diff --git a/.github/scripts/windows-io-trace.ps1 b/.github/scripts/windows-io-trace.ps1
new file mode 100644
index 0000000000..94a11f15c7
--- /dev/null
+++ b/.github/scripts/windows-io-trace.ps1
@@ -0,0 +1,41 @@
+param(
+    [Parameter(Mandatory = $true)]
+    [ValidateSet("mark", "transition", "stop")]
+    [string]$Action,
+
+    [string]$Label,
+
+    [string]$From,
+
+    [string]$To
+)
+
+$ErrorActionPreference = "Stop"
+
+function Invoke-Wpr {
+    param([string[]]$Arguments)
+
+    & wpr @Arguments
+    if ($LASTEXITCODE -ne 0) {
+        throw "wpr $($Arguments -join ' ') failed with exit code $LASTEXITCODE"
+    }
+}
+
+switch ($Action) {
+    "mark" {
+        Invoke-Wpr @("-marker", $Label)
+    }
+    "transition" {
+        Invoke-Wpr @("-marker", "step:end:$From")
+        Invoke-Wpr @("-marker", "step:start:$To")
+    }
+    "stop" {
+        Invoke-Wpr @("-marker", "step:end:$Label")
+
+        $traceRoot = Join-Path $env:RUNNER_TEMP "codex-io-trace"
+        New-Item -ItemType Directory -Force -Path $traceRoot | Out-Null
+        $tracePath = Join-Path $traceRoot "windows-io-$($env:GITHUB_JOB)-$($env:GITHUB_RUN_ID)-$($env:GITHUB_RUN_ATTEMPT).etl"
+        Invoke-Wpr @("-stop", $tracePath)
+        Write-Output "saved Windows I/O trace to $tracePath"
+    }
+}
diff --git a/.github/workflows/bazel.yml b/.github/workflows/bazel.yml
index 3e48b741b7..91f2d3a3bb 100644
--- a/.github/workflows/bazel.yml
+++ b/.github/workflows/bazel.yml
@@ -152,16 +152,41 @@ jobs:
       deployment: false
 
     steps:
+      - name: Start Windows I/O trace
+        shell: pwsh
+        run: |
+          $traceRoot = Join-Path $env:RUNNER_TEMP "codex-io-trace"
+          New-Item -ItemType Directory -Force -Path $traceRoot | Out-Null
+          $null = & wpr -cancel 2>&1
+          & wpr -start FileIO -start DiskIO -filemode -recordtempto $traceRoot
+          if ($LASTEXITCODE -ne 0) {
+            throw "failed to start Windows I/O trace"
+          }
+          & wpr -marker "step:start:checkout"
+          if ($LASTEXITCODE -ne 0) {
+            throw "failed to mark checkout start"
+          }
+
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           persist-credentials: false
 
+      - name: Mark checkout end and wrapper test start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From checkout -To test-buildbuddy-wrapper
+
       - name: Test BuildBuddy Bazel wrapper
         if: matrix.shard == 1
         shell: pwsh
         run: python .github/scripts/test_run_bazel_with_buildbuddy.py
 
+      - name: Mark wrapper test end and Bazel setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From test-buildbuddy-wrapper -To prepare-bazel-ci
+
       - name: Prepare Bazel CI
         id: prepare_bazel
         uses: ./.github/actions/prepare-bazel-ci
@@ -172,6 +197,11 @@ jobs:
           # the PR-blocking critical path after the useful test work is done.
           cache-scope: bazel-test
 
+      - name: Mark Bazel setup end and test shard start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From prepare-bazel-ci -To bazel-test-shard
+
       - name: bazel test shard
         env:
           BAZEL_TEST_SHARD: ${{ matrix.shard }}
@@ -211,6 +241,10 @@ jobs:
             --skip_incompatible_explicit_targets
             --test_tag_filters=-argument-comment-lint
             --test_verbose_timeout_warnings
+            "--action_env=CODEX_IO_TRACE_CACHE_BUST=${GITHUB_RUN_ID}-${GITHUB_RUN_ATTEMPT}-${BAZEL_TEST_SHARD}"
+            "--host_action_env=CODEX_IO_TRACE_CACHE_BUST=${GITHUB_RUN_ID}-${GITHUB_RUN_ATTEMPT}-${BAZEL_TEST_SHARD}"
+            "--test_env=CODEX_IO_TRACE_CACHE_BUST=${GITHUB_RUN_ID}-${GITHUB_RUN_ATTEMPT}-${BAZEL_TEST_SHARD}"
+            --nocache_test_results
             --build_metadata=COMMIT_SHA=${GITHUB_SHA}
             --build_metadata=TAG_windows_test_shard=${BAZEL_TEST_SHARD}
           )
@@ -225,6 +259,11 @@ jobs:
             -- \
             "${selected_targets[@]}"
 
