diff --git a/packages/core/src/__tests__/mcp-host-classification.test.ts b/packages/core/src/__tests__/mcp-host-classification.test.ts
new file mode 100644
index 0000000000..e9b1d19f2f
--- /dev/null
+++ b/packages/core/src/__tests__/mcp-host-classification.test.ts
@@ -0,0 +1,113 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { isLoopbackHost, isPrivateRangeHost } from '../mcp.js';
+
+describe('MCP host classification', () => {
+  it('classifies IPv6 literals by parsed bytes, not spelling', () => {
+    // WHATWG URL parsing canonicalizes hostnames, so the same address
+    // arrives in different spellings depending on how it was written; the
+    // classifier must agree across all of them.
+    for (const spelling of [
+      '[::ffff:127.0.0.1]',
+      '[::FFFF:127.0.0.1]',
+      '[::ffff:7f00:1]',
+      '[0:0:0:0:0:ffff:7f00:1]',
+    ]) {
+      assert.equal(isPrivateRangeHost(spelling), true, spelling);
+      // The asymmetry is deliberate: teaching isLoopbackHost the mapped
+      // spelling would flip its fail-closed cleartext refusal into
+      // fail-open. The mapped loopback must land on the PRIVATE side.
+      assert.equal(isLoopbackHost(spelling), false, spelling);
+    }
+    // The canonical URL round-trip agrees with the raw spellings.
+    assert.equal(isPrivateRangeHost(new URL('https://[::ffff:192.168.1.1]/').hostname), true);
+    assert.equal(isPrivateRangeHost(new URL('https://[64:ff9b::10.0.0.5]/').hostname), true);
+  });
+
+  it('covers the whole fe80::/10 link-local range, not just the fe8 spelling', () => {
+    for (const inner of ['fe80::1', 'fe9a::1', 'feaf::1', 'febf::1']) {
+      assert.equal(isPrivateRangeHost(`[${inner}]`), true, inner);
+    }
+    assert.equal(isPrivateRangeHost('[fec0::1]'), false);
+  });
+
+  it('classifies deprecated IPv4-compatible ::/96 embeddings by their bytes', () => {
+    // RFC 4291 §2.5.5.1 — deprecated, but a byte-level embedding the
+    // classifier must know regardless of whether modern stacks still
+    // translate it: the gate's correctness must not rest on the other
+    // end's network stack.
+    assert.equal(isPrivateRangeHost(new URL('https://[::192.168.1.1]/').hostname), true);
+    assert.equal(isPrivateRangeHost(new URL('https://[::127.0.0.1]/').hostname), true);
+    assert.equal(isPrivateRangeHost('[::c0a8:101]'), true);
+    assert.equal(isPrivateRangeHost('[::7f00:1]'), true);
+    assert.equal(isPrivateRangeHost('[::a00:5]'), true); // ::10.0.0.5
+    // Public v4-compatible stays global; the unspecified address reaches
+    // the local machine on common stacks and is private (fail closed).
+    assert.equal(isPrivateRangeHost('[::808:808]'), false);
+    assert.equal(isPrivateRangeHost('[::]'), true);
+    assert.equal(isPrivateRangeHost('[::2]'), false);
+  });
+
+  it('classifies the remaining translation embeddings and the mapped unspecified address', () => {
+    // Mapped unspecified: connecting to ::ffff:0.0.0.0 reaches a
+    // 127.0.0.1-bound listener on common stacks — private, fail closed.
+    assert.equal(isPrivateRangeHost('[::ffff:0:0]'), true);
+    // RFC 8215 local-use NAT64 space (64:ff9b:1::/48) is reserved for
+    // in-network translation; deployments carve arbitrary RFC 6052 prefix
+    // lengths out of it, so the whole /48 fails closed.
+    assert.equal(isPrivateRangeHost('[64:ff9b:1:c0a8:1:100::]'), true);
+    assert.equal(isPrivateRangeHost('[64:ff9b:1::1]'), true);
+    // RFC 2765 SIIT (::ffff:0:0/96) classifies by its embedded IPv4.
+    assert.equal(isPrivateRangeHost(new URL('https://[0::ffff:0:192.168.1.1]/').hostname), true);
+    assert.equal(isPrivateRangeHost('[::ffff:0:808:808]'), false); // SIIT 8.8.8.8
+  });
+
+  it('treats exactly 0.0.0.0 as machine-local in the plain IPv4 branch too', () => {
+    // `https://0/` canonicalizes to 0.0.0.0, and connecting to it reaches
+    // the local machine — the same rationale as the embedded checks, which
+    // this branch previously never consulted. Only the exact address:
+    // 0.0.0.1 does not reach a local listener.
