diff --git a/packages/backend/src/routes/auth/login.ts b/packages/backend/src/routes/auth/login.ts
index cd2971c..3965b31 100644
--- a/packages/backend/src/routes/auth/login.ts
+++ b/packages/backend/src/routes/auth/login.ts
@@ -1,45 +1,32 @@
+import { LoginRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { buildAuthCookie, generateToken, verifyPassword } from "../../auth/utils";
 import { createSession, getUserByUsername } from "../../db/queries";
-
-const isNonEmptyString = (value: unknown): value is string =>
-    typeof value === "string" && value.trim().length > 0;
+import { errorResponse, parseJsonBody } from "../../validation";
 
 export default async function login(req: BunRequest) {
     if (req.method !== "POST") {
-        return new Response("method not allowed", { status: 405 });
+        return errorResponse("method not allowed", "METHOD_NOT_ALLOWED", 405);
     }
 
-    let body: unknown;
-    try {
-        body = await req.json();
-    } catch {
-        return new Response("invalid JSON", { status: 400 });
-    }
+    const parsed = await parseJsonBody(req, LoginRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!body || typeof body !== "object") {
-        return new Response("invalid request body", { status: 400 });
-    }
-
-    const { username, password } = body as Record<string, unknown>;
-
-    if (!isNonEmptyString(username) || !isNonEmptyString(password)) {
-        return new Response("username and password are required", { status: 400 });
-    }
+    const { username, password } = parsed.data;
 
     const user = await getUserByUsername(username);
     if (!user) {
-        return new Response("invalid credentials", { status: 401 });
+        return errorResponse("invalid credentials", "INVALID_CREDENTIALS", 401);
     }
 
     const ok = await verifyPassword(password, user.passwordHash);
     if (!ok) {
-        return new Response("invalid credentials", { status: 401 });
+        return errorResponse("invalid credentials", "INVALID_CREDENTIALS", 401);
     }
 
     const session = await createSession(user.id);
     if (!session) {
-        return new Response("failed to create session", { status: 500 });
+        return errorResponse("failed to create session", "SESSION_ERROR", 500);
     }
 
     const token = generateToken(session.id, user.id);
diff --git a/packages/backend/src/routes/auth/register.ts b/packages/backend/src/routes/auth/register.ts
index b8c4f2a..af2891a 100644
--- a/packages/backend/src/routes/auth/register.ts
+++ b/packages/backend/src/routes/auth/register.ts
@@ -1,62 +1,33 @@
+import { RegisterRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { buildAuthCookie, generateToken, hashPassword } from "../../auth/utils";
 import { createSession, createUser, getUserByUsername } from "../../db/queries";
-
-const isNonEmptyString = (value: unknown): value is string =>
-    typeof value === "string" && value.trim().length > 0;
+import { errorResponse, parseJsonBody } from "../../validation";
 
 export default async function register(req: BunRequest) {
     if (req.method !== "POST") {
-        return new Response("method not allowed", { status: 405 });
+        return errorResponse("method not allowed", "METHOD_NOT_ALLOWED", 405);
     }
 
-    let body: unknown;
-    try {
-        body = await req.json();
-    } catch {
-        return new Response("invalid JSON", { status: 400 });
-    }
+    const parsed = await parseJsonBody(req, RegisterRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!body || typeof body !== "object") {
-        return new Response("invalid request body", { status: 400 });
-    }
-
-    const { name, username, password, avatarURL } = body as Record<string, unknown>;
-
-    if (!isNonEmptyString(name) || !isNonEmptyString(username) || !isNonEmptyString(password)) {
-        return new Response("name, username, and password are required", { status: 400 });
-    }
-
-    if (username.length < 1 || username.length > 32) {
-        return new Response("username must be 1-32 characters", { status: 400 });
-    }
-
-    if (password.length < 8) {
-        return new Response("password must be at least 8 characters", { status: 400 });
-    }
-
-    const hasUpperCase = /[A-Z]/.test(password);
-    const hasLowerCase = /[a-z]/.test(password);
-    const hasNumber = /[0-9]/.test(password);
-
-    if (!hasUpperCase || !hasLowerCase || !hasNumber) {
-        return new Response("password must contain uppercase, lowercase, and numbers", { status: 400 });
-    }
+    const { name, username, password, avatarURL } = parsed.data;
 
     const existing = await getUserByUsername(username);
     if (existing) {
-        return new Response("username already taken", { status: 400 });
+        return errorResponse("username already taken", "USERNAME_TAKEN", 400);
     }
 
     const passwordHash = await hashPassword(password);
-    const user = await createUser(name, username, passwordHash, avatarURL as string | undefined);
+    const user = await createUser(name, username, passwordHash, avatarURL);
     if (!user) {
-        return new Response("failed to create user", { status: 500 });
+        return errorResponse("failed to create user", "USER_CREATE_ERROR", 500);
     }
 
     const session = await createSession(user.id);
     if (!session) {
-        return new Response("failed to create session", { status: 500 });
+        return errorResponse("failed to create session", "SESSION_ERROR", 500);
     }
 
     const token = generateToken(session.id, user.id);
diff --git a/packages/backend/src/routes/issue/create.ts b/packages/backend/src/routes/issue/create.ts
index dadc370..c758873 100644
--- a/packages/backend/src/routes/issue/create.ts
+++ b/packages/backend/src/routes/issue/create.ts
@@ -1,35 +1,28 @@
+import { IssueCreateRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
-import { createIssue, getProjectByID, getProjectByKey } from "../../db/queries";
+import { createIssue, getProjectByID } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /issue/create?projectId=1&title=Testing&description=Description&status=TO%20DO
-// OR
-// /issue/create?projectKey=projectKey&title=Testing&description=Description&status=TO%20DO
 export default async function issueCreate(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const projectId = url.searchParams.get("projectId");
-    const projectKey = url.searchParams.get("projectKey");
+    const parsed = await parseJsonBody(req, IssueCreateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    let project = null;
-    if (projectId) {
-        project = await getProjectByID(Number(projectId));
-    } else if (projectKey) {
-        project = await getProjectByKey(projectKey);
-    } else {
-        return new Response("missing project key or project id", { status: 400 });
-    }
+    const { projectId, title, description = "", status, assigneeId, sprintId } = parsed.data;
+
+    const project = await getProjectByID(projectId);
     if (!project) {
-        return new Response(`project not found: provided ${projectId ?? projectKey}`, { status: 404 });
+        return errorResponse(`project not found: ${projectId}`, "PROJECT_NOT_FOUND", 404);
     }
 
-    const title = url.searchParams.get("title") || "Untitled Issue";
-    const description = url.searchParams.get("description") || "";
-    const sprintIdParam = url.searchParams.get("sprintId");
-    const sprintId = sprintIdParam ? Number(sprintIdParam) : undefined;
-    const assigneeIdParam = url.searchParams.get("assigneeId");
-    const assigneeId = assigneeIdParam ? Number(assigneeIdParam) : undefined;
-    const status = url.searchParams.get("status") || undefined;
-
-    const issue = await createIssue(project.id, title, description, req.userId, sprintId, assigneeId, status);
+    const issue = await createIssue(
+        project.id,
+        title,
+        description,
+        req.userId,
+        sprintId ?? undefined,
+        assigneeId ?? undefined,
+        status,
+    );
 
     return Response.json(issue);
 }
diff --git a/packages/backend/src/routes/issue/delete.ts b/packages/backend/src/routes/issue/delete.ts
index 68c2625..c75deb6 100644
--- a/packages/backend/src/routes/issue/delete.ts
+++ b/packages/backend/src/routes/issue/delete.ts
@@ -1,18 +1,18 @@
+import { IssueDeleteRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { deleteIssue } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /issue/delete?id=1
 export default async function issueDelete(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
-    if (!id) {
-        return new Response("missing issue id", { status: 400 });
-    }
+    const parsed = await parseJsonBody(req, IssueDeleteRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    const result = await deleteIssue(Number(id));
+    const { id } = parsed.data;
+
+    const result = await deleteIssue(id);
     if (result.rowCount === 0) {
-        return new Response(`no issue with id ${id} found`, { status: 404 });
+        return errorResponse(`no issue with id ${id} found`, "ISSUE_NOT_FOUND", 404);
     }
 
