diff --git a/README.md b/README.md
index 835d6a1..800f9bd 100644
--- a/README.md
+++ b/README.md
@@ -104,13 +104,18 @@ PureSharp uses Roslyn's standard `.editorconfig` mechanism for configuring diagn
 
 | Diagnostic ID | Category | Title | Default Severity |
 |---|---|---|---|
-| **RT0001** | Purity | Static mutable field access | Error |
-| **RT0002** | Purity | Non-pure method call | Error |
-| **RT0003** | Purity | I/O operation | Error |
-| **LVP0001** | Purity | Reassignment to immutable local variable | Error |
-| **LVP0002** | Purity | Immutable local variable missing initializer | Error |
-| **LVP0003** | Naming | Naming suggestion for effectively immutable variables | Warning |
-| **FIF0001** | FluentIf | FluentIf chain not terminated with .Else() | Error |
+| **RT0001** | Purity | Static field access in [PureMethod] | Error |
+| **RT0002** | Purity | Non-pure method call in [PureMethod] | Error |
+| **RT0003** | Purity | I/O operation in [PureMethod] | Error |
+| **LVP0001** | Purity | Reassignment to immutable local variable prohibited | Error |
+| **LVP0002** | Purity | Mandatory initialization of immutable local variable | Error |
+| **LVP0003** | Naming | Suggestion to apply naming convention for immutable local variable | Warning |
+| **FIF0001** | FluentIf | FluentIf chain termination check | Error |
+
+The complete diagnostic contract — full message text, target conditions, the v1.0 public
+contract surface, ID naming and category policy, and the compatibility rules for adding
+or changing a diagnostic — is defined in [`docs/DIAGNOSTICS.md`](docs/DIAGNOSTICS.md),
+which is the single source of truth.
 
 ### Configuration Example
 
diff --git a/docs/DIAGNOSTICS.md b/docs/DIAGNOSTICS.md
new file mode 100644
index 0000000..4cb03d7
--- /dev/null
+++ b/docs/DIAGNOSTICS.md
@@ -0,0 +1,241 @@
+# PureSharp Diagnostic Catalog (SSOT)
+
+**This document is the single source of truth for PureSharp's diagnostic contract.**
+
+Where this catalog, the analyzer source, and the README disagree, this catalog defines
+the intent and the other two must be corrected to match it.
+
+- Applies to: PureSharp v1.0.0
+- Diagnostics are configured through Roslyn's standard `.editorconfig` mechanism
+  (`dotnet_diagnostic.<ID>.severity`). See the README for configuration examples.
+
+## Catalog
+
+Every diagnostic PureSharp can report is listed here. There are no unlisted diagnostics.
+
+### RT — Referential Transparency
+
+Emitted by `ReferentialTransparencyAnalyzer`. These apply **only inside methods marked
+with `[PureMethod]`**.
+
+| Field | Value |
+|---|---|
+| **ID** | `RT0001` |
+| **Category** | `Purity` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | Static field access in `[PureMethod]` |
+| **Message** | Cannot access static, non-readonly field `'{1}'` within `[PureMethod]` method `'{0}'` |
+| **Description** | Referentially transparent methods must not access static mutable state. |
+| **Target** | Field reference where the field is `static` and neither `readonly` nor `const` |
+
+| Field | Value |
+|---|---|
+| **ID** | `RT0002` |
+| **Category** | `Purity` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | Non-pure method call in `[PureMethod]` |
+| **Message** | Cannot call non-pure method `'{1}'` within `[PureMethod]` method `'{0}'` |
+| **Description** | Referentially transparent methods can only call methods marked with the `[PureMethod]` attribute or known pure types. |
+| **Target** | Invocation of a method that is neither `[PureMethod]`-marked nor on a known-pure type |
+
+| Field | Value |
+|---|---|
+| **ID** | `RT0003` |
+| **Category** | `Purity` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | I/O operation in `[PureMethod]` |
+| **Message** | Cannot perform I/O operations within `[PureMethod]` method `'{0}'` |
+| **Description** | Referentially transparent methods must not perform I/O operations such as logging, file access, or network access. |
+| **Target** | Invocation or property access on an I/O type |
+
+### LVP — Local Variable Purity
+
+`LVP0001` and `LVP0002` are emitted by `LocalVariablePurityAnalyzer`;
+`LVP0003` is emitted by `ImmutableNamingSuggestionAnalyzer`.
+
+An **immutable local** is a local variable whose name begins with `_`. The single
+underscore `_` is the discard symbol and is never treated as an immutable local.