+    assert.equal(isPrivateRangeHost('0.0.0.0'), true);
+    assert.equal(isPrivateRangeHost(new URL('https://0/').hostname), true);
+    assert.equal(isPrivateRangeHost('0.0.0.1'), false);
+  });
+
+  it('rejects literals RFC 4291 does not permit instead of mis-parsing them', () => {
+    // Dotted IPv4 belongs only in the low-order 32 bits; an empty zone id
+    // is not a literal. Both fail closed as private.
+    assert.equal(isPrivateRangeHost('[192.168.1.1::]'), true);
+    assert.equal(isPrivateRangeHost('[1:2.2.2.2:3::]'), true);
+    assert.equal(isPrivateRangeHost('[2606:4700::1%]'), true);
+  });
+
+  it('keeps global addresses reachable and fails closed on garbage', () => {
+    assert.equal(isPrivateRangeHost('[2606:4700::1]'), false);
+    assert.equal(isPrivateRangeHost('[::ffff:808:808]'), false); // mapped 8.8.8.8
+    assert.equal(isPrivateRangeHost('[64:ff9b::808:808]'), false);
+    assert.equal(isPrivateRangeHost('[::1]'), false); // isLoopbackHost's positive case
+    assert.equal(isLoopbackHost('[::1]'), true);
+    // An unparseable bracketed literal is treated as private: the SSRF
+    // gate fails closed on spellings it cannot classify.
+    assert.equal(isPrivateRangeHost('[not-an-address]'), true);
+    assert.equal(isPrivateRangeHost('[1::2::3]'), true);
+  });
+});
diff --git a/packages/core/src/mcp.ts b/packages/core/src/mcp.ts
index a9dc697016..dae073c5c2 100644
--- a/packages/core/src/mcp.ts
+++ b/packages/core/src/mcp.ts
@@ -81,25 +81,155 @@ export function isLoopbackHost(hostname: string): boolean {
 /** Private-range and link-local IP LITERALS (RFC 1918, RFC 3927/4291,
  * CGNAT). Hostname-based checks are deliberately out of scope: they would
  * need a resolve here and could still re-resolve differently at request
- * time — callers treat privately-RESOLVING names as accepted risk. */
+ * time — callers treat privately-RESOLVING names as accepted risk.
+ *
+ * IPv6 literals are classified by PARSED BYTES, not spelling: WHATWG URL
+ * parsing canonicalizes hostnames (`[::ffff:127.0.0.1]` arrives here as
+ * `[::ffff:7f00:1]`), so any spelling-based match is one canonicalization
+ * away from missing an address it means to cover. IPv4-mapped (::ffff/96)
+ * and NAT64 (64:ff9b::/96) embeddings classify by their embedded IPv4 —
+ * INCLUDING 127/8, which `isLoopbackHost` deliberately does not learn:
+ * there, an unrecognized spelling must stay "not loopback" so cleartext is
+ * refused (fail closed); here it must stay "private" so the SSRF gate
+ * blocks it (also fail closed) — the two defaults point in opposite
+ * directions on purpose. A bracketed literal that does not parse at all is
+ * therefore treated as private. */
 export function isPrivateRangeHost(hostname: string): boolean {
   const v4 = /^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/u.exec(hostname);
   if (v4) {
-    const [a, b] = [Number(v4[1]), Number(v4[2])];
-    if (a === 10) return true;
-    if (a === 172 && b >= 16 && b <= 31) return true;
-    if (a === 192 && b === 168) return true;
-    if (a === 169 && b === 254) return true;
-    if (a === 100 && b >= 64 && b <= 127) return true;
-    return false;
+    return isPrivateIpv4(Number(v4[1]), Number(v4[2]), Number(v4[3]), Number(v4[4]));
   }
-  if (hostname.startsWith('[')) {
-    const inner = hostname.slice(1, -1).toLowerCase();
-    return inner.startsWith('fc') || inner.startsWith('fd') || inner.startsWith('fe8');
+  if (hostname.startsWith('[') && hostname.endsWith(']')) {
+    const bytes = parseIpv6(hostname.slice(1, -1));
+    if (!bytes) return true;
+    return classifyIpv6(bytes) === 'private';
   }
   return false;
 }
 
+function isPrivateIpv4(a: number, b: number, c: number, d: number): boolean {
+  if (a === 10) return true;
+  if (a === 172 && b >= 16 && b <= 31) return true;
+  if (a === 192 && b === 168) return true;
+  if (a === 169 && b === 254) return true;
+  if (a === 100 && b >= 64 && b <= 127) return true;
+  // Exactly the unspecified address: connecting to 0.0.0.0 reaches the
+  // LOCAL machine on common stacks (`https://0/` canonicalizes to it), so
+  // it fails closed — but only the exact address: 0.0.0.1 does not reach a
+  // local listener, and widening to 0.0.0.0/8 would grow the private set
+  // without buying anything.