-    return new Response(`issue with id ${id} deleted`, { status: 200 });
+    return Response.json({ success: true });
 }
diff --git a/packages/backend/src/routes/issue/update.ts b/packages/backend/src/routes/issue/update.ts
index 9baad23..780fa73 100644
--- a/packages/backend/src/routes/issue/update.ts
+++ b/packages/backend/src/routes/issue/update.ts
@@ -1,41 +1,26 @@
+import { IssueUpdateRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { updateIssue } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /issue/update?id=1&title=Testing&description=Description&assigneeId=2&status=IN%20PROGRESS
-// assigneeId can be "null" to unassign
 export default async function issueUpdate(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
-    if (!id) {
-        return new Response("missing issue id", { status: 400 });
+    const parsed = await parseJsonBody(req, IssueUpdateRequestSchema);
+    if ("error" in parsed) return parsed.error;
+
+    const { id, title, description, status, assigneeId, sprintId } = parsed.data;
+
+    // check that at least one field is being updated
+    if (
+        title === undefined &&
+        description === undefined &&
+        status === undefined &&
+        assigneeId === undefined &&
+        sprintId === undefined
+    ) {
+        return errorResponse("no updates provided", "NO_UPDATES", 400);
     }
 
-    const title = url.searchParams.get("title") || undefined;
-    const description = url.searchParams.get("description") || undefined;
-    const sprintIdParam = url.searchParams.get("sprintId");
-    const assigneeIdParam = url.searchParams.get("assigneeId");
-    const status = url.searchParams.get("status") || undefined;
-
-    // parse sprintId: "null" means unassign, number means assign, undefined means no change
-    let sprintId: number | null | undefined;
-    if (sprintIdParam === "null") {
-        sprintId = null;
-    } else if (sprintIdParam) {
-        sprintId = Number(sprintIdParam);
-    }
-    // same for assigneeId
-    let assigneeId: number | null | undefined;
-    if (assigneeIdParam === "null") {
-        assigneeId = null;
-    } else if (assigneeIdParam) {
-        assigneeId = Number(assigneeIdParam);
-    }
-
-    if (!title && !description && sprintId === undefined && assigneeId === undefined && !status) {
-        return new Response("no updates provided", { status: 400 });
-    }
-
-    const issue = await updateIssue(Number(id), {
+    const issue = await updateIssue(id, {
         title,
         description,
         sprintId,
diff --git a/packages/backend/src/routes/issues/by-project.ts b/packages/backend/src/routes/issues/by-project.ts
index 9f34e53..8d03a0f 100644
--- a/packages/backend/src/routes/issues/by-project.ts
+++ b/packages/backend/src/routes/issues/by-project.ts
@@ -1,14 +1,20 @@
+import { IssuesByProjectQuerySchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getIssuesWithUsersByProject, getProjectByID } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
 export default async function issuesByProject(req: AuthedRequest) {
     const url = new URL(req.url);
-    const projectId = url.searchParams.get("projectId");
+    const parsed = parseQueryParams(url, IssuesByProjectQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    const project = await getProjectByID(Number(projectId));
+    const { projectId } = parsed.data;
+
+    const project = await getProjectByID(projectId);
     if (!project) {
-        return new Response(`project not found: provided ${projectId}`, { status: 404 });
+        return errorResponse(`project not found: ${projectId}`, "PROJECT_NOT_FOUND", 404);
     }
+
     const issues = await getIssuesWithUsersByProject(project.id);
 
     return Response.json(issues);
diff --git a/packages/backend/src/routes/issues/replace-status.ts b/packages/backend/src/routes/issues/replace-status.ts
index f2ba9c3..fb2784f 100644
--- a/packages/backend/src/routes/issues/replace-status.ts
+++ b/packages/backend/src/routes/issues/replace-status.ts
@@ -1,38 +1,21 @@
+import { IssuesReplaceStatusRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getOrganisationMemberRole, replaceIssueStatus } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /issues/replace-status?organisationId=1&oldStatus=TO%20DO&newStatus=IN%20PROGRESS
 export default async function issuesReplaceStatus(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const organisationIdParam = url.searchParams.get("organisationId");
-    const oldStatus = url.searchParams.get("oldStatus");
-    const newStatus = url.searchParams.get("newStatus");
+    const parsed = await parseJsonBody(req, IssuesReplaceStatusRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationIdParam) {
-        return new Response("missing organisationId", { status: 400 });
-    }
+    const { organisationId, oldStatus, newStatus } = parsed.data;
 
-    if (!oldStatus) {
-        return new Response("missing oldStatus", { status: 400 });
-    }
-
-    if (!newStatus) {
-        return new Response("missing newStatus", { status: 400 });
-    }
-
-    const organisationId = Number(organisationIdParam);
-    if (!Number.isInteger(organisationId)) {
-        return new Response("organisationId must be an integer", { status: 400 });
-    }
-
-    // check if user is admin or owner of the organisation
     const membership = await getOrganisationMemberRole(organisationId, req.userId);
     if (!membership) {
-        return new Response("not a member of this organisation", { status: 403 });
+        return errorResponse("not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     if (membership.role !== "owner" && membership.role !== "admin") {
-        return new Response("only admins and owners can replace statuses", { status: 403 });
+        return errorResponse("only admins and owners can replace statuses", "PERMISSION_DENIED", 403);
     }
 
     const result = await replaceIssueStatus(organisationId, oldStatus, newStatus);
diff --git a/packages/backend/src/routes/issues/status-count.ts b/packages/backend/src/routes/issues/status-count.ts
index 5f8b185..8aa0ed5 100644
--- a/packages/backend/src/routes/issues/status-count.ts
+++ b/packages/backend/src/routes/issues/status-count.ts
@@ -1,28 +1,18 @@
+import { IssuesStatusCountQuerySchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getIssueStatusCountByOrganisation, getOrganisationMemberRole } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /issues/status-count?organisationId=1&status=TODO
 export default async function issuesStatusCount(req: AuthedRequest) {
     const url = new URL(req.url);
-    const organisationIdParam = url.searchParams.get("organisationId");
-    const status = url.searchParams.get("status");
+    const parsed = parseQueryParams(url, IssuesStatusCountQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationIdParam) {
-        return new Response("missing organisationId", { status: 400 });
-    }
-
-    if (!status) {
-        return new Response("missing status", { status: 400 });
-    }
-
-    const organisationId = Number(organisationIdParam);
-    if (!Number.isInteger(organisationId)) {
-        return new Response("organisationId must be an integer", { status: 400 });
-    }
+    const { organisationId, status } = parsed.data;
 
