diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 8f6ea6c9..f7037cdf 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -20,3 +20,5 @@ jobs:
           WORKFLOW_STAGE_OPENED: "PR Open"
           WORKFLOW_STAGE_MERGED: "Ready to Test"
           WORKFLOW_STAGE_CLOSED: "Rejected"
+      - name: Run the tests against a mock Teamwork API
+        run: ./tests/run.sh
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index e142fd0f..aca161b8 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -7,6 +7,24 @@
 * [Install Docker](https://docs.docker.com/get-docker/)
 * [Install act tool](https://github.com/nektos/act)
 
+## Running the tests
+
+The tests run the action's Docker image against a mock Teamwork API (`tests/mock-teamwork.py`) and check what it sends:
+
+```
+./tests/run.sh
+```
+
+Each directory in `tests/cases` is a test case:
+
+* `event.json`: the GitHub event the action receives
+* `inputs`: the action's inputs as the `INPUT_*` environment variables GitHub creates for them, plus `GITHUB_EVENT_NAME` and any other environment variables such as `IGNORE_PROJECT_IDS`
+* `expected`: the exit code, errors and requests the mock Teamwork API should end up with
+
+`./tests/run.sh <filter>` only runs the cases whose name contains the filter.
+
+To add a case, create its `event.json` and `inputs`, run `./tests/run.sh --update <case name>` to record its `expected` file, and check that file is right before committing it. The mock's data (which tasks exist, their projects and the workflow stages) is at the top of `tests/mock-teamwork.py`.
+
 ## Usage
 
 Simulate a new pull request
diff --git a/src/teamwork.sh b/src/teamwork.sh
index c7da9e07..4439a1df 100644
--- a/src/teamwork.sh
+++ b/src/teamwork.sh
@@ -16,6 +16,20 @@ teamwork::get_task_id_from_body() {
   echo "$task_ids_str"
 }
 
+# Calls the Teamwork API and prints the response body.
+teamwork::request() {
+  local -r method=$1
+  local -r path=$2
+  local -r payload=${3:-}
+
+  local args=(-sS -X "$method" -u "$TEAMWORK_API_TOKEN"':')
+  if [ -n "$payload" ]; then
+    args+=(-H 'Content-Type: application/json; charset=utf-8' -d "$payload")
+  fi
+
+  curl "${args[@]}" "$TEAMWORK_URI$path"
+}
+
 teamwork::get_project_id_from_task() {
   local -r task_id=$1
 
@@ -25,7 +39,7 @@ teamwork::get_project_id_from_task() {
   fi
 
   response=$(
-    curl "$TEAMWORK_URI/projects/api/v1/tasks/$task_id.json" -u "$TEAMWORK_API_TOKEN"':' |\
+    teamwork::request GET "/projects/api/v1/tasks/$task_id.json" |\
       jq -r '.["todo-item"]["project-id"]'
   )
   echo "$response"
@@ -50,8 +64,7 @@ teamwork::get_workflow_stage() {
   # A single call scoped to the project sideloads every stage across the
   # project's workflow(s); each stage carries its parent workflow id, which the
   # move endpoint also needs.
-  curl -s "$TEAMWORK_URI/projects/api/v3/workflows.json?projectIds=$TEAMWORK_PROJECT_ID&include=stages" \
-    -u "$TEAMWORK_API_TOKEN"':' |\
+  teamwork::request GET "/projects/api/v3/workflows.json?projectIds=$TEAMWORK_PROJECT_ID&include=stages" |\
     jq -r --arg stage_name "$stage_name" \
       '[.included.stages[]? | select(.name | contains($stage_name)) | "\(.workflow.id) \(.id)"][0] // empty'
 }
@@ -78,10 +91,8 @@ teamwork::move_task_to_stage() {
     return
   fi
 
-  response=$(curl -X "PATCH" "$TEAMWORK_URI/projects/api/v3/tasks/$TEAMWORK_TASK_ID/workflows/$workflow_id.json" \
-      -u "$TEAMWORK_API_TOKEN"':' \
-      -H 'Content-Type: application/json; charset=utf-8' \
-      -d "{ \"stageId\": $stage_id, \"positionAfterTask\": -1 }" )
+  response=$(teamwork::request PATCH "/projects/api/v3/tasks/$TEAMWORK_TASK_ID/workflows/$workflow_id.json" \
+      "{ \"stageId\": $stage_id, \"positionAfterTask\": -1 }")
 
   log::message "$response"
 }
@@ -94,10 +105,8 @@ teamwork::add_comment() {
     return
   fi
 
-  response=$(curl -X "POST" "$TEAMWORK_URI/tasks/$TEAMWORK_TASK_ID/comments.json" \
-       -u "$TEAMWORK_API_TOKEN"':' \
-       -H 'Content-Type: application/json; charset=utf-8' \
-       -d "{ \"comment\": { \"body\": \"${body//\"/}\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }" )
+  response=$(teamwork::request POST "/tasks/$TEAMWORK_TASK_ID/comments.json" \
+      "{ \"comment\": { \"body\": \"${body//\"/}\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }")
 
   log::message "$response"
 }
@@ -111,10 +120,8 @@ teamwork::add_tag() {
   fi
 
   if [ "$AUTOMATIC_TAGGING" == true ]; then
-    response=$(curl -X "PUT" "$TEAMWORK_URI/tasks/$TEAMWORK_TASK_ID/tags.json" \
-       -u "$TEAMWORK_API_TOKEN"':' \
-       -H 'Content-Type: application/json; charset=utf-8' \
-       -d "{ \"tags\": { \"content\": \"${tag_name//\"/}\" } }" )
+    response=$(teamwork::request PUT "/tasks/$TEAMWORK_TASK_ID/tags.json" \
+        "{ \"tags\": { \"content\": \"${tag_name//\"/}\" } }")
 
     log::message "$response"
   fi
@@ -129,10 +136,8 @@ teamwork::remove_tag() {
   fi
 
