diff --git a/CLAUDE.md b/CLAUDE.md
index 9a40dd8..a114790 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -33,12 +33,13 @@ Phase 1 (MVP) = static bearer-token auth. Phase 2 adds OAuth 2.1 + PKCE + DCR en
 - **`MEM0_EMBED_DIMS` must match the embedder's real output dimension** (3-small=1536, 3-large=3072). A mismatch causes *silent* search failures, not errors. Changing embedding models requires dropping and recreating the Qdrant collection.
 - **FastMCP = the PrefectHQ `fastmcp` PyPI package**, imported `from fastmcp import FastMCP`. It is NOT the older `mcp.server.fastmcp` module.
 - **Same `MEM0_API_KEY` protects both** the REST endpoints (`require_bearer` dependency) and the MCP endpoint (`StaticTokenVerifier` in Phase 1).
+- **The Phase 2 OAuth `/oauth/authorize` consent step authenticates the resource owner** by requiring `MEM0_API_KEY` (constant-time compare) before issuing a code. Don't remove this — the OAuth endpoints are public, so without it anyone reaching the consent screen could mint a token for the single user's memories.
 
 ## Planned layout (per PRD §3)
 
 - `app/config.py` — `Settings` (pydantic-settings); single source of config truth, reject startup on missing required vars.
 - `app/memory.py` — mem0 wrapper / `_build_config`; **the most tweak-prone file**.
-- `app/mcp_server.py` — the six MCP tools (add/search/list/get/update/delete), each thinly wrapping a mem0 op with `user_id` defaulted.
+- `app/mcp_server.py` — the six MCP tools (add/search/list/get/update/delete), each thinly wrapping a mem0 op with `user_id` defaulted. MCP reads (`search`/`list`) must NOT filter by `agent_id` — all agents share one memory; `agent_id` is a write-only provenance tag on `add`. (REST may still filter reads by `agent_id` for explicit scripted use.)
 - `app/rest.py` — REST router under `/api/v1`.
 - `app/auth.py` — `require_bearer` + `build_verifier()` (Phase 1); composite bearer-or-JWT verifier (Phase 2).
 - `app/oauth.py` / `app/oauth_store.py` — Phase 2 OAuth AS endpoints + SQLite store at `/app/data/oauth.db` (CapRover persistent volume).
diff --git a/app/mcp_server.py b/app/mcp_server.py
index 99cf705..b8f45a5 100644
--- a/app/mcp_server.py
+++ b/app/mcp_server.py
@@ -17,6 +17,10 @@ def add_memory(content: str, agent_id: str | None = None, metadata: dict | None
 
         Use when the user shares preferences, project context, decisions,
         or anything they may want recalled in future conversations.
+
+        agent_id is an optional provenance tag recording which agent wrote the
+        memory. It does NOT partition the store — search and list always span
+        every memory for the user, so all connected agents share one memory.
         """
         kwargs: dict = {"user_id": default_user}
         if agent_id:
@@ -26,20 +30,17 @@ def add_memory(content: str, agent_id: str | None = None, metadata: dict | None
         return memory.add(content, **kwargs)
 
     @mcp.tool
-    def search_memories(query: str, agent_id: str | None = None, limit: int = 10) -> dict:
-        """Search long-term memory by semantic similarity."""
-        filters: dict = {"user_id": default_user}
-        if agent_id:
-            filters["agent_id"] = agent_id
-        return memory.search(query=query, filters=filters, top_k=limit)
+    def search_memories(query: str, limit: int = 10) -> dict:
+        """Search long-term memory by semantic similarity.
+
+        Searches the single shared memory store for the user, across all agents.
+        """
+        return memory.search(query=query, filters={"user_id": default_user}, top_k=limit)
 
     @mcp.tool
-    def list_memories(agent_id: str | None = None) -> dict:
-        """List all stored memories for the current user."""
-        filters: dict = {"user_id": default_user}
-        if agent_id:
-            filters["agent_id"] = agent_id
-        return memory.get_all(filters=filters)
+    def list_memories() -> dict:
+        """List all stored memories for the user (shared across all agents)."""
+        return memory.get_all(filters={"user_id": default_user})
 
