diff --git a/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/.openspec.yaml b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/.openspec.yaml
new file mode 100644
index 0000000..0ba725f
--- /dev/null
+++ b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-06-03
diff --git a/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/proposal.md b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/proposal.md
new file mode 100644
index 0000000..2e7fc7c
--- /dev/null
+++ b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/proposal.md
@@ -0,0 +1,14 @@
+## Why
+
+- After #618 landed the canonical `detectDefaultBaseBranch` in `src/git/index.js`, `src/pr.js` still carried its own drifted `detectBaseBranch` (different candidate list, only-origin ref scope, different fallback). Two base-detectors can return different bases for the same repo. Also, `src/git/index.js` kept a dead `rawRun` import + `void rawRun;` scaffolding deferred from Phase 1.
+
+## What Changes
+
+- Remove `pr.js` `detectBaseBranch`; route PR base resolution through git's canonical `detectDefaultBaseBranch` (imported from `./git`). Drop the now-redundant pr-module test (the detector is covered by `test/git-base-branch.test.js`).
+- Remove the dead `rawRun` import and `void rawRun;` from `git/index.js` (and the stale JSDoc reference).
+
+## Impact
+
+- Surface: PR base detection (`gx pr`). Unifies on the more robust detector (also checks local heads). Common cases unchanged; edge-case differences (`develop` candidate, no-base fallback) converge to the canonical behavior.
+- Risk: low. Zero new test failures vs base (33→33). Detector behavior is spec'd + tested in #618.
+- Deferred (NOT in this PR): A3 sandbox-helper consolidation (3 of 8 helpers DRIFTED between `sandbox/index.js` and `cli/shared/sandbox.js` — needs drift reconciliation, not a partial dedup); A7 launch.js registry-shape (inferred/not-verified defensive code); B5 finish base-branch hoist (marginal perf, finish is not a hot loop).
diff --git a/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/specs/phase5-public-cli-dedup-base-detection-sandbox-launch-rawrun/spec.md b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/specs/phase5-public-cli-dedup-base-detection-sandbox-launch-rawrun/spec.md
new file mode 100644
index 0000000..1ed39d3
--- /dev/null
+++ b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/specs/phase5-public-cli-dedup-base-detection-sandbox-launch-rawrun/spec.md
@@ -0,0 +1,12 @@
+## ADDED Requirements
+
+### Requirement: Single canonical base-branch detector
+Base-branch detection SHALL have one implementation. The PR flow SHALL resolve the base branch via the canonical `detectDefaultBaseBranch` (in `src/git/index.js`) rather than a separate, drift-prone copy.
+
+#### Scenario: PR base uses the canonical detector
+- **WHEN** the PR flow needs a base branch and no explicit `--base` is given
+- **THEN** it resolves the base via `detectDefaultBaseBranch` (origin/HEAD → main/master/dev → DEFAULT_BASE_BRANCH)
+
+#### Scenario: No duplicate detector remains
+- **WHEN** the codebase is searched for base-branch detection
+- **THEN** `src/pr.js` exposes no separate `detectBaseBranch` implementation
diff --git a/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/tasks.md b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/tasks.md
new file mode 100644
index 0000000..490eb97
--- /dev/null
+++ b/openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/tasks.md
@@ -0,0 +1,34 @@
+## Definition of Done
+
+This change is complete only when **all** of the following are true:
+
+- Every checkbox below is checked.
+- The agent branch reaches `MERGED` state on `origin` and the PR URL + state are recorded in the completion handoff.
+- If any step blocks (test failure, conflict, ambiguous result), append a `BLOCKED:` line under section 4 explaining the blocker and **STOP**. Do not tick remaining cleanup boxes; do not silently skip the cleanup pipeline.
+
+## Handoff
+
+- Handoff: change=`agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28`; branch=`agent/<your-name>/<branch-slug>`; scope=`TODO`; action=`continue this sandbox or finish cleanup after a usage-limit/manual takeover`.
+- Copy prompt: Continue `agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28` on branch `agent/<your-name>/<branch-slug>`. Work inside the existing sandbox, review `openspec/changes/agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28/tasks.md`, continue from the current state instead of creating a new sandbox, and when the work is done run `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`.
+
+## 1. Specification
+
+- [x] 1.1 Finalize proposal scope and acceptance criteria for `agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28`.
+- [x] 1.2 Define normative requirements in `specs/phase5-public-cli-dedup-base-detection-sandbox-launch-rawrun/spec.md`.
+
+## 2. Implementation
+
+- [x] 2.1 Implement scoped behavior changes.
+- [x] 2.2 Add/update focused regression coverage.
+
+## 3. Verification
+
+- [x] 3.1 Run targeted project verification commands.
+- [x] 3.2 Run `openspec validate agent-claude-phase5-public-cli-dedup-base-detection-s-2026-06-03-14-28 --type change --strict`.
+- [x] 3.3 Run `openspec validate --specs`.
+
+## 4. Cleanup (mandatory; run before claiming completion)
+
+- [ ] 4.1 Run the cleanup pipeline: `gx branch finish --branch agent/<your-name>/<branch-slug> --base dev --via-pr --wait-for-merge --cleanup`. This handles commit -> push -> PR create -> merge wait -> worktree prune in one invocation.
+- [ ] 4.2 Record the PR URL and final merge state (`MERGED`) in the completion handoff.
+- [ ] 4.3 Confirm the sandbox worktree is gone (`git worktree list` no longer shows the agent path; `git branch -a` shows no surviving local/remote refs for the branch).
diff --git a/src/git/index.js b/src/git/index.js
index feffd27..9237e39 100644
--- a/src/git/index.js
+++ b/src/git/index.js
@@ -13,7 +13,6 @@ const {
   COMPOSE_HINT_FILES,
   LOCK_FILE_RELATIVE,
 } = require('../context');
-const { run: rawRun } = require('../core/runtime');
 