+  if (a === 0 && b === 0 && c === 0 && d === 0) return true;
+  return false;
+}
+
+/** Minimal RFC 4291 text-representation parser: enough to turn any spelling
+ * of an address into its 16 bytes so the classifier never depends on how a
+ * caller (or the URL canonicalizer) chose to write it. Returns undefined
+ * for anything that is not a well-formed address. */
+function parseIpv6(literal: string): Uint8Array | undefined {
+  const zone = literal.indexOf('%');
+  // An empty zone id ('fe80::1%') is not a valid literal; rejecting it
+  // keeps the documented fail-closed default honest.
+  if (zone !== -1 && zone === literal.length - 1) return undefined;
+  const text = (zone === -1 ? literal : literal.slice(0, zone)).toLowerCase();
+  const gap = text.indexOf('::');
+  if (gap !== -1 && text.indexOf('::', gap + 1) !== -1) return undefined;
+  const parseGroups = (part: string, dottedAllowedAtEnd: boolean): number[] | undefined => {
+    if (part === '') return [];
+    const out: number[] = [];
+    const pieces = part.split(':');
+    for (let index = 0; index < pieces.length; index += 1) {
+      const piece = pieces[index] ?? '';
+      if (piece.includes('.')) {
+        // RFC 4291 §2.2: dotted IPv4 only in the low-order 32 bits — the
+        // FINAL piece. '[192.168.1.1::]' must fail, not parse shifted.
+        if (!dottedAllowedAtEnd || index !== pieces.length - 1) return undefined;
+        const dotted = /^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/u.exec(piece);
+        if (!dotted) return undefined;
+        const octets = dotted.slice(1).map(Number);
+        if (octets.some((octet) => octet > 255)) return undefined;
+        out.push(
+          ((octets[0] ?? 0) << 8) | (octets[1] ?? 0),
+          ((octets[2] ?? 0) << 8) | (octets[3] ?? 0),
+        );
+      } else {
+        if (!/^[0-9a-f]{1,4}$/u.test(piece)) return undefined;
+        out.push(Number.parseInt(piece, 16));
+      }
+    }
+    return out;
+  };
+  let groups: number[] | undefined;
+  if (gap === -1) {
+    groups = parseGroups(text, true);
+    if (!groups || groups.length !== 8) return undefined;
+  } else {
+    // Dotted IPv4 can only close the address, so only the tail may carry it.
+    const head = parseGroups(text.slice(0, gap), false);
+    const tail = parseGroups(text.slice(gap + 2), true);
+    if (!head || !tail || head.length + tail.length > 7) return undefined;
+    groups = [...head, ...new Array<number>(8 - head.length - tail.length).fill(0), ...tail];
+  }
+  const bytes = new Uint8Array(16);
+  groups.forEach((group, index) => {
+    bytes[2 * index] = group >> 8;
+    bytes[2 * index + 1] = group & 0xff;
+  });
+  return bytes;
+}
+
+function classifyIpv6(bytes: Uint8Array): 'loopback' | 'private' | 'global' {
+  if (bytes.slice(0, 15).every((byte) => byte === 0) && bytes[15] === 1) return 'loopback';
+  const mapped =
+    bytes.slice(0, 10).every((byte) => byte === 0) && bytes[10] === 0xff && bytes[11] === 0xff;
+  const nat64 =
+    bytes[0] === 0 &&
+    bytes[1] === 0x64 &&
+    bytes[2] === 0xff &&
+    bytes[3] === 0x9b &&
+    bytes.slice(4, 12).every((byte) => byte === 0);
+  // RFC 8215 local-use NAT64 space (64:ff9b:1::/48) is reserved for
+  // translation inside one network — by definition never a global
+  // destination. Deployments carve arbitrary RFC 6052 prefix lengths out
+  // of it, so the embedded IPv4's position is not recoverable here; the
+  // whole /48 fails closed instead.
+  if (
+    bytes[0] === 0 &&
+    bytes[1] === 0x64 &&
+    bytes[2] === 0xff &&
+    bytes[3] === 0x9b &&
+    bytes[4] === 0 &&
+    bytes[5] === 1
+  ) {
+    return 'private';
+  }
+  // The remaining byte-level IPv4 embeddings, classified by the embedded
+  // address: deprecated IPv4-compatible ::/96 (RFC 4291 §2.5.5.1) and the
+  // RFC 2765 SIIT prefix ::ffff:0:0/96. Deprecated is not absent — the
+  // classifier's whole premise is bytes over spellings, and whether the
+  // OTHER end's stack still translates these must not be what the gate's
+  // correctness rests on. `::1` was returned above.