   if [ "$AUTOMATIC_TAGGING" == true ]; then
-    response=$(curl -X "PUT" "$TEAMWORK_URI/tasks/$TEAMWORK_TASK_ID/tags.json" \
-         -u "$TEAMWORK_API_TOKEN"':' \
-         -H 'Content-Type: application/json; charset=utf-8' \
-         -d "{ \"tags\": { \"content\": \"${tag_name//\"/}\" },\"removeProvidedTags\":\"true\" }" )
+    response=$(teamwork::request PUT "/tasks/$TEAMWORK_TASK_ID/tags.json" \
+        "{ \"tags\": { \"content\": \"${tag_name//\"/}\" },\"removeProvidedTags\":\"true\" }")
 
     log::message "$response"
   fi
diff --git a/tests/cases/closed-merged-lightweight/event.json b/tests/cases/closed-merged-lightweight/event.json
new file mode 100644
index 00000000..97732713
--- /dev/null
+++ b/tests/cases/closed-merged-lightweight/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": true
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/closed-merged-lightweight/expected b/tests/cases/closed-merged-lightweight/expected
new file mode 100644
index 00000000..3c02d13c
--- /dev/null
+++ b/tests/cases/closed-merged-lightweight/expected
@@ -0,0 +1,10 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) merged by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/closed-merged-lightweight/inputs b/tests/cases/closed-merged-lightweight/inputs
new file mode 100644
index 00000000..76857bf4
--- /dev/null
+++ b/tests/cases/closed-merged-lightweight/inputs
@@ -0,0 +1,2 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/closed-merged/event.json b/tests/cases/closed-merged/event.json
new file mode 100644
index 00000000..97732713
--- /dev/null
+++ b/tests/cases/closed-merged/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": true
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/closed-merged/expected b/tests/cases/closed-merged/expected
new file mode 100644
index 00000000..5fb0536e
--- /dev/null
+++ b/tests/cases/closed-merged/expected
@@ -0,0 +1,27 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 400
+  invalid JSON: Invalid control character at: line 1 column 25 (char 24)
+  "{ \"comment\": { \"body\": \"\n**octocat** merged a PR: **Add OAuth login**\n[https://github.com/acme/webapp/pull/42](https://github.com/acme/webapp/pull/42)\n\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }"
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Merged"
+
+PUT /tasks/1001/tags.json -> 200
+  removeProvidedTags: "true"
+  tags:
+    content: "PR Open"
+
+PUT /tasks/1001/tags.json -> 200
+  removeProvidedTags: "true"
+  tags:
+    content: "PR Approved"
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 73
diff --git a/tests/cases/closed-merged/inputs b/tests/cases/closed-merged/inputs
new file mode 100644
index 00000000..9d43a813
--- /dev/null
+++ b/tests/cases/closed-merged/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/closed-unmerged-lightweight/event.json b/tests/cases/closed-unmerged-lightweight/event.json
new file mode 100644
index 00000000..09ccdcd9
--- /dev/null
+++ b/tests/cases/closed-unmerged-lightweight/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/closed-unmerged-lightweight/expected b/tests/cases/closed-unmerged-lightweight/expected
new file mode 100644
index 00000000..e4430984
--- /dev/null
+++ b/tests/cases/closed-unmerged-lightweight/expected
@@ -0,0 +1,10 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) closed without merging by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/closed-unmerged-lightweight/inputs b/tests/cases/closed-unmerged-lightweight/inputs
new file mode 100644
index 00000000..76857bf4
--- /dev/null
+++ b/tests/cases/closed-unmerged-lightweight/inputs
@@ -0,0 +1,2 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/closed-unmerged/event.json b/tests/cases/closed-unmerged/event.json
new file mode 100644
index 00000000..09ccdcd9
--- /dev/null
+++ b/tests/cases/closed-unmerged/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/closed-unmerged/expected b/tests/cases/closed-unmerged/expected
new file mode 100644
index 00000000..ab0f25ce
--- /dev/null
+++ b/tests/cases/closed-unmerged/expected
@@ -0,0 +1,23 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 400
+  invalid JSON: Invalid control character at: line 1 column 25 (char 24)
+  "{ \"comment\": { \"body\": \"\n**octocat** closed a PR without merging: **Add OAuth login**\n[https://github.com/acme/webapp/pull/42](https://github.com/acme/webapp/pull/42)\n\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }"
+
+PUT /tasks/1001/tags.json -> 200
+  removeProvidedTags: "true"
+  tags:
+    content: "PR Open"
+
+PUT /tasks/1001/tags.json -> 200
+  removeProvidedTags: "true"
+  tags:
+    content: "PR Approved"
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 74
diff --git a/tests/cases/closed-unmerged/inputs b/tests/cases/closed-unmerged/inputs
new file mode 100644
index 00000000..9d43a813
--- /dev/null
+++ b/tests/cases/closed-unmerged/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/comment-fails/event.json b/tests/cases/comment-fails/event.json
