diff --git a/.crow/archive-missed-packages.yaml b/.crow/archive-missed-packages.yaml
index 673415c..3feb157 100644
--- a/.crow/archive-missed-packages.yaml
+++ b/.crow/archive-missed-packages.yaml
@@ -62,7 +62,7 @@ steps:
       GIT_USER: pat-s
       R_VERSION: 4.5.3
     commands:
-      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3", dependencies = TRUE)'
+      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1", dependencies = TRUE)'
       - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
       - /opt/R/$R_VERSION/bin/R -q -e 'bincraft::process_unarchived_pkgs(Sys.getenv("CODENAME"), Sys.getenv("ARCH"), s3_endpoint = "https://s3.eu-central-003.backblazeb2.com", s3_region = "eu-central-003", s3_bucket = "devxy-rpkgs-binaries", s3_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"), s3_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"), workers = 2L)'
     backend_options:
diff --git a/.crow/build-all-versions-install-deps.yaml b/.crow/build-all-versions-install-deps.yaml
index 1c2df64..3a7d92c 100644
--- a/.crow/build-all-versions-install-deps.yaml
+++ b/.crow/build-all-versions-install-deps.yaml
@@ -75,7 +75,7 @@ steps:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       # Pin the same bincraft version the build steps use, so the precomputed
       # snapshot and the per-agent library stay consistent across the pipeline.
-      - /opt/R/$R_VERSION/bin/R -q -e 'pak::sysreqs_db_update(); pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3"); pak::pak(c("RPostgres", "s3fs", "data.table", "future", "jsonlite")); packageVersion("bincraft")'
+      - /opt/R/$R_VERSION/bin/R -q -e 'pak::sysreqs_db_update(); pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1"); pak::pak(c("RPostgres", "s3fs", "data.table", "future", "jsonlite")); packageVersion("bincraft")'
       - /opt/R/$R_VERSION/bin/R -q -e "source('local/packages-to-build.R'); saveRDS(pkgs, '/mnt/cache/packages/pkgs_to_build.rds'); saveRDS(pkgs[r_minor_sensitive == TRUE], '/mnt/cache/packages/r_minor_sensitive_pkgs.rds'); sprintf('Precomputed %s package versions (%s r-minor-sensitive)', nrow(pkgs), nrow(pkgs[r_minor_sensitive == TRUE]))"
     backend_options:
       docker:
diff --git a/.crow/build-all-versions.yaml b/.crow/build-all-versions.yaml
index f495550..076e482 100644
--- a/.crow/build-all-versions.yaml
+++ b/.crow/build-all-versions.yaml
@@ -121,7 +121,7 @@ steps:
       # to a zero-length value and breaks every metadata query and the sysdeps
       # install). Pin bincraft here, exactly like the R-minor pass below.
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
-      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3")'
+      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1")'
       - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
       - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$R_VERSION/bin/Rscript local/build-all.R $SPLIT_INTO $SPLIT_INDEX $NCPUS 2>&1
       - |
@@ -133,7 +133,7 @@ steps:
           echo "=== R-minor-sensitive pass under R $RV ==="
           LIB="/mnt/cache/R-pkgs-$RMINOR"
           mkdir -p "$LIB"
-          R_LIBS_USER="$LIB" "$(dirname "$RBIN")/R" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3")' || true
+          R_LIBS_USER="$LIB" "$(dirname "$RBIN")/R" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1")' || true
           R_LIBS_USER="$LIB" $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- "$(dirname "$RBIN")/Rscript" local/build-all.R --sensitive-only $SPLIT_INTO $SPLIT_INDEX $NCPUS 2>&1 || true
         done
       # archive missed packages; first arg is the codename (e.g. "alpine324"),
diff --git a/.crow/process-updates.yaml b/.crow/process-updates.yaml
index e4c09ae..b943b2d 100644
--- a/.crow/process-updates.yaml
+++ b/.crow/process-updates.yaml
@@ -192,13 +192,13 @@ steps:
     commands:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - rm -rf /mnt/cache/R-pkgs/00LOCK-* /mnt/cache/R-pkgs/bincraft /mnt/cache/R-pkgs/pkgcache /mnt/cache/pkgcache/R/pkgcache
-      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3")'
+      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1")'
       - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
       - mkdir -p /mnt/cache/pkgcache /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       # rhel-10 ships xwfb-run (Xwayland) instead of xvfb-run; prefer it and start weston, else fall back to xvfb-run
       - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
       # options(future.globals.onReference = NULL): for some reason s3fs::file_delete() throws 'Error: Detected a non-exportable reference ('externalptr') in one of the globals ('FUN' of class 'function') used in the future expression' otherwise
-      - $XVFB $XVFB_ARGS -- /opt/R/$R_VERSION/bin/R -q -e "options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = 'error', repos = structure(c(getOption('repos'),INLA='https://inla.r-inla-download.org/R/stable'))); progressr::handlers('cli'); progressr::handlers(global = TRUE); options(future.globals.onReference = NULL); bincraft::process_cran_updates(interval = $INTERVAL, platform = '${OS}', process_updated = TRUE, process_new = ${PROCESS_NEW}, process_removed = TRUE, r_minor_detection = 'classifier', s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE)"
+      - $XVFB $XVFB_ARGS -- /opt/R/$R_VERSION/bin/R -q -e "options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = 'error', repos = structure(c(getOption('repos'),INLA='https://inla.r-inla-download.org/R/stable'))); progressr::handlers('cli'); progressr::handlers(global = TRUE); options(future.globals.onReference = NULL); bincraft::process_cran_updates(interval = $INTERVAL, platform = '${OS}', process_updated = TRUE, process_new = ${PROCESS_NEW}, process_removed = TRUE, patches = 'local/patches', r_minor_detection = 'classifier',s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE)"
       - |
         PRIMARY_MINOR=$(echo "$R_VERSION" | cut -d. -f1-2)
         for RBIN in /opt/R/[0-9]*/bin/R; do
@@ -208,8 +208,8 @@ steps:
           echo "=== R-minor-sensitive update pass under R $RV ==="
           LIB="/mnt/cache/R-pkgs-$RMINOR"
           mkdir -p "$LIB"
-          R_LIBS_USER="$LIB" "$(dirname "$RBIN")/R" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3")' || true
-          R_LIBS_USER="$LIB" $XVFB $XVFB_ARGS -- "$(dirname "$RBIN")/R" -q -e "options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = NULL); bincraft::process_cran_updates(interval = $INTERVAL, platform = '${OS}', process_updated = TRUE, process_new = FALSE, process_removed = FALSE, r_minor_detection = 'classifier', r_minor_sensitive_only = TRUE, s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE)" || true
+          R_LIBS_USER="$LIB" "$(dirname "$RBIN")/R" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1")' || true
