@@ -184,7 +184,7 @@ outputs:
     description: 'Which state the review head analysis grew from: pr-chain or base.'
     value: ${{ steps.review_analyze.outputs.seed_source }}
   base_source:
-    description: 'How the review base was obtained: saved, committed, or computed.'
+    description: 'How the review base was obtained: saved, ancestor, committed, or computed.'
     value: ${{ steps.review_analyze.outputs.base_source }}
   merge_base_sha:
     description: 'Merge base used as the review comparison baseline.'
@@ -451,6 +451,13 @@ runs:
         CHECKOUT_DIR: ${{ github.workspace }}/.codeboarding-target
         STAGE_DIR: ${{ runner.temp }}/cb-state/${{ github.action }}/out
         FORCE_FULL: ${{ inputs.force_full }}
+        CFG_HASH: ${{ steps.state.outputs.cfg_hash }}
+        # Lets a branch without a usable committed baseline catch up from a saved analysis.
+        ANCESTOR_LOOKUP: ${{ github.server_url == 'https://github.com' && steps.state.outputs.cfg_hash != '' }}
+        GIT_TOKEN: ${{ inputs.github_token }}
+        REPOSITORY: ${{ github.repository }}
+        GH_HOST: ${{ github.server_url }}
+        GITHUB_SERVER_URL: ${{ github.server_url }}
         DEPTH_CAP: ${{ inputs.depth_cap }}
         MODEL: ${{ inputs.model }}
         AGENT_MODEL_INPUT: ${{ inputs.agent_model }}
@@ -539,6 +546,8 @@ runs:
         BASE_FETCH_SECONDS: ${{ steps.fetch_base.outputs.seconds }}
         WARMSTART_DIR: ${{ runner.temp }}/cb-state/${{ github.action }}/warmstart
         RENEW_BASE: ${{ steps.fetch_base.outputs.renew }}
+        # Lets a branch without a usable committed baseline catch up from a saved analysis.
+        ANCESTOR_LOOKUP: ${{ github.server_url == 'https://github.com' && steps.state.outputs.cfg_hash != '' }}
         # For rewriting the progress comment while a base is built from scratch.
         PROGRESS_HEADER: ${{ steps.guard.outputs.comment_id }}
         BASE_REF: ${{ steps.guard.outputs.base_ref }}
 
@@ -72,8 +72,8 @@ never fires.
 | `base_sha` | string | the base branch tip when the event fired — *not* what was compared against |
 | `kind` | string | always `review`, so a reader can tell this artifact from a base or warm-start bundle |
 | `analysed_files_changed` | string | analysed files whose content hash differs between base and head; `unknown` when the analyses cannot say |
-| `base_source` | string | how this run obtained the base graph: `saved` (the artifact for the merge base), `committed` (`.codeboarding/` committed at the merge base, caught up), or `computed` (full analysis in this run) |
-| `base_reason` | string | only with `computed`: `no_baseline` (nothing to seed from) or `incompatible` (a candidate existed but its depth cap differed, or the engine demanded a full run); empty otherwise |
+| `base_source` | string | how this run obtained the base graph: `saved` (the artifact for the merge base), `committed` (`.codeboarding/` committed at the merge base, caught up), `ancestor` (the artifact of an older base-branch commit, caught up), or `computed` (full analysis in this run) |
+| `base_reason` | string | only with `computed`: `no_baseline` (nothing to seed from), `incompatible` (a candidate existed but its depth cap or configuration differed, or the engine demanded a full run) or `too_far_behind` (the nearest saved ancestor is beyond the catch-up bound); empty otherwise |
 | `base_from_sha` | string | the commit whose saved analysis seeded the base; the merge base for `saved`, empty for `computed` or when it lies beyond the fetched history |
 | `catchup_commits` | string | first-parent commits from `base_from_sha` to the merge base that change anything outside `.codeboarding/`; `0` when exact, empty when unknown |
 | `base_seconds` | string | wall time spent obtaining the base, the artifact lookup included |
@@ -109,14 +109,24 @@ them:
 |---|---|
 | the published `codeboarding-base-<cfg>-<merge_base>` artifact with a compatible depth cap | none |
 | no usable artifact — check out the merge base, seed from a compatible baseline committed there, catch up | one incremental, full if Core requires it |
-| no compatible committed baseline either | full analysis directly, at the configured `depth_cap` |
+| no compatible committed baseline either: the nearest `codeboarding-base-<cfg>-<sha>` artifact among the merge base's last 100 first-parent ancestors | one incremental from that commit to the merge base |
+| none within 100 commits either | full analysis directly, at the configured `depth_cap` |
 
