diff --git a/.github/workflows/runtime_build_and_test.yml b/.github/workflows/runtime_build_and_test.yml
index f2f64af45290..f08bfc450968 100644
--- a/.github/workflows/runtime_build_and_test.yml
+++ b/.github/workflows/runtime_build_and_test.yml
@@ -999,6 +999,8 @@ jobs:
     permissions:
       # We use github.token to download the build artifact from a previous runtime_build_and_test.yml run
       actions: read
+      # Used to resolve the merge-base commit through the compare API
+      contents: read
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -1020,21 +1022,40 @@ jobs:
         if: steps.node_modules.outputs.cache-hit != 'true'
       - run: yarn --cwd scripts/release install --frozen-lockfile
         if: steps.node_modules.outputs.cache-hit != 'true'
+      - name: Resolve base commit for comparison
+        # Compare against the commit this pull request diverged from, not the
+        # current tip of the base branch. Otherwise, once main moves ahead,
+        # unrelated commits show up as size changes on every pull request.
+        id: base
+        run: |
+          echo "sha=$(gh api repos/${{ github.repository }}/compare/${{ github.event.pull_request.base.sha }}...${{ github.event.pull_request.head.sha }} --jq .merge_base_commit.sha)" >> "$GITHUB_OUTPUT"
+        env:
+          GH_TOKEN: ${{ github.token }}
       - name: Download artifacts for base revision
+        id: download_base
         # The build could have been generated from a fork, so we must download the build without
         # any verification. This is safe since we only use this for sizebot calculation and the
         # unverified artifact is not used. Additionally this workflow runs in the pull_request
         # trigger so only restricted permissions are available.
         run: |
-          GH_TOKEN=${{ github.token }} scripts/release/download-experimental-build.js --commit=$(git rev-parse ${{ github.event.pull_request.base.sha }}) ${{ (github.event.pull_request.head.repo.full_name != github.repository && '--noVerify') || ''}}
-          mv ./build ./base-build
+          if GH_TOKEN=${{ github.token }} scripts/release/download-experimental-build.js --commit=${{ steps.base.outputs.sha }} ${{ (github.event.pull_request.head.repo.full_name != github.repository && '--noVerify') || ''}}; then
+            mv ./build ./base-build
+          else
+            # Rendered as a warning comment by render-comment.js on the
+            # default branch, which also fails the comment workflow's check.
+            jq --null-input --arg baseSha "${{ steps.base.outputs.sha }}" \
+              '{version: 1, status: "base-build-not-found", baseSha: $baseSha}' \
+              > sizebot-results.json
+            echo "missing=true" >> "$GITHUB_OUTPUT"
+          fi
       - name: Delete extraneous files
         # TODO: The `download-experimental-build` script copies the npm
         # packages into the `node_modules` directory. This is a historical
         # quirk of how the release script works. Let's pretend they
         # don't exist.
         run: rm -rf ./base-build/node_modules
-      - name: Display structure of base-build from origin/main
+      - name: Display structure of base-build
+        if: steps.download_base.outputs.missing != 'true'
         run: ls -R base-build
       - name: Ensure clean build directory
         run: rm -rf build
@@ -1056,6 +1077,9 @@ jobs:
         # posted by runtime_sizebot_comment.yml, which runs on the workflow_run
         # trigger because this job's token is read-only for pull requests from
         # forks and so cannot comment.
+        # Skipped when the base build is missing: the download step already
+        # wrote the not-found results.
+        if: steps.download_base.outputs.missing != 'true'
         run: node ./scripts/sizebot/compare-sizes.js
       - name: Archive sizebot results
         uses: actions/upload-artifact@v4
@@ -1063,3 +1087,8 @@ jobs:
           name: sizebot-results
           path: sizebot-results.json
           if-no-files-found: error
+      # Note: a missing base build does not fail this job on purpose. A failed
+      # job would fail the whole run, and render-comment.js only reads the
+      # results of successful runs, so the warning would never be rendered.
+      # Instead the comment workflow fails on the sizebot-problem.txt marker
+      # after posting the warning, same as for build configuration drift.
diff --git a/.github/workflows/runtime_sizebot_comment.yml b/.github/workflows/runtime_sizebot_comment.yml
index 5d6909650f41..263a2a1cbbb0 100644
--- a/.github/workflows/runtime_sizebot_comment.yml
+++ b/.github/workflows/runtime_sizebot_comment.yml
@@ -95,10 +95,11 @@ jobs:
             const {post} = require(`${process.env.GITHUB_WORKSPACE}/scripts/sizebot/pull-request-comment.js`);
             await post({github, context, core});
 
-      - name: Fail if the build configuration drifted
+      - name: Fail if sizebot reported a problem
         # The comment is posted first, so it explains the problem on the pull
-        # request itself. This step exists so the drift also shows up as a failed
-        # run rather than only in a comment.
+        # request itself. This step exists so the problem (build configuration
+        # drift, or no base build to compare against) also shows up as a
+        # failed run rather than only in a comment.
         if: ${{ steps.resolve.outputs.action == 'continue' && hashFiles('sizebot-problem.txt') != '' }}
         run: |
           cat sizebot-problem.txt
diff --git a/packages/react-devtools-extensions/utils.js b/packages/react-devtools-extensions/utils.js
index 9f02e98d1988..ce12465b35c3 100644
--- a/packages/react-devtools-extensions/utils.js
+++ b/packages/react-devtools-extensions/utils.js
@@ -6,39 +6,21 @@
  */
 
