mcp: answer invalid request ids with -32600 and allow the MCP headers in CORS (DEX-182) - #39761

Release notes for the **v26.48.0** train. One commit is added per
snapshot; the `final` snapshot sets the real dates. **Applied: `rc.1`,
`rc.2`, `rc.3`, `final` — the train is complete.**

- **Content**: 3 Feature sections (upsert hydration; Kafka sink headers
GA; MySQL `EXCLUDE ALL NOT NULL CONSTRAINTS`), 9 Improvements, 2 Guides,
4 Bug Fixes. No **Agent Skills** section — every agent-skills PR in all
four windows was omitted.
- **Scope**: 123 PRs across the four snapshots — rc.1 114 (14 included),
rc.2 5 (0 included), rc.3 1 (1 included), final 3 (0 included).
- **Dates — now real, not provisional**: Cloud **2026-10-29**,
Self-Managed **2026-10-30**. The `rc.1`-anchored provisional estimate
was right, so `final` left both files unchanged. The `v26.48.0` tag's
commit date is 2026-10-28T18:35:05Z, a Wednesday; the first Thursday on
or after it is 2026-10-29, with Self-Managed the day after.
- **Operator compatibility**: row for `v26.48`, release date
`2026-10-30`. Unchanged by rc.2, rc.3, and final.
- **Branch base**: `main`, whose newest published section was `##
v26.46.1` when this branch was cut; v26.47.0 and v26.47.1 have since
merged in. `v26.48.0` stays at the top of the list.

### final increment — empty, and verified empty

`final`'s commit is empty. The tracking issue
([release#627](MaterializeInc/release#627))
lists exactly one item, the unassociated `4edd60a8d` "release: bump to
version v26.48.0", which the release-process-automation rule omits. The
three agent-skills PRs in the window
([#118](MaterializeInc/agent-skills#118),
[#119](MaterializeInc/agent-skills#119),
[#120](MaterializeInc/agent-skills#120)) were
all omitted.

This was **verified rather than assumed**, because a zero-PR snapshot
and a failed fetch produce identical output:
`compare/v26.48.0-rc.3...v26.48.0` returns `total_commits: 1`, and that
commit is the version bump. Nothing merged onto the release branch in
the 42 h 49 m between the `rc.3` tag and the ship tag. Across the train,
every agent-skills PR from
[#111](MaterializeInc/agent-skills#111) to
[#120](MaterializeInc/agent-skills#120) falls in
exactly one window, with no gaps or overlaps.

⚠️ **One decision is still open for the release owner — see "Upsert v2"
below.** Every flag this train held open was re-read in source at the
`v26.48.0` ship tag rather than inherited from the RC snapshots:

| Flag | Default at `v26.48.0` | Source | Held since | Disposition |
| -- | -- | -- | -- | -- |
| `enable_upsert_v2` | **`false`** |
`src/storage-types/src/dyncfgs.rs:505-509` | rc.2 | ⚠️ premise
contradicted — see below |
| `enable_upsert_chunked_stash` | **`false`** |
`src/storage-types/src/dyncfgs.rs:469-475` | rc.2 | same |
| `enable_upsert_paged_spill` | **`false`** |
`src/storage-types/src/dyncfgs.rs:450-455` | rc.2 | same |
| `enable_mcp_protocol_2026_07_28` | **`false`** |
`src/adapter-dyncfgs/src/dyncfgs.rs:295-299` | rc.1 | hold stands —
still omitted |
| `cluster-details-page-cns-173` | n/a (LaunchDarkly) | no source
default | rc.1 | hold stands — still omitted |

**The rc.2 hold on
[#40189](#40189) rests
on a premise this branch now contradicts.** rc.2 omitted it at
Borderline on exactly one ground — every measurement is taken on a path
`enable_upsert_v2` keeps off by default — and asked whether any
environment actually runs upsert v2. The branch answers that without
anyone feeding the answer back: the human-added **"Improved hydration
times and memory efficiency for upsert sources"** section states *"This
change has been rolled out to all Materialize Cloud environments and
applies to all existing and new clusters."* The three flags still read
`false` at the ship tag, which is exactly the
source-default-versus-LaunchDarkly divergence the rollout-lever rule
covers: the published docs call wins, and the sole stated reason for the
omit has fallen away.

