diff --git a/.github/workflows/fork-ci.yml b/.github/workflows/fork-ci.yml
index 1b8e0a0e1601..f21fe8921a0b 100644
--- a/.github/workflows/fork-ci.yml
+++ b/.github/workflows/fork-ci.yml
@@ -6,6 +6,10 @@ name: fork-ci
 on:
   pull_request:
     branches: [fork/main]
+  # The merge queue (WI-1886, docs/ci.md). The queue tests the tree fork/main
+  # will become, so a PR never has to be rebased onto a sibling's merge.
+  merge_group:
+    types: [checks_requested]
   workflow_dispatch:
     inputs:
       # Comma-separated package names to check instead of the diff. Used to
@@ -18,6 +22,12 @@ on:
 permissions:
   contents: read
 
+# A force-push cancels the superseded run. Each queue entry has its own
+# gh-readonly-queue/... ref, so queue entries never cancel each other.
+concurrency:
+  group: fork-ci-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   check:
     name: fmt, clippy, test
@@ -106,11 +116,27 @@ jobs:
         id: crates
         env:
           FORCED_PACKAGES: ${{ inputs.packages }}
+          QUEUE_BASE: ${{ github.event.merge_group.base_sha }}
         shell: bash
         run: |
           set -euo pipefail
-          base="$(git merge-base HEAD origin/fork/main 2>/dev/null || git merge-base HEAD fork/main)"
+          # In the queue, diff the whole group against the queue's base: every
+          # PR ahead of this one in the group is part of the tree under test.
+          if [[ -n "$QUEUE_BASE" ]]; then
+            base="$QUEUE_BASE"
+          else
+            base="$(git merge-base HEAD origin/fork/main 2>/dev/null || git merge-base HEAD fork/main)"
+          fi
           mapfile -t files < <(git diff --name-only "$base"...HEAD)
+          # Path scope: a diff with nothing under codex-rs/ or .github/ (docs,
+          # FORK.md, sdk/) skips the Rust steps. The job still reports, so a
+          # required check is satisfied.
+          rust=false
+          for f in "${files[@]}"; do
+            case "$f" in codex-rs/*|.github/*) rust=true ;; esac
+          done
+          [[ -n "$FORCED_PACKAGES" ]] && rust=true
+          echo "rust=$rust" >> "$GITHUB_OUTPUT"
           if [[ -n "$FORCED_PACKAGES" ]]; then
             mapfile -t packages < <(echo "$FORCED_PACKAGES" | tr ',' '\n')
             echo "forced: ${packages[*]}"
@@ -194,6 +220,7 @@ jobs:
       # the Codex-built artifacts from the upstream release and verifies them
       # against the in-tree checksum manifest, the same as fork-release.
       - name: Configure rusty_v8 artifact overrides and verify checksums
+        if: steps.crates.outputs.rust == 'true'
         uses: ./.github/actions/setup-rusty-v8
         with:
           target: x86_64-unknown-linux-gnu
@@ -203,9 +230,11 @@ jobs:
       # apt package satisfies it. Upstream never builds it with cargo either; it
       # builds //codex-rs/voice-host with Bazel against a bundled GStreamer.
       - name: cargo check
+        if: steps.crates.outputs.rust == 'true'
         run: cargo check --workspace --all-targets --exclude codex-voice-host
 
       - name: cargo fmt
+        if: steps.crates.outputs.rust == 'true'
         shell: bash
         run: |
           set -euo pipefail
@@ -221,10 +250,13 @@ jobs:
       # PR changed fails the build. Compile errors are fatal regardless, which
       # `cargo check` above already enforces.
       - name: cargo clippy
+        if: steps.crates.outputs.rust == 'true'
         shell: bash
+        env:
+          QUEUE_BASE: ${{ github.event.merge_group.base_sha }}
         run: |
           set -euo pipefail
-          base="$(git merge-base HEAD origin/fork/main 2>/dev/null || git merge-base HEAD fork/main)"
+          base="${QUEUE_BASE:-$(git merge-base HEAD origin/fork/main 2>/dev/null || git merge-base HEAD fork/main)}"
           # One changed line range per hunk, as "file start count" with the
           # codex-rs/ prefix stripped. A count of 0 is a pure deletion.
           git diff -U0 "$base"...HEAD -- codex-rs \
@@ -274,6 +306,7 @@ jobs:
           fi
 