     @mcp.tool
     def get_memory(memory_id: str) -> dict:
diff --git a/app/oauth.py b/app/oauth.py
index 6c1889c..2e7fb33 100644
--- a/app/oauth.py
+++ b/app/oauth.py
@@ -10,7 +10,7 @@
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric.rsa import RSAPublicNumbers
 from fastapi import APIRouter, Form, HTTPException, Request
-from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
+from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse, Response
 
 from app import oauth_store
 from app.config import get_settings
@@ -161,55 +161,101 @@ async def register(request: Request) -> JSONResponse:
     )
 
 
-@router.get("/oauth/authorize")
-def authorize_form(
+def _consent_page(
     client_id: str,
     redirect_uri: str,
     code_challenge: str,
-    code_challenge_method: str = "S256",
-    response_type: str = "code",
-    state: str = "",
-    scope: str = "read write",
-) -> HTMLResponse:
-    if response_type != "code":
-        raise HTTPException(status_code=400, detail="unsupported response_type")
-    if code_challenge_method != "S256" or not code_challenge:
-        raise HTTPException(status_code=400, detail="PKCE S256 required")
-    client = oauth_store.get_client(client_id)
-    if not client or redirect_uri not in client["redirect_uris"]:
-        raise HTTPException(status_code=400, detail="invalid client or redirect_uri")
+    state: str,
+    scope: str,
+    error: str = "",
+) -> str:
     e_client_id = html.escape(client_id, quote=True)
     e_redirect_uri = html.escape(redirect_uri, quote=True)
     e_code_challenge = html.escape(code_challenge, quote=True)
     e_state = html.escape(state, quote=True)
     e_scope = html.escape(scope, quote=True)
-    page = f"""
+    error_html = f'<p style="color:red">{html.escape(error, quote=True)}</p>' if error else ""
+    return f"""
     <html><body>
       <h2>Authorize mem0</h2>
+      {error_html}
+      <p>Enter your mem0 API key to grant this client access to your memories.</p>
       <form method="post" action="/oauth/authorize">
         <input type="hidden" name="client_id" value="{e_client_id}">
         <input type="hidden" name="redirect_uri" value="{e_redirect_uri}">
         <input type="hidden" name="code_challenge" value="{e_code_challenge}">
         <input type="hidden" name="state" value="{e_state}">
         <input type="hidden" name="scope" value="{e_scope}">
+        <label>API key: <input type="password" name="password" autofocus></label>
         <button type="submit">Authorize</button>
       </form>
     </body></html>
     """
-    return HTMLResponse(page)
+
+
+def _owner_authenticated(password: str) -> bool:
+    # Single-user: the resource owner proves ownership at the consent step with
+    # the same MEM0_API_KEY that protects the API. An empty configured key must
+    # never authenticate. Hash both sides to fixed-length digests so the compare
+    # is genuinely constant-time (compare_digest can leak length for raw strings).
+    key = get_settings().mem0_api_key
+    if not key:
+        return False
+    provided = hashlib.sha256(password.encode()).digest()
+    expected = hashlib.sha256(key.encode()).digest()
+    return secrets.compare_digest(provided, expected)
+
+
+@router.get("/oauth/authorize")
+def authorize_form(
+    client_id: str,
+    redirect_uri: str,
+    code_challenge: str,
+    code_challenge_method: str = "S256",
+    response_type: str = "code",
+    state: str = "",
+    scope: str = "read write",
+) -> HTMLResponse:
+    if response_type != "code":
+        raise HTTPException(status_code=400, detail="unsupported response_type")
+    if code_challenge_method != "S256" or not code_challenge:
+        raise HTTPException(status_code=400, detail="PKCE S256 required")
+    client = oauth_store.get_client(client_id)
+    if not client or redirect_uri not in client["redirect_uris"]:
+        raise HTTPException(status_code=400, detail="invalid client or redirect_uri")
+    return HTMLResponse(
+        _consent_page(client_id, redirect_uri, code_challenge, state, scope)
+    )
 
