diff --git a/CHANGELOG.md b/CHANGELOG.md
index 603cd6056..e179be986 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Added SPECIFICATION_TESTING.md [#359](https://github.com/opensearch-project/opensearch-api-specification/pull/359)
 - Added StoryValidator to validate stories before running them ([#354](https://github.com/opensearch-project/opensearch-api-specification/issues/354))
 - Added support for `text/plain` responses in `_cat` APIs ([#360](https://github.com/opensearch-project/opensearch-api-specification/pull/360))
+- Added support for `application/yaml` responses ([#363](https://github.com/opensearch-project/opensearch-api-specification/pull/363))
 - Added test for search with seq_no_primary_term ([#367](https://github.com/opensearch-project/opensearch-api-specification/pull/367))
 
 ### Changed
diff --git a/spec/namespaces/cat.yaml b/spec/namespaces/cat.yaml
index 316fd086c..cf87eb996 100644
--- a/spec/namespaces/cat.yaml
+++ b/spec/namespaces/cat.yaml
@@ -848,6 +848,11 @@ components:
             type: array
             items:
               $ref: '../schemas/cat.health.yaml#/components/schemas/HealthRecord'
+        application/yaml:
+          schema:
+            type: array
+            items:
+              $ref: '../schemas/cat.health.yaml#/components/schemas/HealthRecord'
     cat.help@200:
       description: ''
       content:
@@ -864,6 +869,11 @@ components:
             type: array
             items:
               $ref: '../schemas/cat.indices.yaml#/components/schemas/IndicesRecord'
+        application/yaml:
+          schema:
+            type: array
+            items:
+              $ref: '../schemas/cat.indices.yaml#/components/schemas/IndicesRecord'
     cat.master@200:
       description: ''
       content:
diff --git a/spec/schemas/cat.indices.yaml b/spec/schemas/cat.indices.yaml
index c03496f7b..974f4cbfc 100644
--- a/spec/schemas/cat.indices.yaml
+++ b/spec/schemas/cat.indices.yaml
@@ -29,13 +29,13 @@ components:
           type: string
         docs.count:
           description: available docs
-          oneOf:
+          anyOf:
             - type: string
             - nullable: true
               type: string
         docs.deleted:
           description: deleted docs
-          oneOf:
+          anyOf:
             - type: string
             - nullable: true
               type: string
@@ -47,13 +47,13 @@ components:
           type: string
         store.size:
           description: store size of primaries & replicas
-          oneOf:
+          anyOf:
             - type: string
             - nullable: true
               type: string
         pri.store.size:
           description: store size of primaries
-          oneOf:
+          anyOf:
             - type: string
             - nullable: true
               type: string
diff --git a/tests/cat/health.yaml b/tests/cat/health.yaml
index 0a462fe9a..26184eb82 100644
--- a/tests/cat/health.yaml
+++ b/tests/cat/health.yaml
@@ -43,7 +43,7 @@ chapters:
     response:
       status: 200
       content_type: text/plain
-  - synopsis: Cat with a json response.
+  - synopsis: Cat in different formats (format=json).
     method: GET
     path: /_cat/health
     parameters:
@@ -56,3 +56,16 @@ chapters:
           status: yellow
           node.data: '1'
           discovered_cluster_manager: 'true'
+  - synopsis: Cat in different formats (format=yaml).
+    method: GET
+    path: /_cat/health
+    parameters:
+      format: yaml
+    response:
+      status: 200
+      content_type: application/yaml
+      payload:
+        - node.total: '1'
+          status: yellow
+          node.data: '1'
+          discovered_cluster_manager: 'true'
diff --git a/tests/cat/indices.yaml b/tests/cat/indices.yaml
new file mode 100644
index 000000000..ad5072424
--- /dev/null
+++ b/tests/cat/indices.yaml
@@ -0,0 +1,72 @@
+$schema: ../../json_schemas/test_story.schema.yaml
+
+description: Test cat/indices endpoints.
+prologues:
+  - path: /{index}
+    method: PUT
+    parameters:
+      index: books
+    request_body:
+      payload: {}
+epilogues:
+  - path: /books
+    method: DELETE
+    status: [200, 404]
+chapters:
+  - synopsis: Cat with a default text response.
+    method: GET
+    path: /_cat/indices
+    response:
+      status: 200
+      content_type: text/plain
+  - synopsis: Cat with verbose output (v=true).
