () => {
 describe('plan config persistence', () => {
   it('round-trips per-provider plans and clears one provider at a time', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -74,18 +73,12 @@ describe('plan config persistence', () => {
       expect(await readPlan()).toBeUndefined()
       expect(await readPlans()).toEqual({})
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('reads legacy single-plan config as a provider-keyed plan map', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -107,18 +100,12 @@ describe('plan config persistence', () => {
         resetDay: 3,
       })
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('drops a hand-edited all plan when provider-specific plans are present', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -144,18 +131,12 @@ describe('plan config persistence', () => {
       expect(plans.claude).toMatchObject({ id: 'claude-max', provider: 'claude' })
       expect(await readPlan()).toMatchObject({ id: 'claude-max', provider: 'claude' })
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('does not allow an all-provider plan to overlap provider-specific plans', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-plan-test-'))
-    const previousHome = process.env['HOME']
     process.env['HOME'] = dir
 
     try {
@@ -191,11 +172,6 @@ describe('plan config persistence', () => {
       })
       expect((await readPlans()).claude).toBeUndefined()
     } finally {
-      if (previousHome === undefined) {
-        delete process.env['HOME']
-      } else {
-        process.env['HOME'] = previousHome
-      }
       await rm(dir, { recursive: true, force: true })
     }
   })
diff --git a/tests/provider-turn-grouping.test.ts b/tests/provider-turn-grouping.test.ts
index 27ff5a7a..d9dd0d65 100644
--- a/tests/provider-turn-grouping.test.ts
+++ b/tests/provider-turn-grouping.test.ts
@@ -8,18 +8,12 @@ import type { DateRange } from '../src/types.js'
 let home: string
 let cacheDir: string
 let vibeHome: string
-let originalHome: string | undefined
-let originalCacheDir: string | undefined
-let originalVibeHome: string | undefined
 let clearParserCache: (() => void) | undefined
 
 beforeEach(async () => {
   home = await mkdtemp(join(tmpdir(), 'codeburn-turn-group-home-'))
   cacheDir = await mkdtemp(join(tmpdir(), 'codeburn-turn-group-cache-'))
   vibeHome = await mkdtemp(join(tmpdir(), 'codeburn-turn-group-vibe-'))
-  originalHome = process.env['HOME']
-  originalCacheDir = process.env['CODEBURN_CACHE_DIR']
-  originalVibeHome = process.env['VIBE_HOME']
   process.env['HOME'] = home
   process.env['CODEBURN_CACHE_DIR'] = cacheDir
   process.env['VIBE_HOME'] = vibeHome
@@ -29,12 +23,6 @@ afterEach(async () => {
   clearParserCache?.()
   clearParserCache = undefined
   vi.resetModules()
-  if (originalHome === undefined) delete process.env['HOME']
-  else process.env['HOME'] = originalHome
-  if (originalCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-  else process.env['CODEBURN_CACHE_DIR'] = originalCacheDir
-  if (originalVibeHome === undefined) delete process.env['VIBE_HOME']
-  else process.env['VIBE_HOME'] = originalVibeHome
   await rm(home, { recursive: true, force: true })
   await rm(cacheDir, { recursive: true, force: true })
   await rm(vibeHome, { recursive: true, force: true })
diff --git a/tests/providers/antigravity.test.ts b/tests/providers/antigravity.test.ts
index 088c873f..76cba7be 100644
--- a/tests/providers/antigravity.test.ts
+++ b/tests/providers/antigravity.test.ts
@@ -263,7 +263,6 @@ describe('antigravity provider helpers', () => {
 
   it('captures exact Antigravity CLI statusLine usage as fallback calls', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-antigravity-statusline-'))
-    const oldCacheDir = process.env['CODEBURN_CACHE_DIR']
     process.env['CODEBURN_CACHE_DIR'] = dir
 
     try {
@@ -317,15 +316,12 @@ describe('antigravity provider helpers', () => {
       expect(calls[0]!.projectPath).toBeUndefined()
       expect(calls[0]!.costUSD).toBeGreaterThan(0)
     } finally {
-      if (oldCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else process.env['CODEBURN_CACHE_DIR'] = oldCacheDir
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('skips statusLine fallback calls when RPC cache already covered the conversation', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-antigravity-statusline-rpc-dedup-'))
-    const oldCacheDir = process.env['CODEBURN_CACHE_DIR']
     process.env['CODEBURN_CACHE_DIR'] = dir
 