 
 @router.post("/oauth/authorize")
 def authorize_submit(
     client_id: str = Form(...),
     redirect_uri: str = Form(...),
-    code_challenge: str = Form(...),
+    code_challenge: str = Form(""),
     state: str = Form(""),
     scope: str = Form("read write"),
-) -> RedirectResponse:
+    password: str = Form(""),
+) -> Response:
     client = oauth_store.get_client(client_id)
     if not client or redirect_uri not in client["redirect_uris"]:
         raise HTTPException(status_code=400, detail="invalid client or redirect_uri")
+    # Enforce PKCE here too (the GET form does), so a direct POST can't mint a
+    # code that could never be exchanged.
+    if not code_challenge:
+        raise HTTPException(status_code=400, detail="PKCE S256 required")
+    # Authenticate the resource owner before issuing a code. Without this, anyone
+    # who reaches the consent screen could obtain a token for the single user's
+    # memories just by clicking "Authorize".
+    if not _owner_authenticated(password):
+        _log.warning("oauth_consent_rejected", client_id=client_id)
+        return HTMLResponse(
+            _consent_page(
+                client_id, redirect_uri, code_challenge, state, scope,
+                error="Invalid API key.",
+            ),
+            status_code=401,
+        )
     code = secrets.token_urlsafe(32)
     oauth_store.save_code(code, client_id, redirect_uri, code_challenge)
     sep = "&" if "?" in redirect_uri else "?"
diff --git a/docs/DEVELOPER_GUIDE.md b/docs/DEVELOPER_GUIDE.md
index 9e9e82b..de145a5 100644
--- a/docs/DEVELOPER_GUIDE.md
+++ b/docs/DEVELOPER_GUIDE.md
@@ -121,7 +121,10 @@ MCP tools default `user_id` to the single configured user and expose a narrower
   explicitly to `<PUBLIC_BASE_URL>/mcp` so the advertised resource is correct.
 
 The OAuth flow (`app/oauth.py`) is OAuth 2.1 with PKCE (S256 required) and public clients only — no
-client secrets are issued. Endpoints: `/oauth/register` (DCR), `/oauth/authorize` (GET form + POST
+client secrets are issued. The `/oauth/authorize` consent step **authenticates the resource owner**:
+the POST handler requires the `MEM0_API_KEY` (constant-time compared) before issuing a code. Without
+this gate, anyone who reached the public consent screen could mint a token for the single user's
+memories just by clicking "Authorize". Endpoints: `/oauth/register` (DCR), `/oauth/authorize` (GET form + POST
 consent), `/oauth/token` (authorization_code + refresh_token grants), `/.well-known/jwks.json`, and
 the RFC 8414 / RFC 9728 metadata documents. Tokens live 24h; refresh tokens are single-use and
 rotated. `oauth_store.py` persists clients/codes/refresh tokens in SQLite, hashing refresh tokens
diff --git a/docs/USER_GUIDE.md b/docs/USER_GUIDE.md
index 335a198..3d5933f 100644
--- a/docs/USER_GUIDE.md
+++ b/docs/USER_GUIDE.md
@@ -28,7 +28,7 @@ a single service. It gives AI agents and scripts a shared, persistent long-term
 two ways from one process:
 
 - **REST API** under `/api/v1/memories…` — for scripts, n8n, curl, and any HTTP client.
-- **Streamable HTTP MCP** under `/mcp/` — for Claude Code, Claude Desktop, Claude.ai web, and Cowork.
+- **Streamable HTTP MCP** under `/mcp` — for Claude Code, Claude Desktop, Claude.ai web, and Cowork.
 
 Both protocols read and write the **same** memory store, so a fact you save from Claude Code is
 searchable from a curl script and vice versa.
@@ -45,7 +45,10 @@ keyword matches.
 
 Memories can optionally be tagged with:
 
-- `agent_id` — which agent/tool wrote it (e.g. `n8n-flow`, `claude-code`), so you can filter later.
+- `agent_id` — a provenance tag for which agent/tool wrote it (e.g. `n8n-flow`, `claude-code`).
+  Over MCP it is **write-only**: the `search`/`list` tools always span the whole store, so every
+  connected agent (Claude Code, Codex, Claude.ai web, …) shares one memory. The REST API can still
+  filter reads by `agent_id` for scripts that explicitly want a slice.
 - `run_id` — a session or workflow run identifier.
 - `metadata` — arbitrary JSON you attach to a memory.
 
