From 80e2ca7c392b345d759dca58e05c2b8944bb73fe Mon Sep 17 00:00:00 2001
From: Dylan Haskins <dylan@haskins.co.nz>
Date: Sat, 19 Sep 2026 15:04:42 +1200
Subject: [PATCH] fix: don't blanket-disable Claude tool-name prefix for
 non-Anthropic providers (#13856)

* fix: don't blanket-disable Claude tool-name prefix for non-Anthropic providers

The Issue #199/#618 fix that disables the proxy_ tool-name prefix for
requests landing in the general (non-CC-bridge, non-passthrough)
translation branch was scoped to "targetFormat === Claude", but that
condition is true for any provider translating into Claude's wire
format, not just genuine first-party Anthropic traffic. In practice
this let ordinary third-party tool names (e.g. GitHub Copilot's own
client-executed "web_fetch" function tool) pass through unprefixed
and collide with Claude's reserved tool namespace, getting rejected
upstream ("rejected tool(s): web_fetch") for any gh/claude-* model.

#618's actual traffic was real Claude Code (provider "claude") landing
in this fallback branch, so scoping the disable to provider === "claude"
(the same check already used a few lines above in the sibling
isClaudePassthrough branch) keeps that fix intact while restoring
correct prefixing for every other provider that merely targets
Claude's wire format.

Fixes #13835.

* docs: add changelog fragment for #13856
---
 ...56-github-copilot-web-fetch-tool-prefix.md |  1 +
 open-sse/handlers/chatCore.ts                 | 15 +++++++-
 tests/unit/chatcore-translation-paths.test.ts | 34 +++++++++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 changelog.d/fixes/13856-github-copilot-web-fetch-tool-prefix.md

diff --git a/changelog.d/fixes/13856-github-copilot-web-fetch-tool-prefix.md b/changelog.d/fixes/13856-github-copilot-web-fetch-tool-prefix.md
new file mode 100644
index 0000000000..762205297e
--- /dev/null
+++ b/changelog.d/fixes/13856-github-copilot-web-fetch-tool-prefix.md
@@ -0,0 +1 @@
+- **fix(translator):** Third-party tool names (e.g. GitHub Copilot's own `web_fetch` function tool) are no longer sent unprefixed to Claude-wire-format providers outside genuine first-party Anthropic traffic, fixing a `rejected tool(s): web_fetch` 400 for any `gh/claude-*` model ([#13856](https://github.com/diegosouzapw/OmniRoute/pull/13856)) — thanks @dylanhaskins
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 098eb3a26d..7434983c79 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -2494,8 +2494,21 @@ export async function handleChatCore({
       // conflicts with Claude OAuth tools, but in the passthrough path the tools
       // are already in Claude format. Applying the prefix turns "Bash" into
       // "proxy_Bash", which Claude rejects ("No such tool available: proxy_Bash").
+      //
+      // #618's actual traffic was real Claude Code talking to first-party Anthropic
+      // (provider "claude") reaching this fallback branch instead of the dedicated
+      // Claude Code bridge/passthrough branches above. Scoping the disable to
+      // `provider === "claude"` keeps that fix intact while no longer blanket-applying
+      // it to every other provider that merely targets Claude's wire format — a
+      // third-party provider's own ordinary (non-Claude-native) tool names, e.g.
+      // GitHub Copilot's own client-executed "web_fetch" tool, were passing through
+      // unprefixed here and colliding with Claude's reserved tool namespace, since
+      // they were never "already in Claude format" the way this comment assumes.
+      // See #13835.
       if (targetFormat === FORMATS.CLAUDE) {
-        translatedBody._disableToolPrefix = true;
+        if (provider === "claude") {
+          translatedBody._disableToolPrefix = true;
+        }
         normalizeClaudeUpstreamMessages(translatedBody);
       }
 
diff --git a/tests/unit/chatcore-translation-paths.test.ts b/tests/unit/chatcore-translation-paths.test.ts
index 1d5a4eea21..12bab97f0f 100644
--- a/tests/unit/chatcore-translation-paths.test.ts
+++ b/tests/unit/chatcore-translation-paths.test.ts
@@ -1832,6 +1832,40 @@ test("chatCore sets Claude tool prefix disabling, strips empty Anthropic text bl
     ["hello"]
   );
 });
+// #13835: a third-party provider's own ordinary tool name (GitHub Copilot's client-executed
+// "web_fetch" function tool) must still get the proxy_ prefix even though this request lands
+// in the same general (non-claude-passthrough) branch as the "claude" provider test above —
+// only genuine first-party Anthropic traffic (provider "claude") should skip prefixing.
+test("chatCore still prefixes ordinary third-party tool names for non-Anthropic providers targeting Claude", async () => {
+  const { call } = await invokeChatCore({
+    provider: "github",
+    model: "claude-haiku-4.5",
+    endpoint: "/v1/chat/completions",
+    credentials: { apiKey: "gh-key", providerSpecificData: {} },
+    body: {
+      model: "github/claude-haiku-4.5",
+      messages: [{ role: "user", content: "fetch a url" }],
+      tools: [
+        {
+          type: "function",
+          function: {
+            name: "web_fetch",
+            description: "Fetches a URL from the internet.",
+            parameters: {
+              type: "object",
+              properties: { url: { type: "string" } },
+              required: ["url"],
+            },
+          },
+        },
+      ],
+    },
+    responseFormat: "claude",
+  });
+
+  assert.equal(call.body.tools[0].name, "proxy_web_fetch");
+  assert.equal(call.body._toolNameMap, undefined);
+});
 test("chatCore restores prefixed Claude passthrough tool names in upstream responses", async () => {
   const { result } = await invokeChatCore({
     provider: "claude",