diff --git a/packages/core/src/__tests__/session-manager.test.ts b/packages/core/src/__tests__/session-manager.test.ts
index 96d56edff..ed3746342 100644
--- a/packages/core/src/__tests__/session-manager.test.ts
+++ b/packages/core/src/__tests__/session-manager.test.ts
@@ -3,7 +3,7 @@ import { mkdirSync, rmSync, writeFileSync, existsSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 import { randomUUID } from "node:crypto";
-import { createSessionManager } from "../session-manager.js";
+import { createSessionManager, inferProjectId } from "../session-manager.js";
 import { writeMetadata, readMetadata, readMetadataRaw, deleteMetadata } from "../metadata.js";
 import { getSessionsDir, getProjectBaseDir } from "../paths.js";
 import {
@@ -1335,3 +1335,62 @@ describe("PluginRegistry.loadBuiltins importFn", () => {
     expect(importedPackages).toContain("@composio/ao-plugin-runtime-tmux");
   });
 });
+
+describe("inferProjectId", () => {
+  it("matches session ID prefix to project sessionPrefix", () => {
+    const projects = {
+      "my-app": { ...config.projects["my-app"], sessionPrefix: "app" },
+      integrator: {
+        ...config.projects["my-app"],
+        name: "Integrator",
+        sessionPrefix: "int",
+        path: "/tmp/integrator",
+      },
+    };
+    expect(inferProjectId("app-1", projects)).toBe("my-app");
+    expect(inferProjectId("int-42", projects)).toBe("integrator");
+    expect(inferProjectId("app-100", projects)).toBe("my-app");
+  });
+
+  it("falls back to single project when prefix doesn't match", () => {
+    const projects = {
+      "my-app": { ...config.projects["my-app"], sessionPrefix: "app" },
+    };
+    expect(inferProjectId("unknown-5", projects)).toBe("my-app");
+  });
+
+  it("returns empty string when no match and multiple projects", () => {
+    const projects = {
+      "my-app": { ...config.projects["my-app"], sessionPrefix: "app" },
+      integrator: {
+        ...config.projects["my-app"],
+        name: "Integrator",
+        sessionPrefix: "int",
+        path: "/tmp/integrator",
+      },
+    };
+    expect(inferProjectId("unknown-5", projects)).toBe("");
+  });
+
+  it("returns empty string for empty projects config", () => {
+    expect(inferProjectId("app-1", {})).toBe("");
+  });
+
+  it("uses list to infer projectId for legacy sessions without project field", async () => {
+    // Write a session without project field (legacy metadata)
+    writeMetadata(sessionsDir, "app-5", {
+      worktree: "/tmp",
+      branch: "feat/legacy",
+      status: "working",
+      // no project field
+    });
+
+    const sm = createSessionManager({ config, registry: mockRegistry });
+    const sessions = await sm.list();
+
+    const legacySession = sessions.find((s) => s.id === "app-5");
+    expect(legacySession).toBeDefined();
+    // Should infer "my-app" because "app" prefix matches
+    expect(legacySession!.projectId).toBe("my-app");
+  });
+});
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index ed9b47b72..e448a29d9 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -44,7 +44,7 @@ export {
 } from "./tmux.js";
 
 // Session manager — session CRUD
-export { createSessionManager } from "./session-manager.js";
+export { createSessionManager, inferProjectId } from "./session-manager.js";
 export type { SessionManagerDeps } from "./session-manager.js";
 
 // Lifecycle manager — state machine + reaction engine
diff --git a/packages/core/src/session-manager.ts b/packages/core/src/session-manager.ts
index c58a286cd..bdd942891 100644
--- a/packages/core/src/session-manager.ts
+++ b/packages/core/src/session-manager.ts
@@ -84,6 +84,34 @@ function safeJsonParse<T>(str: string): T | null {
   }
 }
 
+/**
+ * Infer projectId from session ID prefix by matching against configured sessionPrefix values.
+ *
+ * Strategy:
+ * 1. Match session ID prefix against each project's sessionPrefix (e.g. "ao-18" matches prefix "ao")
+ * 2. Fallback: if only one project is configured, use that
+ * 3. Returns empty string if no match found
+ */
+export function inferProjectId(
+  sessionId: string,
+  projects: Record<string, ProjectConfig>,
+): string {
+  // Strategy 1: Match session ID prefix against configured sessionPrefix values
+  for (const [projectKey, project] of Object.entries(projects)) {
+    if (project.sessionPrefix && sessionId.startsWith(`${project.sessionPrefix}-`)) {
+      return projectKey;
+    }
+  }
+
+  // Strategy 2: If only one project is configured, use that
+  const projectKeys = Object.keys(projects);
+  if (projectKeys.length === 1) {
+    return projectKeys[0];
+  }
+
+  return "";
+}
+
 /** Valid session statuses for validation. */
 const VALID_STATUSES: ReadonlySet<string> = new Set([
   "spawning",
@@ -116,12 +144,15 @@ function validateStatus(raw: string | undefined): SessionStatus {
 function metadataToSession(
   sessionId: SessionId,
   meta: Record<string, string>,
+  projects: Record<string, ProjectConfig>,
   createdAt?: Date,
   modifiedAt?: Date,
 ): Session {
+  const rawProject = meta["project"] ?? "";
+  const projectId = rawProject || inferProjectId(sessionId, projects);
   return {
     id: sessionId,
-    projectId: meta["project"] ?? "",
+    projectId,
     status: validateStatus(meta["status"]),
     activity: null,
     branch: meta["branch"] || null,
@@ -691,7 +722,7 @@ export function createSessionManager(deps: SessionManagerDeps): SessionManager {
         // If stat fails, timestamps will fall back to current time
       }
 
