diff --git a/src/app/version.js b/src/app/version.js
index 37fcc07..0023563 100644
--- a/src/app/version.js
+++ b/src/app/version.js
@@ -5,6 +5,52 @@ export type VersionInfo = {|
   +minor: number,
   +patch: number,
 |};
+export type GitState = {|
+  +commitHash: string,
+  +commitTimestamp: string, // YYYYmmdd-HHMM, in commit-local time
+  +dirty: boolean, // does the worktree have unstaged/uncommitted changes?
+|};
+export type Environment = "development" | "production" | "test";
+
+/**
+ * Parse the given string as a `GitState`, throwing an error if it is
+ * not valid. The argument should be the result of calling
+ * `JSON.stringify` with a valid `GitState`. Thus, this is a checked
+ * version of `JSON.parse`.
+ */
+export function parseGitState(raw: ?string): GitState {
+  if (typeof raw !== "string") {
+    throw new Error("gitState: not a string: " + String(raw));
+  }
+  const parsed: mixed = Object.freeze(JSON.parse(raw));
+  if (parsed == null || typeof parsed !== "object") {
+    throw new Error("gitState: not a JSON object: " + String(parsed));
+  }
+  // This intermediate variable helps out Flow's inference...
+  const gitState: Object = parsed;
+  if (
+    typeof gitState.commitHash !== "string" ||
+    typeof gitState.commitTimestamp !== "string" ||
+    typeof gitState.dirty !== "boolean" ||
+    Object.keys(gitState).length !== 3
+  ) {
+    throw new Error("gitState: bad shape: " + JSON.stringify(gitState));
+  }
+  return gitState;
+}
+
+/**
+ * Parse the given string as an `Environment`, throwing an error if it
+ * is not valid. The input should be a valid `Environment`.
+ */
+export function parseEnvironment(raw: ?string): Environment {
+  if (raw !== "development" && raw !== "production" && raw !== "test") {
+    throw new Error(
+      "environment: " + (raw == null ? String(raw) : JSON.stringify(raw))
+    );
+  }
+  return raw;
+}
 
 export const VERSION_INFO = Object.freeze({
   major: 0,
diff --git a/src/app/version.test.js b/src/app/version.test.js
new file mode 100644
index 0000000..41c5e44
--- /dev/null
+++ b/src/app/version.test.js
@@ -0,0 +1,109 @@
+// @flow
+
+import {type Environment, parseEnvironment, parseGitState} from "./version";
+
+describe("app/version", () => {
+  // Like `VersionInfo`, but with some extra properties that will
+  // shortly be added to that type.
+  const version = () => ({
+    major: 3,
+    minor: 13,
+    patch: 37,
+    gitState: {
+      commitHash: "d0e1a2d3b4e5",
+      commitTimestamp: "20010203-0405",
+      dirty: true,
+    },
+    environment: "test",
+  });
+
+  describe("parseGitState", () => {
+    it("fails given literal `undefined`", () => {
+      expect(() => parseGitState(undefined)).toThrow(
+        "gitState: not a string: undefined"
+      );
+    });
+    it("fails given literal `null`", () => {
+      expect(() => parseGitState(null)).toThrow("gitState: not a string: null");
+    });
+    it("fails given JSON `null`", () => {
+      expect(() => parseGitState("null")).toThrow(
+        "gitState: not a JSON object: null"
+      );
+    });
+    it("fails given invalid JSON", () => {
+      expect(() => parseGitState("wat")).toThrow(
+        "Unexpected token w in JSON at position 0"
+      );
+    });
+    it("fails given a JSON string", () => {
+      expect(() => parseGitState(JSON.stringify("wat"))).toThrow(
+        "gitState: not a JSON object: wat"
+      );
+    });
+    it("fails given a non-stringified `GitState`", () => {
+      // $ExpectFlowError
+      expect(() => parseGitState(version().gitState)).toThrow(
+        "gitState: not a string: [object Object]"
+      );
+    });
+    it("fails given a JSON object missing a property", () => {
+      const gitState = version().gitState;
+      delete gitState.dirty;
+      expect(() => parseGitState(JSON.stringify(gitState))).toThrow(
+        "gitState: bad shape: {"
+      );
+    });
+    function expectBadShape(gitState) {
+      expect(() => parseGitState(JSON.stringify(gitState))).toThrow(
+        "gitState: bad shape: {"
+      );
+    }
+    it("fails given a JSON object with an extra property", () => {
+      expectBadShape({...version().gitState, wat: "wot"});
+    });
+    it("fails given a JSON object with bad `commitHash`", () => {
+      expectBadShape({...version().gitState, commitHash: true});
+      expectBadShape({...version().gitState, commitHash: 27});
+      expectBadShape({...version().gitState, commitHash: null});
+    });
+    it("fails given a JSON object with bad `commitTimestamp`", () => {
+      expectBadShape({...version().gitState, commitTimestamp: true});
+      expectBadShape({...version().gitState, commitTimestamp: 27});
+      expectBadShape({...version().gitState, commitTimestamp: null});
+    });
+    it("fails given a JSON object with bad `dirty`", () => {
+      expectBadShape({...version().gitState, dirty: "true"});
+      expectBadShape({...version().gitState, dirty: 27});
+      expectBadShape({...version().gitState, dirty: null});
+    });
+    it("parses a valid `GitState`", () => {
+      const gitState = version().gitState;
+      expect(parseGitState(JSON.stringify(gitState))).toEqual(gitState);
+    });
+  });
+
+  describe("parseEnvironment", () => {
+    it("parses each of the valid environments", () => {
+      const allEnvs = {development: true, production: true, test: true};
+      function _unused_staticCheck(x: Environment): true {
+        return allEnvs[x];
+      }
+      for (const env of Object.keys(allEnvs)) {
+        expect(parseEnvironment(env)).toEqual(env);
+      }
+    });
+
+    it("fails given literal `undefined`", () => {
+      expect(() => parseEnvironment(undefined)).toThrow(
+        "environment: undefined"
+      );
+    });
+    it("fails given literal `null`", () => {
+      expect(() => parseEnvironment(null)).toThrow("environment: null");
+    });
+    it("fails given a non-environment string", () => {
+      expect(() => parseEnvironment("wat")).toThrow('environment: "wat"');
+    });
+  });
+});