new file mode 100644
index 00000000..dd7b97e2
--- /dev/null
+++ b/tests/cases/comment-fails/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1003",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/comment-fails/expected b/tests/cases/comment-fails/expected
new file mode 100644
index 00000000..faeb3aba
--- /dev/null
+++ b/tests/cases/comment-fails/expected
@@ -0,0 +1,20 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1003.json -> 200
+
+POST /tasks/1003/comments.json -> 500
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+PUT /tasks/1003/tags.json -> 200
+  tags:
+    content: "PR Open"
+
+GET /projects/api/v3/workflows.json?projectIds=300&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1003/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 72
diff --git a/tests/cases/comment-fails/inputs b/tests/cases/comment-fails/inputs
new file mode 100644
index 00000000..8a3fe736
--- /dev/null
+++ b/tests/cases/comment-fails/inputs
@@ -0,0 +1,6 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/deprecated-board-column-closed/event.json b/tests/cases/deprecated-board-column-closed/event.json
new file mode 100644
index 00000000..09ccdcd9
--- /dev/null
+++ b/tests/cases/deprecated-board-column-closed/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/deprecated-board-column-closed/expected b/tests/cases/deprecated-board-column-closed/expected
new file mode 100644
index 00000000..3d2d9efc
--- /dev/null
+++ b/tests/cases/deprecated-board-column-closed/expected
@@ -0,0 +1,16 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) closed without merging by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 74
diff --git a/tests/cases/deprecated-board-column-closed/inputs b/tests/cases/deprecated-board-column-closed/inputs
new file mode 100644
index 00000000..3a803811
--- /dev/null
+++ b/tests/cases/deprecated-board-column-closed/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
+INPUT_BOARD_COLUMN_CLOSED=Rejected
diff --git a/tests/cases/deprecated-board-column/event.json b/tests/cases/deprecated-board-column/event.json
new file mode 100644
index 00000000..5f80ac3b
--- /dev/null
+++ b/tests/cases/deprecated-board-column/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/deprecated-board-column/expected b/tests/cases/deprecated-board-column/expected
new file mode 100644
index 00000000..395ab4d1
--- /dev/null
+++ b/tests/cases/deprecated-board-column/expected
@@ -0,0 +1,16 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 73
diff --git a/tests/cases/deprecated-board-column/inputs b/tests/cases/deprecated-board-column/inputs
new file mode 100644
index 00000000..28f49173
--- /dev/null
+++ b/tests/cases/deprecated-board-column/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
+INPUT_BOARD_COLUMN_OPENED=Ready to Test
diff --git a/tests/cases/ignored-project/event.json b/tests/cases/ignored-project/event.json
new file mode 100644
index 00000000..a68d412c
--- /dev/null
+++ b/tests/cases/ignored-project/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Tasks: http://teamwork:8080/app/tasks/1001 and http://teamwork:8080/app/tasks/1002",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/ignored-project/expected b/tests/cases/ignored-project/expected
new file mode 100644
index 00000000..19f4cbd7
--- /dev/null
+++ b/tests/cases/ignored-project/expected
@@ -0,0 +1,3 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
diff --git a/tests/cases/ignored-project/inputs b/tests/cases/ignored-project/inputs
new file mode 100644
index 00000000..db11e710
--- /dev/null
+++ b/tests/cases/ignored-project/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
+IGNORE_PROJECT_IDS=100 300
diff --git a/tests/cases/no-task-links/event.json b/tests/cases/no-task-links/event.json
new file mode 100644
index 00000000..4517e6e7
--- /dev/null
+++ b/tests/cases/no-task-links/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Refactors the login form.",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/no-task-links/expected b/tests/cases/no-task-links/expected
new file mode 100644
index 00000000..01fe490f
--- /dev/null
+++ b/tests/cases/no-task-links/expected
@@ -0,0 +1 @@
+exit code: 0
diff --git a/tests/cases/no-task-links/inputs b/tests/cases/no-task-links/inputs
new file mode 100644
index 00000000..9d43a813
--- /dev/null
+++ b/tests/cases/no-task-links/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/opened-lightweight/event.json b/tests/cases/opened-lightweight/event.json
new file mode 100644
index 00000000..d524f93d
--- /dev/null
+++ b/tests/cases/opened-lightweight/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Handle \"null\" in C:\\temp",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/opened-lightweight/expected b/tests/cases/opened-lightweight/expected
new file mode 100644
index 00000000..55e3f681
--- /dev/null
+++ b/tests/cases/opened-lightweight/expected
@@ -0,0 +1,20 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Handle null in C:\temp**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Open"
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 72
diff --git a/tests/cases/opened-lightweight/inputs b/tests/cases/opened-lightweight/inputs
new file mode 100644
index 00000000..8a3fe736
--- /dev/null
+++ b/tests/cases/opened-lightweight/inputs
@@ -0,0 +1,6 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/opened/event.json b/tests/cases/opened/event.json
new file mode 100644
index 00000000..f3579555
--- /dev/null
+++ b/tests/cases/opened/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add \"OAuth\" login",
+    "body": "## Summary\r\nAdds OAuth login for http://teamwork:8080/app/tasks/1001\r\n\r\n```sh\r\n# configure the \"client\" secret\r\nexport CONFIG_DIR=C:\\Users\\dev\\config\r\n```",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/opened/expected b/tests/cases/opened/expected
new file mode 100644
index 00000000..e550bec5
--- /dev/null
+++ b/tests/cases/opened/expected
@@ -0,0 +1,17 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 400
+  invalid JSON: Invalid control character at: line 1 column 25 (char 24)
+  "{ \"comment\": { \"body\": \"\n**octocat** opened a PR: **Add OAuth login**\n[https://github.com/acme/webapp/pull/42](https://github.com/acme/webapp/pull/42)\n`main` \u2b05\ufe0f `feature/oauth`\n\n---\n\n#### Summary\r\nAdds OAuth login for http://teamwork:8080/app/tasks/1001\r\n\r\n```sh\r\n#### configure the client secret\r\nexport CONFIG_DIR=C:\\Users\\dev\\config\r\n```\n\n---\n\n\ud83d\udd22 3 commits / \ud83d\udcdd 5 files updated / \u2795 120 additions / \u2796 8 deletions\n\n\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }"
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Open"
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 72
diff --git a/tests/cases/opened/inputs b/tests/cases/opened/inputs
new file mode 100644
index 00000000..9d43a813
--- /dev/null
+++ b/tests/cases/opened/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/review-approved-lightweight/event.json b/tests/cases/review-approved-lightweight/event.json
new file mode 100644
index 00000000..48e8f8f7
--- /dev/null
+++ b/tests/cases/review-approved-lightweight/event.json
@@ -0,0 +1,31 @@
+{
+  "action": "submitted",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  },
+  "review": {
+    "state": "approved",
+    "body": "Looks good!"
+  }
+}
diff --git a/tests/cases/review-approved-lightweight/expected b/tests/cases/review-approved-lightweight/expected
new file mode 100644
index 00000000..9d4207f1
--- /dev/null
+++ b/tests/cases/review-approved-lightweight/expected
@@ -0,0 +1,14 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) approved by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Approved"
diff --git a/tests/cases/review-approved-lightweight/inputs b/tests/cases/review-approved-lightweight/inputs
new file mode 100644
index 00000000..1f98d2ae
--- /dev/null
+++ b/tests/cases/review-approved-lightweight/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request_review
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/review-approved-no-body/event.json b/tests/cases/review-approved-no-body/event.json
new file mode 100644
index 00000000..ef9793c8
--- /dev/null
+++ b/tests/cases/review-approved-no-body/event.json
@@ -0,0 +1,31 @@
+{
+  "action": "submitted",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  },
+  "review": {
+    "state": "approved",
+    "body": null
+  }
+}
diff --git a/tests/cases/review-approved-no-body/expected b/tests/cases/review-approved-no-body/expected
new file mode 100644
index 00000000..84ff709f
--- /dev/null
+++ b/tests/cases/review-approved-no-body/expected
@@ -0,0 +1,11 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 400
+  invalid JSON: Invalid control character at: line 1 column 25 (char 24)
+  "{ \"comment\": { \"body\": \"\n**octocat** submitted a review to the PR: **Add OAuth login**\n[https://github.com/acme/webapp/pull/42](https://github.com/acme/webapp/pull/42)\n\n---\n\nReview: **approved**\n\n\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }"
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Approved"
diff --git a/tests/cases/review-approved-no-body/inputs b/tests/cases/review-approved-no-body/inputs
new file mode 100644
index 00000000..bf4f6030
--- /dev/null
+++ b/tests/cases/review-approved-no-body/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request_review
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/review-approved/event.json b/tests/cases/review-approved/event.json
new file mode 100644
index 00000000..3e77ae9d
--- /dev/null
+++ b/tests/cases/review-approved/event.json
@@ -0,0 +1,31 @@
+{
+  "action": "submitted",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  },
+  "review": {
+    "state": "approved",
+    "body": "Looks good!\r\nOne nit: rename `cfg` to \"config\"."
