diff --git a/.changeset/did-format-validation.md b/.changeset/did-format-validation.md
new file mode 100644
index 00000000..19b33fa7
--- /dev/null
+++ b/.changeset/did-format-validation.md
@@ -0,0 +1,13 @@
+---
+"@hypercerts-org/sdk-core": minor
+---
+
+Add `isValidDid()` utility function for DID format validation
+
+- Validates DID format (did:method:identifier) with support for numeric method names per W3C spec
+- Exported from `@hypercerts-org/sdk-core` for consumer use
+- `BlobOperationsImpl` constructor now validates `repoDid` and throws `ValidationError` for invalid formats
+
+> **⚠️ Potentially breaking:** callers that previously passed invalid DID strings to `BlobOperationsImpl` (directly or
+> via `Repository`) will now receive a `ValidationError` at construction time instead of silently accepting the value.
+> Use `isValidDid(repoDid)` to check before constructing if needed.
diff --git a/.gitignore b/.gitignore
index f381003b..f3bd65a7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@ coverage.json
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
+package-lock.json
 .DS_Store
 /.idea
 stats.html
diff --git a/packages/sdk-core/src/core/types.ts b/packages/sdk-core/src/core/types.ts
index 1a5d688e..5612946f 100644
--- a/packages/sdk-core/src/core/types.ts
+++ b/packages/sdk-core/src/core/types.ts
@@ -22,6 +22,34 @@ import { z } from "zod";
  */
 export type DID = string;
 
+/**
+ * Validates that a string is a valid DID format.
+ *
+ * DIDs must follow the format: `did:<method>:<method-specific-id>`
+ * where method is lowercase letters and digits, and the identifier contains
+ * alphanumeric characters plus `.`, `_`, `:`, `%`, and `-`.
+ *
+ * @param did - The string to validate
+ * @returns true if the string is a valid DID format
+ *
+ * @example
+ * ```typescript
+ * isValidDid("did:plc:ewvi7nxzyoun6zhxrhs64oiz"); // true
+ * isValidDid("did:web:example.com"); // true
+ * isValidDid("not-a-did"); // false
+ * isValidDid("did:"); // false
+ * ```
+ *
+ * @see https://www.w3.org/TR/did-core/#did-syntax for DID syntax specification
+ */
+export function isValidDid(did: string): boolean {
+  // DID format: did:<method>:<method-specific-id>
+  // Method: lowercase letters and digits (per W3C DID Core spec)
+  // Identifier: alphanumeric plus . _ : % -
+  // method-specific-id must end with at least one non-colon idchar (W3C DID Core 1.0)
+  return /^did:[a-z0-9]+:(?:[a-zA-Z0-9._%-]+:)*[a-zA-Z0-9._%-]+$/.test(did);
+}
+
 /**
  * OAuth session with DPoP (Demonstrating Proof of Possession) support.
  *
diff --git a/packages/sdk-core/src/index.ts b/packages/sdk-core/src/index.ts
index 54166417..32018307 100644
--- a/packages/sdk-core/src/index.ts
+++ b/packages/sdk-core/src/index.ts
@@ -197,7 +197,7 @@ export { InMemoryStateStore } from "./storage/InMemoryStateStore.js";
 
 // Core types and schemas
 export type { DID, Organization, Collaborator, CollaboratorPermissions } from "./core/types.js";
-export { OrganizationSchema, CollaboratorSchema, CollaboratorPermissionsSchema } from "./core/types.js";
+export { OrganizationSchema, CollaboratorSchema, CollaboratorPermissionsSchema, isValidDid } from "./core/types.js";
 export { ATProtoSDKConfigSchema, OAuthConfigSchema, ServerConfigSchema, TimeoutConfigSchema } from "./core/config.js";
 
 // OAuth Permissions System
diff --git a/packages/sdk-core/src/repository/BlobOperationsImpl.ts b/packages/sdk-core/src/repository/BlobOperationsImpl.ts
index 1e8aefd3..0529f492 100644
--- a/packages/sdk-core/src/repository/BlobOperationsImpl.ts
+++ b/packages/sdk-core/src/repository/BlobOperationsImpl.ts
@@ -10,7 +10,8 @@
 import type { Agent } from "@atproto/api";
 import { BlobRef } from "@atproto/lexicon";
 import { CID } from "multiformats/cid";
-import { NetworkError } from "../core/errors.js";
+import { NetworkError, ValidationError } from "../core/errors.js";
+import { isValidDid } from "../core/types.js";
 import type { BlobOperations } from "./interfaces.js";
 
 /**
@@ -68,7 +69,13 @@ export class BlobOperationsImpl implements BlobOperations {
     private repoDid: string,
     private _serverUrl: string,
     private isSDS: boolean,
-  ) {}
+  ) {
+    if (!isValidDid(repoDid)) {
+      throw new ValidationError(
+        `Invalid DID format: "${repoDid}". DIDs must start with "did:" (e.g., "did:plc:abc123")`,
+      );
+    }
+  }
 