+  const compat = bytes.slice(0, 12).every((byte) => byte === 0);
+  const siit =
+    bytes.slice(0, 8).every((byte) => byte === 0) &&
+    bytes[8] === 0xff &&
+    bytes[9] === 0xff &&
+    bytes[10] === 0 &&
+    bytes[11] === 0;
+  if (mapped || nat64 || compat || siit) {
+    const [a, b, c, d] = [bytes[12] ?? 0, bytes[13] ?? 0, bytes[14] ?? 0, bytes[15] ?? 0];
+    // Mapped loopback lands on the PRIVATE side of the gate: isLoopbackHost
+    // stays strict-by-spelling, so this is the check that must catch it.
+    // The embedded unspecified address (0.0.0.0 — reaches the local
+    // machine on common stacks) is private via isPrivateIpv4.
+    if (a === 127) return 'private';
+    return isPrivateIpv4(a, b, c, d) ? 'private' : 'global';
+  }
+  if (((bytes[0] ?? 0) & 0xfe) === 0xfc) return 'private'; // fc00::/7 (ULA)
+  if (bytes[0] === 0xfe && ((bytes[1] ?? 0) & 0xc0) === 0x80) return 'private'; // fe80::/10
+  return 'global';
+}
+
 /** The composed rule the config store enforces, the runtime's fetch guard
  * re-checks per hop, and the editor mirrors onto the URL field: cleartext
  * http is only acceptable where it never leaves the machine. One
diff --git a/packages/mcp/src/__tests__/transport-security.test.ts b/packages/mcp/src/__tests__/transport-security.test.ts
index 41c47b6f3d..bbceead72d 100644
--- a/packages/mcp/src/__tests__/transport-security.test.ts
+++ b/packages/mcp/src/__tests__/transport-security.test.ts
@@ -69,6 +69,46 @@ describe('transport security provenance', () => {
     );
   });
 
+  it('refuses IPv6-mapped spellings of machine-local and private destinations', () => {
+    // WHATWG canonicalizes `[::ffff:127.0.0.1]` to `[::ffff:7f00:1]` before
+    // the gate ever sees it — the classification is by parsed bytes, so
+    // every spelling of the same address lands on the same side. Note the
+    // asymmetry: these are caught as PRIVATE (isLoopbackHost deliberately
+    // stays spelling-strict, so cleartext to the mapped loopback is still
+    // refused by the http branch).
+    for (const url of [
+      'https://[::ffff:127.0.0.1]:8443/latest/meta-data/',
+      'https://[::ffff:192.168.1.1]/router',
+      'https://[0:0:0:0:0:ffff:10.0.0.5]/internal',
+      'https://[64:ff9b::192.168.1.1]/nat64',
+      'https://[fe9a::1]/link-local',
+      // Deprecated IPv4-compatible ::/96 — the third byte-level embedding,
+      // arriving through the same remote-provenance path a real one would.
+      'https://[::192.168.1.1]/compat',
+      'https://[::127.0.0.1]:8443/compat-loopback',
+      'https://[::]/unspecified',
+      'https://[::ffff:0:0]:8443/mapped-unspecified',
+      'https://[64:ff9b:1:c0a8:1:100::]/local-nat64',
+      'https://[0::ffff:0:192.168.1.1]/siit',
+      // `https://0/` canonicalizes to 0.0.0.0 and reaches the local machine.
+      'https://0.0.0.0:8443/unspecified-v4',
+      'https://0/short-form',
+    ]) {
+      assert.throws(() => assertTransportSecurity(new URL(url), remoteRoot), /refused remotely/u);
+    }
+    // Public IPv6 stays reachable — that is what real OAuth endpoints on
+    // IPv6 look like.
+    assert.doesNotThrow(() =>
+      assertTransportSecurity(new URL('https://[2606:4700::1]/token'), remoteRoot),
+    );
+    // Cleartext to the mapped loopback is refused by the strict loopback
+    // predicate, exactly as before.
+    assert.throws(
+      () => assertTransportSecurity(new URL('http://[::ffff:127.0.0.1]:8080/mcp'), remoteRoot),
+      /non-loopback hosts require https/u,
+    );
+  });
+
   it('never allows cleartext http off the machine', () => {
     assert.throws(
       () => assertTransportSecurity(new URL('http://api.example.com/mcp'), remoteRoot),