 A trusted run that computed the base publishes it, so the next pull request
-forking from that commit gets the first row. The rows are `base_source` `saved`,
-`committed` and `computed` in the review metadata, and the review comment says
+forking from that commit gets the first row; that includes a base caught up from
+an ancestor. The rows are `base_source` `saved`, `committed`, `ancestor` and
+`computed` in the review metadata, and the review comment says
 which one ran, with measured times. While a base is computed, the progress
 comment says so in two steps, with the elapsed time and the reason.
 
+The ancestor lookup lists the repository's artifacts once (up to 1,000, newest
+first), keeps those named for this configuration and produced by a run on the
+repository's own code, and walks the merge base's first-parent history, deepening
+the shallow checkout to 101 commits. Sync uses the same lookup when the branch has
+no usable committed baseline, so the first sync after the setup pull request
+merges catches up from the base that pull request's review saved, instead of
+analyzing from scratch.
+
 The configuration hash includes `depth_cap`. The workflow input controls depth
 for both fresh and fallback analyses; stored legacy depth values never override it.
 
 
@@ -124,14 +124,24 @@ analyze_sync() {
   rm -rf "$work"
   seed_state "$CHECKOUT_DIR" "$state"
 
-  if [ "${FORCE_FULL,,}" = true ] || [ "$(depth_cap_from "$state/analysis.json")" != "$DEPTH_CAP" ]; then
-    full "$CHECKOUT_DIR" "$state" "$DEPTH_CAP"
-  else
-    incremental "$CHECKOUT_DIR" "$state"
-    if [ "$REQUIRES_FULL" = true ]; then
-      full "$CHECKOUT_DIR" "$state" "$DEPTH_CAP"
+  REQUIRES_FULL=true
+  if [ "$(printf '%s' "${FORCE_FULL:-false}" | tr '[:upper:]' '[:lower:]')" != true ]; then
+    if [ "$(depth_cap_from "$state/analysis.json")" = "$DEPTH_CAP" ]; then
+      incremental "$CHECKOUT_DIR" "$state"
+    fi
+    # A branch with no usable committed baseline, such as the first sync after the
+    # setup pull request merged, catches up from that pull request's saved base.
+    if [ "$REQUIRES_FULL" = true ] &&
+      seed_from_ancestor "${REPOSITORY:-}" "$(git -C "$CHECKOUT_DIR" rev-parse HEAD 2>/dev/null || true)" "$state" true; then
+      incremental "$CHECKOUT_DIR" "$state"
+      [ "$REQUIRES_FULL" = true ] ||
+        echo "::notice::Caught up from the saved analysis of $ANCESTOR_SHA instead of analyzing from scratch."
     fi
   fi
+  unset GIT_TOKEN
+  if [ "$REQUIRES_FULL" = true ]; then
+    full "$CHECKOUT_DIR" "$state" "$DEPTH_CAP"
+  fi
   # Sync already computes the graph every review of this branch compares against,
   # so publish it instead of making the first pull request recompute it.
   stage "$state" base
@@ -140,8 +150,9 @@ analyze_sync() {
 }
 
 # How far below the merge base this run looks for the commit a saved analysis
-# describes. Past it, a catch-up count is reported as unknown.
-CATCHUP_BOUND=100
+# describes, and for a saved ancestor to catch up from. Past it, a catch-up count
+# is unknown and an ancestor counts as too far behind. It is also the fetch depth.
+CATCHUP_BOUND="${CATCHUP_BOUND:-100}"
 
