diff --git a/TODO.md b/TODO.md
index 02634717d3..7ad7f38a41 100644
--- a/TODO.md
+++ b/TODO.md
@@ -523,6 +523,12 @@ EP/JC/P work; candidates are not speedup claims until measured on both platforms
       data descriptors. A nonseekable libarchive memory-writer fixture reproduces zero size for
       a 1 MiB member on the unchanged parent. Preserve the exact size oracle and parent proof in
       the O19b1 artifact; do not silently admit an unbudgeted seekable central-directory index.
+      The [current-main compatibility investigation](docs/benchmarks/deferred-zip-size-investigation-20261107.md)
+      rejects unconditional payload decoding: supported/unsupported decoder and CRC controls expose
+      new failures in successful listings. Keep unknown versus empty sizes distinct.
+- [ ] Fix deferred bzip2 ZIP content reads through streaming sources. Exact valid fixtures decode
+      through the complete-memory API and Python zipfile but report truncation through MemberReadSource.
+      Preserve the six-case both-host diagnostic and separate this from metadata-only listing.
 - [x] O19b2: Implement and qualify native ZIP seeking with conservative index admission, bounded
       source extents and sequential fallbacks. Reject the uncached policy after repeatable first-read
       regressions; retain all variants and the manual ZIP benchmark in
diff --git a/docs/benchmarks/deferred-zip-size-investigation-20261107.json.gz b/docs/benchmarks/deferred-zip-size-investigation-20261107.json.gz
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diff --git a/docs/benchmarks/deferred-zip-size-investigation-20261107.md b/docs/benchmarks/deferred-zip-size-investigation-20261107.md
new file mode 100644
index 0000000000..cc160e6d51
--- /dev/null
+++ b/docs/benchmarks/deferred-zip-size-investigation-20261107.md
@@ -0,0 +1,114 @@
+# Deferred ZIP sizes: compatibility investigation, 2026-11-07
+
+Do not recover every unknown archive size by decoding its payload during listing. The
+current streaming ZIP reader reports zero for nonempty members whose local headers
+leave sizes to data descriptors. Decoding and counting data blocks recovers sizes for
+stored and deflated examples, but introduces failures in previously successful listings.
+The size defect remains open; this investigation changes no production code.
+
+## Reproduction and controls
+
+The private diagnostic uses exact main `77aa9ef9bc9475adcbdd37cf3004143a96f03060`, the
+pinned libarchive 3.8.1 and `--config=clang_release` with Clang 23.1.4. Six investigative
+GoogleTest cases pass on both macOS and Linux. These cases describe current behavior,
+including defects; they are retained as evidence rather than added to the production
+suite as acceptable behavior.
+
+Each method has a complete one-member ZIP with a known local size and another with a
+signed data descriptor. All contain `data.txt`, six bytes `hello\n`, a matching CRC and
+central directory. Fixtures are built in memory without host-file writes, external
+archivers, or additional dependencies. GoogleTest XML retains their exact bytes.
+
+| Compression | Method | Known-header listing size | Deferred listing size | Deferred streaming content read | Decoding to count blocks |
+| :---------- | -----: | ------------------------: | --------------------: | :------------------------------ | :----------------------- |
+| Stored      |      0 |                         6 |                     0 | Six correct bytes               | Six bytes                |
+| Deflate     |      8 |                         6 |                     0 | Six correct bytes               | Six bytes                |
+| Bzip2       |     12 |                         6 |                     0 | Data loss: truncated body       | Fatal decoding error     |
+| Deflate64   |      9 |                         6 |                     0 | Unsupported decoder             | Decoder failure          |
+
+The known-header listing succeeds for all four methods without requiring their content
+readers. Explicit `archive_read_data_skip` succeeds for every deferred fixture, but
+`archive_entry_size_is_set` remains false and `archive_entry_size` remains zero afterward.
+Reading the size after skipping therefore does not fix this defect.
+
+Python's independent `zipfile` reader fully decodes and checks CRCs for the exact six
+stored/deflate/bzip2 fixtures exported by each host. Both hosts produce identical fixture
+identities. For the two Deflate64 fixtures, Python verifies the ZIP directory, lengths and
+CRC metadata, and zlib verifies the common final stored-block payload. No independent
+Deflate64 decoder was run: this narrowly constructed fixture uses no extended length,
+distance or Huffman features. It does not qualify general Deflate64 support.
