What is the ADS file format?
An .ads file is an Ada package specification - the interface half of an Ada package. Under the GNAT compiler naming convention (the de-facto standard across the Ada ecosystem), every Ada package is split across two files: the specification in .ads and the implementation body in an .adb file.
The .ads file declares what is publicly visible to other compilation units: types, subtypes, constants, subprogram signatures (procedure and function profiles), task types, and generic parameters. It contains no executable code or subprogram bodies - only the declarations that form the contract other packages depend on.
Syntactically, a specification opens with package <Name> is and closes with end <Name>;. Context clauses (with and use statements) at the top list the packages the spec depends on. Ada comments begin with --.
The format is plain UTF-8 (or ASCII) text with MIME type text/x-ada. There are no magic bytes; files are identified by their Ada source structure. The Ada dialect in use - Ada 83, 95, 2005, 2012, or Ada 2022 (ISO/IEC 8652:2023) - is controlled by GNAT compiler flags such as -gnat2022 or a source-level pragma Ada_2022.
.ads files are opened in GNAT Studio, Visual Studio Code with the Ada/SPARK extension, or any plain-text editor. The GNAT toolchain compiles them alongside their .adb bodies; build coordination is handled by a .gpr project file.
Security & safety
RISK: LOWAn .ads file is plain Ada source text and cannot run by itself, so opening one to read it is safe. The normal caution for any source code applies: review code from untrusted sources before compiling and running it, since the surrounding project will produce an executable. There is no DRM, encryption or special handling - treat it like any text/code file. Install the GNAT toolchain only from official sources (AdaCore/GCC).
Format details
in a nutshell- Advertising / ad-serving file - Some ad systems and AdSense-related tooling use .ads for ad-content or ad-tag files; unrelated to Ada source.
- Generic 'address book' / settings export - A few legacy programs save settings or address data with an .ads extension; identify by the originating application.
Programs that open ADS files
Windows3 apps
Notepad
Open-source Open the .ads as text to read or edit it; set language to Ada for syntax highlighting. 
GNAT Studio
Open-source Open the Ada project (.gpr) or the .ads file directly for syntax highlighting, navigation and building with the GNAT toolchain.
Visual Studio Code (Ada extension)
Free Install AdaCore's Ada extension, then open the .ads for highlighting, navigation and the Ada Language Server. 
macOS3 apps
Visual Studio Code (Ada extension)
Free Open the .ads with the Ada extension for highlighting and navigation.
Any text editor (TextEdit/BBEdit)
Built-in Open the .ads as plain text to read the package interface. 
Linux3 apps
GNAT (GCC Ada) + gprbuild
Open-source Read the .ads in any editor; build with gnatmake/gprbuild from the GNAT toolchain (gcc -c file.ads to syntax-check the spec).
GNAT Studio
Open-source Full Ada IDE: open the project and the .ads for editing, navigation and building. Technical details
deep spec| File type | Plain-text Ada source - the specification (interface) half of an Ada package |
| MIME type | text/x-ada (also accepted as text/plain) |
| Text encoding | UTF-8 or ASCII; Latin-1 historically; GNAT encoding selectable via -gnatiX flag |
| Binary signature | None - plain text; identified by Ada keyword structure (package <Name> is … end <Name>;) |
| Typical file size | 1 KB - 100 KB (spec-only declarations; bodies live in the companion .adb) |
| GNAT naming rule | Base name must match the package name, lowercased with dots replaced by dashes - e.g., Ada.Containers maps to ada-containers.ads |
| Structure | Optional with/use context clauses, then package <Name> is, public declarations, optional private section, end <Name>; |
| Content constraint | Declarations only - types, subtypes, constants, subprogram profiles, generics; no executable code or bodies |
| Visibility model | Public part visible to all dependent packages; private section accessible only within the package and its children |
| Language dialect | Ada 83 / 95 / 2005 / 2012 / 2022 selected via GNAT -gnatXX flags or pragma Ada_XXXX in source |
| Compiled output | GNAT produces a .ali (Ada Library Information) file and object file (.o) for each compiled .ads/.adb pair |
| Build coordination | GNAT Project file (.gpr) lists .ads sources and controls per-unit compilation switches |
| Platform support | Cross-platform: Linux, Windows, macOS, and embedded targets (VxWorks, RTEMS, bare-metal ARM, RISC-V) |
| Compiler ecosystem | GNAT (GCC Ada front-end, AdaCore), GNAT LLVM, PTC ObjectAda, Green Hills MULTI Ada |
| Released | 1980 (Ada language); .ads/.adb naming from GNAT (early 1990s) |
| Latest version | Ada 2022 (ISO/IEC 8652:2023) |
| Open standard | Yes · royalty-free |
| Specification | www.adaic.org |
ADS conversions
Community Q&A
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