diff --git a/.changeset/fix-batch-sub-request-urls.md b/.changeset/fix-batch-sub-request-urls.md
new file mode 100644
index 00000000..eb142383
--- /dev/null
+++ b/.changeset/fix-batch-sub-request-urls.md
@@ -0,0 +1,5 @@
+---
+"@proofkit/fmodata": patch
+---
+
+Fix batch sub-request URLs to use canonical FileMaker OData path format. Strips the Otto proxy prefix (`/otto/`) and `.fmp12` file extension from database names in sub-request URLs inside multipart batch bodies, which are processed directly by FileMaker's OData engine. Also fix `InvalidLocationHeaderError` in batch insert/update sub-responses by gracefully handling missing Location headers (returns ROWID -1 instead of throwing).
diff --git a/packages/fmodata/src/client/batch-request.ts b/packages/fmodata/src/client/batch-request.ts
index 191d1232..402bf10d 100644
--- a/packages/fmodata/src/client/batch-request.ts
+++ b/packages/fmodata/src/client/batch-request.ts
@@ -10,6 +10,8 @@ const BOUNDARY_REGEX = /boundary=([^;]+)/;
 const HTTP_STATUS_LINE_REGEX = /HTTP\/\d\.\d\s+(\d+)\s*(.*)/;
 const CRLF_REGEX = /\r\n/;
 const CHANGESET_CONTENT_TYPE_REGEX = /Content-Type: multipart\/mixed;\s*boundary=([^\r\n]+)/;
+const OTTO_PREFIX_REGEX = /^\/otto/;
+const FMPRO_EXT_REGEX = /\.fmp12/;
 
 export interface RequestConfig {
   method: string;
@@ -62,6 +64,18 @@ async function requestToConfig(request: Request): Promise<RequestConfig> {
   };
 }
 
+/**
+ * Transforms a full URL into the canonical path format required by FileMaker's
+ * OData batch processor. Strips proxy prefixes (e.g. /otto/) and the .fmp12
+ * file extension from the database name segment.
+ */
+export function toBatchSubRequestUrl(fullUrl: string): string {
+  const url = new URL(fullUrl);
+  const path = url.pathname.replace(OTTO_PREFIX_REGEX, "");
+  const batchPath = path.replace(FMPRO_EXT_REGEX, "");
+  return `${batchPath}${url.search}`;
+}
+
 /**
  * Formats a single HTTP request for inclusion in a batch
  * @param request - The request configuration
@@ -80,11 +94,15 @@ function formatSubRequest(request: RequestConfig, baseUrl: string): string {
   lines.push("Content-Transfer-Encoding: binary");
   lines.push(""); // Empty line after multipart headers
 
-  // Construct full URL (convert relative to absolute)
+  // Construct sub-request URL as a canonical FileMaker OData path.
+  // Sub-requests inside the batch body are processed directly by FileMaker's
+  // OData engine, so they must not include proxy prefixes (e.g. /otto/) or
+  // the .fmp12 file extension on the database name.
   const fullUrl = request.url.startsWith("http") ? request.url : `${baseUrl}${request.url}`;
+  const subRequestUrl = toBatchSubRequestUrl(fullUrl);
 
   // Add HTTP request line
-  lines.push(`${request.method} ${fullUrl} HTTP/1.1`);
+  lines.push(`${request.method} ${subRequestUrl} HTTP/1.1`);
 
   // For requests with body, add headers
   if (request.body) {
diff --git a/packages/fmodata/src/client/insert-builder.ts b/packages/fmodata/src/client/insert-builder.ts
index 5d75291b..475865ca 100644
--- a/packages/fmodata/src/client/insert-builder.ts
+++ b/packages/fmodata/src/client/insert-builder.ts
@@ -275,14 +275,9 @@ export class InsertBuilder<
       // Check for Location header (for return=minimal)
       if (this.returnPreference === "minimal") {
         const locationHeader = getLocationHeader(response.headers);
-        if (locationHeader) {
-          const rowid = this.parseLocationHeader(locationHeader);
-          // biome-ignore lint/suspicious/noExplicitAny: Type assertion for generic return type
-          return { data: { ROWID: rowid } as any, error: undefined };
-        }
-        throw new InvalidLocationHeaderError(
-          "Location header is required when using return=minimal but was not found in response",
-        );
+        const rowid = locationHeader ? this.parseLocationHeader(locationHeader) : -1;
+        // biome-ignore lint/suspicious/noExplicitAny: Type assertion for generic return type
+        return { data: { ROWID: rowid } as any, error: undefined };
       }
 
