diff --git a/src/types/mapper.spec-d.ts b/src/types/mapper.spec-d.ts
new file mode 100644
index 0000000..2562f6e
--- /dev/null
+++ b/src/types/mapper.spec-d.ts
@@ -0,0 +1,48 @@
+import { expectTypeOf, test } from 'vitest'
+import { Mapper } from '@/types/mapper'
+import { Profile } from '@/types/profile'
+
+declare const broadSourceMapper: Mapper<[
+  Profile<string, number, 'x', string>,
+  Profile<'a', string, 'y', boolean>
+]>
+
+declare const broadSourceKey: string
+
+test('broad source keys do not expose destinations from unrelated profiles', () => {
+  // @ts-expect-error Only source a has destination y.
+  broadSourceMapper.map('b', 'hello', 'y')
+  // @ts-expect-error Only source a has destination y.
+  broadSourceMapper.mapMany('b', ['hello'], 'y')
+
+  expectTypeOf(broadSourceMapper.map('a', 'hello', 'y')).toEqualTypeOf<boolean>()
+  expectTypeOf(broadSourceMapper.mapMany('a', ['hello'], 'y')).toEqualTypeOf<boolean[]>()
+  expectTypeOf(broadSourceMapper.map(broadSourceKey, 1, 'x')).toEqualTypeOf<string>()
+  expectTypeOf(broadSourceMapper.mapMany(broadSourceKey, [1], 'x')).toEqualTypeOf<string[]>()
+
+  // @ts-expect-error Destination y expects a string input.
+  broadSourceMapper.map('a', 1, 'y')
+  // @ts-expect-error Destination y expects string inputs.
+  broadSourceMapper.mapMany('a', [1], 'y')
+})
+
+declare const literalMapper: Mapper<[
+  Profile<'a', number, 'x', string>,
+  Profile<'a', string, 'y', Promise<boolean>>,
+  Profile<'b', number, 'x', boolean>
+]>
+
+declare const unionSourceKey: 'a' | 'b'
+
+test('literal and union keys retain their mapping results', () => {
+  expectTypeOf(literalMapper.map('a', 1, 'x')).toEqualTypeOf<string>()
+  expectTypeOf(literalMapper.map('a', 'hello', 'y')).toEqualTypeOf<Promise<boolean>>()
+  expectTypeOf(literalMapper.mapMany('a', ['hello'], 'y')).toEqualTypeOf<Promise<boolean>[]>()
+  expectTypeOf(literalMapper.map(unionSourceKey, 1, 'x')).toEqualTypeOf<string | boolean>()
+  expectTypeOf(literalMapper.mapMany(unionSourceKey, [1], 'x')).toEqualTypeOf<(string | boolean)[]>()
+
+  // @ts-expect-error Source b has no destination y.
+  literalMapper.map('b', 'hello', 'y')
+  // @ts-expect-error Destination x expects a number input.
+  literalMapper.map('a', 'hello', 'x')
+})
diff --git a/src/types/mapper.ts b/src/types/mapper.ts
index 1fb5786..37465a2 100644
--- a/src/types/mapper.ts
+++ b/src/types/mapper.ts
@@ -1,8 +1,7 @@
 import { ExtractSourceKeys, Profile } from '@/types/profile'
 
 /**
- * Registered profiles grouped once by source key, then destination key, so each `map` call is two
- * property lookups rather than a fresh pass over every registered profile.
+ * Type-level index of profiles by source and destination key, reused when checking mapping calls.
  */
 type ProfileIndex<TProfile> = {
   [TSourceKey in ExtractSourceKeys<TProfile>]: {
@@ -10,11 +9,14 @@ type ProfileIndex<TProfile> = {
   }
 }
 
+// A broad source key can collapse index entries together. Check the selected source key
+// before extracting types so unrelated profiles cannot become valid matches.
+
 /** Extracts the source type from a profile. `unknown` for the destination parameter matches any profile regardless of its destination type. */
-type ProfileSource<TProfile> = TProfile extends Profile<string, infer TSource, string, unknown> ? TSource : never
+type ProfileSource<TProfile, TSourceKey extends string> = TProfile extends Profile<TSourceKey, infer TSource, string, unknown> ? TSource : never
 
-/** Extracts the destination type from a profile. `never` for the source parameter exploits contravariance — any concrete source type satisfies `never extends TSource` in the function position, so the match is unconditional. */
-type ProfileDestination<TProfile> = TProfile extends Profile<string, never, string, infer TDestination> ? TDestination : never
+/** Extracts the destination type from a profile. `never` for the source parameter exploits contravariance — any concrete source type satisfies `never extends TSource` in the function position, so the source type does not restrict the match. */
+type ProfileDestination<TProfile, TSourceKey extends string> = TProfile extends Profile<TSourceKey, never, string, infer TDestination> ? TDestination : never
 
 export type Mapper<TProfiles extends readonly Profile[], TProfile = TProfiles[number]> = {
   register: (profiles: Profile[] | readonly Profile[] | Profile) => void,
@@ -23,9 +25,9 @@ export type Mapper<TProfiles extends readonly Profile[], TProfile = TProfiles[nu
   map: <
     TSourceKey extends keyof ProfileIndex<TProfile> & string,
     TDestinationKey extends keyof ProfileIndex<TProfile>[TSourceKey] & string
-  > (sourceKey: TSourceKey, source: ProfileSource<ProfileIndex<TProfile>[TSourceKey][TDestinationKey]>, destinationKey: TDestinationKey) => ProfileDestination<ProfileIndex<TProfile>[TSourceKey][TDestinationKey]>,
+  > (sourceKey: TSourceKey, source: ProfileSource<ProfileIndex<TProfile>[TSourceKey][TDestinationKey], TSourceKey>, destinationKey: TDestinationKey) => ProfileDestination<ProfileIndex<TProfile>[TSourceKey][TDestinationKey], TSourceKey>,
   mapMany: <
     TSourceKey extends keyof ProfileIndex<TProfile> & string,
     TDestinationKey extends keyof ProfileIndex<TProfile>[TSourceKey] & string
-  > (sourceKey: TSourceKey, sourceArray: ProfileSource<ProfileIndex<TProfile>[TSourceKey][TDestinationKey]>[], destinationKey: TDestinationKey) => ProfileDestination<ProfileIndex<TProfile>[TSourceKey][TDestinationKey]>[],
+  > (sourceKey: TSourceKey, sourceArray: ProfileSource<ProfileIndex<TProfile>[TSourceKey][TDestinationKey], TSourceKey>[], destinationKey: TDestinationKey) => ProfileDestination<ProfileIndex<TProfile>[TSourceKey][TDestinationKey], TSourceKey>[],
 }
diff --git a/src/types/profile.ts b/src/types/profile.ts
index 1217a7e..ec29fd8 100644
--- a/src/types/profile.ts
+++ b/src/types/profile.ts
@@ -10,5 +10,4 @@ export type ProfileKey<T extends Profile> = `${T['sourceKey']}-${T['destinationK
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 export type ExtractSourceKeys<TProfile> = TProfile extends Profile<infer TSourceKey, any, any> ? TSourceKey : never
 
-
 export type Profiles = readonly Profile[]