diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d7702f7..c08767e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,10 +13,24 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      # Pinned, and kept in step with `devVersion` in flake.nix, so CI and
+      # the dev shell agree and a new stable release cannot turn a lint
+      # failure into an unrelated red build.
+      - uses: dtolnay/rust-toolchain@1.99.1
         with:
           components: rustfmt, clippy
       - run: cargo build --all-targets
       - run: cargo test
       - run: cargo fmt --check
       - run: cargo clippy --all-targets -- -D warnings
+
+  msrv:
+    name: Build on the declared MSRV
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      # Must match `rust-version` in Cargo.toml and `msrvVersion` in
+      # flake.nix. Builds only — lint and format output varies by release,
+      # and pinning those to the oldest supported toolchain is not useful.
+      - uses: dtolnay/rust-toolchain@1.88.0
+      - run: cargo test
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 209d90d..bf8b4b9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- A declared minimum supported Rust version of **1.88**, verified by running
+  the test suite against it. The floor comes from the dependency graph
+  (`reqwest` → `url` → `idna` → `icu_*`), not from this crate's own code.
+  CI builds on both the MSRV and the pinned dev toolchain.
 - Wrappers for every one of the API's 34 `Route::resource` groups, plus
   `users` — 265 methods. See `COVERAGE.md` for the per-endpoint inventory.
 - `COVERAGE.md`, a detailed record of what is wrapped, what is not, and why,
diff --git a/CLAUDE.md b/CLAUDE.md
index 43fa234..03bdb09 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -14,6 +14,36 @@ This project uses Nix for toolchain management (Rust via `rust-overlay`):
 nix develop
 ```
 
+### Rust version
+
+Two versions are pinned, and **three places must agree** when either moves:
+
+| | Version | Declared in |
+| --- | --- | --- |
+| Dev toolchain | 1.99.1 | `devVersion` in `flake.nix`, `test` job in `ci.yml` |
+| MSRV | 1.88 | `msrvVersion` in `flake.nix`, `rust-version` in `Cargo.toml`, `msrv` job in `ci.yml` |
+
+The MSRV is set by the **dependency graph**, not by this crate's code:
+`reqwest` pulls in `url` → `idna` → `idna_adapter` → `icu_*`, and the `icu`
+crates require 1.88. Recheck the floor after a dependency bump with:
+
+```sh
+cargo metadata --format-version 1 \
+  | jq -r '.packages[] | select(.rust_version) | "\(.rust_version)\t\(.name)"' \
+  | sort -V | tail -5
+```
+
+Verify a candidate MSRV actually builds before declaring it — the number
+above was confirmed by running the suite, not inferred:
+
+```sh
+CARGO_TARGET_DIR=/tmp/msrv-target nix develop .#msrv --command cargo test
+```
+
+Don't reach for a newer std API without checking it against the MSRV; a
+convenience like `Option::is_none_or` (1.82) is rarely worth raising the
+floor for.
+
 ## Common commands
 
 ```sh
diff --git a/Cargo.toml b/Cargo.toml
index 444aaf5..ae6eaee 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,6 +2,11 @@
 name = "invoiceninjars"
 version = "0.1.0"
 edition = "2021"
+# Set by the dependency graph rather than by this crate's own code: reqwest
+# pulls in url -> idna -> idna_adapter -> icu_*, and the icu crates require
+# 1.88. Verified by running the test suite against it; see the `msrv`
+# devShell in flake.nix.
+rust-version = "1.88"
 description = "An async Rust client for the Invoice Ninja v5 API"
 license = "MIT"
 repository = "https://github.com/bryant/invoiceninjars"
diff --git a/README.md b/README.md
index 8166505..b23d63d 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,8 @@
 
 An async Rust client for the [Invoice Ninja](https://www.invoiceninja.com/) v5 API.
 
+Requires Rust 1.88 or newer.
+
 ## Development
 
 Enter the dev shell (Rust toolchain via `rust-overlay`):
@@ -10,6 +12,14 @@ Enter the dev shell (Rust toolchain via `rust-overlay`):
 nix develop
 ```
 
+The dev toolchain is pinned in `flake.nix` rather than tracking latest
+stable, so it only moves deliberately. To check the crate still builds on
+the declared minimum of 1.88:
+
+```sh
+nix develop .#msrv --command cargo test
+```
+
 ## Usage
 
 ```rust
diff --git a/flake.nix b/flake.nix
index c3f49ee..007ff6f 100644
--- a/flake.nix
+++ b/flake.nix
@@ -16,19 +16,36 @@
           overlays = [ rust-overlay.overlays.default ];
         };
 
-        rustToolchain = pkgs.rust-bin.stable.latest.default.override {
-          extensions = [ "rust-src" "rust-analyzer" "clippy" "rustfmt" ];
+        # Pinned rather than `stable.latest`, so the toolchain only moves
+        # when this line does. `latest` drifts with the flake inputs, which
+        # means a lint can start failing on an unrelated change.
+        devVersion = "1.99.1";
+
+        # Must match `rust-version` in Cargo.toml. The floor comes from the
+        # dependency graph, not from this crate's own code: reqwest pulls in
+        # url -> idna -> idna_adapter -> icu_*, and the icu crates require
+        # 1.88. Check with:
+        #   cargo metadata --format-version 1 \
+        #     | jq -r '.packages[] | select(.rust_version) | .rust_version' \
+        #     | sort -V | tail -1
+        msrvVersion = "1.88.0";
+
+        toolchainFor = version: extensions:
+          pkgs.rust-bin.stable.${version}.default.override { inherit extensions; };
+
+        rustToolchain = toolchainFor devVersion [ "rust-src" "rust-analyzer" "clippy" "rustfmt" ];
+        msrvToolchain = toolchainFor msrvVersion [ ];
+
+        shellFor = toolchain: pkgs.mkShell {
+          packages = [ toolchain pkgs.pkg-config pkgs.openssl ];
+          RUST_SRC_PATH = "${toolchain}/lib/rustlib/src/rust/library";
         };
       in
       {
-        devShells.default = pkgs.mkShell {
-          packages = [
-            rustToolchain
-            pkgs.pkg-config
-            pkgs.openssl
-          ];
+        devShells.default = shellFor rustToolchain;
 
-          RUST_SRC_PATH = "${rustToolchain}/lib/rustlib/src/rust/library";
-        };
+        # Verifies the declared MSRV still builds:
+        #   nix develop .#msrv --command cargo test
+        devShells.msrv = shellFor msrvToolchain;
       });
 }