-      const session = metadataToSession(sessionName, raw, createdAt, modifiedAt);
+      const session = metadataToSession(sessionName, raw, config.projects, createdAt, modifiedAt);
 
       const plugins = resolvePlugins(project);
       // Cap per-session enrichment at 2s — subprocess calls (tmux/ps) can be
@@ -725,7 +756,7 @@ export function createSessionManager(deps: SessionManagerDeps): SessionManager {
         // If stat fails, timestamps will fall back to current time
       }
 
-      const session = metadataToSession(sessionId, raw, createdAt, modifiedAt);
+      const session = metadataToSession(sessionId, raw, config.projects, createdAt, modifiedAt);
 
       const plugins = resolvePlugins(project);
       await ensureHandleAndEnrich(session, sessionId, project, plugins);
diff --git a/packages/web/src/app/api/prs/[id]/merge/route.ts b/packages/web/src/app/api/prs/[id]/merge/route.ts
index 6063f70fe..96ce7c756 100644
--- a/packages/web/src/app/api/prs/[id]/merge/route.ts
+++ b/packages/web/src/app/api/prs/[id]/merge/route.ts
@@ -1,5 +1,6 @@
 import { type NextRequest, NextResponse } from "next/server";
 import { getServices, getSCM } from "@/lib/services";
+import { inferProjectId } from "@composio/ao-core";
 
 /** POST /api/prs/:id/merge — Merge a PR */
 export async function POST(_request: NextRequest, { params }: { params: Promise<{ id: string }> }) {
@@ -18,11 +19,24 @@ export async function POST(_request: NextRequest, { params }: { params: Promise<
       return NextResponse.json({ error: "PR not found" }, { status: 404 });
     }
 
-    const project = config.projects[session.projectId];
+    // Resolve project — use session.projectId, infer from prefix, or fall back to single project
+    let project = config.projects[session.projectId];
+    if (!project) {
+      const inferred = inferProjectId(session.id, config.projects);
+      if (inferred) {
+        project = config.projects[inferred];
+      }
+    }
+
     const scm = getSCM(registry, project);
     if (!scm) {
       return NextResponse.json(
-        { error: "No SCM plugin configured for this project" },
+        {
+          error: "No SCM plugin configured for this project",
+          detail: session.projectId
+            ? `Project "${session.projectId}" not found in config`
+            : `Session "${session.id}" has no project field and prefix could not be matched`,
+        },
         { status: 500 },
       );
     }
diff --git a/packages/web/src/components/Dashboard.tsx b/packages/web/src/components/Dashboard.tsx
index a414fc741..96ca8ebfb 100644
--- a/packages/web/src/components/Dashboard.tsx
+++ b/packages/web/src/components/Dashboard.tsx
@@ -54,7 +54,8 @@ export function Dashboard({ sessions, stats, orchestratorId, projectName }: Dash
       body: JSON.stringify({ message }),
     });
     if (!res.ok) {
-      console.error(`Failed to send message to ${sessionId}:`, await res.text());
+      const text = await res.text().catch(() => "Unknown error");
+      alert(`Failed to send message to ${sessionId}: ${text}`);
     }
   };
 
@@ -64,14 +65,30 @@ export function Dashboard({ sessions, stats, orchestratorId, projectName }: Dash
       method: "POST",
     });
     if (!res.ok) {
-      console.error(`Failed to kill ${sessionId}:`, await res.text());
+      const text = await res.text().catch(() => "Unknown error");
+      alert(`Failed to kill ${sessionId}: ${text}`);
     }
   };
 
   const handleMerge = async (prNumber: number) => {
     const res = await fetch(`/api/prs/${prNumber}/merge`, { method: "POST" });
     if (!res.ok) {
-      console.error(`Failed to merge PR #${prNumber}:`, await res.text());
+      let errorMessage = `Failed to merge PR #${prNumber}`;
+      try {
+        const body = await res.json();
+        if (body.error) {
+          errorMessage += `:\n\n${body.error}`;
+        }
+        if (body.detail) {
+          errorMessage += `\n${body.detail}`;
+        }
+        if (body.blockers && Array.isArray(body.blockers) && body.blockers.length > 0) {
+          errorMessage += `\n\nBlockers:\n${body.blockers.map((b: string) => `• ${b}`).join("\n")}`;
+        }
+      } catch {
+        errorMessage += `:\n\n${await res.text().catch(() => "Unknown error")}`;
+      }
+      alert(errorMessage);
     }
   };