+      - name: Mark test shard end and execution-log upload start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From bazel-test-shard -To upload-bazel-execution-logs
+
       - name: Upload Bazel execution logs
         if: always() && !cancelled()
         continue-on-error: true
@@ -234,10 +273,29 @@ jobs:
           path: ${{ runner.temp }}/bazel-execution-logs
           if-no-files-found: ignore
 
+      - name: Mark execution-log upload end and clean-worktree check start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From upload-bazel-execution-logs -To check-clean-worktree
+
       - name: Check for a clean worktree
         if: always() && !cancelled()
         uses: ./.github/actions/check-clean-worktree
 
+      - name: Stop Windows I/O trace
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action stop -Label check-clean-worktree
+
+      - name: Upload Windows I/O trace
+        if: always() && !cancelled()
+        continue-on-error: true
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
+        with:
+          name: windows-io-bazel-shard-${{ matrix.shard }}
+          path: ${{ runner.temp }}/codex-io-trace
+          if-no-files-found: error
+
   test-windows:
     # Preserve the existing required-check surface while the real work happens
     # in the sharded Windows jobs above.
diff --git a/.github/workflows/v8-canary.yml b/.github/workflows/v8-canary.yml
index d5f0133528..2a05aabe7a 100644
--- a/.github/workflows/v8-canary.yml
+++ b/.github/workflows/v8-canary.yml
@@ -320,14 +320,44 @@ jobs:
             target: aarch64-pc-windows-msvc
 
     steps:
+      - name: Start Windows I/O trace
+        shell: pwsh
+        run: |
+          $traceRoot = Join-Path $env:RUNNER_TEMP "codex-io-trace"
+          New-Item -ItemType Directory -Force -Path $traceRoot | Out-Null
+          $null = & wpr -cancel 2>&1
+          & wpr -start FileIO -start DiskIO -filemode -recordtempto $traceRoot
+          if ($LASTEXITCODE -ne 0) {
+            throw "failed to start Windows I/O trace"
+          }
+          & wpr -marker "step:start:checkout-codex"
+          if ($LASTEXITCODE -ne 0) {
+            throw "failed to mark Codex checkout start"
+          }
+
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
+      - name: Mark Codex checkout end and CI setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From checkout-codex -To setup-ci
+
       - uses: ./.github/actions/setup-ci
 
+      - name: Mark CI setup end and git configuration start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-ci -To configure-git
+
       - name: Configure git for upstream checkout
         shell: bash
         run: git config --global core.symlinks true
 
+      - name: Mark git configuration end and upstream checkout start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From configure-git -To checkout-upstream
+
       - name: Check out upstream rusty_v8
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
@@ -336,18 +366,33 @@ jobs:
           path: upstream-rusty-v8
           submodules: recursive
 
+      - name: Mark upstream checkout end and Python setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From checkout-upstream -To setup-python
+
       - name: Set up Python
         uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6
         with:
           python-version: "3.11"
           architecture: x64
 
+      - name: Mark Python setup end and Codex Rust setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-python -To setup-codex-rust
+
       - name: Set up Codex Rust toolchain for Cargo smoke
         uses: dtolnay/rust-toolchain@e081816240890017053eacbb1bdf337761dc5582 # 1.95.0
         with:
           toolchain: "1.95.0"
           targets: ${{ matrix.target }}
 
+      - name: Mark Codex Rust setup end and rusty_v8 Rust setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-codex-rust -To setup-rusty-v8-rust
+
       - name: Install rusty_v8 Rust toolchain
         env:
           TARGET: ${{ matrix.target }}
@@ -357,20 +402,33 @@ jobs:
           rustup toolchain install 1.91.0 --profile minimal --no-self-update
           rustup target add --toolchain 1.91.0 "${TARGET}"
 
+      - name: Mark rusty_v8 Rust setup end and submodule-status write start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-rusty-v8-rust -To write-submodule-status
+
       - name: Write upstream submodule status
         shell: bash
         working-directory: upstream-rusty-v8
         run: git submodule status --recursive > git_submodule_status.txt
 