+          R_LIBS_USER="$LIB" $XVFB $XVFB_ARGS -- "$(dirname "$RBIN")/R" -q -e "options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = NULL); bincraft::process_cran_updates(interval = $INTERVAL, platform = '${OS}', process_updated = TRUE, process_new = FALSE, process_removed = FALSE, patches = 'local/patches', r_minor_detection = 'classifier',r_minor_sensitive_only = TRUE, s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE)" || true
         done
       - /opt/R/$R_VERSION/bin/R -q -e 'library(bincraft); upload_package_index(codename = "${OS_ID}", s3_endpoint = "https://s3.eu-central-003.backblazeb2.com", s3_region = "eu-central-003", s3_bucket = "devxy-rpkgs-binaries", s3_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"), s3_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"))'
       - |
diff --git a/.crow/weekly-rebuild-missing.yaml b/.crow/weekly-rebuild-missing.yaml
index 34651ba..5d88dec 100644
--- a/.crow/weekly-rebuild-missing.yaml
+++ b/.crow/weekly-rebuild-missing.yaml
@@ -129,12 +129,12 @@ steps:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - mkdir -p /mnt/cache/pkgcache /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
-      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.3") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.3")'
+      - /opt/R/$R_VERSION/bin/R -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.1")'
       - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
       - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
       - /opt/R/$R_VERSION/bin/R -q -e 'pak::pak("httr2")'
       - /opt/R/$R_VERSION/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
-      - $XVFB $XVFB_ARGS -- /opt/R/$R_VERSION/bin/R -q -e "sink(stdout(), type = 'message'); options(crayon.enabled = TRUE, Ncpus = $NCPUS, future.globals.onReference = NULL); pkgs <- readLines('/tmp/rebuild_pkgs.txt'); if (length(pkgs) == 0) { cat('Nothing to rebuild\n'); q('no') }; excluded <- jsonlite::fromJSON('local/excluded-packages.json')[['package']]; pkgs <- setdiff(pkgs, excluded); cat(sprintf('Rebuilding %d packages\n', length(pkgs))); n <- length(pkgs); for (i in seq_along(pkgs)) { x <- pkgs[i]; cat(sprintf('[%d/%d] %s\n', i, n, x)); tryCatch(bincraft::build_binary_package(x, tag_limit = 1L, s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE), error = function(e) cat(sprintf('ERROR building %s - %s\n', x, conditionMessage(e)))) }" 2>&1
+      - $XVFB $XVFB_ARGS -- /opt/R/$R_VERSION/bin/R -q -e "sink(stdout(), type = 'message'); options(crayon.enabled = TRUE, Ncpus = $NCPUS, future.globals.onReference = NULL); pkgs <- readLines('/tmp/rebuild_pkgs.txt'); if (length(pkgs) == 0) { cat('Nothing to rebuild\n'); q('no') }; excluded <- jsonlite::fromJSON('local/excluded-packages.json')[['package']]; pkgs <- setdiff(pkgs, excluded); cat(sprintf('Rebuilding %d packages\n', length(pkgs))); n <- length(pkgs); for (i in seq_along(pkgs)) { x <- pkgs[i]; cat(sprintf('[%d/%d] %s\n', i, n, x)); tryCatch(bincraft::build_binary_package(x, tag_limit = 1L, patches = 'local/patches', s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE), error = function(e) cat(sprintf('ERROR building %s - %s\n', x, conditionMessage(e)))) }" 2>&1
     backend_options:
       docker:
         resources:
diff --git a/.gitignore b/.gitignore
index 94e2c2f..f4124a2 100644
--- a/.gitignore
+++ b/.gitignore
@@ -99,3 +99,6 @@ docs/
 local/test.R
 .DS_Store
 docs/
+
+# Superpowers SDD scratch (briefs, reports, ledger)
+.superpowers/
diff --git a/.markdownlint-cli2.yaml b/.markdownlint-cli2.yaml
index 66e7cc8..032c0ea 100644
--- a/.markdownlint-cli2.yaml
+++ b/.markdownlint-cli2.yaml
@@ -2,3 +2,6 @@
 ignores:
   - LICENSE.md
   - docs/superpowers/**
+  # Internal design docs (specs/plans) are not user-facing reference material.
+  - specs/**
+  - plans/**
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 05b89fe..7448d6c 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -43,3 +43,9 @@ repos:
         entry: YAML filenames must have .yaml extension.
         language: fail
         files: .yml$
+      - id: validate-patches
+        name: validate patch registry
+        entry: Rscript local/validate-patches.R
+        language: system
+        files: ^local/(patches/|validate-patches\.R$)
+        pass_filenames: false
diff --git a/README.md b/README.md
index 278a475..d030948 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,28 @@ For every package+tag combination:
 1. Archive old package versions and keep the latest one in the root
 1. Delete local binaries after successful upload
 
+## Patching packages
+
+Some CRAN packages fail to compile on specific platforms due to compiler- or OS-specific issues unrelated to the package itself.
