diff --git a/crates/tinyagents-harness/src/providers/claude_code/driver.rs b/crates/tinyagents-harness/src/providers/claude_code/driver.rs
index 151208556..7c0b4108e 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/driver.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/driver.rs
@@ -68,6 +68,70 @@ fn push_bounded(acc: &mut String, chunk: &str, max_bytes: usize) {
     }
 }
 
+/// Upper bound on the structured stdout error carried into a returned error.
+const STDOUT_ERROR_CAP: usize = 2_048;
+
+/// Redact every `sk-` style key in `text`, wherever it sits (after `=`, `:`,
+/// quotes, URL separators, ...). A match must start at a non-identifier
+/// boundary (so `task-force` is untouched) and run over the key alphabet.
+fn redact_sk_keys(text: &str) -> String {
+    let is_key_char = |c: char| c.is_ascii_alphanumeric() || c == '-' || c == '_';
+    let mut out = String::with_capacity(text.len());
+    let mut rest = text;
+    let mut prev: Option<char> = None;
+    while let Some(pos) = rest.find("sk-") {
+        let before = rest[..pos].chars().next_back().or(prev);
+        let boundary = before.is_none_or(|c| !(c.is_ascii_alphanumeric() || c == '_'));
+        let tail = &rest[pos + 3..];
+        let key_len = tail.find(|c: char| !is_key_char(c)).unwrap_or(tail.len());
+        if boundary && key_len > 0 {
+            out.push_str(&rest[..pos]);
+            out.push_str("[redacted]");
+            prev = Some(']');
+            rest = &tail[key_len..];
+        } else {
+            out.push_str(&rest[..pos + 3]);
+            prev = Some('-');
+            rest = tail;
+        }
+    }
+    out.push_str(rest);
+    out
+}
+
+/// Remove credentials from a CLI-reported message and bound its length.
+/// Redacts the configured API key wherever it occurs (including inside larger
+/// tokens) and any `sk-` style key, then caps the result.
+fn sanitize_cli_message(raw: &str, api_key: Option<&str>) -> String {
+    let mut text = raw.to_string();
+    if let Some(key) = api_key.filter(|key| !key.is_empty()) {
+        text = text.replace(key, "[redacted]");
+    }
+    let redacted = redact_sk_keys(&text);
+    let mut out = String::new();
+    push_bounded(&mut out, &redacted, STDOUT_ERROR_CAP);
+    out
+}
+
+/// Error text for a nonzero CLI exit: the structured stdout error when the
+/// CLI printed one (sanitized and bounded), otherwise the bounded stderr.
+fn nonzero_exit_message(
+    code: Option<i32>,
+    stdout_error: Option<&str>,
+    stderr: &str,
+    api_key: Option<&str>,
+) -> String {
+    match stdout_error.map(|e| sanitize_cli_message(e, api_key)) {
+        Some(message) if !message.is_empty() => {
+            format!("[claude-code][driver] exit {code:?}: {message}")
+        }
+        _ => format!(
+            "[claude-code][driver] exit {code:?} stderr={}",
+            sanitize_cli_message(stderr.trim(), api_key)
+        ),
+    }
+}
+
 use super::bridge::{ChatMessage, ChatResponse, ProviderDelta};
 use super::event_mapper::EventMapper;
 use super::input_builder::build_stdin;
@@ -612,14 +676,25 @@ pub(crate) async fn run_turn(ctx: TurnContext<'_>) -> anyhow::Result<ChatRespons
     let stderr_text = stderr_task.await.unwrap_or_default();
 
     if !status.success() {
+        // The CLI reports most failures (bad auth, unknown model, invalid
+        // session) as a structured `result`/`error` event on stdout and often
+        // leaves stderr empty. Prefer that message; fall back to stderr only
+        // when stdout carried none.
