Candid Health Software Engineer Interview Guide

The 60-second brief

At Candid Health, a Software Engineer plays a crucial role in rebuilding the complex, often broken infrastructure of healthcare billing. The company's core mission is to make medical billing transparent, fast, and accurate for healthcare providers and patients alike. As an engineer here, you will not just be writing code; you will be designing and scaling the APIs, state machines, and integrations that handle millions of dollars in medical claims. The engineering team works on highly complex problem spaces, such as automating insurance eligibility checks, managing intricate medical coding rules, and building robust pipelines that interface with legacy clearinghouses. The work requires a deep appreciation for data integrity, system reliability, and developer-friendly API design.

This guide is scoped to a Software Engineer candidate at Candid Health.

Focus areas

• TypeScript
• Coding Interviews (Practical Implementation)
• System Design

The Candid Health interview process

Candid Health candidates report 6 rounds over 4-6 weeks. The stages below are what candidates describe, not a published process.

Recruiter Screen

An initial introductory conversation to discuss your background, your interest in Candid Health, and the basics of the role.

What to demonstrate

• An initial introductory conversation to discuss your background, your interest in Candid Health, and the basics of the role
• Depth in TypeScript

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.

Technical Screen / Take-Home

Complete a practical take-home coding exercise or a live remote coding session focused on writing clean, working code in TypeScript.

What to demonstrate

• Complete a practical take-home coding exercise or a live remote coding session focused on writing clean, working code in TypeScript
• Depth in TypeScript

How to prepare

• Answer aloud and timed: Write a program to simulate a state machine that transitions a billing invoice through various stages (e.g., Draft, Submitted, Paid, Rejected) while handling invalid state transitions.
• Answer aloud and timed: Explain the difference between structural typing and nominal typing in TypeScript, and demonstrate how you would leverage type guards to handle runtime API responses.

Coding Interview 2

A live technical session focusing on a slightly more complex, open-ended functional programming task.

What to demonstrate

• A live technical session focusing on a slightly more complex, open-ended functional programming task
• Depth in TypeScript

How to prepare

• Answer aloud and timed: Given a diagram of a simplified, monolithic billing ingestion service, identify the primary bottlenecks and propose a step-by-step plan to decouple it into microservices.
• Answer aloud and timed: How would you modify an existing database schema and API contract to support multi-tenant billing without causing downtime for existing customers?

System Design & Modification

A collaborative session in a diagramming tool to analyze and modify an existing system architecture.

What to demonstrate

• A collaborative session in a diagramming tool to analyze and modify an existing system architecture
• Depth in TypeScript

How to prepare

• Answer aloud and timed: Design a rate-limiting and retry mechanism for an outbound integration with a legacy healthcare clearinghouse API that frequently experiences latency spikes.
• Answer aloud and timed: Walk through how you would introduce an asynchronous queueing system to handle high-volume batch claim submissions while maintaining strict ordering guarantees.

Hiring Manager Interview

A conversational session to discuss your past engineering experiences, leadership style, and technical interests.

What to demonstrate

• A conversational session to discuss your past engineering experiences, leadership style, and technical interests
• Depth in TypeScript

How to prepare

• Prepare two projects you led end to end, each with the decision you owned and what it cost.
• Have three questions about the team's roadmap and how success is measured in the first six months.

The 'WHO' Interview

A highly structured interview walking through your entire career history from high school to the present day.

What to demonstrate

• A highly structured interview walking through your entire career history from high school to the present day
• Depth in TypeScript

How to prepare

• Answer aloud and timed: Why did you decide to transition away from your last role, and what specific criteria are you looking for in your next engineering team?
• Answer aloud and timed: Describe a time when you had a strong technical disagreement with a peer or manager. How did you handle the communication, and what was the ultimate outcome?

Where candidates lose points

Focus on Clean Code Over Clever Hacks

In your coding rounds, prioritize readability, modularity, and proper naming conventions over highly optimized but unreadable one-liners. The interviewers value code that looks like it belongs in a production codebase.

