[MCP] Allow specifying custom headers with streamable http servers (#5241)

This adds two new config fields to streamable http mcp servers:
`http_headers`: a map of key to value
`env_http_headers` a map of key to env var which will be resolved at
request time

All headers will be passed to all MCP requests to that server just like
authorization headers.

There is a test ensuring that headers are not passed to other servers.

Fixes #5180

codex-rs/rmcp-client/src/utils.rs

@@ -6,6 +6,10 @@ use anyhow::Context;
 use anyhow::Result;
 use anyhow::anyhow;
 use mcp_types::CallToolResult;
+use reqwest::ClientBuilder;
+use reqwest::header::HeaderMap;
+use reqwest::header::HeaderName;
+use reqwest::header::HeaderValue;
 use rmcp::model::CallToolResult as RmcpCallToolResult;
 use rmcp::service::ServiceError;
 use serde_json::Value;
@@ -78,6 +82,75 @@ pub(crate) fn create_env_for_mcp_server(
         .collect()
 }
 
+pub(crate) fn build_default_headers(
+    http_headers: Option<HashMap<String, String>>,
+    env_http_headers: Option<HashMap<String, String>>,
+) -> Result<HeaderMap> {
+    let mut headers = HeaderMap::new();
+
+    if let Some(static_headers) = http_headers {
+        for (name, value) in static_headers {
+            let header_name = match HeaderName::from_bytes(name.as_bytes()) {
+                Ok(name) => name,
+                Err(err) => {
+                    tracing::warn!("invalid HTTP header name `{name}`: {err}");
+                    continue;
+                }
+            };
+            let header_value = match HeaderValue::from_str(value.as_str()) {
+                Ok(value) => value,
+                Err(err) => {
+                    tracing::warn!("invalid HTTP header value for `{name}`: {err}");
+                    continue;
+                }
+            };
+            headers.insert(header_name, header_value);
+        }
+    }
+
+    if let Some(env_headers) = env_http_headers {
+        for (name, env_var) in env_headers {
+            if let Ok(value) = env::var(&env_var) {
+                if value.trim().is_empty() {
+                    continue;
+                }
+
+                let header_name = match HeaderName::from_bytes(name.as_bytes()) {
+                    Ok(name) => name,
+                    Err(err) => {
+                        tracing::warn!("invalid HTTP header name `{name}`: {err}");
+                        continue;
+                    }
+                };
+
+                let header_value = match HeaderValue::from_str(value.as_str()) {
+                    Ok(value) => value,
+                    Err(err) => {
+                        tracing::warn!(
+                            "invalid HTTP header value read from {env_var} for `{name}`: {err}"
+                        );
+                        continue;
+                    }
+                };
+                headers.insert(header_name, header_value);
+            }
+        }
+    }
+
+    Ok(headers)
+}
+
+pub(crate) fn apply_default_headers(
+    builder: ClientBuilder,
+    default_headers: &HeaderMap,
+) -> ClientBuilder {
+    if default_headers.is_empty() {
+        builder
+    } else {
+        builder.default_headers(default_headers.clone())
+    }
+}
+
 #[cfg(unix)]
 pub(crate) const DEFAULT_ENV_VARS: &[&str] = &[
     "HOME",