     const membership = await getOrganisationMemberRole(organisationId, req.userId);
     if (!membership) {
-        return new Response("not a member of this organisation", { status: 403 });
+        return errorResponse("not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     const result = await getIssueStatusCountByOrganisation(organisationId, status);
diff --git a/packages/backend/src/routes/organisation/add-member.ts b/packages/backend/src/routes/organisation/add-member.ts
index 516e944..fac4790 100644
--- a/packages/backend/src/routes/organisation/add-member.ts
+++ b/packages/backend/src/routes/organisation/add-member.ts
@@ -1,3 +1,4 @@
+import { OrgAddMemberRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import {
     createOrganisationMember,
@@ -5,53 +6,39 @@ import {
     getOrganisationMemberRole,
     getUserById,
 } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/add-member?organisationId=1&userId=2&role=member
 export default async function organisationAddMember(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const organisationId = url.searchParams.get("organisationId");
-    const userId = url.searchParams.get("userId");
-    const role = url.searchParams.get("role") || "member";
+    const parsed = await parseJsonBody(req, OrgAddMemberRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationId || !userId) {
-        return new Response(
-            `missing parameters: ${!organisationId ? "organisationId " : ""}${!userId ? "userId" : ""}`,
-            { status: 400 },
-        );
-    }
+    const { organisationId, userId, role } = parsed.data;
 
-    const orgIdNumber = Number(organisationId);
-    const userIdNumber = Number(userId);
-
-    if (!Number.isInteger(orgIdNumber) || !Number.isInteger(userIdNumber)) {
-        return new Response("organisationId and userId must be integers", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(orgIdNumber);
+    const organisation = await getOrganisationById(organisationId);
     if (!organisation) {
-        return new Response(`organisation with id ${organisationId} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${organisationId} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    const user = await getUserById(userIdNumber);
+    const user = await getUserById(userId);
     if (!user) {
-        return new Response(`user with id ${userId} not found`, { status: 404 });
+        return errorResponse(`user with id ${userId} not found`, "USER_NOT_FOUND", 404);
     }
 
-    const existingMember = await getOrganisationMemberRole(orgIdNumber, userIdNumber);
+    const existingMember = await getOrganisationMemberRole(organisationId, userId);
     if (existingMember) {
-        return new Response("User is already a member of this organisation", { status: 409 });
+        return errorResponse("user is already a member of this organisation", "ALREADY_MEMBER", 409);
     }
 
-    const requesterMember = await getOrganisationMemberRole(orgIdNumber, req.userId);
+    const requesterMember = await getOrganisationMemberRole(organisationId, req.userId);
     if (!requesterMember) {
-        return new Response("You are not a member of this organisation", { status: 403 });
+        return errorResponse("you are not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     if (requesterMember.role !== "owner" && requesterMember.role !== "admin") {
-        return new Response("Only owners and admins can add members", { status: 403 });
+        return errorResponse("only owners and admins can add members", "PERMISSION_DENIED", 403);
     }
 
-    const member = await createOrganisationMember(orgIdNumber, userIdNumber, role);
+    const member = await createOrganisationMember(organisationId, userId, role);
 
     return Response.json(member);
 }
diff --git a/packages/backend/src/routes/organisation/by-id.ts b/packages/backend/src/routes/organisation/by-id.ts
index 02756c7..95a9d10 100644
--- a/packages/backend/src/routes/organisation/by-id.ts
+++ b/packages/backend/src/routes/organisation/by-id.ts
@@ -1,23 +1,18 @@
+import { OrgByIdQuerySchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getOrganisationById } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /organisation/by-id?id=1
 export default async function organisationById(req: BunRequest) {
     const url = new URL(req.url);
-    const id = url.searchParams.get("id");
+    const parsed = parseQueryParams(url, OrgByIdQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!id) {
-        return new Response("organisation id is required", { status: 400 });
-    }
+    const { id } = parsed.data;
 
-    const organisationId = Number(id);
-    if (!Number.isInteger(organisationId)) {
-        return new Response("organisation id must be an integer", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(organisationId);
+    const organisation = await getOrganisationById(id);
     if (!organisation) {
-        return new Response(`organisation with id ${id} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${id} not found`, "ORG_NOT_FOUND", 404);
     }
 
     return Response.json(organisation);
diff --git a/packages/backend/src/routes/organisation/by-user.ts b/packages/backend/src/routes/organisation/by-user.ts
index 04e0194..5f47abe 100644
--- a/packages/backend/src/routes/organisation/by-user.ts
+++ b/packages/backend/src/routes/organisation/by-user.ts
@@ -1,31 +1,8 @@
 import type { AuthedRequest } from "../../auth/middleware";
-import { getOrganisationsByUserId, getUserById } from "../../db/queries";
+import { getOrganisationsByUserId } from "../../db/queries";
 
-// /organisations/by-user?userId=1
 export default async function organisationsByUser(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const userId = url.searchParams.get("userId");
-
-    if (!userId) {
-        return new Response("userId is required", { status: 400 });
-    }
-
-    const userIdNumber = Number(userId);
-    if (!Number.isInteger(userIdNumber)) {
-        return new Response("userId must be an integer", { status: 400 });
-    }
-
-    // Users can only view their own organisations
-    if (req.userId !== userIdNumber) {
-        return new Response("Access denied: you can only view your own organisations", { status: 403 });
-    }
-
-    const user = await getUserById(userIdNumber);
-    if (!user) {
-        return new Response(`user with id ${userId} not found`, { status: 404 });
-    }
-
-    const organisations = await getOrganisationsByUserId(user.id);
+    const organisations = await getOrganisationsByUserId(req.userId);
 
     return Response.json(organisations);
 }
diff --git a/packages/backend/src/routes/organisation/create.ts b/packages/backend/src/routes/organisation/create.ts
index 048424d..d05d5d5 100644
--- a/packages/backend/src/routes/organisation/create.ts
+++ b/packages/backend/src/routes/organisation/create.ts
@@ -1,37 +1,20 @@
+import { OrgCreateRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { createOrganisationWithOwner, getOrganisationBySlug } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/create?name=Org%20Name&slug=org-name&userId=1&description=Optional%20description
 export default async function organisationCreate(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const name = url.searchParams.get("name");
-    const slug = url.searchParams.get("slug");
-    const userId = url.searchParams.get("userId");
-    const description = url.searchParams.get("description") || undefined;
+    const parsed = await parseJsonBody(req, OrgCreateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!name || !slug || !userId) {
-        return new Response(
-            `missing parameters: ${!name ? "name " : ""}${!slug ? "slug " : ""}${!userId ? "userId" : ""}`,
-            { status: 400 },
-        );
-    }
-
-    const userIdNumber = Number(userId);
-    if (!Number.isInteger(userIdNumber)) {
-        return new Response("userId must be an integer", { status: 400 });
-    }
-
-    // users can only create organisations for themselves (userId cannot be spoofed)
-    if (req.userId !== userIdNumber) {
-        return new Response("access denied: you can only create organisations for yourself", { status: 403 });
-    }
+    const { name, slug, description } = parsed.data;
 
     const existingOrganisation = await getOrganisationBySlug(slug);
     if (existingOrganisation) {
-        return new Response(`organisation with slug "${slug}" already exists`, { status: 409 });
+        return errorResponse(`organisation with slug "${slug}" already exists`, "SLUG_TAKEN", 409);
     }
 
