diff --git a/CHANGELOG.md b/CHANGELOG.md
index 51cbda5e3..747c6227d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -63,6 +63,14 @@
 
 ## [Unreleased]
 
+- Stdio servers from one install share a panel key so a page read from one
+  process can make requests through another. Concurrent malformed-key repairs
+  now select one winner; unavailable storage or a busy repair retains the
+  logged per-process fallback. Thanks to [Jibran Tahir](https://github.com/jibraaan)
+  for #958, [MarvinNL046](https://github.com/MarvinNL046) for the report and
+  proposed diagnosis in #927, and [kvnloo](https://github.com/kvnloo) for identifying
+  the repair race. Windows/Claude Desktop Cowork confirmation remains outstanding.
+
 - Street Level: a street-level imagery layer modelled on the iD editor's photo
   overlay, with Mapillary as its first provider (free client token, CC BY-SA 4.0
   imagery). One right-rail panel holds a chip per provider, shared 360°/flat and
diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index 730586df9..ee3bfbdb2 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -14,6 +14,19 @@ new views from the page that frames it; framing is off unless
 `GEV_EMBED_FRAME_ANCESTORS` allows the framing page. MCP leads with the tools that find
 what to show. See [tools and the MCP server](TOOLS.md).
 
+Stdio processes from one install share the panel key in
+`.gev-cache/mcp-panel-key`, resolved from the checkout rather than the client's
+working directory. First creation is atomic; malformed-file repairs select one
+winner and concurrent processes re-read it. A busy repair waits at most two
+seconds before falling back. If the cache cannot be read/written or repair remains
+busy, stderr reports the fallback to a per-process key; cross-process panels can
+still fail in that mode. An abandoned `.repair-lock` is left intact: after
+confirming no repair process is running, remove it to allow repair on restart.
+HTTP MCP servers retain per-server keys. This key is a panel routing guard, not
+authentication. Large-response continuations remain process-local and require
+consistent tool-call routing. Two-process reproduction is covered locally;
+Windows/Claude Desktop Cowork confirmation remains outstanding (#927).
+
 ## Tools and local MCP server — October 1, 2026
 
 `gods-eye-view/tools` defines queries that answer questions from the app's data,
diff --git a/scripts/format-scope.json b/scripts/format-scope.json
index 7062704a7..693263205 100644
--- a/scripts/format-scope.json
+++ b/scripts/format-scope.json
@@ -940,6 +940,7 @@
   "src/tools/queries/queries.test.mjs",
   "src/tools/mcp/mcp.test.mjs",
   "src/tools/localServer.test.mjs",
+  "src/tools/mcpPanelKey.test.mjs",
   "src/tools/queries/aviation.test.mjs",
   "src/tools/queries/space.test.mjs",
   "src/tools/queries/media.test.mjs",
diff --git a/server/mcp/panelKey.js b/server/mcp/panelKey.js
new file mode 100644
index 000000000..8b34657f7
--- /dev/null
+++ b/server/mcp/panelKey.js
@@ -0,0 +1,117 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import { randomBytes, randomUUID } from 'node:crypto';
+import { fileURLToPath } from 'node:url';
+
+/** Where the install's panel key lives: beside the other local state, which
+ * git ignores, at the checkout root rather than the client's working
+ * directory (an MCP client may start the server anywhere). */
+export const DEFAULT_PANEL_KEY_FILE = fileURLToPath(
+  new URL('../../.gev-cache/mcp-panel-key', import.meta.url),
+);
+
+const KEY_PATTERN = /^[A-Za-z0-9_-]{43}$/;
+const newKey = () => randomBytes(32).toString('base64url');
+const notShared = (error) =>
+  `panel key not shared (${error.code || error.message}); using a key of this process`;
+
+function readKey(file) {
+  const text = fs.readFileSync(file, 'utf8').trim();
+  return KEY_PATTERN.test(text) ? text : null;
+}
+
+/** Serialize malformed-file repair; ordinary first creation still uses link(). */
+function repairKey(file, tmp, key) {
+  const lock = `${file}.repair-lock`;
+  const deadline = Date.now() + 2000;
+  const pause = new Int32Array(new SharedArrayBuffer(4));
+  let fd;
+  for (;;) {
+    try {
+      fd = fs.openSync(lock, 'wx', 0o600);
+      break;
+    } catch (error) {
+      if (error.code !== 'EEXIST') throw error;
+      // Another repair may already have published a usable key.
+      const winner = readKey(file);
+      if (winner) return winner;
+      if (Date.now() >= deadline) {
+        throw Object.assign(new Error('panel key repair is busy'), {
+          code: 'ETIMEDOUT',
+        });
+      }
+      Atomics.wait(pause, 0, 0, 10);
+    }
+  }
+  try {
+    // A repair completed between our earlier read and acquiring the lock.
+    const winner = readKey(file);
+    if (winner) return winner;
+    fs.renameSync(tmp, file);
+    return key;
+  } finally {
+    fs.closeSync(fd);
+    fs.rmSync(lock, { force: true });
+  }
+}
+
+/**
+ * The panel key every stdio server of this install shares.
+ *
+ * A client can start more than one server process for one connection: Claude
+ * Desktop was reported to start two processes; serving the panel's page from
+ * one and sending its `panel_request` calls to the other reproduces #927.
+ * Per-process keys then mismatch, and the panel refuses the request.
+ * The first process to run creates the key, without replacing one
+ * another process created at the same moment; the rest read it. When the file
+ * cannot be read or written, or repair stays busy for two seconds, the process
+ * falls back to its own key, as every process did before.
+ *
+ * As before, the key keeps `panel_request` from clients that only list it; it
+ * is not access control.
+ *
+ * @param {{ file?: string, log?: (line: string) => void }} [options]
+ * @returns {string}
+ */
+export function sharedPanelKey({
+  file = DEFAULT_PANEL_KEY_FILE,
+  log = () => {},
+} = {}) {
+  try {
+    const existing = readKey(file);
+    if (existing) return existing;
+    // Present but not a key: replace it below.
+  } catch (error) {
+    if (error.code !== 'ENOENT') {
+      log(notShared(error));
+      return newKey();
+    }
+  }
+  const key = newKey();
+  let tmp = null;
+  try {
+    fs.mkdirSync(path.dirname(file), { recursive: true });
+    tmp = path.join(
+      path.dirname(file),
+      `.${path.basename(file)}.${randomUUID().slice(0, 8)}.tmp`,
+    );
+    fs.writeFileSync(tmp, `${key}\n`, { flag: 'wx', mode: 0o600 });
+    try {
+      // link() publishes the whole file at once and fails if another process
+      // got there first, so a reader never sees a half-written key and two
+      // processes starting together cannot end up with different keys.
+      fs.linkSync(tmp, file);
+      return key;
+    } catch (error) {
+      if (error.code !== 'EEXIST') throw error;
+      const winner = readKey(file);
+      if (winner) return winner;
+      return repairKey(file, tmp, key);
+    }
+  } catch (error) {
+    log(notShared(error));
+    return key;
+  } finally {
+    if (tmp) fs.rmSync(tmp, { force: true });
+  }
+}
diff --git a/server/mcp/server.js b/server/mcp/server.js
index 0926c865e..3c69dc584 100644
--- a/server/mcp/server.js
+++ b/server/mcp/server.js
@@ -27,14 +27,16 @@ const INSTRUCTIONS =
 export function createLocalMcpServer({
   apiBase = DEFAULT_API_BASE,
   fetchImpl,
+  panelKey = randomBytes(32).toString('base64url'),
 } = {}) {
   const { version } = JSON.parse(
     readFileSync(new URL('../../package.json', import.meta.url), 'utf8'),
   );
-  // Each server makes a new key for its panel. The panel's page carries it
-  // and panel_request requires it; any client may read the page, so it keeps
-  // the tool from clients that only list it, and is not access control.
-  const panelKey = randomBytes(32).toString('base64url');
+  // Unless given one, each server makes a new key for its panel. The panel's
+  // page carries it and panel_request requires it; any client may read the
+  // page, so it keeps the tool from clients that only list it, and is not
+  // access control. The stdio entry passes the install's shared key
+  // (panelKey.js), so separate processes of one client agree.
   return createMcpServer({
     catalog: catalogForSurface(
       composeCatalog({
diff --git a/server/mcp/stdio.js b/server/mcp/stdio.js
index 26f1da8c4..dd0f3fee2 100644
--- a/server/mcp/stdio.js
+++ b/server/mcp/stdio.js
@@ -11,6 +11,7 @@
 import { createInterface } from 'node:readline';
 import { fileURLToPath } from 'node:url';
 import { parseErrorResponse } from '../../src/tools/mcp/index.js';
+import { sharedPanelKey } from './panelKey.js';
 import { createLocalMcpServer } from './server.js';
 import { DEFAULT_API_BASE } from './services.js';
 
@@ -97,7 +98,10 @@ if (process.argv[1] && fileURLToPath(import.meta.url) === process.argv[1]) {
     console.error(
       `God's Eye View MCP server reading ${options.apiBase || DEFAULT_API_BASE}`,
     );
-    await serveStdio(createLocalMcpServer(options), {
+    // A client may start several of these for one connection; they share
+    // the install's panel key so the panel works whichever one it reaches.