+
+The initial Mac diagnostic assumed deferred bzip2 could be decoded like deflate and
+failed that expectation. Its complete failure log is retained. The corrected diagnostic
+separates that discovered limitation, without changing the fixture or production reader.
+
+## Why the shortcut fails
+
+The [ZIP specification](https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT)
+defines data descriptors for streamed entries. A zero local size in that case is not an
+assertion that the file is empty. The central directory contains the fixture's correct
+size, but the current source reader does not select libarchive's seekable ZIP path.
+
+[The pinned libarchive ZIP implementation](https://github.com/libarchive/libarchive/blob/v3.8.1/libarchive/archive_read_support_format_zip.c)
+uses different paths for skipping and reading. Its streamable skip routine can consume a
+known compressed extent or find a signed descriptor without a decoder. Deflate's unknown
+extent uses a decoder while skipping. The public entry's size is not updated by these
+paths. This explains why successful header traversal does not establish payload readability.
+
+The diagnostic also corrupts only the CRC in stored/deflated fixtures. Listing still
+succeeds while reading the payload reports data loss. Replacing header traversal with
+unconditional decoding would add content-validation failures to metadata-only operations.
+This is a behavior change even if stricter validation might be desirable as a separate option.
+
+Deferred bzip2 exposes another existing gap. `ReadMember` over the complete memory bytes
+successfully returns `hello\n`, whereas `MemberReadSource` over the same memory storage
+reports a truncated bzip2 body. Libarchive's streamable bzip2 reader bounds its input by
+`entry_bytes_remaining`, which is zero for this fixture; its seekable reader obtains the
+size from the central directory. The complete memory API versus streaming source API
+must remain separate controls. Calling them both memory reads would hide the difference.
+
+Encrypted and signatureless descriptor cases, arbitrary short reads, ZIP64, malformed
+sizes, sparse offsets and nested replay limits are not qualified by these six cases.
+No universal compatibility or performance claim follows from them.
+
+## Next implementation boundary
+
+1. Preserve the distinction between unknown and empty member sizes. Do not export an
+   unknown size as a trustworthy zero into VFS metadata, summaries or size predicates.
+   Specify which observations require an exact size before changing representation.
+2. Identify cheap, identity-stable random access explicitly. `ReadSource::OpenAt` and
+   `Size` may replay sequential inputs within a shared budget; their mere availability
+   does not justify seeking or rebuilding an index on every member read. Reuse the
+   prior O19b2/O19b3 first-member and all-visible-members controls.
+3. Acquire central-directory sizes only when their source identity, local-member
+   association and budget are proven. Preserve duplicate order, orphan/local-only
+   entries, normalization and source lifetimes. A directory record must not silently
+   substitute a different member for the one exposed by the streaming walk.
+4. Define the sequential fallback and unavailable-size error explicitly. Require exact
+   size when requested, while preserving metadata-only traversal of entries without a
+   usable decoder. Do not make an unbounded payload decode the implicit fallback.
+5. Independently address deferred bzip2 extraction, with both memory and streaming
+   controls. Keep listing, content validation and exact-size acquisition as distinct
+   requirements; do not hide the bug with a zero-size special case.
+
+Before runtime adoption, add desired-behavior regression tests and qualify known/deferred,
+empty/nonempty, signed/signatureless, ZIP64, unsupported/encrypted and corrupt members;
+local/central disagreements and duplicates; short reads and descriptor boundaries;
+nested/decoded sources, failure propagation, and shared memory/replay budgets. Measure
+first/last/repeated member reads, metadata-only listing and size-demanding workloads on
+both hosts. There are no timing, allocation, RSS, full-suite, sanitizer or public-head CI
+results in this investigation. Existing seeking/index regressions remain relevant.
+
+## Retained evidence
+
+[The lossless capsule](deferred-zip-size-investigation-20261107.json.gz) contains 19 hashed
+records: the diagnostic and temporary Bazel rule, relevant exact source/configuration,
+initial failure, final host test/build/XML records and independent fixture validators.
