diff --git a/src/server/sessiond.ts b/src/server/sessiond.ts
index 32d64f1..1a0d5d9 100644
--- a/src/server/sessiond.ts
+++ b/src/server/sessiond.ts
@@ -7,6 +7,7 @@ import { WorkspaceActivityService } from "./activity/workspaceActivityService.js
 import { registerWorkspaceActivityRoutes } from "./activity/workspaceActivityRoutes.js";
 import { SessionEventHub } from "./realtime/sessionEventHub.js";
 import { AuthService } from "./sessions/authService.js";
+import { installGlobalProviderPolicy } from "./sessions/globalProviderPolicy.js";
 import { registerAuthRoutes } from "./sessions/authRoutes.js";
 import { PiSessionService } from "./sessions/piSessionService.js";
 import { createPiSessionManagerGateway } from "./sessions/piSessionManagerGateway.js";
@@ -69,6 +70,9 @@ await runSessionDaemonStartup({
       }),
     });
     auth.subscribe((change) => { sessions.applyAuthChange(change); });
+    // PI WEB only supports globally configured providers: reject every
+    // extension provider registration against the shared daemon-wide runtime.
+    installGlobalProviderPolicy(auth.runtime, (providerId) => { sessions.noteRejectedProviderRegistration(providerId); });
     const terminals = new TerminalService(eventHub, workspaceActivity);
     const runtimeComponent = Object.freeze({
       ...getPiWebRuntimeComponent("sessiond", SESSIOND_RUNTIME_CAPABILITIES),
diff --git a/src/server/sessions/globalProviderPolicy.ts b/src/server/sessions/globalProviderPolicy.ts
new file mode 100644
index 0000000..3e67445
--- /dev/null
+++ b/src/server/sessions/globalProviderPolicy.ts
@@ -0,0 +1,47 @@
+import type { ModelRuntime } from "@earendil-works/pi-coding-agent";
+
+/**
+ * PI WEB supports only globally configured providers: Pi built-ins, agent-dir
+ * `models.json`, and environment credentials. Any provider an extension tries
+ * to register is rejected — the user is told, and everything else the
+ * extension does keeps working.
+ *
+ * Why: all sessions share one daemon-wide {@link ModelRuntime}. Letting one
+ * workspace's extensions mutate it corrupts the provider set of every other
+ * concurrent session (issue #76). Rather than building scoped-provider
+ * isolation, pi-web rejects scoped registrations outright.
+ *
+ * Mechanism: this deliberately shadows the `registerProvider` /
+ * `unregisterProvider` instance methods because Pi 0.80.10 offers no
+ * registration hook. Both Pi call sites (the load-time
+ * `pendingProviderRegistrations` drain in `createAgentSessionServices` and the
+ * late `pi.registerProvider` path through `ModelRegistry`) reach the runtime
+ * through call-time property lookup, so instance shadowing intercepts them
+ * identically. The acceptance test that exercises both paths is the tripwire:
+ * if a Pi upgrade changes these internals, that test fails loudly and this
+ * shim must be revisited.
+ */
+export function installGlobalProviderPolicy(
+  runtime: ModelRuntime,
+  onRejection: (providerId: string) => void,
+): void {
+  runtime.registerProvider = (providerId: string) => {
+    // Swallow: the shared runtime is never mutated, the rejection is surfaced.
+    onRejection(providerId);
+  };
+  runtime.unregisterProvider = () => {
+    // No-op: with every registration rejected, the extension provider layer is
+    // always empty, so there is never anything to unregister.
+  };
+}
+
+/**
+ * User-facing wording for a rejected registration. `cwd` is known only for
+ * rejections raised while loading a workspace's services; late registrations
+ * from session event handlers carry no attribution beyond the provider id.
+ */
+export function providerRejectionMessage(providerId: string, cwd?: string): string {
+  const origin = cwd === undefined ? "registered by an extension" : `registered by an extension in ${cwd}`;
+  return `Provider "${providerId}" ${origin} was ignored — PI WEB only supports globally configured providers. `
+    + "Configure it globally (e.g. agent-dir models.json) to use it here. All other extension features are unaffected.";
+}
diff --git a/src/server/sessions/piSessionService.ts b/src/server/sessions/piSessionService.ts
index c3d87ae..62b3294 100644
--- a/src/server/sessions/piSessionService.ts
+++ b/src/server/sessions/piSessionService.ts
@@ -65,6 +65,7 @@ import {
   type SessionNotificationMutation,
 } from "./sessionNotificationStore.js";
 import { plainTextTheme } from "./plainTextTheme.js";
+import { providerRejectionMessage } from "./globalProviderPolicy.js";
 import { SessionUnreadStore, type SessionUnreadMutation } from "./sessionUnreadStore.js";
 
 /**
@@ -580,14 +581,38 @@ export function createPiWebCustomToolDefinitions(
   ];
 }
 
