diff --git a/CHANGELOG.md b/CHANGELOG.md
index f50ad012..cc419b8f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- **`watch-pr` crashes on repos with non-main default branch** — `fetchFileChanges` hardcoded `origin/main` for git diff, causing failures on repos using `master`, `develop`, or other base branches. Now uses the PR's actual base branch from GitHub metadata.
+
 ## [0.19.0] - 2026-03-04
 
 ### Breaking Changes
diff --git a/packages/cli/src/services/github-fetcher.ts b/packages/cli/src/services/github-fetcher.ts
index 9b02b0da..15ce1f24 100644
--- a/packages/cli/src/services/github-fetcher.ts
+++ b/packages/cli/src/services/github-fetcher.ts
@@ -272,14 +272,17 @@ export class GitHubFetcher {
    * Uses git diff --numstat to get file change statistics.
    *
    * @param _prNumber - PR number (unused - we use git diff from current branch)
+   * @param baseBranch - Base branch name (e.g., 'main', 'master', 'develop')
    * @returns File change context
    */
-  async fetchFileChanges(_prNumber: number): Promise<ChangesContext> {
+  async fetchFileChanges(_prNumber: number, baseBranch: string): Promise<ChangesContext> {
+    const baseRef = `origin/${baseBranch}`;
+
     // Get diff stats using getDiffStats from @vibe-validate/git (architectural compliance)
-    const diffOutput = getDiffStats('origin/main', 'HEAD');
+    const diffOutput = getDiffStats(baseRef, 'HEAD');
 
     // Get commit count using getCommitCount from @vibe-validate/git (architectural compliance)
-    const commitCount = getCommitCount('origin/main', 'HEAD');
+    const commitCount = getCommitCount(baseRef, 'HEAD');
 
     // Parse diff output
     const lines = diffOutput.trim().split('\n').filter((line) => line.length > 0);
diff --git a/packages/cli/src/services/watch-pr-orchestrator.ts b/packages/cli/src/services/watch-pr-orchestrator.ts
index 66fbd4c2..af440459 100644
--- a/packages/cli/src/services/watch-pr-orchestrator.ts
+++ b/packages/cli/src/services/watch-pr-orchestrator.ts
@@ -157,8 +157,8 @@ export class WatchPROrchestrator {
     // Fetch history summary
     const historySummary = await this.historyBuilder.buildSummary(prMetadata.branch);
 
-    // Fetch file changes
-    const changes = await this.fetcher.fetchFileChanges(prNumber);
+    // Fetch file changes (use PR's base branch, not hardcoded 'main')
+    const changes = await this.fetcher.fetchFileChanges(prNumber, prMetadata.base_branch);
 
     // Determine overall status
     let status: PRStatus = 'passed';
@@ -351,8 +351,8 @@ export class WatchPROrchestrator {
       jobs.map(job => this.buildCheckFromJob(job, runId, runDetails.workflow))
     );
 
-    // Fetch file changes
-    const changes = await this.fetcher.fetchFileChanges(prNumber);
+    // Fetch file changes (use PR's base branch, not hardcoded 'main')
+    const changes = await this.fetcher.fetchFileChanges(prNumber, prMetadata.base_branch);
 
     // Determine status
     const status = this.determineOverallStatus(actionChecks);
diff --git a/packages/cli/test/services/github-fetcher.test.ts b/packages/cli/test/services/github-fetcher.test.ts
index 1843ad27..7d20d984 100644
--- a/packages/cli/test/services/github-fetcher.test.ts
+++ b/packages/cli/test/services/github-fetcher.test.ts
@@ -499,7 +499,7 @@ describe('GitHubFetcher', () => {
       vi.spyOn(gitPackage, 'getDiffStats').mockReturnValue(mockDiffOutput);
       vi.spyOn(gitPackage, 'getCommitCount').mockReturnValue(mockCommitCount);
 
-      const changes = await fetcher.fetchFileChanges(123);
+      const changes = await fetcher.fetchFileChanges(123, 'main');
 
       expect(changes).toEqual({
         files_changed: 4,
@@ -523,7 +523,7 @@ describe('GitHubFetcher', () => {
       vi.spyOn(gitPackage, 'getDiffStats').mockReturnValue(mockDiffOutput);
       vi.spyOn(gitPackage, 'getCommitCount').mockReturnValue(mockCommitCount);
 
-      const changes = await fetcher.fetchFileChanges(123);
+      const changes = await fetcher.fetchFileChanges(123, 'main');
 
       expect(changes.top_files).toHaveLength(10);
       expect(changes.files_changed).toBe(20);
@@ -539,7 +539,7 @@ describe('GitHubFetcher', () => {
       vi.spyOn(gitPackage, 'getDiffStats').mockReturnValue(mockDiffOutput);
       vi.spyOn(gitPackage, 'getCommitCount').mockReturnValue(mockCommitCount);
 
-      const changes = await fetcher.fetchFileChanges(123);
+      const changes = await fetcher.fetchFileChanges(123, 'main');
 
       expect(changes.top_files?.[0].new_file).toBe(true);
     });
@@ -552,7 +552,7 @@ describe('GitHubFetcher', () => {
       vi.spyOn(gitPackage, 'getDiffStats').mockReturnValue(mockDiffOutput);
       vi.spyOn(gitPackage, 'getCommitCount').mockReturnValue(mockCommitCount);
 
-      const changes = await fetcher.fetchFileChanges(123);
+      const changes = await fetcher.fetchFileChanges(123, 'main');
 
       expect(changes).toEqual({
         files_changed: 0,
@@ -569,7 +569,20 @@ describe('GitHubFetcher', () => {
         throw new Error('git: not a git repository');
       });
 
-      await expect(fetcher.fetchFileChanges(123)).rejects.toThrow();
+      await expect(fetcher.fetchFileChanges(123, 'main')).rejects.toThrow();
+    });
+
+    it('should use the provided base branch instead of hardcoded main', async () => {
+      const mockDiffOutput = '1\t0\tfile.ts';
+      const mockCommitCount = '1';
+
+      const diffSpy = vi.spyOn(gitPackage, 'getDiffStats').mockReturnValue(mockDiffOutput);
+      const countSpy = vi.spyOn(gitPackage, 'getCommitCount').mockReturnValue(mockCommitCount);
+
+      await fetcher.fetchFileChanges(123, 'master');
+
+      expect(diffSpy).toHaveBeenCalledWith('origin/master', 'HEAD');
+      expect(countSpy).toHaveBeenCalledWith('origin/master', 'HEAD');
     });
   });
 
diff --git a/packages/cli/test/services/watch-pr-orchestrator.test.ts b/packages/cli/test/services/watch-pr-orchestrator.test.ts
index ecf4e474..13bb7b3b 100644
--- a/packages/cli/test/services/watch-pr-orchestrator.test.ts
+++ b/packages/cli/test/services/watch-pr-orchestrator.test.ts
@@ -592,7 +592,7 @@ describe('WatchPROrchestrator', () => {
 
       // Verify PR context is still fetched
       expect(fetchPRSpy).toHaveBeenCalledWith(mockPRNumber);
-      expect(fetchFileChangesSpy).toHaveBeenCalledWith(mockPRNumber);
+      expect(fetchFileChangesSpy).toHaveBeenCalledWith(mockPRNumber, expect.any(String));
     });
   });