diff --git a/src/tomlc17/README.md b/src/tomlc17/README.md
index 83a2d680629..6131fbb7fed 100644
--- a/src/tomlc17/README.md
+++ b/src/tomlc17/README.md
@@ -23,15 +23,58 @@ vendored files are identified by their SHA-256 checksums:
 | `tomlc17.c`  | `e5d61c4960a4ef7254abb4077ffaefd2ba8935459e52314ea67537b5e340ddd5` |
 | `tomlc17.h`  | `9f6cae985cde199a64e0f63d4b395b2738455f9b62d501bb4385ca3a85e34728` |
 
-Use these hashes to identify the exact upstream commit.
+Use these hashes to identify the exact upstream commit.  They are the
+checksums of the **pristine** upstream files; `tomlc17.c` as it sits in this
+directory carries one local change (see below) and hashes to
+`8b4898b2e23046de778e5e989720b8d714bb105f337efce73fb56a827bb7a9f7`.
 
 ## HDF5-local modifications
 
-**None.** The files in this directory are the exact upstream sources with no
-local modifications.  They are intentionally excluded from the HDF5
-clang-format pass (see `.github/workflows/clang-format-check.yml` and
-`bin/format_source`) so that future upstream updates can be dropped in
-without any re-formatting step.
+**One**, in `tomlc17.c` `scan_float()`.  They are otherwise the exact upstream
+sources, and are intentionally excluded from the HDF5 clang-format pass (see
+`.github/workflows/clang-format-check.yml` and `bin/format_source`) so that
+future upstream updates can be dropped in without any re-formatting step.
+
+### `scan_float()`: `ERANGE` alone is not a parse failure
+
+Upstream rejects any `strtod()` that sets `errno`:
+
+```c
+if (errno || *q || q == buffer) {
+  return SETERROR(sp->ebuf, lineno, "error parsing float");
+}
+```
+
+C11 7.22.1.3 leaves it implementation-defined whether `strtod` sets `ERANGE`
+when the result underflows, and glibc sets it for a tiny *inexact* result even
+though the conversion succeeded and returned the correctly rounded value.  The
+effect is that every TOML float literal that rounds to a subnormal double is
+rejected as a syntax error -- `x = 5e-324` as surely as
+`x = 2.2250738585072011e-308`.  Python's `tomllib` accepts all of them.
+
+For HDF5 this reached the filter configuration API through canonicalization:
+`H5Z__rewrite_hexfloats()` rewrites a hex-float literal to `%.17e` decimal, and
+the decimal spelling of a subnormal is inexact even when the hex spelling was
+exact, so `rate = 0x1p-1074` became a parse error on the way to disk.
+
+The local change accepts `ERANGE` when the result is finite and nonzero, and
+still rejects overflow to `+/-HUGE_VAL` and underflow all the way to zero:
+
+```c
+if ((errno && !(errno == ERANGE && fp64 != 0.0 && isfinite(fp64))) || *q ||
+    q == buffer) {
+```
+
+`<math.h>` is already included upstream, so `isfinite` needs no new header.
+The `isfinite` term is load-bearing: `HUGE_VAL` is nonzero, so without it
+overflow would start being accepted.
+
+Covered by `canon-10` in `test/tfilter2.c`, which asserts value transparency
+across the hex-to-decimal rewrite at exact powers of two from 2^-1074 to
+2^1023.
+
+Reported upstream as <https://github.com/cktan/tomlc17/issues/48>.  Drop this
+delta once that is fixed and a release carrying the fix is vendored.
 
 Note: hex-float literal support (C99 `%a` output from `get_config` callbacks)
 is handled in `src/H5Zconfig.c` via a pre-processing step that rewrites
@@ -54,5 +97,7 @@ handed to tomlc17.  This keeps these vendored sources unmodified.
 2. Copy them into this directory, replacing the existing files.
 3. Record the new SHA-256 checksums and tag name in the table above.
 4. Update the "Vendored on" date.
+4a. Re-apply the local `scan_float()` change below, unless upstream has fixed
+   it, and record the new post-patch checksum.
 5. Do **not** run clang-format on these files.
 6. Run the HDF5 test suite (`ctest -R tfilter2`) to verify compatibility.
diff --git a/src/tomlc17/tomlc17.c b/src/tomlc17/tomlc17.c
index e6840ec1a82..8c31ea38084 100644
--- a/src/tomlc17/tomlc17.c
+++ b/src/tomlc17/tomlc17.c
@@ -2401,7 +2401,14 @@ static int scan_float(scanner_t *sp, token_t *tok) {
   errno = 0;
   char *q;
   double fp64 = strtod(buffer, &q);
-  if (errno || *q || q == buffer) {
+  /* HDF5 local change: ERANGE alone is not a parse failure.  glibc raises it
+   * on underflow to a subnormal even though the conversion succeeded and
+   * returned the correctly rounded value, which rejected every subnormal
+   * literal -- "5e-324" and "0x1p-1074" alike.  Accept ERANGE when the result
+   * is finite and nonzero; still reject overflow (+/-HUGE_VAL) and total
+   * underflow to zero, where the value really was not representable. */
+  if ((errno && !(errno == ERANGE && fp64 != 0.0 && isfinite(fp64))) || *q ||
+      q == buffer) {
     return SETERROR(sp->ebuf, lineno, "error parsing float");
   }