-    const organisation = await createOrganisationWithOwner(name, slug, userIdNumber, description);
+    const organisation = await createOrganisationWithOwner(name, slug, req.userId, description);
 
     return Response.json(organisation);
 }
diff --git a/packages/backend/src/routes/organisation/delete.ts b/packages/backend/src/routes/organisation/delete.ts
index 5737916..82c718b 100644
--- a/packages/backend/src/routes/organisation/delete.ts
+++ b/packages/backend/src/routes/organisation/delete.ts
@@ -1,26 +1,20 @@
+import { OrgDeleteRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { deleteOrganisation, getOrganisationById } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/delete?id=1
 export default async function organisationDelete(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
+    const parsed = await parseJsonBody(req, OrgDeleteRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!id) {
-        return new Response("organisation id is required", { status: 400 });
-    }
+    const { id } = parsed.data;
 
-    const organisationId = Number(id);
-    if (!Number.isInteger(organisationId)) {
-        return new Response("organisation id must be an integer", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(organisationId);
+    const organisation = await getOrganisationById(id);
     if (!organisation) {
-        return new Response(`organisation with id ${id} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${id} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    await deleteOrganisation(organisationId);
+    await deleteOrganisation(id);
 
     return Response.json({ success: true });
 }
diff --git a/packages/backend/src/routes/organisation/members.ts b/packages/backend/src/routes/organisation/members.ts
index d13f7ba..3a1d511 100644
--- a/packages/backend/src/routes/organisation/members.ts
+++ b/packages/backend/src/routes/organisation/members.ts
@@ -1,26 +1,21 @@
+import { OrgMembersQuerySchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getOrganisationById, getOrganisationMembers } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /organisation/members?organisationId=1
 export default async function organisationMembers(req: BunRequest) {
     const url = new URL(req.url);
-    const organisationId = url.searchParams.get("organisationId");
+    const parsed = parseQueryParams(url, OrgMembersQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationId) {
-        return new Response("organisationId is required", { status: 400 });
-    }
+    const { organisationId } = parsed.data;
 
-    const orgIdNumber = Number(organisationId);
-    if (!Number.isInteger(orgIdNumber)) {
-        return new Response("organisationId must be an integer", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(orgIdNumber);
+    const organisation = await getOrganisationById(organisationId);
     if (!organisation) {
-        return new Response(`organisation with id ${organisationId} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${organisationId} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    const members = await getOrganisationMembers(orgIdNumber);
+    const members = await getOrganisationMembers(organisationId);
 
     return Response.json(members);
 }
diff --git a/packages/backend/src/routes/organisation/remove-member.ts b/packages/backend/src/routes/organisation/remove-member.ts
index 9122161..f6cd1dd 100644
--- a/packages/backend/src/routes/organisation/remove-member.ts
+++ b/packages/backend/src/routes/organisation/remove-member.ts
@@ -1,50 +1,38 @@
+import { OrgRemoveMemberRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getOrganisationById, getOrganisationMemberRole, removeOrganisationMember } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/remove-member?organisationId=1&userId=2
 export default async function organisationRemoveMember(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const organisationId = url.searchParams.get("organisationId");
-    const userId = url.searchParams.get("userId");
+    const parsed = await parseJsonBody(req, OrgRemoveMemberRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationId || !userId) {
-        return new Response(
-            `missing parameters: ${!organisationId ? "organisationId " : ""}${!userId ? "userId" : ""}`,
-            { status: 400 },
-        );
-    }
+    const { organisationId, userId } = parsed.data;
 
-    const orgIdNumber = Number(organisationId);
-    const userIdNumber = Number(userId);
-
-    if (!Number.isInteger(orgIdNumber) || !Number.isInteger(userIdNumber)) {
-        return new Response("organisationId and userId must be integers", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(orgIdNumber);
+    const organisation = await getOrganisationById(organisationId);
     if (!organisation) {
-        return new Response(`organisation with id ${organisationId} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${organisationId} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    const memberToRemove = await getOrganisationMemberRole(orgIdNumber, userIdNumber);
+    const memberToRemove = await getOrganisationMemberRole(organisationId, userId);
     if (!memberToRemove) {
-        return new Response("User is not a member of this organisation", { status: 404 });
+        return errorResponse("user is not a member of this organisation", "NOT_MEMBER", 404);
     }
 
     if (memberToRemove.role === "owner") {
-        return new Response("Cannot remove the organisation owner", { status: 403 });
+        return errorResponse("cannot remove the organisation owner", "CANNOT_REMOVE_OWNER", 403);
     }
 
-    const requesterMember = await getOrganisationMemberRole(orgIdNumber, req.userId);
+    const requesterMember = await getOrganisationMemberRole(organisationId, req.userId);
     if (!requesterMember) {
-        return new Response("You are not a member of this organisation", { status: 403 });
+        return errorResponse("you are not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     if (requesterMember.role !== "owner" && requesterMember.role !== "admin") {
-        return new Response("Only owners and admins can remove members", { status: 403 });
+        return errorResponse("only owners and admins can remove members", "PERMISSION_DENIED", 403);
     }
 
-    await removeOrganisationMember(orgIdNumber, userIdNumber);
+    await removeOrganisationMember(organisationId, userId);
 
     return Response.json({ success: true });
 }
diff --git a/packages/backend/src/routes/organisation/update-member-role.ts b/packages/backend/src/routes/organisation/update-member-role.ts
index f629ad3..c0acae9 100644
--- a/packages/backend/src/routes/organisation/update-member-role.ts
+++ b/packages/backend/src/routes/organisation/update-member-role.ts
@@ -1,3 +1,4 @@
+import { OrgUpdateMemberRoleRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import {
     getOrganisationById,
@@ -5,56 +6,43 @@ import {
     getUserById,
     updateOrganisationMemberRole,
 } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/update-member-role?organisationId=1&userId=2&role=admin
 export default async function organisationUpdateMemberRole(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const organisationId = url.searchParams.get("organisationId");
-    const userId = url.searchParams.get("userId");
-    const role = url.searchParams.get("role");
-    if (!role || !["admin", "member"].includes(role)) {
-        return new Response("Invalid role: must be either 'admin' or 'member'", { status: 400 });
+    const parsed = await parseJsonBody(req, OrgUpdateMemberRoleRequestSchema);
+    if ("error" in parsed) return parsed.error;
+
+    const { organisationId, userId, role } = parsed.data;
+
+    const organisation = await getOrganisationById(organisationId);
+    if (!organisation) {
+        return errorResponse(`organisation with id ${organisationId} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    if (!organisationId || !userId || !role) {
-        return new Response(
-            `missing parameters: ${!organisationId ? "organisationId " : ""}${!userId ? "userId " : ""}${!role ? "role" : ""}`,
-            { status: 400 },
+    const user = await getUserById(userId);
+    if (!user) {
+        return errorResponse(`user with id ${userId} not found`, "USER_NOT_FOUND", 404);
+    }
+
+    const requesterMember = await getOrganisationMemberRole(organisationId, req.userId);
+    if (!requesterMember) {
+        return errorResponse("you are not a member of this organisation", "NOT_MEMBER", 403);
+    }
+
+    let member = await getOrganisationMemberRole(organisationId, userId);
+    if (!member) {
+        return errorResponse(
+            `user with id ${userId} is not a member of this organisation`,
+            "NOT_MEMBER",
+            404,
         );
     }
 