+The canonical example is `RcppParallel`, whose bundled TBB sources fail on musl (Alpine) and newer compiler/OS combinations.
+Because such packages are often transitive dependencies of many others, a single failure cascades: all dependents fail even though nothing is wrong with the dependent itself.
+
+To address this, frequently-failing packages can be "patched" before they are installed — whether as a direct build target or a transitive dependency pulled in by `pak`.
+
+The patch registry lives in `local/patches/registry.json`.
+Each entry specifies a package and the platforms/versions it applies to, along with either lightweight build-time overrides (environment variables, configure arguments, Makevars) or a source diff (for deeper fixes).
+See `local/patches/README.md` for the complete schema.
+
+Patching uses a two-tier approach:
+
+1. **Lightweight overrides:** environment variables, configure arguments, or Makevars settings applied during build — typically version-independent and fast.
+2. **Source diffs:** unified diff patches applied to the unpacked source before building — more powerful but version-pinned.
+
+The system is implemented in `bincraft`: when a package needs patching, `bincraft` pre-builds it with the patch and serves the patched binary to `pak`, ensuring transitive dependents receive the fixed package.
+This way, the fix cascades to all packages that depend on it.
+
+For the design rationale and architecture, see `specs/2026-06-30-package-patching-design.md`.
+
 ## Build Environment
 
 Binaries are built on a mixed-architecture Kubernetes cluster using CI.
diff --git a/docker/build-one.Dockerfile b/docker/build-one.Dockerfile
index 2cd4d51..e1bf387 100644
--- a/docker/build-one.Dockerfile
+++ b/docker/build-one.Dockerfile
@@ -16,6 +16,9 @@ ARG CACHEBUST
 
 WORKDIR /work
 COPY build-one.R /work/build-one.R
+# Ship the patch registry so build-one.R's `patches = "local/patches"` resolves
+# (build context is `local/`, CWD is /work).
+COPY patches /work/local/patches
 
 RUN --mount=type=secret,id=b2_access,required=true \
     --mount=type=secret,id=b2_secret,required=true \
@@ -37,7 +40,7 @@ RUN --mount=type=secret,id=b2_access,required=true \
       echo "No working virtual display; building without xvfb" >&2; \
     fi; \
     run_build() { if [ "$USE_XVFB" = 1 ]; then $XVFB -a $XVFB_ARGS -- "$@"; else "$@"; fi; }; \
-    ensure_bincraft() { "$1" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.2.1") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.2.1")'; }; \
+    ensure_bincraft() { "$1" -q -e 'if (!requireNamespace("bincraft", quietly = TRUE) || packageVersion("bincraft") != "4.3.0") pak::pak("git::https://codefloe.com/rpkgs/bincraft.git@v4.3.0")'; }; \
     PRIMARY_MINOR=$(echo "$R_VERSION" | cut -d. -f1-2); \
     seen=" $PRIMARY_MINOR "; \
     prc=0; failed=""; \
diff --git a/local/build-all.R b/local/build-all.R
index 6c3725c..c0fe7f7 100644
--- a/local/build-all.R
+++ b/local/build-all.R
@@ -140,6 +140,7 @@ mapply(
       metadata_db_sslmode = "require",
       metadata_db_port = 15432,
       archive = TRUE,
+      patches = "local/patches",
       upload = TRUE,
       store_build_metadata = TRUE
     )
diff --git a/local/build-one.R b/local/build-one.R
index f2d6ebd..07c0b6a 100644
--- a/local/build-one.R
+++ b/local/build-one.R
@@ -103,6 +103,7 @@ for (ver in versions) {
     force = TRUE,
     upload = TRUE,
     archive = TRUE,
+    patches = "local/patches",
     store_build_metadata = TRUE,
     s3_endpoint = s3$s3_endpoint,
     s3_region = s3$s3_region,
diff --git a/local/patches/README.md b/local/patches/README.md
index b094848..af24b2b 100644
--- a/local/patches/README.md
+++ b/local/patches/README.md
@@ -9,7 +9,7 @@ The registry is defined in `registry.json` as an array of patch entries. Each en
 ### Field semantics
 
 | Field | Type | Required | Description |
-|-------|------|----------|-------------|
+| --- | --- | --- | --- |
 | `package` | string | yes | CRAN package name. |
 | `versions` | string | yes | `"*"` for any, a constraint such as `">=5.1.0"`, or an exact version `"5.1.11-2"`. Env-tier fixes are typically `"*"`; source diffs are normally exact or lower-bounded because a diff is pinned to the source it was generated against. |
 | `platforms` | array of strings | yes | Matched against the running build's platform tokens — distro family (`alpine`, `ubuntu`, `redhat`), codename (`ubuntu-2604`, `alpine-324`), and arch (`amd64`, `arm64`). An entry matches if any listed token matches any build token. `["*"]` matches all platforms. |
@@ -34,10 +34,10 @@ To add a new patch entry:
 ## Validation
 
 The registry is validated and applied by bincraft during the build process.
