diff --git a/.github/workflows/rust-ci.yml b/.github/workflows/rust-ci.yml
index 95f65f452..1195b0308 100644
--- a/.github/workflows/rust-ci.yml
+++ b/.github/workflows/rust-ci.yml
@@ -82,8 +82,16 @@ jobs:
       - name: cargo clippy
         run: cargo clippy --target ${{ matrix.target }} --all-features -- -D warnings
 
+      # Build mock-acp-agent to a known location for E2E tests
+      - name: Build mock-acp-agent
+        run: |
+          mkdir -p target/mock-acp-out
+          cargo +nightly build -p mock-acp-agent --profile ci-test --target ${{ matrix.target }} --artifact-dir target/mock-acp-out -Z unstable-options
+
       - name: cargo test
         # continue-on-error: true  # TODO: Fix pre-existing test failures in codex-app-server
+        env:
+          MOCK_ACP_AGENT_BIN: ${{ github.workspace }}/codex-rs/target/mock-acp-out/mock_acp_agent
         run: cargo test --profile ci-test --target ${{ matrix.target }} --all-features
 
       # Save cargo home cache
diff --git a/codex-rs/acp/src/registry.rs b/codex-rs/acp/src/registry.rs
index 32ed71421..e197ec825 100644
--- a/codex-rs/acp/src/registry.rs
+++ b/codex-rs/acp/src/registry.rs
@@ -65,24 +65,40 @@ pub fn get_agent_config(model_name: &str) -> Result<AcpAgentConfig> {
 
     match normalized.as_str() {
         "mock-model" => {
-            // Use full path to mock_acp_agent binary from target directory
-            // This handles both debug and release builds
-            let exe_path = match std::env::current_exe() {
-                Ok(p) => {
-                    let mock_path = p
-                        .parent()
-                        .map(|parent| parent.join("mock_acp_agent"))
-                        .unwrap_or_else(|| std::path::PathBuf::from("mock_acp_agent"));
-                    tracing::debug!("Mock ACP agent path resolved to: {}", mock_path.display());
-                    mock_path
-                }
-                Err(e) => {
-                    tracing::warn!(
-                        "Failed to get current_exe for mock-model: {}, falling back to 'mock_acp_agent'",
-                        e
-                    );
-                    std::path::PathBuf::from("mock_acp_agent")
-                }
+            // Resolve path to mock_acp_agent binary.
+            //
+            // Priority:
+            // 1. MOCK_ACP_AGENT_BIN environment variable (set by CI)
+            // 2. Relative to current executable (for local development)
+            let exe_path = if let Ok(env_path) = std::env::var("MOCK_ACP_AGENT_BIN") {
+                tracing::debug!("Mock ACP agent path from MOCK_ACP_AGENT_BIN: {}", env_path);
+                std::path::PathBuf::from(env_path)
+            } else {
+                // Fall back to resolving relative to current executable.
+                // This handles both:
+                // - Running as `codex` binary: target/{profile}/codex -> target/{profile}/
+                // - Running as test binary: target/{profile}/deps/test -> target/{profile}/
+                let mock_path = std::env::current_exe()
+                    .ok()
+                    .and_then(|p| {
+                        p.parent().map(|parent| {
+                            // Check if we're in a "deps" directory (test binary context)
+                            let in_deps_dir = parent
+                                .file_name()
+                                .map(|name| name == "deps")
+                                .unwrap_or(false);
+
+                            if in_deps_dir {
+                                parent.parent().map(|p| p.join("mock_acp_agent"))
+                            } else {
+                                Some(parent.join("mock_acp_agent"))
+                            }
+                        })
+                    })
+                    .flatten()
+                    .unwrap_or_else(|| std::path::PathBuf::from("mock_acp_agent"));
+                tracing::debug!("Mock ACP agent path resolved to: {}", mock_path.display());
+                mock_path
             };
 
             Ok(AcpAgentConfig {