diff --git a/R/content.R b/R/content.R
index 025926184..4db2adab1 100644
--- a/R/content.R
+++ b/R/content.R
@@ -202,6 +202,8 @@ show_image <- function(x) {
 #' @param arguments Named list of arguments to call the function with.
 #' @param tool ellmer automatically matches a tool request to the tools defined
 #'   for the chatbot. If `NULL`, the request did not match a defined tool.
+#' @param extra Provider-specific metadata associated with the tool request.
+#'   Automatically managed by \pkg{ellmer}.
 ContentToolRequest <- new_class(
   "ContentToolRequest",
   parent = Content,
@@ -209,7 +211,8 @@ ContentToolRequest <- new_class(
     id = prop_string(),
     name = prop_string(),
     arguments = class_list,
-    tool = NULL | ToolDef
+    tool = NULL | ToolDef,
+    extra = class_list
   )
 )
 method(format, ContentToolRequest) <- function(
diff --git a/R/provider-google.R b/R/provider-google.R
index 65110504a..db8eef6e5 100644
--- a/R/provider-google.R
+++ b/R/provider-google.R
@@ -327,10 +327,14 @@ method(value_turn, ProviderGoogleGemini) <- function(
         ContentText(content$text)
       }
     } else if (has_name(content, "functionCall")) {
+      extra <- compact(list(
+        thoughtSignature = content$thoughtSignature
+      ))
       ContentToolRequest(
         content$functionCall$name,
         content$functionCall$name,
-        content$functionCall$args
+        content$functionCall$args,
+        extra = extra
       )
     } else if (has_name(content, "inlineData")) {
       ContentImageInline(
@@ -456,12 +460,13 @@ method(as_json, list(ProviderGoogleGemini, ContentToolRequest)) <- function(
   x,
   ...
 ) {
-  list(
+  compact(list(
     functionCall = list(
       name = x@id,
       args = x@arguments
-    )
-  )
+    ),
+    thoughtSignature = x@extra$thoughtSignature
+  ))
 }
 
 # https://ai.google.dev/api/caching#FunctionResponse
diff --git a/man/Content.Rd b/man/Content.Rd
index 598e3992c..b13bed733 100644
--- a/man/Content.Rd
+++ b/man/Content.Rd
@@ -26,7 +26,8 @@ ContentToolRequest(
   id = stop("Required"),
   name = stop("Required"),
   arguments = list(),
-  tool = NULL
+  tool = NULL,
+  extra = list()
 )
 
 ContentToolResult(value = NULL, error = NULL, extra = list(), request = NULL)
@@ -60,14 +61,14 @@ Automatically managed by \pkg{ellmer}.}
 \item{tool}{ellmer automatically matches a tool request to the tools defined
 for the chatbot. If \code{NULL}, the request did not match a defined tool.}
 
+\item{extra}{Additional data.}
+
 \item{value}{The results of calling the tool function, if it succeeded.}
 
 \item{error}{The error message, as a string, or the error condition thrown
 as a result of a failure when calling the tool function. Must be \code{NULL}
 when the tool call is successful.}
 
-\item{extra}{Additional data.}
-
 \item{request}{The \link{ContentToolRequest} associated with the tool result,
 automatically added by \pkg{ellmer} when evaluating the tool call.}