diff --git a/CHANGELOG.md b/CHANGELOG.md
index 51d1969f..9f0c3efe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -296,6 +296,7 @@ This section lists what is on `main` and not yet on npm.
 - Parallel tool calls that finish behind a slower earlier call are now checkpointed as soon as they settle, keyed by `toolCallId`. Previously only the in-order prefix of finished calls was checkpointed, so a crash while an earlier call was still running made `resume()` re-execute every finished call behind it — side-effecting tools (charges, emails) ran twice. The transcript still records results in the model's call order.
 
 ### Security
+- Session routes: two concurrent first-touch turns on one unowned session no longer both pass the owner check - the claim is atomic now, so the loser is refused (`403 LOUSHO_SESSION_FORBIDDEN`) instead of streaming a turn inside the winner's session; a claim lost while a turn ran clears what it stored and ends the stream on the same coded error. (Eve2-F1)
 - The `@modelcontextprotocol/sdk` peer range is now `^1.31.0` (was `^1.30.1`): 1.30.x is affected by GHSA-6qxp-vccf-f47h (its OAuth client could send credentials to an authorization server chosen by the MCP server), which MCP OAuth (`oauth` on an HTTP server entry) relies on. Upgrade with `npm install @modelcontextprotocol/sdk@^1.31.0`.
 - `fromUIMessages()` (and so `POST <base>/ui`) keeps only `user` and `assistant` messages from the client: a client-sent `system` message (or any other role, or a malformed entry) is dropped instead of reaching the model as instructions. See [AI SDK UI](docs/ai-sdk-ui.md#inbound-conversion). (Eve TOOLS-F9)
 - **Behavior change:** MCP tools now ask for approval by default (`approval: 'always'`). A server's own `readOnlyHint` / `destructiveHint` no longer switch approval off or let a tool run in plan mode unless its entry opts in with `approval: 'annotations'` (loaded descriptors then carry `metadata.mcp.annotationsTrusted: false`). An MCP tool whose `<server>__<tool>` name equals a local tool's is refused with `LOUSHO_CONFIG_INVALID` instead of silently replacing it and its approval gate. Set `approval: 'annotations'` on servers you trust to keep the old behavior. See [MCP](docs/mcp.md#approval-for-mcp-tools). (Eve TOOLS-F11)
diff --git a/src/server/fetchRoutes.ts b/src/server/fetchRoutes.ts
index d965ac5a..63efd3ec 100644
--- a/src/server/fetchRoutes.ts
+++ b/src/server/fetchRoutes.ts
@@ -359,15 +359,18 @@ export async function sessionForbidden(
 /**
  * The default session check: the owner on record decides; without one, a
  * session that is already stored is nobody's to take (its owner may only have
- * been forgotten), and a new one is claimed by a turn (`claim`).
+ * been forgotten), and a new one is claimed by a turn (`claim`) - atomically,
+ * so of two concurrent first touches only the caller whose claim holds may go on.
  */
 async function ownsSession(ctx: ChatRoutesContext, principal: Principal, sessionId: string, claim: boolean, session: AgentSession | undefined): Promise<boolean> {
   const agent = ctx.agent();
   const owner = sessionOwner(agent, sessionId);
   if (owner !== undefined) return samePrincipal(owner, principal);
   if (await sessionStored(ctx, sessionId, session)) return false;
-  if (claim) claimSession(agent, sessionId, principal);
-  return true;
+  // Eve2-F1: the claim itself decides the race - it is atomic with the record,
+  // so a concurrent first touch that registered another owner first is refused
+  // here instead of streaming a turn inside that principal's session.
+  return !claim || claimSession(agent, sessionId, principal);
 }
 
