diff --git a/src/utils/network-classifier.ts b/src/utils/network-classifier.ts
new file mode 100644
index 0000000..ec61f05
--- /dev/null
+++ b/src/utils/network-classifier.ts
@@ -0,0 +1,178 @@
+/**
+ * ADR-107: Network exposure modes as a classified state machine.
+ *
+ * Pure functions consumed by server-bind code, settings UI, and the
+ * MCP `initialize` instructions injector — single source of truth for
+ * "how exposed is the current network configuration?"
+ */
+
+export type BindMode = 'loopback' | 'all' | 'custom';
+
+export type Protocol = 'http' | 'https';
+
+export type CertSource = 'self' | 'user';
+
+export type ResolvedBind = 'loopback' | 'all' | 'custom-loopback' | 'custom-other';
+
+export interface NetworkState {
+  protocol: Protocol;
+  bind: ResolvedBind;
+  certSource: CertSource;
+}
+
+export type VerdictClass = 'ok' | 'warn' | 'jail';
+
+export interface Verdict {
+  class: VerdictClass;
+  reason: string;
+}
+
+const LOOPBACK_ALIASES = new Set(['127.0.0.1', 'localhost', '::1', '::ffff:127.0.0.1']);
+const WILDCARD_ALIASES = new Set(['0.0.0.0', '::']);
+const LOOPBACK_PREFIXES = ['127.'];
+
+function isLoopbackHost(host: string): boolean {
+  const h = host.trim().toLowerCase();
+  if (LOOPBACK_ALIASES.has(h)) return true;
+  return LOOPBACK_PREFIXES.some((p) => h.startsWith(p));
+}
+
+function isWildcardHost(host: string): boolean {
+  return WILDCARD_ALIASES.has(host.trim());
+}
+
+/**
+ * If the user typed a loopback alias or wildcard into the custom field,
+ * collapse it to the corresponding explicit mode. Returns the canonical
+ * mode + the host to persist (blank for non-custom).
+ */
+export function normalizeBindInput(
+  mode: BindMode,
+  customHost: string
+): { mode: BindMode; customHost: string } {
+  if (mode !== 'custom') return { mode, customHost: '' };
+  const trimmed = customHost.trim();
+  if (trimmed === '') return { mode: 'custom', customHost: '' };
+  if (isLoopbackHost(trimmed)) return { mode: 'loopback', customHost: '' };
+  if (isWildcardHost(trimmed)) return { mode: 'all', customHost: '' };
+  return { mode: 'custom', customHost: trimmed };
+}
+
+/**
+ * Resolve settings to the actual host string passed to server.listen().
+ */
+export function resolveListenHost(mode: BindMode, customHost: string): string {
+  switch (mode) {
+    case 'all':
+      return '0.0.0.0';
+    case 'custom': {
+      const trimmed = customHost.trim();
+      return trimmed === '' ? '127.0.0.1' : trimmed;
+    }
+    case 'loopback':
+    default:
+      return '127.0.0.1';
+  }
+}
+
+/**
+ * Resolve settings to the bind axis used by classify().
+ * `custom` collapses to `custom-loopback` or `custom-other` based on
+ * whether the typed host is a loopback alias.
+ */
+export function resolveBindAxis(mode: BindMode, customHost: string): ResolvedBind {
+  if (mode === 'loopback') return 'loopback';
+  if (mode === 'all') return 'all';
+  const trimmed = customHost.trim();
+  if (trimmed === '' || isLoopbackHost(trimmed)) return 'custom-loopback';
+  return 'custom-other';
+}
+
+/**
+ * The full classification table from ADR-107.
+ */
+export function classify(state: NetworkState): Verdict {
+  const { protocol, bind, certSource } = state;
+
+  if (protocol === 'http') {
+    if (bind === 'loopback' || bind === 'custom-loopback') {
+      return {
+        class: 'ok',
+        reason: 'HTTP on loopback — traffic never leaves this machine.'
+      };
+    }
+    if (bind === 'all') {
+      return {
+        class: 'jail',
+        reason:
+          'HTTP bound to every interface. API key and vault content travel in cleartext to anyone on the network.'
+      };
+    }
+    return {
+      class: 'jail',
+      reason:
+        'HTTP bound to a non-loopback address. API key and vault content travel in cleartext on the wire.'
+    };
+  }
+
+  // protocol === 'https'
+  if (bind === 'loopback' || bind === 'custom-loopback') {
+    return {
+      class: 'ok',
+      reason: 'HTTPS on loopback — self-signed certificate is fine here.'
+    };
+  }
+
+  if (certSource === 'user') {
+    return {
+      class: 'ok',
+      reason:
+        bind === 'all'
+          ? 'HTTPS on every interface with a user-supplied certificate — the intended public deployment.'
+          : 'HTTPS on a custom interface with a user-supplied certificate — the intended LAN/remote deployment.'