+    method: GET
+    path: /_cat/indices
+    parameters:
+      v: true
+    response:
+      status: 200
+      content_type: text/plain
+  - synopsis: Cat with headers (h=header1,header2).
+    method: GET
+    path: /_cat/indices
+    parameters:
+      h:
+        - health
+        - status
+    response:
+      status: 200
+      content_type: text/plain
+  - synopsis: Cat displaying all available headers (help=true).
+    method: GET
+    path: /_cat/indices
+    parameters:
+      help: true
+    response:
+      status: 200
+      content_type: text/plain
+  - synopsis: Cat with sorted results.
+    method: GET
+    path: /_cat/indices
+    parameters:
+      s:
+        - status
+    response:
+      status: 200
+      content_type: text/plain
+  - synopsis: Cat in different formats (format=json).
+    method: GET
+    path: /_cat/indices
+    parameters:
+      format: json
+    response:
+      status: 200
+      content_type: application/json
+  - synopsis: Cat in different formats (format=yaml).
+    method: GET
+    path: /_cat/indices
+    parameters:
+      format: yaml
+    response:
+      status: 200
+      content_type: application/yaml
diff --git a/tools/src/tester/ChapterEvaluator.ts b/tools/src/tester/ChapterEvaluator.ts
index c9f60f50a..28ba11917 100644
--- a/tools/src/tester/ChapterEvaluator.ts
+++ b/tools/src/tester/ChapterEvaluator.ts
@@ -17,6 +17,7 @@ import type SchemaValidator from './SchemaValidator'
 import { type StoryOutputs } from './StoryOutputs'
 import { ChapterOutput } from './ChapterOutput'
 import { Operation, atomizeChangeset, diff } from 'json-diff-ts'
+import YAML from 'yaml'
 import _ from 'lodash'
 
 export default class ChapterEvaluator {
@@ -85,7 +86,9 @@ export default class ChapterEvaluator {
 
   #evaluate_payload_body(response: ActualResponse, expected_payload?: Payload): Evaluation {
     if (expected_payload == null) return { result: Result.PASSED }
-    const delta = atomizeChangeset(diff(expected_payload, response.payload))
+    const content_type = response.content_type ?? 'application/json'
+    const payload = this.#deserialize_payload(response.payload, content_type)
+    const delta = atomizeChangeset(diff(expected_payload, payload))
     const messages: string[] = _.compact(delta.map((value, _index, _array) => {
       switch (value.type) {
         case Operation.UPDATE:
@@ -103,6 +106,14 @@ export default class ChapterEvaluator {
     const schema = content?.schema
     if (schema == null && content != null) return { result: Result.PASSED }
     if (schema == null) return { result: Result.FAILED, message: `Schema for "${response.status}: ${response.content_type}" response not found in the spec.` }
-    return this._schema_validator.validate(schema, response.payload)
+    return this._schema_validator.validate(schema, this.#deserialize_payload(response.payload, content_type))
+  }
+
+  #deserialize_payload(payload: any, content_type: string): any {
+    if (payload === undefined) return undefined
+    switch (content_type) {
+      case 'application/yaml': return YAML.parse(payload as string)
+      default: return payload
+    }
   }
 }
diff --git a/tools/tests/tester/fixtures/evals/failed/not_found.yaml b/tools/tests/tester/fixtures/evals/failed/not_found.yaml
index 92ab01556..766fb3b99 100644
--- a/tools/tests/tester/fixtures/evals/failed/not_found.yaml
+++ b/tools/tests/tester/fixtures/evals/failed/not_found.yaml
@@ -10,7 +10,7 @@ chapters:
   - title: This chapter should fail because the operation is not defined in the spec.
     overall:
       result: FAILED
-      message: Operation "GET /_cat/health" not found in the spec.
+      message: Operation "GET /_cat/does_not_exist" not found in the spec.
   - title: This chapter should fail because the parameter is not defined in the spec.
     overall:
       result: FAILED
diff --git a/tools/tests/tester/fixtures/evals/passed.yaml b/tools/tests/tester/fixtures/evals/passed.yaml
index 501041690..a1e264847 100644
--- a/tools/tests/tester/fixtures/evals/passed.yaml
+++ b/tools/tests/tester/fixtures/evals/passed.yaml
@@ -23,7 +23,7 @@ chapters:
         result: PASSED
       payload_schema:
         result: PASSED
-  - title: This GET /_cat chapter should pass.