@@ -165,7 +168,7 @@ and Cowork, which use OAuth (Phase 2).
 
 ```bash
 claude mcp add --scope user --transport http mem0-remote \
-  https://mem0.your-domain.com/mcp/ \
+  https://mem0.your-domain.com/mcp \
   --header "Authorization: Bearer $MEM0_API_KEY"
 ```
 
@@ -175,20 +178,28 @@ Code.
 ### Claude Desktop
 
 Add an entry under the MCP servers section of Claude Desktop's config, pointing at
-`https://mem0.your-domain.com/mcp/` with an `Authorization: Bearer <token>` header (Streamable HTTP
-transport). Restart Claude Desktop to pick it up.
+`https://mem0.your-domain.com/mcp` with an `Authorization: Bearer <token>` header (Streamable HTTP
+transport). Restart Claude Desktop to pick it up. Both `/mcp` and `/mcp/` work; `/mcp` is the
+canonical form.
 
 ### Claude.ai web / Cowork (OAuth)
 
 This requires **Phase 2** (`OAUTH_SIGNING_KEY` set). In the client's connector settings:
 
-1. Add a **custom connector** pointing at `https://mem0.your-domain.com/mcp/`.
+1. Add a **custom connector** pointing at `https://mem0.your-domain.com/mcp`.
 2. Leave the client ID and secret **blank** — the server supports Dynamic Client Registration, so
    the client registers itself automatically.
-3. Complete the consent screen (click **Authorize**) and the redirect back to the client.
+3. On the consent screen, **enter your `MEM0_API_KEY`** in the API key field and click
+   **Authorize**, then let the redirect complete.
 
-The server only allows redirect URIs listed in `OAUTH_ALLOWED_REDIRECT_URIS`, which defaults to the
-official claude.ai and Cowork callback URLs.
+**Why the API key prompt matters (security):** this server is single-user and the consent step
+authenticates *you* as the owner. Because the OAuth endpoints are public, anyone who knows the URL
+could otherwise reach the consent screen; requiring `MEM0_API_KEY` at authorization ensures only the
+holder of that key can mint an access token to your memories. Treat `MEM0_API_KEY` as the master
+credential — anyone with it has full access via either the bearer header or the OAuth flow.
+
+The server also only allows redirect URIs listed in `OAUTH_ALLOWED_REDIRECT_URIS`, which defaults to
+the official claude.ai and Cowork callback URLs.
 
 ### REST / curl / n8n
 
diff --git a/tests/test_mcp.py b/tests/test_mcp.py
index f078b03..811ab67 100644
--- a/tests/test_mcp.py
+++ b/tests/test_mcp.py
@@ -38,14 +38,23 @@ async def test_add_memory_tool(mcp, mem):
 async def test_search_memories_tool(mcp, mem):
     mem.search.return_value = {"results": []}
     async with Client(mcp) as client:
-        await client.call_tool(
-            "search_memories", {"query": "what", "agent_id": "cc", "limit": 7}
-        )
+        await client.call_tool("search_memories", {"query": "what", "limit": 7})
     _, kwargs = mem.search.call_args
-    assert kwargs["filters"] == {"user_id": "ian", "agent_id": "cc"}
+    # Reads are never scoped by agent_id: the store is shared across agents.
+    assert kwargs["filters"] == {"user_id": "ian"}
     assert kwargs["top_k"] == 7
 