     try {
@@ -354,15 +350,12 @@ describe('antigravity provider helpers', () => {
 
       expect(calls).toEqual([])
     } finally {
-      if (oldCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else process.env['CODEBURN_CACHE_DIR'] = oldCacheDir
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('skips singleton statusLine snapshots and deltas monotonic usage', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-antigravity-statusline-runs-'))
-    const oldCacheDir = process.env['CODEBURN_CACHE_DIR']
     process.env['CODEBURN_CACHE_DIR'] = dir
 
     const basePayload = {
@@ -409,15 +402,12 @@ describe('antigravity provider helpers', () => {
       ])
       expect(calls.map(call => call.cachedInputTokens)).toEqual([0, 0])
     } finally {
-      if (oldCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else process.env['CODEBURN_CACHE_DIR'] = oldCacheDir
       await rm(dir, { recursive: true, force: true })
     }
   })
 
   it('treats non-monotonic statusLine usage as a new request snapshot', async () => {
     const dir = await mkdtemp(join(tmpdir(), 'codeburn-antigravity-statusline-reset-'))
-    const oldCacheDir = process.env['CODEBURN_CACHE_DIR']
     process.env['CODEBURN_CACHE_DIR'] = dir
 
     const payload = (
@@ -458,8 +448,6 @@ describe('antigravity provider helpers', () => {
         [200, 30, 500],
       ])
     } finally {
-      if (oldCacheDir === undefined) delete process.env['CODEBURN_CACHE_DIR']
-      else process.env['CODEBURN_CACHE_DIR'] = oldCacheDir
       await rm(dir, { recursive: true, force: true })
     }
   })
diff --git a/tests/providers/crush.test.ts b/tests/providers/crush.test.ts
index 4835f6f3..b86f5358 100644
--- a/tests/providers/crush.test.ts
+++ b/tests/providers/crush.test.ts
@@ -18,19 +18,12 @@ type TestDb = {
 }
 
 let tmpRoot: string
-let originalEnv: string | undefined
 
 beforeEach(async () => {
   tmpRoot = await mkdtemp(join(tmpdir(), 'crush-test-'))
-  originalEnv = process.env['CRUSH_GLOBAL_DATA']
 })
 
 afterEach(async () => {
-  if (originalEnv === undefined) {
-    delete process.env['CRUSH_GLOBAL_DATA']
-  } else {
-    process.env['CRUSH_GLOBAL_DATA'] = originalEnv
-  }
   await rm(tmpRoot, { recursive: true, force: true })
 })
 
diff --git a/tests/providers/cursor-large-db-cap.test.ts b/tests/providers/cursor-large-db-cap.test.ts
index b5b0256e..7b60c3d9 100644
--- a/tests/providers/cursor-large-db-cap.test.ts
+++ b/tests/providers/cursor-large-db-cap.test.ts
@@ -18,16 +18,12 @@ import type { DateRange } from '../../src/types.js'
 const skipReason = isSqliteAvailable() ? null : 'node:sqlite not available — needs Node 22+; skipping'
 
 let tmpDir: string
-let savedBudget: string | undefined
 
 beforeEach(async () => {
   tmpDir = await mkdtemp(join(tmpdir(), 'cursor-cap-'))
-  savedBudget = process.env['CODEBURN_CURSOR_MAX_BUBBLES']
 })
 
 afterEach(async () => {
-  if (savedBudget === undefined) delete process.env['CODEBURN_CURSOR_MAX_BUBBLES']
-  else process.env['CODEBURN_CURSOR_MAX_BUBBLES'] = savedBudget
   await rm(tmpDir, { recursive: true, force: true })
 })
 
diff --git a/tests/providers/devin.test.ts b/tests/providers/devin.test.ts
index 5197140f..4efbe982 100644
--- a/tests/providers/devin.test.ts
+++ b/tests/providers/devin.test.ts
@@ -8,7 +8,6 @@ import { createDevinProvider } from '../../src/providers/devin.js'
 import type { ParsedProviderCall } from '../../src/providers/types.js'
 