 # A depth above 1 also deepens a commit the shallow checkout already holds.
 fetch_commit() {
@@ -187,6 +198,35 @@ catchup_count() {
   echo "$count"
 }
 
+# Replaces $3 with the saved analysis of the nearest first-parent ancestor of $2
+# (or of $2 itself when $4 is true). Sets ANCESTOR_SHA, or when there is none to
+# use, ANCESTOR_REASON: too_far_behind or incompatible.
+ANCESTOR_SHA="" ANCESTOR_REASON=""
+seed_from_ancestor() {
+  local repository="$1" tip="$2" state="$3" include_tip="$4" found dest="$RUNNER_TEMP/codeboarding-ancestor"
+  ANCESTOR_SHA="" ANCESTOR_REASON=""
+  [ "${ANCESTOR_LOOKUP:-false}" = true ] && [ -n "${CFG_HASH:-}" ] && [ -n "${REPOSITORY:-}" ] && [ -n "$tip" ] || return 1
+  fetch_commit "$repository" "$tip" "$(( CATCHUP_BOUND + 1 ))" || true
+  found="$(GH_TOKEN="${GIT_TOKEN:-}" GH_ENTERPRISE_TOKEN="${GIT_TOKEN:-}" TIP_SHA="$tip" DEST="$dest" \
+    INCLUDE_TIP="$include_tip" CATCHUP_BOUND="$CATCHUP_BOUND" \
+    "$ACTION_PATH/scripts/action/find-ancestor-base.sh" || true)"
+  ANCESTOR_SHA="$(awk -F= '$1 == "ancestor_sha" {print $2; exit}' <<< "$found")"
+  if [ -z "$ANCESTOR_SHA" ]; then
+    if grep -qx 'other_cfg_at_tip=true' <<< "$found"; then
+      ANCESTOR_REASON=incompatible
+    elif grep -qx 'too_far_behind=true' <<< "$found"; then
+      ANCESTOR_REASON=too_far_behind
+    fi
+    return 1
+  fi
+  if [ "$(depth_cap_from "$dest/analysis.json")" != "$DEPTH_CAP" ]; then
+    ANCESTOR_SHA="" ANCESTOR_REASON=incompatible
+    return 1
+  fi
+  rm -rf "$state"
+  cp -a "$dest" "$state"
+}
+
 # Rewrites the sticky progress comment while the base is built from scratch. A
 # fork's read-only token makes every call fail, which costs nothing.
 PROGRESS_PID=""
@@ -262,9 +302,20 @@ analyze_review() {
     elif [ -f "$base_state/analysis.json" ]; then
       base_reason=incompatible
     fi
+    # Nothing at the merge base to grow from: catch up from the nearest saved
+    # ancestor, and publish the result under the merge base's own name below.
+    if [ "$REQUIRES_FULL" = true ] && seed_from_ancestor "$REVIEW_BASE_REPO" "$REVIEW_BASE_SHA" "$base_state" false; then
+      incremental "$base_checkout" "$base_state"
+      if [ "$REQUIRES_FULL" = true ]; then
+        base_reason=incompatible
+      else
+        base_source=ancestor base_from_sha="$ANCESTOR_SHA"
+        catchup_commits="$(catchup_count "$ANCESTOR_SHA" "$REVIEW_BASE_SHA")"
+      fi
+    fi
     if [ "$REQUIRES_FULL" = true ]; then
       base_source=computed
-      base_reason="${base_reason:-no_baseline}"
+      base_reason="${base_reason:-${ANCESTOR_REASON:-no_baseline}}"
       trap progress_stop EXIT
       progress_start "$base_started"
       full "$base_checkout" "$base_state" "$DEPTH_CAP"
 