Don't Skip the Clarifying Questions

In both the coding and system design rounds, take a few minutes to ask clarifying questions before writing code or drawing blocks. Some interviewers may intentionally present open-ended prompts to see how you gather requirements.

Do not spend your time memorizing complex algorithmic tricks

Focus on writing clean, modular, and readable TypeScript code.

Questions & practice

Implement a function to parse and validate a complex payload representing a medical claim, ensuring all requir

Medium · Practical Coding & Language Fundamentals

Implement a function to parse and validate a complex payload representing a medical claim, ensuring all required nested fields are present.

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?

Create a utility that performs an efficient object lookup and transformation based on a dynamic set of mapping

Medium · Practical Coding & Language Fundamentals

Create a utility that performs an efficient object lookup and transformation based on a dynamic set of mapping rules.

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

Write a program to simulate a state machine that transitions a billing invoice through various stages (e.g., D

Medium · Practical Coding & Language Fundamentals

Write a program to simulate a state machine that transitions a billing invoice through various stages (e.g., Draft, Submitted, Paid, Rejected) while handling invalid state transitions.

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

Walk through how you would introduce an asynchronous queueing system to handle high-volume batch claim submiss

Medium · System Redesign & Evolution

Walk through how you would introduce an asynchronous queueing system to handle high-volume batch claim submissions while maintaining strict ordering guarantees.

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

Explain why the owner filter ignores the listing index

Medium · composite index · query plans · sort elimination

The only index on resource is (tenant_id, status, updated_at DESC, resource_id DESC). A new endpoint returns one user's resources across all statuses, newest created first: WHERE tenant_id = $1 AND owner_user_id = $2 ORDER BY created_at DESC LIMIT 20. On a tenant with 2M rows it takes 900 ms and EXPLAIN shows a sort above a large scan. Explain precisely why the existing index cannot serve it, give the index that can, and state which of these the new index still will not help: owner_user_id alone across tenants; the same query ordered by updated_at. PostgreSQL 16.

Approach

1. Separate the two jobs an index does. For filtering, a composite btree is seekable only on a left prefix, so with no predicate on status the scan can at best range over tenant_id and test owner_user_id per row; PostgreSQL 16 has no btree skip scan to jump the unconstrained column.
2. For ordering, the index is sorted by (status, updated_at) within a tenant and not by created_at, so the LIMIT cannot stop early: every matching row is read and then sorted. That is the 'Sort Method: top-N heapsort' line, and it is why the plan reads 2M rows to answer with 20.
3. Derive the replacement from the access path — equality, equality, then the ordering column: CREATE INDEX CONCURRENTLY ON resource (tenant_id, owner_user_id, created_at DESC). The scan seeks to the (tenant, owner) range and walks 20 entries in order, so the Sort node disappears along with the row-read.
4. Treat INCLUDE (title, status) as conditional, not free. An index-only scan still visits the heap for any row whose page is not marked all-visible, so on a table taking 1.2k writes/second the win depends on autovacuum keeping the visibility map current, and the wider index costs more on every insert.

Follow-up

• 90% of rows are status='active'. Would a partial index WHERE status = 'active' change your answer, and for which of the three queries?
• A dashboard runs this for 40 owners in one page load. What changes about the design?

Hold a per-tenant active cap against concurrent creates

Hard · write skew · isolation levels · constraints

A tenant on the standard plan may hold at most 50 resources with status='active'. The create handler runs SELECT count(*) FROM resource WHERE tenant_id = $1 AND status = 'active', compares to 50, then inserts. Two creates arrive 3 ms apart on different instances and the tenant lands at 51. Name the anomaly, say whether PostgreSQL 16 READ COMMITTED or REPEATABLE READ prevents it and why, then give an implementation that holds the cap at READ COMMITTED with the exact statements. Finally, say what changes when the cap is 'at most one running export per tenant' on job_run.