 let tmpDir: string
-const originalHome = process.env['HOME']
 
 beforeEach(async () => {
   tmpDir = await mkdtemp(join(tmpdir(), 'devin-provider-'))
@@ -16,8 +15,6 @@ beforeEach(async () => {
 })
 
 afterEach(async () => {
-  if (originalHome === undefined) delete process.env['HOME']
-  else process.env['HOME'] = originalHome
   await rm(tmpDir, { recursive: true, force: true })
 })
 
diff --git a/tests/providers/mistral-vibe.test.ts b/tests/providers/mistral-vibe.test.ts
index b8d49bec..1a393fc6 100644
--- a/tests/providers/mistral-vibe.test.ts
+++ b/tests/providers/mistral-vibe.test.ts
@@ -7,20 +7,12 @@ import { createMistralVibeProvider } from '../../src/providers/mistral-vibe.js'
 import type { ParsedProviderCall } from '../../src/providers/types.js'
 
 let tmpDir: string
-let originalVibeHome: string | undefined
 
 beforeEach(async () => {
   tmpDir = await mkdtemp(join(tmpdir(), 'mistral-vibe-test-'))
-  originalVibeHome = process.env['VIBE_HOME']
-  delete process.env['VIBE_HOME']
 })
 
 afterEach(async () => {
-  if (originalVibeHome === undefined) {
-    delete process.env['VIBE_HOME']
-  } else {
-    process.env['VIBE_HOME'] = originalVibeHome
-  }
   await rm(tmpDir, { recursive: true, force: true })
 })
 
diff --git a/tests/security/prototype-pollution.test.ts b/tests/security/prototype-pollution.test.ts
index 6b6075c4..8a57ac20 100644
--- a/tests/security/prototype-pollution.test.ts
+++ b/tests/security/prototype-pollution.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, afterEach, beforeEach } from 'vitest'
+import { describe, it, expect, afterEach } from 'vitest'
 import { mkdtemp, mkdir, cp, rm } from 'fs/promises'
 import { tmpdir } from 'os'
 import { join } from 'path'
@@ -28,19 +28,9 @@ function makeRange(offsetMs: number): DateRange {
 
 describe('HIGH-1 prototype pollution via unchecked bracket-assign', () => {
   const tmpDirs: string[] = []
-  let originalConfigDir: string | undefined
-
-  beforeEach(() => {
-    originalConfigDir = process.env['CLAUDE_CONFIG_DIR']
-  })
 
   afterEach(async () => {
     delete (Object.prototype as Record<string, unknown>).calls
-    if (originalConfigDir === undefined) {
-      delete process.env['CLAUDE_CONFIG_DIR']
-    } else {
-      process.env['CLAUDE_CONFIG_DIR'] = originalConfigDir
-    }
     while (tmpDirs.length > 0) {
       const d = tmpDirs.pop()
       if (d) await rm(d, { recursive: true, force: true })
diff --git a/tests/session-cache.test.ts b/tests/session-cache.test.ts
index a8f4eeb4..b015322b 100644
--- a/tests/session-cache.test.ts
+++ b/tests/session-cache.test.ts
@@ -28,7 +28,6 @@ beforeEach(() => {
 })
 
 afterEach(async () => {
-  delete process.env['CODEBURN_CACHE_DIR']
   if (existsSync(TMP_DIR)) await rm(TMP_DIR, { recursive: true })
 })
 
@@ -171,11 +170,8 @@ describe('computeEnvFingerprint', () => {
 
   it('changes when env var changes', () => {
     const before = computeEnvFingerprint('claude')
-    const orig = process.env['CLAUDE_CONFIG_DIR']
     process.env['CLAUDE_CONFIG_DIR'] = '/tmp/different'
     const after = computeEnvFingerprint('claude')
-    if (orig === undefined) delete process.env['CLAUDE_CONFIG_DIR']
-    else process.env['CLAUDE_CONFIG_DIR'] = orig
     expect(before).not.toBe(after)
   })
 