+  }
+}
diff --git a/tests/cases/review-approved/expected b/tests/cases/review-approved/expected
new file mode 100644
index 00000000..f24cf9cb
--- /dev/null
+++ b/tests/cases/review-approved/expected
@@ -0,0 +1,11 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 400
+  invalid JSON: Invalid control character at: line 1 column 25 (char 24)
+  "{ \"comment\": { \"body\": \"\n**octocat** submitted a review to the PR: **Add OAuth login**\n[https://github.com/acme/webapp/pull/42](https://github.com/acme/webapp/pull/42)\n\n---\n\nReview: **approved**\nLooks good!\r\nOne nit: rename `cfg` to config.\n\", \"notify\": true, \"content-type\": \"text\", \"isprivate\": false } }"
+
+PUT /tasks/1001/tags.json -> 200
+  tags:
+    content: "PR Approved"
diff --git a/tests/cases/review-approved/inputs b/tests/cases/review-approved/inputs
new file mode 100644
index 00000000..bf4f6030
--- /dev/null
+++ b/tests/cases/review-approved/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request_review
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/review-commented/event.json b/tests/cases/review-commented/event.json
new file mode 100644
index 00000000..aa67d482
--- /dev/null
+++ b/tests/cases/review-commented/event.json
@@ -0,0 +1,31 @@
+{
+  "action": "submitted",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  },
+  "review": {
+    "state": "commented",
+    "body": "Why this change?"
+  }
+}
diff --git a/tests/cases/review-commented/expected b/tests/cases/review-commented/expected
new file mode 100644
index 00000000..19f4cbd7
--- /dev/null
+++ b/tests/cases/review-commented/expected
@@ -0,0 +1,3 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
diff --git a/tests/cases/review-commented/inputs b/tests/cases/review-commented/inputs
new file mode 100644
index 00000000..bf4f6030
--- /dev/null
+++ b/tests/cases/review-commented/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request_review
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/review-dismissed/event.json b/tests/cases/review-dismissed/event.json
new file mode 100644
index 00000000..3ad66cf5
--- /dev/null
+++ b/tests/cases/review-dismissed/event.json
@@ -0,0 +1,31 @@
+{
+  "action": "dismissed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  },
+  "review": {
+    "state": "dismissed",
+    "body": null
+  }
+}
diff --git a/tests/cases/review-dismissed/expected b/tests/cases/review-dismissed/expected
new file mode 100644
index 00000000..73b63c87
--- /dev/null
+++ b/tests/cases/review-dismissed/expected
@@ -0,0 +1,10 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "Review dismissed by octocat"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/review-dismissed/inputs b/tests/cases/review-dismissed/inputs
new file mode 100644
index 00000000..bf4f6030
--- /dev/null
+++ b/tests/cases/review-dismissed/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request_review
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/stage-not-found/event.json b/tests/cases/stage-not-found/event.json
new file mode 100644
index 00000000..5f80ac3b
--- /dev/null
+++ b/tests/cases/stage-not-found/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/stage-not-found/expected b/tests/cases/stage-not-found/expected
new file mode 100644
index 00000000..b1796459
--- /dev/null
+++ b/tests/cases/stage-not-found/expected
@@ -0,0 +1,12 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
diff --git a/tests/cases/stage-not-found/inputs b/tests/cases/stage-not-found/inputs
new file mode 100644
index 00000000..c14aee62
--- /dev/null
+++ b/tests/cases/stage-not-found/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
+INPUT_WORKFLOW_STAGE_OPENED=Code Review
diff --git a/tests/cases/tagging-disabled/event.json b/tests/cases/tagging-disabled/event.json
new file mode 100644
index 00000000..97732713
--- /dev/null
+++ b/tests/cases/tagging-disabled/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "closed",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": true
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/tagging-disabled/expected b/tests/cases/tagging-disabled/expected
new file mode 100644
index 00000000..c39de2a7
--- /dev/null
+++ b/tests/cases/tagging-disabled/expected
@@ -0,0 +1,16 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) merged by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v3/workflows.json?projectIds=100&include=stages -> 200
+
+PATCH /projects/api/v3/tasks/1001/workflows/7.json -> 204
+  positionAfterTask: -1
+  stageId: 73
diff --git a/tests/cases/tagging-disabled/inputs b/tests/cases/tagging-disabled/inputs
new file mode 100644
index 00000000..60904cd2
--- /dev/null
+++ b/tests/cases/tagging-disabled/inputs
@@ -0,0 +1,4 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=false
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/task-links/event.json b/tests/cases/task-links/event.json
new file mode 100644
index 00000000..94875ab9
--- /dev/null
+++ b/tests/cases/task-links/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Linked twice: [http://teamwork:8080/app/tasks/1001](http://teamwork:8080/app/tasks/1001)\nUpper case, old route: HTTP://TEAMWORK:8080/#/tasks/1002\nAgain: http://teamwork:8080/#tasks/1001\nNot task links: subtasks/5, https://github.com/acme/webapp/blob/main/scripts/tasks/2024_cleanup.py\nAnother Teamwork site: https://acme.teamwork.com/app/tasks/1003\nLook-alike host: http://evil-teamwork:8080/app/tasks/1003",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/task-links/expected b/tests/cases/task-links/expected
