diff --git a/.changeset/left-panel-collapse.md b/.changeset/left-panel-collapse.md
new file mode 100644
index 0000000..2fcec61
--- /dev/null
+++ b/.changeset/left-panel-collapse.md
@@ -0,0 +1,5 @@
+---
+"@jmfederico/pi-web": patch
+---
+
+Add a collapse control for the left navigation panel in wide and two-panel layouts.
diff --git a/src/client/src/components/PiWebApp.ts b/src/client/src/components/PiWebApp.ts
index 58cd641..14706c6 100644
--- a/src/client/src/components/PiWebApp.ts
+++ b/src/client/src/components/PiWebApp.ts
@@ -56,6 +56,7 @@ const VIEWPORT_POSITION_REPAIR_DELAY_MS = 250;
 @customElement("pi-web-app")
 export class PiWebApp extends LitElement {
   @state() private state: AppState = initialAppState();
+  @state() private navigationPanelCollapsed = false;
   @state() private workspacePanelCollapsed = false;
   @query("chat-view") private chatView?: ChatView;
   @query("prompt-editor") private promptEditor?: PromptEditor;
@@ -581,6 +582,32 @@ export class PiWebApp extends LitElement {
     `;
   }
 
+  private toggleNavigationPanelCollapse(): void {
+    this.navigationPanelCollapsed = !this.navigationPanelCollapsed;
+  }
+
+  private renderNavigationPanelEdgeControl() {
+    const collapsed = this.navigationPanelCollapsed;
+    const label = collapsed ? "Expand navigation panel" : "Collapse navigation panel";
+    return html`
+      <div class="navigation-panel-edge">
+        <button
+          type="button"
+          class="navigation-panel-edge-button"
+          title=${label}
+          aria-label=${label}
+          aria-controls="navigation-panel"
+          aria-expanded=${String(!collapsed)}
+          @click=${() => { this.toggleNavigationPanelCollapse(); }}
+        >${this.renderNavigationPanelEdgeIcon(collapsed)}</button>
+      </div>
+    `;
+  }
+
+  private renderNavigationPanelEdgeIcon(collapsed: boolean) {
+    return this.renderPanelEdgeIcon(collapsed ? "right" : "left", "navigation-panel-edge-icon");
+  }
+
   private toggleWorkspacePanelCollapse(): void {
     this.workspacePanelCollapsed = !this.workspacePanelCollapsed;
   }
@@ -604,8 +631,12 @@ export class PiWebApp extends LitElement {
   }
 
   private renderWorkspacePanelEdgeIcon(collapsed: boolean) {
-    const path = collapsed ? "M15 18l-6-6 6-6" : "M9 18l6-6-6-6";
-    return html`<svg class="workspace-panel-edge-icon" viewBox="0 0 24 24" aria-hidden="true" focusable="false"><path d=${path}/></svg>`;
+    return this.renderPanelEdgeIcon(collapsed ? "left" : "right", "workspace-panel-edge-icon");
+  }
+
+  private renderPanelEdgeIcon(direction: "left" | "right", className: string) {
+    const path = direction === "left" ? "M15 18l-6-6 6-6" : "M9 18l6-6-6-6";
+    return html`<svg class=${className} viewBox="0 0 24 24" aria-hidden="true" focusable="false"><path d=${path}/></svg>`;
   }
 
   private renderNavigationPanel(autoSwitchToChat: boolean) {
@@ -1286,8 +1317,9 @@ export class PiWebApp extends LitElement {
   override render() {
     const state = this.state;
     return html`
-      <div class=${`shell ${state.mainView === "navigation" ? "navigation-view" : state.mainView === "chat" ? "chat-view" : "workspace-view"}${this.workspacePanelCollapsed ? " workspace-panel-collapsed" : ""}`}>
-        <aside>${this.isMobileNavigationLayout ? null : this.renderNavigationPanel(false)}</aside>
+      <div class=${`shell ${state.mainView === "navigation" ? "navigation-view" : state.mainView === "chat" ? "chat-view" : "workspace-view"}${this.navigationPanelCollapsed ? " navigation-panel-collapsed" : ""}${this.workspacePanelCollapsed ? " workspace-panel-collapsed" : ""}`}>
+        <aside id="navigation-panel">${this.isMobileNavigationLayout ? null : this.renderNavigationPanel(false)}</aside>
+        ${this.renderNavigationPanelEdgeControl()}
         <main class=${state.mainView === "chat" ? "chat-view" : state.mainView === "navigation" ? "navigation-view" : "workspace-view"}>
           ${this.renderContextBar()}
           <div class=${this.mobileTabsFrameClass()}>
diff --git a/src/client/src/components/shared.ts b/src/client/src/components/shared.ts
index 098092e..98eeeb1 100644
--- a/src/client/src/components/shared.ts
+++ b/src/client/src/components/shared.ts
@@ -51,13 +51,13 @@ export interface CompletionItem {
 
 export const appStyles = css`
   :host { position: fixed; top: 0; right: 0; left: 0; display: block; height: 100dvh; box-sizing: border-box; overflow: hidden; padding: env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) env(safe-area-inset-left); color: var(--pi-text); background: var(--pi-bg); font: 14px system-ui, sans-serif; }
-  .shell { display: grid; grid-template-columns: 340px minmax(420px, 1fr) 1px minmax(360px, 42vw); height: 100%; min-height: 0; }
-  aside { grid-column: 1; display: flex; flex-direction: column; min-height: 0; border-right: 1px solid var(--pi-border); overflow: hidden; }
+  .shell { --navigation-panel-width: 340px; --workspace-panel-width: minmax(360px, 42vw); display: grid; grid-template-columns: var(--navigation-panel-width) 1px minmax(420px, 1fr) 1px var(--workspace-panel-width); height: 100%; min-height: 0; }
+  aside { grid-column: 1; display: flex; flex-direction: column; min-height: 0; overflow: hidden; }
   header { flex: 0 0 auto; display: flex; align-items: center; justify-content: space-between; gap: 8px; padding: 12px; border-bottom: 1px solid var(--pi-border); }
   .header-actions { display: flex; align-items: center; gap: 8px; }
   project-list, workspace-list { flex: 0 0 auto; max-height: 26%; overflow: auto; border-bottom: 1px solid var(--pi-border-muted); }
   session-list { flex: 1 1 auto; min-height: 0; overflow: auto; }
-  main { grid-column: 2; display: flex; flex-direction: column; min-width: 0; min-height: 0; }
+  main { grid-column: 3; display: flex; flex-direction: column; min-width: 0; min-height: 0; }
   .context-bar { position: relative; flex: 0 0 auto; min-width: 0; display: none; align-items: center; gap: 0; padding: 6px 0; border-bottom: 1px solid var(--pi-border-muted); background: var(--pi-bg); }
   .context-bar::before, .context-bar::after { content: ""; position: absolute; top: 0; bottom: 0; z-index: 2; width: 20px; opacity: 0; pointer-events: none; transition: opacity .15s ease; }
   .context-bar::before { left: 0; background: linear-gradient(90deg, color-mix(in srgb, var(--pi-shadow-strong) 55%, transparent) 0%, transparent 100%); }
@@ -93,23 +93,31 @@ export const appStyles = css`
   .mobile-navigation-tab, .mobile-navigation-panel { display: none; }
   .mobile-tabs button.selected { border-color: var(--pi-accent); background: var(--pi-selection-bg); }
   .tab-badge { display: inline-block; min-width: 14px; margin-left: 4px; border: 1px solid var(--pi-success-border); border-radius: 999px; background: var(--pi-success-surface); color: var(--pi-success); padding: 0 5px; font-size: 11px; line-height: 16px; text-align: center; }
-  .workspace-panel-edge { grid-column: 3; min-width: 0; min-height: 0; display: flex; align-items: center; justify-content: center; overflow: visible; background: var(--pi-border-muted); z-index: 2; }
-  .workspace-panel-edge-button { position: relative; z-index: 1; box-sizing: border-box; display: grid; place-items: center; width: 18px; height: 48px; padding: 0; border: 1px solid var(--pi-border-muted); border-radius: 999px; background: var(--pi-bg); color: var(--pi-muted); opacity: .75; cursor: pointer; }