diff --git a/tests/setup/env-isolation.ts b/tests/setup/env-isolation.ts
new file mode 100644
index 00000000..079165db
--- /dev/null
+++ b/tests/setup/env-isolation.ts
@@ -0,0 +1,110 @@
+// Vitest setup file: isolates every test from the developer's shell environment.
+//
+// codeburn discovers sessions through a long list of provider-specific env
+// vars (CLAUDE_CONFIG_DIR, CODEX_HOME, CRUSH_GLOBAL_DATA, …) and via HOME /
+// XDG_* / APPDATA / LOCALAPPDATA. Without this file, any value set in the
+// developer's shell (e.g. CLAUDE_CONFIG_DIRS=/Users/me/.claude:…) bleeds into
+// fixture-based tests: the parser reads the developer's REAL sessions instead
+// of the temp-dir fixture, producing nonsense totals and false failures that
+// pass on a clean CI runner.
+//
+// What this file does:
+//   1. Mints an empty sandbox temp dir once per worker.
+//   2. REDIRECTED vars (HOME / XDG_* / APPDATA / LOCALAPPDATA) point at the
+//      sandbox so any fallback to homedir() / platform defaults lands in an
+//      empty filesystem.
+//   3. CLEARED vars (every provider's explicit override) are deleted so a test
+//      that does NOT set one gets "unconfigured" rather than the dev's value.
+//   4. PRESERVED vars (PATH, COLUMNS, …) are snapshotted from the dev's shell
+//      and restored every test. We can't wipe them - Node uses PATH for spawn
+//      and module resolution, terminal code uses COLUMNS - but a test that
+//      mutates them shouldn't leak the change into the next test.
+//   5. Re-asserts the above before EVERY test (global beforeEach), so a test
+//      that mutates an env var doesn't leak its value into the next test.
+//      Tests can freely set process.env['HOME'] = customDir without saving the
+//      previous value - the next test gets a fresh sandbox baseline.
+//
+// CAVEAT: env vars set in a test file's beforeAll() get overwritten by this
+// file's beforeEach before each test runs. Use beforeEach (not beforeAll) when
+// the test body depends on a specific env var value.
+
+import { mkdtempSync } from 'fs'
+import { tmpdir } from 'os'
+import { join } from 'path'
+import { beforeEach } from 'vitest'
+
+const sandbox = mkdtempSync(join(tmpdir(), 'codeburn-test-env-'))
+
+const REDIRECTED = [
+  'HOME',
+  'XDG_CONFIG_HOME',
+  'XDG_DATA_HOME',
+  'XDG_CACHE_HOME',
+  'XDG_STATE_HOME',
+  'APPDATA',
+  'LOCALAPPDATA',
+] as const
+
+const CLEARED = [
+  // Provider session-discovery dirs
+  'CLAUDE_CONFIG_DIR',
+  'CLAUDE_CONFIG_DIRS',
+  'CODEX_HOME',
+  'CRUSH_GLOBAL_DATA',
+  'CODEBUFF_DATA_DIR',
+  'FACTORY_DIR',
+  'GOOSE_PATH_ROOT',
+  'GROK_HOME',
+  'KIRO_HOME',
+  'KIMI_SHARE_DIR',
+  'MUX_ROOT',
+  'QWEN_DATA_DIR',
+  'VIBE_HOME',
+  'WARP_DB_PATH',
+  'ZS_DATA_DIR',
+  // codeburn override dirs / paths
+  'CODEBURN_CACHE_DIR',
+  'CODEBURN_COPILOT_OTEL_DB',
+  'CODEBURN_COPILOT_SESSION_STATE_DIR',
+  'CODEBURN_COPILOT_WS_STORAGE_DIR',
+  'CODEBURN_DESKTOP_SESSIONS_DIR',
+  'CODEBURN_MUX_DIR',
+  'CODEBURN_ANTIGRAVITY_SETTINGS_PATH',
+  // codeburn behavior toggles (set by the dev to tweak local runs)
+  'CODEBURN_COPILOT_DISABLE_OTEL',
+  'CODEBURN_TZ',
+  'CODEBURN_VERBOSE',
+  'CODEBURN_CURSOR_MAX_BUBBLES',
+  'CODEBURN_FORCE_MACOS_MAJOR',
+  // Provider model/credential overrides
+  'KIMI_MODEL_NAME',
+  'AI_GATEWAY_API_KEY',
+  'VERCEL_OIDC_TOKEN',
+  // Read by detectBashBloat - a dev's real shell limit must not bleed in
+  'BASH_MAX_OUTPUT_LENGTH',
+] as const
+
+// Snapshotted from the dev's shell and restored every test. These can't be
+// wiped (Node needs PATH for spawn / module resolution, dashboard/table layout
+// reads COLUMNS) but a test that mutates them shouldn't leak.
+const PRESERVED = ['PATH', 'COLUMNS'] as const
+const preservedSnapshot = new Map<string, string | undefined>()
+for (const key of PRESERVED) preservedSnapshot.set(key, process.env[key])
+
+function applyIsolation(): void {
+  for (const key of REDIRECTED) process.env[key] = sandbox
+  for (const key of CLEARED) delete process.env[key]
+  for (const key of PRESERVED) {
+    const original = preservedSnapshot.get(key)
+    if (original === undefined) delete process.env[key]
+    else process.env[key] = original
+  }
+  // Pin the timezone so date grouping is deterministic regardless of the dev's
+  // shell TZ. Clearing it is not enough (Node falls back to the OS zone); a
+  // non-UTC TZ would otherwise shift day buckets versus a clean CI runner. A
+  // test that needs a specific zone can still set process.env.TZ in beforeEach.
+  process.env.TZ = 'UTC'
+}
+
+applyIsolation()
+beforeEach(applyIsolation)
diff --git a/tests/usage-aggregator.test.ts b/tests/usage-aggregator.test.ts
index 8a577dc1..3a47c7fb 100644
--- a/tests/usage-aggregator.test.ts
+++ b/tests/usage-aggregator.test.ts
@@ -1,39 +1,13 @@
-import { describe, expect, it, beforeAll, afterAll } from 'vitest'
-import { mkdtemp, rm } from 'node:fs/promises'
-import { tmpdir } from 'node:os'
-import { join } from 'node:path'
+import { describe, expect, it, beforeAll } from 'vitest'
 import { buildMenubarPayloadForRange } from '../src/usage-aggregator.js'
 import { getDateRange } from '../src/cli-date.js'
 import { loadPricing } from '../src/models.js'
 