+    };
+  }
+
+  // self-signed on non-loopback
+  return {
+    class: 'warn',
+    reason:
+      bind === 'all'
+        ? 'HTTPS on every interface with a self-signed certificate — encrypted but clients must trust-on-first-use.'
+        : 'HTTPS on a custom interface with a self-signed certificate — encrypted but clients must trust-on-first-use.'
+  };
+}
+
+/**
+ * Convenience: build NetworkState from settings + classify in one call.
+ */
+export function classifyFromSettings(args: {
+  httpsEnabled: boolean;
+  bindMode: BindMode;
+  customBindHost: string;
+  userSuppliedCert: boolean;
+}): Verdict {
+  return classify({
+    protocol: args.httpsEnabled ? 'https' : 'http',
+    bind: resolveBindAxis(args.bindMode, args.customBindHost),
+    certSource: args.userSuppliedCert ? 'user' : 'self'
+  });
+}
+
+/**
+ * The agent-visible warning copy injected into MCP initialize.instructions
+ * when the verdict is `jail`. Returns null otherwise.
+ */
+export function agentInstructionsForVerdict(
+  verdict: Verdict,
+  resolvedHost: string,
+  port: number
+): string | null {
+  if (verdict.class !== 'jail') return null;
+  return [
+    'SECURITY WARNING: This Obsidian MCP server is configured to serve vault contents',
+    `over an unencrypted network interface (${resolvedHost}:${port}). The API key and all`,
+    'document text travel in cleartext over the network. If the user did not intend this,',
+    'advise them to reconfigure the plugin to use HTTPS or to bind to loopback (127.0.0.1) only.'
+  ].join(' ');
+}
diff --git a/tests/network-classifier.test.ts b/tests/network-classifier.test.ts
new file mode 100644
index 0000000..846e8cb
--- /dev/null
+++ b/tests/network-classifier.test.ts
@@ -0,0 +1,183 @@
+import {
+  classify,
+  classifyFromSettings,
+  resolveListenHost,
+  resolveBindAxis,
+  normalizeBindInput,
+  agentInstructionsForVerdict,
+  NetworkState
+} from '../src/utils/network-classifier';
+
+describe('classify() — 9-cell ADR-107 table', () => {
+  const cases: Array<{ name: string; state: NetworkState; expected: 'ok' | 'warn' | 'jail' }> = [
+    { name: '1: http + loopback',            state: { protocol: 'http',  bind: 'loopback',            certSource: 'self' }, expected: 'ok'   },
+    { name: '2: http + custom-other',        state: { protocol: 'http',  bind: 'custom-other',        certSource: 'self' }, expected: 'jail' },
+    { name: '3: http + all',                 state: { protocol: 'http',  bind: 'all',                 certSource: 'self' }, expected: 'jail' },
+    { name: '4: https + loopback + self',    state: { protocol: 'https', bind: 'loopback',            certSource: 'self' }, expected: 'ok'   },
+    { name: '5: https + loopback + user',    state: { protocol: 'https', bind: 'loopback',            certSource: 'user' }, expected: 'ok'   },
+    { name: '6: https + custom-other + self',state: { protocol: 'https', bind: 'custom-other',        certSource: 'self' }, expected: 'warn' },
+    { name: '7: https + custom-other + user',state: { protocol: 'https', bind: 'custom-other',        certSource: 'user' }, expected: 'ok'   },
+    { name: '8: https + all + self',         state: { protocol: 'https', bind: 'all',                 certSource: 'self' }, expected: 'warn' },
+    { name: '9: https + all + user',         state: { protocol: 'https', bind: 'all',                 certSource: 'user' }, expected: 'ok'   }
+  ];
+
+  for (const c of cases) {
+    test(c.name, () => {
+      const v = classify(c.state);
+      expect(v.class).toBe(c.expected);
+      expect(v.reason).toBeTruthy();
+    });
+  }
+
+  test('custom-loopback under http classifies as ok (not jail)', () => {
+    expect(classify({ protocol: 'http', bind: 'custom-loopback', certSource: 'self' }).class).toBe('ok');
+  });
+
+  test('custom-loopback under https + self classifies as ok (not warn)', () => {
+    expect(classify({ protocol: 'https', bind: 'custom-loopback', certSource: 'self' }).class).toBe('ok');
+  });
+});
+
+describe('normalizeBindInput()', () => {
+  test.each([
+    ['127.0.0.1',         'loopback'],
+    ['localhost',         'loopback'],
+    ['LOCALHOST',         'loopback'],
+    ['::1',               'loopback'],
+    ['::ffff:127.0.0.1',  'loopback'],
+    ['127.0.0.99',        'loopback'],
+    ['  127.0.0.1  ',     'loopback']
+  ])('"%s" collapses to loopback mode', (input, expectedMode) => {