 const {execSync} = require('child_process');
-const {existsSync, readFileSync} = require('fs');
+const {readFileSync} = require('fs');
 const {resolve} = require('path');
 
 const GITHUB_URL = 'https://github.com/facebook/react';
+const GIT_COMMIT_HASH_LENGTH = 10;
+
+function shortenCommitHash(commitHash) {
+  return commitHash.trim().slice(0, GIT_COMMIT_HASH_LENGTH);
+}
 
 function getGitCommit() {
-  try {
-    return execSync('git show -s --no-show-signature --format=%h')
-      .toString()
-      .trim();
-  } catch (error) {
-    // Mozilla runs this command from a git archive.
-    // In that context, there is no Git context.
-    // Using the commit hash specified to download-experimental-build.js script as a fallback.
-
-    // Try to read from build/COMMIT_SHA file
-    const commitShaPath = resolve(__dirname, '..', '..', 'build', 'COMMIT_SHA');
-    if (!existsSync(commitShaPath)) {
-      throw new Error(
-        'Could not find build/COMMIT_SHA file. Did you run scripts/release/download-experimental-build.js script?',
-      );
-    }
-
-    try {
-      const commitHash = readFileSync(commitShaPath, 'utf8').trim();
-      // Return short hash (first 7 characters) to match abbreviated commit hash format
-      return commitHash.slice(0, 7);
-    } catch (readError) {
-      throw new Error(
-        `Failed to read build/COMMIT_SHA file: ${readError.message}`,
-      );
-    }
-  }
+  const commitHash = execSync(
+    'git show -s --no-show-signature --format=%H',
+  ).toString();
+  return shortenCommitHash(commitHash);
 }
 
 function getVersionString(packageVersion = null) {
diff --git a/scripts/sizebot/render-comment.js b/scripts/sizebot/render-comment.js
index 32eceef8d859..913634d08d86 100644
--- a/scripts/sizebot/render-comment.js
+++ b/scripts/sizebot/render-comment.js
@@ -26,7 +26,11 @@ const {existsSync, readFileSync, writeFileSync} = require('fs');
 // Results shapes this file knows how to read. `compare-sizes.js` on the pull
 // request branch may be older or newer than this list.
 const SUPPORTED_VERSIONS = new Set([1]);
-const SUPPORTED_STATUSES = new Set(['ok', 'base-artifacts-unavailable']);
+const SUPPORTED_STATUSES = new Set([
+  'ok',
+  'base-artifacts-unavailable',
+  'base-build-not-found',
+]);
 
 const CRITICAL_THRESHOLD = 0.02;
 const SIGNIFICANCE_THRESHOLD = 0.002;
@@ -161,6 +165,15 @@ function validateResults(raw) {
   if (raw.status === 'base-artifacts-unavailable') {
     return {ok: true, results: {status: raw.status}};
   }
+  if (raw.status === 'base-build-not-found') {
+    return {
+      ok: true,
+      results: {
+        status: raw.status,
+        baseSha: isSha(raw.baseSha) ? raw.baseSha : null,
+      },
+    };
+  }
   if (!isSha(raw.baseSha) || !isSha(raw.headSha)) {
     return {ok: false, reason: 'malformed'};
   }
@@ -337,6 +350,23 @@ function renderCompletedReport(context) {
     };
   }
 
+  if (validated.results.status === 'base-build-not-found') {
+    const {baseSha} = validated.results;
+    return {
+      markdown:
+        `No build was found for the base commit${
+          baseSha === null ? '' : ` (${baseSha})`
+        } that this pull request diverged from, so there is no size report. ` +
+        'The build for that commit may have failed, or its artifacts may be ' +
+        'older than the retention window. Rebase the pull request onto a ' +
+        'newer `main` to compare against a base commit that has a build.',
+      missingCriticalPaths: [],
+      problem: `No base build found for ${
+        baseSha === null ? 'the merge-base' : baseSha
+      }`,
+    };
+  }
+
   return renderTable(validated.results);
 }
 
@@ -370,6 +400,7 @@ function renderBody(context) {
   let reportHead;
   let report;
   let missingCriticalPaths = [];
+  let problem;
 
   if (context.action === 'requested') {
     // Only a comment that names the commit it describes holds real numbers. A
@@ -392,6 +423,7 @@ function renderBody(context) {
     const rendered = renderCompletedReport(context);
     report = rendered.markdown;
     missingCriticalPaths = rendered.missingCriticalPaths;
+    problem = rendered.problem;
   }
 
   if (missingCriticalPaths.length > 0) {
@@ -428,6 +460,7 @@ ${REPORT_END}
     assemble,
     reportHead,
     missingCriticalPaths,
+    problem,
   };
 }
 
@@ -454,7 +487,8 @@ function parseReportHead(body) {
 
 function main() {
   const context = JSON.parse(readFileSync(CONTEXT_PATH, 'utf8'));
-  const {body, report, assemble, missingCriticalPaths} = renderBody(context);
+  const {body, report, assemble, missingCriticalPaths, problem} =
+    renderBody(context);
 
   let comment = body;
   if (body.length > MAX_COMMENT_LENGTH) {
@@ -473,6 +507,9 @@ function main() {
       `Missing expected bundles:\n${missingCriticalPaths.join('\n')}\n`
     );
   }
+  if (problem !== undefined) {
+    writeFileSync(PROBLEM_PATH, problem + '\n');
+  }
 
   process.stdout.write(comment);
 }