- **Not already covered.** The published section is about **hydration**
— building a source from scratch, 206 min → 108–113 min, 542 GiB →
183–208 GiB. #40189 is about **catch-up after a backlog**, a different
operation with its own motivating incident
([CPU-320](https://linear.app/materializeinc/issue/CPU-320): a source
that *"could not catch up on any replica size after a producer rate
increase"*). A customer who read the hydration section would learn
nothing about catch-up.
- **Suggested Improvements bullet, to add only if you confirm** — it was
*not* applied in the `final` commit:
> **Faster catch-up for upsert sources with a large backlog**: An upsert
source working through a backlog now drains one timestamp per round
instead of merging and re-pushing the whole backlog each round, so
catch-up time grows linearly rather than quadratically with the backlog;
draining a 100-timestamp tail fell from 6.5 to 0.15 seconds, and a
400-timestamp tail drains in 0.67 seconds.
- **Caveat that must travel with the decision**: the baseline in those
timings is the *paged* flavor on `main`, not the configuration Cloud ran
before this rollout — so they are not a clean before/after for a given
customer. Dropping the two timings and keeping the
linear-versus-quadratic claim holds regardless of baseline.
- **Two weaker links, stated rather than buried**: that the rolled-out
configuration is the one #40189 was measured on is *inferred from the
published section's prose* (its "buffer pool … serve sequential work
from disk" reads as chunked stash plus paged spill), not confirmed by
anyone. And the author declared "Release notes: None", naming
`enable_upsert_v2` — so publishing this would be an **override** of an
author signal, justified by your own later rollout statement.
- **Third consecutive train** to hold this flag: `v26.44.0-rc.1`
([#39041](#39041)),
`v26.45.0-rc.1`
([#39044](#39044)),
`v26.48.0-rc.2` (#40189). If the rollout statement is accurate, the
earlier two are worth revisiting rather than left to age out.

**The other two holds stand, corroborated.**
`enable_mcp_protocol_2026_07_28` still reads `false` at the ship tag and
the five MCP PRs rc.1 held appear nowhere in the section; neither does
the Console cluster-details page stack. In both cases you had the whole
train in view and did not promote them — the confirmation rc.1 asked
for, arriving in the negative.

**No duplicates, re-checked at ship.** The seven PRs rc.1 flagged as
already covered by v26.47.0 are still absent, the `## v26.47.1` section
is untouched, and rc.3's entry
([#40065](#40065))
appears exactly once.

**Two notes on human edits to this branch**, both recorded as fact
rather than objection:

- `524adb8` removed the rc.1 feature section **"Native Google Cloud
Storage for persist"**
([#39983](#39983))
outright. Flagging it only because a published entry disappearing is
easy to miss in a long branch — if that was deliberate, nothing to do.
- Two rc.1 Improvements were promoted to their own feature sections with
docs links and a code sample (Kafka sink headers
[#39908](#39908);
MySQL `EXCLUDE ALL NOT NULL CONSTRAINTS`
[#39873](#39873)), and
a **Guides** section was added that is not in the skill's template. The
lesson for next time: an option whose flag flipped to `true` in the same
release more likely warrants a feature section than the Improvements
bullet the rubric suggests by default.

**Carried forward from rc.3 and still open**: the `K8s recovery: envd on
failing node` nightly — the job that sank
[#40029](#40029), the
first attempt at what #40065 re-lands — **was not checked**; it needs
Buildkite, outside this run. The entry itself was verified correct in
source at both the rc.3 and ship tags, so this is a CI-confidence
question, not a notes-accuracy one.

### rc.3 increment

One Improvement, for #40065: a read-only `environmentd` that reboots to
pick up DDL changes during zero-downtime upgrade catch-up now restarts
in place rather than exiting its container, so Kubernetes no longer
applies crash-restart backoff. Not inherited from rc.1 — rc.1 correctly
omitted the first attempt (#40029) together with its revert (#40061) as
a no-op on `main`; rc.3 re-lands the behavior in narrowed form, verified
as a real change by comparing `entrypoint.sh` at the `v26.47.0` tag (no
retry loop) against `v26.48.0-rc.3` (re-exec on exit 75). The draft's
closing "so zero-downtime upgrades catch up faster" was cut: no cited PR
measures an upgrade duration, so the entry claims only the mechanism.

- **Not a Bug Fix.** 0dt upgrades completed on v26.47.0; the catch-up
reboots merely collected backoff delay. Existing behavior improved, so
Improvements.
- **Residual, deliberately unpublished.** A stale read-only process in a
replaced pod still restarts in place on these halts until the kubelet
kills its container, competing with its replacement for the generation's
replicas. The `MZ_RESTARTED_IN_PLACE` marker check keeps it from booting
writable, so it cannot fence the leader — the failure #40061 reverted.
Worth watching if 0dt reports look odd after this ships.

### rc.2 increment — empty, by design

rc.2's commit is empty: no entry was published and the provisional dates
did not move. It exists so later runs can see from the commit subject
that rc.2 was applied. Its single materialize PR, #40189, is the open
item discussed under the `final` increment above.

