diff --git a/CHANGELOG.md b/CHANGELOG.md
index cdd7f09c85..eaa3633edb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 * ![Enhancement][badge-enhancement] The padding of the various container elements in the HTML style has been reduced, to improve the look of the generated HTML pages. ([#1814][github-1814], [#1818][github-1818])
 * ![Bugfix][badge-bugfix] Build failures now only show fatal errors, rather than all errors. ([#1816][github-1816])
+* ![Bugfix][badge-bugfix] Disable git terminal prompt when detecting remote HEAD branch for ssh remotes, and allow ssh-agent authentication (by appending rather than overriding ENV). ([#1821][github-1821])
 
 ## Version `v0.27.17`
 
@@ -1032,6 +1033,7 @@
 [github-1814]: https://github.com/JuliaDocs/Documenter.jl/issues/1814
 [github-1816]: https://github.com/JuliaDocs/Documenter.jl/pull/1816
 [github-1818]: https://github.com/JuliaDocs/Documenter.jl/pull/1818
+[github-1821]: https://github.com/JuliaDocs/Documenter.jl/pull/1821
 <!-- end of issue link definitions -->
 
 [julia-38079]: https://github.com/JuliaLang/julia/issues/38079
diff --git a/src/Documenter.jl b/src/Documenter.jl
index b4bd6d4d47..4f0302d14a 100644
--- a/src/Documenter.jl
+++ b/src/Documenter.jl
@@ -532,9 +532,12 @@ function deploydocs(;
 
     # Try to figure out default branch (see #1443 and #1727)
     if devbranch === nothing
+        env = copy(ENV)
+        env["GIT_TERMINAL_PROMPT"] = "0"
+        env["GIT_SSH_COMMAND"] = get(env, "GIT_SSH_COMMAND", "ssh -o \"BatchMode yes\"")
         str = try
             read(pipeline(ignorestatus(
-                setenv(`git remote show origin`, ["GIT_TERMINAL_PROMPT=0"]; dir=root)
+                setenv(`git remote show origin`, env; dir=root)
             ); stderr=devnull), String)
         catch
             ""