+    const panelKey = sharedPanelKey({ log: (line) => console.error(line) });
+    await serveStdio(createLocalMcpServer({ ...options, panelKey }), {
       input: process.stdin,
       output: process.stdout,
       log: (line) => console.error(`<- ${line}`),
diff --git a/src/tools/mcpPanelKey.test.mjs b/src/tools/mcpPanelKey.test.mjs
new file mode 100644
index 000000000..d70fdb3e2
--- /dev/null
+++ b/src/tools/mcpPanelKey.test.mjs
@@ -0,0 +1,200 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { spawn } from 'node:child_process';
+import { sharedPanelKey } from '../../server/mcp/panelKey.js';
+import { createLocalMcpServer } from '../../server/mcp/server.js';
+
+const PANEL_KEY_MODULE = new URL(
+  '../../server/mcp/panelKey.js',
+  import.meta.url,
+).href;
+
+function tempKeyFile(t) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gev-panel-key-'));
+  t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+  return path.join(dir, 'state', 'mcp-panel-key');
+}
+
+const readPage = async (server) => {
+  const page = await server.handle({
+    jsonrpc: '2.0',
+    id: 1,
+    method: 'resources/read',
+    params: { uri: 'ui://gods-eye-view/globe' },
+  });
+  return page.result.contents[0].text.match(/"panelKey":"([^"]+)"/)[1];
+};
+const panelRequest = (server, key) =>
+  server.handle({
+    jsonrpc: '2.0',
+    id: 2,
+    method: 'tools/call',
+    params: {
+      name: 'panel_request',
+      arguments: { key, path: '/panel/index.html' },
+    },
+  });
+const serverWith = (panelKey) =>
+  createLocalMcpServer({
+    apiBase: 'http://127.0.0.1:5000',
+    fetchImpl: async () => new Response('ok'),
+    ...(panelKey ? { panelKey } : {}),
+  });
+
+test('a panel page from one server process is refused by another without a shared key (#927)', async () => {
+  const pageServer = serverWith();
+  const requestServer = serverWith();
+  const key = await readPage(pageServer);
+  const refused = await panelRequest(requestServer, key);
+  assert.equal(refused.result.isError, true);
+  assert.match(
+    refused.result.content[0].text,
+    /Only the God's Eye View panel may make this request/,
+  );
+});
+
+test('servers sharing the install key accept each other’s panel page', async (t) => {
+  const file = tempKeyFile(t);
+  const pageServer = serverWith(sharedPanelKey({ file }));
+  const requestServer = serverWith(sharedPanelKey({ file }));
+  const key = await readPage(pageServer);
+  const opened = await panelRequest(requestServer, key);
+  assert.equal(opened.result.isError, false);
+});
+
+test('the shared key is created once, kept, and private to the user', (t) => {
+  const file = tempKeyFile(t);
+  const first = sharedPanelKey({ file });
+  assert.match(first, /^[A-Za-z0-9_-]{43}$/);
+  assert.equal(sharedPanelKey({ file }), first);
+  assert.equal(fs.readFileSync(file, 'utf8').trim(), first);
+  if (process.platform !== 'win32')
+    assert.equal(fs.statSync(file).mode & 0o777, 0o600);
+  // No staging files are left beside it.
+  assert.deepEqual(fs.readdirSync(path.dirname(file)), ['mcp-panel-key']);
+});
+
+test('a malformed key file is replaced', (t) => {
+  const file = tempKeyFile(t);
+  fs.mkdirSync(path.dirname(file), { recursive: true });
+  fs.writeFileSync(file, 'not a key\n');
+  const key = sharedPanelKey({ file });
+  assert.match(key, /^[A-Za-z0-9_-]{43}$/);
+  assert.equal(sharedPanelKey({ file }), key);
+});
+
+test('when the key cannot be stored, the process uses a key of its own', (t) => {
+  const file = tempKeyFile(t);
+  // A file where the directory should be makes every write fail.