 /**
@@ -399,8 +402,18 @@ export function claimOnCommit(ctx: ChatRoutesContext, principal: Principal | und
       yield* run;
     } finally {
       const agent = ctx.agent();
-      if (await sessionStored(ctx, session.id, session)) claimSession(agent, session.id, principal);
-      else releaseSession(agent, session.id, principal);
+      if (await sessionStored(ctx, session.id, session)) {
+        // Eve2-F1: the claim taken before the turn can have been forgotten past
+        // the owner cap and taken by another principal while it ran; this
+        // turn's transcript is then readable as theirs, so a lost re-claim
+        // clears it again and the stream ends on a coded error, never a clean `done`.
+        if (!claimSession(agent, session.id, principal)) {
+          await session.clear().catch(() => undefined);
+          throw new SDKError(`Session '${session.id}' was claimed by another principal while this turn ran; the transcript it stored was cleared.`, 'LOUSHO_SESSION_FORBIDDEN');
+        }
+      } else {
+        releaseSession(agent, session.id, principal);
+      }
     }
   })();
 }
diff --git a/src/server/requestGuard.test.ts b/src/server/requestGuard.test.ts
index 3441fd49..ec640291 100644
--- a/src/server/requestGuard.test.ts
+++ b/src/server/requestGuard.test.ts
@@ -9,9 +9,11 @@ import { defineTool } from '../tools/defineTool';
 import { mockModel } from '../testing';
 import { memoryStore } from '../storage/agentStore';
 import { createRouteHandler, type RouteHandlerOptions } from './routeHandler';
-import { serveFetch } from './fetchRoutes';
+import { handleChatFetch, serveFetch } from './fetchRoutes';
 import { claimSession, MAX_SESSION_OWNERS, releaseSession, sessionOwner, setSessionOwnerLimit } from './requestGuard';
-import type { AuthFn } from '../auth';
+import { MemorySessionStore } from '../session/sessionStore';
+import { textOf } from '../providers/content';
+import type { AuthFn, Principal } from '../auth';
 
 const deploy = defineTool({ name: 'deploy', description: 'Deploys', input: z.object({}), needsApproval: true, execute: () => 'shipped' });
 
@@ -219,6 +221,89 @@ describe('default session owner', () => {
     }
   });
 