       // For 204 responses without return=minimal, FileMaker doesn't return the created entity
@@ -295,11 +290,14 @@ export class InsertBuilder<
       };
     }
 
-    // If we expected return=minimal but got a body, that's unexpected
+    // If we expected return=minimal but got a body (e.g. batch sub-responses
+    // where FM returns 204-with-body, converted to 200 by parsedToResponse),
+    // try to extract ROWID from the Location header or return -1.
     if (this.returnPreference === "minimal") {
-      throw new InvalidLocationHeaderError(
-        "Expected 204 No Content for return=minimal, but received response with body",
-      );
+      const locationHeader = getLocationHeader(response.headers);
+      const rowid = locationHeader ? this.parseLocationHeader(locationHeader) : -1;
+      // biome-ignore lint/suspicious/noExplicitAny: Type assertion for generic return type
+      return { data: { ROWID: rowid } as any, error: undefined };
     }
 
     // Use safeJsonParse to handle FileMaker's invalid JSON with unquoted ? values
diff --git a/packages/fmodata/tests/batch-sub-request-url.test.ts b/packages/fmodata/tests/batch-sub-request-url.test.ts
new file mode 100644
index 00000000..727e06ce
--- /dev/null
+++ b/packages/fmodata/tests/batch-sub-request-url.test.ts
@@ -0,0 +1,51 @@
+import { describe, expect, it } from "vitest";
+import { formatBatchRequest, toBatchSubRequestUrl } from "../src/client/batch-request";
+
+describe("toBatchSubRequestUrl", () => {
+  it("strips /otto/ prefix and .fmp12 extension", () => {
+    const result = toBatchSubRequestUrl(
+      "https://host.example.com/otto/fmi/odata/v4/GMT_Web.fmp12/bookings?$top=1&$select=_GMTNum",
+    );
+    expect(result).toBe("/fmi/odata/v4/GMT_Web/bookings?$top=1&$select=_GMTNum");
+  });
+
+  it("strips .fmp12 extension without /otto/ prefix", () => {
+    const result = toBatchSubRequestUrl("https://host.example.com/fmi/odata/v4/GMT_Web.fmp12/bookings");
+    expect(result).toBe("/fmi/odata/v4/GMT_Web/bookings");
+  });
+
+  it("handles URLs without /otto/ or .fmp12", () => {
+    const result = toBatchSubRequestUrl("https://host.example.com/fmi/odata/v4/MyDB/contacts");
+    expect(result).toBe("/fmi/odata/v4/MyDB/contacts");
+  });
+
+  it("preserves query parameters", () => {
+    const result = toBatchSubRequestUrl(
+      "https://host.example.com/otto/fmi/odata/v4/MyDB.fmp12/contacts?$filter=name eq 'test'&$top=10",
+    );
+    expect(result).toBe("/fmi/odata/v4/MyDB/contacts?$filter=name%20eq%20%27test%27&$top=10");
+  });
+});
+
+describe("formatBatchRequest sub-request URLs", () => {
+  it("uses canonical paths without /otto/ prefix or .fmp12 in sub-requests", () => {
+    const baseUrl = "https://host.example.com/otto/fmi/odata/v4/GMT_Web.fmp12";
+    const { body } = formatBatchRequest(
+      [{ method: "GET", url: `${baseUrl}/bookings?$top=1&$select=_GMTNum` }],
+      baseUrl,
+    );
+
+    // The sub-request line must use the canonical path
+    expect(body).toContain("GET /fmi/odata/v4/GMT_Web/bookings?$top=1&$select=_GMTNum HTTP/1.1");
+    // Must NOT contain the otto prefix or .fmp12 in the request line
+    expect(body).not.toContain("/otto/");
+    expect(body).not.toContain(".fmp12");
+  });
+
+  it("handles relative URLs by prepending baseUrl then transforming", () => {
+    const baseUrl = "https://host.example.com/otto/fmi/odata/v4/MyDB.fmp12";
+    const { body } = formatBatchRequest([{ method: "GET", url: "/contacts?$top=5" }], baseUrl);
+
+    expect(body).toContain("GET /fmi/odata/v4/MyDB/contacts?$top=5 HTTP/1.1");
+  });
+});