 /**
  * Result of a synchronous child-process spawn.
@@ -90,8 +89,8 @@ const { run: rawRun } = require('../core/runtime');
  * answer from cache. `cachedSpawn` falls through to `cp.spawnSync` for any
  * command not on its read-only allowlist (writes like `git commit`,
  * `git push`, `git checkout`, `git worktree add/remove`), so this is a
- * strict perf optimization that preserves `rawRun`'s signature
- * `(cmd, args, opts) -> spawnSync result`.
+ * strict perf optimization preserving the `(cmd, args, opts) -> spawnSync
+ * result` signature.
  *
  * @param {string} cmd Executable to invoke.
  * @param {ReadonlyArray<string>} args Argument vector.
@@ -107,9 +106,6 @@ function run(cmd, args, options = {}) {
     timeout: options.timeout,
   });
 }
-// Keep rawRun referenced so a future write that intentionally bypasses the
-// cache stays trivial. (Currently nothing in this module needs to bypass.)
-void rawRun;
 
 /**
  * Run `git -C <repoRoot> <args>` and throw unless `allowFailure` is set.
diff --git a/src/pr.js b/src/pr.js
index fe7873c..4d91f73 100644
--- a/src/pr.js
+++ b/src/pr.js
@@ -13,6 +13,7 @@ const {
   resolveRepoRoot,
   currentBranchName,
   hasOriginRemote,
+  detectDefaultBaseBranch,
 } = require('./git');
 
 const DEFAULT_POLL_INTERVAL_MS = 5_000;
@@ -105,26 +106,6 @@ function pushBranch(repoRoot, branch, { setUpstream = true } = {}) {
   };
 }
 
-function detectBaseBranch(repoRoot, fallback = 'main') {
-  // Prefer remote HEAD; fall back to common base names; finally `fallback`.
-  const remoteHead = run('git', ['symbolic-ref', '--short', 'refs/remotes/origin/HEAD'], {
-    cwd: repoRoot, allowFailure: true,
-  });
-  if (remoteHead.status === 0) {
-    const value = (remoteHead.stdout || '').trim();
-    if (value.startsWith('origin/')) {
-      return value.slice('origin/'.length);
-    }
-  }
-  for (const candidate of ['main', 'dev', 'develop', 'master']) {
-    const exists = run('git', ['rev-parse', '--verify', `refs/remotes/origin/${candidate}`], {
-      cwd: repoRoot, allowFailure: true,
-    });
-    if (exists.status === 0) return candidate;
-  }
-  return fallback;
-}
-
 function defaultPrTitleFromCommit(repoRoot, branch) {
   const result = run('git', ['log', '-1', '--pretty=%s', branch], {
     cwd: repoRoot, allowFailure: true,
@@ -201,7 +182,7 @@ function openPullRequest(options) {
     }
   }
 
-  const base = options.base || detectBaseBranch(repoRoot);
+  const base = options.base || detectDefaultBaseBranch(repoRoot);
   const title = options.title || defaultPrTitleFromCommit(repoRoot, branch);
   const body = options.body || defaultPrBodyFromCommits(repoRoot, branch, base);
 
@@ -367,7 +348,6 @@ module.exports = {
   findOpenPrForBranch,
   findLatestPrForBranch,
   pushBranch,
-  detectBaseBranch,
   openPullRequest,
   getPullRequestStatus,
   enableAutoMerge,
diff --git a/test/pr-module.test.js b/test/pr-module.test.js
index 1ae50ca..6a642f8 100644
--- a/test/pr-module.test.js
+++ b/test/pr-module.test.js
@@ -32,16 +32,6 @@ function makeRepo() {
   return dir;
 }
 
-test('detectBaseBranch falls back to main when no origin is configured', () => {
-  const repoRoot = makeRepo();
-  try {
-    const base = prModule.detectBaseBranch(repoRoot);
-    assert.equal(base, 'main');
-  } finally {
-    fs.rmSync(repoRoot, { recursive: true, force: true });
-  }
-});
-
 test('defaultPrTitleFromCommit returns latest commit subject', () => {
   const repoRoot = makeRepo();
   try {