+  - title: This GET /_cat chapter returns text/plain and should pass.
     overall:
       result: PASSED
     request:
@@ -37,7 +37,38 @@ chapters:
         result: PASSED
       payload_schema:
         result: PASSED
-
+  - title: This GET /_cat/health chapter returns application/json and should pass.
+    overall:
+      result: PASSED
+    request:
+      parameters:
+        format:
+          result: PASSED
+      request_body:
+        result: PASSED
+    response:
+      status:
+        result: PASSED
+      payload_body:
+        result: PASSED
+      payload_schema:
+        result: PASSED
+  - title: This GET /_cat/health chapter returns application/yaml and should pass.
+    overall:
+      result: PASSED
+    request:
+      parameters:
+        format:
+          result: PASSED
+      request_body:
+        result: PASSED
+    response:
+      status:
+        result: PASSED
+      payload_body:
+        result: PASSED
+      payload_schema:
+        result: PASSED
 epilogues:
   - title: DELETE /books
     overall:
diff --git a/tools/tests/tester/fixtures/specs/excerpt.yaml b/tools/tests/tester/fixtures/specs/excerpt.yaml
index 4914a30b5..136f08511 100644
--- a/tools/tests/tester/fixtures/specs/excerpt.yaml
+++ b/tools/tests/tester/fixtures/specs/excerpt.yaml
@@ -9,9 +9,22 @@ paths:
       x-operation-group: cat.help
       x-version-added: '1.0'
       description: Returns help for the Cat APIs.
+      parameters:
+        - $ref: '#/components/parameters/cat.help::query.format'
       responses:
         '200':
           $ref: '#/components/responses/cat.help@200'  
+  /_cat/health:
+    get:
+      operationId: cat.health.0
+      x-operation-group: cat.health
+      x-version-added: '1.0'
+      description: Returns health for the Cat APIs.
+      parameters:
+        - $ref: '#/components/parameters/cat.health::query.format'
+      responses:
+        '200':
+          $ref: '#/components/responses/cat.health@200'
   /{index}:
     delete:
       operationId: indices.delete.0
@@ -73,6 +86,21 @@ components:
         text/plain:
           schema:
             type: string
+    cat.health@200:
+      content:
+        text/plain:
+          schema:
+            type: string
+        application/json:
+          schema:
+            type: array
+            items:
+              type: object
+        application/yaml:
+          schema:
+            type: array
+            items:
+              type: object
     indices.delete@200:
       description: ''
       content:
@@ -101,6 +129,11 @@ components:
               - shards_acknowledged
               - acknowledged
   parameters:
+    cat.health::query.format:
+      in: query
+      name: format
+      schema:
+        type: string
     indices.delete::path.index:
       in: path
       name: index
diff --git a/tools/tests/tester/fixtures/stories/failed/not_found.yaml b/tools/tests/tester/fixtures/stories/failed/not_found.yaml
index b210f74e9..6f1b75739 100644
--- a/tools/tests/tester/fixtures/stories/failed/not_found.yaml
+++ b/tools/tests/tester/fixtures/stories/failed/not_found.yaml
@@ -7,7 +7,7 @@ epilogues:
     status: [200, 404]
 chapters:
   - synopsis: This chapter should fail because the operation is not defined in the spec.
-    path: /_cat/health
+    path: /_cat/does_not_exist
     method: GET
   - synopsis: This chapter should fail because the parameter is not defined in the spec.
     path: /{index}
diff --git a/tools/tests/tester/fixtures/stories/passed.yaml b/tools/tests/tester/fixtures/stories/passed.yaml
index 17f1e57d1..c4a3c5c2b 100644
--- a/tools/tests/tester/fixtures/stories/passed.yaml
+++ b/tools/tests/tester/fixtures/stories/passed.yaml
@@ -11,6 +11,26 @@ chapters:
     method: PUT
     parameters:
       index: books
-  - synopsis: This GET /_cat chapter should pass.
+  - synopsis: This GET /_cat chapter returns text/plain and should pass.
     path: /_cat
     method: GET
+  - synopsis: This GET /_cat/health chapter returns application/json and should pass.
+    path: /_cat/health
+    parameters:
+      format: json
+    method: GET
+    response:
+      status: 200
+      content_type: application/json
+      payload:
+        - node.total: '1'
+  - synopsis: This GET /_cat/health chapter returns application/yaml and should pass.
+    path: /_cat/health
+    parameters:
+      format: yaml
+    method: GET
+    response:
+      status: 200
+      content_type: application/yaml
+      payload:
+        - node.total: '1'