-  .workspace-panel-edge-button:hover, .workspace-panel-edge-button:focus-visible { color: var(--pi-text); background: var(--pi-surface-hover); opacity: 1; }
+  .navigation-panel-edge, .workspace-panel-edge { min-width: 0; min-height: 0; display: flex; align-items: center; justify-content: center; overflow: visible; background: var(--pi-border-muted); z-index: 2; }
+  .navigation-panel-edge { grid-column: 2; }
+  .workspace-panel-edge { grid-column: 4; }
+  .navigation-panel-edge-button, .workspace-panel-edge-button { position: relative; z-index: 1; box-sizing: border-box; display: grid; place-items: center; width: 18px; height: 48px; padding: 0; border: 1px solid var(--pi-border-muted); border-radius: 999px; background: var(--pi-bg); color: var(--pi-muted); opacity: .75; cursor: pointer; }
+  .navigation-panel-edge-button:hover, .navigation-panel-edge-button:focus-visible, .workspace-panel-edge-button:hover, .workspace-panel-edge-button:focus-visible { color: var(--pi-text); background: var(--pi-surface-hover); opacity: 1; }
+  .shell.navigation-panel-collapsed .navigation-panel-edge-button { transform: translateX(calc(50% - .5px)); }
   .shell.workspace-panel-collapsed .workspace-panel-edge-button { transform: translateX(calc(-50% + .5px)); }
-  .workspace-panel-edge-icon { width: 12px; height: 12px; fill: none; stroke: currentColor; stroke-width: 2.2; stroke-linecap: round; stroke-linejoin: round; pointer-events: none; }
-  workspace-panel { grid-column: 4; min-width: 0; min-height: 0; overflow: hidden; }
+  .navigation-panel-edge-icon, .workspace-panel-edge-icon { width: 12px; height: 12px; fill: none; stroke: currentColor; stroke-width: 2.2; stroke-linecap: round; stroke-linejoin: round; pointer-events: none; }
+  workspace-panel { grid-column: 5; min-width: 0; min-height: 0; overflow: hidden; }
   @media (min-width: 1181px) {
-    .shell.workspace-panel-collapsed { grid-template-columns: 340px minmax(420px, 1fr) 1px; }
+    .shell.navigation-panel-collapsed { --navigation-panel-width: 0px; }
+    .shell.navigation-panel-collapsed > aside { display: none; }
+    .shell.workspace-panel-collapsed { --workspace-panel-width: 0px; }
     .shell.workspace-panel-collapsed > workspace-panel { display: none; }
   }
   @media (max-width: 1180px) {
-    .shell { grid-template-columns: 340px minmax(0, 1fr); grid-template-rows: auto minmax(0, 1fr); }
+    .shell { grid-template-columns: var(--navigation-panel-width) 1px minmax(0, 1fr); grid-template-rows: auto minmax(0, 1fr); }
+    .shell.navigation-panel-collapsed { --navigation-panel-width: 0px; }
+    .shell.navigation-panel-collapsed > aside { display: none; }
     aside { grid-row: 1 / 3; }
-    main { grid-column: 2; grid-row: 1 / 3; }
+    .navigation-panel-edge { grid-row: 1 / 3; }
+    main { grid-column: 3; grid-row: 1 / 3; }
     .mobile-tabs-frame { display: flex; }
     .shell.workspace-view main { grid-row: 1; min-height: auto; }
-    .shell.workspace-view > workspace-panel { grid-column: 2; grid-row: 2; display: flex; border-left: 0; }
+    .shell.workspace-view > workspace-panel { grid-column: 3; grid-row: 2; display: flex; border-left: 0; }
     .shell:not(.workspace-view) > workspace-panel { display: none; }
     .workspace-panel-edge { display: none; }
     main.workspace-view chat-view, main.workspace-view prompt-editor, main.workspace-view status-bar,
@@ -118,7 +126,7 @@ export const appStyles = css`
   }
   @media (max-width: 760px) {
     .shell { grid-template-columns: minmax(0, 1fr); }
-    aside { display: none; }
+    aside, .navigation-panel-edge { display: none; }
     main, .shell.workspace-view > workspace-panel { grid-column: 1; }
     .context-bar { display: flex; }
     .mobile-navigation-tab { display: block; }