-    const orgIdNumber = Number(organisationId);
-    const userIdNumber = Number(userId);
-
-    if (!Number.isInteger(orgIdNumber) || !Number.isInteger(userIdNumber)) {
-        return new Response("organisationId and userId must be integers", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(orgIdNumber);
-    if (!organisation) {
-        return new Response(`organisation with id ${organisationId} not found`, { status: 404 });
-    }
-
-    const user = await getUserById(userIdNumber);
-    if (!user) {
-        return new Response(`user with id ${userId} not found`, { status: 404 });
-    }
-
-    const requesterMember = await getOrganisationMemberRole(orgIdNumber, req.userId);
-    if (!requesterMember) {
-        return new Response("You are not a member of this organisation", { status: 403 });
-    }
-
-    let member = await getOrganisationMemberRole(orgIdNumber, userIdNumber);
-    if (!member) {
-        return new Response(`User with id ${userId} is not a member of this organisation`, { status: 404 });
-    }
-
     if (requesterMember.role !== "owner" && requesterMember.role !== "admin") {
-        return new Response("Only owners and admins can update member roles", { status: 403 });
+        return errorResponse("only owners and admins can update member roles", "PERMISSION_DENIED", 403);
     }
 
-    member = await updateOrganisationMemberRole(orgIdNumber, userIdNumber, role);
+    member = await updateOrganisationMemberRole(organisationId, userId, role);
 
     return Response.json(member);
 }
diff --git a/packages/backend/src/routes/organisation/update.ts b/packages/backend/src/routes/organisation/update.ts
index 502b64d..55b8355 100644
--- a/packages/backend/src/routes/organisation/update.ts
+++ b/packages/backend/src/routes/organisation/update.ts
@@ -1,62 +1,28 @@
-import { ISSUE_STATUS_MAX_LENGTH } from "@issue/shared";
+import { OrgUpdateRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getOrganisationById, updateOrganisation } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /organisation/update?id=1&name=New%20Name&description=New%20Description&slug=new-slug
 export default async function organisationUpdate(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
-    const name = url.searchParams.get("name") || undefined;
-    const description = url.searchParams.get("description") || undefined;
-    const slug = url.searchParams.get("slug") || undefined;
-    const statusesParam = url.searchParams.get("statuses");
+    const parsed = await parseJsonBody(req, OrgUpdateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    let statuses: Record<string, string> | undefined;
-    if (statusesParam) {
-        try {
-            const parsed = JSON.parse(statusesParam);
-            if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
-                return new Response("statuses must be an object", { status: 400 });
-            }
-            const entries = Object.entries(parsed);
-            if (entries.length === 0) {
-                return new Response("statuses must have at least one status", { status: 400 });
-            }
-            if (!entries.every(([key, value]) => typeof key === "string" && typeof value === "string")) {
-                return new Response("statuses values must be strings", { status: 400 });
-            }
-            if (entries.some(([key]) => key.length > ISSUE_STATUS_MAX_LENGTH)) {
-                return new Response(`status must be <= ${ISSUE_STATUS_MAX_LENGTH} characters`, {
-                    status: 400,
-                });
-            }
-            statuses = parsed;
-        } catch {
-            return new Response("invalid statuses format (must be JSON object)", { status: 400 });
-        }
-    }
+    const { id, name, description, slug, statuses } = parsed.data;
 
-    if (!id) {
-        return new Response("organisation id is required", { status: 400 });
-    }
-
-    const organisationId = Number(id);
-    if (!Number.isInteger(organisationId)) {
-        return new Response("organisation id must be an integer", { status: 400 });
-    }
-
-    const existingOrganisation = await getOrganisationById(organisationId);
+    const existingOrganisation = await getOrganisationById(id);
     if (!existingOrganisation) {
-        return new Response(`organisation with id ${id} does not exist`, { status: 404 });
+        return errorResponse(`organisation with id ${id} does not exist`, "ORG_NOT_FOUND", 404);
     }
 
     if (!name && !description && !slug && !statuses) {
-        return new Response("at least one of name, description, slug, or statuses must be provided", {
-            status: 400,
-        });
+        return errorResponse(
+            "at least one of name, description, slug, or statuses must be provided",
+            "NO_UPDATES",
+            400,
+        );
     }
 
-    const organisation = await updateOrganisation(organisationId, {
+    const organisation = await updateOrganisation(id, {
         name,
         description,
         slug,
diff --git a/packages/backend/src/routes/project/by-creator.ts b/packages/backend/src/routes/project/by-creator.ts
index 36c8db8..8424f35 100644
--- a/packages/backend/src/routes/project/by-creator.ts
+++ b/packages/backend/src/routes/project/by-creator.ts
@@ -1,23 +1,18 @@
+import { ProjectByCreatorQuerySchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getProjectsByCreatorID, getUserById } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /projects/by-creator?creatorId=1
 export default async function projectsByCreator(req: BunRequest) {
     const url = new URL(req.url);
-    const creatorId = url.searchParams.get("creatorId");
+    const parsed = parseQueryParams(url, ProjectByCreatorQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!creatorId) {
-        return new Response("creatorId is required", { status: 400 });
-    }
+    const { creatorId } = parsed.data;
 
-    const creatorIdNumber = Number(creatorId);
-    if (!Number.isInteger(creatorIdNumber)) {
-        return new Response("creatorId must be an integer", { status: 400 });
-    }
-
-    const creator = await getUserById(creatorIdNumber);
+    const creator = await getUserById(creatorId);
     if (!creator) {
-        return new Response(`user with id ${creatorId} not found`, { status: 404 });
+        return errorResponse(`user with id ${creatorId} not found`, "USER_NOT_FOUND", 404);
     }
 
     const projects = await getProjectsByCreatorID(creator.id);
diff --git a/packages/backend/src/routes/project/by-organisation.ts b/packages/backend/src/routes/project/by-organisation.ts
index 2b55e05..9af0246 100644
--- a/packages/backend/src/routes/project/by-organisation.ts
+++ b/packages/backend/src/routes/project/by-organisation.ts
@@ -1,33 +1,27 @@
+import { ProjectByOrgQuerySchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getOrganisationById, getOrganisationsByUserId, getProjectsByOrganisationId } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /projects/by-organisation?organisationId=1
 export default async function projectsByOrganisation(req: AuthedRequest) {
     const url = new URL(req.url);
-    const organisationId = url.searchParams.get("organisationId");
+    const parsed = parseQueryParams(url, ProjectByOrgQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!organisationId) {
-        return new Response("organisationId is required", { status: 400 });
-    }
+    const { organisationId } = parsed.data;
 
-    const orgIdNumber = Number(organisationId);
-    if (!Number.isInteger(orgIdNumber)) {
-        return new Response("organisationId must be an integer", { status: 400 });
-    }
-
-    const organisation = await getOrganisationById(orgIdNumber);
+    const organisation = await getOrganisationById(organisationId);
     if (!organisation) {
-        return new Response(`organisation with id ${organisationId} not found`, { status: 404 });
+        return errorResponse(`organisation with id ${organisationId} not found`, "ORG_NOT_FOUND", 404);
     }
 
-    // Check if user has access to this organisation
     const userOrganisations = await getOrganisationsByUserId(req.userId);
-    const hasAccess = userOrganisations.some((item) => item.Organisation.id === orgIdNumber);
+    const hasAccess = userOrganisations.some((item) => item.Organisation.id === organisationId);
     if (!hasAccess) {
-        return new Response("Access denied: you are not a member of this organisation", { status: 403 });
+        return errorResponse("access denied: you are not a member of this organisation", "NOT_MEMBER", 403);
     }
 