+  it('a concurrent first touch on an unowned session loses the claim: refused, and none of its input is streamed or stored (Eve2-F1)', async () => {
+    // The first transcript load is delayed so both owner checks observe the
+    // session as unowned and unstored before either claim runs; the claim
+    // itself must then decide the race - one caller wins, the other must be
+    // refused rather than stream a turn inside the winner's session.
+    let delayed = false;
+    const sessions = new class extends MemorySessionStore {
+      override async load(id: string) {
+        if (!delayed) {
+          delayed = true;
+          await new Promise((resolve) => setTimeout(resolve, 100));
+        }
+        return super.load(id);
+      }
+    }();
+    const store = memoryStore();
+    store.sessions = sessions;
+    const agent = createAgent({
+      provider: mockModel([(req) => `echo:${req.messages.filter((m) => m.role === 'user').map((m) => textOf(m)).join('|')}`]),
+      store,
+    });
+    const ctx = { name: 'test', agent: () => agent };
+    const post = (input: string) => new Request('http://srv/chat', { method: 'POST', headers: asJson, body: json({ sessionId: 'race-1', input }) });
+    const caller = (id: string): Principal => ({ id, type: 'user', authenticator: 'custom' });
+    const [aliceRes, malloryRes] = await Promise.all([
+      handleChatFetch(post('ALICE_MARKER'), ctx, caller('alice')),
+      handleChatFetch(post('MALLORY_MARKER'), ctx, caller('mallory')),
+    ]);
+    // Exactly one caller may win the claim; the other gets the session 403, not a stream.
+    expect([aliceRes?.status, malloryRes?.status].sort()).toEqual([200, 403]);
+    const aliceWon = aliceRes?.status === 200;
+    const winner = aliceWon ? aliceRes! : malloryRes!;
+    const loser = aliceWon ? malloryRes! : aliceRes!;
+    const loserMarker = aliceWon ? 'MALLORY_MARKER' : 'ALICE_MARKER';
+    expect(await loser.json()).toMatchObject({ code: 'LOUSHO_SESSION_FORBIDDEN' });
+    // The loser never ran a turn: none of its input is in the winner's stream or in the stored transcript.
+    expect(await winner.text()).not.toContain(loserMarker);
+    expect(JSON.stringify(await sessions.load('race-1'))).not.toContain(loserMarker);
+    expect(sessionOwner(agent, 'race-1')).toMatchObject({ id: aliceWon ? 'alice' : 'mallory' });
+    // And after the race the loser is still a stranger, exactly as in the sequential case.
+    const retry = await handleChatFetch(post('late-attempt'), ctx, caller(aliceWon ? 'mallory' : 'alice'));
+    expect(retry?.status).toBe(403);
+  });
+
+  it('a claim lost while a turn ran clears what it stored and ends the stream on a coded error (Eve2-F1)', async () => {
+    // With the owner cap at one, a claim on another session evicts 's' while
+    // alice's turn is still running; mallory then takes the unowned, still
+    // unstored id through the session-less { message } route. Alice's commit
+    // must not leave her transcript under mallory's claim.
+    const previous = setSessionOwnerLimit(1);
+    try {
+      let openGate!: () => void;
+      const gate = new Promise<void>((resolve) => (openGate = resolve));
+      const sessions = new MemorySessionStore();
+      const store = memoryStore();
+      store.sessions = sessions;
+      const agent = createAgent({
+        provider: mockModel([(req) => (textOf(req.messages.at(-1)!) === 'alice-secret' ? gate.then(() => 'alice-turn') : 'other-turn')], { onExhausted: 'repeat-last' }),
+        store,
+      });
+      const ctx = { name: 'test', agent: () => agent };
+      const caller = (id: string): Principal => ({ id, type: 'user', authenticator: 'custom' });
+      const post = (body: unknown) => new Request('http://srv/chat', { method: 'POST', headers: asJson, body: json(body) });
+
+      const aliceRes = (await handleChatFetch(post({ sessionId: 's', input: 'alice-secret' }), ctx, caller('alice')))!;
+      expect(aliceRes.status).toBe(200);
+      const aliceBody = aliceRes.text(); // consuming the SSE is what runs the turn
+      await (await handleChatFetch(post({ sessionId: 'x', input: 'evict' }), ctx, caller('alice')))!.text(); // claims 'x', forgetting 's'
+      expect(sessionOwner(agent, 's')).toBeUndefined();
+      const malloryRes = await handleChatFetch(post({ sessionId: 's', message: 'mallory was here' }), ctx, caller('mallory'));
+      expect(malloryRes?.status).toBe(200);
+      expect(sessionOwner(agent, 's')).toMatchObject({ id: 'mallory' });
+
+      openGate();
+      const body = await aliceBody;
+      expect(body).toContain('LOUSHO_SESSION_FORBIDDEN');
+      expect(JSON.stringify((await sessions.load('s')) ?? [])).not.toContain('alice-secret');
+      expect(sessionOwner(agent, 's')).toMatchObject({ id: 'mallory' });
+    } finally {
+      setSessionOwnerLimit(previous);
+    }
+  });
+
   it('a turn that stores nothing leaves the session unclaimed', async () => {
     const model = mockModel([{ error: new Error('provider down') }, 'Hi Alice.']);
     const agent = createAgent({ provider: model, store: memoryStore() });
diff --git a/src/server/requestGuard.ts b/src/server/requestGuard.ts
index f038bb61..9999aa46 100644
--- a/src/server/requestGuard.ts
+++ b/src/server/requestGuard.ts
@@ -187,16 +187,25 @@ export function sessionOwner(agent: object, sessionId: string): Principal | unde
   return owner;
 }
 
-/** Records `principal` as the owner of session `sessionId` of `agent`, unless it has one. */
-export function claimSession(agent: object, sessionId: string, principal: Principal): void {
+/**
+ * Records `principal` as the owner of session `sessionId` of `agent` when it
+ * has none, and answers whether `principal` holds the claim afterward: `true`
+ * when it was just taken (or was `principal`'s already), `false` when another
+ * principal owns it. Eve2-F1: the check-and-set is one synchronous step, so a
+ * `false` answer means a concurrent first touch won the id and this caller
+ * must be refused - never streamed a turn inside that principal's session.
+ */
+export function claimSession(agent: object, sessionId: string, principal: Principal): boolean {
   let sessions = owners.get(agent);
   if (!sessions) owners.set(agent, (sessions = new Map()));
-  if (sessions.has(sessionId)) return;
+  const owner = sessions.get(sessionId);
+  if (owner !== undefined) return samePrincipal(owner, principal);
   sessions.set(sessionId, principal);
   for (const oldest of sessions.keys()) {
     if (sessions.size <= ownerLimit) break;
     sessions.delete(oldest);
   }
+  return true;
 }
 
 /** Forgets `principal`'s claim on session `sessionId` of `agent` (a turn that stored nothing); another principal's claim is kept. */