       - name: cargo test
+        if: steps.crates.outputs.rust == 'true'
         shell: bash
         # codex-core's tests overflow the default thread stack (fatal runtime
         # error: stack overflow). Upstream sets the same 8 MiB floor on its
diff --git a/FORK.md b/FORK.md
index ef9e553a3af2..e3b61347b4b8 100644
--- a/FORK.md
+++ b/FORK.md
@@ -94,3 +94,8 @@ runs GitHub-hosted. `fork-upstream-drift` is the check that the recorded base ta
 "Upstream fixes taken through" marker, is still the newest stable tag
 (WI-1125). It has no schedule: run it on `workflow_dispatch`, or from the
 sync runbook.
+
+`fork-ci` supports a merge queue (`merge_group`), skips its Rust steps when a
+PR touches nothing under `codex-rs/` or `.github/`, and cancels superseded runs
+per ref. How it runs on PRs, in the queue, and around upstream syncs:
+`docs/ci.md`.
diff --git a/docs/ci.md b/docs/ci.md
new file mode 100644
index 000000000000..c8f352e36244
--- /dev/null
+++ b/docs/ci.md
@@ -0,0 +1,61 @@
+# Fork CI: pull requests, the merge queue, and `fork/main`
+
+Fork-only. Upstream's CI is disabled on this fork (see `FORK.md`,
+"Workflows"); `fork-ci` is the one check that runs on pull requests into
+`fork/main`. This file covers how it is meant to run with a merge queue
+(WI-1886).
+
+## What runs where
+
+| Event | `fork-ci` |
+|---|---|
+| `pull_request` into `fork/main` | Rust steps only when the diff touches `codex-rs/` or `.github/`; fmt/clippy/nextest only for the packages the diff changes |
+| `merge_group` (the queue) | Rust steps for the packages the **whole queued group** changes, diffed against the queue's base (`merge_group.base_sha`), plus `cargo check --workspace` |
+| `workflow_dispatch` | the packages named in `packages`, or the diff |
+
+A pull request that touches only docs, `FORK.md` or `sdk/` still reports
+`fmt, clippy, test` as passed, with the Rust steps skipped, so a required
+check never waits on it.
+
+### Why the queue is package-scoped too
+
+Vogt's queue runs everything. Here, "everything" is a workspace-wide nextest
+over hundreds of upstream crates, at roughly an hour on the 30-vCPU runner, and
+upstream lint and test debt makes a full run fail regardless of the PR. The
+queue keeps the package selection but widens the diff to the whole group, and
+`cargo check --workspace --all-targets` still compiles every crate, so a
+cross-crate API break fails the queue.
+
+### Concurrency
+
+`fork-ci` cancels in progress per ref. A force-push to a PR cancels the
+superseded run. Each queue entry has its own `gh-readonly-queue/fork/main/...`
+ref, so entries never cancel each other.
+
+### Runners
+
+Same-repo PRs, queue entries and `workflow_dispatch` run on the self-hosted GCP
+Spot `rust-heavy` runner. A PR from another fork stays on GitHub-hosted
+`ubuntu-24.04`, so untrusted code never reaches a persistent runner. A
+`merge_group` event is always trusted: only a maintainer can enqueue.
+
+## Landing
+
+`land-pr` (workspace `.agent-tools/bin/land-pr`) detects a merge queue from
+the base branch's rulesets. It enqueues with `gh pr merge --auto` and follows
+the entry's `merge_group` run. Don't rebase a queued PR by hand.
+
+## Upstream syncs
+
+A rebase of `fork/main` onto a new upstream tag is a force-push
+(`docs/fork-upstream-sync.md`), not a queued merge. Drain or pause the queue
+before you push the rebase. The ruleset below bypasses only the repository
+admin role, which is what lets the `--force-with-lease` push through.
+
+## Turning the queue on (operator)
+
+`fork/main` has no branch protection and no ruleset today, so `fork-ci` is not
+required and the queue is off. The workflow supports the queue without
+needing it. The exact ruleset is in the WI-1886 PR description: one branch
+ruleset on `fork/main` that requires `fmt, clippy, test` and adds a
+`merge_queue` rule.