         anyhow::bail!(
-            "[claude-code][driver] exit {:?} stderr={}",
-            status.code(),
-            stderr_text.trim()
+            "{}",
+            nonzero_exit_message(
+                status.code(),
+                mapper.error.as_deref(),
+                &stderr_text,
+                ctx.anthropic_api_key.as_deref(),
+            )
         );
     }
     if let Some(err) = mapper.error.clone() {
-        anyhow::bail!("[claude-code][driver] {}", err);
+        anyhow::bail!(
+            "[claude-code][driver] {}",
+            sanitize_cli_message(&err, ctx.anthropic_api_key.as_deref())
+        );
     }
 
     // Do not make a session durable until Claude has accepted the launch and
diff --git a/crates/tinyagents-harness/src/providers/claude_code/driver_tests.rs b/crates/tinyagents-harness/src/providers/claude_code/driver_tests.rs
index 450ae3308..1ac96fe65 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/driver_tests.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/driver_tests.rs
@@ -350,3 +350,166 @@ fn the_shape_of_the_line_is_reported() {
     assert!(shape("panic: claude-code crashed").contains("non-json"));
     assert!(shape("   ").contains("blank"));
 }
+
+// ---- nonzero exit with a structured stdout error (openhuman#5712) ----
+
+#[cfg(unix)]
+async fn run_fake_claude(script_body: &str, api_key: Option<&str>) -> anyhow::Result<ChatResponse> {
+    use std::os::unix::fs::PermissionsExt;
+    let dir = tempfile::tempdir().expect("tempdir");
+    let bin = dir.path().join("claude");
+    std::fs::write(&bin, format!("#!/bin/sh\ncat >/dev/null\n{script_body}\n")).expect("script");
+    std::fs::set_permissions(&bin, std::fs::Permissions::from_mode(0o755)).expect("chmod");
+    let messages = [ChatMessage::user("hello")];
+    run_turn(TurnContext {
+        bin_path: bin,
+        workspace_dir: dir.path().join("ws"),
+        project_dir: dir.path().join("project"),
+        thread_id: "t-5712".into(),
+        model: "sonnet".into(),
+        append_system_prompt: None,
+        messages: &messages,
+        session_store: Arc::new(SessionStore::open(&dir.path().join("ws"))),
+        stream: None,
+        anthropic_api_key: api_key.map(str::to_string),
+        persist_session: false,
+        mcp_provider: None,
+    })
+    .await
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_surfaces_structured_stdout_error() {
+    let err = run_fake_claude(
+        r#"echo '{"type":"result","subtype":"success","is_error":true,"result":"Invalid API key - please run /login"}'; exit 1"#,
+        None,
+    )
+    .await
+    .expect_err("nonzero exit must fail");
+    let text = err.to_string();
+    assert!(
+        text.contains("Invalid API key - please run /login"),
+        "{text}"
+    );
+    assert!(text.contains("exit Some(1)"), "{text}");
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_surfaces_error_event_message() {
+    let err = run_fake_claude(
+        r#"echo '{"type":"error","error":"model not found"}'; exit 2"#,
+        None,
+    )
+    .await
+    .expect_err("nonzero exit must fail");
+    assert!(err.to_string().contains("model not found"), "{err}");
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_without_stdout_error_falls_back_to_stderr() {
+    let err = run_fake_claude(r#"echo 'boom on stderr' >&2; exit 1"#, None)
+        .await
+        .expect_err("nonzero exit must fail");
+    let text = err.to_string();
+    assert!(text.contains("stderr=boom on stderr"), "{text}");
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_stdout_error_keeps_secrets_out() {
+    let err = run_fake_claude(
+        r#"echo '{"type":"result","is_error":true,"result":"auth failed for key sk-ant-live-key-1234 and my-configured-secret"}'; exit 1"#,
+        Some("my-configured-secret"),
+    )
+    .await
+    .expect_err("nonzero exit must fail");
+    let text = err.to_string();
+    assert!(text.contains("auth failed"), "{text}");
