Copilot Connection Reset Behind Zscaler: Diagnose the Proxy Safely

Official documentation checked October 14, 2026.

A “connection reset by peer” error while using Copilot behind Zscaler means a connection was closed unexpectedly. It does not, by itself, identify Zscaler as the source or prove that a root certificate is missing. A blocked destination, interrupted secure WebSocket, inspection incompatibility, timeout, or another network component can produce similar symptoms.

Use the sequence below with your network administrator. Do not disable Zscaler Client Connector, endpoint protection, or certificate validation to make the message disappear.

1. Identify the failing request before changing proxy settings

Record the affected experience: Copilot Chat in the browser, the Copilot app, or a feature inside Word, Excel, Outlook, or Teams. Include the client version, account type, error text, timestamp with time zone, and whether the failure happens at sign-in, when submitting a prompt, or partway through a response.

1. Save your work, then retry once with a harmless test prompt. Record whether the page itself loads and whether ordinary Microsoft 365 functions still work.
2. Ask the network team to correlate that time and device with the Zscaler transaction or connection logs. Capture the destination, matched rule, forwarding path, and recorded failure reason; a screenshot of the browser error alone cannot establish these.
3. If the team approves a comparison, repeat the same test on a known-good managed device or an approved test network. Keep the account and client as similar as possible.

A successful comparison narrows the investigation; it is not permission to move confidential prompts to an unmanaged network. Browser and desktop results should be logged separately because they may use different policies and connection paths.

2. Check current Microsoft endpoints and secure WebSocket connectivity

Microsoft’s current Copilot network requirements cover Microsoft 365 endpoints and secure WebSocket connections, including *.office.com and *.cloud.microsoft. Microsoft requires the *.cloud.microsoft wildcard rather than an improvised list of individual hosts. The Copilot app uses copilot.cloud.microsoft.

Ask the administrator to use the connectivity tests linked from that requirements page and compare the result with gateway logs. A normal HTTPS page opening does not establish that a long-lived WSS connection works. Microsoft specifically identifies interfering TLS inspection and aggressive proxy timeouts as possible failure causes.

Use the organization’s actual forwarding configuration when interpreting the result. Do not replace its proxy or PAC settings with a generic Zscaler hostname and port copied from a troubleshooting article. A tunnel deployment, explicit proxy, and application-specific route should not be treated as interchangeable.

If a response is merely generic or omits work documents, investigate account, entitlement, and document access separately. Poor grounding is not evidence that a gateway stripped an authorization header.

3. Investigate certificates only when the evidence points to trust

If the client reports an untrusted issuer or another certificate-chain error, have IT inspect the certificate presented for the actual failing destination. Distinguish an expired certificate, a wrong host name, and a missing trusted issuer. Installing a root certificate will not repair all three.

Zscaler’s TLS inspection deployment guidance calls for planned certificate distribution and testing. Applications can use different trust stores, and certificate pinning or mutual TLS can make interception unsuitable. The relevant question is whether this specific application and connection support the configured inspection policy.

• Obtain any required certificate only through your organization’s authenticated IT distribution process. Have IT verify its identity and fingerprint independently.
• Use the approved device-management deployment and correct trust store. Do not import an unknown certificate from a forum, a pop-up, or an error-page download.
• Test the correction on the affected pilot device before wider deployment. Record the certificate and policy change so it can be reviewed or rolled back.

Trusting a new root affects more than Copilot. A user should not add a root to the machine-wide store simply because a connection was reset.

4. Apply a scoped repair and retest the original operation

Once logs identify the cause, the network team can correct the relevant destination rule, supported protocol handling, timeout, or certificate deployment. Where inspection is incompatible, any exception needs a security review of the affected traffic and compensating controls. It should not become a blanket bypass for all Microsoft services or all users.

Zscaler’s Client Connector operations guidance recommends defined bypass procedures and isolated test profiles. Follow that controlled approach, with a named owner, limited test population, expiry or review point, and rollback plan.

Retest the same client and harmless prompt, then the originally failing feature using approved data. Confirm that the reset is gone and required access controls still apply. A successful chat alone does not prove that gateway inspection or DLP covers every Office integration.

If the failure remains, escalate with sanitized timestamps, destinations, rule results, client versions, and diagnostic outcomes. Network traces can contain cookies, tokens, URLs, and document information: collect and share them only through the organization’s approved support channel.