Approach

1. Name it: write skew. The two transactions read an overlapping set and write disjoint rows, so there is no row-level conflict for the engine to detect and each commit is individually legal.
2. Rule out the levels precisely. READ COMMITTED takes a fresh snapshot per statement and takes no lock on the counted rows, so both see 49. PostgreSQL's REPEATABLE READ is snapshot isolation: it removes non-repeatable reads and phantoms within the snapshot but still admits write skew, because the anomaly is not a re-read of a changed row, it is a read of a set that a concurrent transaction invalidates. Only SERIALIZABLE closes it, by tracking the read dependency and aborting one transaction with SQLSTATE 40001 — a guarantee that exists only if the application re-runs the whole transaction from the read.
3. Convert the set predicate into a single-row conflict: keep tenant.active_resource_count and run UPDATE tenant SET active_resource_count = active_resource_count + 1 WHERE tenant_id = $1 AND active_resource_count < 50 in the same transaction as the INSERT. Zero affected rows is the cap, returned as 409. The row lock serialises the decision at any isolation level, and contention is bounded to one tenant's row — which is also the fair-scheduling unit, unlike a global counter that would convoy every tenant behind one row.
4. State the cost you just took on: a counter is a second source of truth that can drift, so every path that changes status must adjust it inside the same transaction, and a periodic reconciliation has to exist, with resource_revision as the authority for what the count should have been.

Follow-up

• A resource moves from archived back to active. Which statements change, and what breaks if the counter update and the status change land in different transactions?
• The cap becomes plan-dependent and a plan can change mid-month. Where does the number 50 live, and who reads it?

Explain the difference between structural typing and nominal typing in TypeScript, and demonstrate how you wou

Medium · Practical Coding & Language Fundamentals

Explain the difference between structural typing and nominal typing in TypeScript, and demonstrate how you would leverage type guards to handle runtime API responses.

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

Given a diagram of a simplified, monolithic billing ingestion service, identify the primary bottlenecks and pr

Medium · System Redesign & Evolution

Given a diagram of a simplified, monolithic billing ingestion service, identify the primary bottlenecks and propose a step-by-step plan to decouple it into microservices.

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?

How would you modify an existing database schema and API contract to support multi-tenant billing without caus

Medium · System Redesign & Evolution

How would you modify an existing database schema and API contract to support multi-tenant billing without causing downtime for existing customers?

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

Design a rate-limiting and retry mechanism for an outbound integration with a legacy healthcare clearinghouse

Medium · System Redesign & Evolution

Design a rate-limiting and retry mechanism for an outbound integration with a legacy healthcare clearinghouse API that frequently experiences latency spikes.

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

p99 jumped on one listing filter while p50 stayed flat

Medium · query planning · composite index · percentiles

After a release that added an owner_user_id filter to the resource listing, p99 rose from 90 ms to 1.9 s while p50 stayed at 40 ms. Traffic and row counts are unchanged. resource carries the index (tenant_id, status, updated_at DESC, resource_id DESC). The new query filters tenant_id and owner_user_id, orders by updated_at DESC, resource_id DESC, and takes 20 rows. On PostgreSQL, explain the shape of the regression, prove it from a query plan, and give the index you would add.

Approach

1. Start from the shape. A flat p50 with a moved p99 means a subset of requests changed cost, not all of them, so the first job is naming the subset. Bucket the endpoint's latency by the tenant's row count; the natural hypothesis is that large tenants are a small share of requests and all of the tail.
2. Get the plan for the new query on a large tenant with EXPLAIN (ANALYZE, BUFFERS). Expect an index scan over the tenant's range, a filter discarding most of it, then a Sort feeding the Limit, possibly reporting Sort Method: external merge Disk. Read actual rows on the scan node, not estimated.
3. Explain why the existing index cannot serve it. A composite B-tree is seekable only as a left prefix, and with no equality predicate on status the scan cannot treat updated_at as an ordering, because rows in the tenant's range are ordered by status first. Everything matching must be read and sorted before LIMIT 20 can apply, so a tenant with 400,000 rows pays 400,000 rows to return 20.
4. Add (tenant_id, owner_user_id, updated_at DESC, resource_id DESC). Equality on the first two columns leaves the index ordered by updated_at within that pair, so the plan becomes an index scan that stops after 20 rows with no Sort node. PostgreSQL can scan a B-tree backwards, so the DESC markers matter only if the two sort columns ever disagree in direction; keeping them explicit documents the order the keyset cursor depends on.

Follow-up

• The endpoint paginates with OFFSET. What does page 500 cost with your index, and what does the keyset version cost?
• How would you have caught this before release, given that a 10,000-row seed database produces the same plan shape at an unnoticeable cost?

Suggested practice checklist

Built from the rounds and topics Candid Health candidates report.

Day 1 — Map the Candid Health loop

• Write out the reported sequence: Recruiter Screen, Technical Screen / Take-Home, Coding Interview 2, System Design & Modification, Hiring Manager Interview, The 'WHO' Interview.
• 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 6 reported rounds, with the weakest marked.

Day 2 — Work TypeScript

• Spend the session on TypeScript, which Candid Health candidates report being tested on.
• Write one worked example in TypeScript and time yourself on it.

Deliverable: One timed worked example in TypeScript.

Day 3 — Work Coding Interviews (Practical Implementation)

• Spend the session on Coding Interviews (Practical Implementation), which Candid Health candidates report being tested on.
• Write one worked example in Coding Interviews (Practical Implementation) and time yourself on it.

Deliverable: One timed worked example in Coding Interviews (Practical Implementation).

Day 4 — Work System Design

• Spend the session on System Design, which Candid Health candidates report being tested on.
• Write one worked example in System Design and time yourself on it.

Deliverable: One timed worked example in System Design.

Day 5 — Answer out loud: Practical Coding & Language Fundamentals

• Answer aloud, timed: Implement a function to parse and validate a complex payload representing a medical claim, ensuring all required nested fields are present.
• Answer aloud, timed: Create a utility that performs an efficient object lookup and transformation based on a dynamic set of mapping rules.

Deliverable: Spoken answers to 2 reported Practical Coding & Language Fundamentals question(s), under time.

Day 6 — Answer out loud: System Redesign & Evolution

• Answer aloud, timed: Given a diagram of a simplified, monolithic billing ingestion service, identify the primary bottlenecks and propose a step-by-step plan to decouple it into microservices.
• Answer aloud, timed: How would you modify an existing database schema and API contract to support multi-tenant billing without causing downtime for existing customers?

Deliverable: Spoken answers to 2 reported System Redesign & Evolution question(s), under time.

Day 7 — Answer out loud: Chronological Career & Behavioral Reflection

• Answer aloud, timed: Starting from your time in high school and college, what motivated you to pursue computer science, and what were the key turning points in your early career?
• Answer aloud, timed: For each of your previous professional roles, what were the high points, the low points, and the achievements you are most proud of?

Deliverable: Spoken answers to 2 reported Chronological Career & Behavioral Reflection question(s), under time.

Behavioural preparation

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

Starting from your time in high school and college, what motivated you to pursue computer science, and what we

Medium · Chronological Career & Behavioral Reflec

Starting from your time in high school and college, what motivated you to pursue computer science, and what were the key turning points in your early career?

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?

For each of your previous professional roles, what were the high points, the low points, and the achievements

Medium · Chronological Career & Behavioral Reflec

For each of your previous professional roles, what were the high points, the low points, and the achievements you are most proud of?

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?

Why did you decide to transition away from your last role, and what specific criteria are you looking for in y

Medium · Chronological Career & Behavioral Reflec

Why did you decide to transition away from your last role, and what specific criteria are you looking for in your next engineering team?

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?

Describe a time when you had a strong technical disagreement with a peer or manager. How did you handle the co

Medium · Chronological Career & Behavioral Reflec

Describe a time when you had a strong technical disagreement with a peer or manager. How did you handle the communication, and what was the ultimate outcome?

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?