-    const projects = await getProjectsByOrganisationId(orgIdNumber);
+    const projects = await getProjectsByOrganisationId(organisationId);
 
     return Response.json(projects);
 }
diff --git a/packages/backend/src/routes/project/create.ts b/packages/backend/src/routes/project/create.ts
index ca954c0..9a7a048 100644
--- a/packages/backend/src/routes/project/create.ts
+++ b/packages/backend/src/routes/project/create.ts
@@ -1,33 +1,25 @@
-import type { BunRequest } from "bun";
+import { ProjectCreateRequestSchema } from "@issue/shared";
+import type { AuthedRequest } from "../../auth/middleware";
 import { createProject, getProjectByKey, getUserById } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /project/create?key=KEY&name=Testing&creatorId=1&organisationId=1
-export default async function projectCreate(req: BunRequest) {
-    const url = new URL(req.url);
-    const key = url.searchParams.get("key");
-    const name = url.searchParams.get("name");
-    const creatorId = url.searchParams.get("creatorId");
-    const organisationId = url.searchParams.get("organisationId");
+export default async function projectCreate(req: AuthedRequest) {
+    const parsed = await parseJsonBody(req, ProjectCreateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!key || !name || !creatorId || !organisationId) {
-        return new Response(
-            `missing parameters: ${!key ? "key " : ""}${!name ? "name " : ""}${!creatorId ? "creatorId " : ""}${!organisationId ? "organisationId" : ""}`,
-            { status: 400 },
-        );
-    }
+    const { key, name, organisationId } = parsed.data;
 
-    // check if project with key already exists in the organisation
     const existingProject = await getProjectByKey(key);
-    if (existingProject?.organisationId === parseInt(organisationId, 10)) {
-        return new Response(`project with key ${key} already exists`, { status: 400 });
+    if (existingProject?.organisationId === organisationId) {
+        return errorResponse(`project with key ${key} already exists in this organisation`, "KEY_TAKEN", 400);
     }
 
-    const creator = await getUserById(parseInt(creatorId, 10));
+    const creator = await getUserById(req.userId);
     if (!creator) {
-        return new Response(`creator with id ${creatorId} not found`, { status: 404 });
+        return errorResponse(`creator not found`, "CREATOR_NOT_FOUND", 404);
     }
 
-    const project = await createProject(key, name, creator.id, parseInt(organisationId, 10));
+    const project = await createProject(key, name, creator.id, organisationId);
 
     return Response.json(project);
 }
diff --git a/packages/backend/src/routes/project/delete.ts b/packages/backend/src/routes/project/delete.ts
index 18ab66f..e20c7a4 100644
--- a/packages/backend/src/routes/project/delete.ts
+++ b/packages/backend/src/routes/project/delete.ts
@@ -1,21 +1,20 @@
+import { ProjectDeleteRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { deleteProject, getProjectByID } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /project/delete?id=1
 export default async function projectDelete(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
+    const parsed = await parseJsonBody(req, ProjectDeleteRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!id) {
-        return new Response(`project id is required`, { status: 400 });
-    }
+    const { id } = parsed.data;
 
-    const existingProject = await getProjectByID(Number(id));
+    const existingProject = await getProjectByID(id);
     if (!existingProject) {
-        return new Response(`project with id ${id} does not exist`, { status: 404 });
+        return errorResponse(`project with id ${id} does not exist`, "PROJECT_NOT_FOUND", 404);
     }
 
-    await deleteProject(Number(id));
+    await deleteProject(id);
 
-    return new Response(`project with id ${id} deleted successfully`, { status: 200 });
+    return Response.json({ success: true });
 }
diff --git a/packages/backend/src/routes/project/update.ts b/packages/backend/src/routes/project/update.ts
index 23ffd73..2782ad1 100644
--- a/packages/backend/src/routes/project/update.ts
+++ b/packages/backend/src/routes/project/update.ts
@@ -1,45 +1,46 @@
+import { ProjectUpdateRequestSchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getProjectByID, getProjectByKey, getUserById, updateProject } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /project/update?id=1&key=NEW&name=new%20name&creatorId=1&organisationId=1
 export default async function projectUpdate(req: BunRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
-    const key = url.searchParams.get("key") || undefined;
-    const name = url.searchParams.get("name") || undefined;
-    const creatorId = url.searchParams.get("creatorId") || undefined;
-    const organisationId = url.searchParams.get("organisationId") || undefined;
+    const parsed = await parseJsonBody(req, ProjectUpdateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!id) {
-        return new Response(`project id is required`, { status: 400 });
-    }
+    const { id, key, name, creatorId, organisationId } = parsed.data;
 
-    const existingProject = await getProjectByID(Number(id));
+    const existingProject = await getProjectByID(id);
     if (!existingProject) {
-        return new Response(`project with id ${id} does not exist`, { status: 404 });
+        return errorResponse(`project with id ${id} does not exist`, "PROJECT_NOT_FOUND", 404);
     }
 
     if (!key && !name && !creatorId && !organisationId) {
-        return new Response(`at least one of key, name, creatorId, or organisationId must be provided`, {
-            status: 400,
-        });
+        return errorResponse(
+            "at least one of key, name, creatorId, or organisationId must be provided",
+            "NO_UPDATES",
+            400,
+        );
     }
 
-    const projectWithKey = key ? await getProjectByKey(key) : null;
-    if (projectWithKey && projectWithKey.id !== Number(id)) {
-        return new Response(`a project with key "${key}" already exists`, { status: 400 });
+    if (key) {
+        const projectWithKey = await getProjectByKey(key);
+        if (projectWithKey && projectWithKey.id !== id) {
+            return errorResponse(`a project with key "${key}" already exists`, "KEY_TAKEN", 400);
+        }
     }
 
-    const newCreator = creatorId ? await getUserById(Number(creatorId)) : null;
-    if (creatorId && !newCreator) {
-        return new Response(`user with id ${creatorId} does not exist`, { status: 400 });
+    if (creatorId) {
+        const newCreator = await getUserById(creatorId);
+        if (!newCreator) {
+            return errorResponse(`user with id ${creatorId} does not exist`, "USER_NOT_FOUND", 400);
+        }
     }
 
-    const project = await updateProject(Number(id), {
+    const project = await updateProject(id, {
         key,
         name,
-        creatorId: newCreator?.id,
-        organisationId: organisationId ? Number(organisationId) : undefined,
+        creatorId,
+        organisationId,
     });
 
     return Response.json(project);
diff --git a/packages/backend/src/routes/project/with-creator.ts b/packages/backend/src/routes/project/with-creator.ts
index 032f108..cb2ee13 100644
--- a/packages/backend/src/routes/project/with-creator.ts
+++ b/packages/backend/src/routes/project/with-creator.ts
@@ -1,23 +1,18 @@
+import { ProjectByIdQuerySchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getProjectWithCreatorByID } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /project/with-creator?id=1
 export default async function projectWithCreatorByID(req: BunRequest) {
     const url = new URL(req.url);
-    const id = url.searchParams.get("id");
+    const parsed = parseQueryParams(url, ProjectByIdQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!id) {
-        return new Response("project id is required", { status: 400 });
-    }
+    const { id } = parsed.data;
 