   /**
    * Uploads a blob to the server.
diff --git a/packages/sdk-core/tests/core/types.test.ts b/packages/sdk-core/tests/core/types.test.ts
new file mode 100644
index 00000000..cc658429
--- /dev/null
+++ b/packages/sdk-core/tests/core/types.test.ts
@@ -0,0 +1,80 @@
+import { describe, it, expect } from "vitest";
+import { isValidDid } from "../../src/core/types.js";
+
+describe("isValidDid", () => {
+  describe("valid DIDs", () => {
+    it("should accept did:plc format", () => {
+      expect(isValidDid("did:plc:abc123")).toBe(true);
+    });
+
+    it("should accept did:web format", () => {
+      expect(isValidDid("did:web:example.com")).toBe(true);
+    });
+
+    it("should accept DID with alphanumeric identifier", () => {
+      expect(isValidDid("did:plc:ewvi7nxzyoun6zhxrhs64oiz")).toBe(true);
+    });
+
+    it("should accept DID with dots in identifier", () => {
+      expect(isValidDid("did:web:sub.example.com")).toBe(true);
+    });
+
+    it("should accept DID with colons in identifier", () => {
+      expect(isValidDid("did:web:example.com:user:123")).toBe(true);
+    });
+
+    it("should accept DID with percent-encoded characters", () => {
+      expect(isValidDid("did:example:abc%20def")).toBe(true);
+    });
+
+    it("should accept DID with hyphens and underscores", () => {
+      expect(isValidDid("did:example:my-test_id")).toBe(true);
+    });
+
+    it("should accept DID with method containing digits", () => {
+      expect(isValidDid("did:key2:abc123")).toBe(true);
+    });
+
+    it("should accept DID with method containing multiple digits", () => {
+      expect(isValidDid("did:btc1:xyz789")).toBe(true);
+    });
+
+    it("should accept DID with method that is all digits", () => {
+      expect(isValidDid("did:123:identifier")).toBe(true);
+    });
+  });
+
+  describe("invalid DIDs", () => {
+    it("should reject empty string", () => {
+      expect(isValidDid("")).toBe(false);
+    });
+
+    it("should reject string not starting with did:", () => {
+      expect(isValidDid("not-a-did")).toBe(false);
+    });
+
+    it("should reject did: without method", () => {
+      expect(isValidDid("did:")).toBe(false);
+    });
+
+    it("should reject did:method without identifier", () => {
+      expect(isValidDid("did:plc:")).toBe(false);
+    });
+
+    it("should reject DID with trailing colon in identifier", () => {
+      expect(isValidDid("did:example:abc:")).toBe(false);
+    });
+
+    it("should reject method with uppercase letters", () => {
+      expect(isValidDid("did:PLC:abc123")).toBe(false);
+    });
+
+    it("should reject random URL", () => {
+      expect(isValidDid("https://example.com")).toBe(false);
+    });
+
+    it("should reject AT-URI", () => {
+      expect(isValidDid("at://did:plc:abc123/collection/rkey")).toBe(false);
+    });
+  });
+});
diff --git a/packages/sdk-core/tests/repository/BlobOperationsImpl.test.ts b/packages/sdk-core/tests/repository/BlobOperationsImpl.test.ts
index 199a50c7..d5be4ec1 100644
--- a/packages/sdk-core/tests/repository/BlobOperationsImpl.test.ts
+++ b/packages/sdk-core/tests/repository/BlobOperationsImpl.test.ts
@@ -1,7 +1,7 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import type { Agent } from "@atproto/api";
 import { BlobOperationsImpl } from "../../src/repository/BlobOperationsImpl.js";
-import { NetworkError } from "../../src/core/errors.js";
+import { NetworkError, ValidationError } from "../../src/core/errors.js";
 import { createMockAgent, TEST_REPO_DID, TEST_PDS_URL, TEST_SDS_URL } from "../utils/mocks.js";
 
 describe("BlobOperationsImpl", () => {
@@ -13,6 +13,26 @@ describe("BlobOperationsImpl", () => {
     blobOps = new BlobOperationsImpl(mockAgent as unknown as Agent, TEST_REPO_DID, TEST_PDS_URL, false);
   });
 
+  describe("constructor", () => {
+    it("should accept valid DID", () => {
+      expect(
+        () => new BlobOperationsImpl(mockAgent as unknown as Agent, "did:plc:abc123", TEST_PDS_URL, false),
+      ).not.toThrow();
+    });
+
+    it("should throw ValidationError for invalid DID", () => {
+      expect(() => new BlobOperationsImpl(mockAgent as unknown as Agent, "not-a-did", TEST_PDS_URL, false)).toThrow(
+        ValidationError,
+      );
+    });
+
+    it("should include helpful error message with the invalid DID", () => {
+      expect(() => new BlobOperationsImpl(mockAgent as unknown as Agent, "invalid", TEST_PDS_URL, false)).toThrow(
+        /Invalid DID format: "invalid"/,
+      );
+    });
+  });
+
   describe("upload", () => {
     it("should upload a blob successfully", async () => {
       const mockBlob = new Blob(["test content"], { type: "text/plain" });