diff --git a/apps/mobile/src/features/threads/projectThreadCreationValidation.ts b/apps/mobile/src/features/threads/projectThreadCreationValidation.ts
new file mode 100644
index 00000000000..e4ad776e23d
--- /dev/null
+++ b/apps/mobile/src/features/threads/projectThreadCreationValidation.ts
@@ -0,0 +1,56 @@
+import { EnvironmentId, ProjectId } from "@t3tools/contracts";
+import * as Schema from "effect/Schema";
+
+export class ProjectThreadTaskRequiredError extends Schema.TaggedErrorClass<ProjectThreadTaskRequiredError>()(
+  "ProjectThreadTaskRequiredError",
+  {
+    environmentId: EnvironmentId,
+    projectId: ProjectId,
+    environmentMode: Schema.Literals(["local", "worktree"]),
+  },
+) {
+  override get message(): string {
+    return "Enter a task before starting the thread.";
+  }
+}
+
+export class ProjectThreadBaseBranchRequiredError extends Schema.TaggedErrorClass<ProjectThreadBaseBranchRequiredError>()(
+  "ProjectThreadBaseBranchRequiredError",
+  {
+    environmentId: EnvironmentId,
+    projectId: ProjectId,
+  },
+) {
+  override get message(): string {
+    return "Select a base branch before creating a worktree.";
+  }
+}
+
+export const ProjectThreadCreationValidationError = Schema.Union([
+  ProjectThreadTaskRequiredError,
+  ProjectThreadBaseBranchRequiredError,
+]);
+export type ProjectThreadCreationValidationError = typeof ProjectThreadCreationValidationError.Type;
+
+export function validateProjectThreadCreation(input: {
+  readonly environmentId: EnvironmentId;
+  readonly projectId: ProjectId;
+  readonly environmentMode: "local" | "worktree";
+  readonly branch: string | null;
+  readonly initialMessageText: string;
+}): ProjectThreadCreationValidationError | null {
+  if (input.initialMessageText.trim().length === 0) {
+    return new ProjectThreadTaskRequiredError({
+      environmentId: input.environmentId,
+      projectId: input.projectId,
+      environmentMode: input.environmentMode,
+    });
+  }
+  if (input.environmentMode === "worktree" && !input.branch) {
+    return new ProjectThreadBaseBranchRequiredError({
+      environmentId: input.environmentId,
+      projectId: input.projectId,
+    });
+  }
+  return null;
+}
diff --git a/apps/mobile/src/features/threads/use-project-actions.ts b/apps/mobile/src/features/threads/use-project-actions.ts
index a0c19d9fe8b..9531567f447 100644
--- a/apps/mobile/src/features/threads/use-project-actions.ts
+++ b/apps/mobile/src/features/threads/use-project-actions.ts
@@ -21,6 +21,7 @@ import { makeTurnCommandMetadata } from "../../lib/commandMetadata";
 import { uuidv4 } from "../../lib/uuid";
 import { useAtomCommand } from "../../state/use-atom-command";
 import { setPendingConnectionError } from "../../state/use-remote-environment-registry";
+import { validateProjectThreadCreation } from "./projectThreadCreationValidation";
 
 function deriveThreadTitleFromPrompt(value: string): string {
   const trimmed = value.trim();
@@ -52,15 +53,16 @@ export function useCreateProjectThread() {
       const initialMessageText = input.initialMessageText.trim();
       const nextTitle = deriveThreadTitleFromPrompt(input.initialMessageText);
 
-      if (initialMessageText.length === 0) {
-        const error = new Error("Enter a task before starting the thread.");
-        setPendingConnectionError(error.message);
-        return AsyncResult.failure(Cause.fail(error));
-      }
-      if (input.envMode === "worktree" && !input.branch) {
-        const error = new Error("Select a base branch before creating a worktree.");
-        setPendingConnectionError(error.message);
-        return AsyncResult.failure(Cause.fail(error));
+      const validationError = validateProjectThreadCreation({
+        environmentId: input.project.environmentId,
+        projectId: input.project.id,
+        environmentMode: input.envMode,
+        branch: input.branch,
+        initialMessageText,
+      });
+      if (validationError !== null) {
+        setPendingConnectionError(validationError.message);
+        return AsyncResult.failure(Cause.fail(validationError));
       }
 
       const isWorktree = input.envMode === "worktree";