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FAIR VALUE MEASUREMENTS - Investment Portfolio by Level (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Mar. 31, 2025
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | $ 1,877,907 | [1],[2],[3],[4],[5] | $ 1,785,299 | [6],[7],[8],[9],[10]
Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 1,877,907 |  | 1,785,299 |
Recurring |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Total Investments | 1,877,907 |  | 1,785,299 |
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Total Investments | 0 |  | 0 |
Recurring | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Total Investments | 0 |  | 0 |
Recurring | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Total Investments | 1,877,907 |  | 1,785,299 |
Recurring | First lien loans |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 1,687,942 |  | 1,586,622 |
Recurring | First lien loans | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | First lien loans | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | First lien loans | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 1,687,942 |  | 1,586,622 |
Recurring | Second lien loans |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 17,170 |  | 18,066 |
Recurring | Second lien loans | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Second lien loans | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Second lien loans | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 17,170 |  | 18,066 |
Recurring | Subordinated debt |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 1,139 |  | 1,218 |
Recurring | Subordinated debt | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Subordinated debt | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Subordinated debt | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 1,139 |  | 1,218 |
Recurring | Preferred equity |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 66,474 |  | 102,918 |
Recurring | Preferred equity | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Preferred equity | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Preferred equity | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 66,474 |  | 102,918 |
Recurring | Common equity & warrants |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 101,440 |  | 76,475 |
Recurring | Common equity & warrants | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Common equity & warrants | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  | 0 |
Recurring | Common equity & warrants | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 101,440 |  | $ 76,475 |
Recurring | Earnout |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 3,742 |  |  |
Recurring | Earnout | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  |  |
Recurring | Earnout | Significant Other Observable Inputs (Level 2) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | 0 |  |  |
Recurring | Earnout | Significant Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investment owned, fair value | $ 3,742 |  |  |
[1] | All debt investments are income-producing, unless otherwise noted. Equity investments are non-income producing, unless otherwise noted. (2)All of the Company’s investments, the investments of Capital Southwest SPV LLC ("SPV"), the investments of SBIC I (as defined below) and the investments of SBIC II (as defined below) are pledged as collateral for the Company’s senior secured revolving credit facility, the SPV's financing credit facility or in support of the SBA-guaranteed debentures to be issued by Capital Southwest SBIC I, LP ("SBIC I") and Capital Southwest SBIC II, LP ("SBIC II"), the Company's wholly-owned subsidiaries that operate as small business investment companies, respectively.
[2] | Non-Control/Non-Affiliate investments are generally defined by the 1940 Act as investments that are neither control investments nor affiliate investments. Investments are classified as non-control/non-affiliate investments, unless otherwise noted. At September 30, 2025, the Company held $1,488.4 million of non-control/non-affiliate investments, which represented approximately 79.3% of the Company’s investment assets. The fair value of these investments as a percent of net assets is 157.1%.
[3] | The Company's investment portfolio is comprised entirely of debt and equity securities of privately held companies for which quoted prices falling within the categories of Level 1 and Level 2 inputs are not readily available. Therefore, the Company values all of its portfolio investments at fair value, as determined in good faith by the valuation committee comprised of certain officers of the Company (the "Valuation Committee") as the valuation designee of the Board of Directors (the "Valuation Designee") pursuant to Rule 2a-5 under the Investment Company Act of 1940, as amended (the “1940 Act”), using significant unobservable Level 3 inputs. Refer to Note 4 - Fair Value Measurements for further discussion.
[4] | Equity ownership may be held in shares or units of a company that is either wholly owned by the portfolio company or under common control by the same parent company to the portfolio company.
[5] | Unless otherwise noted, all portfolio company headquarters are based in the United States.
[6] | Equity ownership may be held in shares or units of a company that is either wholly owned by the portfolio company or under common control by the same parent company to the portfolio company.
[7] | Unless otherwise noted, all portfolio company headquarters are based in the United States.
[8] | All debt investments are income-producing, unless otherwise noted. Equity investments are non-income producing, unless otherwise noted.
[9] | Non-Control/Non-Affiliate investments are generally defined by the 1940 Act as investments that are neither control investments nor affiliate investments. Investments are classified as non-control/non-affiliate investments, unless otherwise noted. At March 31, 2025, the Company held $1,436.3 million of non-control/non-affiliate investments, which represented approximately 80.5% of the Company’s investment assets. The fair value of these investments as a percent of net assets is 162.6%.
[10] | The Company's investment portfolio is comprised entirely of debt and equity securities of privately held companies for which quoted prices falling within the categories of Level 1 and Level 2 inputs are not readily available. Therefore, the Company values all of its portfolio investments at fair value, as determined in good faith by the valuation committee comprised of certain officers of the Company (the "Valuation Committee") as the valuation designee of the Board of Directors (the "Valuation Designee") pursuant to Rule 2a-5 under the Investment Company Act of 1940, as amended (the “1940 Act”), using significant unobservable Level 3 inputs. Refer to Note 4 - Fair Value Measurements for further discussion.

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