new file mode 100644
index 00000000..8bc306ea
--- /dev/null
+++ b/tests/cases/task-links/expected
@@ -0,0 +1,73 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1002.json -> 200
+
+POST /tasks/1002/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/5.json -> 404
+
+POST /tasks/5/comments.json -> 404
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/2024.json -> 404
+
+POST /tasks/2024/comments.json -> 404
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1003.json -> 200
+
+POST /tasks/1003/comments.json -> 500
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1003.json -> 200
+
+POST /tasks/1003/comments.json -> 500
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/task-links/inputs b/tests/cases/task-links/inputs
new file mode 100644
index 00000000..76857bf4
--- /dev/null
+++ b/tests/cases/task-links/inputs
@@ -0,0 +1,2 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/task-not-found/event.json b/tests/cases/task-not-found/event.json
new file mode 100644
index 00000000..c9ba3a23
--- /dev/null
+++ b/tests/cases/task-not-found/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Tasks: http://teamwork:8080/app/tasks/9999 and http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/task-not-found/expected b/tests/cases/task-not-found/expected
new file mode 100644
index 00000000..57c82da5
--- /dev/null
+++ b/tests/cases/task-not-found/expected
@@ -0,0 +1,19 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/9999.json -> 404
+
+POST /tasks/9999/comments.json -> 404
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
+
+GET /projects/api/v1/tasks/1001.json -> 200
+
+POST /tasks/1001/comments.json -> 201
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/task-not-found/inputs b/tests/cases/task-not-found/inputs
new file mode 100644
index 00000000..76857bf4
--- /dev/null
+++ b/tests/cases/task-not-found/inputs
@@ -0,0 +1,2 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/cases/unsupported-event/event.json b/tests/cases/unsupported-event/event.json
new file mode 100644
index 00000000..da136fb5
--- /dev/null
+++ b/tests/cases/unsupported-event/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "synchronize",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/unsupported-event/expected b/tests/cases/unsupported-event/expected
new file mode 100644
index 00000000..19f4cbd7
--- /dev/null
+++ b/tests/cases/unsupported-event/expected
@@ -0,0 +1,3 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 200
diff --git a/tests/cases/unsupported-event/inputs b/tests/cases/unsupported-event/inputs
new file mode 100644
index 00000000..9d43a813
--- /dev/null
+++ b/tests/cases/unsupported-event/inputs
@@ -0,0 +1,5 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_AUTOMATIC_TAGGING=true
+INPUT_WORKFLOW_STAGE_OPENED=PR Open
+INPUT_WORKFLOW_STAGE_MERGED=Ready to Test
+INPUT_WORKFLOW_STAGE_CLOSED=Rejected
diff --git a/tests/cases/wrong-token/event.json b/tests/cases/wrong-token/event.json
new file mode 100644
index 00000000..5f80ac3b
--- /dev/null
+++ b/tests/cases/wrong-token/event.json
@@ -0,0 +1,27 @@
+{
+  "action": "opened",
+  "number": 42,
+  "pull_request": {
+    "number": 42,
+    "title": "Add OAuth login",
+    "body": "Task: http://teamwork:8080/app/tasks/1001",
+    "html_url": "https://github.com/acme/webapp/pull/42",
+    "head": {
+      "ref": "feature/oauth",
+      "repo": {
+        "full_name": "acme/webapp"
+      }
+    },
+    "base": {
+      "ref": "main"
+    },
+    "commits": 3,
+    "changed_files": 5,
+    "additions": 120,
+    "deletions": 8,
+    "merged": false
+  },
+  "sender": {
+    "login": "octocat"
+  }
+}
diff --git a/tests/cases/wrong-token/expected b/tests/cases/wrong-token/expected
new file mode 100644
index 00000000..a86eb133
--- /dev/null
+++ b/tests/cases/wrong-token/expected
@@ -0,0 +1,10 @@
+exit code: 0
+
+GET /projects/api/v1/tasks/1001.json -> 401
+
+POST /tasks/1001/comments.json -> 401
+  comment:
+    body: "PR [**Add OAuth login**](https://github.com/acme/webapp/pull/42) opened by **octocat**"
+    content-type: "text"
+    isprivate: false
+    notify: true
diff --git a/tests/cases/wrong-token/inputs b/tests/cases/wrong-token/inputs
new file mode 100644
index 00000000..1e001d12
--- /dev/null
+++ b/tests/cases/wrong-token/inputs
@@ -0,0 +1,3 @@
+GITHUB_EVENT_NAME=pull_request
+INPUT_TEAMWORK_API_TOKEN=wrong-token
+INPUT_LIGHTWEIGHT_COMMENT=true
diff --git a/tests/mock-teamwork.py b/tests/mock-teamwork.py
new file mode 100644
index 00000000..2058c2ba
--- /dev/null
+++ b/tests/mock-teamwork.py
@@ -0,0 +1,105 @@
+"""Mock Teamwork API for tests/run.sh.
+
+Answers from the fixed data below and records every request it receives, in a
+readable form, to the file given as its only argument.
+"""
+import base64
+import json
+import re
+import sys
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+LOG_PATH = sys.argv[1]
+AUTHORIZATION = "Basic " + base64.b64encode(b"test-token:").decode()
+
+# Task id -> project id. Requests about any other task fail with a 404.
+TASKS = {"1001": 100, "1002": 200, "1003": 300}
+# Adding a comment to these tasks fails with a server error.
