diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f2d0bb5..1f8a4e6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -6,6 +6,13 @@ name: CI
 # Pull requests from forks (outside contributors) run the full gate, once the maintainer approves
 # the run, so the review sees it; so do Dependabot's (.github/dependabot.yml), which nobody tests
 # locally. The heavy job stays off pull requests.
+#
+# What runs when (docs/releasing.md, "What CI runs when"): the `plan` job picks a base, the last
+# `develop` commit whose CI run passed (a pull request: its merge base; nightly and manual runs:
+# none), and `cargo xtask affected` maps the change since it to crates. `cargo xtask ci --affected`
+# then lints and tests those crates and their dependents only; the heavy and oracle jobs run when
+# the change reaches the solvers, the oracle or everything (Cargo.toml/Cargo.lock/xtask/workflows).
+# Nightly and manual runs, and every release (release.yml), run all of it.
 on:
   pull_request:
     paths:
@@ -26,7 +33,7 @@ on:
       - 'docs/features.md'
       - 'docs/oracle.md'
   schedule:
-    # Nightly, on the default branch (develop): the heavy tests.
+    # Nightly, on the default branch (develop): the whole gate, the heavy tests and the oracle.
     - cron: '23 3 * * *'
     # Weekly: the oracle on macOS too, so results that differ by platform show up in CI (the
     # fixtures are often blessed on a Mac; #115).
@@ -37,7 +44,8 @@ permissions:
   contents: read
 
 # A newer merge cancels the run still going for the one before it, so a burst of merges is tested
-# once, at the tip. Nightly and manual runs are grouped apart and never cancelled by a merge.
+# once, at the tip; nothing is missed, since the tip's run tests everything since the last green
+# run. Nightly and manual runs are grouped apart and never cancelled by a merge.
 concurrency:
   group: ci-${{ github.ref }}-${{ github.event_name }}
   cancel-in-progress: ${{ github.event_name == 'push' || github.event_name == 'pull_request' }}
@@ -46,13 +54,75 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
+  plan:
+    name: Plan (affected crates)
+    if: github.event.schedule != '41 4 * * 0' && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository || github.event.pull_request.user.login == 'dependabot[bot]')
+    runs-on: ubuntu-24.04
+    timeout-minutes: 15
+    permissions:
+      contents: read
+      # To find the last green run on develop.
+      actions: read
+    outputs:
+      base: ${{ steps.base.outputs.base }}
+      everything: ${{ steps.affected.outputs.everything }}
+      solver: ${{ steps.affected.outputs.solver }}
+      wasm: ${{ steps.affected.outputs.wasm }}
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+        with:
+          key: plan
+          save-if: ${{ github.ref == 'refs/heads/develop' }}
+      - name: Pick the base
+        id: base
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          EVENT: ${{ github.event_name }}
+          PR_BASE: ${{ github.base_ref }}
+        run: |
+          base=""
+          case "$EVENT" in
+            push)
+              # The newest develop commit whose push run passed and that this commit contains.
+              # Runs that were cancelled or failed are skipped, so their changes are tested here.
+              for sha in $(gh api "repos/$GITHUB_REPOSITORY/actions/workflows/ci.yml/runs?branch=develop&event=push&status=success&per_page=100" --jq '.workflow_runs[].head_sha'); do
+                if [ "$sha" != "$GITHUB_SHA" ] && git merge-base --is-ancestor "$sha" HEAD; then
+                  base="$sha"
+                  break
+                fi
+              done
+              ;;
+            pull_request)
+              base="$(git merge-base "origin/$PR_BASE" HEAD || true)"
+              ;;
+          esac
+          # Nightly and manual runs, or no green run to start from: everything.
+          echo "base=$base" >>"$GITHUB_OUTPUT"
+          echo "### Base: \`${base:-none (everything)}\`" >>"$GITHUB_STEP_SUMMARY"
+      - name: cargo xtask affected
+        id: affected
+        env:
+          BASE: ${{ steps.base.outputs.base }}
+        run: |
+          if [ -n "$BASE" ]; then
+            cargo xtask affected --base "$BASE" --github-output | tee -a "$GITHUB_STEP_SUMMARY"
+          else
+            printf 'everything=true\nsolver=true\nwasm=true\n' >>"$GITHUB_OUTPUT"
+          fi
+
   ci:
     name: cargo xtask ci
-    if: github.event.schedule != '41 4 * * 0' && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository || github.event.pull_request.user.login == 'dependabot[bot]')
+    needs: plan
     runs-on: ubuntu-24.04
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt, clippy
@@ -60,12 +130,18 @@ jobs:
       - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           save-if: ${{ github.ref == 'refs/heads/develop' }}
-      - run: cargo xtask ci
+      - name: cargo xtask ci (the affected crates, or everything)
+        env:
+          BASE: ${{ needs.plan.outputs.base }}
+        run: |
+          if [ -n "$BASE" ]; then cargo xtask ci --affected "$BASE"; else cargo xtask ci; fi
 
   heavy:
-    # The heavy verification and corpus tests (#[ignore]d) and the doc tests.
+    # The heavy verification and corpus tests (#[ignore]d) and the doc tests, when the change
+    # reaches them.
     name: cargo xtask test --heavy
-    if: github.event.schedule != '41 4 * * 0' && github.event_name != 'pull_request'
+    needs: plan
+    if: github.event_name != 'pull_request' && (needs.plan.outputs.solver == 'true' || needs.plan.outputs.everything == 'true')
     runs-on: ubuntu-24.04
     timeout-minutes: 60
     steps:
@@ -79,9 +155,11 @@ jobs:
   oracle:
     # Verification and validation (docs/verification.md): every case against its reference and
     # its fixture, and docs/oracle.md must be the table the cases produce. Cases marked `slow`
-    # run here too while the whole set stays within a few minutes; `--quick` skips them.
+    # run here too while the whole set stays within a few minutes; `--quick` skips them. It runs
+    # when the change reaches the solvers or the oracle's package.
     name: cargo xtask oracle
-    if: github.event.schedule != '41 4 * * 0' && (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.repository || github.event.pull_request.user.login == 'dependabot[bot]')
+    needs: plan
+    if: needs.plan.outputs.solver == 'true' || needs.plan.outputs.everything == 'true'
     runs-on: ubuntu-24.04
     timeout-minutes: 45
     steps: