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The Thaddeus Foundation Software Engineer Interview Guide

The Thaddeus Foundation Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

As a Software Engineer at The Thaddeus Foundation, you will play a pivotal role in developing and maintaining technology solutions that drive the organization’s mission forward. This role is crucial as it directly impacts the foundation's ability to deliver innovative, user-friendly products that serve its community. You will collaborate with cross-functional teams to design, build, and enhance systems that facilitate outreach and support efforts, making a meaningful difference in the areas the foundation serves. In this position, you can expect to work on a variety of projects that involve complex problem-solving and the application of your technical skills. You will engage with modern technologies, such as React Native, and will have the opportunity to contribute to projects that require both creativity and analytical thinking.

This guide is scoped to a Software Engineer candidate at The Thaddeus Foundation.

Focus areas

  • Behavioral Interviewing
  • React Native
  • HR Screening

The The Thaddeus Foundation interview process

The Thaddeus Foundation candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Screening

The first step involves a preliminary assessment of the candidate's qualifications and fit for the role.

What to demonstrate

  • The first step involves a preliminary assessment of the candidate's qualifications and fit for the role
  • Depth in Behavioral Interviewing

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Behavioral Discussions

Candidates will engage in discussions focused on their teamwork and problem-solving approaches.

What to demonstrate

  • Candidates will engage in discussions focused on their teamwork and problem-solving approaches
  • Depth in Behavioral Interviewing

How to prepare

  • Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
  • Re-read the description of the behavioral discussions above and write down what you would ask to confirm before it.

Technical Assessments

Depending on the team, candidates may undergo technical evaluations to assess their skills.

What to demonstrate

  • Depending on the team, candidates may undergo technical evaluations to assess their skills
  • Depth in Behavioral Interviewing

How to prepare

  • Answer aloud and timed: Explain a recent project where you utilized a specific technology or framework.
  • Answer aloud and timed: How do you approach debugging a complex issue in your code?

Where candidates lose points

Research the Foundation

Understanding the mission and values will help you frame your answers in a way that resonates with the interviewers.

Practice Behavioral Questions

Prepare specific examples that highlight your experiences and achievements in previous roles.

Be Honest About Your Skills

If you encounter questions about areas where you're less experienced, express a willingness to learn and improve.

Engage with Interviewers

Ask insightful questions about the foundation's projects and priorities to demonstrate your interest.

Questions & practice

Write a function that reverses a string.

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

How would you find the first non-repeating character in a string?

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Explain the difference between depth-first search and breadth-first search.

Medium · Coding / Algorithms

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Keep soft-deleted accounts from blocking re-registration

Easy · soft delete · unique constraints · engine differences

app_user holds user_id, tenant_id, email CITEXT, password_hash (NULL for SSO principals), email_verified_at, auth_version, status ('invited','active','suspended','deactivated'), created_at, updated_at, deleted_at. Two live accounts for one address inside a tenant must be impossible, but an address freed by a soft delete must be reusable, and the same tenant may delete and re-register it repeatedly. Write the uniqueness DDL for PostgreSQL 16, then the equivalent for MySQL 8 where partial indexes do not exist, and say what each permits once three deleted rows already hold that address.

Approach

  1. Start from what is actually unique: not (tenant_id, email), but (tenant_id, email) among live rows. PostgreSQL says that directly — CREATE UNIQUE INDEX app_user_live_email ON app_user (tenant_id, email) WHERE deleted_at IS NULL. A full constraint over the same two columns burns the address permanently the first time someone deletes an account.
  2. Keep case-insensitivity in the type or the index, never in the application: CITEXT as given, or UNIQUE (tenant_id, lower(email)) as an expression index where the extension is unavailable. A case-sensitive unique column is exactly how two accounts for one human appear.
  3. For MySQL 8 the predicate has to move inside the key: add a discriminator column that is a constant 0 while the row is live and is set to user_id on delete, with UNIQUE (tenant_id, email, deleted_marker). Live rows share the constant and still collide; deleted rows differ from each other and stop colliding.
  4. State the NULL variant and its dependency: leaving the marker NULL for deleted rows also works, because a unique index treats NULLs as distinct — true in MySQL, and true in PostgreSQL only under the default NULLS DISTINCT, which PostgreSQL 15 lets you reverse. Check the polarity against the three existing deleted rows: constant-on-live is what preserves the collision you want, and reversing it silently admits duplicate live accounts.

Follow-up

  • A deleted account re-registers with the same address the next day. Do the old resource rows follow the new user_id, and how does the API keep the two principals apart?
  • How do you honour an erasure request while resource_revision.actor_user_id still references this table?

Stop tag and share joins from fanning out a page

Medium · join fan-out · semi-joins · many-to-many

resource_tag is (resource_id, tag_id) with PK (resource_id, tag_id); resource_share is (resource_id, shared_with_user_id, permission). The tagged-and-shared listing inner-joins resource to both, filters tenant_id, tag_id = ANY($2) and shared_with_user_id = $3, orders by updated_at DESC and takes 50. Pages come back with fewer than 50 distinct resources and the total in the header is far too high. Explain the row multiplication, rewrite both the page query and the count query so each is correct, and name the index each one needs. PostgreSQL 16.

Approach

  1. Do the arithmetic against the predicates that are actually there. An inner join emits one row per matching child row, and both joins are filtered: tag_id = ANY($2) admits only the requested tags, shared_with_user_id = $3 admits one user's share rows. So a resource holding three of the requested tags and shared with $3 once yields three rows, not one — the multiplier is its count of matching tags times its share rows for that single user, and that second factor is 1 unless the table admits duplicate (resource_id, shared_with_user_id) pairs. LIMIT 50 then limits rows rather than resources, and COUNT(*) counts pairs — the header is the product, not the population.
  2. Reject DISTINCT as the fix. It deduplicates after the product has been built, so the planner must materialise and sort the fanned-out set before the LIMIT can apply, and it leaves any SUM or AVG in the same select list wrong.
  3. Rewrite both filters as semi-joins, keeping resource as the only row source: AND EXISTS (SELECT 1 FROM resource_tag rt WHERE rt.resource_id = r.resource_id AND rt.tag_id = ANY($2)) and the same shape against resource_share. A semi-join stops at the first match per resource and preserves the driving index order, so ORDER BY updated_at DESC, resource_id DESC LIMIT 50 still stops after 50 rows.
  4. Count with the same predicates and no join at all: SELECT count(*) FROM resource r WHERE r.tenant_id = $1 AND r.status = 'active' AND EXISTS (...) AND EXISTS (...). Nothing multiplies a resource, so the number is the population.

Follow-up

  • The filter changes from 'any of these tags' to 'all of these tags'. Rewrite it and state what it costs relative to the ANY form.
  • A resource can be shared with the same user twice under different permissions. Does your count change, and should it?

What programming languages are you most comfortable with, and why?

Medium · Technical / Domain Questions

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Explain a recent project where you utilized a specific technology or framework.

Medium · Technical / Domain Questions

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How would you design a system to manage user data securely?

Medium · Problem-Solving / Case Studies

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Given a set of requirements, how would you approach the design of a new feature?

Medium · Problem-Solving / Case Studies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Can you walk us through the steps you would take to optimize an existing application?

Medium · Problem-Solving / Case Studies

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How do you approach debugging a complex issue in your code?

Medium · Technical / Domain Questions

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics The Thaddeus Foundation candidates report.

Day 1 — Map the The Thaddeus Foundation loop

  • Write out the reported sequence: Initial Screening, Behavioral Discussions, Technical Assessments.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work Behavioral Interviewing

  • Spend the session on Behavioral Interviewing, which The Thaddeus Foundation candidates report being tested on.
  • Write one worked example in Behavioral Interviewing and time yourself on it.

Deliverable: One timed worked example in Behavioral Interviewing.

Day 3 — Work React Native

  • Spend the session on React Native, which The Thaddeus Foundation candidates report being tested on.
  • Write one worked example in React Native and time yourself on it.

Deliverable: One timed worked example in React Native.

Day 4 — Work HR Screening

  • Spend the session on HR Screening, which The Thaddeus Foundation candidates report being tested on.
  • Write one worked example in HR Screening and time yourself on it.

Deliverable: One timed worked example in HR Screening.

Day 5 — Answer out loud: Behavioral / Leadership

  • Answer aloud, timed: Describe a time you faced a significant challenge at work. How did you overcome it?
  • Answer aloud, timed: Can you give an example of how you have worked effectively within a team?

Deliverable: Spoken answers to 2 reported Behavioral / Leadership question(s), under time.

Day 6 — Answer out loud: Technical / Domain Questions

  • Answer aloud, timed: What programming languages are you most comfortable with, and why?
  • Answer aloud, timed: Explain a recent project where you utilized a specific technology or framework.

Deliverable: Spoken answers to 2 reported Technical / Domain Questions question(s), under time.

Day 7 — Answer out loud: Problem-Solving / Case Studies

  • Answer aloud, timed: How would you design a system to manage user data securely?
  • Answer aloud, timed: Given a set of requirements, how would you approach the design of a new feature?

Deliverable: Spoken answers to 2 reported Problem-Solving / Case Studies question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe a time you faced a significant challenge at work. How did you overcome it?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Can you give an example of how you have worked effectively within a team?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

How do you prioritize your tasks when managing multiple deadlines?

Medium · Behavioral / Leadership

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?