+    assert!(!text.contains("sk-ant-live-key-1234"), "{text}");
+    assert!(!text.contains("my-configured-secret"), "{text}");
+}
+
+#[test]
+fn stdout_error_is_bounded() {
+    let long = "e".repeat(STDOUT_ERROR_CAP * 3);
+    let message = nonzero_exit_message(Some(1), Some(&long), "", None);
+    assert!(message.len() < STDOUT_ERROR_CAP + 128, "{}", message.len());
+}
+
+#[test]
+fn sk_keys_are_redacted_after_separators() {
+    for raw in [
+        "key:sk-ant-abc123 failed",
+        "token=sk-ant-abc123",
+        "https://x.test/?k=sk-ant-abc123&y=1",
+        "\"sk-ant-abc123\"",
+        "ANTHROPIC_API_KEY=sk-ant-abc123,",
+    ] {
+        let out = sanitize_cli_message(raw, None);
+        assert!(!out.contains("abc123"), "{raw} -> {out}");
+        assert!(out.contains("[redacted]"), "{raw} -> {out}");
+    }
+    assert_eq!(
+        sanitize_cli_message("task-force ask-me sk-", None),
+        "task-force ask-me sk-"
+    );
+}
+
+#[test]
+fn configured_key_embedded_in_larger_token_is_redacted() {
+    let out = sanitize_cli_message("bad my-secret_suffix and pre-my-secret.", Some("my-secret"));
+    assert!(!out.contains("my-secret"), "{out}");
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn zero_exit_is_error_result_is_sanitized_and_bounded() {
+    let big = "x".repeat(5_000);
+    let script = format!(
+        r#"echo '{{"type":"result","subtype":"success","is_error":true,"result":"bad key topsecret99 sk-ant-zzz {big}"}}'; exit 0"#
+    );
+    let err = run_fake_claude(&script, Some("topsecret99"))
+        .await
+        .expect_err("is_error must fail");
+    let text = err.to_string();
+    assert!(
+        !text.contains("topsecret99") && !text.contains("sk-ant-zzz"),
+        "{text}"
+    );
+    assert!(text.len() < 2_300, "len {}", text.len());
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_stderr_fallback_is_redacted_and_bounded() {
+    let script = r#"echo "auth failed key=topsecret99 sk-ant-live-1234" >&2; head -c 5000 /dev/zero | tr '\0' x >&2; exit 1"#;
+    let err = run_fake_claude(script, Some("topsecret99"))
+        .await
+        .expect_err("nonzero exit must fail");
+    let text = err.to_string();
+    assert!(
+        !text.contains("topsecret99") && !text.contains("sk-ant-live-1234"),
+        "{text}"
+    );
+    assert!(text.len() < 2_300, "len {}", text.len());
+}
+
+#[cfg(unix)]
+#[tokio::test]
+async fn nonzero_exit_surfaces_object_error_message_sanitized() {
+    let err = run_fake_claude(
+        r#"echo '{"type":"error","error":{"message":"denied for topsecret99 sk-ant-xyz"}}'; exit 2"#,
+        Some("topsecret99"),
+    )
+    .await
+    .expect_err("nonzero exit must fail");
+    let text = err.to_string();
+    assert!(text.contains("denied for"), "{text}");
+    assert!(
+        !text.contains("topsecret99") && !text.contains("sk-ant-xyz"),
+        "{text}"
+    );
+}
diff --git a/crates/tinyagents-harness/src/providers/claude_code/event_mapper.rs b/crates/tinyagents-harness/src/providers/claude_code/event_mapper.rs
index 29648c0c6..2d4f3bf65 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/event_mapper.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/event_mapper.rs
@@ -108,7 +108,7 @@ impl EventMapper {
                 subtype,
                 usage,
                 total_cost_usd,
-                ..
+                raw,
             } => {
                 let mut parsed = usage.as_ref().map(parse_usage);
                 // CC stream emits `total_cost_usd` on the terminal `result`
@@ -120,8 +120,20 @@ impl EventMapper {
                     usage.charged_amount_usd = cost;
                 }
                 self.usage = parsed;
-                if subtype.as_deref() == Some("error") && self.error.is_none() {
-                    self.error = Some("claude reported `result.subtype=error`".into());
+                // A failed turn is reported as `subtype=error` or as
+                // `is_error=true` with the human-readable reason in `result`.