 describe('buildMenubarPayloadForRange', () => {
-  // Point HOME / config at an empty temp dir so the payload is built from an
-  // empty dataset. This keeps the test deterministic and fast regardless of how
-  // much real session data the developer's machine has for "today" (parsing a
-  // heavy day previously pushed this past its timeout).
-  const saved: Record<string, string | undefined> = {}
-  let tmp: string
-
   beforeAll(async () => {
-    tmp = await mkdtemp(join(tmpdir(), 'codeburn-agg-test-'))
-    for (const key of ['HOME', 'CLAUDE_CONFIG_DIR', 'XDG_CONFIG_HOME', 'CODEBURN_CACHE_DIR']) {
-      saved[key] = process.env[key]
-    }
-    process.env['HOME'] = tmp
-    process.env['CLAUDE_CONFIG_DIR'] = join(tmp, '.claude')
-    process.env['XDG_CONFIG_HOME'] = join(tmp, '.config')
-    process.env['CODEBURN_CACHE_DIR'] = join(tmp, 'cache')
     await loadPricing()
   })
 
-  afterAll(async () => {
-    for (const [key, value] of Object.entries(saved)) {
-      if (value === undefined) delete process.env[key]
-      else process.env[key] = value
-    }
-    await rm(tmp, { recursive: true, force: true })
-  })
-
   it('returns a valid payload and skips optimize findings when optimize:false', async () => {
     const payload = await buildMenubarPayloadForRange(getDateRange('today'), { provider: 'all', optimize: false })
     expect(typeof payload.current.label).toBe('string')
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 00000000..b56c0158
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    // Runs once per worker before any test. Scrubs the developer's shell so
+    // session-discovery env vars (CLAUDE_CONFIG_DIRS, HOME, XDG_*, every
+    // provider-specific *_HOME) don't bleed real local data into fixtures.
+    setupFiles: ['./tests/setup/env-isolation.ts'],
+  },
+})