+    const out = normalizeBindInput('custom', input);
+    expect(out.mode).toBe(expectedMode);
+    expect(out.customHost).toBe('');
+  });
+
+  test.each([
+    ['0.0.0.0',  'all'],
+    ['::',       'all'],
+    ['  0.0.0.0', 'all']
+  ])('"%s" collapses to all mode', (input, expectedMode) => {
+    const out = normalizeBindInput('custom', input);
+    expect(out.mode).toBe(expectedMode);
+    expect(out.customHost).toBe('');
+  });
+
+  test('arbitrary LAN address stays custom and is trimmed', () => {
+    const out = normalizeBindInput('custom', '  192.168.1.50  ');
+    expect(out.mode).toBe('custom');
+    expect(out.customHost).toBe('192.168.1.50');
+  });
+
+  test('empty custom stays custom with empty host', () => {
+    expect(normalizeBindInput('custom', '')).toEqual({ mode: 'custom', customHost: '' });
+    expect(normalizeBindInput('custom', '   ')).toEqual({ mode: 'custom', customHost: '' });
+  });
+
+  test('non-custom modes blank the customHost', () => {
+    expect(normalizeBindInput('loopback', '192.168.1.5')).toEqual({ mode: 'loopback', customHost: '' });
+    expect(normalizeBindInput('all', '192.168.1.5')).toEqual({ mode: 'all', customHost: '' });
+  });
+});
+
+describe('resolveListenHost()', () => {
+  test('loopback → 127.0.0.1', () => {
+    expect(resolveListenHost('loopback', '')).toBe('127.0.0.1');
+  });
+
+  test('all → 0.0.0.0', () => {
+    expect(resolveListenHost('all', '')).toBe('0.0.0.0');
+  });
+
+  test('custom → trimmed customHost', () => {
+    expect(resolveListenHost('custom', '  192.168.1.50  ')).toBe('192.168.1.50');
+  });
+
+  test('custom with empty host falls back to loopback (defensive)', () => {
+    expect(resolveListenHost('custom', '')).toBe('127.0.0.1');
+    expect(resolveListenHost('custom', '   ')).toBe('127.0.0.1');
+  });
+});
+
+describe('resolveBindAxis()', () => {
+  test('loopback mode → loopback axis', () => {
+    expect(resolveBindAxis('loopback', '')).toBe('loopback');
+  });
+
+  test('all mode → all axis', () => {
+    expect(resolveBindAxis('all', '')).toBe('all');
+  });
+
+  test('custom with empty host → custom-loopback (defensive)', () => {
+    expect(resolveBindAxis('custom', '')).toBe('custom-loopback');
+  });
+
+  test('custom with loopback alias → custom-loopback', () => {
+    expect(resolveBindAxis('custom', '127.0.0.1')).toBe('custom-loopback');
+    expect(resolveBindAxis('custom', 'localhost')).toBe('custom-loopback');
+  });
+
+  test('custom with LAN address → custom-other', () => {
+    expect(resolveBindAxis('custom', '192.168.1.50')).toBe('custom-other');
+  });
+});
+
+describe('classifyFromSettings()', () => {
+  test('default install (http + loopback) → ok', () => {
+    const v = classifyFromSettings({
+      httpsEnabled: false,
+      bindMode: 'loopback',
+      customBindHost: '',
+      userSuppliedCert: false
+    });
+    expect(v.class).toBe('ok');
+  });
+
+  test('http + all → jail', () => {
+    const v = classifyFromSettings({
+      httpsEnabled: false,
+      bindMode: 'all',
+      customBindHost: '',
+      userSuppliedCert: false
+    });
+    expect(v.class).toBe('jail');
+  });
+
+  test('https + all + self-signed → warn', () => {
+    const v = classifyFromSettings({
+      httpsEnabled: true,
+      bindMode: 'all',
+      customBindHost: '',
+      userSuppliedCert: false
+    });
+    expect(v.class).toBe('warn');
+  });
+
+  test('https + all + user cert → ok', () => {
+    const v = classifyFromSettings({
+      httpsEnabled: true,
+      bindMode: 'all',
+      customBindHost: '',
+      userSuppliedCert: true
+    });
+    expect(v.class).toBe('ok');
+  });
+});
+
+describe('agentInstructionsForVerdict()', () => {
+  test('returns null for ok', () => {
+    expect(agentInstructionsForVerdict({ class: 'ok', reason: 'x' }, '127.0.0.1', 3001)).toBeNull();
+  });
+
+  test('returns null for warn', () => {
+    expect(agentInstructionsForVerdict({ class: 'warn', reason: 'x' }, '0.0.0.0', 3443)).toBeNull();
+  });
+
+  test('returns warning string for jail, includes host and port', () => {
+    const s = agentInstructionsForVerdict({ class: 'jail', reason: 'x' }, '0.0.0.0', 3001);
+    expect(s).not.toBeNull();
+    expect(s).toContain('SECURITY WARNING');
+    expect(s).toContain('0.0.0.0:3001');
+    expect(s).toContain('cleartext');
+  });
+});