diff --git a/src/lib/init/tools/apply-patchset.ts b/src/lib/init/tools/apply-patchset.ts
index ff37d3f1f..c09c87078 100644
--- a/src/lib/init/tools/apply-patchset.ts
+++ b/src/lib/init/tools/apply-patchset.ts
@@ -14,9 +14,66 @@ import type { InitToolDefinition, ToolContext } from "./types.js";
 const EMPTY_AUTH_TOKEN_RE =
   /^(SENTRY_AUTH_TOKEN[ \t]*=[ \t]*)(?:['"]?[ \t]*['"]?)?[ \t]*$/m;
 const PATH_SEGMENT_RE = /[/\\]/u;
+const WINDOWS_DRIVE_RE = /^[A-Za-z]:/;
 
 const VALID_PATCH_ACTIONS = new Set(["create", "modify", "delete"]);
 
+function validatePatchPath(filePath: unknown): string | undefined {
+  if (typeof filePath !== "string" || filePath.length === 0) {
+    return "Invalid patch path: expected a non-empty project-relative path";
+  }
+  if (filePath.includes("\\")) {
+    return `Invalid patch path "${filePath}": use project-relative POSIX paths`;
+  }
+  if (WINDOWS_DRIVE_RE.test(filePath) || path.posix.isAbsolute(filePath)) {
+    return `Invalid patch path "${filePath}": absolute paths are not allowed`;
+  }
+  const segments = filePath.split("/");
+  if (
+    segments.some(
+      (segment) => segment.length === 0 || segment === "." || segment === ".."
+    )
+  ) {
+    return `Invalid patch path "${filePath}": path segments must not be empty, "." or ".."`;
+  }
+  return;
+}
+
+function validatePatch(patch: unknown, cwd: string): ToolResult | undefined {
+  if (!patch || typeof patch !== "object") {
+    return {
+      ok: false,
+      error:
+        "Invalid patch path: expected a patch object with a project-relative path",
+    };
+  }
+
+  const candidate = patch as { action?: unknown; path?: unknown };
+  const pathError = validatePatchPath(candidate.path);
+  if (pathError) {
+    return { ok: false, error: pathError };
+  }
+  const patchPath = candidate.path as string;
+  try {
+    safePath(cwd, patchPath);
+  } catch (error) {
+    return {
+      ok: false,
+      error: error instanceof Error ? error.message : String(error),
+    };
+  }
+  if (
+    typeof candidate.action !== "string" ||
+    !VALID_PATCH_ACTIONS.has(candidate.action)
+  ) {
+    return {
+      ok: false,
+      error: `Unknown patch action: "${String(candidate.action)}" for path "${patchPath}"`,
+    };
+  }
+  return;
+}
+
 /**
  * Apply a batch of file creates, modifications, and deletes.
  */
@@ -29,12 +86,9 @@ export async function applyPatchset(
   }
 
   for (const patch of payload.params.patches) {
-    safePath(payload.cwd, patch.path);
-    if (!VALID_PATCH_ACTIONS.has(patch.action)) {
-      return {
-        ok: false,
-        error: `Unknown patch action: "${patch.action}" for path "${patch.path}"`,
-      };
+    const validationError = validatePatch(patch, payload.cwd);
+    if (validationError) {
+      return validationError;
     }
   }
 
@@ -66,12 +120,9 @@ function applyPatchsetDryRun(payload: ApplyPatchsetPayload): ToolResult {
   const applied: Array<{ path: string; action: string }> = [];
 
   for (const patch of payload.params.patches) {
-    safePath(payload.cwd, patch.path);
-    if (!VALID_PATCH_ACTIONS.has(patch.action)) {
-      return {
-        ok: false,
-        error: `Unknown patch action: "${patch.action}" for path "${patch.path}"`,
-      };
+    const validationError = validatePatch(patch, payload.cwd);
+    if (validationError) {
+      return validationError;
     }
     applied.push({ path: patch.path, action: patch.action });
   }
diff --git a/test/lib/init/tools/filesystem-tools.test.ts b/test/lib/init/tools/filesystem-tools.test.ts
index d0d67a25b..ca08bfcc8 100644
--- a/test/lib/init/tools/filesystem-tools.test.ts
+++ b/test/lib/init/tools/filesystem-tools.test.ts
@@ -128,6 +128,98 @@ describe("filesystem tools", () => {
     ).toContain("sntrys_test_token_123");
   });
 
+  test("rejects unsafe apply-patchset paths before writing", async () => {
+    const unsafePaths: unknown[] = [
+      null,
+      "../../outside.txt",
+      "/tmp/outside.txt",
+      "C:/repo/package.json",
+      "C:\\repo\\package.json",
+      "apps/../package.json",
+      "./package.json",
+      "apps//package.json",
+    ];
+
+    for (const unsafePath of unsafePaths) {
+      const result = await executeTool(
+        {
+          type: "tool",
+          operation: "apply-patchset",
+          cwd: testDir,
+          params: {
+            patches: [
+              {
+                path: unsafePath as never,
+                action: "create",
+                patch: "should not be written\n",
+              },
+            ],
+          },
+        },
+        makeContext(testDir)
+      );
+
+      expect(result.ok).toBe(false);
+      expect(result.error).toContain("Invalid patch path");
+    }
+  });
+
+  test("applies patchsets to nested project-relative files", async () => {
+    fs.mkdirSync(path.join(testDir, "Cary.ConversionFunnels.API"));
+    fs.writeFileSync(
+      path.join(
+        testDir,
+        "Cary.ConversionFunnels.API",
+        "Cary.ConversionFunnels.API.csproj"
+      ),
+      "<Project></Project>\n"
+    );
+
+    const result = await executeTool(
+      {
+        type: "tool",
+        operation: "apply-patchset",
+        cwd: testDir,
+        params: {
+          patches: [
+            {
+              path: "Directory.Packages.props",
+              action: "create",
+              patch: "<Project></Project>\n",
+            },
+            {
+              path: "Cary.ConversionFunnels.API/Cary.ConversionFunnels.API.csproj",
+              action: "modify",
+              edits: [
+                {
+                  oldString: "<Project></Project>",
+                  newString:
+                    '<Project><ItemGroup><PackageReference Include="Sentry.AspNetCore" /></ItemGroup></Project>',
+                },
+              ],
+            },
+          ],
+        },
+      },
+      makeContext(testDir)
+    );
+
+    expect(result.ok).toBe(true);
+    expect(
+      fs.readFileSync(path.join(testDir, "Directory.Packages.props"), "utf-8")
+    ).toContain("<Project>");
+    expect(
+      fs.readFileSync(
+        path.join(
+          testDir,
+          "Cary.ConversionFunnels.API",
+          "Cary.ConversionFunnels.API.csproj"
+        ),
+        "utf-8"
+      )
+    ).toContain("Sentry.AspNetCore");
+  });
+
   test("greps and globs files inside the project", async () => {
     fs.mkdirSync(path.join(testDir, "src"));
     fs.writeFileSync(