+/**
+ * Correlates provider registrations rejected by the global provider policy
+ * with the services load that triggered them. `begin` returns the mutable list
+ * the policy listener appends to while the load is in flight; `end` detaches
+ * it. Implemented by {@link PiSessionService}, which owns the in-flight set.
+ */
+interface ProviderRejectionTracker {
+  begin(): string[];
+  end(rejectedProviderIds: string[]): void;
+}
+
 function createDefaultRuntimeFactory(
   modelRuntime: ModelRuntime,
   sessionManagers: Pick<PiSessionManagerGateway, "open">,
   spawn?: SpawnSessionFn,
   subsessions?: SubsessionToolDeps,
+  providerRejections?: ProviderRejectionTracker,
 ): PiWebCreateAgentSessionRuntimeFactory {
   return async ({ cwd, agentDir, sessionManager, sessionStartEvent, initialModel, delegationToolsEnabled }) => {
-    const services = await createAgentSessionServices({ cwd, agentDir, modelRuntime });
+    const rejectedProviderIds = providerRejections?.begin();
+    let services: AgentSessionServices;
+    try {
+      services = await createAgentSessionServices({ cwd, agentDir, modelRuntime });
+    } finally {
+      if (rejectedProviderIds !== undefined) providerRejections?.end(rejectedProviderIds);
+    }
+    // Surface each provider the policy rejected during this load as a session
+    // warning, through the same diagnostics pipeline as other runtime setup
+    // issues. The registration was ignored; nothing else about the load changes.
+    for (const providerId of new Set(rejectedProviderIds ?? [])) {
+      services.diagnostics.push({ type: "warning", message: providerRejectionMessage(providerId, cwd) });
+    }
     const resolvedDelegationToolsEnabled = delegationToolsEnabled
       ?? await sessionAllowsDelegationTools(sessionManager, sessionManagers);
     const customTools = createPiWebCustomToolDefinitions(cwd, resolvedDelegationToolsEnabled, spawn, subsessions);
@@ -704,6 +729,8 @@ export class PiSessionService implements SessionRouteService {
   private readonly now: () => Date;
   private readonly notificationStore: SessionNotificationStore;
   private readonly notificationGenerationBySession = new WeakMap<PiAgentSession, SessionNotificationGeneration>();
+  /** Rejection lists of in-flight services loads; see {@link noteRejectedProviderRegistration}. */
+  private readonly pendingProviderRejectionLoads = new Set<string[]>();
   private readonly unreadStore: SessionUnreadStore;
   private readonly unreadPublicationRetryInitialMs: number;
   private readonly pendingUnreadMutations: SessionUnreadMutation[] = [];
@@ -742,6 +769,16 @@ export class PiSessionService implements SessionRouteService {
         check: (parentSessionId, sessionId, parentSessionFile) => this.checkSubsession(parentSessionId, sessionId, parentSessionFile),
         read: (parentSessionId, sessionId, query, parentSessionFile) => this.readSubsession(parentSessionId, sessionId, query, parentSessionFile),
       },
+      {
+        begin: () => {
+          const rejectedProviderIds: string[] = [];
+          this.pendingProviderRejectionLoads.add(rejectedProviderIds);
+          return rejectedProviderIds;
+        },
+        end: (rejectedProviderIds) => {
+          this.pendingProviderRejectionLoads.delete(rejectedProviderIds);
+        },
+      },
     );
     this.createAgentRuntime = deps.createAgentRuntime ?? defaultCreateAgentRuntime;
     this.workspaceActivity = deps.workspaceActivity;
@@ -786,6 +823,34 @@ export class PiSessionService implements SessionRouteService {
     return this.notificationStore.catalogSnapshot();
   }
 
+  /**
+   * Handle a provider registration rejected by the daemon-wide global provider
+   * policy (installed on the shared model runtime by sessiond).
+   *
+   * A rejection raised while at least one services load is in flight is
+   * recorded onto every in-flight load, which surfaces it as that session's
+   * load warning. The shim cannot attribute a registration to a specific
+   * extension or load, so overlapping loads may each report the same provider
+   * id; that over-reports but never drops a rejection.
+   *
+   * With no load in flight this is a late registration from a bound session's
+   * extension event handler. It cannot be attributed to a session either, so
+   * the notice is broadcast to every active session's notification inbox.
+   */
+  noteRejectedProviderRegistration(providerId: string): void {
+    if (this.pendingProviderRejectionLoads.size > 0) {
+      for (const load of this.pendingProviderRejectionLoads) load.push(providerId);
+      return;
+    }
+    const message = providerRejectionMessage(providerId);
+    for (const record of this.active.values()) {
+      const generation = this.notificationGenerationBySession.get(record.runtime.session);
+      if (generation === undefined) continue;
+      const added = this.notificationStore.addNotification(generation, message, "warning");
+      this.publishNotificationMutations(added.mutations);
+    }
+  }
+
   async unreadCatalog(): Promise<SessionUnreadCatalogSnapshot> {
     await this.publishUnreadMutations([]);
     return this.unreadStore.durableCatalogSnapshot();