+Every record was decoded and hash-checked after packaging. The capsule is 103,735 bytes;
+SHA-256: `b0bd93614cf62b23828be6700ef86cfa45133b70974df1ca0d6c35997ffb8cf2`.
+
+Reconstruct the base checkout, apply the retained private build-rule patch and diagnostic
+source, and run `bazel test @xff_archive//:deferred_zip_investigation_test --config=clang_release`.
+Run the retained `validate-fixtures.py` against that test's XML for the independent check.
+The diagnostic is intentionally not a production acceptance test: several expectations
+record bugs that the future implementation must change.
diff --git a/docs/performance-analysis.md b/docs/performance-analysis.md
index 241c82e569..65f1b7719d 100644
--- a/docs/performance-analysis.md
+++ b/docs/performance-analysis.md
@@ -4985,3 +4985,18 @@ GitHub permits up to 100 pending runs; overflow still cancels additional request
 canceled revisions need explicit reconciliation/backfill, not a claim that changing the workflow
 recovered them. Measurement commands, sampling, CPU selection, artifact identity, PR checks and
 publication policies are unchanged. There is no runtime or elapsed-time improvement claim.
+
+### Streamed ZIP sizes: reject unconditional content decoding
+
+[The current-main compatibility investigation](benchmarks/deferred-zip-size-investigation-20261107.md)
+reproduces zero deferred sizes and unchanged public entry sizes after skipping. Six private
+investigative cases pass on both hosts, and independent Python ZIP decoding validates the exact
+stored/deflate/bzip2 fixtures. Decoding every unknown payload would break successful unsupported-method
+listings and introduce CRC failures to metadata-only walks. Deferred bzip2 additionally reads through
+the complete-memory API but fails through the streaming source API for the same valid archive.
+
+Reject that blanket fallback before timing it. Keep unknown-size representation, metadata demand,
+identity-stable random access, visible-local-member association and shared read budgets as explicit
+requirements. The separate deferred-bzip2 content bug is tracked. All 19 source/configuration/log
+records are retained losslessly; no runtime implementation, speedup, full-suite/sanitizer qualification
+or public-head CI claim is made. O19b3 and streamed-size correctness remain open.
diff --git a/release-site.json b/release-site.json
index db089aaf7a..58fc2101e4 100644
--- a/release-site.json
+++ b/release-site.json
@@ -146,7 +146,8 @@
     "docs/benchmark-chart-sensitivity-audit.md": "docs/benchmark-chart-sensitivity-audit.html",
     "docs/benchmark-extended-history-campaign.md": "docs/benchmark-extended-history-campaign.html",
     "docs/benchmarks/find-listing-optimization-20261107.md": "docs/benchmarks/find-listing-optimization-20261107.html",
-    "docs/benchmarks/lazy-content-state-20261107.md": "docs/benchmarks/lazy-content-state-20261107.html"
+    "docs/benchmarks/lazy-content-state-20261107.md": "docs/benchmarks/lazy-content-state-20261107.html",
+    "docs/benchmarks/deferred-zip-size-investigation-20261107.md": "docs/benchmarks/deferred-zip-size-investigation-20261107.html"
   },
   "generated_html": {
     "XFF.md": {
@@ -184,6 +185,7 @@
     "docs/benchmarks/extended-history-revisions-20261107.json": "docs/benchmarks/extended-history-revisions-20261107.json",
     "docs/benchmarks/extended-history-viewer-check-20261107.json": "docs/benchmarks/extended-history-viewer-check-20261107.json",
     "docs/benchmarks/find-listing-optimization-20261107.json": "docs/benchmarks/find-listing-optimization-20261107.json",
-    "docs/benchmarks/lazy-content-state-20261107.json": "docs/benchmarks/lazy-content-state-20261107.json"
+    "docs/benchmarks/lazy-content-state-20261107.json": "docs/benchmarks/lazy-content-state-20261107.json",
+    "docs/benchmarks/deferred-zip-size-investigation-20261107.json.gz": "docs/benchmarks/deferred-zip-size-investigation-20261107.json.gz"
   }
 }