-    const projectId = Number(id);
-    if (!Number.isInteger(projectId)) {
-        return new Response("project id must be an integer", { status: 400 });
-    }
-
-    const projectWithCreator = await getProjectWithCreatorByID(projectId);
+    const projectWithCreator = await getProjectWithCreatorByID(id);
     if (!projectWithCreator || !projectWithCreator.Project) {
-        return new Response(`project with id ${id} not found`, { status: 404 });
+        return errorResponse(`project with id ${id} not found`, "PROJECT_NOT_FOUND", 404);
     }
 
     return Response.json(projectWithCreator);
diff --git a/packages/backend/src/routes/sprint/create.ts b/packages/backend/src/routes/sprint/create.ts
index 566c160..07076d3 100644
--- a/packages/backend/src/routes/sprint/create.ts
+++ b/packages/backend/src/routes/sprint/create.ts
@@ -1,63 +1,29 @@
+import { SprintCreateRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { createSprint, getOrganisationMemberRole, getProjectByID } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /sprint/create?projectId=1&name=Sprint%201&startDate=2025-01-01T00:00:00.000Z&endDate=2025-01-14T23:59:00.000Z
 export default async function sprintCreate(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const projectId = url.searchParams.get("projectId");
-    const name = url.searchParams.get("name");
-    const color = url.searchParams.get("color") || undefined;
-    const startDateParam = url.searchParams.get("startDate");
-    const endDateParam = url.searchParams.get("endDate");
+    const parsed = await parseJsonBody(req, SprintCreateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!projectId || !name || !startDateParam || !endDateParam) {
-        return new Response(
-            `missing parameters: ${!projectId ? "projectId " : ""}${!name ? "name " : ""}${
-                !startDateParam ? "startDate " : ""
-            }${!endDateParam ? "endDate" : ""}`,
-            { status: 400 },
-        );
-    }
+    const { projectId, name, color, startDate, endDate } = parsed.data;
 
-    const projectIdNumber = Number(projectId);
-    if (!Number.isInteger(projectIdNumber)) {
-        return new Response("projectId must be an integer", { status: 400 });
-    }
-
-    const project = await getProjectByID(projectIdNumber);
+    const project = await getProjectByID(projectId);
     if (!project) {
-        return new Response(`project not found: provided ${projectId}`, { status: 404 });
+        return errorResponse(`project not found: ${projectId}`, "PROJECT_NOT_FOUND", 404);
     }
 
     const membership = await getOrganisationMemberRole(project.organisationId, req.userId);
     if (!membership) {
-        return new Response("not a member of this organisation", { status: 403 });
+        return errorResponse("not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     if (membership.role !== "owner" && membership.role !== "admin") {
-        return new Response("only owners and admins can create sprints", { status: 403 });
+        return errorResponse("only owners and admins can create sprints", "PERMISSION_DENIED", 403);
     }
 
-    const trimmedName = name.trim();
-    if (trimmedName === "") {
-        return new Response("name cannot be empty", { status: 400 });
-    }
-
-    const startDate = new Date(startDateParam);
-    if (Number.isNaN(startDate.valueOf())) {
-        return new Response("startDate must be a valid date", { status: 400 });
-    }
-
-    const endDate = new Date(endDateParam);
-    if (Number.isNaN(endDate.valueOf())) {
-        return new Response("endDate must be a valid date", { status: 400 });
-    }
-
-    if (startDate > endDate) {
-        return new Response("endDate must be after startDate", { status: 400 });
-    }
-
-    const sprint = await createSprint(project.id, trimmedName, color, startDate, endDate);
+    const sprint = await createSprint(project.id, name, color, new Date(startDate), new Date(endDate));
 
     return Response.json(sprint);
 }
diff --git a/packages/backend/src/routes/sprints/by-project.ts b/packages/backend/src/routes/sprints/by-project.ts
index d929d48..b4e0e3e 100644
--- a/packages/backend/src/routes/sprints/by-project.ts
+++ b/packages/backend/src/routes/sprints/by-project.ts
@@ -1,28 +1,23 @@
+import { SprintsByProjectQuerySchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getOrganisationMemberRole, getProjectByID, getSprintsByProject } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /sprints/by-project?projectId=1
 export default async function sprintsByProject(req: AuthedRequest) {
     const url = new URL(req.url);
-    const projectId = url.searchParams.get("projectId");
+    const parsed = parseQueryParams(url, SprintsByProjectQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!projectId) {
-        return new Response("missing projectId", { status: 400 });
-    }
+    const { projectId } = parsed.data;
 
-    const projectIdNumber = Number(projectId);
-    if (!Number.isInteger(projectIdNumber)) {
-        return new Response("projectId must be an integer", { status: 400 });
-    }
-
-    const project = await getProjectByID(projectIdNumber);
+    const project = await getProjectByID(projectId);
     if (!project) {
-        return new Response(`project not found: provided ${projectId}`, { status: 404 });
+        return errorResponse(`project not found: ${projectId}`, "PROJECT_NOT_FOUND", 404);
     }
 
     const membership = await getOrganisationMemberRole(project.organisationId, req.userId);
     if (!membership) {
-        return new Response("not a member of this organisation", { status: 403 });
+        return errorResponse("not a member of this organisation", "NOT_MEMBER", 403);
     }
 
     const sprints = await getSprintsByProject(project.id);
diff --git a/packages/backend/src/routes/timer/end.ts b/packages/backend/src/routes/timer/end.ts
index aad6c82..663778d 100644
--- a/packages/backend/src/routes/timer/end.ts
+++ b/packages/backend/src/routes/timer/end.ts
@@ -1,21 +1,20 @@
-import { calculateBreakTimeMs, calculateWorkTimeMs } from "@issue/shared";
+import { calculateBreakTimeMs, calculateWorkTimeMs, TimerEndRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { endTimedSession, getActiveTimedSession } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// POST /timer/end
 export default async function timerEnd(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const issueId = url.searchParams.get("issueId");
-    if (!issueId || Number.isNaN(Number(issueId))) {
-        return new Response("missing issue id", { status: 400 });
-    }
-    const activeSession = await getActiveTimedSession(req.userId, Number(issueId));
+    const parsed = await parseJsonBody(req, TimerEndRequestSchema);
+    if ("error" in parsed) return parsed.error;
+
+    const { issueId } = parsed.data;
+
+    const activeSession = await getActiveTimedSession(req.userId, issueId);
 
     if (!activeSession) {
-        return new Response("no active timer", { status: 400 });
+        return errorResponse("no active timer", "NO_ACTIVE_TIMER", 400);
     }
 
-    // already ended - return existing without modification
     if (activeSession.endedAt) {
         return Response.json({
             ...activeSession,
@@ -27,7 +26,7 @@ export default async function timerEnd(req: AuthedRequest) {
 
     const ended = await endTimedSession(activeSession.id, activeSession.timestamps);
     if (!ended) {
-        return new Response("failed to end timer", { status: 500 });
+        return errorResponse("failed to end timer", "END_FAILED", 500);
     }
 
     return Response.json({
diff --git a/packages/backend/src/routes/timer/get-inactive.ts b/packages/backend/src/routes/timer/get-inactive.ts
index 9b88b59..aaec18e 100644
--- a/packages/backend/src/routes/timer/get-inactive.ts
+++ b/packages/backend/src/routes/timer/get-inactive.ts
@@ -1,17 +1,18 @@
-import { calculateBreakTimeMs, calculateWorkTimeMs } from "@issue/shared";
+import { calculateBreakTimeMs, calculateWorkTimeMs, TimerGetQuerySchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getInactiveTimedSessions } from "../../db/queries";
+import { parseQueryParams } from "../../validation";
 
