From b818378a33f9bc5c41ae90ec2188f9b3fee364fd Mon Sep 17 00:00:00 2001
From: Steven Enamakel <enamakel@tinyhumans.ai>
Date: Fri, 10 Jul 2026 21:11:15 +0000
Subject: [PATCH] harness(openai): surface the full transport error source
 chain
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A reqwest send failure renders only its outermost context in Display (e.g.
"error sending request for url (...)"), while the actionable cause — a
"connection refused (os error 111)" errno, a TLS failure, a DNS error — lives
one or more links down the source() chain. send_checked stringified just {e},
so a host classifying on the TinyAgentsError::Model message (loopback-offline
detection, transport retryability) could not see it.

Add error_source_chain(err) which walks the full source() chain joined by ": ",
and use it when mapping the send failure. Unit-tested; fmt + clippy clean.
---
 src/harness/providers/openai/mod.rs       |  2 +-
 src/harness/providers/openai/test.rs      | 30 +++++++++++++++++++++++
 src/harness/providers/openai/transport.rs | 29 ++++++++++++++++++++--
 3 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/src/harness/providers/openai/mod.rs b/src/harness/providers/openai/mod.rs
index 787eddf..df50a41 100644
--- a/src/harness/providers/openai/mod.rs
+++ b/src/harness/providers/openai/mod.rs
@@ -75,7 +75,7 @@ use convert::*;
 use sse::*;
 #[cfg(test)]
 use transport::{
-    auth_headers, effective_temperature, glob_match, merge_provider_options,
+    auth_headers, effective_temperature, error_source_chain, glob_match, merge_provider_options,
     merge_system_into_user, request_timeout,
 };
 
diff --git a/src/harness/providers/openai/test.rs b/src/harness/providers/openai/test.rs
index 5ae6268..42147e6 100644
--- a/src/harness/providers/openai/test.rs
+++ b/src/harness/providers/openai/test.rs
@@ -1475,3 +1475,33 @@ fn merge_provider_options_prefers_overrides_and_handles_nulls() {
         json!(["top_k", 40])
     );
 }
+
+#[test]
+fn error_source_chain_walks_all_causes() {
+    use std::fmt;
+    // A transport error whose outer Display hides the actionable errno one link
+    // down the `source()` chain (the reqwest/hyper shape).
+    #[derive(Debug)]
+    struct Outer(std::io::Error);
+    impl fmt::Display for Outer {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            write!(f, "error sending request for url (http://localhost:1234/)")
+        }
+    }
+    impl std::error::Error for Outer {
+        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+            Some(&self.0)
+        }
+    }
+    let io = std::io::Error::new(
+        std::io::ErrorKind::ConnectionRefused,
+        "connection refused (os error 111)",
+    );
+    let chain = error_source_chain(&Outer(io));
+    assert!(chain.contains("error sending request for url (http://localhost:1234/)"));
+    assert!(chain.contains("connection refused (os error 111)"));
+    assert_eq!(
+        chain,
+        "error sending request for url (http://localhost:1234/): connection refused (os error 111)"
+    );
+}
diff --git a/src/harness/providers/openai/transport.rs b/src/harness/providers/openai/transport.rs
index 10cdb90..c969125 100644
--- a/src/harness/providers/openai/transport.rs
+++ b/src/harness/providers/openai/transport.rs
@@ -742,8 +742,12 @@ impl OpenAiModel {
         url: &str,
     ) -> Result<reqwest::Response> {
         let response = builder.send().await.map_err(|e| {
-            let error =
-                self.provider_error(format!("{what} to {url} failed: {e}"), None, None, None);
+            let error = self.provider_error(
+                format!("{what} to {url} failed: {}", error_source_chain(&e)),
+                None,
+                None,
+                None,
+            );
             TinyAgentsError::Model(self.provider_failure_message(&error))
         })?;
 
@@ -853,6 +857,27 @@ pub(super) fn request_timeout(timeout_ms: Option<u64>, streaming: bool) -> Optio
     }
 }
 
+/// Renders an error together with its full `source()` cause chain, joined by
+/// `": "`.
+///
+/// `reqwest`/`hyper` transport errors surface only their outermost context in
+/// `Display` (e.g. `error sending request for url (...)`), while the actionable
+/// detail — `connection refused (os error 111)`, a TLS failure, a DNS error —
+/// lives one or more links down the `source()` chain. A plain `{e}` drops it, so
+/// callers that classify on the message (loopback-offline detection, transport
+/// retryability) never see it. Walking the chain here keeps the whole shape in
+/// the `TinyAgentsError::Model` string. Pure, so it is unit-testable.
+pub(super) fn error_source_chain(err: &dyn std::error::Error) -> String {
+    let mut rendered = err.to_string();
+    let mut source = err.source();
+    while let Some(inner) = source {
+        rendered.push_str(": ");
+        rendered.push_str(&inner.to_string());
+        source = inner.source();
+    }
+    rendered
+}
+
 /// Merges baked `defaults` under a request's own `overrides` provider options.
 ///
 /// Keys present in `overrides` win over `defaults`. A `Null` on either side