@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+# Finds the nearest first-parent ancestor of TIP_SHA with a saved base analysis
+# under this configuration and downloads it into DEST, so a run without an exact
+# base can catch up from it instead of analyzing from scratch. Best effort.
+#
+# Prints key=value lines on stdout and everything else on stderr:
+#   ancestor_sha      the commit whose analysis is now in DEST, or empty
+#   too_far_behind    true when a compatible ancestor exists only beyond the bound
+#   other_cfg_at_tip  true when TIP_SHA has a saved analysis under another configuration
+#
+# Needs the tip's first-parent history in CHECKOUT_DIR, at least CATCHUP_BOUND deep.
+set -euo pipefail
+: "${REPOSITORY:?}" "${CFG_HASH:?}" "${TIP_SHA:?}" "${CHECKOUT_DIR:?}" "${DEST:?}"
+BOUND="${CATCHUP_BOUND:-100}"
+rm -rf "$DEST"
+
+api() { gh api -H 'Accept: application/vnd.github+json' "$@"; }
+export GH_HOST="${GH_HOST:-github.com}"
+GH_HOST="${GH_HOST#*://}"
+
+ancestor="" too_far=false other_cfg=false
+report() {
+  printf 'ancestor_sha=%s\ntoo_far_behind=%s\nother_cfg_at_tip=%s\n' "$ancestor" "$too_far" "$other_cfg"
+}
+trap report EXIT
+
+# One listing of every artifact, newest first. The name filter on this endpoint is
+# exact, so a prefix needs the whole list. The provenance rule is fetch-state.sh's:
+# only artifacts from a run on this repository's own code, since a fork's workflow
+# can upload under any name and its bytes would reach a pickle loader.
+trusted='.artifacts[]?
+  | select(.expired == false)
+  | select(.workflow_run != null)
+  | select(.workflow_run.head_repository_id == .workflow_run.repository_id)
+  | select(.name | startswith("codeboarding-base-"))
+  | .name'
+names="" page=1
+while [ "$page" -le "${MAX_PAGES:-10}" ]; do
+  if ! listing="$(api "repos/$REPOSITORY/actions/artifacts?per_page=100&page=$page" 2>/dev/null)"; then
+    echo "::warning::Could not list artifacts in $REPOSITORY; not looking for an older saved analysis." >&2
+    exit 0
+  fi
+  names="$names$(jq -r "$trusted" <<< "$listing" 2>/dev/null || true)"$'\n'
+  returned="$(jq -r '.artifacts | length' <<< "$listing" 2>/dev/null || echo 0)"
+  [ "${returned:-0}" -eq 100 ] || break
+  page=$(( page + 1 ))
+done
+
+prefix="codeboarding-base-$CFG_HASH-"
+saved="$(grep "^$prefix" <<< "$names" | cut -c$(( ${#prefix} + 1 ))- || true)"
+if grep "^codeboarding-base-.*-$TIP_SHA\$" <<< "$names" | grep -vq "^$prefix"; then
+  other_cfg=true
+fi
+[ -n "$saved" ] || exit 0
+
+# Distance 0 is the tip itself, which a review already looked up by its exact
+# name; sync has no such lookup and passes INCLUDE_TIP=true.
+distance=0
+for commit in $(git -C "$CHECKOUT_DIR" rev-list --first-parent --max-count=$(( BOUND + 1 )) "$TIP_SHA" 2>/dev/null); do
+  if { [ "$distance" -gt 0 ] || [ "${INCLUDE_TIP:-false}" = true ]; } && grep -qx "$commit" <<< "$saved"; then
+    ancestor="$commit"
+    break
+  fi
+  distance=$(( distance + 1 ))
+done
+
+if [ -z "$ancestor" ]; then
+  # Saved analyses of other branches are no reason to call this one far behind,
+  # so ask whether the newest one is an ancestor at all.
+  newest="$(head -n 1 <<< "$saved")"
+  if [ "$(api "repos/$REPOSITORY/compare/$newest...$TIP_SHA" --jq .status 2>/dev/null || true)" = ahead ]; then
+    too_far=true
+  fi
+  exit 0
+fi
+
+# Downloaded by its exact name, through the same checks as every other lookup.
+if ! GITHUB_OUTPUT="" ARTIFACT_NAME="$prefix$ancestor" DEST="$DEST" \
+  "$(dirname "$0")/fetch-state.sh" >&2 || [ ! -f "$DEST/analysis.json" ]; then
+  ancestor=""
+fi