-// GET /timer?issueId=123
 export default async function timerGetInactive(req: AuthedRequest) {
     const url = new URL(req.url);
-    const issueId = url.searchParams.get("issueId");
-    if (!issueId || Number.isNaN(Number(issueId))) {
-        return new Response("missing issue id", { status: 400 });
-    }
-    const sessions = await getInactiveTimedSessions(Number(issueId));
+    const parsed = parseQueryParams(url, TimerGetQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!sessions[0] || !sessions) {
+    const { issueId } = parsed.data;
+
+    const sessions = await getInactiveTimedSessions(issueId);
+
+    if (!sessions || sessions.length === 0) {
         return Response.json(null);
     }
 
diff --git a/packages/backend/src/routes/timer/get.ts b/packages/backend/src/routes/timer/get.ts
index 723f786..3e02783 100644
--- a/packages/backend/src/routes/timer/get.ts
+++ b/packages/backend/src/routes/timer/get.ts
@@ -1,15 +1,21 @@
-import { calculateBreakTimeMs, calculateWorkTimeMs, isTimerRunning } from "@issue/shared";
+import {
+    calculateBreakTimeMs,
+    calculateWorkTimeMs,
+    isTimerRunning,
+    TimerGetQuerySchema,
+} from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { getActiveTimedSession } from "../../db/queries";
+import { parseQueryParams } from "../../validation";
 
-// GET /timer?issueId=123
 export default async function timerGet(req: AuthedRequest) {
     const url = new URL(req.url);
-    const issueId = url.searchParams.get("issueId");
-    if (!issueId || Number.isNaN(Number(issueId))) {
-        return new Response("missing issue id", { status: 400 });
-    }
-    const activeSession = await getActiveTimedSession(req.userId, Number(issueId));
+    const parsed = parseQueryParams(url, TimerGetQuerySchema);
+    if ("error" in parsed) return parsed.error;
+
+    const { issueId } = parsed.data;
+
+    const activeSession = await getActiveTimedSession(req.userId, issueId);
 
     if (!activeSession) {
         return Response.json(null);
diff --git a/packages/backend/src/routes/timer/toggle.ts b/packages/backend/src/routes/timer/toggle.ts
index 9f0070f..6405a2c 100644
--- a/packages/backend/src/routes/timer/toggle.ts
+++ b/packages/backend/src/routes/timer/toggle.ts
@@ -1,20 +1,23 @@
-import { calculateBreakTimeMs, calculateWorkTimeMs, isTimerRunning } from "@issue/shared";
+import {
+    calculateBreakTimeMs,
+    calculateWorkTimeMs,
+    isTimerRunning,
+    TimerToggleRequestSchema,
+} from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { appendTimestamp, createTimedSession, getActiveTimedSession } from "../../db/queries";
+import { parseJsonBody } from "../../validation";
 
-// POST /timer/toggle?issueId=123
 export default async function timerToggle(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const issueId = url.searchParams.get("issueId");
-    if (!issueId || Number.isNaN(Number(issueId))) {
-        return new Response("missing issue id", { status: 400 });
-    }
+    const parsed = await parseJsonBody(req, TimerToggleRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    const activeSession = await getActiveTimedSession(req.userId, Number(issueId));
+    const { issueId } = parsed.data;
+
+    const activeSession = await getActiveTimedSession(req.userId, issueId);
 
     if (!activeSession) {
-        // no active session, create new one with first timestamp
-        const newSession = await createTimedSession(req.userId, Number(issueId));
+        const newSession = await createTimedSession(req.userId, issueId);
         return Response.json({
             ...newSession,
             workTimeMs: 0,
@@ -23,10 +26,9 @@ export default async function timerToggle(req: AuthedRequest) {
         });
     }
 
-    // active session exists, append timestamp (toggle)
     const updated = await appendTimestamp(activeSession.id, activeSession.timestamps);
     if (!updated) {
-        return new Response("failed to update timer", { status: 500 });
+        return Response.json({ error: "failed to update timer", code: "UPDATE_FAILED" }, { status: 500 });
     }
 
     const running = isTimerRunning(updated.timestamps);
diff --git a/packages/backend/src/routes/user/by-username.ts b/packages/backend/src/routes/user/by-username.ts
index fc703ce..d0c3c99 100644
--- a/packages/backend/src/routes/user/by-username.ts
+++ b/packages/backend/src/routes/user/by-username.ts
@@ -1,18 +1,18 @@
+import { UserByUsernameQuerySchema } from "@issue/shared";
 import type { BunRequest } from "bun";
 import { getUserByUsername } from "../../db/queries";
+import { errorResponse, parseQueryParams } from "../../validation";
 
-// /user/by-username?username=someusername
 export default async function userByUsername(req: BunRequest) {
     const url = new URL(req.url);
-    const username = url.searchParams.get("username");
+    const parsed = parseQueryParams(url, UserByUsernameQuerySchema);
+    if ("error" in parsed) return parsed.error;
 
-    if (!username) {
-        return new Response("username is required", { status: 400 });
-    }
+    const { username } = parsed.data;
 
     const user = await getUserByUsername(username);
     if (!user) {
-        return new Response(`User with username '${username}' not found`, { status: 404 });
+        return errorResponse(`user with username '${username}' not found`, "USER_NOT_FOUND", 404);
     }
 
     return Response.json(user);
diff --git a/packages/backend/src/routes/user/update.ts b/packages/backend/src/routes/user/update.ts
index 94127ca..4010ce4 100644
--- a/packages/backend/src/routes/user/update.ts
+++ b/packages/backend/src/routes/user/update.ts
@@ -1,25 +1,28 @@
-import type { UserRecord } from "@issue/shared";
+import { UserUpdateRequestSchema } from "@issue/shared";
 import type { AuthedRequest } from "../../auth/middleware";
 import { hashPassword } from "../../auth/utils";
 import { getUserById } from "../../db/queries";
+import { errorResponse, parseJsonBody } from "../../validation";
 
-// /user/update?id=1&name=NewName&password=NewPassword&avatarURL=...
 export default async function update(req: AuthedRequest) {
-    const url = new URL(req.url);
-    const id = url.searchParams.get("id");
-    if (!id) {
-        return new Response("id is required", { status: 400 });
-    }
+    const parsed = await parseJsonBody(req, UserUpdateRequestSchema);
+    if ("error" in parsed) return parsed.error;
 
-    const user = await getUserById(Number(id));
+    const { name, password, avatarURL } = parsed.data;
+
+    const user = await getUserById(req.userId);
     if (!user) {
-        return new Response("user not found", { status: 404 });
+        return errorResponse("user not found", "USER_NOT_FOUND", 404);
+    }
+
+    if (!name && !password && avatarURL === undefined) {
+        return errorResponse(
+            "at least one of name, password, or avatarURL must be provided",
+            "NO_UPDATES",
+            400,
+        );
     }
 
-    const name = url.searchParams.get("name") || undefined;
-    const password = url.searchParams.get("password") || undefined;
-    const avatarURL =
-        url.searchParams.get("avatarURL") === "null" ? null : url.searchParams.get("avatarURL") || undefined;
     let passwordHash: string | undefined;
     if (password !== undefined) {
         passwordHash = await hashPassword(password);
@@ -29,8 +32,8 @@ export default async function update(req: AuthedRequest) {
     const updatedUser = await updateById(user.id, { name, passwordHash, avatarURL });
 
     if (!updatedUser) {
-        return new Response("failed to update user", { status: 500 });
+        return errorResponse("failed to update user", "UPDATE_FAILED", 500);
     }
 
-    return Response.json(updatedUser as UserRecord);
+    return Response.json(updatedUser);
 }