codex-exec: allow resume --last to read prompt #6717 (#6719)

### Description

- codex exec --json resume --last "<prompt>" bailed out because clap
treated the prompt as SESSION_ID. I removed the conflicts_with flag and
reinterpret that positional as a prompt when
--last is set, so the flow now keeps working in JSON mode.
(codex-rs/exec/src/cli.rs:84-104, codex-rs/exec/src/lib.rs:75-130)
- Added a regression test that exercises resume --last in JSON mode to
ensure the prompt is accepted and the rollout file is updated.
(codex-rs/exec/tests/suite/resume.rs:126-178)

### Testing

  - just fmt
  - cargo test -p codex-exec
  - just fix -p codex-exec
  - cargo test -p codex-exec

#6717

Signed-off-by: Dmitri Khokhlov <dkhokhlov@cribl.io>

codex-rs/exec/src/cli.rs

@@ -101,7 +101,7 @@ pub struct ResumeArgs {
     pub session_id: Option<String>,
 
     /// Resume the most recent recorded session (newest) without specifying an id.
-    #[arg(long = "last", default_value_t = false, conflicts_with = "session_id")]
+    #[arg(long = "last", default_value_t = false)]
     pub last: bool,
 
     /// Prompt to send after resuming the session. If `-` is used, read from stdin.

codex-rs/exec/src/lib.rs

@@ -82,7 +82,21 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     let prompt_arg = match &command {
         // Allow prompt before the subcommand by falling back to the parent-level prompt
         // when the Resume subcommand did not provide its own prompt.
-        Some(ExecCommand::Resume(args)) => args.prompt.clone().or(prompt),
+        Some(ExecCommand::Resume(args)) => {
+            let resume_prompt = args
+                .prompt
+                .clone()
+                // When using `resume --last <PROMPT>`, clap still parses the first positional
+                // as `session_id`. Reinterpret it as the prompt so the flag works with JSON mode.
+                .or_else(|| {
+                    if args.last {
+                        args.session_id.clone()
+                    } else {
+                        None
+                    }
+                });
+            resume_prompt.or(prompt)
+        }
         None => prompt,
     };
 

codex-rs/exec/tests/suite/resume.rs

@@ -123,6 +123,60 @@ fn exec_resume_last_appends_to_existing_file() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[test]
+fn exec_resume_last_accepts_prompt_after_flag_in_json_mode() -> anyhow::Result<()> {
+    let test = test_codex_exec();
+    let fixture =
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/cli_responses_fixture.sse");
+
+    // 1) First run: create a session with a unique marker in the content.
+    let marker = format!("resume-last-json-{}", Uuid::new_v4());
+    let prompt = format!("echo {marker}");
+
+    test.cmd()
+        .env("CODEX_RS_SSE_FIXTURE", &fixture)
+        .env("OPENAI_BASE_URL", "http://unused.local")
+        .arg("--skip-git-repo-check")
+        .arg("-C")
+        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg(&prompt)
+        .assert()
+        .success();
+
+    // Find the created session file containing the marker.
+    let sessions_dir = test.home_path().join("sessions");
+    let path = find_session_file_containing_marker(&sessions_dir, &marker)
+        .expect("no session file found after first run");
+
+    // 2) Second run: resume the most recent file and pass the prompt after --last.
+    let marker2 = format!("resume-last-json-2-{}", Uuid::new_v4());
+    let prompt2 = format!("echo {marker2}");
+
+    test.cmd()
+        .env("CODEX_RS_SSE_FIXTURE", &fixture)
+        .env("OPENAI_BASE_URL", "http://unused.local")
+        .arg("--skip-git-repo-check")
+        .arg("-C")
+        .arg(env!("CARGO_MANIFEST_DIR"))
+        .arg("--json")
+        .arg("resume")
+        .arg("--last")
+        .arg(&prompt2)
+        .assert()
+        .success();
+
+    let resumed_path = find_session_file_containing_marker(&sessions_dir, &marker2)
+        .expect("no resumed session file containing marker2");
+    assert_eq!(
+        resumed_path, path,
+        "resume --last should append to existing file"
+    );
+    let content = std::fs::read_to_string(&resumed_path)?;
+    assert!(content.contains(&marker));
+    assert!(content.contains(&marker2));
+    Ok(())
+}
+
 #[test]
 fn exec_resume_by_id_appends_to_existing_file() -> anyhow::Result<()> {
     let test = test_codex_exec();