+
+| Field | Value |
+|---|---|
+| **ID** | `LVP0001` |
+| **Category** | `Purity` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | Reassignment to immutable local variable prohibited |
+| **Message** | Cannot reassign a value to immutable local variable `'{0}'` |
+| **Description** | Local variables starting with an underscore (`_`) are treated as immutable, and reassignment after declaration is prohibited. |
+| **Target** | Assignment, compound assignment, or increment/decrement targeting an immutable local |
+
+| Field | Value |
+|---|---|
+| **ID** | `LVP0002` |
+| **Category** | `Purity` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | Mandatory initialization of immutable local variable |
+| **Message** | Immutable local variable `'{0}'` must be initialized at the time of declaration |
+| **Description** | Local variables starting with an underscore (`_`) are treated as immutable and must be assigned an initial value at declaration. |
+| **Target** | Declaration of an immutable local with no initializer |
+
+| Field | Value |
+|---|---|
+| **ID** | `LVP0003` |
+| **Category** | `Naming` |
+| **Default severity** | `Warning` |
+| **Enabled by default** | Yes |
+| **Title** | Suggestion to apply naming convention for immutable local variable |
+| **Message** | Local variable `'{0}'` is effectively immutable. Consider starting its name with an underscore `'_'`. |
+| **Description** | Local variables that are never reassigned can have their immutability explicitly shown by starting the name with an underscore. |
+| **Target** | A local variable that is never reassigned but whose name does not begin with `_` |
+
+### FIF — FluentIf
+
+Emitted by `FluentIfAnalyzer`.
+
+| Field | Value |
+|---|---|
+| **ID** | `FIF0001` |
+| **Category** | `FluentIf` |
+| **Default severity** | `Error` |
+| **Enabled by default** | Yes |
+| **Title** | FluentIf chain termination check |
+| **Message** | FluentIf chain must be terminated with `'.Else(...)'` |
+| **Description** | Method chains starting with `Fluent.If()` must always end with `.Else()`. |
+| **Target** | A `Fluent.If()` chain not terminated by `.Else(...)` |
+
+## Summary table
+
+| ID | Category | Default severity | Analyzer | v1.0 public contract |
+|---|---|---|---|---|
+| `RT0001` | Purity | Error | `ReferentialTransparencyAnalyzer` | Yes |
+| `RT0002` | Purity | Error | `ReferentialTransparencyAnalyzer` | Yes |
+| `RT0003` | Purity | Error | `ReferentialTransparencyAnalyzer` | Yes |
+| `LVP0001` | Purity | Error | `LocalVariablePurityAnalyzer` | Yes |
+| `LVP0002` | Purity | Error | `LocalVariablePurityAnalyzer` | Yes |
+| `LVP0003` | Naming | Warning | `ImmutableNamingSuggestionAnalyzer` | Yes |
+| `FIF0001` | FluentIf | Error | `FluentIfAnalyzer` | Yes |
+
+## v1.0 public contract
+
+All seven diagnostics above are **part of the v1.0.0 public contract**. For each of them,
+the following are contract surface that consumers may depend on:
+
+- The **ID** string
+- The **category**
+- The **default severity**
+- The **fact that the diagnostic exists and is enabled by default**
+
+The following are **not** contract surface and may change in any release:
+
+- The exact wording of the title, message, and description (including translations)
+- The internal analyzer class that emits the diagnostic
+- The precise implementation strategy used to detect the condition
+
+Message wording is deliberately excluded so that clarity improvements and localization
+work are not breaking changes. Consumers must match on the diagnostic **ID**, never on
+message text.
+
+## ID naming convention
+
+```
+<PREFIX><NNNN>
+```
+
+- **PREFIX** — a short uppercase prefix identifying the rule family:
+  | Prefix | Family |
+  |---|---|
+  | `RT` | Referential Transparency — purity of `[PureMethod]` bodies |
+  | `LVP` | Local Variable Purity — immutable local variable rules |
+  | `FIF` | FluentIf — `Fluent.If()` chain rules |
+- **NNNN** — a zero-padded four-digit number, allocated sequentially within the family
+  starting at `0001`.
+
+Rules:
+
+- A prefix identifies the **rule family**, not the analyzer class. `LVP0003` is emitted
+  by `ImmutableNamingSuggestionAnalyzer` but belongs to the LVP family because it is a
+  local-variable purity concern.
+- Numbers are **never reused**, including after a diagnostic is removed.
+- A new rule family requires a new prefix and an entry in the table above.
+
+## Category policy
+
+| Category | Meaning |
+|---|---|
+| `Purity` | The code violates a purity or immutability guarantee. Correctness-affecting. |
+| `Naming` | The code is correct, but a naming convention would express intent better. Advisory. |
+| `FluentIf` | The code misuses the FluentIf API surface. Correctness-affecting. |
+
+Guidance:
+
+- `Purity` and `FluentIf` diagnostics default to `Error` — they indicate the code does
+  not satisfy a guarantee PureSharp is asked to enforce.
+- `Naming` diagnostics default to `Warning` or lower — they are suggestions and must
+  never block a build by default.
+- A diagnostic's category must not change once the diagnostic is part of a released
+  public contract; see the compatibility rules below.
+
+## Compatibility rules
+
+### Adding a new diagnostic
+
+Adding a diagnostic that is **enabled by default** can fail a build that previously
+succeeded, so it is treated as a breaking change.
+
+- Allocate the next unused number in the family; never reuse a retired number.
+- Add it to this catalog and to the README table in the same change.
+- Add positive and negative tests in the same change.
+- Prefer introducing a new default-`Error` diagnostic only in a **major** release.
+  In a minor release, introduce it as `Warning` or `isEnabledByDefault: false` first.
+
+### Changing an existing diagnostic
+
+| Change | Allowed in | Notes |
+|---|---|---|
+| Title / message / description wording | Any release | Not contract surface. Consumers must not match on text. |
+| Translation updates | Any release | Same as above. |
+| Widening detection (more code now reported) | Major release | Can break a previously passing build. |
+| Narrowing detection (less code now reported) | Minor release | Does not break builds; may weaken a guarantee, so document it. |
+| Raising default severity | Major release | Can break a previously passing build. |
+| Lowering default severity | Minor release | Document the reduced guarantee. |
+| Changing category | Major release | Category is contract surface; it appears in consumer tooling. |
+| Changing ID | Never | Retire the old ID and allocate a new one instead. |
+
+### Removing a diagnostic
+
+- Removal is a **major**-release change.
+- The retired ID stays listed in this catalog under a "Retired" heading with the version
+  in which it was removed, and its number is never reused.
+- Consumers suppressing the retired ID in `.editorconfig` must not start failing; an
+  unknown `dotnet_diagnostic` entry is ignored by Roslyn, so no migration is required.
+
+### Change checklist
+
+Any change touching a `DiagnosticDescriptor` must update, in the same commit:
+
+- [ ] The descriptor in the analyzer source
+- [ ] This catalog
+- [ ] The README diagnostic table (and translated READMEs where they carry the table)
+- [ ] Tests covering the new or changed behaviour
+
+## Retired diagnostics
+
+None. No diagnostic has been removed from PureSharp.
+
+## Consistency verification
+
+The three representations of the diagnostic contract must agree:
+
+1. `DiagnosticDescriptor` definitions in `src/PureSharp.Core/*Analyzer.cs`
+2. Resource strings in `src/PureSharp.Core/Resources/DiagnosticResources.resx`
+   (and per-language `.resx` files)
+3. This catalog and the README table
+
+Each descriptor must reference the resource keys belonging to **its own ID** —
+`<ID>_Title`, `<ID>_MessageFormat`, and `<ID>_Description`. A descriptor referencing
+another diagnostic's resource key is a defect: it surfaces the wrong explanatory text to
+consumers even though the ID, category, and severity look correct.
diff --git a/src/PureSharp.Core/ImmutableNamingSuggestionAnalyzer.cs b/src/PureSharp.Core/ImmutableNamingSuggestionAnalyzer.cs
index accbbda..da43993 100644
--- a/src/PureSharp.Core/ImmutableNamingSuggestionAnalyzer.cs
+++ b/src/PureSharp.Core/ImmutableNamingSuggestionAnalyzer.cs
@@ -22,7 +22,7 @@ public class ImmutableNamingSuggestionAnalyzer : DiagnosticAnalyzer
         category: "Naming",
         defaultSeverity: DiagnosticSeverity.Warning,
         isEnabledByDefault: true,
-        description: new LocalizableResourceString(nameof(DiagnosticResources.FIF0001_Description), DiagnosticResources.ResourceManager, typeof(DiagnosticResources)));
+        description: new LocalizableResourceString(nameof(DiagnosticResources.LVP0003_Description), DiagnosticResources.ResourceManager, typeof(DiagnosticResources)));
 
     /// <inheritdoc />
     public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics =>