diff --git a/.github/workflows/main-ci.yml b/.github/workflows/main-ci.yml
index 4f00a9101c..a19cb50abc 100644
--- a/.github/workflows/main-ci.yml
+++ b/.github/workflows/main-ci.yml
@@ -63,8 +63,9 @@ jobs:
     if: needs.check-changes.outputs.web-changed == 'true'
     uses: ./.github/workflows/web-tests.yml
     with:
-      base_sha: ${{ github.event_name == 'pull_request' && github.event.pull_request.base.sha || github.event.before }}
-      head_sha: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+      base_sha: ${{ github.event.before || github.event.pull_request.base.sha }}
+      diff_range_mode: ${{ github.event.before && 'exact' || 'merge-base' }}
+      head_sha: ${{ github.event.after || github.event.pull_request.head.sha || github.sha }}
 
   style-check:
     name: Style Check
diff --git a/.github/workflows/web-tests.yml b/.github/workflows/web-tests.yml
index 443a22ea93..fb85d6e175 100644
--- a/.github/workflows/web-tests.yml
+++ b/.github/workflows/web-tests.yml
@@ -6,6 +6,9 @@ on:
       base_sha:
         required: false
         type: string
+      diff_range_mode:
+        required: false
+        type: string
       head_sha:
         required: false
         type: string
@@ -89,6 +92,7 @@ jobs:
       - name: Check app/components diff coverage
         env:
           BASE_SHA: ${{ inputs.base_sha }}
+          DIFF_RANGE_MODE: ${{ inputs.diff_range_mode }}
           HEAD_SHA: ${{ inputs.head_sha }}
         run: node ./scripts/check-components-diff-coverage.mjs
 
diff --git a/web/__tests__/check-components-diff-coverage.test.ts b/web/__tests__/check-components-diff-coverage.test.ts
index 9ce8b30ceb..79f6c8fd26 100644
--- a/web/__tests__/check-components-diff-coverage.test.ts
+++ b/web/__tests__/check-components-diff-coverage.test.ts
@@ -1,4 +1,5 @@
 import {
+  buildGitDiffRevisionArgs,
   getChangedBranchCoverage,
   getChangedStatementCoverage,
   getIgnoredChangedLinesFromSource,
@@ -7,6 +8,11 @@ import {
 } from '../scripts/check-components-diff-coverage-lib.mjs'
 
 describe('check-components-diff-coverage helpers', () => {
+  it('should build exact and merge-base git diff revision args', () => {
+    expect(buildGitDiffRevisionArgs('base-sha', 'head-sha', 'exact')).toEqual(['base-sha', 'head-sha'])
+    expect(buildGitDiffRevisionArgs('base-sha', 'head-sha')).toEqual(['base-sha...head-sha'])
+  })
+
   it('should parse changed line maps from unified diffs', () => {
     const diff = [
       'diff --git a/web/app/components/share/a.ts b/web/app/components/share/a.ts',
diff --git a/web/scripts/check-components-diff-coverage-lib.mjs b/web/scripts/check-components-diff-coverage-lib.mjs
index 56318f38a2..48abc14613 100644
--- a/web/scripts/check-components-diff-coverage-lib.mjs
+++ b/web/scripts/check-components-diff-coverage-lib.mjs
@@ -3,6 +3,12 @@ import path from 'node:path'
 
 const DIFF_COVERAGE_IGNORE_LINE_TOKEN = 'diff-coverage-ignore-line:'
 
+export function buildGitDiffRevisionArgs(base, head, mode = 'merge-base') {
+  return mode === 'exact'
+    ? [base, head]
+    : [`${base}...${head}`]
+}
+
 export function parseChangedLineMap(diff, isTrackedComponentSourceFile) {
   const lineMap = new Map()
   let currentFile = null
diff --git a/web/scripts/check-components-diff-coverage.mjs b/web/scripts/check-components-diff-coverage.mjs
index 6a861c4f52..8cf3c96223 100644
--- a/web/scripts/check-components-diff-coverage.mjs
+++ b/web/scripts/check-components-diff-coverage.mjs
@@ -2,6 +2,7 @@ import { execFileSync } from 'node:child_process'
 import fs from 'node:fs'
 import path from 'node:path'
 import {
+  buildGitDiffRevisionArgs,
   getChangedBranchCoverage,
   getChangedStatementCoverage,
   getIgnoredChangedLinesFromFile,
@@ -24,6 +25,7 @@ const APP_COMPONENTS_COVERAGE_PREFIX = 'app/components/'
 const SHARED_TEST_PREFIX = 'web/__tests__/'
 const STRICT_TEST_FILE_TOUCH = process.env.STRICT_COMPONENT_TEST_TOUCH === 'true'
 const EXCLUDED_MODULES_LABEL = [...EXCLUDED_COMPONENT_MODULES].sort().join(', ')
+const DIFF_RANGE_MODE = process.env.DIFF_RANGE_MODE === 'exact' ? 'exact' : 'merge-base'
 
 const repoRoot = repoRootFromCwd()
 const webRoot = path.join(repoRoot, 'web')
@@ -216,6 +218,7 @@ function buildSummary({
     '### app/components Diff Coverage',
     '',
     `Compared \`${baseSha.slice(0, 12)}\` -> \`${headSha.slice(0, 12)}\``,
+    `Diff range mode: \`${DIFF_RANGE_MODE}\``,
     '',
     `Excluded modules: \`${EXCLUDED_MODULES_LABEL}\``,
     `Excluded file kinds: \`${COMPONENT_COVERAGE_EXCLUDE_LABEL}\``,
@@ -365,7 +368,7 @@ function buildSkipSummary(changedExcludedSourceFiles) {
 }
 
 function getChangedFiles(base, head) {
-  const output = execGit(['diff', '--name-only', '--diff-filter=ACMR', `${base}...${head}`, '--', 'web/app/components', 'web/__tests__'])
+  const output = execGit(['diff', '--name-only', '--diff-filter=ACMR', ...buildGitDiffRevisionArgs(base, head, DIFF_RANGE_MODE), '--', 'web/app/components', 'web/__tests__'])
   return output
     .split('\n')
     .map(line => line.trim())
@@ -373,7 +376,7 @@ function getChangedFiles(base, head) {
 }
 
 function getChangedLineMap(base, head) {
-  const diff = execGit(['diff', '--unified=0', '--no-color', '--diff-filter=ACMR', `${base}...${head}`, '--', 'web/app/components'])
+  const diff = execGit(['diff', '--unified=0', '--no-color', '--diff-filter=ACMR', ...buildGitDiffRevisionArgs(base, head, DIFF_RANGE_MODE), '--', 'web/app/components'])
   return parseChangedLineMap(diff, isTrackedComponentSourceFile)
 }