+  fs.writeFileSync(path.dirname(path.dirname(file)) + '/state', 'blocker');
+  const logged = [];
+  const a = sharedPanelKey({ file, log: (line) => logged.push(line) });
+  const b = sharedPanelKey({ file, log: (line) => logged.push(line) });
+  assert.match(a, /^[A-Za-z0-9_-]{43}$/);
+  assert.notEqual(a, b);
+  assert.equal(logged.length, 2);
+  assert.match(logged[0], /panel key not shared/);
+});
+
+test('processes starting at the same moment agree on one key', async (t) => {
+  const file = tempKeyFile(t);
+  const script =
+    `const { sharedPanelKey } = await import(${JSON.stringify(PANEL_KEY_MODULE)});` +
+    `process.stdout.write(sharedPanelKey({ file: ${JSON.stringify(file)} }));`;
+  const run = () =>
+    new Promise((resolve, reject) => {
+      const child = spawn(
+        process.execPath,
+        ['--input-type=module', '-e', script],
+        { stdio: ['ignore', 'pipe', 'inherit'] },
+      );
+      let out = '';
+      child.stdout.on('data', (chunk) => (out += chunk));
+      child.on('error', reject);
+      child.on('close', (code) =>
+        code === 0 ? resolve(out) : reject(new Error(`exit ${code}`)),
+      );
+    });
+  const keys = await Promise.all(Array.from({ length: 8 }, run));
+  assert.equal(new Set(keys).size, 1);
+  assert.equal(fs.readFileSync(file, 'utf8').trim(), keys[0]);
+});
+
+test('concurrent malformed-file repairs return the same persisted key', async (t) => {
+  const file = tempKeyFile(t);
+  fs.mkdirSync(path.dirname(file), { recursive: true });
+  fs.writeFileSync(file, 'not a key\n');
+  const script = `
+    import fs from 'node:fs';
+    const { sharedPanelKey } = await import(${JSON.stringify(PANEL_KEY_MODULE)});
+    const [file, marker, other] = process.argv.slice(1);
+    const original = fs.readFileSync;
+    let reads = 0;
+    fs.readFileSync = function (target, ...args) {
+      const text = original.call(this, target, ...args);
+      if (target === file && ++reads === 2) {
+        // Both processes capture the actual malformed contents before either
+        // can replace them. The old unchecked rename returns two different keys.
+        fs.writeFileSync(marker, 'ready');
+        const deadline = Date.now() + 5000;
+        while (!fs.existsSync(other)) {
+          if (Date.now() >= deadline) throw new Error('repair barrier timed out');
+          Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 5);
+        }
+      }
+      return text;
+    };
+    process.stdout.write(sharedPanelKey({ file }));
+  `;
+  const run = (index) =>
+    new Promise((resolve, reject) => {
+      const marker = (id) => path.join(path.dirname(file), `ready-${id}`);
+      const child = spawn(
+        process.execPath,
+        [
+          '--input-type=module',
+          '-e',
+          script,
+          file,
+          marker(index),
+          marker(1 - index),
+        ],
+        { stdio: ['ignore', 'pipe', 'inherit'] },
+      );
+      t.after(() => child.kill());
+      let out = '';
+      child.stdout.on('data', (chunk) => (out += chunk));
+      child.on('error', reject);
+      child.on('close', (code) =>
+        code === 0 ? resolve(out) : reject(new Error(`exit ${code}`)),
+      );
+    });
+  const keys = await Promise.all([run(0), run(1)]);
+  assert.equal(new Set(keys).size, 1);
+  assert.equal(fs.readFileSync(file, 'utf8').trim(), keys[0]);
+  assert.equal(fs.existsSync(`${file}.repair-lock`), false);
+  assert.equal(
+    fs.readdirSync(path.dirname(file)).some((name) => name.endsWith('.tmp')),
+    false,
+  );
+});
+
+test('an unavailable repair lock falls back without hanging or removing its owner’s lock', (t) => {
+  const file = tempKeyFile(t);
+  fs.mkdirSync(path.dirname(file), { recursive: true });
+  fs.writeFileSync(file, 'not a key\n');
+  fs.writeFileSync(`${file}.repair-lock`, 'held');
+  const logged = [];
+  const key = sharedPanelKey({ file, log: (line) => logged.push(line) });
+  assert.match(key, /^[A-Za-z0-9_-]{43}$/);
+  assert.match(logged[0], /ETIMEDOUT/);
+  assert.equal(fs.readFileSync(file, 'utf8'), 'not a key\n');
+  assert.equal(fs.readFileSync(`${file}.repair-lock`, 'utf8'), 'held');
+  assert.deepEqual(fs.readdirSync(path.dirname(file)).sort(), [
+    'mcp-panel-key',
+    'mcp-panel-key.repair-lock',
+  ]);
+});