 
+async def test_read_tools_do_not_expose_agent_id(mcp):
+    # Reads must not be scopable by agent_id, or a client's model can partition
+    # the shared store and break cross-agent memory.
+    async with Client(mcp) as client:
+        tools = {t.name: t for t in await client.list_tools()}
+    for name in ("search_memories", "list_memories"):
+        props = (tools[name].inputSchema or {}).get("properties", {})
+        assert "agent_id" not in props, name
+
+
 async def test_list_memories_tool(mcp, mem):
     mem.get_all.return_value = {"results": []}
     async with Client(mcp) as client:
diff --git a/tests/test_oauth.py b/tests/test_oauth.py
index 8df661d..3259ea7 100644
--- a/tests/test_oauth.py
+++ b/tests/test_oauth.py
@@ -131,6 +131,7 @@ def test_full_authorize_token_flow(oauth_client):
             "client_id": client_id,
             "redirect_uri": ALLOWED_URI,
             "code_challenge": challenge,
+            "password": "test-bearer-token",
         },
         follow_redirects=False,
     )
@@ -158,6 +159,59 @@ def test_full_authorize_token_flow(oauth_client):
     assert payload["scope"] == "read write"
 
 
+def test_authorize_rejects_wrong_password(oauth_client):
+    # The consent step must authenticate the resource owner: a wrong API key
+    # must not yield an authorization code.
+    client_id = _register(oauth_client)
+    _, challenge = _pkce()
+    resp = oauth_client.post(
+        "/oauth/authorize",
+        data={
+            "client_id": client_id,
+            "redirect_uri": ALLOWED_URI,
+            "code_challenge": challenge,
+            "password": "not-the-key",
+        },
+        follow_redirects=False,
+    )
+    assert resp.status_code == 401
+    assert "location" not in resp.headers
+
+
+def test_authorize_rejects_missing_password(oauth_client):
+    client_id = _register(oauth_client)
+    _, challenge = _pkce()
+    resp = oauth_client.post(
+        "/oauth/authorize",
+        data={
+            "client_id": client_id,
+            "redirect_uri": ALLOWED_URI,
+            "code_challenge": challenge,
+        },
+        follow_redirects=False,
+    )
+    assert resp.status_code == 401
+    assert "location" not in resp.headers
+
+
+def test_authorize_rejects_empty_pkce(oauth_client):
+    # A direct POST with an empty code_challenge must not mint a code that could
+    # never be exchanged (PKCE is enforced on POST, not just the GET form).
+    client_id = _register(oauth_client)
+    resp = oauth_client.post(
+        "/oauth/authorize",
+        data={
+            "client_id": client_id,
+            "redirect_uri": ALLOWED_URI,
+            "code_challenge": "",
+            "password": "test-bearer-token",
+        },
+        follow_redirects=False,
+    )
+    assert resp.status_code == 400
+    assert "location" not in resp.headers
+
+
 def test_authorize_form_escapes_state(oauth_client):
     client_id = _register(oauth_client)
     _, challenge = _pkce()
@@ -185,7 +239,8 @@ def test_pkce_mismatch_rejected(oauth_client):
     _, challenge = _pkce()
     resp = oauth_client.post(
         "/oauth/authorize",
-        data={"client_id": client_id, "redirect_uri": ALLOWED_URI, "code_challenge": challenge},
+        data={"client_id": client_id, "redirect_uri": ALLOWED_URI,
+              "code_challenge": challenge, "password": "test-bearer-token"},
         follow_redirects=False,
     )
     code = resp.headers["location"].split("code=")[1].split("&")[0]
@@ -227,7 +282,8 @@ def test_code_is_single_use(oauth_client):
     verifier, challenge = _pkce()
     resp = oauth_client.post(
         "/oauth/authorize",
-        data={"client_id": client_id, "redirect_uri": ALLOWED_URI, "code_challenge": challenge},
+        data={"client_id": client_id, "redirect_uri": ALLOWED_URI,
+              "code_challenge": challenge, "password": "test-bearer-token"},
         follow_redirects=False,
     )
     code = resp.headers["location"].split("code=")[1].split("&")[0]
@@ -247,7 +303,8 @@ def _obtain_tokens(oauth_client) -> dict:
     verifier, challenge = _pkce()
     resp = oauth_client.post(
         "/oauth/authorize",
-        data={"client_id": client_id, "redirect_uri": ALLOWED_URI, "code_challenge": challenge},
+        data={"client_id": client_id, "redirect_uri": ALLOWED_URI,
+              "code_challenge": challenge, "password": "test-bearer-token"},
         follow_redirects=False,
     )
     code = resp.headers["location"].split("code=")[1].split("&")[0]