@@ -24,6 +24,14 @@ describe("NativeToolCallParser.parseDynamicMcpTool name for API history", () =>
 		expect(result?.name).toBe(declared)
 	})
 
+	it("parses the declared name of a tool on a server with a very long name", () => {
+		const declared = buildMcpToolName("s".repeat(60), TOOL)
+
+		const result = NativeToolCallParser.parseDynamicMcpTool({ id: "toolu_4", name: declared, arguments: "{}" })
+
+		expect(result).toMatchObject({ type: "mcp_tool_use", name: declared })
+	})
+
 	it("keeps a short name verbatim", () => {
 		const result = NativeToolCallParser.parseDynamicMcpTool({
 			id: "toolu_3",
 
@@ -18,6 +18,52 @@ describe("long MCP tool names", () => {
 		expect(name.slice(55)).toMatch(/^_[0-9a-f]{8}$/)
 	})
 
+	it.each([
+		{ toolLength: 50, shortened: false },
+		{ toolLength: 51, shortened: true },
+	])(
+		"treats a $toolLength-character tool as shortened: $shortened at the 64-character limit",
+		({ toolLength, shortened }) => {
+			const tool = "t".repeat(toolLength)
+			const uncut = `mcp--weather--${tool}`
+			const name = buildMcpToolName("weather", tool)
+
+			expect(uncut).toHaveLength(14 + toolLength)
+			expect(name).toHaveLength(64)
+			if (shortened) {
+				expect(name).toMatch(/_[0-9a-f]{8}$/)
+			} else {
+				expect(name).toBe(uncut)
+			}
+		},
+	)
+
+	describe("with a long server name", () => {
+		it.each([46, 47, 48, 49, 50, 70])("keeps both separators for a %i-character server name", (serverLength) => {
+			const server = `s${"x".repeat(serverLength - 1)}`
+			const name = buildMcpToolName(server, TOOL)
+			const parsed = parseMcpToolName(name)
+
+			expect(name).toHaveLength(64)
+			expect(name).toMatch(/_[0-9a-f]{8}$/)
+			expect(parsed).not.toBeNull()
+			expect(server.startsWith(parsed!.serverName)).toBe(true)
+			expect(parsed!.toolName.length).toBeGreaterThan(0)
+			expect(resolveMcpToolByName([{ name: server, tools: [{ name: TOOL }] }], name)).toEqual({
+				serverName: server,
+				toolName: TOOL,
+			})
+		})
+
+		it("drops a hyphen at the cut so it does not merge with the separator", () => {
+			const server = `${"a".repeat(46)}-${"b".repeat(10)}`
+			const name = buildMcpToolName(server, TOOL)
+
+			expect(name).not.toContain("---")
+			expect(parseMcpToolName(name)?.serverName).toBe("a".repeat(46))
+		})
+	})
+
 	it("is deterministic and keeps tools that share a 55-character prefix distinct", () => {
 		const first = buildMcpToolName(SERVER, `${TOOL}_a`)
 		const second = buildMcpToolName(SERVER, `${TOOL}_b`)
 
@@ -151,7 +151,17 @@ export function buildMcpToolName(serverName: string, toolName: string): string {
 	}
 
 	const hash = createHash("sha256").update(fullName).digest("hex").slice(0, HASH_SUFFIX_LENGTH)
-	return `${fullName.slice(0, MAX_TOOL_NAME_LENGTH - HASH_SUFFIX_LENGTH - 1)}_${hash}`
+	const budget = MAX_TOOL_NAME_LENGTH - HASH_SUFFIX_LENGTH - 1
+	const prefix = `${MCP_TOOL_PREFIX}${MCP_TOOL_SEPARATOR}`
+	// Cap the server part so the second separator and at least one tool character survive;
+	// otherwise the shortened name no longer parses as mcp--server--tool and the call is dropped.
+	// A trailing hyphen is removed so it cannot merge with the separator.
+	const maxServerLength = budget - prefix.length - MCP_TOOL_SEPARATOR.length - 1
+	const server = sanitizeMcpName(serverName).slice(0, maxServerLength).replace(/-+$/, "")
+	const head = `${prefix}${server}${MCP_TOOL_SEPARATOR}`
+	const tool = sanitizeMcpName(toolName).slice(0, budget - head.length)
+
+	return `${head}${tool}_${hash}`
 }
 
 /**