+      - name: Mark submodule-status write end and source-cache restore start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From write-submodule-status -To restore-source-cache
+
       - name: Restore upstream source-build cache
-        uses: actions/cache@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5
+        uses: actions/cache/restore@668228422ae6a00e4ad889ee87cd7109ec5666a7 # v5
         with:
           path: |
             ${{ env.CARGO_TARGET_DIR }}/sccache
             ${{ env.CARGO_TARGET_DIR }}/${{ matrix.target }}/release/gn_out
-          key: rusty-v8-source-${{ matrix.target }}-${{ matrix.runner }}-sandbox-${{ hashFiles('upstream-rusty-v8/Cargo.lock', 'upstream-rusty-v8/build.rs', 'upstream-rusty-v8/git_submodule_status.txt') }}
-          restore-keys: |
-            rusty-v8-source-${{ matrix.target }}-${{ matrix.runner }}-sandbox-
+          key: rusty-v8-source-io-trace-${{ matrix.target }}-${{ github.run_id }}-${{ github.run_attempt }}
+
+      - name: Mark source-cache restore end and sccache setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From restore-source-cache -To setup-sccache
 
       - name: Install and start sccache
         shell: pwsh
@@ -389,12 +447,35 @@ jobs:
           . $basename/sccache --start-server
           echo "$(pwd)/$basename" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
 
+      - name: Mark sccache setup end and Chromium clang setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-sccache -To setup-chromium-clang
+
       - name: Install Chromium clang for ARM64 MSVC cross build
         if: matrix.target == 'aarch64-pc-windows-msvc'
         shell: bash
         working-directory: upstream-rusty-v8
         run: python3 tools/clang/scripts/update.py
 
+      - name: Mark Chromium clang setup end and cold-graph setup start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From setup-chromium-clang -To force-cold-source-graph
+
+      - name: Force cold source-build graph
+        shell: pwsh
+        env:
+          TARGET: ${{ matrix.target }}
+        run: |
+          Remove-Item -Recurse -Force -ErrorAction SilentlyContinue (Join-Path $env:CARGO_TARGET_DIR "sccache")
+          Remove-Item -Recurse -Force -ErrorAction SilentlyContinue (Join-Path $env:CARGO_TARGET_DIR "$env:TARGET/release/gn_out")
+
+      - name: Mark cold-graph setup end and upstream build start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From force-cold-source-graph -To build-upstream-rusty-v8
+
       - name: Build upstream rusty_v8 sandbox release pair
         env:
           SCCACHE_IDLE_TIMEOUT: 0
@@ -404,6 +485,11 @@ jobs:
         working-directory: upstream-rusty-v8
         run: cargo +1.91.0 build --locked --release --target "${TARGET}" --features v8_enable_sandbox
 
+      - name: Mark upstream build end and staging start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From build-upstream-rusty-v8 -To stage-upstream-pair
+
       - name: Stage upstream sandbox release pair
         env:
           TARGET: ${{ matrix.target }}
@@ -416,6 +502,11 @@ jobs:
             --output-dir "dist/${TARGET}" \
             --sandbox
 
+      - name: Mark staging end and Cargo smoke link start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From stage-upstream-pair -To smoke-link-cargo
+
       - name: Smoke link staged artifact with Cargo
         env:
           TARGET: ${{ matrix.target }}
@@ -437,12 +528,36 @@ jobs:
             cargo +1.95.0 test -p codex-v8-poc --target "${TARGET}" --features sandbox --no-run
           )
 
+      - name: Mark Cargo smoke link end and staged-artifact upload start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From smoke-link-cargo -To upload-staged-artifacts
+
       - name: Upload staged artifacts
         uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7
         with:
           name: v8-canary-${{ needs.metadata.outputs.v8_version }}-ptrcomp-sandbox-${{ matrix.target }}
           path: dist/${{ matrix.target }}/*
 
+      - name: Mark staged-artifact upload end and clean-worktree check start
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action transition -From upload-staged-artifacts -To check-clean-worktree
+
       - name: Check for a clean worktree
         if: always() && !cancelled()
         uses: ./.github/actions/check-clean-worktree
+
+      - name: Stop Windows I/O trace
+        if: always() && !cancelled()
+        shell: pwsh
+        run: ./.github/scripts/windows-io-trace.ps1 -Action stop -Label check-clean-worktree
+
+      - name: Upload Windows I/O trace
+        if: always() && !cancelled()
+        continue-on-error: true
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7
+        with:
+          name: windows-io-v8-source-${{ matrix.target }}
+          path: ${{ runner.temp }}/codex-io-trace
+          if-no-files-found: error