### rc.1 — standing warnings

⚠️ **Seven PRs in rc.1's tracking issue were already covered by the
v26.47.0 train, and none is republished here.** v26.47.0 took five RCs
whose later ones were cherry-picks onto the release branch; the same PRs
appear in this issue because they are also what landed on `main` for
v26.48.0. Four of the seven **shipped to Cloud in v26.47.0 on
2026-10-22** — #37653, #39910, #39927, #40036 — so republishing any of
them would put duplicate text in front of customers eight days later.
The other three (#39903, #39906, #40046) were held at Borderline in
v26.47.0 and are held again here, each reason re-checked rather than
inherited. The "snapshots are disjoint by construction" guarantee holds
*within* a train, not across trains, so this was caught mechanically by
grepping every PR number in the issue against the v26.47.0 snapshot
directories. All seven are listed under their own heading in
`rc.1/omitted.md`.

**Open questions for the release owner**

- **Upsert v2** (#40189, rc.2) — see the `final` increment above. The
rollout statement on this branch has changed the picture since rc.2;
this is the one item that needs a decision before merge.
- **MCP protocol revision 2026-07-28** (#39698, #39702, #39703, #39759,
#39761) — the stack is complete and `enable_mcp_protocol_2026_07_28`
still defaults to **`false`** at the ship tag. Omitted, with the absence
of any MCP text on this branch taken as confirmation. Drafted wording
remains in `rc.1/omitted.md` if the flag flips.
- **Redesigned cluster details page in the Console** (#39446, #39507,
#39508, #39510, #39607) — omitted for the same reason. Note that the
Improvements section does carry **"New cluster details page on
Self-Managed"**, added by hand, which links the v26.47 section rather
than re-describing the page.
- **Schema projection on the Console's Freshness page** (#40031) — a
genuine new way to diagnose lag, but the author states the whole
Freshness page "is currently behind a feature flag that is only enabled
for certain customers in impersonation mode". **Is the page generally
reachable yet?** This is the same page v26.47.0 held for the same
reason.
- **Time zone data 2026e timing** (#40045) — the author flags the
Canadian zone changes as effective **2026-11-01**, three days after the
Cloud date. Tight, but it holds.

*Resolved since rc.1: the Kafka sink headers framing question — the GA
section on this branch answers it.*

**Verification notes** — rc.1's Step 5 pass cut one drafted entry and
rewrote four:

- **Cut**: a Bug Fix drafted for #40032 ("halt with exit code 168 when a
live reader loses its lease"). The PR's own Problem section is about
alerting — `mz_persist_lease_timeout_read` cannot distinguish a dead
reader from a stuck one — and the follow-up it names is a Cloud alerting
change. Nothing a customer observes changes.
- **tzdata** (#40045): the draft said the Canadian zones move to
"permanent daylight saving time", the author's prose. The PR's data
table shows a permanent **−05/EST** and **−06/CST** — summer *offsets*
kept year-round, but with another zone's standard-time *abbreviations*,
which is what a user sees from `AT TIME ZONE`. Reworded to what the
table supports.
- **Hydration memory** (#39923): scoped to the consolidation step and
the measured outcome (~628 GiB peak, no restart, against every
pre-change run exiting on the 849 GiB limit). The PR explicitly says the
fix "covers this consolidation path, not the sink's whole footprint";
[CPU-317](https://linear.app/materializeinc/issue/CPU-317) tracks the
rest.
- **Kafka clients** (#40026): "55 to 24" is measured on a 4-worker,
11-source setup. The CPU throttling and degraded freshness are observed
symptoms of the *old* behavior at a few hundred sources, not a measured
post-change result at that scale, so the entry states the count as the
outcome and the throttling as what the count drove.
- **`environmentd` accept backlog** (#40035): the author's hedge is kept
— "less likely to be restarted", not eliminated, because the PR says
"This makes the queue fill later, not never."

**Judgment calls a reviewer may want to revisit** — #39605 is filed as
an **Improvement**, not a Bug Fix: nothing was incorrect, an unbounded
operation gained a bound and a knob. #40101 is filed as a **Bug Fix**
rather than omitted under the internal-logging rule, because credentials
reaching the logs is a security defect with a user-visible remedy; only
credentials a user inlined rather than wrapped in a `SECRET` were
affected.

Snapshot drafts (with PR links) in mz-skills:
- rc.1:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.1/release_notes.md)
/
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.1/omitted.md)
- rc.2:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.2/release_notes.md)
/
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.2/omitted.md)
- rc.3:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.3/release_notes.md)
/
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.3/omitted.md)
- final:
[release_notes.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/final/release_notes.md)
/
[omitted.md](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/final/omitted.md)

**Borderline PRs omitted:** rc.1: 12 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.1/omitted.md#borderline),
rc.2: 1 —
[omitted.md#borderline](https://github.com/MaterializeInc/mz-skills/blob/main/data/mz-release-notes/v26.48.0/rc.2/omitted.md#borderline),
rc.3: 0, final: 0

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <[email protected]>
Co-authored-by: Valerie Chiang <[email protected]>
Co-authored-by: Pranshu Maheshwari <[email protected]>
Co-authored-by: Pranshu Maheshwari <[email protected]>