-For manual validation, use:
+For manual validation, run the validator from the repo root:
 
-```r
-x <- jsonlite::fromJSON("local/patches/registry.json", simplifyVector = FALSE)
+```bash
+Rscript local/validate-patches.R
 ```
 
-This loads the registry; inspect the structure to verify correctness.
+This validates the schema, referenced patch-file existence, and checks for duplicate entries across platforms and versions.
diff --git a/local/validate-patches.R b/local/validate-patches.R
new file mode 100644
index 0000000..5f5aace
--- /dev/null
+++ b/local/validate-patches.R
@@ -0,0 +1,56 @@
+#!/usr/bin/env Rscript
+# Validate local/patches/registry.json: schema, referenced patch files, and
+# ambiguous overlaps. Exits 1 on any problem. Used by pre-commit and CI.
+
+dir <- "local/patches"
+registry_file <- file.path(dir, "registry.json")
+if (!file.exists(registry_file)) {
+  cat("No registry.json found; nothing to validate.\n")
+  quit(status = 0L)
+}
+
+or_q <- function(x) if (is.null(x)) "?" else x
+
+reg <- jsonlite::fromJSON(registry_file, simplifyVector = FALSE)
+required <- c("package", "versions", "platforms", "reason")
+errs <- character(0L)
+
+for (i in seq_along(reg)) {
+  e <- reg[[i]]
+  missing <- setdiff(required, names(e))
+  if (length(missing) > 0L) {
+    errs <- c(errs, sprintf(
+      "entry %d (%s): missing %s", i,
+      if (is.null(e$package)) "?" else e$package, toString(missing)
+    ))
+  }
+  if (!is.null(e$patch)) {
+    p <- file.path(dir, e$patch)
+    if (!file.exists(p)) {
+      errs <- c(errs, sprintf("entry %d (%s): patch file '%s' missing",
+                              i, e$package, p))
+    }
+  }
+}
+
+# Ambiguous overlap: two entries for the same package with identical platforms
+# and versions.
+keys <- vapply(reg, function(e) {
+  sprintf(
+    "%s|%s|%s",
+    or_q(e$package),
+    paste(sort(as.character(unlist(e$platforms))), collapse = ","),
+    or_q(e$versions)
+  )
+}, character(1L))
+dups <- keys[duplicated(keys)]
+if (length(dups) > 0L) {
+  errs <- c(errs, sprintf("ambiguous duplicate entries: %s", toString(unique(dups))))
+}
+
+if (length(errs) > 0L) {
+  cat("Patch registry validation FAILED:\n")
+  cat(paste0("  - ", errs, "\n"))
+  quit(status = 1L)
+}
+cat(sprintf("Patch registry OK (%d %s).\n", length(reg), if (length(reg) == 1L) "entry" else "entries"))