+                let failed = subtype.as_deref() == Some("error")
+                    || raw.get("is_error").and_then(Value::as_bool) == Some(true);
+                if failed && self.error.is_none() {
+                    let reason = raw
+                        .get("result")
+                        .and_then(Value::as_str)
+                        .map(str::trim)
+                        .filter(|reason| !reason.is_empty());
+                    self.error = Some(match reason {
+                        Some(reason) => reason.to_string(),
+                        None => "claude reported `result.subtype=error`".into(),
+                    });
                 }
                 self.finished = true;
                 Vec::new()
diff --git a/crates/tinyagents-harness/src/providers/claude_code/event_mapper_tests.rs b/crates/tinyagents-harness/src/providers/claude_code/event_mapper_tests.rs
index 17d60f4de..aeb673e42 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/event_mapper_tests.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/event_mapper_tests.rs
@@ -160,3 +160,15 @@ fn final_assistant_message_is_skipped() {
     });
     assert!(deltas.is_empty());
 }
+
+#[test]
+fn is_error_result_carries_its_reason_as_the_error() {
+    let mut m = EventMapper::new();
+    m.handle(ClaudeCodeEvent::Result {
+        subtype: Some("success".into()),
+        usage: None,
+        total_cost_usd: None,
+        raw: json!({"type":"result","subtype":"success","is_error":true,"result":"Invalid API key"}),
+    });
+    assert_eq!(m.error.as_deref(), Some("Invalid API key"));
+}
diff --git a/crates/tinyagents-harness/src/providers/claude_code/stream_parser.rs b/crates/tinyagents-harness/src/providers/claude_code/stream_parser.rs
index 2bd8ad508..32c8372c2 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/stream_parser.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/stream_parser.rs
@@ -219,7 +219,11 @@ impl StreamJsonParser {
             "error" => ClaudeCodeEvent::Error {
                 message: v
                     .get("error")
-                    .and_then(Value::as_str)
+                    .and_then(|e| {
+                        e.as_str()
+                            .or_else(|| e.get("message").and_then(Value::as_str))
+                    })
+                    .or_else(|| v.get("message").and_then(Value::as_str))
                     .unwrap_or("claude-code error")
                     .to_string(),
             },
diff --git a/crates/tinyagents-harness/src/providers/claude_code/stream_parser_tests.rs b/crates/tinyagents-harness/src/providers/claude_code/stream_parser_tests.rs
index 1e97518b1..3b6078f7d 100644
--- a/crates/tinyagents-harness/src/providers/claude_code/stream_parser_tests.rs
+++ b/crates/tinyagents-harness/src/providers/claude_code/stream_parser_tests.rs
@@ -243,3 +243,13 @@ fn feed_bytes_still_replaces_genuinely_invalid_bytes() {
         "0xFF must decode to the replacement character, not vanish"
     );
 }
+
+#[test]
+fn error_event_with_object_payload_keeps_its_message() {
+    let mut parser = StreamJsonParser::new();
+    let events = parser.feed("{\"type\":\"error\",\"error\":{\"message\":\"rate limited\"}}\n");
+    assert!(matches!(
+        events.as_slice(),
+        [ClaudeCodeEvent::Error { message }] if message == "rate limited"
+    ));
+}
diff --git a/crates/tinyagents-integration-tests/tests/dependency_boundary.rs b/crates/tinyagents-integration-tests/tests/dependency_boundary.rs
index e9d653e75..c4f0705dd 100644
--- a/crates/tinyagents-integration-tests/tests/dependency_boundary.rs
+++ b/crates/tinyagents-integration-tests/tests/dependency_boundary.rs
@@ -43,11 +43,15 @@ const KNOWN_GENERIC_CLAUDE_CODE_CHAT_MESSAGE_DEBT: &[(&str, usize)] = &[
     ),
     (
         "crates/tinyagents-harness/src/providers/claude_code/driver.rs",
-        71,
+        135,
     ),
     (
         "crates/tinyagents-harness/src/providers/claude_code/driver.rs",
-        240,
+        304,
+    ),
+    (
+        "crates/tinyagents-harness/src/providers/claude_code/driver_tests.rs",
+        363,
     ),
     (
         "crates/tinyagents-harness/src/providers/claude_code/input_builder.rs",