+FAILING_COMMENTS = {"1003"}
+# Every project uses the same workflow (id 7), with these stages.
+STAGES = {
+    str(stage_id): {"id": stage_id, "name": name, "workflow": {"id": 7, "type": "workflows"}}
+    for stage_id, name in [(71, "To Do"), (72, "PR Open"), (73, "Ready to Test"), (74, "Rejected")]
+}
+
+
+def render(value, indent=1):
+    """Renders a JSON body as indented "key: value" lines. Multi-line strings are
+    shown line by line after a "|" gutter, with control characters such as \\r
+    shown as their Unicode pictures (␍)."""
+    pad = "  " * indent
+    if not isinstance(value, dict):
+        return [pad + json.dumps(value, ensure_ascii=False)]
+    lines = []
+    for key in sorted(value):
+        item = value[key]
+        if isinstance(item, dict):
+            lines += [f"{pad}{key}:", *render(item, indent + 1)]
+        elif isinstance(item, str) and "\n" in item:
+            lines.append(f"{pad}{key}:")
+            for line in item.split("\n"):
+                line = "".join(chr(0x2400 + ord(c)) if ord(c) < 0x20 else c for c in line)
+                lines.append(f"{pad}  | {line}".rstrip())
+        else:
+            lines.append(f"{pad}{key}: {json.dumps(item, ensure_ascii=False)}")
+    return lines
+
+
+class Handler(BaseHTTPRequestHandler):
+    def log_message(self, *args):
+        pass
+
+    def handle_request(self):
+        raw = self.rfile.read(int(self.headers.get("Content-Length") or 0)).decode()
+        status, response, details = self.respond(raw)
+        # Record before replying, so the log is complete once the action exits
+        with open(LOG_PATH, "a") as log:
+            log.write("\n" + "\n".join([f"{self.command} {self.path} -> {status}", *details]) + "\n")
+
+        body = json.dumps(response).encode() if response is not None else b""
+        self.send_response(status)
+        if body:
+            self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(body)))
+        self.end_headers()
+        self.wfile.write(body)
+
+    do_GET = do_POST = do_PUT = do_PATCH = handle_request
+
+    def respond(self, raw):
+        """Returns the status, response body and recorded details of a request."""
+        details = []
+        if raw:
+            if not self.headers.get("Content-Type", "").startswith("application/json"):
+                return 415, {"MESSAGE": "Expected a JSON body"}, ["  " + json.dumps(raw)]
+            try:
+                details = render(json.loads(raw))
+            except ValueError as error:
+                return 400, {"MESSAGE": "Invalid JSON"}, [f"  invalid JSON: {error}", "  " + json.dumps(raw)]
+
+        if self.headers.get("Authorization") != AUTHORIZATION:
+            return 401, {"MESSAGE": "Unauthorized"}, details
+
+        task = re.search(r"/tasks/(\d+)", self.path)
+        if task and task[1] not in TASKS:
+            return 404, {"MESSAGE": "Not found"}, details
+
+        request = f"{self.command} {self.path}"
+        if task := re.fullmatch(r"GET /projects/api/v1/tasks/(\d+)\.json", request):
+            return 200, {"todo-item": {"id": int(task[1]), "project-id": TASKS[task[1]]}}, details
+        if re.fullmatch(r"GET /projects/api/v3/workflows\.json\?projectIds=\d+&include=stages", request):
+            return 200, {"workflows": [{"id": 7, "name": "Development"}], "included": {"stages": STAGES}}, details
+        if task := re.fullmatch(r"POST /tasks/(\d+)/comments\.json", request):
+            if task[1] in FAILING_COMMENTS:
+                return 500, {"MESSAGE": "Internal server error"}, details
+            return 201, {"STATUS": "OK", "commentId": "1"}, details
+        if re.fullmatch(r"PUT /tasks/\d+/tags\.json", request):
+            return 200, {"STATUS": "OK"}, details
+        if re.fullmatch(r"PATCH /projects/api/v3/tasks/\d+/workflows/7\.json", request):
+            return 204, None, details
+        return 404, {"MESSAGE": "Unknown route"}, details
+
+
+server = HTTPServer(("", 8080), Handler)
+print("listening", flush=True)
+server.serve_forever()
diff --git a/tests/run.sh b/tests/run.sh
new file mode 100755
index 00000000..f2cd198c
--- /dev/null
+++ b/tests/run.sh
@@ -0,0 +1,157 @@
+#!/usr/bin/env bash
+# Runs the action's Docker image against a mock Teamwork API
+# (tests/mock-teamwork.py) for every case in tests/cases, and compares the exit
+# code, stderr and requests the mock received with the case's expected file.
+#
+# A case is a directory with:
+#   event.json  the GitHub event payload
+#   inputs      KEY=VALUE lines: GITHUB_EVENT_NAME, the INPUT_* env vars GitHub
+#               creates for the action's inputs, and any other env such as
+#               IGNORE_PROJECT_IDS. Inputs that aren't set are empty.
+#   expected    what the run should produce
+#
+# Usage: tests/run.sh [--update] [filter]
+#   --update  rewrites the expected files from the current output
+#   filter    only runs the cases whose name contains it
+set -euo pipefail
+
+ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+IMAGE=github-sync:test
+NETWORK=github-sync-test
+MOCK=github-sync-teamwork
+
+update=false
+filter=
+for arg in "$@"; do
+  if [ "$arg" == "--update" ]; then
+    update=true
+  else
+    filter=$arg
+  fi
+done
+
+stop_mock() {
+  docker rm --force "$MOCK" >/dev/null 2>&1 || true
+  docker network rm "$NETWORK" >/dev/null 2>&1 || true
+}
+
+work=$(mktemp -d)
+trap 'stop_mock; rm -rf "$work"' EXIT
+
+# The action's args, in action.yml order, as the input each one comes from
+# (empty for the '' placeholders). The GitHub runner builds them the same way.
+arg_inputs=()
+while IFS= read -r item || [ -n "$item" ]; do
+  case $item in
+    "''") arg_inputs+=("") ;;
+    "\${{ inputs."*" }}")
+      item=${item#"\${{ inputs."}
+      arg_inputs+=("${item%" }}"}") ;;
+    *)
+      echo "Unsupported item in action.yml's runs.args: $item" >&2
+      exit 1 ;;
+  esac
+done < <(sed -n '/^  args:/,/^[^ #]/ s/^    - //p' "$ROOT/action.yml")
+
+# Runs the action for the case in $1 the way GitHub does: inputs as INPUT_* env
+# vars, passed by name only, and as positional args built from action.yml.
+run_case() (
+  local -r dir=$1
+
+  export INPUT_TEAMWORK_URI=http://teamwork:8080
+  export INPUT_TEAMWORK_API_TOKEN=test-token
+  local env_names=(INPUT_TEAMWORK_URI INPUT_TEAMWORK_API_TOKEN)
+  local line
+  while IFS= read -r line || [ -n "$line" ]; do
+    case $line in ''|'#'*) continue ;; esac
+    export "${line%%=*}=${line#*=}"
+    env_names+=("${line%%=*}")
+  done < "$dir/inputs"
+
+  local docker_args=(--rm --network "$NETWORK" --volume "$dir/event.json:/github/workflow/event.json:ro"
+    --env GITHUB_REPOSITORY=acme/webapp --env GITHUB_EVENT_PATH=/github/workflow/event.json)
+  local env_name
+  for env_name in "${env_names[@]}"; do
+    docker_args+=(--env "$env_name")
+  done
+
+  local action_args=() input value
+  for input in "${arg_inputs[@]}"; do
+    value=
+    if [ -n "$input" ]; then
+      env_name="INPUT_$input"
+      value=${!env_name:-}
+    fi
+    action_args+=("$value")
+  done
+
+  docker run "${docker_args[@]}" "$IMAGE" "${action_args[@]}"
+)
+
+docker build --quiet --tag "$IMAGE" "$ROOT" >/dev/null
+
+stop_mock
+docker network create "$NETWORK" >/dev/null
+docker run --detach --name "$MOCK" --network "$NETWORK" --network-alias teamwork \
+  --volume "$ROOT/tests/mock-teamwork.py:/mock-teamwork.py:ro" --volume "$work:/out" \
+  python:3.13-alpine python /mock-teamwork.py /out/requests >/dev/null
+for _ in $(seq 50); do
+  if docker logs "$MOCK" 2>&1 | grep -q listening; then
+    break
+  fi
+  sleep 0.1
+done
+if ! docker logs "$MOCK" 2>&1 | grep -q listening; then
+  echo "The mock Teamwork API didn't start:" >&2
+  docker logs "$MOCK" >&2
+  exit 1
+fi
+
+ran=0
+failed=0
+for dir in "$ROOT"/tests/cases/*/; do
+  dir=${dir%/}
+  name=$(basename "$dir")
+  case $name in *"$filter"*) ;; *) continue ;; esac
+  ran=$((ran + 1))
+
+  : > "$work/requests"
+  code=0
+  run_case "$dir" > "$work/stdout" 2> "$work/stderr" || code=$?
+  {
+    echo "exit code: $code"
+    if [ -s "$work/stderr" ]; then
+      echo
+      echo "stderr:"
+      sed 's/^/  /' "$work/stderr"
+    fi
+    cat "$work/requests"
+  } > "$work/actual"
+
+  if $update; then
+    cp "$work/actual" "$dir/expected"
+    echo "updated  $name"
+  elif diff -u "$dir/expected" "$work/actual" > "$work/diff" 2>&1; then
+    echo "ok       $name"
+  else
+    failed=$((failed + 1))
+    echo "FAIL     $name"
+    sed 's/^/    /' "$work/diff"
+    echo "    action stdout:"
+    sed 's/^/      /' "$work/stdout"
+  fi
+done
+
+echo
+if ((ran == 0)); then
+  echo "No cases match '$filter'"
+  exit 1
+fi
+if $update; then
+  echo "Updated $ran expected file(s), review them before committing"
+elif ((failed > 0)); then
+  echo "$failed of $ran case(s) failed"
+  exit 1
+else
+  echo "All $ran case(s) passed"
+fi